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Chemical Engineering Engineering
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Microwave-Assisted Synthesis of Pd Nanoparticles Supported on Microwave-Assisted Synthesis of Pd Nanoparticles Supported on
Fe3O4, Co3O4, and Ni(OH)2 Nanoplates and Catalysis Application Fe3O4, Co3O4, and Ni(OH)2 Nanoplates and Catalysis Application
for CO oxidation for CO oxidation
Hany A. Elazab The British University in Egypt, [email protected]
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Recommended Citation Recommended Citation Elazab, Hany A., "Microwave-Assisted Synthesis of Pd Nanoparticles Supported on Fe3O4, Co3O4, and Ni(OH)2 Nanoplates and Catalysis Application for CO oxidation" (2014). Chemical Engineering. 111. https://buescholar.bue.edu.eg/chem_eng/111
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RESEARCH PAPER
Microwave-assisted synthesis of Pd nanoparticles supportedon Fe3O4, Co3O4, and Ni(OH)2 nanoplates and catalysisapplication for CO oxidation
Hany A. Elazab • Sherif Moussa •
B. Frank Gupton • M. Samy El-Shall
Received: 27 November 2013 / Accepted: 21 May 2014 / Published online: 12 June 2014
� Springer Science+Business Media Dordrecht 2014
Abstract In this paper, we report a simple, versatile,
and rapid method for the synthesis of Pd nanoparticle
catalysts supported on Fe3O4, Co3O4, and Ni(OH)2
nanoplates via microwave irradiation. The important
advantage of microwave dielectric heating over con-
vective heating is that the reactants can be added at
room temperature (or slightly higher temperatures)
without the need for high-temperature injection. Fur-
thermore, the method can be used to synthesize metal
nanoparticle catalysts supported on metal oxide nano-
particles in one step. We also demonstrate that the
catalyst-support interaction plays an important role in
the low temperature oxidation of CO. The current
results reveal that the Pd/Co3O4 catalyst has particularly
high activity for CO oxidation as a result of the strong
interaction between the Pd nanoparticles and the Co3O4
nanoplates. Optimizations of the size, composition, and
shape of these catalysts could provide a new family of
efficient nanocatalysts for the low temperature oxida-
tion of CO.
Keywords CO oxidation � Microwave synthesis �Pd nanoparticles � Magnetite Fe3O4 nanoparticles �Hexagonal Co3O4 nanoplates � Hexagonal Ni(OH)2
nanoplates
Introduction
Heterogeneous catalysis processes have a wide range of
applications ranging from hydrocarbon refining, fuel-
cell technologies, production of chemicals and phar-
maceuticals, and removal of harmful gases and volatile
organics for the control of pollution in the environment
(Ertl 2009; Somorjai and Li 2010; Gao and Goodman
2012). For example, the catalyzed oxidation of CO by
O2 is an important process in pollution control since
small exposure (ppm) to this odorless invisible gas can
be lethal (World Health Organization 1999). Significant
advances in air quality and environmental protection
have been achieved through the introduction of the
three-way catalytic converters in automobiles where
Pd, Pt, and Rd nanoparticles are the main active
catalysts (Haag et al. 1999; Shelef and McCabe 2000;
Somorjai and Li 2010; Freund et al. 2011). Palladium is
among the most investigated catalysts as it plays an
important role in many different heterogeneous
Electronic supplementary material The online version ofthis article (doi:10.1007/s11051-014-2477-0) contains supple-mentary material, which is available to authorized users.
H. A. Elazab � B. F. Gupton � M. S. El-Shall
Department of Chemical Engineering, Virginia
Commonwealth University, Richmond, VA 23284, USA
S. Moussa � B. F. Gupton � M. S. El-Shall (&)
Department of Chemistry, Virginia Commonwealth
University, Richmond, VA 23284, USA
e-mail: [email protected]
M. S. El-Shall
Department of Chemistry, Faculty of Science, King
Abdulaziz University, Jeddah 21589, Saudi Arabia
123
J Nanopart Res (2014) 16:2477
DOI 10.1007/s11051-014-2477-0
catalysis processes such as carbon–carbon cross-cou-
pling reactions, energy conversion, and storage in
addition to the catalytic oxidation of CO, a prototype
reaction important in many applications such as proton-
exchange fuel cells, WGS reaction, and catalytic
convertor applications (Choudhary and Goodman
2002; Bianchini and Shen 2009; Molnar 2011; Jin
et al. 2012).
