Embed Size (px)
Citation preview
I
Keywords
Photocatalysis; Visible light; Localized surface plasmon resonance; Plasmonic
photocatalysts; Plasmonic metal nanoparticles; Gold nanoparticles; Alloy
nanoparticles; Cross-coupling reactions; Nitrobenzene reduction; Alcohol oxidation;
Esterification; Interband absorption; Nonplasmonic metal nanoparticles; Organic
synthesis
II
Abstract
Photocatalysis is particularly intriguing in the realm of green chemical science,
because it combines the efficiency of catalysis with the use of light energy.
Photocatalysts that can drive the synthesis of fine chemicals with visible light—the
reliable, abundant and green energy source that produces no pollution—at ambient
temperature, are of great interest. However, it is still a challenging goal to develop new
heterogeneous photocatalysts that exhibit high activity for the synthesis of fine organic
chemicals under visible light irradiation and moderate conditions. This project aims to
develop new metal nanoparticle photocatalysts and investigate some typical significant
organic reactions using the catalysts under visible light irradiation.
Firstly, we incorporated palladium (Pd)—a metal component with an intrinsic
catalytic ability for many chemical reactions, into plasmonic-metal (gold) nanoparticles
to obtain gold-palladium alloy nanoparticles (Au-Pd alloy NPs). These highly efficient
Au-Pd alloy NPs photocatalyst can strongly absorb visible light to catalyze various
cross-coupling reactions at ambient temperatures. Among those well-known cross-
coupling reactions, we focused on a systematic investigation of the Au-Pd alloy NP
catalyzed Suzuki cross-coupling reaction under visible light irradiation at very low
reaction temperatures. We found that the performance of the alloy catalyst depends on
the alloy composition, light intensity and wavelength. When the Au:Pd molar ratio is
1:1.62 in the catalyst, the reaction exhibits the best turnover frequency (TOF) and photo
quantum yield (QY). The application of the Au-Pd alloy NP photocatalysts has also
been extended to other more challenging catalytic reaction under mild conditions. We
found that the direct oxidative esterification of aliphatic alcohols can be driven by
visible light irradiation where molecular oxygen is the benign oxidant, using Au-Pd
III
alloy NPs supported on phosphate doped hydrotalcite (HT) photocatalysts. The
phosphate doped HT support can effectively act as basic site for the catalytic reactions
in base-free conditions. Notably, these heterogeneous catalysts are easily recycled and
can be conveniently reused, which is an important aspect in the development of
practical and cost-effective catalytic oxidation processes.
The application of Au-based plasmonic-metal photocatalyst is not only limited to
Au-Pd alloy NPs, but can also be extend to other alloy NPs. Thus we developed Au-Cu
alloy NPs, and found that Au-Cu alloy NPs can selectively change the reaction pathway
for the reduction of nitroaromatics under visible light irradiation―directly to aromatic
amines rather than to unavoidable azo- or azoxy-derivatives. The Au/Cu composition in
the alloy NPs can be finely tuned to obtain the optimal photocatalytic activity and
maintain surface Cu stability in air, with Au/Cu=2.6/0.4 exhibits the best performance.
This work suggests that the alloy NPs can provide new pathways in selective
photocatalytic processes.
Apart from alloy NP photocatalysts, this thesis also includes the development for
supported Au NP photocatalysts based on the previous study of our group. By using the
support of HT with ions exchange, we found that visible light can drive selective
reduction of aromatic nitro compounds to azoxy compounds using the action of HT
support Au NPs under mild conditions. Thus, we can efficiently control the product
selectivity of the reduction of aromatic nitro compounds. Moreover, supported gold
nanoparticles (Au/Al2O3) can drive esterification from aldehydes and alcohols by
visible light at ambient temperatures. The photocatalytic efficiencies strongly depend
on the Au loading, particle sizes of the AuNPs, irradiance and wavelength of the light
irradiation.
IV
Finally, a breakthrough was made in the area of metal NP photocatalysts recently.
We discovered that irradiation with light can significantly enhance the intrinsic
catalytic performance of nonplasmonic transition metals (Pd, Pt, Rh, and Ir) NPs at
ambient temperatures for several types of reactions. These metal NPs strongly absorb
the light mainly through interband electronic transitions. The excited electrons interact
with the reactant molecules on the particles to accelerate these reactions. The rate of the
catalyzed reaction depends on the concentration and energy of the excited electrons,
which can be increased by increasing the light intensity or by reducing the irradiation
wavelength. Since NPs of nonplasmonic metals have been widely used for various
applications in catalytic reactions, the reported discovery may significantly broaden the
application of catalytic processes driven by solar energy.
Overall, the discovery of these metal NP photocatalysts for organic synthesis
reveals new photocatalytic mechanisms for the controlled transformation of chemical
compounds. The prospect of sunlight irradiation driving chemical reactions may
provide opportunity for the organic synthesis via a more controlled, simplified, and
greener process.
V
List of Publications
Journal Publications
1. Q. Xiao, Z. Liu, A. Bo, S. Zavahir, S. Sarina, S. Bottle, J. D. Riches, H. Y. Zhu*,
Catalytic Transformation of Aliphatic Alcohols to Corresponding Esters in O2
under Neutral Conditions Using Visible Light Irradiation, Journal of the American
Chemical Society, 2015, 137, 1956-1966 (IF=11.444).
2. Q. Xiao, S. Sarina, A. Bo, J. Jia, H. Liu, D. P. Arnold, Y. Huang, H. Wu, H. Zhu*,
Visible Light Driven Cross-Coupling Reactions at Lower Temperatures Using a
Photocatalyst of Palladium and Gold Alloy Nanoparticles, ACS Catalysis, 2014, 4,
1725-1734 (IF=7.572).
3. Q. Xiao, S. Sarina, E. Jaatinen, J. Jia, D. P. Arnold, H. Liu, H. Zhu*, Efficient
Photocatalytic Suzuki Cross-coupling Reactions on Au-Pd Alloy Nanoparticles
under Visible Light Irradiation, Green Chemistry, 2014, 16, 4272-4285 (IF=6.852).
4. Q. Xiao, E. Jaatinen, H. Zhu*, Direct Photocatalysis for Organic Synthesis using
Plasmonic Metal Nanoparticles Irradiated with Visible Light, Chemistry-An Asian
Journal, 2014, 9, 3046-3064 (IF=3.935).
5. S. Sarina, H. Zhu*, Q. Xiao, E. Jaatinen, J. Jia, Y. Huang, Z. Zheng, H. Wu,
Viable Photocatalysts under Solar-Spectrum Irradiation: Nonplasmonic Metal
Nanoparticles, Angewandte Chemie International Edition, 2014, 53, 2935-2940
(IF=11.336).
6. S. Sarina, H. Zhu*, E. Jaatinen, Q. Xiao, H. Liu, J. Jia, C. Chen, J. Zhao,
Enhancing Catalytic Performance of Palladium in Gold and Palladium Alloy
Nanoparticles for Organic Synthesis Reactions through Visible Light Irradiation at
Ambient Temperatures, Journal of the American Chemical Society, 2013, 135,
5793-5801 (IF=11.444).
7. W. Cui, Q. Xiao, S. Sarina, W. Ao, M. Xie, H. Zhu*, Z. Bao, Au-Pd Alloy
Nanoparticle Catalyzed Selective Oxidation of Benzyl Alcohol and Tandem
Synthesis of Imines at Ambient Conditions, Catalysis Today, 2014, 235, 152-159
(IF=3.309).
8. Y. Zhang, Q. Xiao, Y. Bao, Y. Zhang, S. Bottle, S. Sarina, Z. Bao, H. Zhu*, Direct
Photocatalytic Conversion of Aldehydes to Esters using Supported Gold
VI
Nanoparticles under Visible Light Irradiation at Room Temperature, Journal of
Physical Chemistry C, 2014, 118, 19062-19069 (IF=4.835).
Manuscript Submitted
9. Q. Xiao, S. Sarina, E. R. Waclawik, J. F. Jia, J. Chang, J. D. Riches, H. Y. Zhu*,
The Alloying of Small Amounts of Cu into Au Nanoparticles Alters the Reaction
Pathway of the Photocatalytic Reduction of Nitroaromatics for Sole Amine
Products.
10. Q. Xiao, A. Bo, Z. Zheng, W. Martens, H. Zhu*, Visible Light Driven Selective
Reduction of Aromatic Nitro to Azoxy Compounds using Supported Gold
Nanoparticles: a Promotional Effect of Phosphate and Transition Metal Ions in
Hydrotalcite Support.
Conferences and Presentations
11. Q. Xiao, S. Sarina, H. Zhu*, Enhancing Catalytic Performance of Palladium in
Gold and Palladium Alloy Nanoparticles for Suzuki Reactions under Visible Light
Irradiation, XIth European Congress on Catalysis (EuropaCat Lyon 2013), Lyon,
France, September 1-6, 2013 (Oral presentation).
12. Q. Xiao, H. Zhu*, Visible Llight Photocatalytic Process for Cross-Coupling
Reactions at Green Mild Conditions, 3rd International Symposium on Green
Chemistry (ISGC 2015), La Rochelle, France, May 3-7, 2015 (Oral presentation).
VII
QUT Verified Signature
VIII
Acknowledgements
First and foremost I would like to express my heartfelt gratitude to my principle
supervisor Prof. Huaiyong Zhu, for his excellent guidance, caring, patience, and
providing me with an excellent atmosphere for doing research; for his always pushing
me forward, and passing on the research values and the dreams that he has given to me.
I would never have been able to achieve such progress without his guidance and
support during the last three years.
Thanks also go to my associate supervisor Dr. Xuebin Ke, for his guidance,
support and patience towards the completion of my work.
I would like to thank my senior colleague Dr. Sarina Sarina, who also as a good
friend, was always willing to help and give her best suggestions on my research.
Many thanks to A/Prof. Esa Jaatinen, A/Prof. Dennis P. Arnold, A/Prof. Eric R.
Waclawik, A/Prof. Aijun Du, Dr. Hongwei Liu, Dr. Jamie Riches, Dr. Wayde Martens,
Dr. Zhanfeng Zheng and Prof. Jianfeng Jia for their collaboration, advice and valuable
suggestion particularly in the method of conducting a research.
Special thanks, of course, go to my dear colleagues: Arixin Bo, Yiming Huang,
Zhe Liu and Fathima Sifani Zavahir, who lent me a helping hand in conducting the lab
works.
Sincere thanks also extend to Dr. Chris Carvalho, Mrs. Leonora Newby and Dr.
Llew Rintoul for their training and help with the instruments technology, Mr. Tony
Raftery and Dr. Barry Wood (UQ) for their kind assistance on XRD and XPS.
IX
I wish to thank the QUT for supporting the tuition fee and CSC for living
allowance. Appreciates also give to the funding from Australian Research Council
(ARC) for the research.
Finally, my deepest gratitude goes to my mom, for her generous support and
continuous encouragement throughout my entire university education.
X
Table of Contents
Keywords .......................................................................................................................... I
Abstract ........................................................................................................................... II
List of Publications ......................................................................................................... V
Statement of Original Authorship ................................................................................ VII
Acknowledgements ..................................................................................................... VIII
Table of Contents ............................................................................................................ X
INTRODUCTORY REMARKS .................................................................................. XII
CHAPTER 1 .................................................................................................................... 1
INTRODUCTION AND LITERATURE REVIEW ........................................................ 1
1.1 Introductory Remarks ........................................................................................ 1
1.2 Article 1 ............................................................................................................. 2
CHAPTER 2 .................................................................................................................. 47
SUPPORTED GOLD BASED ALLOY NANOPARTICLE PHOTOCATALYSTS
FOR ORGANIC SYNTHESIS BY VISIBLE LIGHT .................................................. 47
2.1 Introductory Remarks ...................................................................................... 47
2.2 Article 2 ........................................................................................................... 51
2.3 Article 3 ........................................................................................................... 77
2.4 Article 4 ......................................................................................................... 120
2.5 Article 5 ......................................................................................................... 156
XI
CHAPTER 3................................................................................................................. 187
SUPPORTED GOLD NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS BY VISIBLE LIGHT ............................................................................ 187
3.1 Introductory Remarks .................................................................................... 187
3.2 Article 6 ......................................................................................................... 189
3.3 Article 7 ......................................................................................................... 207
CHAPTER 4................................................................................................................. 223
NONPLASMONIC METAL NANOPARTICLE PHOTOCATALYSTS FOR
ORGANIC SYNTHESIS BY VISIBLE LIGHT ......................................................... 223
4.1 Introductory Remarks .................................................................................... 223
4.2 Article 8 ......................................................................................................... 225
CHAPTER 5
CONCLUSIONS & FUTURE WORK ........................................................................ 252
Conclusions .............................................................................................................. 252
Future Work.............................................................................................................. 255
XII
INTRODUCTORY REMARKS
This thesis “Visible Light Photocatalytic Synthesis of Fine Organic Chemicals
with New Photocatalysts” investigated new metal nanoparticle photocatalysts and their
application in organic synthesis using visible light. The aim of this thesis is to develop
new photocatalysts instead of traditional semiconductor photocatalysts to utilize visible
light to drive chemical reactions under mild reaction conditions:
• To develop new photocatalysts, such as plasmonic-metal nanoparticle
(such as gold) and its alloy with other catalytically active transition metal
nanoparticles
• To develop new transition metal nanoparticle photocatalysts (so-called
nonplasmonic nanoparticles, such as palladium and platinum)
• The application of these new photocatalysts to drive the synthesis of fine
organic chemicals
• To study the mechanism of the photocatalytic reactions
This thesis will show that plasmonic-metal nanoparticle (such as gold) as well as
its alloy with other catalytically active transition metal nanoparticles can be used as
efficient visible light photocatalysts. More importantly, those transition metal
nanoparticles (so-called nonplasmonic nanoparticles, such as palladium and platinum)
which are widely used as thermally activated catalysts for the synthesis of important
organic compounds can also enhance the efficiency of organic reactions under visible
light irradiation. This thesis will highlight that these two kinds of metal nanoparticle
photocatalysts are able to drive many useful organic chemical reactions by visible light,
while traditional semiconductor photocatalysts are mainly used for simple
XIII
oxidation/reduction and dye degradations. The apparently different reaction mechanism
for the metal nanoparticle photocatalysts will also be highlighted. The significance of
this study is to find an alternative processs to drive chemical reactions under green mild
condition by visible light or even sunlight, which is an important aspect in the view of
sustainable and green chemistry.
This thesis is a collection of published works submitted by the author to various
scientific journals. Thus, the general formatting follows the style of the specific
journals. Repetition and redundancy in the introductory sections of each paper is
unavoidable owing to the close relationships between the subject matter published.
The flow chart on the next page is a graphical representation of how this thesis
structured.
XIV
CHAPTER 1: INTRODUCTION AND LITERATURE REVIEW A review of the literatures relating to the latest developments in direct photocatalysis using plasmonic-metal nanoparticles for organic synthesis.
CHAPTER 2: SUPPORTED GOLD BASED ALLOY NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS BY VISIBLE LIGHT Papers on the Au-Pd alloy nanoparticle photocatalysts for cross-coupling reactions and direct esterification of aliphatic alcohols. A study on Au-Cu alloy nanoparticle photocatalysts for direct nitrobenzene reduction to aniline.
CHAPTER 3: SUPPORTED GOLD NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC SYNTHESIS BY VISIBLE
LIGHT Papers on using supported Au nanoparticles to drive nitrobenzene reduction to azoxybenzene and esterification from aldehydes and alcohols by visible light.
CHAPTER 4: NONPLASMONIC METAL NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC SYNTHESIS BY VISIBLE
LIGHT Extend the application of plasmonic-metal nanoparticle photocatalysts to transition metal nanoparticles. Use nonplasmonic metal nanoparticles to enhance the efficiency of organic reactions under visible light irradiation.
CONCLUSIONS & FUTURE WORK Final conclusions are made based on all the scientific papers compiled in this thesis. The individual conclusions of each paper are summarised. Avenues for future work are also suggested.
1
CHAPTER 1
INTRODUCTION AND LITERATURE REVIEW
1.1 Introductory Remarks
This chapter includes one review article:
Article 1 is an invited Focus Review by Chemistry–An Asian Journal. This Focus
Review summarizes the latest developments in direct photocatalysis using plasmonic-
metal nanoparticles. Plasmonic-metal nanoparticles are recognized as a new form of
medium that is particularly efficient in harvesting light energy for chemical processes
due to their strong light absorption over a range of the visible and UV regions of the
solar spectrum. Recently, a conceptual breakthrough was made: plasmonic-metal
nanoparticles such as Au, Ag and Cu can be used directly as visible light
photocatalysts, and the photocatalytic mechanisms are distinct from those for
traditional semiconductor photocatalysts. The progress in this new burgeoning research
area is of great interest. In this Focus Review, recent developments in the direct
photocatalysis of plasmonic-metal nanoparticles are described, with a focus on the role
of the localized surface plasmon resonance (LSPR) effect in plasmonic-metal
nanoparticles and their applications in organic transformations. The role of light
irradiation in the catalyzed reactions and the light-excited energetic electron reaction
mechanisms will be highlighted.
2
1.2 Article 1
DOl: 10.1002/asia.201402310
Direct Photocatalysis for Organic Synthesis by Using Plasmonic-Metal Nanoparticles Irradiated with Visible Light
Qi Xiao, Esa Jaatinen, and Huaiyong Zhu*[�l
Chem. Arian J. 2014, 9, 3046-3064 Wiley Online Library 3046 C 2014 Wiley-VCHVerlag GmbH&Co. KGaA, Weinheim
3
4
47
CHAPTER 2
SUPPORTED GOLD BASED ALLOY NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC SYNTHESIS BY VISIBLE LIGHT
2.1 Introductory Remarks
This chapter includes four articles:
Article 2 (published on ACS Catalysis, 2014, 4, 1725−1734) is the first report of
visible light driven various cross-coupling reactions using Au-Pd alloy nanoparticles
under ambient temperatures. Palladium (Pd)-catalyzed cross-coupling reactions are
among the most important methods in organic synthesis for the formation of carbon–
carbon bonds. In this paper, we report the discovery of highly efficient and green
photocatalytic processes by which cross-coupling reactions, including Sonogashira,
Stille, Hiyama, Ullmann, and Buchwald–Hartwig reactions, can be driven with visible
light at temperatures slightly above room temperature using alloy nanoparticles of gold
and Pd on zirconium oxide, thus achieving high yields. These catalytic cross-coupling
processes are due to the interaction of light-excited electrons of the catalyst with the
reactant molecules, while high temperatures are not a prerequisite for driving them. The
reaction rate depends on the number of light-excited electrons and the number of
reactant molecules on the catalyst surface. The number of reactant molecules on the
surface depends mainly on the affinity of the surface for the reactants. Pd sites have a
strong affinity for those reactant molecules. This finding reveals the possibility of green
cross-coupling reactions driven by solar energy, and it is also a breakthrough to drive
48
the widely-used cross-coupling reactions with heterogeneous catalysts with visible light
under mild conditions.
In Article 3 (published on Green Chemistry, 2014, 16, 4272−4285), we focused
on a systematic investigation of the Au-Pd alloy nanoparticle catalyzed Suzuki cross-
coupling reaction under visible light irradiation at very lower reaction temperature (30
°C). We found that the performance of the alloy catalyst depends on the alloy
composition, light intensity and wavelength. When the Au:Pd molar ratio is 1:1.62 in
the catalyst, it exhibits the best turnover frequency (TOF) and photo quantum yield
(Q.Y.). The results of both the free electron-gas model analysis and density functional
theory (DFT) simulation indicate that the Au-Pd alloy nanostructure increases the
charge heterogeneity of the NP surface, which enhances interaction between the alloy
NPs and the reactant molecules adsorbed on the nanoparticles. The strong interaction
facilitates the transfer of light-excited electrons on the alloy nanoparticles to the
reactant molecules adsorbed on the particles, and such electron transfer weakens the C–
I bond of the reactant molecules and facilitates the reactions. Understanding this
mechanism is useful for developing photocatalytic versions of other cross-coupling
reactions.
The application of the Au-Pd alloy nanoparticle photocatalysts has also been
extend to other more challengeable catalytic reaction under mild conditions. In Article
4 (published on J. Am. Chem. Soc., 2015, 137, 1956−1966), we reported that the direct
oxidative esterification of aliphatic alcohols can be driven by visible light irradiation
and molecular oxygen as benign oxidant using recyclable Au-Pd alloy nanoparticles
supported on phosphate doped hydrotalcite photocatalysts. The phosphate doped
hydrotalcite support can effectively act as basic site for the catalytic reactions in base-
free conditions. Esterification is one of the most fundamentally important reactions in
49
organic synthesis. Traditionally, esters are prepared by the reaction of activated acid
derivatives with alcohols, multistep process that often produces large amounts of
unwanted by-products. The direct catalytic esterification of non-activated aliphatic
alcohols with molecular oxygen is rather challenging, especially in the absence of
additional base. Herein we report visible light driven direct oxidative esterification of
aliphatic alcohols, which proceed highly selective under mild conditions with oxygen
as oxidant. To the best of our knowledge, no results regarding the Au-Pd alloy
nanoparticles catalyzed direct oxidative esterification of aliphatic alcohols, let alone
base-free (no additive) under visible light irradiation, have been reported. Notably,
these heterogeneous catalysts are easily recycled and can be conveniently reused,
which is an important aspect in the development of practical and cost-effective catalytic
oxidation processes. This result represents a milestone towards greener commercial
process for clean and efficient production of aliphatic esters.
The application of Au-based plasmonic-metal photocatalyst is not only limited to
Au-Pd alloy nanoparticles, but can also be extend to other alloy nanoparticles (Au-Cu
alloy). In 2010 we have discovered that supported Au nanoparticles can efficiently
drive reduction of nitroaromatics to azo-compounds by visible light at ambient reaction
conditions (Angew. Chem. Int. Ed., 2010, 49, 9657). In Article 5 (a submitted
manuscript), by alloying trace copper (Cu) with Au, we found that the obtained Au-Cu
alloy nanoparticles can selectively change the reaction pathway for the reduction of
nitroaromatics under visible light irradiation―directly to aromatic amines rather than
to unavoidable azo- or azoxy-derivatives. The Au/Cu composition in the alloy
nanoparticles can be finely tuned to obtain the optimal photocatalytic activity and
maintain surface Cu stability in air, with Au/Cu=2.6/0.4 exhibits the best performance.
The Au-Cu alloy nanoparticles absorb visible light, and the light excited energetic
50
electrons on the NP surface activate the reactants. Tuning light intensity and
wavelength can obtain different reaction activity. Using density functional theory
(DFT) calculations we confirmed that on the Au-Cu alloy surface, the intermediate
nitrosobenzene was strongly adsorbed by the Cu atoms on the surface, it is readily to
follow the direct route to produce aniline on the Au-Cu surface rather than follow the
condensation route on pure Au surface. The stronger adsorption energy of the product
aniline on Au-Cu alloy surface could also facilitate its formation. This work suggests
that by alloying the plasmonic metal nanoparticles catalysts will find more applications
and provide new mechanisms in selective photocatalytic processes.
51
2.2 Article 2
+QJij!QijiiifiM pubs.acs.org/a.cscatalysis
Visible Light-Driven Cross-Coupling Reactions at Lower Temperatures Using a Photocatalyst of Palladium and Gold Alloy Nanoparticles Q! Xiao/ Sarina Sarina/ Arixin Bo/ Jianfeng Jia/ Hongwei Liu/ Dennis P. Arnold,t Yiming Huang, t Haishun Wu,:t: and Huaiyong Zhu*'t tschool of Chentistry, Physics and Mechanical Engineering, Faculty of Science and Technology, Queensland University of Technology, Brisbane, QLD 4001, Australia *School of Chentical and Material Science, Shanxi Normal University, Linfen 041004, China
52
Abstract: Palladium (Pd)-
catalyzed cross-coupling reactions
are among the most important
methods in organic synthesis. We
report the discovery of highly
efficient and green photocatalytic
processes by which cross-coupling
reactions, including Sonogashira, Stille, Hiyama, Ullmann, and Buchwald-Hartwig
reactions can be driven with visible light at temperatures slightly above room
temperature using alloy nanoparticles of gold and Pd on zirconium oxide, thus
achieving high yields. The alloy nanoparticles absorb visible light and their conduction
electrons gain energy, which is available at the surface Pd sites. Results of the density
functional theory calculations indicate that transfer of the light excited electrons from
the nanoparticle surface to the reactant molecules adsorbed on the nanoparticle surface
activates the reactants. When the light intensity was increased, a higher reaction rate
was observed, because of the increased population of photoexcited electrons. The
irradiation wavelength also has an important impact on the reaction rates. Ultraviolet
irradiation can drive some reactions with chlorobenzene substrate, while visible light
irradiation failed to, and substantially improve the yields of the reactions with the
bromobenzene substrate. The discovery reveals the possibility of using low-energy and
-density sources such as sunlight to drive chemical transformations.
Keywords: alloy nanoparticles; cross-coupling; photocatalysis; surface plasmon
resonance; visible light
53
1. Introduction
Palladium-based catalysts are widely used in cross-coupling reactions for the formation
of carbon-carbon bonds. A large variety of homogeneous catalytic systems based on
Pd(II) or Pd(0) have become powerful and versatile tools in modern organic synthesis,1-
3 being employed from the total synthesis of natural products to the preparation of new
materials to bio-organic chemistry.4,5 In addition, recent years have seen the imperative
to develop “green” cross-coupling reactions using more environmentally friendly
catalysts and methods.4 For example, Pd nanoparticles (PdNPs) have emerged as
promising catalysts that can function under moderate conditions and be recycled more
readily,6,7 although the catalytic efficiency of the NPs is often not as good as that of the
homogeneous Pd catalysts. Many of the catalytic reactions (with homogeneous Pd
catalysts or PdNP catalysts) are thermally driven to achieve viable efficiency, but the
heating also has negative side effects. For example, it makes the synthesis reactions
energy-intensive as we have to heat the entire reaction system, including the reactor
and solvent. High reaction temperatures may also increase the extent of formation of
unwanted side products in some reactions.8-11 Furthermore, heating may compromise
catalysts’ stability and reusability. Thus, new catalytic systems based on efficient,
recyclable catalysts and green energy sources for effective chemical transformations
are highly desirable but remain a significant challenge.
Light is one potentially sustainable energy source with which to drive chemical
reactions.12-19 Very recently, we discovered that Au-Pd alloy NPs can strongly absorb
visible light and efficiently enhance the extents of conversion of several reactions,
including the Suzuki-Miyaura cross-coupling reactions, at temperatures slightly above
room temperature.19 Two other research groups also found almost at the same time that
it is an effective approach to use irradiation to accelerate Suzuki cross-coupling
reactions by using Au-Pd bimetallic nanostructures.20,21 We believe that the conduction
electrons of the NPs gain the energy of the incident light, generating electrons at high
energy levels (light -excited electrons). These light -excited electrons are available at
the surface Pd sites of the alloy NPs. The surface Pd sites have good affinity for the
reactant molecules, and the electrons at these sites enhance their intrinsic ability to
activate the reactant molecules. The charge heterogeneity of the alloy NP surface,
because of the different electronegativities of gold and palladium, also plays a key role
54
in the catalytic reactions. The proposed mechanism of light-excited electrons of alloy
NPs and the interaction of the adsorbed reactant molecules with the excited electrons
and the NP surface is not specific to the Suzuki-Miyaura cross-coupling reaction. It is
rational to hypothesize that the Au-Pd alloy structure is likely to be efficient for driving
a variety of Pd-catalyzed cross-coupling reactions with visible light. A challenge is that
the activation of various reactant molecules for different reactions may not be the same,
but the study of several cross-coupling reactions can provide insight into the
mechanism of the photocatalytic cross-coupling processes in general.
In this study, five different cross-coupling reactions were investigated to confirm
the general applicability of Au-Pd alloy NP photocatalysts under visible light
irradiation, namely the Sonogashira, Stille, Hiyama, and Ullmann C-C couplings and
the Buchwald-Hartwig amination (C-N cross-coupling). We found that visible light can
drive these cross-coupling reactions with the alloy NP catalyst at temperatures slightly
above room temperature, and the performance of the alloy catalyst depends on the
intensity and wavelength of the light irradiation. A mechanism is proposed on the basis
of the results of the density functional theory (DFT) calculations and experimental
observation: light absorption of the alloy NPs generates excited electrons, and the light
-excited electrons with sufficient energy are able to transfer into the lowest unoccupied
molecular orbital (LUMO) of the reactant molecules adsorbed on the NPs, weakening
the chemical bonds of the molecules and facilitating the reactions.
2. Results and Discussion
2.1 Catalyst Synthesis and Characterization
In this study, Au-Pd NPs were supported on zirconia (ZrO2) powder as
photocatalysts (for the detailed method, see the Experimental Section). The NPs of pure
gold and pure palladium on the ZrO2 support (AuNPs@ZrO2 and PdNPs@ZrO2) were
also prepared under synthetic conditions similar to those used for the synthesis of the
alloy NPs. Figure 1 shows the transmission electron microscopy (TEM) analysis of the
alloy NPs; the Au-Pd alloy NPs are distributed evenly on the ZrO2 particle surface, and
the mean diameters of the Au-Pd alloy NPs are <7 nm (Figure 1b). The elemental
composition of the as-prepared Au-Pd alloy NPs was studied using energy dispersion
55
X-ray spectroscopy (EDX). As shown in Figure 1e, line scan analysis for a typical Au-
Pd alloy NP shows that Au and Pd are distributed fairly uniformly in an alloy NP.
Figure 1. (a) TEM image of the Au−Pd alloy NPs. (b) Particle size distribution of the
Au−Pd alloy NPs. (c and d) High -resolution TEM (HR-TEM) images of the Au-Pd
alloy NPs. (e) Line profile analysis of a typical Au−Pd NP providing information about
the elemental composition and Au/Pd distribution of the NP.
The important feature of the Au−Pd alloy NPs, which makes them useful in
photocatalysis, is that they strongly absorb visible light mainly through the localized
surface plasmon resonance (LSPR) effect of AuNPs.19−21 Figure 2 shows the diffuse
reflectance ultraviolet−visible extinction (DR UV-vis) spectra of the samples. Here, the
spectrum of the Au-Pd alloy NP sample is clearly different from the spectra of the pure
metal NPs. The ZrO2 support exhibited little absorption of light with wavelengths
56
longer than 400 nm. Hence, the light absorption in the spectra of the Au−Pd alloy
NPs@ZrO2 samples, which is the difference between light absorption of the alloy
NPs@ZrO2 and that of the ZrO2 support alone, is due to the absorption of the alloy
NPs.22,23 The difference between the two spectra of supported and colloid metal NPs
can be attributed to scattering. Because the distances between the alloy NPs on the
ZrO2 support are much smaller than those between the alloy NPs in their colloid
suspension, the light scattering is much stronger for the supported alloy NPs than that
for the unsupported NPs.24 The light scattering is generally more significant at longer
wavelengths (<600 nm).
Figure 2. DR UV-vis spectra of the Au−Pd alloy NPs@ZrO2 catalyst and their
comparison with pure AuNPs@ZrO2, PdNPs@ZrO2, and Au−Pd alloy NPs without the
ZrO2 support in an aqueous suspension.
2.2 Photocatalytic Reactions
3-Iodotoluene was used as the aryl halide substrate to react with various coupling
partners using Au-Pd alloy NPs under visible light irradiation (Table 1). The reactions
were also conducted in the dark but with other conditions identical. For example, the
temperature of the reaction mixture in the dark was kept the same as that of the reaction
mixture under light (45 °C) by a water bath. The data in Table 1 show the results of
visible light -enhanced cross-coupling reactions using the Au-Pd alloy NP catalyst.
57
Table 1. Performance of Au-Pd Alloy NPs, AuNPs, and PdNPs for Cross-Coupling
Reactions under Visible Light Irradiation (red numbers) and in the Dark (black
numbers).
aThe yields were calculated from the product content and the aryl iodide conversions
measured by gas chromatography (GC). A Nelson halogen lamp (wavelengths of 400-
750 nm) was used as the visible light source and the light intensity was measured to be
0.45 W/cm2. The reaction was conducted at 45±2 °C for 24 h. For the detailed reaction
conditions, see the Experimental Section. bTOF (turnover frequency) values were
calculated on the basis of the amount of Pd metal.
It is evident that irradiation increased the extents of conversion of all the cross-
coupling reactions (compared with the same reactions conducted in the dark). Control
experiments using the support ZrO2 (without Au−Pd alloy NPs) as the catalyst were
performed. No conversion was observed for the reaction when the system was
illuminated with light or when the reaction was conducted in the dark because ZrO2 has
a large band gap (5 eV) and exhibits negligible light absorption in the visible range.25
Undoubtedly, the catalytic activity is due to the Au−Pd alloy NPs, and the catalytic
enhancement observed when the system was irradiated is due to the alloy NPs.
There have been many reports of cross-coupling reactions catalyzed via
homogeneous or heterogeneous processes, but most of them need to be conducted at
elevated temperatures (≥100 °C), even under reflux conditions.26-30 This study shows
that visible light irradiation can drive the same reactions on the Au−Pd alloy NPs under
58
much milder reaction conditions (45 °C), achieving very good yields, and no additional
additives such as cocatalysts or phosphine ligands are required. The photocatalytic
process at low reaction temperatures proposed in this study is thermodynamically
preferred. Thus, utilizing light energy to promote the efficiency of this process is a
green approach due to its lower energy input.
As shown in Table 1, for all the reactions PdNP catalysts exhibited better activity
under irradiation than in the dark. The absorption of visible light and UV light by
PdNPs can excite the electron interband transition,31 and these excited electrons at the
surface of the PdNPs enhance the catalytic ability of the PdNPs. AuNPs exhibit no
catalytic activity for these reactions, whether under light irradiation or in the dark,
except for the Buchwald-Hartwig reaction(17 % under light, 15 % in the dark). The
Au−Pd alloy NPs exhibited superior catalytic activity when irradiate; the calculated
TOF values for Au−Pd alloy NPs are much higher than those of pure AuNPs or PdNPs
(approximately 1.7-4.2-fold) under light irradiation. This can be attributed to the fact
that the charge heterogeneity of the alloy NP surface is greater than those of AuNP and
PdNP surfaces,19 which leads to a stronger interaction between the alloy NPs and
reactant molecules.32,33 The performance of the Au−Pd alloy NPs strongly depends on
the Au/Pd ratio for the reactions in the presence and absence of light. It has been shown
that when the Au/Pd molar ratio is 1/1.86, the surface charge heterogeneity is the
greatest and results in optimal catalytic activity.19
It is known that AuNPs exhibit strong visible light absorption due to the LSPR
effect.34,35 LSPR is the resonant light-induced coherent oscillation of charges at the
metal-dielectric interface, established when the frequency of the incident light matches
the frequency of metal surface electrons oscillating against the restoring force of their
positive nuclei. Therefore, in addition to creating a NP surface with greater surface
charge heterogeneity, gold in the Au−Pd alloy NPs enhances the ability of the NP to
harvest the light energy [compared with that of PdNPs (see Figure 2)]. When the alloy
NPs are irradiated with light, the conduction electrons gain the energy of the incident
light yielding Pd sites with light -excited electrons at the alloy NP surface. Hence, the
intrinsic catalytic activity of the Pd sites is significantly enhanced at low reaction
temperatures.
59
Table 2. Au−Pd Alloy NPs Catalyzed Cross−Coupling Reactions with Different Aryl
Halides under Visible Light Irradiation (red numbers) and in the Dark (black numbers)
[values in parentheses are the TOF values (h-1).][b]
aThe yields were calculated from the product content and the aryl halide conversions
measured by GC. The products were analyzed by GC and mass spectrometry. For
detailed reaction conditions, see the Experimental Section. bTOF values were calculated
on the basis of amount of Pd metal. cReaction time of 14 h.
The general applicability of the Au−Pd alloy NP-photocatalyzed cross-coupling
reaction was investigated with a series of differently substituted aryl halides. As shown
in Table 2, the light irradiation remarkably promoted the reaction in each case. High
selectivities for the desired cross-coupling products were achieved regardless of
whether the substituents were electron donors or acceptors. Thus the visible light
60
photocatalytic process using Au−Pd alloy NPs can drive various cross-coupling
reactions with a broad range of substrates.
It is well-known that activation of C−Br and C−Cl bonds is much more difficult
than activation of the C−I bond and in general requires harsher reaction conditions in
the heterogeneous catalysis system.36 In this study, we examined the catalytic activity
of Au−Pd alloy NPs for reactions using bromobenzene and chlorobenzene as substrates
with light irradiation. Visible light can activate bromobenzene effectively, but the
yields are much lower than those using iodobenzene (Table 3, entries 1−3).