The performance of a heterogeneous catalyst for
CO oxidation including activity, selectivity, and
stability depends on various factors such as the type
of the support, the metal precursor, the preparation
conditions, the pre-treatment conditions, and the
catalytic reaction conditions (Somorjai and Li 2010;
Freund et al. 2011; Gao and Goodman 2012). Among
these factors, the nature of the support is expected to
play an important role given the strong tendency of CO
molecules to adsorb on the surface of the metal
nanoparticle catalyst, and therefore the turnover relies
on having surface sites open for O2 adsorption and
dissociation (Somorjai and Li 2010; Freund et al.
2011; Gao and Goodman 2012). Metal oxides have
been used as catalyst supports for Pd and other CO
oxidation catalysts since they provide a convenient
way to disperse the catalyst nanoparticles and prevent
their agglomeration and sintering during the reactions
(Campbell et al. 2000; Santra and Goodman 2003;
Farmer and Campbell 2010; Somorjai and Li 2010;
Freund et al. 2011; Gao and Goodman 2012; Jin et al.
2012). In addition, metal oxides can influence the
mechanism of the catalytic oxidation through the
metal–metal oxide interface where electron transfer
processes can take place. In fact, recent studies have
shown that the metal oxide can directly participate
along with the metal nanoparticles in the CO oxidation
process through concerted or sequential mechanisms
(Freund et al. 2011; Green et al. 2011; Rodriguez
2011; Widmann and Behm 2011).
Fe3O4 nanoparticles present an interesting appli-
cation as a catalyst support for CO oxidation since
the presence of the Fe2? ion in Fe3O4 provides some
reducible oxide characters. Recently, a Pd/Fe3O4
hybrid nanocatalyst has been shown to significantly
improve the oxidation of CO at temperatures
\100 �C (Chen et al. 2012). Similarly, several
reports have shown that cobalt oxide-based catalysts
are generally active toward CO oxidation; however,
their activity depends on the cobalt oxidation state
and the phase formation of cobalt under the reaction
conditions (Jernigan and Somorjai 1994; Radwan
et al. 2007; Jia et al. 2011; An et al. 2013). Co3O4
shows significant activity in the oxidation of CO at
low temperatures (Jansson 2000; Jansson et al. 2002;
Xie et al. 2009; Jia et al. 2011). Also, the Ni(OH)2-
based materials could provide catalyst support
applications for CO oxidation (Radwan et al. 2007;
Zhou et al. 2009). These materials have many
important applications such as in batteries where
they are considered the primary electrode materials
and also in supercapacitor applications (Zhou et al.
2009; Wang et al. 2010).
In this paper, we report a simple method to prepare
Pd nanoparticle catalysts supported on Fe3O4, Co3O4,
and Ni(OH)2 nanoparticles and nanoplates, and com-
pare their catalytic activities for CO oxidation. This
work introduces the method and demonstrates that the
shape and morphology of the support nanoparticles
can have a significant effect on the activity of the Pd
catalysts.
The approach utilized in the present work is based
on microwave synthesis of nanoparticles from metal
salts in solutions. Microwave irradiation (MWI)
methods provide simple and fast routes to the synthe-
sis of nanomaterials since no high temperature or high
pressure is needed. Furthermore, MWI is particularly
useful for a controlled large-scale synthesis that
minimizes the thermal gradient effects (Kappe 2004;
Glaspell et al. 2005; Abdelsayed et al. 2008, 2009;
Herring et al. 2011; Qiu et al. 2011; Siamaki et al.
2011). The heating of a substance by MWI depends on
the ability of the material (solvent or reagent) to
absorb microwave radiation and convert it into heat.
Due to the difference in the solvent and reactant
dielectric constants, selective dielectric heating can
provide significant enhancement in reaction rates. By
using metal precursors that have large microwave
absorption cross sections relative to the solvent, very
high effective reaction temperatures can be achieved.
The rapid transfer of energy directly to the reactants
(faster than they are able to relax) causes an instan-
taneous internal temperature rise. Thus, the activation
energy is essentially decreased as compared with
conductive heating, and the reaction rate increases
accordingly. Furthermore, reaction parameters such as
temperature, time, and pressure can be controlled
easily (Kappe 2004; Glaspell et al. 2005; Abdelsayed
et al. 2008, 2009; Herring et al. 2011; Qiu et al. 2011;
Siamaki et al. 2011).
2477 Page 2 of 11 J Nanopart Res (2014) 16:2477
123
Experimental
All chemicals used in the experiments were purchased
and used as received without further purifications.
Palladium nitrate (10 wt% in 10 wt% HNO3,
99.999 %), Fe(NO3)3�9H2O, Co(NO3)2�6H2O,
Ni(NO3)2�6H2O, and hydrazine hydrate (80 %) were
obtained from Sigma Aldrich. Deionized water (D.I.
H2O, *18 MX) was used for all experiments. For all
the syntheses described here, a conventional micro-
wave oven (2.45 GHz) operating at 1,000 W was
used. For the Pd/Fe3O4 system, 5, 20, 40, and 50 wt%
Pd catalysts were prepared by adding 687.2, 578.7,
434.1, 361.7 mg of Fe(NO3)3�9H2O into 97, 388, 776,
and 970 ll of the Pd nitrate solutions, respectively, in
the presence of 20 ml deionized water, and the
solutions were sonicated for 1 h followed by the
addition of 600 ll hydrazine hydrate at room temper-
ature. The reaction mixture was microwaved in 60 s
cycles for a total reaction time of 7 min. For the Pd/
Co3O4 system, 10, 20, 30, 40, and 50 wt% Pd catalysts
were prepared by adding 443.9, 394.6, 345.2, 295.9,
and 246.6 mg of Co(NO3)2�6H2O into 194, 388, 582,
776, and 970 ll of the Pd nitrate solutions, respec-
tively, in the presence of 20 ml deionized water, and
the solutions were sonicated for 1 h followed by the
addition of 600 ll hydrazine hydrate at room temper-
ature. The reaction mixture was microwaved in 60 s
cycles for a total reaction time of 5 min. For the Pd/
Ni(OH)2 system, 30, 50, and 70 wt% Pd catalysts were
prepared by adding 346.7, 247.6, and 148.6 mg of
Ni(NO3)2�6H2O into 582, 970, and 1,358 ll of the Pd
nitrate solutions, respectively, in the presence of 20 ml
deionized water, and the solutions were sonicated for
1 h followed by the addition of 600 ll hydrazine
hydrate at room temperature. The reaction mixture
was microwaved in 60 s cycles for a total reaction time
of 9 min. In all cases, the supported catalyst product
was washed using hot deionized water 2–3 times and
then ethanol 2–3 times, and finally the product was
dried in an oven at 80 �C for 2 h.
Characterization
TEM images were obtained using a Joel JEM-1230
electron microscope operated at 120 kV equipped
with a Gatan UltraScan 4000SP 4 K 9 4 K CCD
camera. X-ray diffraction (SA-XRD) patterns were
measured at room temperature with an X’Pert Philips
Materials Research Diffractometer using Cu Ka1
radiation. The X-ray photoelectron spectroscopy
(XPS) spectra were measured on a Thermo Fisher
Scientific ESCALAB 250 using a monochromatic Al
KR. All the TEM, XRD, and XPS measurements were
obtained on the as-prepared samples without any
treatment.
Catalysis measurements
For the CO catalytic oxidation, the sample was placed
inside a Thermolyne 2100 programmable tube furnace
reactor as described in the previous publications
(Glaspell et al. 2005; Abdelsayed et al. 2006; Yang
et al. 2006; Glaspell et al. 2008; Abdelsayed et al.
2009). The sample temperature was measured by a
thermocouple placed near the sample. In a typical
experiment, a gas mixture consisting of 3.5 wt% CO
and 20 wt% O2 in helium was passed over the sample,
while the temperature was ramped. The gas mixture
was set to flow over the sample at a rate of 100 cc/min
controlled via MKS digital flow meters. The conver-
sion of CO to CO2 was monitored using an infrared gas
analyzer (ACS, Automated Custom Systems Inc.). All
the catalytic activities were measured (using a 50 mg
sample) after a heat treatment of the catalyst at 110 �C
in the reactant gas mixture for 15 min in order to
remove moisture and adsorbed impurities.
Results and discussion
Figure 1a displays a typical TEM image of the
unsupported Pd nanoparticles prepared by the MWI-
assisted chemical reduction of Pd(NO3)2 using hydra-
zine hydrate (HH). As expected, in the absence of a
capping agent or a support, the nanoparticles show a
significant degree of aggregation although the primary
particles have small sizes in the range of 6–8 nm.