Surprisingly, we found that ultraviolet (UV) irradiation with higher intensity and
energy could not only substantially improve the yields of the reactions (Table 2, entries
1−3) but also activate the reactions with chlorobenzenes that can hardly be activated
under visible light irradiation (Table 3, entries 4 and 5). These results indicate that one
can improve the catalytic activities by increasing light intensity and using shorter
wavelength light.
Table 3. Examples of Bromobenzene and Chlorobenzene as Substrates for
Cross−Coupling Reactions Using Au−Pd Alloy NPs under Visible Light and UV Light
Irradiation
aThe yields were calculated from the product content and the aryl halide conversions
measured by GC. The values in parentheses are the data for reactions controlled under
the same conditions in the dark. Reaction temperature of 65 °C. bThe UV light reaction
was conducted under UV lamp (UVP Blak-Ray B100AP High Intensity UV Lamp, 100
W, 365 nm UV) irradiation with a light intensity of 0.9 W/cm2 and the other reaction
conditions were kept the same. cTOF values were calculated on the basis of amount of
Pd metal.
61
2.3 Impact of Light Intensity and Wavelength
We investigated the dependence of catalytic activity on light intensity, and the results
of the representative examples of Sonogashira, Hiyama, and Stille reactions are
depicted in Figure 3. When the irradiation intensity was increased from 0.1 to 0.2, 0.3,
0.4, and 0.5 W/cm2 with other reaction conditions unchanged, the extent of conversion
of the reactions on the Au−Pd alloy NPs increased. There is a positive relationship
between the intensity and reaction rate.
Figure 3. Dependence of the catalytic activity of Au−Pd alloy NPs for (a) Sonogashira,
(b) Hiyama, and (c) Stille cross-coupling reactions on the intensity of light irradiation.
62
The numbers with percentages show the contribution of the light irradiation effect. 3-
Iodotoluene was used as the aryl halide substrate to react with the corresponding
coupling partner under visible light irradiation. The reaction conversions were based on
the average of two experiments. In the reactions for determining the light-intensity
dependence, a photometer was used to measure the light intensity; the other
experimental conditions were kept the same. For the reaction conditions, see the
Experimental Ssection.
As shown in Figure 3, the results clearly exhibit an almost linear dependence. The
contributions of light irradiation to the extent of conversion efficiency were calculated
by subtracting the conversion of the reaction in the dark from the overall extent of
conversion observed when the system was irradiated, with both reactions occurring at
an identical reaction temperature. Here the conversion of the reaction in the dark is
regarded as the contribution of the thermal effect. The relative contributions of light
and thermal processes to the conversion efficiencies are shown in Figure 3. We can see
that the higher the light intensity, the greater the contribution of irradiation to the
overall conversion rate. When the light intensity is 0.1 W/cm2, the light contributions
for these reactions were only 47 % (Sonogashira), 20 % (Hiyama) and 14 % (Stille),
and when the light intensity increased to 0.5 W/cm2, 90, 89 and 63 %, respectively of
the conversion are due to irradiation. A stronger light intensity will induce a larger
population of electrons at higher energy levels and create a stronger electromagnetic
field around the NPs (surface enhancement effect), as reported for AuNPs.37 The
transfer of the light-excited electrons of a metal nanoparticle to molecules adsorbed on
the nanoparticles is well -known.38,39 Such transfer induces the reaction of the
molecules. The surface enhancement effect also contributes to a stronger interaction
between the NPs and reactant molecules, and thus enhanced catalytic activity of the
coupling reactions.
A useful tool for determining whether an observed reaction occurs via a
photoinduced process or a thermocatalytic process is the action spectrum, which should
show one-to-one mapping between the wavelength-dependent photocatalytic rate and
the light extinction spectrum.40,41 In this study, the reaction rates of the photocatalyic
cross-coupling reactions using Au−Pd alloy NPs at 40±2 °C under irradiation with
different wavelengths were determined. Five LED lamps with wavelengths 400±5,
63
470±5, 530±5, 590±5 and 620±5 nm were used, and the rates were converted to the
apparent quantum efficiencies (AQEs). The AQE was calculated using the relationship
AQE (%)=[(Ylight-Ydark)/(the number of incident photons)]×100, where Ylight and Ydark
are the amounts of products formed under light irradiation and dark conditions,
respectively. The plot of AQE versus the respective wavelengths is the action spectrum
of the reaction. The action spectra of Sonogashira and Stille reactions are shown in
Figure 4 as representative examples. Each action spectrum in the figure is compared
with the light absorption spectrum of the Au−Pd alloy NPs and the spectrum of AuNPs.
Figure 4. Action spectra for (a) Sonogashira and (b) Stille cross-coupling reactions.
The light absorption spectra (left axis) are DR-UV−vis spectra of AuNPs (purple) and
the Au−Pd alloy NPs (blue). 3-Iodotoluene was used as the aryl halide substrate to react
with corresponding coupling partners under visible light irradiation. The AQE values
were calculated on the basis of the average of three experiments. For details, see the
Experimental Section.
64
The action spectra exhibit a few interesting features. First, action spectra of the
cross-coupling reactions do not follow the absorption spectrum of the Au−Pd alloy
NPs@ZrO2 catalyst, which includes a substantial contribution from scattering. It means
that the scattering has little impact on the catalytic performance. Second, a
correspondence is observed between the AQE of the Sonogashira reaction and the light
absorption of AuNPs (Figure 4a), which show the characteristic LSPR absorption peak
in the range between 500 and 550 nm.34,35 The PdNPs exhibited a high catalytic activity
for the Sonogashira reaction in the dark (Table 1), indicating that the PdNPs can readily
activate reactants of this reaction. Given that alloying with Au results in a loss of most
of the activity in the dark and AuNPs exhibited no activity for this reaction (Table 1),
the reaction apparently takes place only at the Pd sites and is not affected by the higher
surface charge heterogeneity of the alloy NPs. Thus, the function of gold in the alloy
NP is apparently only to absorb the light energy. The light absorption of Au−Pd alloy
NPs is obviously stronger than that of the pure PdNPs at all wavelengths (Figure 2).
Gold nanostructures exhibit strong LSPR absorption of visible light, which can excite
electrons to high energy levels. In the alloy NPs, these light excited -electrons can
migrate to the surface Pd sites where they function like the same light -excited
electrons of the PdNPs, resulting in a significant enhancement of the catalytic
performance of the alloy NPs. The action spectrum suggests that the enhancement of
the catalytic performance is mainly due to the LSPR absorption of gold in the alloy
NPs. The situation of the Stille reaction is similar to that of the Sonogashira reaction
(Table 1); the action spectrum of the Stille reaction likewise follows the absorption
spectrum of AuNPs (Figure 4b). Here, we can confirm that it is the Au that acts as an
antenna that harvests visible light enhancing the reaction yield in alloy NPs-catalyzed
reactions.
The dependence of photocatalytic activity on light intensity and wavelength
indicates that electrons excited by light absorption are responsible for the observed
photocatalytic activity.42 Because the rate of the catalyzed reactions is expected to
depend on the population of electrons with sufficient energy to initiate the reaction of
the reactant molecules, one can increase the number of light -excited electrons by
applying a high light intensity. The electron energy sufficient to initiate the reaction of
the molecules on the metal NPs is dependent on the actual reaction in question. Tuning
the irradiation wavelength can increase the number of light -excited electrons with
65
sufficient energy to induce the reaction and may also assist us in understanding the
mechanism of the reactions.
Figure 5. Dependence of catalytic activity on different reaction temperatures for (a)
Sonogashira and (b) Hiyama reaction under a thermal heating process in the dark
(triangles) and the light irradiation process (circles). 3-Iodotoluene was used as the aryl
halide substrate to react with corresponding coupling partners under light irradiation
and in the dark. The light intensity was 0.45 W/cm2.
2.4 Impact of Temperature
Another important feature of the photocatalytic process on metal NP catalysts is that
the photocatalytic activity of the NPs can be increased by elevating the reaction
temperature.43,44 This feature is also observed from the alloy NP photocatalysts this
study. For example, the extent of conversion of the Sonogashira reaction at 30 °C was
only 11 %, whereas the conversion reached 100 % when the temperature was increased
66
to 60 °C in the same time interval (Figure 5a, blue dots). At a higher temperature, more
electrons of the metal NPs populate higher energy levels, but these electrons can still
gain energy from the incident light. Thus, upon irradiation, the number of electrons
with sufficient energy to initiate reaction of the molecules adsorbed on the metal NPs is
greater. At a low reaction temperature, the photoexcitation contributes dominantly to
the photocatalytic activity and the photothermal effect would contribute much less.20
For many catalyzed reactions involving the interaction between light -excited electrons
of a catalyst with reactant molecules, high reaction temperatures are not a prerequisite
to drive them. Light irradiation can yield excited electrons with sufficient energy at low
reaction temperatures and facilitate the reactions of the reactants. The metal NPs have
the capacity to couple the stimuli of light irradiation and heat to drive the catalytic
reaction.43 This property not only distinguishes them from semiconductor
photocatalysts but also reveals the potential of the NPs to utilize the infrared radiation
in sunlight, which accounts for a large fraction of the solar spectrum and could be used
to heat the NPs, further facilitating the reaction.
Because the photocatalytic activity of the alloy NP photocatalysts varies with
temperature, the apparent activation energies of Sonogashira and Hiyama reactions
were estimated by using the Arrhenius equation and kinetic data for photocatalytic
reactions conducted over the range of 30-60 °C. As shown in Figure 6, the difference
between the activation energies of the light-enhanced process and the process in the
dark (ΔEa) indicates the reduction in the measured activation energy due to irradiation.
For example, the estimated activation energy is ~217 kJ/mol for the Sonogashira
reaction in the dark (Figure 6a), while it is ~120 kJ/mol for the photocatalytic reaction
under visible light irradiation. Thus, light irradiation can reduce the activation energy of
the Sonogashira reaction by 97 kJ/mol, which represents 44 % of the “uncatalyzed”
activation energy. Similarly, the activation energy of the Hiyama reaction was reduced
by 66 % (Figure 6b). The fact that irradiation substantially reduces the activation
energy demands that the photocatalytic process have a mechanism from that of the
process in the dark.
67
Figure 6. Apparent activation energies of (a) Sonogashira and (b) Hiyama reactions
calculated for the photoreaction and the reaction in the dark. The contribution of light
irradiation is calculated from the difference in the extents of conversion of two
processes (with and without light) and presented as a percentage (Y axis). 3–
Iodotoluene was used as the aryl halide substrate to react with corresponding coupling
partners under light irradiation and in the dark. The light intensity was 0.45 W/cm2.
2.5 Proposed Mechanism
The cross-coupling reactions involve two essential steps: breaking the carbon−iodine
bond in aryl iodide, and activation of the coupling partner molecules that then
facilitates transmetalation.45,46 It has been postulated that the transfer of an electron
from the Pd atoms to the halogen atoms is involved in facilitating carbon−halogen bond
cleavage in the Pd heterocatalysis.20 This is effectively the heteroanalog of the familiar
homogeneous oxidative addition step involving ligated Pd(0). In the present study
when the PdNPs were irradiated with light, enhanced catalytic performance was
observed. Because the absorption of light by PdNPs excites the electrons of the PdNPs
to the high -energy band, it is deduced that the light -excited electrons at the surface of
the PdNPs can enhance the catalytic ability. Given that AuNPs exhibit no catalytic
activity for most of the reactions, it is reasonable to believe that the light -excited
electrons at the surface Pd sites of the alloy NPs facilitate the reactions. The linear
dependence of the photoinduced reaction rate on the light intensity, observed in Figure
3, usually suggests an electron-driven chemical process on the metal surface.42 Also,
transient electron transfer from a light-excited metal NP to a chemically adsorbed
molecule is well -known.38,39 Electrons of alloy NPs are excited by light irradiation; the
excited electrons transfer from the alloy NP surface to the LUMO of a molecule
68
adsorbed on the NP, activating the substrate. The electron can finally return to the metal
at a lower energy (Figure 7a-c). We therefore performed DFT calculations on the
transfer of one electron from the NP surface to the reactant iodobenzene molecule. The
simulation suggests that when one electron enters an unoccupied orbital, the C-I bond
will become longer (from 0.214 to 0.300 nm), so the cleavage of the C-I bond will be
much easier (Figure 7d). For the reactions in which the rate-determining step is C-I
bond cleavage, irradiation will facilitate the transfer of an electron from the NP to the
adsorbed aryl iodide molecule, yielding a transient negative ion species. Loss of iodide
ion will afford either an adsorbed phenyl radical or a true organometallic
aryl−palladium iodide complex on the surface and then activate the reactions (Figure
7e).
Figure 7. Proposed mechanism for the photocatalytic cross-coupling reactions. (a)
Light irradiation excites electrons of an alloy NP to high energy levels, and the transfer
of the excited electrons with sufficient energy from the alloy NP to the LUMO of
molecules adsorbed on the NP can take place, activating the reaction. The electrons
finally return to the metal NP at a lower energy.38 (b) At higher reaction temperatures,
more excited electrons populate higher energy levels of the metal NPs, which can be
readily transferred to the LUMO of the absorbed molecule. (c) Under higher irradiation
intensity, more electrons of the metal NPs populate higher energy levels, resulting in
more electron transfers to the LUMO of the absorbed molecule and higher reaction
69
rates. (d) DFT calculations on the reactant iodobenzene before (left) and after (right)
the transfer of one electron from the NP surface to the reactant molecule. The C−I bond
is elongated because of the electron transfer, which facilitates the activation of
reactions. (e) Schematic diagram of the pathway for the photocatalytic cross-coupling
reactions.
Given that the PdNPs exhibited catalytic activity for most of the reactions in the
dark, it is believed that activating iodobenzene molecules on the surface Pd sites is not
difficult. The fact that the Buchwald-Hartwig coupling reaction on PdNPs did not occur
in the dark suggests that the surface Pd sites cannot activate aniline in the dark. The
significant catalytic activity of PdNPs for this reaction under light should be due to the
activation of aniline on the PdNP surface under irradiation. Aniline has a strong
interaction with AuNP surfaces and is often used for the surface enhanced Raman
spectral study,34 and this explains the observed catalytic activity of the AuNPs in the
dark. The fact that the Au−Pd alloy NPs exhibited catalytic activity superior to that of
both the AuNPs and the PdNPs in the dark indicates that the charge heterogeneity of the
alloy NP surface also enhances the catalytic activity.
The heterogeneous catalytic processes of the coupling reactions examined in this
study have multiple steps and are by no means well -understood.47,48 In this regard, we
focused only on possible roles for the light -excited electrons afforded at the NP
surfaces by irradiation. The delineation of subsequent steps is a profound challenge but
is beyond the scope of this preliminary survey.
3. Conclusions
The findings in this study demonstrate that irradiation of Au-Pd alloy NPs can
significantly enhance the intrinsic catalytic activity of Pd at lower temperatures for a
number of Pd-catalyzed cross-coupling reactions. An outstanding feature of the Au−Pd
alloy NPs is their ability to efficiently concentrate the energy of a photon flux to a very
small volume and to transfer this energy to the adsorbed molecules to induce their
reaction on the surface. These catalytic cross-coupling processes are due to the
interaction of light excited electrons of the catalyst with the reactant molecules, while
high temperatures are not a prerequisite for driving them. The reaction rate depends on
the number of the light excited electrons and the number of the reactant molecules on
70
the catalyst surface. The number of the reactant molecules on the surface depends
mainly on the affinity of the surface for the reactants. Pd sites have strong affinity for
many organic molecules. The number of the light -excited electrons can be increased by
applying high light intensity. Although we focused on Au−Pd nanostructures, the
discussed mechanisms may be universal, and similar principles could be used in the
design of various photocatalysts comprising any of a range of plasmonic metals and a
catalytically active transition metal.37 Thus we have been able to utilize a low –energy
and -density source to drive a wide range of useful chemical transformations. In these
reaction systems, the energy efficiency is high, because of the specific absorption of the
light only by the NP catalysts, and not by the solvent, the oxide support, the atmosphere
or the reaction vessel. The findings reported here reveal the possibility of green cross-
coupling reactions driven by visible light at temperatures slightly above room
temperature.
Experimental Section
4.1 Chemicals
Zirconium(IV) oxide (ZrO2, <100 nm particle size, TEM), gold(III) chloride trihydrate
(HAuCl4∙3H2O, ≥99.9 % trace metal basis), palladium(II) chloride (PdCl2,
ReagentPlus, 99 %), sodium borohydride, powder (NaBH4, ≥98.0 %), hydrochloric
acid [HCl, 32 % (w/w), analytical reagent, Chem-Supply] and N,N-dimethylformamide
(DMF, anhydrous, 99.8 %, total impurities, < 0.005 % water) were purchased from
Sigma-Aldrich (unless otherwise noted) and used as received without further
purification. The water used in all experiments was prepared by being passed through
an ultrapurification system.
4.2 Preparation of Catalysts
Au−Pd alloy NPs (1.5 wt% Au−1.5 wt% Pd supported on ZrO2, Au/Pd molar ratio of
1/1.86) were prepared by the impregnation-reduction method. ZrO2 powder (2.0 g) was
dispersed into a HAuCl4 (15.2 mL, 0.01 M) and PdCl2 (28.3 mL, 0.01 M) aqueous
solution under magnetic stirring at room temperature. A lysine (16 mL, 0.53 M)
aqueous solution was then added to the mixture while it was vigorously stirred for 30
min, and the pH value was determined to be 8−9. To this suspension was added
dropwise a freshly prepared aqueous NaBH4 (3 mL, 0.35 M) solution in 20 min,
71
followed by an addition of an HCl (3 mL, 0.3 M) solution. The mixture was aged for 24
h, and then the solid was separated by centrifugation, washed with water (three times)
and ethanol (once), and dried at 60 °C in a vacuum oven for 24 h. The dried powder
was used directly as a catalyst. Pure Au NPs (3 wt%) and Pd NPs (3 wt%) were
prepared via a similar method but using different quantities of HAuCl4 and PdCl2
aqueous solutions.
4.3 Characterization of Catalysts
A transmission electron microscope (TEM) study and line profile analysis by an energy
dispersion X-ray (EDX) spectrum technique of the photocatalysts were conducted on a
Philips CM200 TEM with an accelerating voltage of 200 kV.Eelement line scanning
was conducted on a Bruker energy dispersion X-ray (EDX) scanner attached to a
JEOL-2200FS TEM with scanning beam diameter of ≥ 1.0 nm. The Au and Pd contents
of the prepared catalysts were determined by energy dispersion X-ray spectrum (EDS)
technology using the attachment to a FEI Quanta 200 environmental scanning electron
microscope (SEM). Diffuse reflectance UV−visible (DR-UV−vis) spectra of the sample
powders were examined with a Varian Cary 5000 spectrometer with BaSO4 as a
reference.
4.4 Photocatalytic Reactions
A 25 mL Pyrex round bottom flask was used as the reaction container, and after the
reactants and catalyst had been added, the flask was sealed with a rubber septum cap.
The flask was irradiated with magnetic stirring using a halogen lamp (from Nelson,
wavelength in the range of 400-750 nm) as the visible light source and the light
intensity was measured to be 0.45 W/cm2. The temperature of the reaction system was
carefully controlled with an air conditioner attached to the reaction chamber. The
reaction system under light illumination was maintained at the same temperature as the
corresponding reaction system in the dark to ensure that the comparison is meaningful.
All the reactions in the dark were conducted using a water bath placed above a
magnetic stirrer to control the reaction temperature; the reaction flask was wrapped
with aluminum foil to avoid exposure of the reaction mixture to light. At given
irradiation time intervals, 2 mL aliquots were collected and then filtered through a
Millipore filter (pore size of 0.45 μm) to remove the catalyst particulates. The liquid-
72
phase products were analyzed by an Agilent 6890 gas chromatography (GC) HP-5
column to measure the change in the concentrations of reactants and products. An
Agilent HP5973 mass spectrometer was used to identify the product. For the reactions
using H2O as the solvent, the product was extracted with dichloromethane (CH2Cl2)
before GC analysis. The GC conversions and selectivities were calculated from the
product content and the aryl halide conversions.
4.4.1 Sonogashira Cross-Coupling Reactions. Aryl iodide (1 mmol), alkyl alkyne (1.2
mmol), photocatalysts (50 mg), cetyltrimethylammonium bromide (CTAB) (1 mmol)
and K3PO4 (2 mmol) were added to 10 mL of H2O. The reaction temperature was 45±2
°C, under a 1 atm argon atmosphere, with a reaction time of 24 h.
4.4.2 Hiyama Cross-Coupling Reactions. Aryl iodide (1 mmol), trimethoxyphenylsilane
(1.5 mmol), photocatalysts (50 mg), and tetrabutylammonium fluoride (TBAF) (1.2
mmol) were added to 5 mL of toluene. The reaction temperature was 45±2 °C, with a
reaction time of 24 h.
4.4.3 Stille Cross-Coupling Reactions. Aryl iodide (1 mmol), tributylphenylstannane
(1.2 mmol), photocatalysts (50 mg), cetyltrimethylammonium bromide (CTAB) (1
mmol), and NaOH (3 mmol) were added to 10 mL of H2O. The reaction temperature
was 45±2 °C, 1 atm argon atmosphere, with a reaction time of 24 h.
4.4.4 Ullmann Cross-Coupling Reactions. Aryl iodide (1 mmol), photocatalysts (50
mg), and NaOH (3 mmol) were added to 10 mL of an EtOH/H2O mixture [1/1 (v/v)].
The reaction temperature was 45±2 °C, with a reaction time of 24 h.
4.4.5 Buchwald-Hartwig Cross-Coupling Reactions. Aryl iodide (1 mmol), aniline (1.2
mmol), photocatalysts (50 mg), and potassium tert-butoxide (t-BuOK) (3 mmol) were
added to 10 mL of N,N-dimethylformamide (DMF). The reaction temperature was
45±2 °C, with a reaction time of 24 h.
4.4.6 Action Spectrum Experiments. LED lamps (Tongyifang, Shenzhen, China) with
wavelengths 400±5 nm (TYF-H030 G45), 470±5 nm (TYF-H030 G35), 530±5 nm
(TYF-H030 G35), 590±5 nm (TYF-H030 G38), and 620±5 nm (TYF-H030 G32) were
used as the light source. The light intensity was measured to be 0.50 W/cm2, and the
other reaction conditions were identical to those of typical reaction procedures.
73
4.5 DFT Calculation
The geometries of all the species were optimized at the level of DFT with Becke’s49
three-parameter exchange and Lee-Yang-Parr correlation functional50 implemented in
Orca51. Ahlrichs’ triple -ع valence basis set52 TZVP was employed to describe the
orbitals of all atoms involved. The iodoben-zene molecule and its corresponding
negative ion were fully optimized as defined by the B3LYP/TZVP method.
The authors declare no competing financial interest
Acknowledgements. The authors gratefully acknowledge financial support from the
Australian Research Council (ARC DP110104990).
REFERENCES
(1) De Mejere, A.; Diederich, F. Metal-Catalysed Cross-Coupling Reactions, Second
Edition; Wiley-VCH: Weinheim, Germany, 2004.
(2) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem. Int. Ed. 2005, 44, 4442-
4489.
(3) Johansson-Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.; Snieckus, V. Angew.
Chem. Int. Ed. 2012, 51, 5062-5085.
(4) Buchwald, S. L. Acc. Chem. Res. 2008, 41, 1439-1439.
(5) Magano, J.; Dunetz, J. R. Chem. Rev. 2011, 111, 2177-2250.
(6) Balanta, A.; Godard, C.; Claver, C. Chem. Soc. Rev. 2011, 40, 4973-4985.
(7) Molnar, A. Chem. Rev. 2011, 111, 2251-2320.
(8) Campbell, C. T.; Parker, S. C.; Starr, D. E. Science 2002, 298, 811-814.
(9) Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R. E.;
Elam, J. W.; Meyer, R. J.; Redfern, P. C.; Teschner, D.; Schlögl, R.; Pellin, M. J.;
Curtiss, L. A.; Vajda, S. Science 2010, 328, 224-228.
(10) Wittstock, A.; Zielasek, V.; Biener, J.; Friend, C. M.; Baumer, M. Science 2010,
327, 319-322.
(11) Lu, C. L.; Prasad, K. S.; Wu, H. L.; Ho, J. A. A.; Huang, M. H. J. Am. Chem. Soc.
2010, 132, 14546-14553.
(12) Narayanama, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102-113.
(13) Pirnot, M. T.; Rankic, D. A.; Martin, D. B. C.; MacMillan, D. W. C. Science 2013,
339, 1593-1596.
74
(14) Nagib, D. A.; MacMillan, D. W. C. Nature 2011, 480, 224-228.
(15) Shih, H. W.; Vander Wal, M. N.; Grange, R. L.; MacMillan, D. W. C. J. Am.
Chem. Soc. 2010, 132, 13600-13603.
(16) Palmisano, G.; Augugliaro, V.; Pagliarob, M.; Palmisano. L. Chem. Commun.
2007, 33, 3425-3437.
(17) Hoffmann, N. Chem. Rev. 2008, 108, 1052-1103.
(18) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527-532.
(19) Sarina, S.; Zhu, H.; Jaatinen, E.; Xiao, Q.; Liu, H.; Jia, J.; Chen, C.; Zhao, J. J.
Am. Chem. Soc. 2013, 135, 5793-5801.
(20) Wang, F.; Li, C.; Chen, H.; Jiang, R.; Sun, L. D. Li, Q. Wang, J.; Yu, J. C.; Yan,
C. H. J. Am. Chem. Soc. 2013, 135, 5588-5601.
(21) Huang, X.; Li, Y.; Chen, Y.; Zhou, H.; Duan, X.; Huang, Y. Angew. Chem. Int.
Ed. 2013, 52, 6063-6067.
(22) Lee, Y. W.; Kim, N. H.; Lee, K. Y.; Kwon, K.; Kim, M.; Han, S. W. J. Phys.
Chem. C 2008, 112, 6717-6722.
(23) Chen, Y. H.; Tseng, Y. H.; Yeh, C. S. J. Mater. Chem. 2002, 12, 1419-1422.
(24) Prodan, E.; Radloff, C.; Halas, N. J.; Norlander, P. Science 2003, 302, 419-422.
(25) Emeline, A.; Kataeva, G. V.; Rudakova, A. S.; Ryabchuk, V. K.; Serpone, N.
Langmuir 1998, 14, 5011-5022.
(26) For Sonogashira reactions, see: (a) Ciriminna, R.; Pandarus, V.; Gingras, G.;
Béland, F.; Demma Carà, P.; Pagliaro, M. ACS Sustainable Chem. Eng. 2013, 1,
57-61. (b) Pandarus, V.; Gingras, G.; Béland, F.; Ciriminna, R.; Pagliaro, M. Org.
Process Res. Dev. 2012, 16, 117-122. (c) Srinivas, K.; Srinivas, P.; Prathima, P. S.;
Balaswamy, K.; Sridhar, B.; Rao, M. M. Catal. Sci. Technol. 2012, 2, 1180-1187.
(d) Schweizer, S.; Becht, J. M.; Le Drian, C. Adv. Synth. Catal. 2007, 349, 1150-
1158.
(27) For Stille reactions, see: (a) Espinet, P.; Echavarren, A. M. Angew. Chem. Int. Ed.
2004, 43, 4704-4734; (b) DelPozo, J.; Carrasco, D.; Pérez-Temprano, M. H.;
García-Melchor, M.; Álvarez, R.; Casares, J. A.; Espinet, P. Angew. Chem. Int. Ed.
2013, 52, 2189-2193.
(28) For Hiyama reactions, see: (a) Diebold, C.; Derible, A.; Becht, J. M.; Le Drian. C.
Tetrahedron 2012, 69, 264-267. (b) Zhang, L.; Li, P.; Li, H.; Wang, L. Catal. Sci.
Technol. 2012, 2, 1859-1864.
75
(29) For Buchwald–Hartwig reactions, see: (a) Paul, P.; Sengupta, P.; Bhattacharya, S.
J. Organomet. Chem. 2013, 724, 281-288. (b) Liu, X.; Zhang, S. Synlett 2011, 8,
1137-1142. (c) Rout, L.; Jammi, S.; Punniyamurthy, T. Org. Lett. 2007, 9, 3397-
3399. (d) Pan, K.; Ming, H.; Yu, H.; Huang, H.; Liu, Y.; Kang. Z. Dalton Trans.
2012, 41, 2564-2566. (e) Kidwai, M.; Mishra, N. K.; Bhardwaj, S.; Jahan, A.;
Kumar, A.; Mozumdar, S. ChemCatChem 2010, 2, 1312-1317. (f) Jammi, S.;
Sakthivel, S.; Rout, L.; Mukherjee, T.; Mandal, S.; Mitra, R.; Saha, P.;
Punniyamurthy. T. J. Org. Chem. 2009, 74, 1971-1976.
(30) For Ullmann reactions, see: (a) Layek, K.; Maheswaran, H.; Kantam, M. L. Catal.
Sci. Technol. 2013, 3, 1147-1150. (b) Wang, L.; Lu, W. Org. Lett. 2009, 11, 1079-
1082. (c) Gädda, T. M.; Kawanishi Y.; Miyazawaa, A. Synth. Commun. 2012, 42,
1259-1267.
(31) Fan, G.; Qu, S.; Wang, Q.; Zhao, C.; Zhang, L.; Li, Z. J. Appl. Phys. 2011, 109,
023102.
(32) Chen, M. S.; Kumar, D.; Yi, C. W.; Goodman, D. W. Science 2005, 310, 291-293.
(33) Tang, W. J.; Henkelman, G. J. Chem. Phys. 2009, 130, 194504.
(34) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729-7744.
(35) Mulvaney, P. Langmuir 1996, 12, 788-800.
(36) Yin, L.; Liebscher, J. Chem. Rev. 2007, 107, 133-173.
(37) Sarina, S.; Waclawik, E. R.; Zhu, H. Green Chem. 2013, 15, 1814-1833.
(38) Brus, L. Acc. Chem. Res. 2008, 41, 1742-1749.
(39) Lindstrom, C. D.; Zhu, X. Y. Chem. Rev. 2006, 106, 4281-4300.
(40) Tanaka, A.; Sakaguchi, S.; Hashimoto, K.; Kominami, H. ACS Catal. 2013, 3, 79-
85.
(41) Kowalska, E.; Abea, R.; Ohtania, B. Chem. Commun. 2009, 2, 241-243.
(42) Christopher, P.; Xin, H. L.; Linic, S. Nat. Chem. 2011, 3, 467-472.
(43) Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10, 911-921.
(44) Christopher, P.; Xin, H. L.; Marimuthu, A.; Linic, S. Nat. Mater. 2012, 11, 1044-
1050.
(45) Xue, L. Q.; Lin, Z. Y. Chem. Soc. Rev. 2010, 39, 1692-1705.
(46) Diederich, F.; Stang, P. Metal-catalyzed Cross-coupling Reactions; Wiley-VCH:
Weinheim, Germany, 1998.
(47) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. Adv. Synth. Catal. 2006, 348,
609-679.
76
(48) Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J. M.; Polshettiwar, V.
Chem. Soc. Rev. 2011, 40, 5181-5203.
(49) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(50) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789.
(51) Neese, F. WIREs Comput. Mol. Sci. 2012, 2, 73-78.
(52) Schaefer, A.; Huber, S.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829-5835.
77
2.3 Article 3
Green Chemistry
PAPER
lii\ CrossMark \1!!!1' ftti.fof updates
Cite this: Green Chem., 2014, 16,
4272
Efficient photocatalytic Suzuki cross-coupling reactions on Au-Pd alloy nanoparticles under visible light irradiationt Oi Xiao," Sarina Sarina," Esa Jaatinen,a Jianfeng J1a,b Dennis P. Arnold,a Hongwei Lluc and Huaiyong Zhu*a
78
Abstract
We report herein highly efficient
photocatalysts comprising supported
nanoparticles (NPs) of gold (Au) and
palladium (Pd) alloys, which utilize
visible light to catalyse the Suzuki
cross-coupling reactions at ambient
temperature. The alloy NPs strongly
absorb visible light, energizing the conduction electrons of NPs which produce
highly energetic electrons at the surface sites. The surface of the energized NPs
activates the substrates and these particles exhibit good activity on a range of
typical Suzuki reaction combinations. The photocatalytic efficiencies strongly
depend on the Au:Pd ratio of the alloy NPs, irradiation light intensity and
wavelength. The results show that the alloy nanoparticles efficiently couple
thermal and photonic energy sources to drive Suzuki reactions. Results of the
density functional theory (DFT) calculations indicate that transfer of the light-
excited electrons from the nanoparticle surface to the reactant molecules
adsorbed on the nanoparticle surface activates the reactants. The knowledge
acquired in this study may inspire further studies of new efficient photocatalysts
and a wide range of organic syntheses driven by sunlight.
Introduction
Cross-coupling reactions are powerful and versatile tools in modern organic synthesis
for the formation of carbon-carbon bonds.1 Among various cross-coupling reactions,
79
the Suzuki reaction, discovered by 2010 Nobel laureate Akira Suzuki,2 is of great
significance in synthetic chemistry for biaryl compounds, which have many
applications as intermediates in the preparation of materials,3 natural products,4 and
bioactive compounds.5 The use of well-tolerated functionalized aryl halides to react
with innocuous boronic acids, which are generally non-toxic and thermally, air-, and
moisture-stable, is the practical advantage of the Suzuki reaction, relative to many other
cross-coupling processes.6 In the past, the Suzuki reactions were typically performed
under homogeneous conditions using phosphine ligand/palladium as catalytic system,
which has shown relatively high activity and selectivity.7 However, the separation and
recovery of the catalyst represent key issues for sustainable development of the
chemical industry. Hence huge efforts have been made to develop heterogeneous Pd
catalysts for Suzuki reactions,8 including the immobilization or stabilization of Pd NPs
on different supports, such as high surface-area silica,9 carbon nanotubes,10 polymers,11
metal oxides,12 double hydroxides,13 dendrimers14 and magnetic nanomaterials.15 For
instance, gold-palladium bimetallic NPs were reported as highly active catalysts for
Suzuki reactions.16 Nonetheless, many of these reported processes with heterogeneous
catalysts require relative elevated reaction temperature and prolonged reaction time. In
some cases, the active sites on the porous solid supports are not readily accessible for
the reactants.8 Therefore, highly active, easily separable and reusable catalyst systems,
combined with mild, green chemistry techniques, are still considered as an important
objective for Suzuki reactions.
Photocatalysis is particularly intriguing in the realm of green chemical science,
because it combines the efficiency of catalysis with the potential use of sunlight.17
Photocatalysis driven by visible light is an ideal process, as a result of the abundance of
visible light, its benign environmental impact, and sustainability. Recently, visible light
photocatalytic reactions have shown great potential in organic synthesis.18 We have
been particularly interested in using photocatalysis as a new means to drive chemical
reactions for organic synthetic transformations with high activity and selectivity, such
as visible light induced oxidation and reduction of aryl aromatics using gold NP
photocatalysts.19 Very recently, we have incorporated Pd into AuNPs to form Au-Pd
alloy NPs and utilized the ensemble properties of the alloy NPs to drive several
reactions, including Suzuki reactions, with visible light at ambient temperatures.20 It
was found that the intrinsic catalytic activity of Pd can be significantly enhanced under
light irradiation of alloy NPs due to the electronic heterogeneity at the alloy NP surface
80
and the energy absorbed from the incident light. Two other research groups also found
almost at the same time that it is an effective approach to use irradiation to accelerate
Suzuki cross–coupling reactions by using Au-Pd bimetallic nanostructures.21 In our
previous study, we believed that the conduction electrons of the alloy NPs gain the
energy of the incident light, generating electrons at high energy levels (“energetic
electrons”). These energetic electrons are available at the surface Pd sites of the alloy
NPs. The surface sites have good affinity for the reactant molecules and the energetic
electrons at these sites enhance the intrinsic ability of the metal sites to activate the
reactant molecules. The charge heterogeneity of the alloy NP surface, due to the
different electronegativities of gold and palladium, also plays a key role in the catalytic
reactions. This tentatively proposed mechanism of light-excited electrons on alloy NPs
and the interaction of the reactant molecules adsorbed on the NPs with the excited
electrons and the NP surface are still by no means well understood. The delineation of
detailed steps involved in the reaction is quite a profound challenge.
In this study, we focused on a systematic investigation of Au-Pd alloy NP catalysed
Suzuki reaction under visible light irradiation. We found that the performance of the
alloy catalyst depends on the alloy composition, light intensity and wavelength.