Figure 1b (i–iii) displays TEM images of the
Fe3O4, Co3O4, and Ni(OH)2 nanoparticles, respec-
tively, prepared using HH reduction under MWI in the
absence of Pd ions. Additional TEM images are given
in Figure S1 (Supplementary Materials). In the case of
Fe3O4, [Fig. 1b(i)], most of the nanoparticles have
irregular shapes with diameters between 25 and
30 nm, and a few have a hexagonal plate shape. This
is similar to the morphology of the Fe3O4 nanoparticle
prepared by the HH reduction of FeCl3 under MWI as
J Nanopart Res (2014) 16:2477 Page 3 of 11 2477
123
reported by Wang et al. (2007). However, for the
Co3O4 nanoparticles, more well-defined hexagonal
shape particles are formed as shown in Fig. 1b(ii) with
the angles of adjacent edges very close to 120�. For the
Ni(OH)2 nanoparticles, smaller particles (6–8 nm)
with irregular triangles and hexagonal shapes are
formed as shown in Fig. 1b(iii).
Figure 1b(iv–vi) displays the TEM images of the
Pd nanoparticles supported on the Fe3O4, Co3O4, and
Ni(OH)2 nanoplates, respectively, prepared using the
MWI-assisted chemical reduction method. It is clear
that in the three catalyst systems, most of the Pd
particles appear to be deposited on the support
nanoplates.
Figure 2a displays the XRD patterns of the as-
synthesized Pd, Fe3O4, 20 wt% Pd/Fe3O4, and
50 wt% Pd/Fe3O4 nanoparticles prepared by the HH
reduction under MWI. The Pd nanoparticles show the
typical fcc pattern of Pd of crystalline particles
(JCPDS-46-1043). The 100 % Fe3O4 nanoparticles
show the characteristic peaks for the spinal Fe3O4
phase (ICCD-00-003-0863) in addition to a small
percentage of the a-Fe2O3 phase as indicated by the 2hpeaks at 33.3 and 49.6 (JCPDS-33-0664). For the
20 wt% Pd/Fe3O4 and 50 wt% Pd/Fe3O4 nanoparti-
cles, peaks due to Pd and Fe3O4 are present with no
indication of the presence of the a-Fe2O3 phase.
To characterize the surface composition of the
supported nanocatalysts, we carried out XPS mea-
surements as shown in Fig. 2b, c for the 50 wt% Pd/
Fe3O4 catalyst. The data reveal the presence of Fe(III)
as indicated by the observed peaks at 724.2 and
710.5 eV corresponding to the binding energies of the
2p1/2 and 2p3/2 electrons, respectively. The broad
Fe(III) 2p3/2 peak centered at 710.5 eV most likely
contains contributions from the Fe(II) 2p3/2 which
normally occurs at *708 eV. For Pd, the observed
binding energies of 334.8 and 340.1 eV indicate the
presence of 82 % Pd0 and 18 % Pd2? (due to the
presence of surface layer of PdO). However, these
values are slightly lower than the binding energies of
Pd 3d electrons in pure Pd nanoparticles where the
values of 335 and 341.1 eV have been reported for Pd0
and Pd2?, respectively (Chen et al. 2012). The
decrease in the binding energy of the Pd 3d electron
in the Pd/Fe3O4 supported catalyst indicates that the
Pd in the supported catalyst is more electron rich than
in pure Pd nanoparticles. This could be due to electron
transfer from Fe2O3 to Pd consistent with similar
results obtained for the Pd/Fe2O3 hybrid nanocatalysts
prepared by a seed-mediated process (Chen et al.
2012), and also for Pd nanoparticles grown by vapor
phase deposition on ordered crystalline Fe3O4 films
(Schalow et al. 2005).
Fig. 1 a TEM of Pd nanoparticles prepared by HH reduction of Pd(NO3)2 without a support under MWI. b TEM of nanoparticles
prepared by HH reduction of i Fe3O4, ii Co3O4, iii Ni(OH)2, iv 50 wt% Pd/Fe3O4, v 30 wt% Pd/Co3O4, and (vi) 50 wt% Pd/Ni(OH)2
2477 Page 4 of 11 J Nanopart Res (2014) 16:2477
123
Figure 3a compares the catalytic oxidation of CO
over Fe3O4 and Pd nanoparticles as well as 5, 20, 40,
and 50 wt% Pd nanoparticles supported on Fe3O4. The
catalytic activities of pure Pd and pure support
nanoparticles were measured as a benchmark for
comparison with the supported catalysts.
It is clear that both the individual Fe3O4 and Pd
nanoparticles show poor activities with 100 % con-
version temperatures of 273 and 179 �C, respec-
tively. In the case of pure Pd, this is mostly due to the
aggregation of the Pd nanoparticles and the complete
coverage by CO molecules on the catalyst surface so
there are not many open sites available for the
adsorption of the O2 molecules. The data also show
that the 5 wt% Pd/Fe3O4 catalyst has lower activity
than the pure Pd nanoparticle catalyst indicating that
not enough Pd nanoparticles are interacting with the
Fe3O4 nanoparticles to create a sufficient number of
active interfaces for the CO oxidation. Increasing the
Pd wt% up to 50 % increases the activity as shown in
Fig. 3a. However, catalysts containing more than 50
wt% Pd show similar behavior to the pure Pd
nanoparticle catalyst confirming the importance of
the support in dispersing the catalyst nanoparticles
and decreasing their tendency for aggregation and
sintering. Therefore, it appears that the 50 wt% Pd/
Fe3O4 catalyst provides reasonable optimization
between the adsorption of the CO and O2 molecules
on the Pd-Fe3O4 interfaces to allow efficient oxida-
tion of CO.
To confirm the stronger interaction between the Pd
and Fe3O4 nanoparticles prepared simultaneously
using the MWI method as compared to the physical
mixing of individual Pd and Fe3O4 nanoparticles, we
measured the catalytic activity of a 50 wt% physical
mixture of Pd and Fe3O4 nanoparticles prepared
separately under identical conditions to the prepara-
tion of the supported 50 wt% Pd/Fe3O4 catalyst. As
shown in Fig. 3b, the physical mixture exhibits a
significantly lower activity than the supported catalyst
prepared simultaneously using the MWI. This pro-
vides evidence for stronger interaction between the Pd
and Fe3O4 nanoparticles, which is reflected in the
enhanced catalytic activity.
740 730 720 710 700
2P1/2 , Sat.
2P3/2 , Sat.
Fe3O4 (2P1/2 )
Inte
nsity
(a.
u.)
Binding Energy (eV)
Fe3O4(2P3/2)(b)
345 340 335 330
Pd+2 (3d3/2)
Pd0(3d3/2)
Pd +2 (3d5/2 )
Inte
nsity
(a.
u.)
Binding Energy (eV)
Pd0(3d5/2)81.6% Pd0
18.4 % Pd+2
(c)
10 20 30 40 50 60 70 80 90
* *31
1
440
511
422
400
222220
22231
1
22020
0
111
Inte
nsity
(a.
u)
2θ (degree)
Pd nanoparticles Fe3O4 nanoplates
50 wt % Pd / Fe3O4
20 wt % Pd / Fe3O4
(a)Fig. 2 a XRD patterns of
Pd nanoparticles, Fe3O4
nanoplates, 20 wt% Pd/
Fe3O4, and 50 wt% Pd/
Fe3O4 (*due to Fe2O3).
b XPS binding energy (Fe
2p region) for the 50 wt%
Pd/Fe3O4 catalyst. c XPS
binding energy (Pd
3d region) for the 50 wt%
Pd/Fe3O4 catalyst
J Nanopart Res (2014) 16:2477 Page 5 of 11 2477
123
The catalytic activity of the 50 wt% Pd/Fe3O4
catalyst prepared here using the simple one-step MWI-
assisted chemical reduction is comparable to that of
the Pd/Fe2O3 hybrid nanocatalysts prepared by a seed-
mediated process for the synthesis of Pd core nano-
particles followed by deposition of Fe3O4 surface
0 40 80 120 160 200 240 2800
20
40
60
80
100
% C
O C
onve
rsio
n
Fe3O4 nanoplates Pd nanoparticles 5 wt% Pd/Fe3O4
50 wt% Pd/Fe3O4
40 wt% Pd/Fe3O4
20 wt% Pd/Fe3O4
Catalyst Temperature (°C)Catalyst Temperature (°C)
(a)
0 40 80 120 160 200 2400
10
20
30
40
50
60
70
80
90
100
% C
O C
onve
rsio
n
(b)
In Situ MWI synthesis 50 wt% Pd + Fe3O4 mixture
50 wt% Pd/Fe3O4
Fig. 3 a CO catalytic oxidation by Pd nanoparticles supported on Fe3O4 with different compositions. b Comparison between the
activity of the 50 wt% Pd/Fe3O4 catalyst and a 50 wt% mixture of Pd and Fe3O4 nanoparticles
10 20 30 40 50 60 70 80 90
2θ (degree)
Inte
nsity
(a.