Theoretical calculations show that the alloying of gold and palladium enhances the
interaction between the reactant molecules and the alloy NPs. The strong interaction
facilitates the transfer of light-excited electrons on the alloy NPs to the reactant
molecules adsorbed on the NPs, and such electron transfer weakens the C–I bond of the
reactant molecules and facilitates the reactions. Understanding this mechanism is useful
for developing photocatalytic versions of other cross-coupling reactions.
Results and discussion
Catalysts preparation and characterization
Photocatalysts made of Au and Pd alloy NPs with various Au:Pd ratios on ZrO2 support
were prepared via an impregnation-reduction procedure as described in our previous
paper.20 The different Au:Pd ratios were achieved by using different quantities of
HAuCl4 or PdCl2 aqueous solutions under otherwise unchanged experimental
conditions. The Au and Pd contents in the alloy NPs are given in Table 1. The metal
element contents of the catalysts were determined by energy-dispersive X-ray
spectroscopy (EDS). For comparison, catalysts of pure Au NP or pure Pd NP on ZrO2
81
were also prepared by a similar method. The Au-Pd molar ratios of the samples were
derived from the metal content and are also listed in Table 1. The specific surface areas
of the photocatalysts were estimated from N2 physical sorption data using the Brauner-
Emmet-Teller (BET) method.
The BET specific surface areas of the samples are similar to that of the pure ZrO2
support (Table 1). The ZrO2 support has a moderate specific surface area, and loading
with the metal NPs does not cause significant change in the overall specific surface
area of samples, and the overall specific surface area seems not to influence
substantially the photocatalytic activity of the samples in the present study.
Table 1. Metal content, Au:Pd ratio and specific surface area of the photocatalysts.
Catalyst Au[wt%] Pd[wt%] Au:Pd
[molar ratio]
Surface area
[m2g-1]a
Alloy-1 1.63 1.43 1:1.62 10.54
Alloy-2 1.11 3.07 1:5.14 10.35
Alloy-3 5.72 0.99 1:0.33 10.59
Alloy-4 5.03 2.66 1:0.99 10.55
Au-ZrO2 3.20 0 1:0 13.38
Pd-ZrO2 0 3.12 0:1 9.99 a The specific surface area of pure ZrO2 support was measured to be 11.17 m2g-1.
Transmission electron microscopy (TEM) analysis of the NPs (Figure 1A) shows
Au-Pd alloy NPs uniformly dispersed on the surfaces of the ZrO2 crystals. The mean
diameters of the pure Au, pure Pd and the Au-Pd alloy NPs are less than 7 nm. High
resolution TEM (HR-TEM) in Figure 1B and 1C also confirmed the formation of Au-
Pd alloy NP on the ZrO2 crystal surface. Line profile analysis of the energy dispersive
X–ray (EDX) spectrum for a typical Au-Pd alloy NP shows that the NP consists of both
Au and Pd distributed spherically around a common centre which means that the two
metals exist as binary alloy NPs in this sample (Figure 1D).
82
Figure 1. Catalysts characterization. (A) TEM image of the Au-Pd alloy NPs. (B)
HR−TEM image of the typical Au-Pd alloy NPs. (C) HR−TEM image of an alloy
particle indicated in Figure 1B (green square). (D) Line profile analysis of the EDX
spectrum of a typical Au−Pd NP indicated in Figure 1B (blue dotted line) providing the
information on the elemental composition and Au/Pd distribution of the NP.
Figure 2 shows the X-ray diffraction (XRD) patterns of the catalysts with different
Au:Pd ratios on ZrO2. All diffraction peaks can be indexed to a monoclinic structure of
ZrO2 crystal (JCPDS, No. 65-2357), no reflection peaks of Au and Pd were observed
by the XRD patterns, because the metal content is low and the metal diffraction peaks
may be interfered with the diffraction peaks of ZrO2, this result suggests that the
detection of metal NP signals in XRD patterns is also closely related to the supporting
materials.
Figure 2. XRD patterns of catalysts with different Au:Pd ratios on ZrO2. Vertical bars
represent the standard diffraction data for monoclinic ZrO2 (JCPDS, No. 65-2357).
83
Figure 3. UV-vis spectra of Au-Pd NPs with different Au:Pd ratios on ZrO2.
The formation of Au-Pd alloy NPs is also supported by the light absorption
properties of the samples, as shown in Figure 3. ZrO2 has a band-gap of about 5 eV,19a
and exhibits weak absorption of visible light of wavelengths above 400 nm and
therefore, the ZrO2 support itself does not contribute to photocatalytic activity. In
contrast, all the NP photocatalysts display strong absorption in the UV and visible
ranges of the spectrum, indicating that solar energy is strongly coupled to the metal
NPs. The absorption peak at 520 nm in the spectrum of the pure Au NP sample is due
to the characteristic localized surface plasmon resonance (LSPR) absorption of Au
NPs.22 The presence of the support and its interaction with the Au NPs can shift and
broaden this peak. The LSPR absorption band of Pd NPs is deep within the UV
wavelength range, so its light absorption at solar wavelengths occurs through both
LSPR and interband electron transition contributions.23 The spectrum of the Au-Pd
alloy NPs sample is clearly different from the spectra of the pure metal NPs. The
dielectric constant of the NPs changes, and the plasmon resonance is determined by the
dielectric constant of the alloy NPs. In the spectra of the alloy NP samples, the
characteristic Au LSPR absorption peak at 520 nm is much weaker compared with the
spectrum of pure Au sample. For example Alloy-1 the molar ratio of Au:Pd is 1:1.62.
The number of Pd atoms in this alloy is much larger than that of Au atoms. Thus, the
absorption due to the LSPR effect of gold nanostructure has limited contribution to the
overall light absorption. Nonetheless, the absorption of the alloy NPs in visible range is
more intense than that observed for pure Pd sample. This means that the alloy NPs have
better ability to gain light energy, which is important to their catalytic performance
under visible light irradiation.
84
Catalyst screening in Suzuki cross-coupling reactions
The photocatalytic activity of Alloy-1 for the Suzuki cross-coupling reaction between
3-iodotoluene and phenylboronic acid was tested at 30 °C and the results are
summarized in Table 2 (entry 1). An excellent yield (96 %) of the desired cross-
coupling product 3-methylbiphenyl was achieved under visible light irradiation within
6 h; only trace homo-coupling product of phenylboronic acid was detected. Control
experiments showed that the yield decreased to 37 % when the reaction was conducted
in the dark while other experimental conditions remained identical. This confirms that
visible light irradiation significantly facilitates the Suzuki cross-coupling reactions
using alloy photocatalyst.
Table 2. Catalysts screening in Suzuki cross-coupling reactions a
I + (HO)2B
Incident light,Photocatalysts
H3C H3CK2CO3, DMF:H2O=3:1
Entry Catalyst Incident
light
Conv.
[%]b TONc
TOF
[h-1]c
Q.Y.
[%]d
1 Alloy-1 Visible 96 87 14.5 4.4
Dark 37 34 5.7
2 Alloy-2 Visible 40 34 5.7 1.8
Dark 10 8 1.3
3 Alloy-3 Visible 28 33 5.5 1.3
Dark 6 6 1.0
4 Alloy-4 Visible 55 56 9.3 2.5
Dark 17 17 2.8
5 Au-ZrO2 Visible 2 3 0.5 0.1
Dark 0 0 0
6 Pd-ZrO2 Visible 26 18 3.0 1.2
Dark 11 8 1.3 a Reaction conditions: 3-iodotoluene (1 mmol), phenylboronic acid (1.5 mmol),
photocatalysts (50 mg, containing 3% of metals), base K2CO3 (3 mmol), 20 mL of
solvent N,N-dimethylformamide (DMF)/H2O (V:V=3:1), 30 °C, 6 h in argon
atmosphere, the light intensity 0.5 W/cm2. b GC conversion: (aryl iodide
converted)/(initial amount of aryl iodide)×100. c TON and TOF were calculated based
on the total amount of metal(s). d Q.Y.: quantum yield, the calculation method of Q.Y.
85
is given in ESI.
To investigate the effect of alloy NP photocatalyst composition, the coupling of 3-
iodotoluene with phenylboronic acid was used as a model reaction to screen the effect
of catalysts with different compositions. Photocatalysts with varying Au-Pd
composition show differences in the reaction conversion rates (Table 2, entries 1-6).
The highest yield of the target product was found when using Alloy-1 (Au:Pd molar
ratio = 1:1.62) as the catalyst, showing relatively high turnover frequency (TOF) and
photo quantum yield (Q.Y.). Pure Pd catalyst showed poor performance (26 %) and
pure Au catalyst exhibited little activity (with a conversion of 2 %). Therefore,
palladium is an indispensable active component of the catalyst for Suzuki cross-
coupling reaction. The photocatalytic properties of Au-Pd alloy NPs are much better
than those of the pure metal NPs of either component metal or that of a mechanical
mixture of the pure metal NPs of the two elements in the same ratio and quantity. The
underlying cause of this apparent improvement in reactivity and its dependence on
alloy composition is of great interest, and is believed to be related to the electron
redistribution between the two metals, which will be further discussed below.
Compared with some literature reported reaction conditions of heterogeneous Pd
catalysts for Suzuki cross-couplings (Table 3), it can be seen that, Pd NPs supported on
various supports can drive the Suzuki coupling under elevated temperatures, while Au-
Pd alloy NPs can drive the reaction at only 30℃ with visible light irradiation achieving
high conversion. Thus the Au-Pd alloy NP photocatalyst is much greener and efficient.
Table 3. Comparison of the reaction conditions and achieved conversion of
heterogeneous Pd based catalysts reported in literatures for Suzuki cross-coupling
reactions using iodobenzene and phenylboronic acid as substrates
Entry Catalysts Conditions a Conv. [%]
1 0.1 mol% Pd/MCM-419c 78℃, 5h 93%
2 LDH-DS-Pd 13b 80℃, 5h 93%
3 XL-Pd 11a 115℃ 86%
4 G3-OH(Pd)10 24 Reflux, 24h 71%
5 G4-OH Pd NPs 25 78℃, 18h 98%
6 Pd-G-3 14a Reflux, 24h 47%
7 0.01mol% Pd-CD 26 60℃, 24h 100%
86
8 Pd-Ln 27 95℃, 4h 97%
9 Silica-APTS-Pd 28 100℃, 2h 99%
10 3% Pd/MWCNT 29 Reflux, 2h 95%
11 Polymer-anchored Pd 30 80℃, 5h 100%
12 Pd-NP-1 31 Reflux, 4h 95%
13 Pd-S-GaAs(001) 32 80℃, 12h 89%
14 PdTNs 33 20℃, 1d 100%
15 Au-Pd alloy NPs
in present study
30℃, 6h
Visible light 98%
a Reaction conditions: reaction temperature, reaction time.
The influence of reaction conditions
The influence of several critical reaction conditions in the cross-coupling reaction of 3-
iodotoluene with phenylboronic acid, such as solvents, bases, and reaction atmosphere,
have been investigated using Alloy-1 catalyst under visible light irradiation. First, the
reaction was carried out in argon, oxygen and air atmosphere, respectively, and we
observed that inert gas atmosphere promoted the reaction markedly (Table 4, entries 1-
3). In the oxygen atmosphere, more homocoupling and oxidation products of
phenylboronic acid were detected by GC-MS. Several commonly used solvents were
used for the reaction while other reaction conditions were maintained unchanged. The
polar aprotic solvents, such as N,N-dimethylformamide (DMF), dimethyl sulfoxide
(DMSO) and dioxane afforded low yields of the coupling product (even trace yield in
dioxane), the nonpolar solvent toluene also gave a trace yield (Table 4, entry 12, 13, 16
and 17). In contrast, much higher yield was observed in the polar protic solvents such
as ethanol (Table 4, entry 14). Interestingly, we found that the reactions in both pure
DMF and H2O were sluggish (Table 4, entry 12, 15), however, a mixed solvent of DMF
and H2O led to a profound increase in the activity. Excellent yield and selectivity were
achieved when the volume ratio of DMF and H2O was 3:1 (Table 4, entry 1). Other
mixed solvents such as EtOH-H2O and DMSO-H2O have also been tested, and
exhibited decent activity (Table 4, entry 18, 19). The superior function of the mixed
solvent may be due to the increased solubility of the reactants and bases, as the ability
to dissolve bases in water for activating arylboronic acid can enhance the rate of the
reaction in the aqueous medium. The influence of mixed solvent on the Suzuki cross-
87
coupling reactions has been reported in the literature.34
For the various bases added into the reaction system, the best results were obtained
with K2CO3 as the base in DMF:H2O (V:V=3:1) solvent at given temperature (Table 4,
entry 1). Na3PO4 also furnished the coupling products in good activity (Table 4, entry
8). In contrast, Na2CO3, Cs2CO3 and hydroxide bases led to relatively lower yields and
selectivities (Table 4, entry 4, 5, 10 and 11) and other weak bases such as NaOAc, KI
and NaF gave very poor activity (Table 4, entry 6, 7 and 9).
Table 4. Influence of the reaction conditions for Suzuki cross-coupling reactions a
I + (HO)2B
Incident light,Alloy-1
H3C H3Cbase, solvent, 30 °C
Entry Base Solvent Atmosphere Conv. [%] b Sel. [%] c
1 K2CO3 DMF:H2O
(3:1) Ar 96 ＞99
2 K2CO3 DMF:H2O
(3:1) O2 25 78
3 K2CO3 DMF:H2O
(3:1) air 45 91
4 Na2CO3 DMF:H2O
(3:1) Ar 79 90
5 Cs2CO3 DMF:H2O
(3:1) Ar 86 90
6 NaOAc DMF:H2O
(3:1) Ar 13 70
7 KI DMF:H2O
(3:1) Ar 8 0
8 Na3PO4 DMF:H2O
(3:1) Ar 89 ＞99
9 NaF DMF:H2O
(3:1) Ar 4 ＞99
10 NaOH DMF:H2O
(3:1) Ar 75 92
11 KOH DMF:H2O Ar 86 90
88
(3:1)
12 K2CO3 DMF Ar 10 69
13 K2CO3 DMSO Ar 34 96
14 K2CO3 EtOH Ar 73 45
15 K2CO3 H2O Ar 5 ＞99
16 K2CO3 Toluene Ar trace --
17 K2CO3 Dioxane Ar trace --
18 K2CO3 EtOH:H2O
(1:1) Ar 60 ＞99
19 K2CO3 DMSO:H2O
(3:1) Ar 96 95
a Reaction conditions: 3-iodotoluene (1 mmol), phenylboronic acid (1.5 mmol), Alloy-1
photocatalysts (50 mg, containing 3% of metals), base (3 mmol), 20 mL of solvent,
environment temperature 30 °C, reaction time 6 h, the light intensity 0.5 W/cm2. b GC
conversion: (aryl iodide converted)/(initial amount of aryl iodide)×100. c Selectivity:
(target product formed)/(total amount of product formed)×100.
The scope of Suzuki cross-coupling reactions with different substituents
With the optimized conditions in hand, the photocatalytic performance of Alloy-1 for
various substrates was investigated. All the reactions were carried out at 30 °C in argon
using K2CO3 as base and DMF aqueous solution (DMF:H2O =3:1) as reaction medium.
The results are summarized in Table 5. The cross-coupling reactions of aryl iodides
with electron withdrawing and electron donating substituents and phenylboronic acid
afford the desired biaryls in good yields in various reaction periods. The catalytic
system is also effective for electronically diverse arylboronic acids (Table 5, entries 10-
12). Most of the reactions can be accomplished within 6 h, but the coupling reaction of
4-iodoaniline (Table 5, entry 8) and 4-iodophenol (Table 5, entry 9) with phenylboronic
acid led only to modest conversions. In most cases, the formation of homo-coupling
products was not observed, and the extent of this side reaction did not exceeded 2 %
except for the case of 4-(N,N-dimethylamino)phenylboronic acid with iodobenzene
(Table 5, entry 12).
89
Table 5. The scope of Suzuki cross-coupling reactions with different substituents
IR1
+ (HO)2BR2
R1
R2
K2CO3, DMF:H2O=3:1, 30 °C
1 2
Incident light, Alloy-1
a GC conversion: (aryl iodide converted)/(initial amount of aryl iodide)×100;
Selectivity: (target product formed)/(total amount of product formed)×100. The
values in the parentheses are the data for dark reactions. Reaction conditions:
aryl iodide (1 mmol), aryl boronic acid (1.5 mmol), Alloy-1 photocatalysts (50
mg, containing 3% of metals), base K2CO3 (3 mmol), 20 mL of solvent N,N-
dimethylformamide (DMF)/H2O (V:V=3:1), environment temperature 30 °C,
argon atmosphere, the light intensity 0.5 W/cm2. b Reaction time 6 h. c Reaction
time 2 h. d Reaction time 4 h. e Reaction time 22 h. f 1,4-diiodobenzene (1
mmol), phenylboronic acid (2 mmol), reaction time 6 h, the other reaction
conditions kept identical.
We also tried to use aryl bromides as substrate to achieve the reaction under visible
light irradiation, however, the usual reaction conditions above achieved poor
conversion (< 5 %). Thus we tried to improve the conditions and found that using a
stronger base NaOH combined with cetyltrimethylammonium bromide (CTAB) and
H2O as solvent can drive the reaction effectively. CTAB helps in bringing
90
bromobenzene into the aqueous reaction solution.21a As can be seen from Table 6, we
can efficiently enhance the catalytic activity of the reactions using aryl bromide
substrates using alloy NPs under light irradiation.
Table 6. Photocatalytic Suzuki cross-coupling reactions using aryl bromides as
substrates
BrR1
+ (HO)2BR2
R1
R2
NaOH, CTAB, H2O, 30 °C
1 2
Incident light, Alloy-1
a GC conversion: (aryl bromide converted)/(initial amount of aryl bromide)×100;
Selectivity: (target product formed)/(total amount of product formed)×100. The
values in the parentheses are the data for dark reactions. Reaction conditions:
aryl bromide (1 mmol), aryl boronic acid (1 mmol), Alloy-1 photocatalysts (50
mg, containing 3% of metals), NaOH (3 mmol), cetyltrimethylammonium
bromide (CTAB) (1 mmol), 10 mL H2O, environment temperature 30 °C,
reaction time: 3 h, the light intensity 0.5 W/cm2.
The influence of light intensity and wavelength
We investigated the dependence of the catalytic activity on the light intensity and the
results are depicted in Figure 4. When the irradiation intensity was increased from 0.1
to 0.2, 0.3, 0.4 and 0.5 W/cm2 with other reaction conditions unchanged, the conversion
of the reactions on the Au-Pd alloy NPs increased. There is a positive relationship
between the intensity and reaction rate. The results clearly show an almost linear
dependence. We calculated the contributions of the light irradiation to the conversion
efficiency by subtracting the conversion of the reaction in the dark from the overall
conversion observed when the system was irradiated, with both reactions occurring at
identical reaction temperature. Here the conversion of the reaction in the dark is
91
regarded as the contribution of thermal effect. The relative contributions of light and
thermal processes to the conversion efficiencies are shown in Figure 4. We can see that
the higher the light intensity, the greater the contribution of irradiation to the overall
conversion rate. When the light intensity is 0.1 W/cm2, the light contribution was only
27 %, and when the light intensity increased to 0.5 W/cm2, 67 % of the conversion is
due to irradiation. A stronger light intensity will induce a larger population of electrons
in high energy levels and create a stronger electromagnetic field around the NPs (field
enhancement effect), as reported for AuNPs.35 The field enhancement effect also
contributes to a stronger interaction between the NPs and reactant molecules, and thus
enhanced catalytic activity of the coupling reactions.20
Figure 4. The dependence of the catalytic activity of Au-Pd alloy NPs for Suzuki
reaction on the intensity of the light irradiation. The numbers with percentages show
the contribution of the light irradiation effect. Reaction conditions: 3-iodotoluene (1
mmol), phenylboronic acid (1.5 mmol), Alloy-1 photocatalysts (50 mg, containing 3%
of metals), K2CO3 (3 mmol), 20 mL of solvent DMF/H2O (V:V=3:1), 30 °C, 4 h in
argon atmosphere. In the reactions to determine the light-intensity dependence, a
photometer was used to measure the light intensity; the other experimental conditions
were kept identical.
Figure 5. The reaction conversion comparison of varying catalysts using different light
sources.
92
In addition, the photocatalytic reactions were conducted under high intensity white
LED irradiation (the LED light intensity is 5.0 W/cm2, and the wavelength range is
between 400 nm and 700 nm as shown in Figure S2, SI). There is no apparent
difference in conversions between the two kinds of light sources when using Alloy-1,
Alloy-3 and Alloy-4 catalysts (Figure 5). In contrast, when using Alloy-2 (much higher
Pd content, Au:Pd molar ratio=1:5.14) for the photocatalytic procedure, the reaction
under high intensity LED (67 %) showed much better conversion than the normal
halogen lamp (40 %). We investigated the difference of wavelength range between the
LED and halogen lamps and it is possible that the absence of the wavelength range near
500 nm for the white LED light may strongly reduce the LSPR effect of Au NPs in the
alloy NPs, thus Alloy-1, Alloy-3 and Alloy-4 show sluggish difference when using
high intensity LED irradiation. The underlying cause of the apparent improvement in
reactivity of Alloy-2 is believed to be a synergistic effect of light induced inter-band
electron transitions of Pd NPs and the charge redistribution between gold and
palladium. These results imply that energetic electrons derived from Pd sites on the
alloy surface may play a significant role in the photocatalytic process.
A useful tool for determining whether an observed reaction occurs via a photo-
induced process or a thermo-catalytic process is the action spectrum, which should
show one-to-one mapping between the wavelength-dependent photocatalytic rate and
the light extinction spectrum.36 In the present study, the reaction rates of the
photocatalyic Suzuki reaction using Alloy-1 at 30 °C under irradiation with different
wavelengths were determined. Five LED lamps with wavelengths 400±5 nm, 470±5
nm, 530±5 nm, 590±5 nm and 620±5 nm, respectively, were used and the rates are
converted to the apparent quantum efficiencies (AQE). The AQE was calculated as:
AQE (%)=[(Ylight–Ydark)/(the number of incident photons)]×100, where Ylight and Ydark
are the amounts of products formed under light irradiation and dark conditions,
respectively. The plot of the AQE versus the respective wavelengths is the action
spectrum of the reaction. As shown in Figure 6, the action spectrum of the Suzuki
reaction matchs well with the absorption spectrum of Au-Pd alloy NPs, higher activity
is observed at the wavelengths where the Au-Pd alloy NPs strongly absorb the light.
When this reaction was conducted in the dark both PdNPs and AuNPs exhibited much
lower catalytic activities than the alloy NPs (Table 2). This can be attributed to the
higher surface charge heterogeneity of the alloy NPs, which contributes positively to
the reaction.20 When irradiated with light, the reaction yield increases significantly. The
93
most significant enhancement is observed from the alloy NPs, which tracks the light
absorption by the alloy NPs. Furthermore, the shorter wavelength with higher energy
photons are able to excite electrons to higher energy levels that can then be transferred
to the adsorbed reactant molecules more readily as proposed in the mechanism section.
Therefore when the light intensity is constant, the shorter wavelength, the higher AQE.
Generally the light absorption of short wavelengths mainly excite inter-band electron
transition,23b which enhance the yield of the reaction significantly, and the overall
enhancement tracks the light absorption by the alloy NPs.
Figure 6. Action spectrum for Suzuki cross–coupling reactions (red points). The light
absorption spectrum (left axis) is the diffuse reflectance UV−visible (DR−UV−vis)
spectrum of the Au-Pd alloy NPs (blue curve). 3–Iodotoluene was used as the aryl
halide substrate to react with phenylboronic acid under visible light irradiation.
The dependence of photocatalytic activity on light intensity and wavelength indicate
that energetic electrons excited by light absorption are responsible for the observed
photocatalytic activity.37 Since the reaction rate is expected to depend on the population
of electrons with sufficient energy to initiate reaction of the reactant molecules, one can
increase the number of energetic electrons by applying higher light intensity. Tuning
the irradiation wavelength can also increase the number of energetic electrons and may
also assist us to understand the mechanism of the reactions.
The influence of Au:Pd composition ratio
The results in Table 2 reveal the strong dependence of photocatalytic performance of
Au-Pd alloy catalysts on the Au:Pd molar ratio for the Suzuki reaction. Of the four
alloys studied, it was found that the highest yield of target products was achieved when
the alloy NPs have the Au:Pd molar ratio of 1:1.62. At this molar ratio, the ratio of the
94
electrons in Au to those in Pd is 1:0.89 (near 1:1). Alloy NPs with other Au:Pd ratios
exhibited much lower activities. A possible explanation for the superior catalytic
activities of Au-Pd alloy NPs to NPs consisting of either pure component is that Au can
isolate active Pd sites within bimetallic systems.38 However, this does not explain why
the optimal catalytic activity was observed when the Au:Pd electron ratio is near 1:1. It
is possible that a charge redistribution on the alloy particle surface can contribute to the
increase of the photocatalytic activity.20 Such a charge redistribution stems from the
electronic structure of the metals. As illustrated in Scheme 1, pure palladium has a
slightly larger work function (ΦPd ~5.6 eV) than pure gold (ΦAu ~5.3 eV),39 so once the
two metals are in contact, conduction electrons will flow between gold and palladium
until equilibrium is reached (Φ*) with the electron chemical potential equal everywhere
in the alloy NP. A consequence of such a charge-redistribution is that the palladium
atoms at the NP's surface will be electron rich sites and the gold atoms at the surface
will be slightly positively charged. The heterogeneity in charge distribution may
enhance interaction between reactant molecules and the alloy NPs when the reactant
molecules are electrophilic or nucleophilic.20 The enhanced interaction is able to lower
the activation energy of the reaction and thus increase the catalytic activity.40
Furthermore, the Fermi level in alloy NP (Φalloy) is higher than that in pure PdNP (ΦPd),
so that the transfer of electrons at the Fermi level of the alloy NPs to reactant molecule
adsorbed on the NPs is easier, compared with that from the Fermi level of pure PdNPs
to the adsorbed molecule. The light absorption of gold results in energetic conduction
electrons, which are in even higher energy level Φalloy* and have a higher driving force
to migrate to the Pd sites on the surface. This further increases the possibility of
electron transfer from the alloy NPs to the reactant molecules. The detailed transfer of
the light-induced electrons to the adsorbed reactant molecule will be further discussed
below.
The underlying cause of this apparent improvement in reactivity and its dependence
on alloy composition is of great interest, and is believed to be related to the electron
redistribution between the two metals. The electron redistribution can be estimated by
using a free electron-gas model.41 The analysis reveals that the number of electrons
transferred between the two metals, (ΔN), is a maximum when the ratio of the electrons
of the two metals in the alloy NPs is approximately equal (details are provided in ESI
Text S1). As shown in Figure 7 the electron transfer (ΔN) predicted by the model is a
function of the gold electron concentration (%) in the Au-Pd alloy NPs, and/or Au:Pd
95
molar ratio. More importantly, there is a strong correlation between the number of
electrons transferred ∆N and the conversion efficiency of reactants over the alloy NP
photocatalysts: the photocatalytic conversions (refer to the axis on the right hand side)
of the photocatalysts for the Suzuki reaction are given (the red symbol) in Figure 7. As
discussed for the NPs investigated here, alloy NPs with Au:Pd electron ratio near 1:1
(Au:Pd molar ratio: 1:1.62) possess the largest electron transfer number ∆N and display
significant Au-Pd ionic bond character because of Pd rich surface of the NP. These
electronic properties lead to the strongest interaction between the reactant and the metal
NPs facilitating the photocatalytic reaction. This could explain why the activity of
Alloy-1 is much higher than that of the catalysts with other Au:Pd ratios.
Scheme 1. The conduction electrons of Au NPs exist at the NP's surface and a small
fraction of the conduction electrons distribute in the energy levels above Fermi level at
ambient temperature. The surface electronic properties of the alloy NPs are different
from those of pure gold NPs as there are Pd islets on the alloy NP's surface. The Pd
sites are electron-rich because Pd (ΦPd ~5.6 eV) has a slightly larger work function than
gold (ΦAu ~5.3 eV) and electrons will flow from gold to palladium until equilibrium is
reached (the chemical potentials of the electrons are equal in the two metals, being
Φalloy). The Fermi level Φalloy in the alloy NPs is higher than that in pure palladium. Au
NPs strongly absorb the visible light mainly due to the LSPR effect and inter-band
electron transitions, by which the conduction electrons gain the energy of light
irradiation (more conduction electrons distribute to high energy levels). The energetic
conduction electrons in gold can migrate to the Pd sites on the surface. It follows that
the light flux to the NPs predominantly results in a surface (indicated by red colour)
with high energy electrons. The surface Pd sites with energetic electrons could exhibit
significantly enhanced catalytic activity.
96
Figure 7. Electron transfer from gold to palladium in the alloy NPs, expressed as ∆N,
varies with the composition of the alloy NPs (the curve). ∆N reaches a maximum when
the Au:Pd molar ratio is 1:1.62. The Au:Pd molar ratio in the alloy NPs (horizontal
axis) and photocatalytic conversions (red symbols) of the Suzuki cross-coupling
reactions in the present study (vertical axis on the right) are given respectively, the
reaction conversions were based on the average values of 3 runs for each experiment.
We also carried out simulations using the density functional theory (DFT) for
electronic states with and without light irradiation; the irradiation wavelength range
between 532 and 535 nm was chosen, which is around the LSPR absorption of Au.
Calculation capacity limitations of our DFT simulation necessitated the examination of
a Pd32, Au32, and Au12Pd20 cluster. The Au:Pd ratio of the Au12Pd20 cluster is 1.67,
close to the ratio of 1:1.62 for the optimal Alloy-1 photocatalyst. The detailed
calculation method and the calculated Mulliken charge distributions are given in ESI.
The DFT simulation results confirm that charge heterogeneity exists even in the
monometallic Pd clusters and monometallic Au clusters, and the alloy structure of Au
and Pd increases the charge heterogeneity of the NP surface. This result is consistent
with that of the free electron-gas model analysis and previous reports.42 Furthermore,
light irradiation can strongly promote the charge heterogeneity in Au-Pd alloy NPs
compare with pure AuNPs (Figure S4, ESI).
From point of view of reaction kinetics, the visible light absorption by the
photocatalyst can contribute to reducing the activation energy. Hence, the
photocatalytic process has a lower activation energy compared to the corresponding
reaction under heating (thermal process). In the present study, the cross-coupling of 3-
iodotoluene with phenylboronic acid was used as the model reaction to investigate the
kinetics at different temperatures: 20, 30, 40 and 50 °C, and the Arrhenius equation was
97
applied to derive the apparent activation energies of the reaction under light irradiation
and thermal reaction in the dark. The difference between the activation energy of the
two processes indicates the contribution of the light irradiation. The apparent activation
energy is ~49.2 kJ/mol for the cross-coupling reaction in the dark, while it is ~33.7
kJ/mol for the photocatalytic reaction under visible light, respectively. The activation
energy of the photocatalytic cross-coupling reaction is 15.5 kJ/mol lower as illustrated
in Figure 8. This further confirms that the enhanced charge heterogeneity is able to
lower the activation energy of the reaction and thus increase the catalytic activity.
Figure 8. Apparent activation energy reduction of Suzuki reaction caused by the light
irradiation on Au-Pd alloy NP photocatalyst.
Leaching tests
Although various metal NPs have been applied in Suzuki reaction catalysis, the nature
of the true metal species in catalytic cycles remains to be elucidated.8,43 It is debated if
the catalysis stems from the NPs themselves (in other words, heterogeneous catalysis
process), or from trace Pd leached into solution (homogeneously catalysis process).16a
It is a highly challenging task to discriminate the actions of homogeneous Pd-complex
catalysts with single Pd atoms from soluble NP catalysts in the present study of NP
catalysis. This can be ascribed to the possible dynamic exchanges linking different
types of metal species in solution and the NPs acting as reservoirs.16a,44 There is support
for both of these pathways because they may be intertwined, since leaching may be
assisted by one or several steps of the catalytic cycle.44 Current studies employ mainly
the “filtrate transfer” test,45 elemental analysis of the reaction solution, or both, to
examine catalyst leaching. Although there is no strict rule for the metal NPs catalyzed
reactions, the absence of evidence against catalyst leaching may lead to questioning of
98
reaction heterogeneity as well as the identity of actual catalytic species.46
Herein, a sequential reactivity study (“filtrate transfer” test) was developed to
pinpoint the main origin of the efficient catalysis. Two identical catalytic reactions
were initiated using the normal procedure: the mixture for the reaction conducted under
visible light was labelled as A, and that for the dark reaction was labelled as B. After
0.5 h, the reactions were interrupted and the catalysts were separated from the solutions
by centrifugation. The clarified supernatant solution from the photocatalytic reaction
was divided into two equal parts to give solutions A1 and A2, and similarly the
supernatant solution from the dark reaction was divided into solutions B1 and B2. Then
A1 and B1 were re-irradiated for 2.5 h, A2 and B2 were left in the dark to continue
reaction for 2.5 h.
As shown in Figure 9, the photocatalytic reaction achieved 96 % conversion in our
typical reaction, which is much higher than those reactions interrupted and proceeded
without catalysts. This suggests that the heterogeneous Au-Pd alloy NPs is essential in
the photocatalytic reaction to achieve high yield. On the other hand, we found that for
both A and B series solutions, the reaction still proceeded to some extent after the solid
catalyst was removed from the reaction mixture regardless of whether the reaction was
initially conducted in the dark or under irradiation. The conversion achieved after the
removal of the solid catalyst, indicated by the increases in conversion, is attributed to
trace component peeled off from the catalyst during the first 0.5 h reaction. The
greatest conversion rate was achieved in A1. Nonetheless, the conversion rates
achieved by the peeled trace component in the supernatant are much less than that of
the typical reaction under light irradiation. This indicates that the reaction is
predominately catalysed by the supported alloy NPs.
In supernatant left in the dark, the reaction is catalysed by the leached component or
alloy NPs peeled off from the catalyst (A2 and B2). The increases in the conversion
rate of the two supernatants are similar, being 17% (= 45%–28%) and 15% (= 25%–
10%), respectively. Interestingly, the conversion rates of the irradiated supernatants are
higher than those of the corresponding supernatants in the dark (comparisons of B1
with B2, and A1 with A2). This fact suggests that light irradiation can enhance the
catalytic performance of the leached component or peeled alloy NPs, as well.
99
Figure 9. The conversion rates of the supernatants without the solid catalyst. The
reaction conversions were based on the average values of 2 runs for each experiment.
The trace component peeled off from the solid catalyst could be fine alloy NPs or
Pd2+ ions, but the centrifugation could not separate the species from liquids. We
investigated the reaction using Pd2+ ions (Table 7). The PdCl2 catalysed reactions were
conducted both under light irradiation and in the dark, and the conversions were nearly
the same. Au-ZrO2 was also added to the PdCl2 catalysed reaction system, but it did not
affect the catalytic performance (Table 7). Hence, the differences between the
conversions of B1 and B2 (20 %) and between those of A1 and A2 (5 %) are not
contributed by Pd2+ ions, but by the peeled Au-Pd NPs in the supernatant. We repeated
the experiments but removing the solid catalyst after reaction proceeded for one hour,
and compared the results with those shown above (see ESI Figure S5). The extension of
the initial reaction stage from 0.5 h to 1.0 h, gave the similar results.
Table 7. The study of the role of homogeneous Pd2+ ions in the photocatalysis process
Reaction conditions a Conversion (%) d
PdCl2 dark reaction b 81.5
PdCl2 photoreaction b 81.1
PdCl2 + Au-ZrO2 dark reaction c 81.4
PdCl2 + Au-ZrO2 photoreaction c 82.5 aGeneral reaction conditions: 3-iodotoluene (0.5 mmol), phenylboronic acid
(0.75 mmol), catalysts, K2CO3 (1.5 mmol), 10 mL of solvent DMF/H2O
(V:V=3:1), 30 °C, reaction time: 20 min. bcatalyst: 0.5 mL PdCl2 (0.01M)
100
solution. ccatalysts: 0.5 mL PdCl2 (0.01M) solution + 25 mg Au-ZrO2 (1.5 % wt). dGC conversion: (aryl iodide converted)/(initial amount of aryl iodide)×100.
Reusability tests
The reusability of the Au-Pd alloy NP catalysts was examined using bromobenzene as
substrate under visible light irradiation. The results with Alloy-1 are illustrated in
Figure 10. The catalyst was used for five runs with each run conditions kept identical.
After each run of the experiment, the catalyst was separated by centrifugation,
exhaustively washed with water and ethanol twice, and then dried at 60 °C for reuse.