u)
(a)
400
22211
1
20142
2
10344
051
1220 31
1
22231
1
22020
011
1Pd nanoparticlesCo3O4 nanoplates
50 wt % Pd / Co3O430 wt % Pd / Co3O420 wt % Pd / Co3O4
345 340 335 330
(c)
Binding Energy (eV)
Inte
nsity
(a.
u.)
Pd+2(3d3/2)Pd+2(3d5/2)
Pd 0(3d3/2)
Pd0(3d5/2)86.26% Pd0
13.74% Pd+2
810 800 790 780 770
Co2+(2P1/2)
Co2+(2P3/2)
Co3+(2P1/2)Co3+(2P3/2)
(b)
Binding Energy (eV)
Inte
nsity
(a.
u.)
Fig. 4 a XRD patterns of
Pd nanoparticles, Co3O4
nanoplates, and Pd
supported on Co3O4
nanoplates. b XPS (Fe 2p)
and c XPS (Pd 3d) of
50 wt% Pd/Co3O4 catalyst
2477 Page 6 of 11 J Nanopart Res (2014) 16:2477
123
layers in solution and then thermal annealing at
300 �C (Chen et al. 2012). The 100 % conversion of
CO into CO2 was measured for the Pd/Fe3O4 hybrid
catalyst at 125 �C as compared to 128 �C for the
50 wt% Pd/Fe3O4 catalyst prepared in this work.
Figure 4a shows the XRD patterns of the Pd,
Co3O4, 20 wt% Pd/Co3O4, 30 wt% Pd/Co3O4, and
50 wt% Pd/Co3O4 nanoparticles prepared by the HH
reduction under MWI. The 100 % Co3O4 nanoparti-
cles show the characteristic peaks for the spinal Co3O4
phase (ICCD-00-030-044300). For the 20, 30, and
50 wt% Pd/Co3O4 nanoparticles, peaks due to Pd and
Co3O4 are present and with no indication of the
presence of other phases.
The Co and Pd XPS data for the 50 wt% Pd/Co3O4
catalyst are shown in Fig. 4b, c for the Co-2p and Pd-
3d electron binding energies, respectively. The data
reveal the presence of Co(II) and Co(III) as indicated
by the observed peaks at 796.8 and 780.7 eV corre-
sponding to the Co(II) 2p1/2 and 2p3/2 binding
energies, respectively, and at 802.6 and 786.2 eV
corresponding to the Co(III) 2p1/2 and 2p3/2 binding
energies, respectively. For Pd, the observed binding
energies of 340.7 and 335.4 eV indicate the presence
of 86 % Pd0 (3d5/2), and 342.5 and 337.2 eV indicate
the presence of 14 % Pd2? (due to the presence of PdO
on the surface).