The results show that the catalyst can be reused without losing activity significantly and
the product selectivity can be maintained >99%. The Au-Pd alloy NPs are reusable
photocatalysts for Suzuki reactions. These results also suggest that the leaching of the
alloy NPs from the photocatalyst during the photocatalytic reactions should be
negligible.
Figure 10. The reusability of the Au-Pd alloy NPs for Suzuki reaction using
bromobenzene as substrate under visible light irradiation. Reaction conditions: aryl
bromide (1 mmol), aryl boronic acid (1 mmol), Alloy-1 photocatalysts (50 mg,
containing 3% of metals), NaOH (3 mmol), cetyltrimethylammonium bromide (CTAB)
(1 mmol), 10 mL H2O, environment temperature 30 °C, reaction time: 3 h, the light
intensity 0.5 W/cm2.
Mechanism
It is well known that light-excited electrons of plasmonic metal NPs can populate
unoccupied orbitals of the molecules adsorbed on the NPs yielding transient anionic
species.47 We propose tentative reaction pathways for the photocatalytic Suzuki
101
reaction using Au-Pd alloy NPs photocatalyst. When the alloy NPs are irradiated with
light, the conduction electrons are elevated into excited states (hot electrons) through
absorption of light energy. This increases the NPs’ ability to induce reactions involving
the adsorbed reactant molecules, due to the enhanced surface charge heterogeneity.
Several previous studies suggested that the heterogeneous metal NP's surface was the
true active catalyst, and that activation of aryl halides was possible due to synergistic
anchimeric (adsorption of the aryl moiety to the NP surface can serve as an anchor,
enhancing the chemical interaction of the carbon-halogen bond with a separate but
nearby active site) and electronic (the adsorption of aryl moiety can influence the
electron density of the catalytic surface, which can facilitate the carbon-halogen bond
activation) effects occurring in the presence of adsorbed species on the catalyst
surface.44,48 The oxidative addition is known as the rate-determining step in Suzuki
reactions, and it is said that this step in the heterocatalysis involves the electron transfer
from the Pd atoms to the halide atoms.21a Furthermore, the redox properties of
transition metals are known to be influenced by irradiation with light.49 Palladium(0)
d10 complexes are relatively well studied and are known to possess long-lived (triplet)
excited states both in solution and in the solid state. Due to their excited state lifetimes
in the microsecond range, these complexes can undergo bimolecular photochemical
reactions, including electron transfer and atom transfer reactions with halocarbons.49
Thus, they can readily react with halogenated hydrocarbons and aromatic compounds
under irradiation with visible light.
We therefore performed DFT calculations on the transfer of light-excited electron
from the NPs surface to the reactant iodobenzene molecule (the detailed calculation
methods and results are given in ESI). The simulation suggests that when one electron
enters an unoccupied orbital, the C–I bond will elongate to 0.300 nm from 0.214 nm
(Scheme 2A), so the cleavage of the C–I bond will be much easier. For the reactions in
which the rate-determining step is the C–I bond activation, irradiation will facilitate
electron transfer from the NP to the adsorbed aryl iodide molecule, yielding a transient
radical anion. Loss of iodide ion will afford either an adsorbed phenyl radical or a true
organometallic aryl−palladium iodide complex on the surface. Once the reaction is
initiated, all the remaining steps may proceed following the well-accepted
mechanism.50 Reaction with the base gives the intermediate, which via transmetalation
with the boronate complex forms diaryl organopalladium species. Finally, reductive
elimination of the desired product restores the initial alloy surface completing the
102
photocatalysis cycle (Scheme 2B).
Scheme 2. (A) Light irradiation excites electrons of an alloy NP to high energy levels,
and the transfer of the excited electrons with sufficient energy from the alloy NP to the
LUMO of molecules adsorbed on the NP can take place, activating the reaction. The
electrons finally return to the metal NP at a lower energy.47a DFT calculations show
that the C−I bond is elongated due to the electron transfer; (B) Proposed catalytic cycle
for Suzuki reactions using the Au-Pd alloy NPs under light irradiation.
Conclusions
In summary, it is found that visible light can efficiently enhance the performance of
Au-Pd alloy NPs supported by ZrO2 for Suzuki cross-coupling reactions at low
temperatures. The combination of light absorption of alloy NPs, the enhanced
interaction between the reactants and the NPs as well as the intrinsic catalytic activity
of the transition metal leads to a unique structure where the absorption of visible light
can yield energetic electrons available at catalytically active transition metal sites on
the NP surface promoting the reactions of the molecules adsorbed on the NPs. The
photocatalytic Suzuki reactions achieved superior activity when the ratio of the number
of electrons from Au and Pd in alloy NPs is nearly equal, and showed good feasibility
on a range of substrates. The dependence of photocatalytic activity on light intensity
103
and wavelength indicate that energetic electrons excited by light absorption are
responsible for the observed photocatalytic activity. The results of both the free
electron-gas model analysis and DFT simulation indicate that the Au-Pd alloy
nanostructure increases the charge heterogeneity of the NP surface, which enhances
interaction between the alloy NPs and the reactant molecules adsorbed on the NPs. A
mechanism is proposed based on the results of the DFT calculations and experimental
observation: light absorption of the alloy NPs generates energetic electrons and the
excited electrons with sufficient energy are able to transfer to the reactant molecules
adsorbed on the NPs, weakening the chemical bonds of the molecules and facilitating
the reactions. Using NP photocatalysts and visible light to drive the synthesis of biaryl
compounds represents a new controlled, simplified, and sustainable process in the
realm of green chemistry. The knowledge acquired in this study may inspire further
studies in new efficient photocatalysts of gold and other transition metals for a wide
range of organic syntheses driven by sunlight.
Experimental section
Catalysts preparation
Catalysts with 3 wt% of pure gold nanoparticles on ZrO2, 3 wt% of pure palladium
nanoparticles on ZrO2 and Au-Pd alloy photocatalysts with different Au:Pd ratios on
ZrO2 were prepared by impregnation-reduction method. For example, Alloy-1
(1.5wt%Au-1.5wt%Pd supported on ZrO2) was prepared by the following procedure:
ZrO2 powder (2.0 g) was dispersed into HAuCl4 aqueous solution (0.01M, 15.2 mL)
and PdCl2 aqueous solution (0.01 M, 28.3 mL) was added while stirring magnetically.
Aqueous lysine (0.53 M, 20 mL) was then added with vigorous stirring, which was
continued for 30 min. To this suspension, aqueous NaBH4 solution (0.35 M, 10 mL)
was added dropwise over 20 min, followed by an addition of hydrochloric acid (0.3 M,
10 mL). During the reduction process, the white ZrO2 powder became black, then dark
grey. The mixture was let stand for 24 h and then the solid was separated by
centrifugation, washed with water and ethanol, and dried at 60 °C. The dried solid was
used directly as catalyst. Catalysts with other Au:Pd ratios were prepared in a similar
method but using different quantities of HAuCl4 aqueous solution or PdCl2 aqueous
solution.
104
Catalysts characterization
TEM study and line profile analysis by energy dispersion X-ray spectrum technique of
the photocatalysts were carried out on a Philips CM200 TEM with an accelerating
voltage of 200 kV. The Au and Pd contents of the prepared catalysts were determined
by EDS technology using the attachment to a FEI Quanta 200 Environmental SEM.
The element line scanning was conducted on a Bruker EDX scanner attached to JEOL-
2200FS TEM with scanning beam diameter of 1.0 nm. X-ray diffraction (XRD)
patterns of the sample powders were collected using a Philips PANalytical X’pert Pro
diffractometer. CuKα radiation (λ= 1.5418 Å) and a fixed power source (40 kV and 40
mA) were used. DR-UV-vis spectra of the sample powders were examined by a Varian
Cary 5000 spectrometer.
General procedure for photocatalytic reactions
A 25 mL Pyrex round bottom flask was used as the reaction container, after adding
reactants and catalyst, the flask was sealed with a rubber septum cap. The flask was
then transferred into a reactor chamber and irradiated with magnetic stirring using a
halogen lamp (Nelson, wavelength in the range 400–750 nm) as the visible light source.
The light intensity was measured to be 0.5 W/cm2, light illuminance 94000 lux, the
light intensity was kept constant in all photocatalytic process except for the
experiments investigating the impact of intensity. The light intensity can be adjusted by
manipulating the distance between the lamp and the reaction flask, a photometer (TES
1332A) was used to measure the light intensity. The temperature of the reaction system
was carefully controlled with an air conditioner attached to the reaction chamber. The
environmental temperature in the reactor chamber was maintained at 30 ℃. The
solution temperature in the flask was measured to be 31±1.5 ℃. At given irradiation
time intervals, 2 mL aliquots were collected, centrifuged, and then filtered through a
Millipore filter (pore size 0.45 μm) to remove the catalyst particulates. The filtrates
were analyzed by an Agilent 6890 gas chromatograph with HP-5 column. An Agilent
HP5973 mass spectrometer was used to determine and analyze the product
compositions. For the reactions using H2O as solvent, the product was extracted with
dichloromethane (CH2Cl2) before GC analysis. All the dark reactions were conducted
using a water bath placed above a magnetic stirrer to control the reaction temperature
105
identical to the photocatalytic reactions; the reaction flask was wrapped with
aluminium-foil to prevent ingress of light.
Acknowledgements
The authors gratefully acknowledge financial support from the Australian
Research Council (ARC DP110104990).
Notes and references a School of Chemistry, Physics and Mechanical Engineering, Faculty of Science
and Technology, Queensland University of Technology, Brisbane, QLD 4001,
Australia. E-mail: [email protected]; Fax: +61 7 3138 1804; Tel: +61 7 3138
1581.
† Electronic supplementary information (ESI) available: Fig. S1–S5, DFT
calculations, See DOI: 10.1039/c4gc00588k.
1 (a) A. de Meijere and F. Diederich, Metal-Catalyzed Cross-Coupling Reactions,
Vol. 1-2, Wiley-VCH, Weinheim, 2004; (b) G.C. Fu and A. F. Littke, Angew. Chem.
Int. Ed. 2002, 41, 4176-4211.
2 (a) N. Miyaura, K. Yamada and A. Suzuki, Tetrahedron Lett. 1979, 20, 3437-3440;
(b) for the 2010 Nobel Prize, see:
http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2010/.
3 S. Kotha and K. Lahiri, Eur. J. Org. Chem. 2007, 8, 1221-1236.
4 A. Fihri, P. Meunier and J. C. Hierso, Coord. Chem. Rev. 2007, 251, 2017-2055.
5 C. J. Li, Chem. Rev. 2005, 105, 3095-3166.
6 A. F. Littke, C. Dai and G. C. Fu, J. Am. Chem. Soc. 2000, 122, 4020-4028.
7 (a) S. L. Buchwald, Acc. Chem. Res. 2008, 41, 1439; (b) L. Yin and J. Liebscher,
Chem. Rev. 2007, 107, 133-173; (c) N. Miyaura and A. Suzuki, Chem. Rev. 1995,
95, 2457-2483.
8 A. Fihri, M. Bouhrara, B. Nekoueishahraki, J. M. Basset and V. Polshettiwar. Chem.
Soc. Rev. 2011, 40, 5181-5203.
9 (a) P. Han, X. Wang, X. Qiu, X. Ji and L. Gao, J. Mol. Catal. A: Chem. 2007, 272,
136-141; (b) Z. Zheng, H. Li, T. Liu and R. Cao, J. Catal. 2010, 270, 268-274; (c)
D. D. Das and A. Sayari, J. Catal. 2007, 246, 60-65.
106
10 V. I. Sokolov, E. G. Rakov, N. A. Bumagin and M. G. Vinogradov, Fullerenes,
Nanotubes, Carbon Nanostruct. 2010, 18, 558-563.
11 (a) R. Najman, J. K. Cho, A. F. Coffey, J. W. Davies and M. Bradley, Chem.
Commun. 2007, 47, 5031-5033; (b) I. P. Beletskaya, A. N. Kashin, I. A. Khotina
and A. R. Khokhlov, Synlett. 2008, 10, 1547-1552; (c) G. Wei, W. Zhang, F. Wen,
Y. Wang and M. Zhang, J. Phys. Chem. C 2008, 112, 10827-10832.
12 (a) M. L. Kantam, S. Roy, M. Roy, B. Sreedhar and B. M. Choudary, Adv. Synth.
Catal. 2005, 347, 2002-2008; (b) A. Gniewek, J. J. Ziolkowski, A. M. Trzeciak, M.
Zawadzki, H. Grabowska and J. Wrzyszcz, J. Catal. 2008, 254, 121-130.
13 (a) M. L. Kantam, S. Roy, M. Roy, B. Sreedhar, B. M. Choudary and R. L. De, J.
Mol. Catal. A: Chem. 2007, 273, 26-31; (b) S. Y. Liu, Q. Z. Zhou, Z. N. Jin, H. J.
Jiang and X. Z. Jiang, Chin. J. Catal. 2010, 31, 557-561.
14 (a) K. R. Gopidas, J. K. Whitesell and M. A. Fox, Nano Lett. 2003, 3, 1757-1760;
(b) L. Wu, B. L. Li, Y. Y. Huang, H. F. Zhou, Y. M. He and Q. H. Fan, Org. Lett.
2006, 8, 3605-3608.
15 (a) S. E. Garcia-Garrido, J. Francos, V. Cadierno, J. M. Basset and V. Polshettiwar,
ChemSusChem. 2011, 4, 104-111; (b) V. Polshettiwar and R. S. Varma,
Tetrahedron 2010, 66, 1091-1097.
16 (a) P. P. Fang, A. Jutand, Z. Q. Tian and C. Amatore, Angew. Chem. Int. Ed. 2011,
50, 12184-12188; (b) T. S. A. Heugebaert, S. De Corte, T. Sabbe, T. Hennebel, W.
Verstraete, N. Boon and C. V. Stevens, Tetrahedron Lett. 2012, 53, 1410-1412.
17 (a) G. Palmisano, V. Augugliaro, M. Pagliaro and L. Palmisano, Chem. Commun.
2007, 33, 3425-3437; (b) M. Fagnoni, D. Dondi, D. Ravelli and A. Albini, Chem.
Rev. 2007, 107, 2725-2756; (c) S. Navalon, R. Martin, M. Alvaro and H. Garcia,
ChemSusChem, 2011, 4, 650-657.
18 (a) D. A. Nicewicz and D. W. C. MacMillan, Science 2008, 322, 77-80; (b) D. A.
Nagib, M. E. Scott and D. W. C. MacMillan, J. Am. Chem. Soc. 2009, 131, 10875-
10877; (c) H. W. Shih, M. N. Vander Wal, R. L. Grange and D. W. C. MacMillan,
J. Am. Chem. Soc. 2010, 132, 13600-13603; (d) M. Rueping, C. Vila, R. M.
Koenigs, K. Poscharny and D. C. Fabry, Chem. Commun. 2011, 47, 2360-2362; (e)
M. Rueping, S. Zhu and R. M. Koenigs, Chem. Commun. 2011, 47, 8679-8681; (f)
P. V. Pham, D. A. Nagib and D. W. C. MacMillan, Angew. Chem. Int. Ed. 2011, 50,
6119-6122; (g) C. Dai, J. M. R. Narayanam and C. R. J. Stephenson, Nat. Chem.
107
2011, 3, 140-145; (h) R. S. Andrews, J. J. Becker and M. R. Gagné, Org. Lett. 2011,
13, 2406-2409.
19 (a) X. Chen, H. Y. Zhu, J. C. Zhao, Z. F. Zheng and X. P. Gao, Angew. Chem. Int.
Ed. 2008, 47, 5353-5356; (b) H. Y. Zhu, X. B. Ke, X. Z. Yang, S. Sarina and H. W.
Liu, Angew. Chem. Int. Ed. 2010, 49, 9657-9661; (c) S. Sarina, H. Y. Zhu, Z. F.
Zheng, S. Bottle, J. Chang, X. B. Ke, J. C. Zhao, Y. N. Huang, A. Sutrisno, M.
Willans and G. R. Li, Chem. Sci. 2012, 3, 2138-2146.
20 S. Sarina, H. Zhu, E. Jaatinen, Q. Xiao, H. Liu, J. Jia, C. Chen and J. Zhao, J. Am.
Chem. Soc., 2013, 135, 5793-5801.
21 (a) F. Wang, C. Li, H. Chen, R. Jiang, L. D. Sun, Q. Li, J. Wang, J. C. Yu and C. H.
Yan, J. Am. Chem. Soc., 2013, 135, 5588-5601; b) X. Huang, Y. Li, Y. Chen, H.
Zhou, X. Duan and Y. Huang, Angew. Chem. Int. Ed., 2013, 52, 6063-6067.
22 (a) P. Mulvaney, Langmuir 1996, 12, 788-800; (b) S. Eustis and M. A. El-Sayed,
Chem. Soc. Rev. 2006, 35, 209-217; (c) P. V. Kamat, J. Phys. Chem. B 2002, 106,
7729-7744.
23 (a) K. Patel, S. Kapoor, D. P. Dave and T. Mukherjee, Res. Chem. Intermed. 2006,
32, 103-113; (b) S. Sarina, H. Y. Zhu, Q. Xiao, E. Jaatinen, J. Jia, Y. Huang, Z.
Zheng and H. Wu, Angew. Chem. Int. Ed. 2014, 53, 2935–2940.
24 Y. Li and M. A. El-Sayed, J. Phys. Chem. B, 2001, 105, 8938-8943.
25 M. Pittelkow, K. Moth-Poulsen, U. Boas and J. B. Christensen, Langmuir, 2003, 19,
7682-7684.
26 J. D. Senra, L. F. B. Malta, M. E. H. M. da Costa, R. C. Michel, L. C. S. Aguiar, A.
B. C. Simas and O. A. C. Antunes, Adv. Synth. Catal., 2009, 351, 2411-2422.
27 S. L. Huang, A. Q. Jia and G. X. Jin, Chem. Commun., 2013, 49, 2403-2405.
28 L. Zhang, L. Wang, H. Li and P. Li, Synth. Commun. 2008, 38, 1498-1511.
29 H. B. Pan, C. H. Yen, B. Yoon, M. Sato and C. M. Wai, Synth. Commun. 2006, 36,
3473-3478.
30 S. M. Islam, P. Mondal, K. Tuhina, A. S. Roy, S. Mondal and D. Hossain, J. Inorg.
Organomet. Polym. 2010, 20, 264-277.
31 V. Chandrasekhar, R. S. Narayanan and P. Thilagar, Organometallics 2009, 28,
5883-5888.
32 N. Hoshiya, N. Isomura, M. Shimoda, H. Yoshikawa, Y. Yamashita, K. Iizuka, S.
Tsukamoto, S. Shuto and M. Arisawa, ChemCatChem 2009, 1, 279-285.
108
33 F. Lu, J. Ruiz and D. Astruc, Tetrahedron Lett. 2004, 45, 9443-9445.
34 (a) L. F. Liu, Y. H. Zhang and B.W. Xin, J. Org. Chem. 2006, 71, 3994-3997; (b) Y.
Li, X. M. Hong, D. M. Collard and M. A. El-Sayed, Org. Lett. 2000, 2, 2385-2388.
35 S. Sarina, E. R. Waclawik and H. Zhu, Green Chem. 2013, 15, 1814-1833.
36 (a) A. Tanaka, S. Sakaguchi, K. Hashimoto and H. Kominami, ACS Catal. 2013, 3,
79-85; (b) E. Kowalska, R. Abea and B. Ohtania, Chem. Commun. 2009, 2, 241-
243.
37 P. Christopher, H. L. Xin and S. Linic, Nat. Chem. 2011, 3, 467-472.
38 M. S. Chen and D.W. Goodman, Science 2004, 306, 252-255.
39 Handbook of Chemistry and Physics, 91st ed. 2010-2011, section 12-114.
40 C. S. Cho, W. X. Ren and S. C. Shim, Bull. Korean Chem. Soc. 2005, 26, 1611-
1613.
41 K. Yamada, K. Miyajima and F. Mafun, J. Phys. Chem. C 2007, 111, 11246-11251.
42 (a) A. Kotsifa, T. I. Halkides, D. I. Kondarides and X. E. Verykios, Catal. Lett.,
2002, 79, 113; (b) P. P. Fang, J. F. Li, X. D. Lin, J. R. Anema, D. Y. Wu, B. Ren
and Z. Q. Tian, J. Electro. Chem., 2012, 665, 70.
43 J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem. 2003, 198, 317-341.
44 (a) R. G. Finke and S. Ozkar, Coord. Chem. Rev. 2004, 248, 135-146; (b) N. T. S.
Phan, M. Van Der Sluys and C. W. Jones, Adv. Synth. Catal. 2006, 348, 609-679.
45 The “filtrate transfer” test refers to the following experimental conditions: (1)
initiate a nanoparticle-catalyzed reaction under normal conditions; (2) interrupt the
reaction before completion by removing the heterogeneous catalyst (usually by
filtration or centrifugation); (3) treat the filtrate under normal reaction conditions
without any addition of heterogeneous catalyst; and (4) if the reactant conversion in
the filtrate remains unchanged over the usual reaction completion time, then the
active catalyst is considered to be heterogeneous with minimal leaching.
46 H. Cong and J. A. Porco, Jr. ACS Catal. 2012, 2, 65-70.
47 (a) L. Brus, Acc. Chem. Res. 2008, 41, 1742; (b) M. Turner, V. B. Golovko, O. P.
Vaughan, P. Abdulkin, A. Berenguer-Murcia, M. S. Tikhov, B. F. Johnson and R.
M. Lambert, Nature, 2008, 454, 981; (c) P. Christopher, H. L. Xin, A. Marimuthu
and S. Linic, Nat. Mater., 2012, 11, 1044.
109
48 C. R. LeBlond, A. T. Andrews, Y. K. Sun and J. R. Sowa, Org. Lett. 2001, 3, 1555-
1557.
49 D. M. Roundhill, Photochemistry and photophysics of metal complexes. Plenum
Press, New York, (1994).
50 (a) M. B. Thathagar, J. E. ten Elshof and G. Rothenberg, Angew. Chem. Int. Ed.
2006, 45, 2886-2890; (b) A. V. Gaikwad, A. Holyuigre, M. B. Thathagar, J. E. ten
Elshof and G. Rothenberg, Chem. Eur. J. 2007, 13, 6908-6913; (c) S. MacQuarrie,
J. H. Horton, J. Barnes, K. McEleney, H. P. Loock and C. M. Crudden, Angew.
Chem. Int. Ed. 2008, 47, 3279-3282; (d) Z. Niu, Q. Peng, Z. Zhuang, W. He and Y.
Li, Chem. Eur. J. 2012, 18, 9813-9817.
110
Electronic Supplementary Information
Efficient photocatalytic Suzuki cross-coupling reactions on Au-Pd
alloy nanoparticles under visible light irradiation
Qi Xiao,a Sarina Sarina,a Esa Jaatinen,a Jianfeng Jia,a Dennis P. Arnold,a Hongwei
Liu,a and Huaiyong Zhu a*
a School of Chemistry, Physics and Mechanical Engineering, Faculty of Science
and Technology, Queensland University of Technology, Brisbane, QLD 4001,
Australia.
E-mail: [email protected]; Fax: +61 7 3138 1804; Tel: +61 7 3138 1581.
LEGENDS
Figure S1. Absorption intensity of Au-Pd alloy NPs on ZrO2 and irradiation intensity
of incandescent light (including the calculation method of quantum yield)
Figure S2. The wavelength range of the high intensity LED
Figure S3. The geometries of iodobenzene molecule and iodobenzene negative ion
based on DFT calculation
Figure S4. The optimized geometry and the natural charge distributions of the Au32
cluster and Au12Pd20 clusters in ground state and considered excited state
Figure S5. Comparison of the results for the reactions interrupted at 0.5 h and 1h
Text S1. Estimation of Au-Pd alloy NPs’ ionic property by free gas model
Text S2. DFT calculation of charge distribution in Au-Pd alloy nanoparticle.
111
Figure S1.
Absorption intensity of Au-Pd alloy NPs on ZrO2 (red curve) and irradiation intensity
of incandescent light (black curve). The overlapped area indicates the distribution of
the absorbed photons.
The calculation method of quantum yield:
The light intensity measured at the reaction system was 0.50 W/cm2 (which included
both the absorbed and scattered light). The overall energy of the photons of the
irradiation on the reaction system was derived from the product of the light intensity
and section area of the reactor, which under irradiation. The overlap of the light source
and the absorption spectrum of catalysts provide the distribution of the absorbed
photons over the wavelength range between 400 nm and 800 nm, as shown in the figure.
We could estimate the mean wavelength of the absorbed photons from the distribution
(after being normalized). The mean energy of the photons could be calculated from the
mean wavelength. The number of the photons introduced in the reaction system in our
study was calculated from the ratio of the overall energy of the photons and mean
energy of the photons. The number of molecules formed was determined by the light
induced conversion (calculated by difference of photoreaction and thermal reaction).
Thus the apparent quantum yield was from the ratio of the number of molecules formed
to the number of the photons introduced in the reaction system.
112
Figure S2.
The wavelength range of the high intensity LED.
113
Figure S3.
The geometries of iodobenzene molecule (left) and iodobenzene negative ion (right)
(bond length in Å). Detailed DFT calculation method, see Supplementary Text below.
114
Figure S4.
The optimized geometry and the natural charge distributions of the Au32 cluster (A) and
Au12Pd20 clusters (B) in ground state and considered excited state.
115
Figure S5.
Comparison of the results for the reactions interrupted at 0.5 h (A and A1) and 1.0 h (C
and C1).
The reaction was interrupted at 1.0 h, and the catalyst was removed by centrifugation,
the solution was labelled as C, 0.5 mL sample was collected for GC test. The rest
solution without catalyst was re-irradiated as typical procedure, and after the reaction,
sample was collected for GC test (labelled as C1). We compared the results with that
for the reaction interrupted at 0.5 h (solution A and A1). We can see that after 1.0 h, the
reaction proceeded smoothly with catalyst, and the reaction increased from 28 % (A) to
47 % (C). After removing catalyst, the conversion for C1 is 61 %, and the conversion
for A1 is 50 %. The difference between A and A1 is 22 %, which is much higher than
that between C and C1 (14 %). Thus the catalytic efficiency of the supernatant in 1.0 h
is lower than that in 0.5 h, the reaction did not proceed too much for the solution C,
which means that less alloy NPs was peeled off after 0.5 h. Moreover, from the
reusability of the Au-Pd alloy NPs (Figure 10 in main text), we can also confirm that
the catalytic activity doesn’t lose too much during the reaction. All of these results can
support our conclusion that visible light can stimulate the reaction in the initial phase
(within 0.5 h), and the trace of alloy NPs may be peeled off in the supernatant, but
overall the main contribution of the activity results from the photocatalytic response of
heterogeneous Au-Pd alloy NPs.
116
Text S1. Estimation of Au-Pd alloy NPs’ ionic property by free gas model.
The electron redistribution of the Au-Pd bond is dependent on the magnitude of the
electron transferred between the two metals. An estimate of magnitude of the charge
transferred can be obtained with the free electron gas model,1 with the change in the
number of electrons given by:
(1)
where D(εF) is the density of electron states at the Fermi energies for the two
metals and:
(2a)
(2b)
(2c)
where ФPd and ФAu are the work functions of pure palladium and gold, respectively,
and Ф* is the work function of the alloy once charge equilibrium is reached.
Effectively Δa and Δb give the shift in Fermi level (chemical potential) of the two
metals at their interface upon contact. The density of states of a free electron gas at the
Fermi level 2 is given by (3):
(3)
where N is the number of electrons, so for the two metals the densities are:
(3a)
(3b)
Combining Equations 3a and 3b with Equation 1 the ratio Fermi level shift is given
by:
bDaDN PdFAuF ∆=∆=∆ )()( ,, εε
*Φ−Φ=∆ Pda
Aub Φ−Φ=∆ *
AuPdba Φ−Φ=∆+∆
FF
NDε
ε23)( =
AuF
AuAuF
ND,
, 23)(ε
ε =
PdPdF
ND,
, 23)(ε
ε =
117
(4)
In the alloy systems in this study, the relative concentration of Pd and Au is varied.
If the relative concentration of Pd in the alloy is x, then that of Au will be 1-x and
Equation 4 becomes:
(5)
By combining Equation 5 with Equations 2c and 1, the total change in electron
concentration can be evaluated:
(6)
where K is a constant of proportionality. Therefore, the net increase in electron
concentration on the Pd outer-shell of the nanoparticle will be:
(7)
A plot of N∆ as a function of the gold concentration in the Au-Pd alloy NPs, 1-x,
is shown in Figure 6. The maximum charge transfer occurs at approximately x = 0.5 (i.e.
Au:Pd ratio of the alloy particles is 1:1).
References
1. K. Yamada, K. Miyajima, F. Mafun, J. Phys. Chem. C 2007, 111, 11246-11251.
2. C. Kittle, Introduction to Solid State Physics, 8th ed. Wiley and Sons, New York,
2005.
AuF
Au
Pd
NN
ba
,
,
εε
=∆∆
AuF
xx
ba
,
,
1 εε
−=
∆∆
( ) ( ) K
xx
xK
xx
xN
AuF
AuPd
6.53.5
11
3.56.56.52
3
112
3
,
,,
−+
−=
−+
Φ−Φ=∆
εεε
( )xxK
xx
xKN −′
−+
′=∆ 14.0~9.0
11
4.0
118
Text S2. Density function theory (DFT) calculation of charge distribution in Au-
Pd alloy nanoparticle.
An Au32 cluster and a corresponding Au12Pd20 alloy cluster were constructed to
mimic the Au and Au-Pd nanoparticles. The geometry of the Au32 and Au12Pd20 were
optimized by PBE1 method of density functional theory implemented in CP2K2 code.
The Molecular optimized double zetha-valence Shorter Range basis sets3 with a
polarization function was used to describe the valance orbitals and Goedecker-Teter-
HutterPseudo-potential4was used to describe the core electrons.The excited state
calculations on as optimized structures were performed in the framework of Time-
Dependent density functional theory with B3LYP5,6 functional provided by Gaussian09
package7. In this stage, Lanl2dz basis set8was selected to describe the atomic orbital of
Au and Pdatoms.The excited states with excited wavelength of 534nm for Au32 and 532
nm for Au12Pd20 were considered in our calculations. The optimized geometry of the
Au32 and Au12Pd20 clustersand the natural charge distributions9 of them in ground state
and considered excited statewere depicted in Figure S4.
References
1. Perdew, J. P; Burke, K; Ernzerhof, M., Physical Review Letters, 77 (18), 3865-3868
(1996).
2. CP2K version 2.4, the CP2K developers group (2013), http://www.cp2k.org/ .
3. D. Becke, Density-functional thermochemistry. III. The role of exact exchange, J.
Chem. Phys.1993, 98, 5648-5652.
4. VandeVondele, J; Hutter, J. J. Chem. Phys., 127 (11), 114105 (2007).
5. Krack, M., Theoretical Chemistry Accounts, 114 (1-3), 145-152 (2005).
6. Lee, C., Yang, W., and Parr, R. G., Phys. Rev. B, 1998, 3, 785-789.
7. Hay, P. J. and Wadt, W. R., J. Chem. Phys.1985, 82, 299-310.
8. Frisch, M. J., Trucks, G. W., Schlegel, H. B., G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji,
M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L.
Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.
Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E.
119
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J.
B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O.
Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K.
Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S.
Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and
D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010.
9. Reed, A. E., Weinstock, R. B., and Weinhold, F., J. Chem. Phys., 83 (1985) 735-46.
120
2.4 Article 4
121
Catalytic Transformation of Aliphatic Alcohols to
Corresponding Esters in O2 under Neutral Conditions Using
Visible Light Irradiation
Qi Xiao,† Zhe Liu,† Arixin Bo,† Fathima Sifani Zavahir,† Sarina Sarina,† Steven
Bottle,† James D. Riches,‡,§ and Huaiyong Zhu*,†
† School of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia
‡ Institute for Future Environments, Queensland University of Technology, Brisbane,
QLD 4001, Australia
§ School of Earth, Environmental and Biological Sciences, Queensland University of
Technology, Brisbane, QLD 4001, Australia
KEYWORDS: aliphatic alcohol, esterification, photocatalysis, alloy nanoparticles,
hydrotalcite
ABSTRACT: Selective
oxidation of aliphatic alcohols
under mild and base-free
conditions is a challenging
process for organic synthesis.
Herein, we report a one-pot
process for the direct oxidative esterification of aliphatic alcohols, that is significantly
enhanced by visible light irradiation at ambient temperatures. The new methodology
uses heterogenerous photocatalysts of gold-palladium alloy nanoparticles on a
phosphate modified hydrotalcite support and molecular oxygen as a benign oxidant.
The alloy nanoparticles absorb visible light, and the light-excited metal electrons on the
nanoparticle surface activate the reactants. Tuning the intensity and wavelength of the
irradiation can remarkably change the reaction activity. Shorter wavelength light (< 550
nm) drives the reaction more efficiently than light of longer wavelength (e.g. 620 nm)
especially at low temperatures. The phosphate exchanged hydrotalcite support provides
sufficient basicity for the catalytic reactions, thus the addition of base is not required.
The photocatalysts are efficient and readily recyclable. The findings reveal the first
122
example of using “green” oxidants and light energy to drive direct oxidative
esterification of aliphatic alcohols under base-free mild conditions.
1. INTRODUCTION
Esterification is one of the most essential reactions in organic synthesis.1-3
Traditionally, esters are prepared by the reaction of activated acid derivatives with
alcohols,4 a multistep process that often produces large amounts of unwanted by-
products. Typically alcohols are readily available as bulk chemicals and so represent
attractive starting materials for large scale production. In this regard, the direct
conversion of alcohols into esters represents a significant advance towards green,
economic, and sustainable processes.5-9 One such process involves the selective
oxidation of aliphatic alcohols to corresponding carbonyl compounds. Among the
possible alcohol oxidation reactions, the catalytic and selective oxidation of aliphatic
alcohols with molecular oxygen is rather challenging, especially at neutral pH, and
when employing only moderate reaction conditions.10 To date, only a very few
examples are known for the direct self-esterification of aliphatic alcohols. Of these
studies, most involve homogeneous catalytic systems (Scheme 1), such as those using
iodide or bromide as oxidant,11-14 or transition metal complex catalysts (Pd, Ru, Rh and
Ir etc.).15-23 Furthermore a mild base must be added to counter the acid generated in the
process.11-23 More significantly, most homogeneous catalysis requires harsh reaction
conditions, such as high temperatures and high pressures.15-23
Scheme 1. Direct oxidative esterification of aliphatic alcohol (1-octanol as example)
OH
Au-Pd@HT-PO43-
O
O
Our work
Reported work
Visible light, no additive, 1 atm O2, 55°CMild reaction conditions
Homogeneous: halides oxidant or Pd, Ru, Rh, Ir complex
Heterogeneous: Co3O4-N@C, K3PO4, 1 bar O2,120°C
O
O
Beller's method
additive, (high temperature, high pressure)
123
Therefore, the development of an environmentally benign heterogeneous catalyst for
the esterification of aliphatic alcohols continues to attract significant interest. For
example, Beller and co-workers demonstrated that easily reusable Co3O4-N@C
catalysts can efficiently drive the direct oxidative esterification of aliphatic alcohols.24
Whilst efficient, this reaction needs to be conducted under 1 bar O2 and 120 °C
(Scheme 1).24 Stahl and co-workers reported a heterogeneous catalyst consisting of
Pd/charcoal in combination with bismuth (III) nitrate and tellurium metal that
efficiently esterifies aliphatic alcohols.25 This process could be achieved under much
milder conditions (1 atm O2, 50 °C) compared with Beller’s method, but a strong base,
potassium methoxide, was used and the mixed solid catalysts were not easily recycled
after the reaction. Overall, the use of heterogeneous catalysts for the direct oxidative
esterification of aliphatic alcohols is rarely reported.
Herein we describe a visible light driven, one-pot process for direct oxidative
esterification of aliphatic alcohols that uses molecular oxygen as oxidant, and which
exhibits high product selectivity under mild conditions. Gold-palladium alloy
nanoparticles (Au-Pd alloy NPs) are used as the catalyst for the esterification of
aliphatic alcohols, which proceeds without the addition of base and under visible light
irradiation (Scheme 1).
The discovery of this novel catalytic process derives from our recent development of
novel Au-Pd alloy NP photocatalyzed aryl alcohol oxidations using visible light.26
While investigating the selective oxidation of benzyl alcohol, we observed the trace
formation of esters as side products. Addition of a base to remove acid was seen to
increase the ester yield. We envisioned that Au-Pd alloy NPs could be useful for the
direct self-esterification of aliphatic alcohols, driven by visible light irradiation. This
challenging goal would be of clear significance and broad interest especially if the
addition of base could be avoided. In this regard, an integrated photocatalyst design,
which relies on the synergy of the metal NPs and support material, should in principle
be particularly effective toward this goal. Based on the fact that K3PO4 or K2CO3 have
been used as additives to enhance the catalyst performance,12,13,16,18,20,24 we theorized
that the addition of base may not be required if basic sites are present as part of the NP
supporting material. By combining the Au-Pd alloy NPs with such supports we could
thereby provide a new heterogeneous photocatalyst for base-free oxidation reactions.
In the present study, we have used ion exchange to introduce phosphate anions into
hydrotalcite (with a formula of [Mg6Al2(OH)16]CO3·mH2O and abbreviated HT) to
124
obtain a unique acid buffering support material (HT-PO43-), and then loaded Au-Pd
alloy NPs onto the HT-PO43- support material. This unique structure can effectively
couple the basic sites of the support material with the photocatalytic properties of the
alloy NPs, being used for the direct self-esterification of aliphatic alcohols under
irradiation. Thus, the direct esterification of aliphatic alcohols can be driven without
any additive under visible light irradiation and benign reaction conditions. Notably,
these heterogeneous catalysts can be easily recycled and conveniently reused, which is
an important aspect in the development of practical and cost-effective catalytic
oxidation processes. The results reveal a potential route towards greener commercial
process for clean and efficient production of aliphatic esters.
2. EXPERIMENTAL SECTION
2.1 Preparation of Catalysts
Mg-Al HT: The Mg-Al HT with an Mg/Al ratio of 3 was produced using a sol-gel
process following established procedures with some modification.27,28 For this, an
acidic aqueous solution of metal nitrates was prepared by dissolving Mg(NO3)2•6H2O
(115.4 g, 0.45 mol) and Al(NO3)3•9H2O (56.3 g, 0.15 mol) in 0.6 L of deionized water.
A second alkaline solution was prepared from NaOH (60.0 g, 1.5 mol) and Na2CO3
(26.5 g, 0.25 mol) in 1.0 L of deionized water. Both solutions were heated to 75 °C. For
precipitation, the nitrate and alkaline solutions were added dropwise to 400 mL of
water at 75 °C, giving a pH of 10. The suspension was aged for 3 h at 85 °C under
vigorous stirring. After being cooled down to RT, the gel was filtered and loaded into
an autoclave that was subsequently kept at 80 °C for 16 h. The hydrothermally treated
gel was washed with 350 mL of deionized water until the washings reached a pH of 7.
The resultant white precipitate was dried in oven overnight at 80 °C and ground to a
powder size.
The HT was calcined to 450 °C (heating rate 10 °C•min-1) in a flow of 100 mL•min-1
dry air for 8 h in preparation yielding mixed oxides of magnesium and aluminum,
which were used for the subsequent ion exchange process.
Phosphate modified HT (HT-PO43-): The calcined HT (2.0 g) was dispersed into 50
mL Na3PO4 aqueous solution (0.02 mmol/L), the suspension was stirred at room
temperature for 12 h, then the solid was washed and dried at 110 °C for 10 h, the
resultant solid was ground and denoted as HT-PO43-.
125
Au−Pd@HT-PO43- catalyst was prepared by an impregnation–reduction method. HT-
PO43- powder (2.0 g) was dispersed into HAuCl4 (15.2 mL, 0.01 M) and PdCl2 (28.3
mL, 0.01 M) aqueous solution under magnetic stirring at room temperature. An
aqueous solution of Lysine (16 mL, 0.53 M) was then added with vigorous stirring for
30 min whereupon the pH value was shown to be 8−9. To this suspension, a freshly
prepared aqueous NaBH4 (3 mL, 0.35 M) solution was added drop wise over 20 min.
The mixture was aged for 24 h, and then the solid was separated by centrifugation,
washed with water (3 times) and ethanol (once), and dried at 60 °C in a vacuum oven
for 24 h. The dried powder was used directly as catalyst. Monometallic Au/Pd catalysts
were prepared in a similar method using HAuCl4 and PdCl2 aqueous solutions,
respectively.
2.2 Characterization of Catalysts
The particle size and morphology of the catalyst samples was characterized with a
JEOL2100 transmission electron microscope (TEM), equipped with a Gatan Orius
SC1000 CCD camera. Scanning electron microscope (SEM) imaging, elemental
mapping and EDS were performed using a ZEISS Sigma SEM at accelerating voltages
of 20 kV. X-ray diffraction (XRD) patterns of the samples were recorded on a Philips
PANalytical X’Pert PRO diffractometer using CuKa radiation (λ=1.5418 Å) at 40 kV
and 40 mA. The diffraction data were collected from 5 to 75° with a resolution of
0.01°(2θ). Nitrogen physisorption isotherms were measured at -196 °C on the Tristar II
3020. Prior to each measurement, the sample was degassed at 150 °C for 16 h under
high vacuum. The specific surface area was calculated by the (Brauner-Emmet-Teller)
BET method from the data in a P/P0 range between 0.05 and 0.2. Temperature-
programmed desorption of ammonia (NH3-TPD) was conducted on Micromeritics
AutoChem II 2920 Chemisorption Analyzer to determine the acidic properties of the
catalysts. Catalyst samples were activated at 450 °C for 1 h in vacuum. Ammonia was
adsorbed at 1 mbar and 100 °C for 1 h. For desorption, the samples were heated to the
corresponding temperature from 100 to 600 °C at a rate of 10 K•min-1; desorbing gases
were monitored with a Pfeifer mass spectrometer. Diffuse reflectance UV−visible
(DR−UV−vis) spectra of the sample powders were examined with a Varian Cary 5000
spectrometer with BaSO4 as a reference.
2.3 Photocatalytic Reactions
126
A 20 mL Pyrex glass tube (ϕ, 12 mm) was used as the reaction container, and after
the reactants and catalyst had been added, the tube was sealed with a rubber septum
cap. The tube was irradiated with magnetic stirring using a halogen lamp (from Nelson,
wavelength in the range 400–750 nm) as the visible light source and the light intensity
was measured to be 0.5 W/cm2. The temperature of the reaction system was carefully
controlled with an air conditioner attached to the reaction chamber. The reaction system
under light illumination was maintained at the same temperature as the corresponding
reaction system in the dark to ensure that the comparison is meaningful. All the
reactions in the dark were conducted using a water bath placed above a magnetic stirrer
to control the reaction temperature; the reaction tube was wrapped with aluminum foil
to avoid exposure of the reaction mixture to light. At given irradiation time intervals,
0.5 mL aliquots were collected, and then filtered through a Millipore filter (pore size
0.45 μm) to remove the catalyst particulates. The liquid−phase products were analyzed
by an Agilent 6890 gas chromatography (GC) with HP−5 column to measure the
change in the concentrations of reactants and products. An Agilent HP5973 mass
spectrometer was used to identify the product.
Action spectrum experiments: Light emission diode (LED) lamps (Tongyifang,
Shenzhen, China) with wavelengths 360±5 nm, 400±5 nm, 470±5 nm, 530±5 nm,
590±5 nm and 620±5 nm were used as the light source. The environmental temperature
was measured to be 45±2 °C, and all the other reaction conditions were identical to
those of typical reaction procedures. The AQE was calculated as: AQE = [(Ylight–
Ydark)/(the number of incident photons)]×100%, where Ylight and Ydark are the number
of products formed under irradiation and dark conditions, respectively.
3. RESULTS AND DISSCUSSION
3.1 Catalyst Synthesis and Characterization
HT solids are known to possess surface basic properties that can be fine-tuned by
their compositions.29-31 In this study, the phosphate modified HT support (HT-PO43-)
was prepared by utilizing the ‘‘memory effect’’ of HT. The HT solid was calcined to
450 °C yielding mixed oxides of magnesium and aluminum. When the mixed oxide
powder was dispersed into Na3PO4 aqueous solution, the layered double hydroxide
structure was restored but the anions between the layers are phosphate anions. The
resultant solid was used as support of Au-Pd alloy catalysts (Scheme 2).
127
Scheme 2. Preparation of Au-Pd@HT-PO43- catalysts.
Figure 1 shows the representative TEM images of the catalysts; the Au-Pd alloy NPs
are distributed on the HT surface (Figure 1a), and the average diameter of the Au-Pd
alloy NPs is approximately 4~5 nm (Figure 1b). The high resolution TEM (HR-TEM)
images reveal the atom lattices of Au-Pd alloy NPs (Figure 1c and d). The lattice fringe
spacing of 0.23 nm corresponds to the interplanar distance of (111) planes in the Au-Pd
alloy lattice (Figure 1d).
Figure 1. (a) TEM image of the Au-Pd@HT-PO4
3- catalyst. (b) Particle size
distribution of the Au-Pd alloy NPs based on the statistical analysis from TEM images
(by measuring >350 isolate particles in the images of the sample). (c) HR−TEM image
of the Au-Pd alloy NPs. (d) HR-TEM image of an alloy particle indicated in Figure 1c
(red square).
To investigate the elemental composition in the as-prepared photocatalyst, energy-
dispersive X-ray spectroscopy (EDX) elemental mappings of the Au-Pd@HT-PO43-
catalyst were performed (Figure 2a). EDX elemental mapping of the SEM image shows
that all the components are homogeneously distributed throughout the sample. The
128
phosphorus element is clearly evidenced in the mapping data and the phosphorus
signals indicate that the doped phosphate anion is uniformly distributed in the HT
sample. The percentages of Mg, Al, P, Au and Pd elements in the catalysts were also
analyzed from the EDX spectrum (Figure 2b), the Mg/Al ratio (3/1) and Au/Pd ratio
(1/1) are matched with the initial experimental design.
Figure 2. (a) Scanning electron microscopy (SEM) image of a typical Au-Pd@HT-
PO43- sample and the corresponding energy dispersive spectrometer (EDX) mapping of
Mg, Al, P, Au and Pd elements; (b) EDX spectrum.
Figure 3. The XRD patterns of the photocatalysts.
The well-defined layered structure characteristic of HT is confirmed for all samples
by the X-ray diffraction (XRD) patterns (Figure 3). It is clear that all diffraction peaks
could be indexed to the HT structure and the structure was restored after introducing
phosphate anions and remained unchanged after the metal NPs were loaded, although
129
the intensity of the diffraction peaks decreased, and their widths increased. No
reflections assignable to Au and Pd are present in the XRD patterns, possibly because
the low metal content was below the detection limit and/or due to poor crystallinity of
the metal NPs on the surface of HT.
Figure 4. The normalized diffuse reflectance ultraviolet−visible (DR-UV/Vis)
extinction spectra of the photocatalysts.
The formation of Au-Pd alloy NPs is also supported by the light absorption
properties of the samples, as shown in Figure 4. The HT-PO43- support exhibits weak
absorption of visible light of wavelengths above 400 nm and therefore, the support
itself cannot contribute to photocatalytic activity. In contrast, all the NP photocatalysts
display strong absorption in the UV and visible ranges of the spectrum, indicating that
the NPs are able to utilize most of the irradiation energy delivered in the solar
spectrum. The presence of the support and its interaction with the NPs can substantially
shift and broaden the light absorption peaks.32 The spectrum of the Au-Pd alloy NPs
sample is clearly different from the spectra of metal NPs of either pure component. The
dielectric constant of the alloy NPs is different from the pure metal NPs and the light
absorption of the metal NPs depends substantially on their dielectric constant. In the
spectra of the alloy NP samples, the characteristic localized surface plasmon resonance
(LSPR) absorption peak of Au NPs at about 520 nm is weaker compared with the
spectrum of a pure Au sample. The main absorption peak of the pure Pd NPs on the
support is at about 300 nm in the UV region, and its light absorption at solar
wavelengths occurs through the light excitation of the electron to higher energy
levels.33 Obviously the absorption of the alloy NPs in the visible range is more intense
than that observed for pure Pd sample. This means that the alloy NPs have a better
130
ability to gain light energy to enhance their catalytic performance when irradiated with
visible light.
3.2 Photocatalytic Performance
Exploratory photocatalytic experiments with the different catalytic materials were
performed using the oxidative esterification of 1-octanol to give octyl octanoate as a
model reaction. For comparison, the reactions were also conducted in the dark with
other conditions kept identical. For example, the temperature of the reaction in the dark
was kept the same as that of the reaction under irradiation by using a water bath.
To understand the effect of various bases on the performance of the catalytic system,
we used Au-Pd@ZrO2 catalysts with various base additives (Table S1, SI), and found
that when using K3PO4 as the base the photocatalytic reaction exhibited the best
performance, and this is in agreement with literature report that K3PO4 is the optimal
base additive for the oxidative esterification.20,24 The performance of the alloy NPs on
the unmodified HT was also examined. As can be seen in Table 1, Au-Pd@HT catalyst
exhibits a slightly better activity under irradiation (entry 1) than in the dark for the
direct oxidative esterification of 1-octanol. Adding K3PO4 increases the reaction
activity of this catalyst (entry 2). To our delight, the alloy NPs on the PO43- modified
HT, Au-Pd@HT-PO43- catalyst, exhibit optimal performance for the one-pot oxidative
esterification. Excellent conversion (94%) and good selectivity (76%) was achieved
without any additional base additive under visible light irradiation (entry 3). A much
lower activity was observed for the reaction in the dark (entry 3). The catalytic
activities of the monometallic Au and Pd catalyst as well as a mixture of monometallic
Au and Pd catalysts (entry 4-6) are obviously lower than that of the alloy NP catalyst.
Interestingly, Pd@HT-PO43- is a superior catalyst to Au@HT-PO4
3- both under the
visible light irradiation and in the dark. This could be attributed to the better ability of
Pd NPs to activate molecular oxygen,34-36 and the fact that irradiation can enhance the
catalytic performance of Pd NPs.33
These results indicate that the alloying of the two metals can further enhance the
catalytic activity of the Au-Pd alloy NPs, which is in line with our previous reported
results.26 The alloy NP surface has greater charge heterogeneity than pure metal NP
surface, which leads to a stronger interaction between the alloy NPs and reactant
molecules facilitating the reaction. Compared with recent reported heterogeneous
catalysts for the direct oxidative esterification of 1-octanol at 120 °C under 1 bar of O2,
131
which achieved a octyl octanoate yield of 75%,24 the Au-Pd alloy NP photocatalyst in
the present study is efficient and much more environmentally friendly. We also
conducted the reactions with Au-Pd@HT-PO43- catalyst under simulated sunlight
irradiation: under a irradiance (light intensity) of 0.45 W/cm2 (50 °C), 43% conversion
was obtained, with 61% octyl octanoate selectivity, which is much higher than that of
the control experiment in the dark (Table S2). To the best of our knowledge, the metal
NPs or Au-Pd alloy NPs on HT-based materials have not been found to be efficient
base-free oxidation catalysts under mild conditions. The present study is also the first
successful example of the direct oxidative esterification of aliphatic alcohols under
base-free conditions with visible light. Notably, the novel catalyst system is highly
stable and can be reused several times (see below).
Table 1. Activity test and catalyst screening for oxidative esterification of 1-octanol.
OH O
OPhotocatalyst, hv
Additive, O2, 55°C, 24h
Entry Catalyst Additive Incident
light
Conversion
(%)
Selectivity
(%)
1 Au-Pd@HT − Visible 48 54
− Dark 45 3
2 Au-Pd@HT K3PO4 Visible 78 62
K3PO4 Dark 57 25
3 Au-Pd@HT-PO43-
− Visible 94 76
− Dark 62 42
4 Au@HT-PO43-
− Visible 35 72
− Dark 24 68
5 Pd@HT-PO43-
− Visible 84 58
− Dark 60 31
6 Au@HT-PO4
3-
+Pd@HT-PO43-
− Visible 54 63
− Dark 46 34
7 HT-PO43-
− Visible 0 0
− Dark 0 0
Reaction conditions: photocatalyst 100 mg, reactant 0.2 mmol, additive 50 mg, solvent
α,α,α-trifluorotoluene 2 mL, 1 atm O2, environment temperature 55 °C, reaction time
132
24 h, and the light intensity was 0.5 W/cm2. The conversions and selectivity were
calculated from the product formed and the reactant converted measured by gas
chromatography (GC).
The direct oxidative esterification of a series of aliphatic alcohols catalyzed by the
Au-Pd@HT-PO43- catalyst was also investigated. As shown in Table 2, good yields of
the corresponding aliphatic esters were achieved. Notably, the product yields for the
reactions under irradiation are much higher than those of typical thermal reactions
undertaken in the dark.
Table 2. Photocatalytic base-free direct oxidative esterification of various aliphatic
alcohols.a
R OH
R=aliphatic
Au-Pd@HT-PO43-, hv
1 atm O2, 55°C, 24hR O R
O
ester Entry Ester Yield (%)b
1 O
O
73 (40)
2 O
O
75 (44)
3 O
O
72 (26)
4 O
O
47 (13)
5 O
O
53 (12)
a Reaction conditions: Au-Pd@HT-PO43- photocatalyst 100 mg , reactant 0.2 mmol, 2
mL α,α,α-trifluorotoluene, 1 atm O2, environment temperature 55 °C, reaction time 24
h, and the light intensity was 0.5 W/cm2. b The yields were calculated from the product
formed and the reactant converted measured by GC, the values in parentheses are the
results in the dark.
The oxidative esterification of aryl alcohols using Au-Pd@HT-PO43- catalyst under
visible light irradiation was also investigated. Here, again the catalyst is active and
highly selective under visible light irradiation, and benzyl alcohol reacts smoothly with
133
methanol, ethanol, benzyl alcohol as well as 1-octanol, yielding corresponding aryl
esters in different solvents in yields up to 76% (Table 3).
Table 3. Photocatalytic base-free oxidative esterification of aryl alcohols.a
Entry Reactant Product Yield (%)b
1c OH
O
O
76 (50)
2c OH
OMe O
O
OMe 40 (16)
3d OH
O
O
51 (23)
4e OH
O
O
43 (17)
5f OH
O
O
30 (3)
a Reaction conditions: Au-Pd@HT-PO43- photocatalyst 50 mg, reactant 0.2 mmol, 2 mL
solvent, 1 atm O2, environment temperature 55 °C, reaction time 24 h. The light
intensity was 0.5 W/cm2. b The yields were calculated from the product formed and the
reactant converted measured by GC, the values in parentheses are the results in the
dark. c Methanol as solvent. d Ethanol as solvent. e n-heptane as solvent. f 1-octanol as
solvent.
Table 4. Photocatalytic base-free oxidation of secondary aliphatic alcohols to ketones.a
R R'
R=alkyl
Au-Pd@HT-PO43-, hv
1 atm O2, 55°C, 24hR R'
OOH
Entry Reactant Product Yield (%)b
1 OH
O
100 (19)
2 OH
O
91 (17) a Reaction conditions: Au-Pd@HT-PO4
3- photocatalyst 50 mg, reactant 0.2 mmol, 2 mL
solvent α,α,α-trifluorotoluene, 1 atm O2, environment temperature 55 °C, reaction time
24 h. The light intensity was 0.5 W/cm2. b The yields were calculated from the product
134
formed and the reactant converted measured by GC, the values in parentheses are the
results in the dark.
The Au-Pd@HT-PO43- photocatalysts are also effective for the oxidation of
secondary aliphatic alcohols to ketones. Excellent aliphatic ketone yields were achieved
under base-free mild reaction conditions (Table 4).
A key attraction to heterogeneous catalysis is the possibility of catalyst recycling. We
carried out a series of experiments using oxidative esterification of 1-octanol under
irradiation to demonstrate the recyclability of Au-Pd@HT-PO43- catalyst. Briefly, after
each reaction cycle, Au-Pd@HT-PO43- catalyst was separated by centrifugation, and
washed thoroughly by ethanol twice and dried for subsequent reactions. As shown in
Figure 5a, the catalyst was reused for several cycles without significant loss of activity.
From the typical TEM image of the Au-Pd@HT-PO43- catalyst after recycled (Figure
5b), the Au-Pd alloy NPs still distribute evenly on the HT surface, no obvious
agglomeration was observed.
Figure 5. (a) The photocatalytic activity the Au-Pd@HT-PO4
3- catalyst after 5
recycled; (b) Representative TEM image of the Au-Pd@HT-PO43- catalyst after
recycled. The dark particles are the alloy NPs.
3.3 Influence of the Supports
Since the oxidative esterification can proceed in the absence of added base, the
surface properties of the support materials of the photocatalysts are expected to play a
critical role in catalyst performance. Specific surface areas of the photocatalysts
(derived from nitrogen sorption data) and the surface acidity of the samples (measured
by NH3 temperature-programmed desorption, NH3-TPD) are provided in Table 5. The
specific surface area is not the decisive factor on the performance as the optimal
photocatalyst has a relatively small specific surface area. Basic sites on the support
surface alone cannot simply be dominate factor enhancing the catalyst performance
since MgO and HT have basic surface sites but Au-Pd@MgO and Au-Pd@HT exhibit
135
low and moderate activity, respectively. We also found that the oxidative esterification
activity of the Au-Pd@HT-PO43- catalyst can be greatly inhibited by the addition of
benzoic acid or pyridine (Scheme S1). This is likely to arise from interference with the
basic/acidic sites of the support by reaction with the benzoic acid or pyridine31 and
potentially by surface complexation of the benzoate or pyridinium conjugate ions
limiting access by the alcohol substrate. Therefore a moderate population of
basic/acidic sites on the supports is necessary for the catalysis and the basic surface
sites from PO43- are superior to other basic sites for the catalytic performance. The
unique surface character of the Au-Pd@HT-PO43- catalyst is essential for facilitating
the base-free direct oxidative esterification of aliphatic alcohols. The content of PO43- in
the HT support also affects the catalytic activity (Table S3). According to the results of
EDX analysis, when the phosphorus content is 0.2 wt% of the catalyst, the catalyst
exhibits the best performance; further increasing the amount of PO43- can suppress the
conversion rate and ester selectivity. Thus, a small amount of phosphate exchanged into
the HT support can provide necessary basic/acidic sites for the catalyzed reactions and
thus avoid the need to add base for efficient conversion.
Table 5. Activity test of different supports for base-free oxidative esterification of 1-
octanol.a
OH O
OPhotocatalyst, hv
1atm O2, 55°C, 24h
Entry Catalyst
BET
surface area
(m2/g)b
Acid
density
(mmol/g)c
Incident
light
Conv.
(%)
Select.
(%)
Product
yield
(%)
1 Au-Pd@HT-
PO43-
13 0.91 Visible 94 76 72
Dark 62 42 26
2 Au-Pd@HT 59 1.00 Visible 48 54 26
Dark 45 3 2
3 Au-Pd@Al2O3 201 1.07 Visible 51 92 47
Dark 33 5 2
4 Au-Pd@MgO 3 0.03 Visible 20 34 7
Dark 3 0 0
5 Au-Pd@ZrO2 20 0.20 Visible 38 47 19
Dark 33 36 12 a Reaction conditions: photocatalyst 100 mg, reactant 0.2 mmol, solvent α,α,α-
136
trifluorotoluene 2 mL, 1 atm O2, environment temperature 55 °C, reaction time 24 h.
The light intensity was 0.5 W/cm2. The conversions and selectivity were calculated
from the product formed and the reactant converted measured by GC. b Determined by
adsorption-desorption of nitrogen. c Determined by NH3-TPD.
3.4 Impact of Light Intensity and Wavelength
The dependence of catalytic activity on light intensity (irradiance) was investigated,
and the results of the oxidative esterification of 1-octanol under different irradiances
are depicted in Figure 6. When the irradiance was raised, the reaction yields increased.
There is a positive relationship between the irradiance and reaction rate. The
contributions of irradiation to the conversion efficiency were calculated by subtracting
the reaction yield achieved in the dark from the overall yield of the irradiated system
when reactions were conducted at identical reaction temperature. Here the conversion
of the reaction in the dark is regarded as the contribution of the thermal effect. The
relative contributions of visible light irradiation to the conversion efficiencies are
shown in Figure 6.
Figure 6. The dependence of the catalytic activity of Au-Pd@HT-PO4
3- photocatalyst
for the oxidative esterification of 1-octanol on the intensity of irradiation.
The numbers with percentages show the contribution of the irradiation effect. It can
be seen that the higher the irradiance, the greater the contribution of irradiation to the
overall reaction rate. When the irradiance is 0.3 W/cm2, the light contribution for the
reaction is only 46%, and when the irradiance increased to 0.7 W/cm2, 74% of the
product yield is due to irradiation. A greater irradiance provides more light-excited
energetic electrons and creates a stronger electromagnetic field around the NPs
(electromagnetic field enhancement effect), as reported for AuNPs.37-39
137
The light excited metal electrons may facilitate the reaction via two pathways: release
energy to the lattice to thereby heat the NPs (photo-thermal effect) or transfer to the
reactant molecules that are adsorbed on the NP directly causing the reaction of the
molecules (excited electron transfer). If the reaction is due to the photo-thermal effect,
the wavelength of the light employed in the irradiation has little impact on the reaction
rate when the irradiance is identical. A useful tool for determining whether a reaction is
induced due to the photo-thermal effect is the action spectrum, which shows the
relationship between the wavelength of incident light and the photocatalytic rate.40,41 In
this study, the reaction rates of the photocatalyic oxidative esterification of 1-octanol
using Au-Pd@HT-PO43- at 45±2 °C under irradiation with different wavelengths were
determined. The obtained reaction rates were converted to the apparent quantum
efficiencies (AQEs).33,42 The plot of AQE versus the respective wavelengths is the
action spectrum of the reaction. The action spectrum shows that the dependence of the
AQE of the reaction catalyzed by the Au-Pd alloy catalysts on the wavelength of
irradiation does not follow the absorption spectrum of the supported Au-Pd@HT-PO43-
catalyst (Figure 7). The highest activity is observed at shorter wavelengths, thus the
photo-thermal effect is not the main pathway for the reaction.
Figure 7. Photocatalytic action spectrum for the oxidative esterification of 1-octanol
using Au-Pd@HT-PO43- photocatalyst. The light absorption spectrum (left axis) is the
DR−UV/vis spectra of the supported Au-Pd@HT-PO43- catalyst (black curve).
Since the irradiance and reaction temperature were held constant at each wavelength
(Figure 7), the total input energy gained by the photocatalysts under irradiation at
different wavelengths was identical in a given reaction period. The impact of external
heating has been excluded from AQE values, as the number of product molecules
formed in the dark was deducted. Assuming that the input photon energy is totally
138
converted into a thermal effect, the catalytic activity caused by the photo-thermal effect
should be similar. The extraordinary high enhancement in activity with shorter-
wavelength photons indicates that photo-enhancement at 360 nm is not only due to a
simple photo-thermal effect.33 It is more likely that the photocatalytic esterification of
1-octanol irradiated with light wavelength short than 550 nm mainly proceeds via the
excited electron transfer.
The photons with a shorter wavelength are able to excite metal electrons to higher
energy levels, and these electrons have more chances to transfer to the anti-bonding
orbitals, located above the bonding orbitals on the energy scale, inducing reaction.
When the energy of the anti-bonding orbital is high, the electrons excited by light with
a longer wavelength do not have sufficient energy for the injection. They will relax to
low energy levels and release energy to heat the lattice of the alloy NPs, enhancing the
reaction only by the photo-thermal effect. Thus, the photo-thermal contribution to the
AQE obtained under the longest wavelength (620 nm) is the largest, compared to that
from the other wavelengths, Therefore, by comparing the AQE observed at short
wavelengths with that observed at long wavelengths, we can estimate the contribution
from the excited electron transfer.33 The AQE values at 590 nm and 620 nm are low,
and the AQE values at short wavelengths (<550 nm) are much larger. The large
difference between the AQE values at short and long wavelengths means that most of
the chemical transformation at short wavelength is via the excited electron transfer
pathway.
We compare the oxidative esterification of 1-octanol using Au-Pd@HT-PO43-
photocatalyst at various temperatures under 400 nm and 620 nm LED irradiation,
respectively, in Figure S3. It can be seen that the catalytic performance of the reaction
irradiated with 400 nm wavelength is much better than that irradiated with 620 nm
wavelength and in the dark, especially at lower reaction temperature. The dependence
of the catalytic activity on the reaction temperature for the reaction irradiated with
long-wavelength (620 nm) is very similar with that in the dark.
The dependence of photocatalytic activity on irradiance and wavelength indicates
that electrons excited by light absorption are mainly responsible for the observed
photocatalytic activity.43 Because the rate of the catalyzed reactions is expected to
depend on the population of electrons with sufficient energy to initiate the reaction of
the reactant molecules, one can increase the number of these “hot” electrons by
applying a high irradiance or tuning the irradiation wavelength to accelerate the
139
reaction. This knowledge may also assist us in understanding the mechanism of the
catalytic reactions.33,37-39,42
Figure 8. (a) The catalytic activities of the oxidative esterification of 1-octanol using
Au-Pd@HT-PO43- photocatalyst at different temperatures under visible light irradiation
and in the dark. The numbers with percentages show the contribution of the irradiation
effect. (b) Comparison of action spectra for the oxidative esterification of 1-octanol
using Au-Pd@HT-PO43- photocatalyst at two different temperatures. (c) A schematic of
the effect of increasing temperature on the gain of vibrational energy, thus facilitate the
reaction.
140
3.4 Impact of Temperature
An important feature of the photocatalytic process on metal NP catalysts is that the
catalytic activity of the NPs can be increased by elevating the reaction temperature.44,45
This feature is also observed from the Au-Pd alloy NP photocatalysts.42 In the present
study, we conducted the oxidative esterification of 1-octanol using Au-Pd@HT-PO43-
photocatalyst at various temperatures both under irradiation and in the dark. As shown
in Figure 8a, raising the reaction temperature can achieve higher product yields, for
both the irradiated reactions and the reaction in the dark. We calculated the contribution
of the irradiation effect by the value of the yield difference between the light reaction
and the dark reaction divided by the total yield under irradiation. For example, when at
55 °C, the difference between the light reaction and dark reaction is 46%, accounting
for 64% of the total yield. To further understand the effect of reaction temperature, we
compare the action spectra for the photocatalytic reaction at two different temperatures
(45 °C and 70 °C, Figure 8b). Much lower AQEs were obtained at a higher temperature
as the increased number of product molecules formed in the dark is removed from the
values when the AQE of the irradiated reaction is calculated. The AQE value barely
varies with the wavelength of irradiation. As shown in Figure 8c, at a higher
temperature, light excites more electrons to higher energy levels, and the probability of
the transfer of the excited electron from the metal to the adsorbed molecules to initiate
their reaction is higher than that at lower reaction temperature.43 More importantly, at
higher temperatures the relative population of excited vibrational states of the adsorbed
reactant molecule increases according to the Bose-Einstein distribution.45 This means
that, on average, the reactant molecule will require less energy to surmount the
activation barrier, and the activation of the alcohol molecule should be much easier. At
high temperatures the contribution from the thermal effect (phonon-driven) can be
greater than that from the light irradiation as the reactant molecules may gain most
energy from heating to overcome the activation energy barrier. In this case, the thermal
energy is sufficient to induce a significant population of vibrationally excited states of
the reactant molecules.43,46 While the light-excited electrons can also contribute to
accelerating the reactions.47,48 The light-excited hot electrons in the high-energy tail of
a thermal Fermi-Dirac distribution can induce reactions by transient population of
normally unoccupied states.47 We can see that light contribution is 72% when the
reaction is limited to 35 °C (Figure 8a). At lower temperatures, excite electron transfer
dominates the photocatalytic activity and the thermal effect (and photothermal effect as
141
well) contribute much less.48 For many catalyzed reactions in which the interaction of
the light-excited hot electrons of a catalyst with reactant molecules induces the
reaction, high reaction temperatures are not a prerequisite for efficacy.
The alloy NPs have the capacity to couple the stimuli of irradiation and heat to drive
the catalytic reaction due to the continuum of metal electron energy levels.42,44 This
property not only distinguishes them from semiconductor photocatalysts, but also
demonstrates the potential of the NPs to utilize the infrared radiation of sunlight, which
accounts for a larger fraction of the solar spectrum and could be used to heat the NPs,
further facilitating the reaction.
Figure 9. The time course for the catalytic activities [reaction conversion and product
selectivity (red: ester, grey: aldehyde)] of the oxidative esterification of 1-octanol using
142
Au-Pd@HT-PO43- photocatalyst under irradiation with 400 nm LED (a), 620 nm LED
(b) and in the dark (c) at 50 °C.
3.5 Proposed Reaction Pathway
To investigate the reaction pathway, we studied the evolution of the products during
the time course of the oxidative esterification of 1-octanol using Au-Pd@HT-PO43-
photocatalyst under irradiation with a short-wavelength LED (400 nm) and a long-
wavelength LED (620 nm), respectively (Figure 9a and 9b), and the results are
compared with that of the reaction in the dark (Figure 9c). We found that the
conversion of the reaction irradiated with short-wavelength is higher than that
irradiated with long-wavelength and in the dark. Furthermore the aldehyde is the main
intermediate during the reaction course, both under light irradiation and in the dark.
This suggests that the alcohol is first oxidized to aldehyde during the process and then
the aldehyde further reacts with another alcohol molecule to achieve the direct
esterification.
Scheme 3. Proposed reaction pathway.
A tentative mechanism for the direct oxidative esterification of alcohols is proposed
based on literature precedent13,15,21,24 and depicted in Scheme 3. The oxidative
esterification reaction may proceed through an oxidation of alcohol to aldehyde (IV)
and then a condensation reaction between aldehyde and another molecule of alcohol
143
which results in the formation of hemiacetal intermediate (V), followed by oxidative
dehydrogenation to give the corresponding ester. In this case, the alcohol molecule is
adsorbed on the Au-Pd alloy NP surface because the alloy NP surface has a strong
binding affinity. The selective oxidation of alcohol to aldehyde on the alloy NP surface
is likely to proceed first (I-IV). Irradiation can facilitate the cleavage of O-H bond for
the insertion step, which leads to the formation of metal alkoxide and metal hydride
species (II). The surface basic sites of the support can bind the hydrogen atom of the
reactant molecule and also facilitate the O-H cleavage on the metal surface as per
reports in the literature.49 Light excited electrons can promote hydrogen abstraction
from the α-H of the metal alkoxy species, which then yields the aldehyde (III-IV). DFT
calculations also suggest that the transfer a light excited electron from metal NP to the
reactant molecule adsorbed on the surface can facilitate the cleavage of the C-H bond.26
The presence of surface basic sites also lowers the barrier for the activation of the C-H
bond of the metal alkoxide intermediate to form the aldehyde over metal NP surface.47
Then the formed aldehyde on the metal NP surface can react with another molecule of
alcohol and form the hemiacetal intermediate (V).13,15,21,24 Finally, the final ester
product can be obtained by oxidative dehydrogenation of the hemiacetal intermediate,
and the metal active sites are regenerated as oxygen reacts with hydrogen on the metal
NP surface.
Irradiation also enhances the surface charge heterogeneity of the alloy NPs, which
means that interactions between the alloy NPs and reactant molecules are enhanced.9
There is a much higher probability that the reactant molecules are adsorbed on the Pd
sites on the alloy NPs surface, and Pd sites have a better ability to attract hydrogen,
which can promote the hydrogen abstraction steps (both III and V). Overall, the light-
excited electrons may facilitate the hydrogen abstraction steps, which could assist the
direct oxidative esterification when the alcohol molecules are adsorbed on the surface
of metal NPs.
4. CONCLUSIONS
In summary, a stable and reusable catalyst of Au-Pd alloy NPs supported on
phosphate anion modified hydrotalcite has been found to be active and selective for the
direct oxidative esterification of aliphatic alcohols under visible light irradiation using
molecular oxygen as a benign oxidant. The novel catalyst can be prepared readily by
utilizing the “memory effect” of hydrotalcite, followed by the impregnation-reduction
144
of gold and palladium salts to form Au-Pd alloy NPs on the surface of phosphate anion
modified hydrotalcite. The catalyst exhibits superior performance for the synthesis of a
variety of esters with aliphatic alcohols when irradiated with visible light: achieving
good yields without any additives. It is also applicable for oxidative esterification of
aryl alcohols and oxidation of secondary aliphatic alcohols to ketones. These catalytic
processes are due to the interaction of light-excited electrons from the catalyst reacting
with the substrate molecules, and high temperatures and high pressures are not
required. The oxidative esterification of aliphatic alcohols involves selective
conversion of aliphatic alcohols to the corresponding aldehydes and subsequent
esterification of the aldehydes with unreacted alcohol. The reaction rate depends on the
number and the energy level of light-excited electrons, which can be tuned by the
incident light intensity and wavelength. The base-free catalytic process is simple, cost-
effective, and environmentally benign.
ASSOCIATED CONTENT
Supporting Information. Supplementary Tables S1-S3, Figures S1-S3 and Scheme
S1.
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]
Notes
The authors declare no competing finical interest.
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the Australian Research Council
(ARC DP110104990), Institute of Coal Chemistry, Chinese Academy of Sciences for
TPD measurement, supported by the Foundation of State Key Laboratory of Coal
Conversion (Grant No. J14-15-605).
REFERENCES
(1) Larock, R. C. Comprehensive Organic Transformations, VCH: New York, 1989;
p.966 and references therein.
(2) March, J. Advanced Organic Chemistry, Wiley, New York, 1985.
145
(3) Lloyd, W. G. J. Org. Chem. 1967, 32, 2816–2819.
(4) Larock, R. C. Comprehensive Organic Transformations: A Guide to Functional
Group Preparations, 2nd ed., Wiley-VCH, New York, 1999.
(5) Xu, B.; Liu, X.; Haubrich, J.; Madix, R. J.; Friend, C. M. Angew., Chem. Int. Ed.
2009, 48, 4206–4209.
(6) Oliveira, R. L.; Kiyohara, P. K.; Rossi, L. M. Green Chem. 2009, 11, 1366–1370.
(7) Miyamura, H.; Yasukawa, T.; Kobayashi, S.; Green Chem. 2010, 12, 776–778.
(8) Zweifel, T.; Naubron, J. V.; Grutzmacher, H. Angew. Chem., Int. Ed. 2009, 48,
559–563.
(9) Arita, S.; Koike, T.; Kayaki, Y.; Ikariya, T. Chem. Asian J. 2008, 3, 1479–1485.
(10) Lu, T.; Du, Z.; Liu, J.; Ma, H.; Xu. J. Green Chem. 2013, 15, 2215–2221.
(11) Liu, X.; Wu J.; Shang Z. Synth. Commun. 2012, 42, 75–83.
(12) Mori, N.; Togo, H. Tetrahedron 2005, 61, 5915–5925.
(13) Barluenga, J.; González-Bobes, F.; Murguía, M. C.; Ananthoju, S. R.; González, J.
M. Chem. Eur. J. 2004, 10, 4206–4213.
(14) Do, Y.; Ko, S. -B.; Hwang, I. -C.; Lee, K. -E.; Lee, S. W.; Park, J.
Organometallics 2009, 28, 4624–4627.
(15) Izumi, A.; Obora, Y.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 2006, 47, 9199–
9201.
(16) Liu, C.; Tang, S.; A. Lei, W. Chem. Commun. 2013, 49, 1324–1326.
(17) Esteruelas, M. A.; García-Obregón, T.; Herrero, J.; Oliván, M. Organometallics
2011, 30, 6402–6407.
(18) Gowrisankar, S.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed. 2011, 50, 5139–
5143.
(19) Schleker, P. P. M.; Honeker, R.; Klankermayer, J.; Leitner, W. ChemCatChem
2013, 5, 1762–1764.
(20) Shahane, S.; Fischmeister, C.; Bruneau, C. Catal. Sci. Technol. 2012, 2, 1425–
1428.
(21) Nielsen, M.; Junge, H.; Kammer, A.; Beller, M. Angew. Chem., Int. Ed. 2012, 51,
5711–5713.
(22) Meijer, R. H.; Ligthart, G. B. W. L.; Meuldijk, J.; Vekemans, J. A. J. M.; Hulshof,
L. A.; Mills, A. M.; Kooijman, H.; Spek, A. L. Tetrahedron 2004, 60, 1065–1072.
(23) Van Doorslaer, C.; Schellekens, Y.; Mertens, P.; Binnemans, K.; De Vos, D. Phys.
Chem. Chem. Phys. 2010, 12, 1741–1749.
146
(24) Jagadeesh, R. V.; Junge, H.; Pohl, M. -M.; Radnik, J.; Brückner, A.; Beller, M. J.
Am. Chem. Soc. 2013, 135, 10776–10782.
(25) Powell, A. B.; Stahl, S. S. Org. Lett. 2013, 15, 5072–5075.
(26) Sarina, S.; Zhu, H.; Jaatinen, E.; Xiao, Q.; Liu, H.; Jia, J.; Chen, C.; Zhao, J. J.
Am. Chem. Soc. 2013, 135, 5793–5801.
(27) Bezen, M. C. I.; Breitkopf, C.; Lercher. J. A. ACS Catal. 2011, 1, 1384–1393.
(28) Orthman, J.; Zhu, H. Y.; Lu. G. Q. Sep. Purif. Technol. 2003, 31, 53–59.
(29) Gupta, N. K.; Nishimura, S.; Takagaki, A.; Ebitani, K. Green Chem. 2011, 13,
824–827.
(30) Noujima, A.; Mitsudome, T.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Angew.
Chem., Int. Ed. 2011, 50, 2986–2989.
(31) Debecker, D. P.; Gaigneaux, E. M.; Busca, G. Chem. Eur. J. 2009, 15, 3920–3935.
(32) Prodan, E.; Radloff, C.; Halas, N. J.; Norlander, P. Science 2003, 302, 419−422.
(33) Sarina, S.; Zhu, H. Y.; Xiao, Q.; Jaatinen, E.; Jia, J.; Huang, Y.; Zheng, Z.; Wu, H.
Angew. Chem., Int. Ed. 2014, 53, 2935–2940.
(34) Long, R., Mao, K., Gong, M., Zhou, S., Hu, J., Zhi, M., You, Y., Bai, S., Jiang, J.,
Zhang, Q., Wu, X.; Xiong, Y. Angew. Chem., Int. Ed. 2014, 53, 3205–3209.
(35) Penner, S.; Bera, P.; Pedersen, S.; Ngo, L. T.; Harris, J. J. W.; Campbell, C. T. J.
Phys. Chem. B 2006, 110, 24577–24584.
(36) Zhang, Y.; Yang, Z.; Wu, M. Phys. Chem. Chem. Phys. 2014, 16, 20532–20536.
(37) Sarina, S.; Waclawik, E. R.; Zhu, H. Y. Green Chem. 2013, 15, 1814−1833.
(38) Xiao, Q.; Jaatinen, E.; Zhu, H. Y. Chem. Asian J. 2014, 9, 3046–3064.
(39) Kale, M. J.; Avanesian, T.; Christopher, P. ACS Catal. 2014, 4, 116–128.
(40) Kowalska, E.; Abea, R.; Ohtania, B. Chem. Commun. 2009, 2, 241-243.
(41) Tanaka, A.; Sakaguchi, S.; Hashimoto, K.; Kominami, H. ACS Catal. 2013, 3, 79–
85.
(42) Xiao, Q.; Sarina, S., Bo, A.; Jia, J.; Liu, H.; Arnold, D. P.; Huang, Y.; Wu, H.;
Zhu, H. Y. ACS Catal. 2014, 4, 1725–1734.
(43) Christopher, P.; Xin, H. L.; Linic, S. Nat. Chem. 2011, 3, 467–472.
(44) Linic, S.; Christopher, P.; Ingram, D. B. Nat. Mater. 2011, 10, 911−921.
(45) Christopher, P.; Xin, H. L.; Marimuthu, A.; Linic, S. Nat. Mater. 2012, 11,
1044−1050.
(46) Olsen, T.; Gavnholt, J.; Schiøtz, J. Phys. Rev. B 2009, 79, 035403.
147
(47) Bonn, M.; Funk, S.; Hess, C.; Denzler, D. N.; Stampf, C.; Scheffler, M.; Wolf, M.;
Ertl, G. Science 1999, 285, 1042−1045.
(48) Wang, F.; Li, C.; Chen, H.; Jiang, R.; Sun, L. D.; Li, Q.; Wang, J.; Yu, J. C.; Yan,
C. H. J. Am. Chem. Soc. 2013, 135, 5588−5601.
(49) Zope, B. N.; Hibbitts, D. D.; Neurock, M., Davis, R. J. Science 2010, 330, 74–78.
148
Supporting Information
Catalytic Transformation of Aliphatic Alcohols to Corresponding
Esters in O2 under Neutral Conditions Using Visible Light Irradiation
Qi Xiao,† Zhe Liu,† Arixin Bo,† Fathima Sifani Zavahir,† Sarina Sarina,† Steven
Bottle,† James D. Riches,‡,§ and Huaiyong Zhu*,†
† School of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia
‡ Institute for Future Environments, Queensland University of Technology, Brisbane,
QLD 4001, Australia
§ School of Earth, Environmental and Biological Sciences, Queensland University of
Technology, Brisbane, QLD 4001, Australia
Table of content:
Supplementary Tables
Tables S1 to S3
Supplementary Figures
Figures S1 to S3
Supplementary Schemes
Scheme S1
149
Table S1. Base additive activity test using Au-Pd@ZrO2 for oxidative esterification of
1-octanol.
OH O
OPhotocatalyst, hv
Additive, O2, 55°C, 24h
Entry Catalyst Additive Incident
light
Conversion
(%)
Selectivity
(%)
1 Au-Pd@ZrO2 − Visible 38 47
− Dark 33 36
2 Au-Pd@ZrO2 NaOH Visible 80 97
NaOH Dark 78 96
3 Au-Pd@ZrO2 Na2CO3 Visible 51 41
Na2CO3 Dark 48 20
4 Au-Pd@ZrO2 K2CO3 Visible 78 55
K2CO3 Dark 44 12
5 Au-Pd@ZrO2 Cs2CO3 Visible 62 40
Cs2CO3 Dark 17 0
6 Au-Pd@ZrO2 K3PO4 Visible 92 87
K3PO4 Dark 33 46
Reaction conditions: photocatalyst 100 mg, reactant 0.2 mmol, additive 50 mg, solvent
α,α,α-trifluorotoluene 2 mL, 1 atm O2, environment temperature 55 °C, reaction time
24 h. The light intensity was 0.5 W/cm2. The conversions and selectivity were
calculated from the product formed and the reactant converted measured by GC.
150
Table S2. The catalytic performance for Au-Pd@HT-PO43- catalyst in the oxidative
esterification of 1-octanol under simulated sunlight irradiation and in the dark.
Conversion
(%)
Selectivity (%) Yield (%)
aldehyde ester aldehyde ester
Dark 29 82 18 24 5
Sunlight 43 39 61 17 27
Reaction conditions: Au-Pd@HT-PO43- photocatalyst 100 mg, reactant 0.2 mmol,
solvent α,α,α-trifluorotoluene 2 mL, 1 atm O2, environment temperature 40 °C, reaction
time 24 h. The simulated sunlight intensity was measured to be 0.45 W/cm2. The
conversions and selectivity were calculated from the product formed and the reactant
converted measured by GC.
151
Table S3. The effect of PO43- anion concentration for Au-Pd@HT-PO4
3- catalyst in the
photocatalytic oxidative esterification of 1-octanol.
Entry P (wt%)a Conversion (%) Selectivity (%)
1 0.2 94 76
2 0.7 50 39
3 2.8 28 35
Reaction conditions: photocatalyst 100 mg, reactant 0.2 mmol, solvent α,α,α-
trifluorotoluene 2 mL, 1 atm O2, environment temperature 55 °C, reaction time 24 h.
The light intensity was 0.5 W/cm2. The conversions and selectivity were calculated
from the product formed and the reactant converted measured by GC. a Determined by
EDS spectrum (see Figure 2 and Figure S2).
152
Figure S1. Adsorption/desorption isotherms of the samples
The isotherm exhibits an H3-type hysteresis at high relative pressure, which is typical
for aggregates of plate-like particles (D. Meloni, R. Monaci, V. Solinas, A. Auroux, E.
Dumitriu, Appl. Catal. A: Gen. 350 (2008) 86.), the enclosure of adsorption/desorption
branches at relatively high p/p0 = 0.85, for Au-Pd@HT, could be attributed to the
presence of large mesoporous structure and/or some macropores. This kind of
hysteresis is typical for the presence of open large pores, which allow easy diffusion of
the reactants through the materials. The Au-Pd@HT-PO43- sample show an enclosure of
adsorption/desorption branches at relatively low p/p0 = 0.45. The fast increase in the
amount of adsorbed nitrogen in the range of very low relative pressures is an indication
of the presence of some microporosity.
153
Figure S2. SEM image of typical Au-Pd@HT-PO43- samples with higher PO4
3- anion
concentration [(a) P: 0.7 wt%; (b) P: 2.8 wt%] and the corresponding EDS mapping
spectrum.
154
Figure S3. The catalytic activities of the oxidative esterification of 1-octanol using Au-
Pd@HT-PO43- photocatalyst at different temperatures under monochromic LED
(wavelength=400 nm, 620 nm) irradiation and in the dark: (a) the relationship between
final ester yield and the reaction temperature; reaction conversion and product
selectivity irradiated with 400 nm LED (b), 620 nm LED (c) and in the dark (d) at
different temperatures.
155
Scheme S1. Benzonic acid or pyridine poisoning experiments for the oxidative
esterification of 1-octanol using Au-Pd@HT-PO43- photocatalyst.
(a) Benzonic acid poisoning experiment
OH O
OAu-Pd@HT-PO4
3-, hv
1 atm O2, 55°C, 24hBenzoic acid (0.1 mmol)
0.2 mmolConversion:10%, Selectivity:15%
(b) Pyridine poisoning experiment
OH O
OAu-Pd@HT-PO4
3-, hv
1 atm O2, 55°C, 24hPyridine (0.1 mmol)
0.2 mmolConversion:26%, Selectivity:44%
156
2.5 Article 5
157
The Alloying of Small Amounts of Cu into Au Nanoparticles
Alters the Reaction Pathway of the Photocatalytic Reduction
of Nitroaromatics for Sole Amine Products**
Qi Xiao, Sarina Sarina, Eric R. Waclawik, Jian-Feng Jia, Jin Chang, James D. Riches,
and Huai-Yong Zhu*
Abstract: It has been found that supported
gold (Au) nanoparticles (NPs) can efficiently
catalyze reductive coupling of nitroaromatics
to yield azo-compounds under visible light
irradiation at ambient temperature. Herein,
we report that alloying small amounts of
copper (Cu) into Au NPs can alter the reaction pathway of the catalytic reaction
system: nitroaromatics are transformed directly to aromatic amines without azo- or
azoxy- byproducts. The Au/Cu composition in the alloy NPs can be finely tuned to
achieve optimal photocatalytic activity and maintain a stable surface of Cu in air. The
alloy NPs with Au/Cu=2.6/0.4 exhibits the best performance. Both of the experimental
and density functional theory (DFT) simulation results suggest that direct reduction of
nitrobenzene to aniline is much more favorable on the Au-Cu alloy surface. This work
highlights that we may achieve high selectivity of a specific product by alloying metal
NPs. Such an approach will assist us in utilizing visible light to efficiently drive various
synthesis reactions.
Aromatic amines are highly valuable chemical intermediates widely used in the
manufacture of pharmaceuticals, polymers, dyes and cosmetics.[1] Traditionally, amines
can be synthesized by transition metal-catalyzed hydrogenation of nitro compounds.[2]
Actually the reduction of nitroaromatics is one of the essential and widely studied
processes in organic synthesis. Various supported metal catalysts such as Au/CeO2,[3a]
Au/Al2O3,[3b] Pt/Al2O3,[3c] Au/Fe2O3,[3d] Au/ZrO2[3e] and Au/C[3f] have been studied for
the process. High pressures of hydrogen and/or high reaction temperatures are typically
required for reasonable reaction efficiency.[3d] The reduction of nitroaromatics under
moderate conditions has been explored as an alternative. We have reported a facile
photocatalytic reductive coupling of nitroaromatic compounds using supported Au NPs
at ambient reaction conditions.[3e] The Au NPs exhibited high catalytic activity for the
158
transformation of nitroaromatics directly to corresponding azo-compounds under
visible light irradiation, since the supported Au NPs can strongly absorb light by the
localized surface plasmon resonance (LSPR) effect. These results open up a new
avenue for synthesis of organic compounds via visible light-driven catalytic
processes.[4] Very recently it has been found that Cu NPs supported on graphene also
exhibit high photocatalytic activity for the reductive coupling of nitroaromatics to
aromatic azo-compounds under irradiation of solar spectrum.[5] However, the main
products in these reaction systems are mainly reductive coupling products, such as azo-
/azoxy- compounds; amines appeared as over-reduced products. It is difficult to
achieve high selectivity to amine products by these catalysts, and new high
performance photocatalysts need to be designed for selectively producing amines under
visible light.
One effective approach to achieve efficient Au based photocatalysts for various
reactions is alloying another metal into Au NPs.[6] Among the reported alloy bimetallic
NPs, Au-Cu alloy NPs are of particular interest, because of their high activity and the
low cost of Cu. However, O2 in air can oxidize surface Cu atoms of the Au-Cu alloy
NPs, which will result in the rapid loss of their activity during reactions exposed to
air.[7] It is known that the Cu oxidation rate depends on alloy NP composition, where
increasing amounts of Au can improve catalyst stability.[8] In bulk Au-Cu alloys Au can
protect Cu from oxidation by limiting Cu2O surface island growth.[9] Thus, it is possible
to obtain Au-Cu alloy NPs that have a low Cu content and are stable in air. The Au-Cu
alloy NPs have been used as catalysts for CO and alcohol oxidation reactions.[10] In the
present study we found that Au-Cu alloy NPs can efficiently drive the reduction of
nitroaromatics directly to aryl amines under visible light irradiation, with sole product
aryl amines. It implies that the small fraction of Cu in the alloy switches the reductive
coupling of nitroaromatics to reduction yielding aryl amines.
A series of Au-Cu alloy NPs supported on zirconium oxide (ZrO2) catalysts were
prepared in the present study. The total metal amount (Au+Cu) of the catalyst was
controlled at 3 wt%, while the Au:Cu ratio was varied to obtain several alloy catalysts
(labelled Au3-xCux@ZrO2, e.g. Au2.6Cu0.4@ZrO2 catalyst contains 2.6 wt% Au and 0.4
wt% Cu). Figure 1a shows a typical transmission electron microscopy (TEM) image of
a Au2.6Cu0.4@ZrO2 catalyst. The mean size of the alloy NPs is about 5 nm. The lattice
159
fringes of Au-Cu alloy NPs can be observed from the high resolution TEM (HR-TEM)
image in Figure 1b. The lattice fringe spacing of 0.22 nm corresponds to the interplanar
distance of AuCu(111) planes.[11] The energy dispersive X-ray spectroscopy (EDS) line
scan analysis for a Au−Cu alloy NP (inset in Figure 1a) indicates that Au and Cu are
distributed fairly uniformly in an alloy NP. This is also apparent in the EDS mapping
shown in Figure S1, the content of Au is much higher than that of Cu.
Figure 1. a) Typical TEM image of Au2.6Cu0.4@ZrO2; inset: line profile analysis
providing information about the elemental composition and Au/Cu distribution of a NP.
b) HR-TEM images of the typical Au2.6Cu0.4 alloy NP. c) XPS profile of Cu species in
the Au2.6Cu0.4 alloy NPs and d) diffuse reflectance ultraviolet-visible light (UV/Vis)
absorption spectra of Au2.6Cu0.4@ZrO2 and monometallic Au3.0@ZrO2 catalyst.
The pattern of X-ray photoelectron spectroscopy (XPS) is shown in Figure 1c. The
binding energies of Cu 2p1/2 at around 952.0 eV and Cu 2p3/2 at 932.5 eV can be
attributed to the Cuo state, therefore Cu exists as metallic state in the Au2.6Cu0.4 alloy
NPs.[5] The monometallic Au NP catalyst exhibits a distinctive light absorption band at
525 nm due to the LSPR effect (Figure 1d). In comparison, a red-shift of the band is
observed for the Au-Cu alloy NPs, which is also evidence of the alloying (Figure 1d
and Figure S2).[12]
The photocatalytic performance of Au3-x-Cux alloy NP catalysts for the reduction of
nitrobenzene to aniline under visible light irradiation, as a representative reaction, is
summarized in Figure S3. Isopropyl alcohol is both the reducing agent and solvent for
160
this reaction. The Au3-x-Cux alloy NP catalysts exhibit high activity for the reduction
and no by-products of azo- and azoxy-derivatives were obtained. In addition, the
aniline yield of the light irradiated reaction is significantly higher than that of the
reaction in the dark. Evidently, light irradiation makes a great contribution to the
reaction. Au2.6Cu0.4@ZrO2 catalysts exhibit the optimal performance under light
irradiation with an aniline yield of 100% (Table 1, entry 1). A further increase in Cu
loadings results in declined performance (Figure S3), and may cause oxidation of Cu at
the NP surface, which can suppress the catalytic activity. Monometallic Au3.0@ZrO2
catalyst can effectively catalyze the reductive coupling of nitrobenzene with main
product of azobenzene as we reported previously,[3e] but the selectivity to aniline is low
(Table 1, entry 2). These results suggest that the direct photocatalytic reduction of
nitrobenzene using Au-Cu alloy NP catalysts is distinctly different from the reductive
coupling of nitrobenzene catalyzed by Au NP catalysts although other reaction
conditions are similar. To verify this, we compared the time-conversion and selectivity
plots for the same reaction using Au2.6Cu0.4@ZrO2 and Au3.0@ZrO2 catalyst,
respectively (Figure 2).
Table 1. Photocatalytic reduction of nitroaromatics to aryl amines using Au2.6Cu0.4@ZrO2 catalyst[a]
Entry Reactant Product Yield (%)[b]
1 NO2 NH2 100 (28)
2[c] NO2 NH2 26 (5)
3 NO2Me NH2Me 95 (36)
4 NO2MeO NH2MeO 89 (40)
5 NO2Cl NH2Cl 63 (24)
6 NO2Br NH2Br 90 (20)
7 NO2I NH2I 40 (0)
8 NO2HOH2C NH2HOH2C 45 (8)
[a] The reactions were conducted in an argon atmosphere at 40°C using 2 mL of isopropyl alcohol mixed with 0.025 mmol KOH, 0.1 mmol nitroaromatics, and 50 mg
161
of Au2.6Cu0.4@ZrO2 catalyst. The irradiation intensity was 0.5 W/cm2, and the reaction time was 6 h. [b] Yield measured by GC analysis, and the aryl amines selectivity was 100%. The values in the parentheses are the yields for the control experiments in the dark. [c] Monometallic Au3.0@ZrO2 as catalyst, the other reaction conditions were kept identical.
Figure 2. Time-conversion plot for nitrobenzene reduction using a) Au2.6Cu0.4@ZrO2
and b) Au3.0@ZrO2 catalyst.
As can be seen, when Au2.6Cu0.4@ZrO2 catalyst was used (Figure 2a), the
conversion of nitrobenzene increased as the reaction proceeded, while the selectivity to
aniline directly reached 100% from the initial reaction stage and remained at 100%
until the reaction completed. In contrast, pure Au NPs exhibited no product selectivity
to aniline in early reaction stage (Figure 2b). The nitrobenzene conversion reached 100%
within 3 h, which is much faster than Au-Cu alloy NPs, however the main product is
azobenzene. Azoxybenzene is afforded in the initial stage of the reaction (0.5 h). As the
reaction proceeded, the selectivity of azoxybenzene dropped substantially while the
selectivity of azobenzene increases remarkably (0.5~2 h). The selectivity of azobenzene
reached 100% within 2 h, and then declined while aniline emerged as the further
162
reduced product. The aniline selectivity is much lower even at the end of the six hour
reaction, compared with that using Au-Cu alloy NPs as the photocatalyst.
The general applicability of Au2.6Cu0.4@ZrO2 catalyst for the photocatalytic
reduction of nitroaromatics is also illustrated by the results shown in Table 1, entries 3–
8. Visible light irradiation promoted each reaction, the yields of the light irradiated
reactions are much higher than that of the reactions in the dark for all reactions. The
products are corresponding aryl amines, no coupling product such as azo- or azoxy-
compounds were detected. For the reduction of halogen-containing nitroaromatics,
dehalogenation could sometimes be inevitable. In our case, the dehalogenation was
suppressed and quantitative conversion of these substrates can be realized (entries 5-7).
Evidently, the Au-Cu alloy NP catalyst is an efficient photocatalyst for reduction of
nitroaromatics directly to aryl amines under visible light and moderate reaction
conditions.
The performance of recycled Au2.6Cu0.4@ZrO2 catalyst was monitored for five
successive rounds. A slight performance decline was detected (Figure S4). The TEM
images of the used catalyst after the 5 rounds show no obvious change in morphology
and no NP aggregation (Figure S5). When the used catalyst was heated in a mixture of
H2 (5 vol%) and Ar atmosphere at 450°C for 0.5 h, its catalytic activity was restored to
the level of the fresh catalyst with 100% aniline yield (Figure S4). The recovery of the
catalyst was also supported by the light absorption property (Figure S6). This suggests
that a slight oxidation of Cu might occur, most likely at the surface of the NPs during
the photocatalytic reaction which led to a slight decline of the photocatalytic activity.
We studied the dependence of the catalytic activity of reduction of nitrobenzene to
aniline on light irradiance (Figure S7). When the irradiation intensity (irradiance) was
raised, the aniline yields increased. As shown in Figure S7, the higher the irradiance,
the greater the contribution of irradiation to the overall reaction rate. The light
contribution for the reaction is only 20% under an irradiance of 0.2 W/cm2, while it is
78% when the irradiance is 0.7 W/cm2. A stronger irradiance will excite more electrons
at high energy levels and create a stronger electromagnetic field around the NPs
(electromagnetic field enhancement effect), which can facilitate the reactions, as
reported for AuNPs.[4]
163
Figure 3a is the action spectrum for the apparent quantum efficiency (AQE) of the
reduction of nitrobenzene to aniline. It shows which wavelength of the irradiation is
most effective for driving the reduction. The AQE for aniline formation well matches
with the light absorption of the alloy NP catalyst. Therefore, it can be concluded that
aniline formation catalyzed by the alloy photocatalyst was driven by the light
absorption of Au-Cu alloy NPs on ZrO2 due to the LSPR effect.
Figure 3. Photocatalytic action spectra for the reduction of a) nitrobenzene and b) 4-
nitrobenzyl alcohol using Au2.6Cu0.4@ZrO2 photocatalyst. c) The calculated electron
density distribution of HOMO and LUMO orbitals, the corresponding energy levels
relative to the vacuum level for nitrobenzene and 4-nitrobenzyl alcohol. The LUMO of
164
4-nitrobenzyl alcohol molecule is higher than that of nitrobenzene molecule, thus it
requires light-excited energetic electrons to higher energy levels to activate it.
An exception was observed in the action spectrum for the reaction using 4-
nitrobenzyl alcohol as substrate (Figure 3b): the reaction AQE values do not follow the
trend of the light absorption band of the alloy NP catalyst, instead, the AQE is greater
under shorter wavelengths (such as 400 nm and 460 nm) although the most intensive
light absorption of the catalyst appears at about 560 nm. We noted that the yield of 4-
nitrobenzyl alcohol reduction is much lower than that of nitrobenzene reduction (Table
1). This means that the reaction of 4-nitrobenzyl alcohol is more difficult to be
activated with metal electrons excited by the LSPR light absorption, compared to the
nitrobenzene reduction. The metal electrons excited by light with shorter wavelength
are more effective in activating 4-nitrobenzyl alcohol molecules for the reaction.
A possible reaction mechanism is that the light excited electrons may inject into the
lowest unoccupied molecular orbital (LUMO) of the reactant molecules adsorbed on
the metal NPs inducing their reaction, if they have sufficient energy.[4b,c,13] The LUMO
of 4-nitrobenzyl alcohol is higher (-2.7 eV) on the energy scale than that of
nitrobenzene (-2.9 eV), thus only the electrons that are at the energy levels above that
of the LUMO level can inject into the LUMO (Figure 3c). The Fermi level of Au is -5.1
eV, which is about 2.4 eV and 2.2 eV below the LUMO levels of 4-nitrobenzyl alcohol
and nitrobenzene, respectively. Only the metal electrons that gain energy greater than
2.4 eV from the light absorption have the potential to induce the reaction of 4-
nitrobenzyl alcohol (the Fermi level of the alloy NPs is higher than that of Au and thus
the energy required for the injection < 2.4 eV). At identical irradiance, the absorbed
shorter wavelengths can excite more metal electrons to the energy levels high enough
for the injection than the LSPR absorption which excite most of the metal electrons to
the energy levels about 2 eV above the Fermi level. Hence, the irradiation with shorter
wavelengths can result in greater reaction rates.
The reaction pathway for the reduction of nitroaromatic compounds using
nitrobenzene as the example is proposed. As shown in Scheme 1, the reduction
catalyzed by Au-Cu alloy NPs under light irradiation may proceed along a direct route:
the nitrobenzene is reduced to the nitrosobenzene and further to the corresponding
165
phenylhydroxylamine in two very fast consecutive steps. Finally, the
phenylhydroxylamine is reduced to aniline. Whereas the reactions catalyzed by
monometallic Au NPs involves the coupling of one nitroso compound molecule with an
intermediate hydroxylamine molecule to give a azoxybenzene molecule, which is
further reduced in the consecutive steps to the corresponding azo, hydrazo, and finally
an aniline compound molecule.
Scheme 1. Possible reaction pathways for the reduction of aromatic nitro compounds to
the corresponding anilines, red: direct route, purple: coupling route.
Nitrosobenzene reduction was conducted using Au-Cu alloy NPs and Au NPs,
respectively, with other experimental conditions maintained identical to those for the
nitrobenzene reduction. As shown in Scheme S1, azoxybenzene was the main product
in both cases, and the product selectivities are very similar. We found that in the
presence of KOH, nitrosobenzene is very reactive, forming large amount of
azoxybenzene (62%) rapidly, as soon as KOH was added. Thus, concentrated
nitrosobenzene in the reaction prefers the coupling route for the both catalysts. In such
a pathway, the final product aniline should be yielded after the coupling step (azoxy→
azo→ hydrazo→ aniline). According to the literature, the hydrogenation of
nitrosobenzene is much faster than the reduction of nitrobenzene.[14,15] This is the
reason why nitrosobenzene was not detected during the reaction. The nitrosobenzene
then transforms rapidly to phenylhydroxylamine. Only when the concentration of
nitrosobenzene is high enough will the coupling to form azoxybenzene proceed.[16]
Thus in the case of using Au-Cu alloy NPs as photocatalysts for the reduction, the
nitrosobenzene formed on the NP surface should be rapidly converted into
phenylhydroxylamine, which accumulates on the surface and is then transformed to
166
aniline. It is also suggested that the reaction follows a direct nitro→ hydroxylamine→
aniline route without formation of the intermediate nitrosobenzene.[14,17]
To further understand the reason why aniline is the sole product of the reaction
catalyzed by Au-Cu alloy NPs, we performed a density functional theory (DFT)
simulation study on adsorption of nitrosobenzene and phenylhydroxylamine on Au and
Au-Cu cluster surfaces (Scheme 2 and Table S1), as they are common intermediates in
both the direct route and the coupling route. The detailed simulation method was
provided in SI.
Scheme 2. Simulated nitrosobenzene (PhNO) and phenylhydroxylamine (PhNHOH) on
Au and Au-Cu cluster (the full optimized geometry is shown in Figure S8).
The simulation results suggest that the adsorption energies for both nitrosobenzene
and phenylhydroxylamine on Au-Cu alloy NPs are greater than that on Au NPs. For
example, the adsorption energy of nitrosobenzene on the Au-Cu surface is 0.96 eV and
greater than that on the Au surface (0.73 eV). When nitrosobenzene was adsorbed on
the Au-Cu alloy surface, the O atom attaches to the surface Cu atom, and the Cu–O
distance is 2.07 Å and in a range suitable for chemical bond formation. Whereas when
nitrosobenzene molecules attach to surface Au atoms, the Au–O distance is 2.85 Å. The
Cu–O distance for phenylhydroxylamine adsorbed on Au-Cu alloy surface is 2.33 Å,
much shorter than the Au–O distance on Au surface (3.06 Å), but too large to form a
chemical bond. Thus, once the intermediates of nitrosobenzene and
167
phenylhydroxylamine formed on the Au-Cu alloy surface, they are strongly adsorbed
by the surface Cu atoms. Such adsorption impedes the coupling of the intermediate
molecules, which requires migration of the molecules, but favors reducing the
intermediate to aniline directly. For the Au NP catalyst, the adsorbed intermediate
molecules can migrate to form azo bonds, because of the homogeneous Au surface and
weaker adsorption energy. Thus the coupling route is preferred.
The stronger adsorption of the final product aniline on Au-Cu alloy surface could
also facilitate its formation (Figure S9 and Table S1). This was further confirmed by
the experimental results that the Au-Cu alloy NPs can catalyze the reduction of
azobenzene to the sole product aniline with 70% yield within 16h under visible light
irradiation, whereas pure Au NPs are sluggish to obtain aniline (Scheme S2). This
suggests that it is much more favorable to form aniline on Au-Cu alloy surface and pure
Au NPs can suppress the hydrogenation to aniline to some degree.
It is also worth noting that the photocatalytic coupling of nitrobenzene on the pure
Au NP surface does not consume the reducing agent except for the initial stage, while
the direct reduction of nitrobenzene to aniline does (Figure S10). Isopropyl alcohol was
the reducing agent in the photocatalytic process providing hydrogen and being oxidized
to acetone. Indeed, the content of acetone in the reaction system catalyzed by the Au-
Cu alloy NPs increased gradually as the reaction proceeded (Figure S10), in proportion
with the nitrobenzene conversion (or aniline yield) shown in Figure 2a. The isopropyl
alcohol was firstly oxidized to acetone yielding H–Au NP[3e] or H–Cu NP[5] species;
then the H–Au NP or H–Cu NP species are capable of reacting with the oxygen atoms
of N–O bonds in the nitrobenzene to yield the intermediates nitrosobenzene or
phenylhydroxylamine on the NP surface. The light excited metal electrons provide the
activation energy required for the cleavage of the N–O bond, and the H–Au NP or H–
Cu NP species also provide H required for reduction of the intermediates to the final
product aniline.
In summary, a green photocatalytic process for the reduction of nitroaromatics to
aryl amines, driven by light irradiation, without heating and pressurized reagents, can
be achieved using Au-Cu alloy NP catalyst prepared simply by alloying small amounts
of Cu into Au NPs. The small fraction of Cu is able to switch the reaction pathway to
achieve 100% selectivity to the corresponding amines, which is distinctly different
168
from the coupling route observed in the system using pure Au NPs as photocatalyst. A
similar effect was observed in a preliminary study when a small fraction of Cu was
alloyed with silver (Ag) NPs. Higher aniline yield was achieved for nitrobenzene
reduction catalyzed by Ag2.6Cu0.4@ZrO2 catalyst than that by Ag3.0@ZrO2 catalyst
although the aniline yield is much lower compared with that in the present study
(Further investigation of the Ag-Cu alloy NP catalysts is in progress). This highlights
that the reaction pathway change caused by alloying may appear in other systems and
may represent a potential approach to optimize the efficiency of photocatalytic
reduction. The catalytic system described here may present a new strategy toward the
development of new heterogeneous catalysts, and also contribute to understand the
development of photocatalytic systems for more complex organic reactions.
Acknowledgements
This work was supported by the Australian Research Council (ARC DP110104990).
We thank Arixin Bo for assistance with SEM-EDS characterization.
Keywords: Au-Cu alloy • photocatalysis • reduction • reaction mechanisms • visible
light
[1] P. F. Vogt, J. J. Gerulis, Ullmann's Encyclopedia of Industrial Chemistry in
Aromatic Amines, Wiley-VCH Verlag GmbH & Co., Weinheim, 2005.
[2] H. U. Blaser, H. Steiner, M. Studer, ChemCatChem 2009, 1, 210.
[3] a) M. Makosch, J. Sa, C. Kartusch, G. Richner, J. A. van Bokhoven, K.
Hungerbuhler, ChemCatChem 2012, 4, 59–63; b) X. Huang, X. P. Liao, B. Shi,
Green Chem. 2011, 13, 2801–2805; c) P. Serna, M. Boronat, A. Corma, Top. Catal.
2011, 54, 439–446; d) A. Corma, P. Serna, Science 2006, 313, 332–334; e) H. Y.
Zhu, X. B. Ke, X. Z. Yang, S. Sarina, H. W. Liu, Angew. Chem. 2010, 122, 9851–
9855; Angew. Chem. Int. Ed. 2010, 49, 9657–9661; f) Y. Choi, H. S. Bae, E. Seo, S.
Jang, K. H. Park, B. S. Kim, J. Mater. Chem. 2011, 21, 15431–15436.
[4] a) S. Sarina, E. R. Waclawik, H. Y. Zhu, Green Chem. 2013, 15, 1814−1833; b) Q.
Xiao, E. Jaatinen, H. Y. Zhu, Chem. Asian J. 2014, DOI: 10.1002/asia.201402310;
c) M. J. Kale, T. Avanesian, P. Christopher, ACS Catal. 2014, 4, 116–128; d) S.
169
Linic, P. Christopher, D. B. Ingram, Nat. Mater. 2011, 10, 911–921; e) C. Wang, D.
Astruc, Chem. Soc. Rev. 2014, 43, 7188–7216.
[5] X. N. Guo, C. H. Hao, G. Q. Jin, H. Y. Zhu, X. Y. Guo, Angew. Chem. 2014, 126,
2004–2008; Angew. Chem. Int. Ed. 2014, 53, 1973–1977.
[6] a) Y. Shen, S. Zhang, H. Li, Y. Ren, H. Liu, Chem. Eur. J. 2010, 16, 7368–7371;
b) K. Kaizuka, H. Miyamura, S. Kobayashi, J. Am. Chem. Soc. 2010, 132, 15096–
15098; c) S. Sarina, H. Y. Zhu, E. Jaatinen, Q. Xiao, H. W. Liu, J. F. Jia, C. Chen,
J. Zhao, J. Am. Chem. Soc. 2013, 135, 5793–5801; d) N. K. Chaki, H. Tsunoyama,
Y. Negishi, H. Sakurai, T. Tsukuda, J. Phys. Chem. C 2007, 111, 4885–4888.
[7] X. Liu, A. Wang, L. Li, T. Zhang, C. Y. Mou, J. F. Lee, J. Catal. 2011, 278, 288–
296.
[8] Z. Xu, E. Lai, Y. Shao-Horn, K. Hamad-Schifferli, Chem. Commun. 2012, 48,
5626–5628.
[9] L. Wang, J. C. Yang, J. Mater. Res. 2005, 20, 1902.
[10]a) Y. Sugano, Y. Shiraishi, D. Tsukamoto, S. Ichikawa, S. Tanaka, T. Hirai, Angew.
Chem. 2013, 125, 5403–5407; Angew. Chem. Int. Ed. 2013, 52, 5295–5299; b) X.
Li, S. S. S. Fang, J. Teo, Y. L. Foo, A. Borgna, M. Lin, Z. Zhong, ACS Catal. 2012,
2, 360–369; c) J. C. Bauer, D. Mullins, M. Li, Z. Wu, E. A. Payzant, S. H.
Overbury, S. Dai, Phys. Chem. Chem. Phys. 2011, 13, 2571–2581; d) W. Li, A.
Wang, X. Liu, T. Zhang, Appl. Catal. A 2012, 146, 433–434.
[11] S. Khanal, N. Bhattarai, D. McMaster, D. Bahena, J. J. Velazquez-Salazar, M.
Jose-Yacaman, Phys. Chem. Chem. Phys. 2014, 16, 16278–16283.
[12] S. Pramanik, M. K. Mishra, G. De, CrystEngComm 2014, 16, 56–63.
[13] a) L. B. Zhao, Y. F. Huang, X. M. Liu, J. R. Anema, D. Y. Wu, B. Ren, Z. Q. Tian,
Phys. Chem. Chem. Phys. 2012, 14, 12919–12929; b) L. B. Zhao, M. Zhang, Y. F.
Huang, C. T. Williams, D. Y. Wu, B. Ren, Z. Q. Tian, J. Phys. Chem. Lett. 2014, 5,
1259–1266.
[14] A. Corma, P. Concepción, P. Serna, Angew. Chem. 2007, 119, 7404–7407; Angew.
Chem. Int. Ed. 2007, 46, 7266–7269.
[15] X. Liu, H. Q. Li, S. Ye, Y. M. Liu, H. Y. He, Y. Cao, Angew. Chem. 2014, 126,
7754–7758; Angew. Chem. Int. Ed. 2014, 53, 7624–7628.
[16]M. Makosch, J. Sa, C. Kartusch, G. Richner, J. A. van Bokhoven, K. Hungerbühler,
ChemCatChem 2012, 4, 59–63.
[17] E. A. Gelder, S. D. Jackson, C. M. Lok, Chem. Commun. 2005, 522–524.
170
Supporting Information for
The Alloying of Small Amounts of Cu into Au Nanoparticles Alters the
Reaction Pathway of the Photocatalytic Reduction of Nitroaromatics
for Sole Amine Products
Qi Xiao,[a] Sarina Sarina,[a] Eric R. Waclawik,[a] Jian-Feng Jia,[b] Jin Chang,[a] James
D. Riches[c] and Huai-Yong Zhu*[a]
*correspondence to: [email protected] [a]School of Chemistry, Physics and Mechanical Engineering, Science and Engineering
Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia [b]School of Chemical and Material Science, Shanxi Normal University, Linfen 041004,
China [c]Institute for Future Environments, School of Earth, Environmental and Biological
Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia
Table of content:
Materials and Methods
S1 Chemicals
S2 Preparation of Catalysts
S3 Characterization of Catalysts
S4 Photocatalytic Reactions
S5 Theoretical Calculation
Supplementary Figures
Figures S1 to S10
Supplementary Schemes
Schemes S1 and S2
Supplementary Tables
Table S1 and S2
171
Materials and Methods:
S1 Chemicals
Zirconium (IV) oxide (ZrO2, <100 nm particle size, TEM), gold(III) chloride hydrate
(HAuCl4·xH2O, 99.999% trace metals basis), Sodium borohydride (NaBH4, ≥98.0 %),
copper(II) nitrate hydrate (Cu(NO3)2·xH2O, ≥99.9 % trace metals basis). All the
chemicals used in the experiments were purchased from Sigma-Aldrich (unless
otherwise noted) and used as received without further purification. The water used in
all experiments was prepared by passing through an ultra-purification system.
S2 Preparation of Catalysts
Au2.6Cu0.4@ZrO2 catalyst: ZrO2 powder (1.0 g) was dispersed into a mixture of
HAuCl4 (13.2 mL, 0.01 M) and Cu(NO3)2 (6.25 mL, 0.01 M) aqueous solution under
magnetic stirring at room temperature. A lysine (3 mL, 0.1 M) aqueous solution was
then added into the mixture with vigorous stirring for 30 min, the pH value was 8−9.
To this suspension, a freshly prepared NaBH4 (2 mL, 0.35 M) aqueous solution was
added drop wise. The mixture was aged for 24 h and then the solid was separated by
centrifugation, washed with water (three times) and ethanol (once), and dried at 60°C in
a vacuum oven for 24 h. The dried powder was subjected to thermal treatment in a
mixture of H2 (5 vol%) and Ar at 450°C for 0.5 h. The obtained powder was used
directly as Au2.6Cu0.4@ZrO2 catalyst. All the other catalysts were prepared via the same
methods with different quantities of HAuCl4 and Cu(NO3)2 aqueous solutions.
S3 Characterization of Catalysts
The size, morphology and composition of the catalyst samples was characterized with a
JEOL2100 transmission electron microscope (TEM), equipped with a Gatan Orius
SC1000 CCD camera and an Oxford energy dispersive X-ray spectrometer (EDS). The
Au and Cu contents of the prepared catalysts were determined by energy dispersion
X−ray spectrum (EDS) technology using the attachment to a FEI Quanta 200
environmental scanning electron microscope (SEM). Diffuse reflectance UV−visible
(DR−UV−vis) spectra of the sample powders were examined with a Varian Cary 5000
spectrometer with BaSO4 as a reference.
S4 Photocatalytic Reactions
172
A 20 mL Pyrex glass tube was used as the reaction container, and after the reactants
and catalyst had been added, the tube was sealed with a rubber septum cap. The
reaction mixture was stirred magnetically and irradiated using a halogen lamp (from
Nelson, wavelength in the range 400–750 nm) as the visible light source and the light
intensity was measured to be 0.5 W/cm2. The temperature of the reaction system was
carefully controlled with an air conditioner attached to the reaction chamber. The
control reaction system in the dark was maintained at the same temperature to ensure
that the comparison is meaningful. All the reactions in the dark were conducted using a
water bath placed above a magnetic stirrer to control the reaction temperature; the
reaction tube was wrapped with aluminum foil to avoid exposure of the reaction
mixture to light. At given irradiation time intervals, 0.5 mL aliquots were collected, and
then filtered through a Millipore filter (pore size 0.45 μm) to remove the catalyst
particulates. The liquid−phase products were analyzed by an Agilent 6890 gas
chromatography (GC) with HP−5 column to measure the change in the concentrations
of reactants and products. An Agilent HP5973 mass spectrometer was used to identify
the product. The acetone concentration was tested by Agilent 6890 GC with DB−Wax
column.
Reaction conditions: nitrobenzene in isopropyl alcohol (IPA) solution (0.05 M) 2 mL
(containing nitrobenzene 0.1 mmol), KOH in IPA solution (0.1 M) 0.25 mL (KOH
0.025 mmol), and catalyst 0.05 g were added to the reaction tube, the temperature was
40°C, under a 1 atm argon atmosphere, with a reaction time of 6 h.
Light emitting diode (LED) lamps (Tongyifang, Shenzhen, China) with wavelengths
400±5 nm (TYF-H030 G45), 470±5 nm (TYF-H030 G35), 530±5 nm (TYF-H030
G35), 590±5 nm (TYF-H030 G38) and 620±5 nm (TYF-H030 G32) were used as the
light source to investigate the catalytic performance under different wavelength (Action
spectrum experiments). The AQE was calculated as: AQE = [(Ylight–Ydark)/(the number
of incident photons)]×100%, where Ylight and Ydark are the number of products formed
under light irradiation and dark conditions, respectively.
S5 Theoretical Calculation
The optimized geometries and electronic properties of nitrobenzene and 4-nitrobenzyl
alcohol were calculated using the density functional theory (DFT) with B3LYP
functional and 6-31G basis set, as implemented in the Gaussian 09 program package.
173
Supplementary Figures
Figure S1. (a) SEM image of a typical Au-Cu alloy catalyst; (b) EDS spectrum; (c)
The corresponding EDS mapping of Zr, O, Au and Cu elements.
174
Figure S2. UV/Vis extinction spectra of the Au-Cu alloy catalysts.
175
Figure S3. Photocatalytic reduction of nitrobenzene to aniline using Au-Cu alloy NPs
@ZrO2 catalysts with varied composition.
176
Figure S4. The photocatalytic stability of Au2.6Cu0.4@ZrO2 catalyst in five cycles and
after H2 heat treatment for nitrobenzene reduction to aniline.
177
Figure S5. TEM image of the Au2.6Cu0.4@ZrO2 catalyst after recycled.
178
Figure S6. UV/Vis extinction spectra of the Au2.6Cu0.4@ZrO2 catalyst after recycled
and H2 heat treatment.
179
Figure S7. The dependence of the catalytic activity of Au2.6Cu0.4@ZrO2 photocatalyst
for the reduction of nitrobenzene to aniline on the intensity of the light irradiation. The
percentages inside of the figure show the contribution of the light irradiation effect.
180
Figure S8. The optimized geometry of the Au103Cu cluster.
181
Figure S9. The simulated aniline (PhNH2) on Au104 and CuAu103. When aniline
molecule is adsorbed on Au-Cu alloy surface, the alone electron pair on the N atom is
attracted by the Cu atom and the length of N–Cu bond is 2.23 Å. However, when
aniline molecule is close to the surface of Au surface, the alone electron pair on the N
atom was repelled obviously as shown in the figure. The adsorption energy of aniline
on Au-Cu alloy surface is 1.22 eV, while the adsorption energy on Au surface is 0.99
eV (Table S1).
182
Figure S10. The time-conversion plot for acetone formation during the reduction of
nitrobenzene under visible light irradiation using Au2.6Cu0.4@ZrO2 and Au3.0@ZrO2
catalyst respectively.
183
Supplementary Schemes
NOKOH, AuCu alloy NPs
NH2
NN
NN
O
Aniline 5%
Azobenzene 3%
Azoxybenzene 85%
hv, 40°C, 3h
NH2
NN
NN
O
Aniline 4%
Azobenzene 4%
Azoxybenzene 83%
NOKOH, Au NPs
a)
b)
hv, 40°C, 3h
Scheme S1. Nitrosobenzene as a reactant for the reduction using Au2.6Cu0.4@ZrO2 (a)
and Au3.0@ZrO2 (b) catalyst under light irradiation (the results are shown as the
product yield).
184
NN
KOH, AuCu alloy NPs
hv, 40°C, 16hNH2 Aniline Selectivity 100%
NN
KOH, AuNPs NN
H
HHydrazobenzene Selectivity 90%
a)
b)
NH2 Aniline Selectivity 10%
Conversion 55%
Conversion 70%
hv, 40°C, 16h
Scheme S2. Azobenzene as a reactant for the reduction using Au2.6Cu0.4@ZrO2 (a) and
Au3.0@ZrO2 (b) catalyst under light irradiation.
185
Supplementary Tables
Table S1. The calculated adsorption energy
Adsorption energy (eV)
On Au surface On Au-Cu surface
nitrosobenzene 0.73 0.96
phenylhydroxylamine 0.96 1.18
aniline 0.99 1.22
186
Table S2. DFT simulation parameter
Parameter Value
Method Mixed Gaussian and Plane-wave (GPW)
Functional PBE
Dispersive interaction correction DFT-D3
Pseudo-potential Goedecker-Teter-Hutter(GTH)
Gaussian-type basis set sets Molecular optimized double zetha-valence
Shorter Range basis sets with a
polarization function (DZVP-MOLOPT-
SR)
Plane-wave cut-off 400Ry
Electron density convergence
criteria
1.0 × 10-7
Convergence criteria for geometry
optimization
maximum force 4.5× 10-5 Hartree/Bohr
RMS force 3.0 ×10-5 Hartree/Bohr
maximum coordinate change 3.0 × 10-4
Bohr
RMS coordinate change 1.5 × 10-4 Bohr
187
CHAPTER 3
SUPPORTED GOLD NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS BY VISIBLE LIGHT
3.1 Introductory Remarks
This chapter includes two articles:
Our group reported in 2010 that the supported Au nanoparticles exhibit superior
performance in the photocatalytic reductive coupling of nitro aromatic compounds to
produce the corresponding azo compounds under both visible and UV irradiation
(Angew. Chem. Int. Ed., 2010, 49, 9657). This work highlighted that this reductive
coupling process can be driven by light under ambient conditions. However, in this
reaction process, the selectivity to the intermediate azoxybenzene is not controllable
(products are azobenzenes), we envisioned the design of a high-performance catalyst
for the selective reduction of nitro compounds to azoxy compounds under much milder
and greener reaction conditions. In Article 6 (a submitted manuscript) we found that
visible light can drive selective reduction of aromatic nitro compounds to azoxy
compounds using the action of hydrotalcite (HT) support gold nanoparticles under mild
conditions. The photocatalytic activity strongly depends on the exchanged ions in the
support, as well as light wavelength and intensity. Thus, we can efficiently control the
product selectivity of the reduction of aromatic nitro compounds to various reduced
final products by finely tuning the support (Article 6) and alloying Au with Cu (Article
5 in Chapter 2).
188
Article 7 (published on Journal of Physical Chemistry C, 2014, 118, 19062-
19069) reported that visible light can drive esterification from aldehydes and alcohols
using supported gold nanoparticles (Au/Al2O3) as photocatalysts at ambient
temperatures. The Au nanoparticles absorb visible light due to the localized surface
plasmon resonance (LSPR) effect, and the conduction electrons of the Au nanoparticles
gain the energy of the incindent light. The energetic electrons, which concentrate at the
nanoparticles’ surface, facilitate the activation of a range of aldehyde and alcohol
substrates. The photocatalytic efficiencies strongly depend on the Au loading, particle
sizes of the AuNPs, irradiance and wavelength of the light irradiation.
189
3.2 Article 6
190
Visible light driven selective reduction of aromatic nitro to
azoxy compounds using supported gold nanoparticles: a
promotional effect of phosphate and transition metal ions in
hydrotalcite support
Qi Xiao, Arixin Bo, Zhanfeng Zheng, Wayde Martens and Huaiyong Zhu*
Visible light can drive selective reduction of aromatic nitro compounds to
azoxy compounds using the action of hydrotalcite (HT) support gold
nanoparticles (AuNPs) under mild conditions. The photocatalytic activity
strongly depends on the exchanged ions in the support, as well as light
wavelength and intensity.
Aromatic azoxy compounds have been widely utilised as dyes, analytical reagents,
reducing agents, stabilizers, and polymerization inhibitors.1 Generally, Azoxy
compounds can be prepared from their corresponding amines, hydroxylamines and azo,
nitro, and nitroso compounds, and the synthesis of these compounds is often conducted
at high temperatures using strong base and transition-metal agents.2 Furthermore, the
harsh reaction conditions may result in over-reduced products such as azobenzene or
aniline.3 From a green chemistry point of view, it is an attractive and challenging goal
to develop highly active, easily separable and reusable catalyst systems that can
perform such desirable syntheses of aromatic azoxy compounds under more controlled,
simplified, and greener conditions.
Recently, we discovered that the supported AuNPs exhibit superior performance in
the photocatalytic reductive coupling of nitro aromatic compounds to produce the
corresponding azo compounds under both visible and UV irradiation.4 During this
process, reductive coupling of nitrobenzene to azoxybenzene firstly took place, and
then azobenzene was afforded by loss of the oxygen atom of the azoxybenzene N-O
bond. This work highlighted that this reductive coupling process can be driven by light
under ambient conditions. AuNPs absorb visible light mainly due to the localised
surface plasmon resonance (LSPR) effect.5 However, in this reaction process, the
selectivity to the intermediate azoxybenzene is not controllable (products are
191
azobenzenes); we envisioned the design of a high-performance catalyst for the selective
reduction of nitro compounds to azoxy compounds under much milder and greener
reaction conditions. Herein, we find an effective pathway for the selective coupling of
nitrobenzenes to azoxybenzenes with supported AuNPs under visible light irradiation.
Namely, an inorganic material of HT supporting AuNPs (Au/HT) for the highly
efficient photocatalytic reduction of nitrobenzenes into azoxybenzenes.
HT (Mg6Al2(OH)16CO3•nH2O) type layered double hydroxides, which have been
demonstrated as a promising support material for AuNP in many catalytic reactions,6
are known to possess surface basic properties that can be fine-tuned by their
compositions.7 Recently, it was reported that doping transition-metal cations into HT
can afford a strong synergistic effect between AuNPs and HT support, which
apparently enhance the catalytic activity.8 However, the nature of the gold-support
interactions and the effects of HT composition on the catalytic performance are still
unknown. Recently, a BiVO4 lattice doped with phosphate to enhance photocatalytic
activity was reported, the PO4 oxoanion dopant greatly improves the charge-transfer
characteristics.9 In this communication, both addition of phosphate anions and
transition metal cations to HTs are explored to develop excellent supports for AuNPs.
Indeed, we found that these modified HT supported AuNPs can be used as efficient
visible light photocatalysts for selective reduction of aromatic nitro to azoxy
compounds.
The HT precursor was prepared by the homogeneous precipitation method.10 A series
of phosphate and transition metal modified PO43--M-HT (M = Ga3+, Fe3+, Cu2+, Zn2+)
supports was prepared by using the calcination–reconstruction process (known as
‘‘memory effect’’ of HT) (see the ESI for the details). Various HT-supported gold
catalysts were prepared by a modified deposition–precipitation approach with reduction
by NaBH4. The well-defined layered structure characteristic of HT is confirmed for all
samples by x-ray diffraction (XRD). It is clear that all diffraction peaks could be
indexed to the HT structure and the structure remained unchanged after the ion
exchanged and Au loaded. (Fig.S1 and S2, ESI) No reflections assignable to Au were
present in the XRD patterns, possibly because the low Au content was below the
detection limit and/or due to poor crystallinity of the AuNPs on the surface of HT.
Transmission electron micrographs (TEM) show evidence of high dispersion of gold
nanoparticles with mean sizes between 5-7 nm (Fig. S3, ESI), indicating that the
influence of the support on the AuNP size is relatively small. The UV/Vis absorption
192
spectra of the as-prepared Au/PO43--M-HT samples are shown in Fig. S4 (ESI). The
absorption peak at 520 nm in the spectrum of the samples is due to the LSPR
absorption of the AuNPs.5 The presence of the support and its interaction with the
AuNPs can strongly shift and broaden the absorption peaks.
Table 1 The catalytic properties of various Au catalysts
Entry Catalysts Visible light No light Conv. (%) Selec. (%) Conv. (%) Selec. (%)
1 Au/HT 42 (13) 97 29 98 2 Au/PO4
3--Zn2+-HT 54 (41) 98 13 86 3 Au/PO4
3--Fe3+-HT 52 (40) 97 12 77 4 Au/PO4
3--Ga3+-HT 56 (54) >99 2 >99 5 Au/PO4
3--Cu2+-HT 0 0 0 0 6 Au/Zn2+-HT 16 (13) >99 3 >99 7 Au/Fe3+-HT 13 (13) 74 0 0 8 Au/Ga3+-HT 13 (5) >99 8 >99 9 Au/Cu2+-HT 0 0 0 0
10 Au/PO43--HT 4 (2) >99 2 99
11 Au/ZrO2 42 (24) 54 18 84 12 Au/CeO2 30 (4) 94 26 98
Reduction reaction was conducted in an argon atmosphere at 40 °C using 15 mL of
isopropyl alcohol mixed with 1.5 mL 0.1M KOH/isopropyl alcohol, 1.5 mmol
nitrobenzene, and 50 mg catalyst. Reaction time: 5 h. The values in the parentheses are
the conversion under visible light irradiation subtracting that without light.
We applied the obtained Au/PO43--M-HT catalysts for the selective reduction of
nitrobenzenes. The reaction was performed in isopropyl alcohol under visible light
irradiation with an argon atmosphere at 40°C for 5 h. For comparison control thermal
reactions were conducted without light and the other experimental conditions kept
identical. The parent M-HT, PO43--HT and HT supports did not convert nitrobenzene
under identical conditions. Interestingly, all the phosphate and transition metal ions
containing Au/PO43--M-HT catalysts exhibited much higher activity than the
phosphate-free or transition metal-free catalysts (Table 1). Though Au/HT exhibited
good conversion (42%) and selectivity under light irradiation, the thermal reaction
without light can also drive the reduction (29%). We use the conversions under visible
193
light irradiation subtracting that without light to determine the contribution of light
effect (the values in the parentheses in Table 1), all the catalysts with both phosphate
and transition metal ions exhibited obvious increased activity over those without or just
with one component of the additive ions. This confirms that the phosphate and
transition metal ions play a synergistic effect in the Au/PO43--M-HT catalysts,
especially for the reaction under light. However, the catalysts with Cu2+ show no
activity at all (entry 5 and 9), for comparison, supported semiconductor catalysts such
as Au/ZrO2 and Au/CeO2 also give reasonable conversions, but the selectivity or the
contribution of light effect is poor (entry 11 and 12).
Considering that Au/PO43--Ga3+-HT photocatalysts exhibit the best performance
according to the reaction conversion under light irradiation, we will use it as the model
catalyst to further evaluate the reductions by extending the substrate scope. As can be
seen from Table S1 (ESI), Au/PO43--Ga3+-HT can successfully drive several examples
of nitroaromatic compounds for reductive coupling yielding corresponding azoxy
compounds with good selectivity under visible irradiation.
In order to better understand the effect of light irradiation, we applied optical filter
glass with different cut off wavelength to clarify the influence of wavelength range on
the photocatalytic activity of Au/PO43--Ga3+-HT for the reduction reaction. As shown in
Fig. 1a, when the light with wavelength below 490 nm was removed (the working
wavelength range will be 490-800 nm), the conversion of the reaction decreased to
35%; when cut off the wavelength below 550 nm and 600 nm, the conversion
decreased to 27% and 17% respectively; considering that the thermal reaction at this
temperature is 2%, we found that the main contribution of light irradiation to the
photocatalytic activity comes from light in the range of 490-600 nm, accounts for the
54% of the total conversion rate, while light in the wavelength ranges of 400-490 nm
and 600-800 nm contribute 7% and 40%, respectively (pie chart in Fig. 1a). The energy
absorbed by the AuNPs from light in the wavelength range between 490-600 nm was
estimated from the overlap of the absorption spectrum of the AuNP with the spectral
irradiance of incandescent light used (Fig. 1b), to be 40.8% of the total light energy
absorbed by the NPs. Given that the LSPR peak of AuNPs is in this wavelength range,
these results suggest that AuNPs functions as an antenna for visible light absorption.
194
Fig. 1 (a) The dependence of the catalytic activity of the Au/PO43--Ga-HT catalyst for
the reaction on the wavelength of the light irradiation. Both light driven reaction and
the thermal reaction in the dark were conducted at 40°C. The inset pie chart is the
contribution to the conversion efficiency for the specific wavelength ranges used. (b)
The energy absorbed by the AuNPs from irradiation was estimated from the overlap
area of the absorption spectrum of the AuNP (curve a) with the spectral irradiance of
incandescent light used (curve b).
Fig. 2 (a) Photocatalytic rate as a function of light intensity for various temperatures. (b)
Light intensity dependent activity of Au/PO43--Ga-HT catalyst for the reaction at 40°C,
the percentage numbers in red show the contribution of light irradiation.
The impact of the light intensity on the catalyst performance was also investigated
while keeping other experimental conditions unchanged. Fig. 2a shows the rate of
nitrobenzene reduction over the Au/PO43--Ga3+-HT catalyst as a function of light
intensity at different temperatures (30, 40, 50 and 60°C, respectively). When the light
intensity increased (the reaction temperature of the reaction mixture was controlled at
45°C, the only parameter changed is light intensity), the reaction conversion increased
linearly up to a light intensity of 0.7 W/cm2. Further increase in light intensity results in
much greater rate increases, and the relation between light intensity and reaction rate
becomes nonlinear. This is a feature of the chemical processes driven by the light
195
excited electrons of metals.11 It is also possible that when the light intensity is very high,
multi-photon absorption occurs, increasing the number of excited metal electrons with
sufficient energy to drive the reactions. The light induced enhancement on the
conversion was calculated by subtracting the observed conversion of a reaction
performed in the dark from the conversion observed under light irradiation controlled at
the same temperature (40°C). This allows the photo-induced and thermal contributions
to the conversions to be determined and expressed as a percentage for each process, as
shown in Fig.2b. It shows clearly that higher light intensities, results in a larger light
enhanced contribution to the total conversion rate.
We have also evaluated the reusability of the Au/PO43--Ga3+-HT catalyst, which was
successfully recycled three times in the reaction without apparent loss of activity and
selectivity. Subsequently, the recycled catalyst was tested by XRD and TEM (Fig. S7
and S8, ESI). XRD results confirm retention of the original HT structure and TEM
shows that the average size and distribution of the Au nanoparticles was unchanged.
These results demonstrate that Au/PO43--M-HT catalyst is stable and practical visible
light photocatalyst for reduction of nitrobenzene to azoxybenzene.
Scheme 1. Proposed mechanism for the photocatalytic reduction.
The proposed mechanism for the visible light driven reduction of nitrobenzenes to
azoxybenzenes is shown in Scheme 1. The hydrogen atom abstracted form isopropyl
alcohol is first adsorbed on the AuNP surface, forming relatively stable H-AuNP
species.5 The linear dependence of the photo-induced reaction rate on the light intensity,
observed in Fig. 2, usually suggests an electron-driven chemical process on metal
surface.11 Also, light-excited transient electrons can transfer from the NP surface to a
chemically adsorbed molecule is well-known.12 This means that the excited conduction
electrons can interact strongly with the electrophilic nitro groups of nitrobenzene
molecules, and assist the cleavage of the N-O bonds by the H-AuNP species on the
196
AuNPs. Increasing the irradiation intensity and temperature produces more excited
electrons, which can results in higher conversion rates. Thus, nitrobenzene is reduced to
nitrosobenzene, which then in turn is quickly converted to N-hydroxybenznamine. N-
hydroxybenznamine can readily couple with nitrosobenzne to form dihydroxy
intermediate, which can dehydrate to give azoxybenzene. The reaction didn't proceed
further to form azobenzene or aniline, this is due to the reaction barrier of the
hydrogenation of azoxybenzene to azobenzene is much higher than that of former
steps.13 A significant merit of our catalytic system used here is that the high selectivity
to azoxybenzene can be maintained constantly under mild reaction conditions. The
PO43- oxoanion may increase the charge redistribution and internal electric field inside
the support,9 which can facilitate the light-excited electron transfer. In addition, the
introduction of phosphate and transition metal ions to hydrotalcite support can strongly
affect the properties of the basic sites on the catalysts, thus producing a synergistic
effect on the photocatalytic activity and product selectivity.
In conclusion, an environmental sustainable and industrial friendly visible light
driven process for azoxy compounds production has been developed using hydrotalcite-
supported gold nanoparticles. The exchange of phosphate and transition metal ions into
the support increases product selectivity and promotes the light irradiation effect. We
envision that this type of photocatalysts will be an important catalyst for the selective
synthesis of azoxy compounds under mild conditions, and that these unique
nanomaterials might lead to the development of novel plasmonic photocatalyst systems
for other reactions as well.
The authors gratefully acknowledge the financial support from the Australian
Research Council (ARC DP110104990).
Notes and references a School of Chemistry, Physics and Mechanical Engineering, Queensland
University of Technology, Brisbane, QLD 4001, Australia. E-mail:
[email protected]; Fax: +61 7 3138 1804; Tel: +61 7 3138 1581.
† Electronic Supplementary Information (ESI) available: Experimental details and
results, XRD, XPS, TEM and UV data.
197
1 (a) J. M. Huang, J. F. Kuo and C. Y. Chen, J. Appl. Polym. Sci., 1995, 55, 1217;
(b) A. Rezaeifard, M. Jafarpour, M. A. Naseri and R. Shariati, Dyes Pigments,
2008, 76, 840.
2 F. A. Khan and Ch. Sudheer. Tetrahedron Lett., 2009, 50, 3394.
3 (a) A. Albini, E. Fasani, M. Moroni and S. Pietra, J. Org. Chem., 1986, 51, 88;
(b) M. Cifelli, G. Cinacchi and L. De Gaetani, J. Chem. Phys., 2006, 125,
164912.
4 H. Y. Zhu, X. B. Ke, X. Z. Yang, S. Sarina and H. W. Liu, Angew. Chem. Int.
Ed., 2010, 49, 9657.
5 D. K. Roper, W. Ahn and M. Hoepfner, J. Phys. Chem. C, 2007, 111, 3636.
6 (a) N. K. Gupta, S. Nishimura, A. Takagaki and K. Ebitani, Green Chem., 2011,
13, 824; (b) A. Noujima, T. Mitsudome, T. Mizugaki, K. Jitsukawa and K.
Kaneda, Angew. Chem. Int. Ed., 2011, 50, 2986.
7 D. P. Debecker, E. M. Gaigneaux and Guido Busca, Chem. Eur. J., 2009, 15,
3920.
8 P. Liu, Y. Guan, R. van Santen, C. Li and E. J. M. Hensen, Chem. Commun.,
2011, 47, 11540.
9 W. J. Jo, J. W. Jang, K. J. Kong, H. J. Kang, J. Y. Kim, H. Jun, K. P. S. Parmar
and J. S. Lee, Angew. Chem. Int. Ed., 2012, 51, 3147.
10 (a) M. C. I. Bezen; C. Breitkopf; J.A. Lercher. ACS Catal., 2011, 1, 1384; (b) J.
Orthman; H. Y. Zhu; G. Q. Lu. Sep. Purif. Technol., 2003, 31, 53.
11 P. Christopher, H. L. Xin, A. Marimuthu and S. Linic, Nat. Mater., 2012, 11,
1044.
12 (a) L. Brus, Acc. Chem. Res., 2008, 41, 1742; (b) C. D. Lindstrom and X. Y. Zhu,
Chem. Rev., 2006, 106, 4281.
13 (a) L. Hu, X. Cao, L. Chen, J. Zheng, J. Lu, X. Sun and H. Gu, Chem. Commun.,
2012, 48, 3445; (b) A. Corma, P. Concepción and P. Serna, Angew. Chem., Int.
Ed., 2007, 46, 7266.
198
Electronic Supplementary Information (ESI)
Visible light driven selective reduction of aromatic nitro to azoxy
compounds using supported gold nanoparticles: a promotional effect
of phosphate and transition metal ions in hydrotalcite support
Qi Xiao, Arixin Bo, Zhanfeng Zheng, Wayde Martens and Huaiyong Zhu*
School of Chemistry, Physics and Mechanical Engineering, Faculty of Science and
Technology, Queensland University of Technology, Brisbane, QLD 4001,
Australia.
E-mail: [email protected]; Fax: +61 7 3138 1804; Tel: +61 7 3138 1581.
LEGENDS
Text S1. Experimental details: catalyst preparation, catalyst characterization,
photocatalytic reaction
Figure S1. XRD patterns of various Au/PO43--M-HT catalysts
Figure S2. XRD patterns of various HT support
Figure S3. TEM images and Au particle size distributions for various Au/PO43--M-HT
catalysts
Figure S4. UV/Vis absorption spectra of as-prepared Au/PO43--M-HT catalysts
Figure S5. XPS spectra of Au/PO43--Ga3+-HT catalysts
Figure S6. Time-conversion plot for nitrobenzene reduction using Au/PO43-- Ga3+-HT
catalysts
Figure S7. XRD patterns comparison of recycled Au/PO43--Ga3+-HT catalysts
Figure S8. TEM image of recycled Au/PO43--Ga3+-HT catalysts
Table S1. Photocatalytic reduction of nitrobenzenes with different substrates using
Au/PO43--Ga3+-HT catalyst
199
Text S1 Experimental details
(1) Catalyst preparation
Hydrotalcite (HT) support. The Mg-Al HT mixed oxide precursor with an Mg/Al ratio
of 3 was provided using a sol-gel process following the procedure described in
references with some modification.[(a) M. C. I. Bezen; C. Breitkopf; J.A. Lercher. ACS
Catal. 2011, 1, 1384–1393; (b) J. Orthman; H. Y. Zhu; G. Q. Lu. Sep. Purif. Technol.
2003, 31, 53–59.] For this, an acidic aqueous solution of metal nitrates was prepared by
dissolving Mg(NO3)2·6H2O (115.39 g, 0.45 mol) and Al(NO3)3·9H2O (56.27 g, 0.15
mol) in 0.6 L of deionized water. A second alkaline solution was prepared from NaOH
(60.00 g, 1.5 mol) and Na2CO3 (26.50 g, 0.25 mol) in 1.0 L of deionized water. Both
solutions were heated to 75 °C. For precipitation, the nitrate and alkaline solutions were
added dropwise to 400 mL of water at 75 °C, giving a pH of 10. The suspension was
aged for 3 h at 85 °C under vigorous stirring. After cooling to RT, the gel was filtered
and loaded into an autoclave. Hydrothermal synthesis was carried out for 16 h at 80 °C.
The gel was washed with 350 mL of deionized water until a pH of 7 of the washing
water was reached. The white precipitate was freeze-dried and ground.
The HT precursor was calcined to 450 °C (heating rate 10 °C·min-1) in a flow of 100
mL·min-1 synthetic air for 8 h and ready for ion exchange.
PO43--M-HT. A prepared aqueous Mn+ nitrate solution (0.5 mmol Mn+/g HT precursor)
was added to the calcined HT precursor in a Schlenk flask. The mixture was stirred at
RT for 12 h, then the solid was washed and dried at 110 °C for 10h, the resultant
product was denoted as M-HT. Before doping with PO4 oxoanion, the M-HT
precursors were ground and calcined to 450°C again in a flow of synthetic air for 8 h.
The calcined M-HT (2.0 g) was dispersed into 50 mL Na3PO4 aqueous solution (0.02
mmol/L), the mixture was stirred at RT for 12 h, then the solid was washed and dried at
110 °C for 10h, the resultant solid was ground and denoted as PO43--M-HT.
Au/PO43--M-HT catalysts. Catalysts with 3 wt% of gold nanoparticles on HT were
prepared by impregnation-reduction method. 2.0 g PO43--M-HT powder was dispersed
into 15.2 mL of 0.01 M HAuCl4 aqueous solution while magnetically stirring. 20 mL of
0.53 M lysine was then added into the mixture with vigorous stirring for 30 min. To
this suspension, 10 mL of 0.35 M NaBH4 solution was added dropwise in 20 min,
200
followed by an addition of 10 mL of 0.3 M hydrochloric acid. The mixture was aged
for 24 h and then the solid was separated, washed with water and ethanol, and dried at
60 °C. The dried solid was used directly as catalyst.
(2) Catalyst characterization
X-ray diffraction (XRD) patterns of the samples were recorded on a Philips
PANalytical X’Pert PRO diffractometer using CuKa radiation (λ=1.5418 Å) at 40 kV
and 40 mA. The diffraction data were collected from 5 to 75° with a resolution being
0.01°(2θ). Nitrogen physisorption isotherms were measured at -196 °C on the Tristar II
3020. Prior to each measurement, the sample was degassed at 150 °C for 16 h under
high vacuum. The specific surface area was calculated by the BET method from the
data in a P/P0 range between 0.05 and 0.2. The XPS data were recorded on an
ESCALAB 250 spectrometer and AlKa radiation was used as the X-ray source. The C1s
peak at 284.8 eV was used as a reference for the calibration of the binding energy scale.
Transmission electron microscopy (TEM) images were taken with a Philips CM200
Transmission electron microscope employing an accelerating voltage of 200 kV. The
specimens were fine powders deposited onto a copper microgrid coated with a holey
carbon film. The content of gold on a zeolite was determined by energy-dispersive X-
ray spectroscopy (EDS) attached on an FEI Quanta 200 scanning electron microscope
(SEM). The diffuse reflectance UV/Vis (DR-UV/Vis) spectra were recorded on a Cary
5000 UV/Vis-NIR Spectrophotometer.
(3) Photocatalytic reaction
The reaction was conducted in a 25 mL round-bottomed Pyrex glass flask with a sealed
spigot and a magnetic stirrer. The reaction temperature was controlled by an air-
conditioner within a sealed wooden box. A 500 W Halogen lamp was used as the
incandescent light source. The light intensity in the reaction position was set at 0.45
W/cm2 and could be adjusted by changing the distance between the reactor and the
light source. The wavelength range was tuned by using various glass filters to cut off
the irradiation below a certain value of wavelength.
Catalytic reduction of nitrobenzene to azoxybenzene was conducted under the argon
atmosphere. Typically, 1.5 mmol nitrobenzene, 15 mL isopropanol as solvent, and 1.5
mL of 0.1 M KOH solution in isopropanol were mixed in the reactor, followed by
201
adding 50 mg of the catalyst and purging with argon gas, and then stirred during
reaction and illuminated with the incandescent light.
202
Fig. S1 XRD patterns of various Au/PO43--M-HT catalysts.
Fig. S2 XRD patterns of various HT support.
203
Fig. S3 TEM images and Au particle size distributions for various Au/PO43--M-HT
catalysts.
204
Fig. S4 UV/Vis absorption spectra of as-prepared Au/PO43--M-HT catalysts.
Fig. S5 XPS spectra of Au/PO43--Ga3+-HT catalyst, the broad peak close to Au4f5/2 is
due to Al2p energy loss peak.
205
Table S1 Photocatalytic reduction of nitrobenzenes with different substrates using
Au/PO43--Ga3+-HT catalyst
Reactant Main product Conv.
(%)
Sel.
(%)
NO2H3C N
NO
H3CCH3
56 98
NO2Cl N
NO
ClCl
58 >99
NO2H3CO N
NO
H3COOCH3
61 96
NO2H3COC N
NO
H3COCCOCH3
60 97
Reduction reaction was conducted in an argon atmosphere at 40 °C using 15 mL of
isopropyl alcohol mixed with 1.5 mL 0.1M KOH/isopropyl alcohol, 1.5 mmol
nitrobenzene, and 50 mg catalyst. Reaction time: 5 h.
Fig. S6 Time-conversion plot for nitrobenzene reduction using Au/PO43-- Ga3+-HT
catalysts.
206
Fig. S7 XRD patterns comparison of recycled Au/PO43--Ga3+-HT catalysts.
Fig. S8 TEM image of recycled Au/PO43-- Ga3+-HT catalysts.
207
3.3 Article 7
THE
I:
'" t
': .
s T4· . , __
AuNPs?7-29
208
216
Supporting Information for
Direct Photocatalytic Conversion of Aldehydes to Esters using Supported Gold Nanoparticles under Visible Light Irradiation at Room Temperature Yulin Zhang,a Qi Xiao,b Yongsheng Bao,a Yajing Zhang,a Steven Bottle,b Sarina Sarina,b Zhaorigetu Bao,*a and Huaiyong Zhu*b
217
Chemicals
Chloroauric acid (99.999%, HAuCl4) was purchased from Sinopharm Chemical
Reagent Company. L-lysine (98.0%), sodium borohydride (99.99%, NaBH4),
potassium hydroxide (99.0%, KOH) were purchased from Sigma-Aldrich. The supports
of cerium (IV) oxide (nanopowder, 99.95%, CeO2), zirconium (IV) oxide (nanopowder,
<100 nm of particle size, ZrO2), titanium (IV) oxide (nanopowder, 99.5%), and
aluminum oxide (γ-Al2O3) were purchased from Shanghai Aladdin Reagent Company.
All the chemicals were used as received without further purification.
Catalyst Characterization
TEM images were recorded with a Jeol JEM-1210 transmission electron microscope
employing an accelerating voltage of 200 kV. The samples were suspended in ethanol
and dried on holey carbon-coated Cu grids. The composition of samples was
determined by using the energy-dispersive X-ray spectroscopy attachment of
transmission electron microscope.
The X-ray photoelectron spectroscopy (XPS) was measured with ESCALAB210 of
British VG Company. All binding energies were referenced to the C (1s) hydrocarbon
peak at 285.00 eV. The UV-visible spectra were examined by Shimadzu UV-2550
spectrophotometer in the range of 200–800 nm at room temperature with BaSO4 as the
reference.
The specific surface areas of the samples were derived from the nitrogen sorption data
of the samples at liquid nitrogen temperature, using the Brunauer–Emmett–Teller (BET)
method from the data in a relative pressure (P/P0) range between 0.06 and 0.30.
The amount of surface acidity was measured by NH3 temperature-programmed
desorption (NH3-TPD) at ambient pressure. The sample (50 mg) was pretreated at
218
300 °C for 30 min and cooled to 80 °C in flowing He. At this temperature, sufficient
pulses of NH3 were injected until adsorption saturation, followed by purging with He
for about 2 h. The temperature was then raised from 80 to 800 °C at a rate of 10 °C
/min to desorb NH3. The NH3 desorbed was detected by with a thermal conductivity
detector (TCD).
The Au content was determined by the HITACHI Z-8000-type polarized Zeeman
atomic absorption spectrophotometer (AAS) of Hitachi company. The Au sensitive
wavelength is 2428 Å.
219
Figure S1. TEM images of Au-NPs on different oxide supports. (a) Au/CeO2, (b). Au/TiO2. The length of the scale bars in the images is 50 nm. (c).Au/CeO2, (d). Au/TiO2. The length of the scale bars in the images is 20 nm. (e), (f) are Au particle size distribution of Au/CeO2 and Au/TiO2, respectively (the particle size distributions were determined from TEM images by meauring >200 isolate particles obtained from images from distinct quadrants of the grid.).
220
Figure S2. UV-vis diffuse reflectance spectra of 3 wt% Au-NPs on different supports.
300 400 500 600 700 800
Abso
rban
ce (a
.u.) 520nm
CeO2TiO2
Al2O3
Au/TiO2
Au/CeO2
Au/Al2O3
Wavelength (nm)
221
Figure S3. Contributions to activation energy of light irradiation on esterification of benzaldehyde with ethanol. The reactions were conducted both under irradiation (photocatalytic process) and in the dark (thermal process) at various temperatures to examine the kinetics. According to the first-order reaction rate equation, the values of the rate constants were calculated. Furthermore, the Arrhenius equation was applied to derive the apparent activation energies of the reductions via photocatalytic and thermal processes.
222
Table S1. Influences of gold loading on esterification of benzaldehyde with supported Au-NPs
Au Loading (wt%) Visible light Dark
Conv. (%) Sel (%) Conv. (%) Sel. (%)
1 12.4 94.5 2.5 99
2 36.0 96.6 3.5 99
3 78.3 99.4 4.4 99.7
Reaction conditions: benzaldehyde (0.5 mmol) and Au/Al2O3 30 mg in ethanol (5 mL) at room temperature, Reaction time 12 h, Light intensity 0.34 W·cm-2 ; Conversion and selectivity were determined by GC.
223
CHAPTER 4
NONPLASMONIC METAL NANOPARTICLE PHOTOCATALYSTS FOR ORGANIC
SYNTHESIS BY VISIBLE LIGHT
4.1 Introductory Remarks
This chapter includes one article:
Article 8 (published on Angew. Chem. Int. Ed., 2014, 53, 2935–2940) is the 1st
example of nonplasmonic metal nanoparticle visible-light photocatalysts. Nanoparticles
of nonplasmonic transition metals, such as palladium (Pd), platinum (Pt), rhodium
(Rh), and iridium (Ir), supported by oxide solids are widely used as catalysts for the
synthesis of important organic compounds. However, until now, the use of light
irradiation to enhance the efficiency of organic reactions with nonplasmonic metal
nanoparticles has been largely overlooked. In this paper, we discovered that irradiation
with light can significantly enhance the intrinsic catalytic performance of nonplasmonic
transition metals (Pd, Pt, Rh, and Ir) nanoparticles at ambient temperatures for several
types of reactions. These metal nanoparticles strongly absorb the light mainly through
interband electronic transitions. The excited electrons interact with the reactant
molecules on the particles to accelerate these reactions. The rate of the catalyzed
reaction depends on the concentration and energy of the excited electrons, which can be
increased by increasing the light intensity or by reducing the irradiation wavelength.
The metal nanoparticles can also effectively couple thermal and light energy sources to
more efficiently drive chemical transformations. Since nanoparticles of nonplasmonic
metals have been widely used for various applications, the reported discovery may
224
significantly broaden the application of catalytic processes driven by light, and most
importantly, our study indicated that plasmonic excitation is not the only mechanism
involved when the irradiation of metal particles with light leads to enhanced catalytic
activity.
225
4.2 Article 8
226
232
Supplemental Information for
Viable Photocatalysts under Solar Spectrum Irradiation: Non-
plasmonic Metal Nanoparticles
Sarina Sarina 1, Huai-Yong Zhu 1*, Qi Xiao 1, Esa Jaatinen 1, Jianfeng Jia 2, Yiming
Huang 1, Zhanfeng Zheng 1, Haishun Wu2
*correspondence to: [email protected]
1Chemistry discipline, Queensland University of Technology, Brisbane, Qld 4001,
Australia 2School of Chemical and Material Science, Shanxi Normal University, Linfen 041004,
China
Table of content:
Materials and Methods
S1 Chemicals
S2 Catalysts Preparation
S3 Catalysts Characterization
S4 Photocatalytic Reactions
Supplementary Figures and Tables
Figures S1 to S8, Table S1
Supplementary Text
Text S1 and S2
233
Materials and Methods:
Chemicals
Zirconium (IV) oxide (ZrO2, <100 nm particle size, TEM), Palladium (II) chloride
(PdCl2, ReagentPlus®, 99%), Sodium borohydride, powder (NaBH4, ≥98.0 %),
Platinum (III) chloride trihydrate (HPtCl3·3H2O, ≥99.9 % trace metals basis), Rhodium
(III) chloride and Iridium (III) chloride. Hydrochloric acid (HCl, 32 % w/w, analytical
reagent, Chem−Supply, Australia). All the chemicals used in the experiments were
purchased from Sigma-Aldrich (unless otherwise noted) and used as received without
further purification. The water used in all experiments was prepared by passing through
an ultra-purification system.
Catalysts Characterization
TEM study and line profile analysis by energy dispersion X−ray spectrum technique of
the photocatalysts were carried out on a Philips CM200 TEM with an accelerating
voltage of 200 kV. The metal content of the prepared catalysts were determined by
EDS technology using the attachment to a FEI Quanta 200 Environmental SEM. The
element line scanning was conducted on a Bruker EDX scanner attached to
JEOL−2200FS TEM with scanning beam diameter down to 1.0 nm. Diffuse reflectance
UV−visible (DR−UV−vis) spectra of the sample powders were examined by a Varian
Cary 5000 spectrometer with BaSO4 as a reference.
Photocatalytic Reactions
Oxidant free dehydrogenation of benzyl alcohol to aldehyde: 100 mg catalyst, the
metal content is 3 wt%; 0.5 mmol reactant in 5 ml triflourotoluene solvent; 1 atm Ar
atmosphere. Oxygen was removed from the reaction mixture prior to introducing Ar
and the reaction proceeded 48 h for all catalysts under irradiation of various light and in
the dark at 45º±2ºC.
Degradation of phenol: 50 mg catalyst, the metal content is 3 wt%; 0.5 mmol of
reactant in 10 ml aqueous solution; the reaction proceeded for 24 h for all catalysts
under irradiation of various light and in the dark at 30±1ºC.
234
Suzuki-Miyaura coupling: Aryl iodide (1 mmol), arylboronic acid (1.5 mmol),
photocatalysts (50 mg) and K2CO3 (3 mmol) were added to 20 mL N,N-
dimethylformamide (DMF)/H2O (V:V=3:1). For the reactions using aryl bromide and
aryl chloride, NaOH (3 mmol) was used as base, cetyltrimethylammonium bromide
(CTAB) (1 mmol) helped in bringing aryl bromide into the solvent (H2O 10mL).
Reaction temperature: 30±2 °C, reaction time: 24h for aryl iodine and bromide, 16h for
aryl chloride.
Hiyama Cross-coupling Reactions: Aryl iodide (1 mmol), trimethoxyphenylsilane
(1.5 mmol), photocatalysts (50 mg), cetyltrimethylammonium bromide (CTAB) (1
mmol) and tetrabutylammonium fluoride (TBAF) (1.2 mmol) were added to 5 mL
toluene. Reaction temperature: 45±2 °C, 24h.
Buchwald-Hartwig Cross-coupling: Aryl iodide (1 mmol), aniline (1.2 mmol),
photocatalysts (50 mg) and potassium tert-butoxide (t-BuOK) (3 mmol) were added to
10 mL N,N-dimethylformamide (DMF). Reaction temperature: 45±2 °C, 1 atm oxygen
atmosphere, 24 h.
235
Fig. S1 Light absorption spectra of the metal nanoparticles in a colloid suspension
(dash line) and supported on ZrO2 (solid line). The light absorption of pure ZrO2 is also
shown in the figure and reveals that the ZrO2 support exhibits negligible light
absorption at wavelengths longer than 370 nm. Therefore, it can be concluded that the
light absorption of the M@ZrO2 samples observed in the UV-Visible spectra is due to
the absorption of the metal NPs (the difference between light absorption of M@ZrO2
and that of the ZrO2 support alone). The difference between the supported and colloidal
metal NP spectra for each sample is attributed to scattering caused by closely spaced
NPs and NP aggregates 1. In the unsupported colloidal samples the NPs are essentially
single particles with diameters less than 10 nm. Mie theory shows that visible light
absorption is significantly greater than scattering for single particles of this size. For the
supported samples, the metal NPs are much more closely packed leading to NP
aggregation (see Fig. S2, SI), which results in significantly higher scattering at longer
wavelengths.
Reference
1. Funston, A. M., Novo, C., Davis, T. J. & Mulvaney, P. Plasmon coupling of gold
nanorods at short distances and in different geometries. Nano Lett. 9, 1651-1658
(2009).
236
Fig. S2. (a-e) Transmission electron microscopy (TEM) image of the metal NP catalyst
and (f) particle size distribution of the metal NP catalyst.
237
Fig. S3 XPS analysis of the metal nanoparticles supported on ZrO2.
References
1 Metallic state Pd NP
a) J. Kanongo, L. Selegard, C. Vahlberg, K. Uvdal, H. Saha, S. Basu, XPS study of
palladium sensitized nano porous silicon thin film, Bull. Mater. Sci., 33, 2010, pp. 647-
651. b) M. Brun, A. Berthet, J. C. Bertolini, XPS, AES and Auger parameter of Pd and
PdO, J. Electr. Spectr. Related Phenom., 104 (1999) 55–60.
2 Metallic state Pt NP
C. Dablemont, P. Lang, C. Mangeney, J. Piquemal, V. Petkov, F. Herbst, G. Viau,
FTIR and XPS Study of Pt Nanoparticle Functionalization and Interaction with
Alumina, Langmuir, 24, 2008, 5832-5841.
238
3 Metallic state Rh NP
Y. Wang, Z. Song, D. Ma, H. Luo, D. Liang, X. Bao, Characterization of Rh-based
catalysts with EPR, TPR, IR and XPS, J. Mol. Catal. A: Chem., 149, 1999. 51–61
4 Metallic state Ir NP and Rh NP
a) I. S. Park, M. S. Kwon, Kyung Yeon Kang, J. S. Lee, J. Park, Rhodium and Iridium
Nanoparticles Entrapped in Aluminum Oxyhydroxide Nanofibers: Catalysts for
Hydrogenations of Arenes and Ketones at Room Temperature with Hydrogen Balloon,
Adv. Syn. & Catal., 349, 2007, 2039-2047. b) R. Zanoni, R. Psaro, C. Dossi, L.
Garlaschelli, R. Della Pergola, D. Roberto, XPS characterization of SiO2-supported
iridium produced in situ from Ir4(CO)12, J. Cluster Sci., 1, 1990, 241-247.
239
Fig. S4. Wavelength output of visible light sources: incandescent lamp and LED lamp.
240
Table S1 Detailed data analysis of action spectra:
PdNPs catalysed benzyl alcohol dehydrogenation (Figure 4a)
Wavelength (nm) 350±5 400±5 620±5
AQE (%) 0.17 0.078 0.02
AQE (wavelength)/ AQE (620nm) 8.5 3.9 1
Photon Energy (PE, eV) 3.54 3.10 2.00
PE (wavelength)/PE (620nm) 1.77 1.55 1
AQE ratio / PE ratio 8.5/1.77=4.8 3.9/1.55=2.52 1
PtNPs catalysed benzyl amine oxidative coupling (Figure 4b)
Wavelength (nm) 350±5 400±5 620±5
AQE (%) 0.39 0.28 0.03
AQE (wavelength)/ AQE (620nm) 13 9.3 1
AQE ratio / PE ratio 7.34 6.02 1
RhNPs catalysed benzyl amine oxidative coupling (Figure 4c)
Wavelength (nm) 350±5 400±5 620±5
AQE (%) 0.5 0.15 0.01
AQE (wavelength)/ AQE (620nm) 50 15 1
AQE ratio / PE ratio 28.2 9.68 1
IrNPs catalysed benzyl alcohol dehydrogenation (Figure 4d)
Wavelength (nm) 350±5 400±5 620±5
AQE (%) 0.25 0.175 0.02
AQE (wavelength)/ AQE (620nm) 12.5 8.8 1
AQE ratio / PE ratio 7.1 5.65 1
The AQE (ratio of the number of product molecules formed due to the irradiation to the
number of photons absorbed by the metal nanoparticles) under irradiation with a
wavelength of 400 ±5 nm is much higher than that under the irradiation with a
241
wavelength of 620 ±5 nm. For instance, the AQE of benzyl amine oxidative coupling
catalysed by PtNPs irradiated with light of 400 nm wavelength is 9.3 times the AQE
under irradiation of 620 nm, while the photon energy of the 400 nm light is only 1.55
times the energy of the photons with the 620 nm wavelength. The activity enhancement
by the short wavelength photons is extraordinary high, this indicating that the
enhancement achieved with 400 nm irradiation is mostly due to the photoexcitation
rather than photothermal effect. There is a trend that the shorter the wavelength, the
larger the AQE value. It demonstrates that the contribution from photoexitation
increases significantly with decreasing wavelength. For other reactions catalysed by
different metal nanoparticles the similar situations are observed (as shown in a table
added in SI). The AQE (catalytic activity) at the longest wavelength (620 ±5 nm)
provide the close approximation of the contribution from photothermal effect of the
metal nanoparticles, as the AQE at long wavelengths are low and the changes in the
values are very limited, compared with the AQE at short wavelengths (<500 nm).
242
Fig. S5. We divided the conversion under irradiation with light by the conversion under
conventional heating to calculate the light irradiation enhancement (Angew Chem. Int.
Ed. 2013, 52, 6063). Figure S5 shows the results with the data of the present study,
where the ratio of the reaction rates is plotted as a function of irradiance. The results in
figure a1, b1 and c1 clearly show linear dependences of the light irradiation
enhancement factor on the irradiation irradiance for all three reactions at various
temperatures and thus demonstrate that the photo-excitation of the metal electrons is
the primary factor responsible for the light-enhanced activity.
243
Fig. S6. The infrared absorption spectra of benzyl alcohol on PdNPs on ZrO2
(Pd@ZrO2 in the upper panel) and RhNPs on ZrO2 (RhNP@ZrO2 in the lower panel).
The infrared spectra of pure benzyl alcohol and benzaldehyde are also provided for
comparison. The main differences between the spectra of benzyl alcohol on PdNP
sample and pure benzyl alcohol are strong absorption between 1700 cm-1 and 1730 cm-1,
changes in regions around 1000 cm-1 and 680 - 750 cm-1, which indicates strong
chemical adsorption of benzyl alcohol on the PdNP sample. In contrast, all the infrared
absorption peaks of benzyl alcohol can be observed from the spectrum of benzyl
alcohol on RhNPs on ZrO2 except for that the peaks at 1007 cm-1 and 1028 cm-1, which
are attributed to C-O stretching vibrations, shift slightly. This suggests a physical
adsorption of benzyl alcohol on RhNP sample.
244
Fig. S7. The relation between the conversion under light irradiation and the conversion
in the dark. a: visible light, including incandescent and LED lamps; b: UV light and the
conversion in the dark for the reactions in Figs. 2 and 3 using the NPs of the four
metals. The red lines indicate the situation where light irradiation has no contribution,
the two conversions are identical. In general the visible light (incandescent and LED
lights) induced enhancement is greater for higher thermal conversion rates (the
conversion rates in the dark). However, the same trend was not obvious for the
reactions under UV irradiation.
245
Fig. S8. The catalytic performance of metal NPs on ZrO2 support for oxidative
coupling of benzylamine and oxidant free dehydrogenation of benzyl alcohol under
irradiation of simulated sunlight source (from Electro Powerpacs, SLO-BLO, Mode
No. 1163). Reaction condition: 0.5 mmol of reactant, 100 mg of catalyst, reaction
temperature was 45°C; the irradiancewas 0.45 W/m2; and the conversion after 24 h was
determined.
246
Supplementary Text S1
The contribution of the interband absorption to the absorption by metal NPs in
visible and UV range
Inter-band absorption is a common feature of both plasmonic and non-plasmonic
transition metal NPs and occurs when a single electron absorbs a photon and is excited
from one energy band to another 1-4. Non-plasmonic metal NPs exhibit significant
absorption of visible and UV irradiation 5. In general, the overall UV-Vis absorption of
the NPs is a complicated combination of both processes and it is not correct to simply
add the bound electron absorption to the free electron absorption to obtain the total
absorption 6. The contributions from the LSPR effect and inter-band transition,
respectively, vary from metal to metal. For AgNPs, the absorption in the visible is
dominated by the LSPR effect displaying a strong plasmon peak at around 400 nm
(details are provided in SI). In contrast, for PtNPs, the absorption between 200 and 800
nm is dominated by the inter-band absorption contribution. While for AuNPs and
PdNPs, both free and bound electron absorption play significant roles in visible light
absorption.
References
1. Pakizeh, T. Optical absorption of nanoparticles described by an electronic local
interband transition. J. Opt. 15, 025001 (2013).
2. Pakizeh, T., Langhammer, C., Zorić, I., Apell, P. & Käll, M. Intrinsic fano
interference of localized plasmons in Pd nanoparticles. Nano Lett. 9, 882-886
(2009).
3. Weaver, J. H. Optical properties of Rh, Pd, Ir, and Pt. Phys.Rev. B 11, 1416-1425
(1975).
4. Weaver, J. H. Optical investigation of the electronic structure of bulk Rh and Ir.
Phys.Rev. B 15, 4115-4118 (1977).
5. Creighton, J. A. & Eadon, D. G. Ultraviolet–visible absorption spectra of the
colloidal metallic elements. J. Chem. Soc., Faraday Trans. 87, 3881-3891 (1991).
247
6. Pinchuk, A., Plessen, G. V. & Kreibig, U. Influence of interband electronic
transitions on the optical absorption in metallic nanoparticles. J. Phys. D: Appl.
Phys. 37, 3133-3139 (2004).
Calculated extinction spectra of Ag, Pt, Rh and Pd are shown below:
This treatment follows along the lines of Pinchuk et al, J. Phys. D: Appl. Phys.
37, pp. 3133-3139 (2004). The total absorption of the metal is determined by the
combination of the optical properties of the bound electrons (inter-band transitions),
and that of the free electrons (i.e. LSPR or Drude term). In general, the final absorption
depends on these two sets of properties in a complicated way and it is not correct to
simply say that the total absorption is the sum of the bound electron absorption added
to the free electron absorption. This simplification applies sometimes (i.e. in the case of
silver and platinum over distinct wavelength ranges) but not in all cases (eg. such as
with gold and palladium). Nonetheless we can separate the optical properties (eg.
permittivity) of bound and free electrons to determine which is dominant in given
metals for specific wavelengths. This is possible as we have the measured optical
properties for all the metals and a theoretical description of the optical properties of the
free electron behaviour. Therefore by subtracting the free electron properties from the
measured values we get the optical properties of the bound electrons.
1. Silver
Abso
rban
ce
Wavelength (nm)
Ag Absorption
Ag Bound Only
248
This graph shows the absorption spectrum of silver from 200 nm to 800 nm.
The blue curve is the total absorption while the red curve is the absorption spectrum of
silver if we could turn off the LSPR (i.e. only the bound electrons contribute to the
relative permittivity as given by equation S1.2). Here we see that the absorption is
completely dominated by the LSPR, so here we can say that around 400 nm all the
absorption is LSPR.
2. Platinum
This graph shows the absorption spectrum of platinum from 200 nm to 800 nm.
The blue curve is total absorption of platinum and the red curve is the absorption of the
platinum if the LSPR could be turned off. Here we see the exact opposite of silver in
that the spectrum from 200 nm to 800 nm is almost completely dominated by bound
electron absorption.
3 Rhodium
Abso
rban
ce
Wavelength (nm)
Pt Absorption
Pt Bound Only
249
The total absorption spectrum of Rhodium (blue curve), and the absorption in
the absence of any free electrons as calculated from Drude theory (red curve). Like
Platinum, for wavelengths greater than 400 nm the absorption is dominated by the
contributions from the bound electrons.
4. Palladium
This graph shows the absorption spectrum of palladium from 250 nm to 800 nm.
The blue curve is total absorption of palladium and the red curve is the absorption of
the palladium in the absence of any free electron contribution . Here it is evident that
while the bound electron contribution is still dominant at wavelengths greater than 400
nm, that the free electron behaviour does play a role.
Abso
rban
ce
Wavelength (nm)
Rh Absorption
Rh Bound Only
Abso
rban
ce
Wavelength (nm)
Pd Absorption
Pd Bound Only
250
References
7. Pakizeh, T. Optical absorption of nanoparticles described by an electronic local
interband transition. J. Opt. 15, 025001 (2013).
8. Pakizeh, T., Langhammer, C., Zorić, I., Apell, P. & Käll, M. Intrinsic fano
interference of localized plasmons in Pd nanoparticles. Nano Lett. 9, 882-886
(2009).
9. Weaver, J. H. Optical properties of Rh, Pd, Ir, and Pt. Phys.Rev. B 11, 1416-1425
(1975).
10. Weaver, J. H. Optical investigation of the electronic structure of bulk Rh and Ir.
Phys.Rev. B 15, 4115-4118 (1977).
11. Creighton, J. A. & Eadon, D. G. Ultraviolet–visible absorption spectra of the
colloidal metallic elements. J. Chem. Soc., Faraday Trans. 87, 3881-3891 (1991).
12. Pinchuk, A., Plessen, G. V. & Kreibig, U. Influence of interband electronic
transitions on the optical absorption in metallic nanoparticles. J. Phys. D: Appl.
Phys. 37, 3133-3139 (2004).
13. Bohren, C. F. & Huffman, D. R. Absorption and scattering of light by small
particles. (Wiley-VCH, Weinheim, Germany, 2004)
14. Palik, E. D. Handbook of optical constants of solids. (Academic press, San Diego,
USA 1985)
251
Supplementary Text S2
Detailed DFT calculation methods:
To model the PhCH2OH- transient anion, all the associated species were optimized at
the level of density functional theory (DFT) with Becke’s 1 three-parameter exchange
and Lee-Yang-Parr correlation functional 2 implemented in Gaussian 09 package 3. 6-
311++G(d,p) basis set was employed to describe the orbital of all atoms involved. The
energy to break the bond between C and α-H in PhCH2OH was calculated favouring the
reaction of PhCH2OH = PhCHOH + H, while in PhCH2OH- favouring the reaction of
PhCH2OH- = PhCHOH + H- .
Iodobenzene molecule and its corresponding negative ions were fully optimized
under as defined B3LYP/TZVP method. The geometry of all the species were
optimized at the level of DFT with Becke’s 1 three–parameter exchange and Lee–
Yang–Parr correlation functional 2 implemented in Orca 4. Ahlrichs’ triple zeta valence
basis set 5 TZVP was employed to describe the orbitals of all atoms involved.
References
1. A. D. Becke, J. Chem. Phys. 98, 5648 (1993).
2. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 37, 785 (1988).
3. M. J. Frisch, et al., Gaussian 09, C.01, Gaussian, Inc., Wallingford CT (2010).
4. F. Neese, Wiley interdisciplinary Reviews – Comp. Mol. Sci., 2, 73-78 (2012)
5. A. Schaefer, S. Huber, R. Ahlrichs,. J. Chem. Phys. 100, 5829-5835 (1994).
252
CHAPTER 5
CONCLUSIONS & FUTURE WORK
Conclusions
In this thesis, several new metal nanoparticle photocatalysts have been developed
and used for various organic synthesis reactions under visible light irradiation:
In Chapter 2, first, we found an effective approach to broaden the application of
AuNP photocatalysts is to incorporate a metal with an intrinsic catalytic ability as an
alloy with the Au NP base, to catalyze various chemical reactions with sunlight. We
successfully realized the coupling of light absorption of AuNP and catalytic property of
Pd in alloy structures and drove several kinds of cross-coupling reactions. An
outstanding feature of the Au-Pd alloy NPs is their ability to efficiently concentrate the
energy of a photon flux into a very small volume and to transfer this energy to adsorbed
molecules inducing their reaction on the surface. These catalytic cross-coupling
processes are due to the interaction of light-excited electrons of the catalyst with the
reactant molecules, while high temperatures are not a prerequisite for driving them. The
reaction rate depends on the number of light-excited electrons and the number of
reactant molecules on the catalyst surface. The number of reactant molecules on the
surface depends mainly on the affinity of the surface for the reactants. Pd sites have a
strong affinity for many organic molecules. The number of light-excited electrons can
be increased by applying high light intensity. Besides, a stable, inexpensive, and
253
reusable Au-Pd alloy NPs supported on the phosphate anion doped hydrotalcite surface
catalyst is shown to be active and selective for the direct oxidative esterification of
aliphatic alcohols under visible light irradiation using 1 atm of molecular oxygen as
benign oxidant. Finally, we further extended the alloy NP photocatalysts to Au-Cu
alloys, and the as-prepared Au-Cu alloy NPs can drive reduction of nitroaromatics to
aryl amines under visible light irradiation in a direct route, which is apparently from the
condensation route that using pure Au NPs. The photocatalytic reaction pathway can be
finely tuned by addition very few amount of Cu into Au, which can maintain Cu’s
stability on the surface and keep high catalytic activity as well. The LSPR absorption of
Au-Cu alloy plays an important role in visible light absorption, tuning light intensity
and wavelength can obtain different reaction activity. The catalytic system described
here promotes a sophisticated multi-step reaction process by controlled manipulation of
the reaction pathways, and may present a new strategy toward the development of new
heterogeneous catalysts, and also contribute to understand the development of
photocatalytic systems for more complex organic reactions.
Overall, this study of alloy NP photocatalysts provides a general guiding
principle for determining the applicability of the alloy NP photocatalysts as well as a
clue for designing suitable photocatalysts made from gold alloyed with other transition
metals. The knowledge acquired in this study may inspire further studies in new
efficient photocatalysts and a wide range of organic synthesis driven by sunlight. The
component of the new photocatalysts, especially the light harvesting component,
should not be limited to Au only. Many other noble metals NP with LSPR effect, for
example, Ag and Cu, can also be alloyed with Pd to form new photocatalyst structures.
In Chapter 3, the photocatalytic application of pure Au NPs was extended by
applying different support. The phosphate and transition metal ions doped Au/PO43--
254
Ga3+-HT catalysts can be used for selective synthesis of azoxy compounds from nitro
compounds by visible light under mild conditions. The PO43- oxoanion may increase
the charge redistribution and internal electric field inside the support, which can
facilitate the light-excited electron transfer. In addition, the introduction of phosphate
and transition metal ions to hydrotalcite (HT) support can strongly affect the properties
of the basic sites on the catalysts, thus producing a synergistic effect on the
photocatalytic activity and product selectivity. Moreover, visible light irradiation can
also drive various aldehydes (both aromatic aliphatic aldehydes) and alcohols into the
corresponding esters in high yields using Au/Al2O3 catalyst.
In Chapter 4, it was discovered that irradiation with light can significantly
enhance the intrinsic catalytic performance of nonplasmonic transition metal NPs at
ambient temperatures for several types of reactions. These transition metal NPs
strongly absorb the light mainly through interband electronic transitions. The excited
electrons interact with the reactant molecules on the particles to accelerate these
reactions. The rate of the catalyzed reaction depends on the concentration and energy of
the excited electrons, which can be increased by increasing the light intensity or by
reducing the irradiation wavelength. The metal NPs can also effectively couple thermal
and light energy sources to more efficiently drive chemical transformations. This study
indicated that plasmonic excitation is not the only mechanism involved when the
irradiation of metal particles with light leads to enhanced catalytic activity.
255
Future Work
Although considerable achievements have been made on metal NP photocatalysts,
more work still needs to be done in the future to improve the photocatalytic
performance and to clarify the photocatalytic mechanism. Future work can be proposed
from the following aspects:
1. The development of relationships between reactant electronic structure and
photocatalytic signatures is of significant importance for designing photocatalytic
systems that allow for unique control of reaction selectivity. More experimental and
theoretical efforts on the elaboration of these processes are strongly desired, as the
understanding of these will greatly help to optimize the enhancement effects in the
further application of these new metal NP photocatalysts.
2. A well-known feature of plasmonic nanostructures is their tuneable LSPR
wavelength with particle geometry such as composition, shape and size etc. This means
it is possible in principle to design nanostructures that can absorb the entire solar
spectrum more efficiently by manipulating these properties in catalyst preparation. The
dependence of direct plasmon driven photocatalytic characteristics (efficiencies,
wavelength dependence, reaction selectivity etc.) on the structure of the plasmonic NPs
is expected to be a focus of future research.
3. Many experiments so far on plasmon-metal photocatalytic reactions have
mainly relied on the plasmonic properties of gold and silver. Practical implementation
of plasmon-enhanced chemical reactions will require the use of inexpensive, earth-
abundant elements such as aluminium and copper. Their LSPR properties and chemical
stabilities for catalytic chemical reactions are important issues to be considered in the
future.
256
4. The detailed experimental analysis of contributions from different driving
forces of photo-enhanced chemical processes inside those plasmonic and non-
plasmonic metal NP photocatalytic systems are not clear or much less known. High
energetic electrons excited by incident photons and electron thermal effect (temperature
increase) resulting from light absorption are the two main contribution routes that
photons appear to induce enhanced chemical reactions. Thus, clarifying the responsible
driving forces for metal NP photocatalysis should be considered in the future study.
![Alkyne-Protected AuPd Alloy Nanoparticles for ... · Alkyne-Protected AuPd Alloy Nanoparticles for Electrocatalytic Reduction of Oxygen Christopher P. Deming,[a] Albert Zhao,[b] Yang](https://img.pdfslide.us/doc/110x75/60150270d73ac56ce159f152/alkyne-protected-aupd-alloy-nanoparticles-for-alkyne-protected-aupd-alloy-nanoparticles.jpg)
![InCl3-Catalyzed [2+3] Cycloaddition Reaction: A Rapid ...Rapid Synthesis of 5-Substituted 1H-tetrazole under Microwave Irradiation VIJAY S. PATILA, KAMALAKAR P. NANDREA, AMULRAO U](https://img.pdfslide.us/doc/110x75/60b38cd29424f023f120693f/incl3-catalyzed-23-cycloaddition-reaction-a-rapid-rapid-synthesis-of-5-substituted.jpg)