Figure 5a compares the catalytic oxidation of CO
over Co3O4 and Pd nanoparticles as well as 10, 20, and
30 wt% Pd nanoparticles supported on Co3O4. Unlike
the pure Fe3O4 nanoparticles, the Co3O4 nanoparticles
exhibit a significant activity for CO oxidation with T50
(the temperature at which CO conversion reaches
50 %) and T100 occurring at 117 and 140 �C, respec-
tively. The same trend of increasing catalytic activity
with increasing the Pd wt% in the catalyst is also
observed in the Pd/Co3O4 system as in the Pd/Fe3O4
system. However, significant activity is observed for
the 20 wt% Pd/Co3O4 catalyst especially at lower
temperatures. For example, Fig. 5a shows a 10 %
conversion of CO into CO2 at 70 �C and a T50 of about
110 �C for the 20 wt% Pd/Co3O4 catalyst. It appears
that Co3O4 is responsible for the low temperature
oxidation of CO since this behavior is also weakly
observed in pure Co3O4 nanoparticles as shown in
Fig. 5a. It should be noted that in spite of the high
oxidation activity of pure Co3O4 nanoparticles (T100
occurring at 140 �C), the 30 wt% Pd/Co3O4 catalyst
provides slightly higher activity with T100 occurring at
127 �C. The lowest T100 obtained for the 30 wt% Pd/
Co3O4 catalyst at 127 �C is similar to that obtained for
the 50 wt% Pd/Fe3O4 catalyst. This indicates that
Co3O4 nanoparticles provide a more active support for
the Pd nanocatalyst than Fe3O4 nanoparticles, and
hence a lower amount of the Pd nanoparticles can be
used to achieve the same T100. Also, similar to the
result obtained for the 50 wt% Pd/Fe3O4 catalyst, the
physical mixture of Pd and Co3O4 nanoparticles
results in lower activity than that of the supported
50 wt% Pd/Co3O4 catalyst, as shown in Fig. 5b,
indicating that the simultaneous reduction of Pd and
Co nitrates under MWI produces supported Pd nano-
particles on the Co3O4 nanoplates and not simply a
mixture of Pd and Co3O4 nanoparticles.
Figure 6a displays the XRD patterns of the Pd,
Ni(OH)2, 30 wt% Pd/Ni(OH)2, 50 wt% Pd/Ni(OH)2,
and 70 wt% Pd/Ni(OH)2 nanoparticles prepared by
0 40 80 120 160 200 2400
20
40
60
80
100
% C
O C
onve
rsio
n
Catalyst Temperature (°C)
Catalyst Temperature (°C)
(b)50 wt% Pd/Co3O4In Situ MWI synthesis 50 wt% Pd + Co3O4 mixture
0 40 80 120 160 2000
20
40
60
80
100 (a)
% C
O C
onve
rsio
n
Pd nanoparticles 10 wt% Pd/ Co3O4
20 wt% Pd/ Co3O4
30 wt% Pd/ Co3O4
Co3O4 nanoplates
Fig. 5 a CO catalytic oxidation by Pd nanoparticles supported
on Co3O4 with different compositions. b Comparison between
the activity of the 50 wt% Pd/Co3O4 catalyst and a 50 wt%
mixture of Pd and Co3O4 nanoparticles
J Nanopart Res (2014) 16:2477 Page 7 of 11 2477
123
the HH reduction under MWI. The 100 % Ni(OH)2
nanoparticles show the characteristic peaks for the
hexagonal phase (ICCD-00-001-1047). Small diffrac-
tion peaks due to Ni(OH)2 can be observed in the XRD
pattern of the 30 wt% Pd/Ni(OH)2 sample as shown in
Fig. 6a. However, for samples containing more than
30 wt% Pd, the XRD patterns are dominated by weak
and broad Pd diffraction peaks. This may suggest the
partial formation of a Pd–Ni alloy with the main
diffraction peak between the 2h values corresponding
to the 111 and 101 planes of Pd and Ni(OH)2,
respectively. The formation of bimetallic alloys under
the microwave-assisted synthesis of metal nanoparti-
cles has been previously reported (Abdelsayed et al.
2009).
Figure 6b shows that the binding energies of the Ni
2p1/2 and 2p3/2 electrons are 860.7 and 878.8 eV,
respectively, indicating that Ni is present as Ni(OH)2
but there are peaks at 872.1, 853.3, and 855.2 eV
indicating the presence of NiO at the surface of the
nanoparticles.
The Pd XPS spectra of the 50 wt% Pd/Ni(OH)2
catalyst shown in Fig. 6c indicate the presence of only
69 % Pd0 and higher percentage of Pd2? as compared
to the other Pd catalysts supported on Fe3O4 and
Co3O4 nanoplates. This could indicate inefficient
reduction of the Pd ions in the presence of Ni(OH)2
under the microwave conditions. The partial formation
of an oxidized Pd–Ni alloy suggested by the XRD
results would also decrease the percentage of Pd0 in
the Pd/Ni(OH)2 catalyst.
Figure 7a compares the catalytic oxidation of CO
over Ni(OH)2 and Pd nanoparticles as well as 30, 50,
and 70 wt% Pd nanoparticles supported on Ni(OH)2.
The pure Ni(OH)2 nanoparticles show low activity for
CO oxidation with T50 and T100 occurring at 213 �C
and 244 �C, respectively. This is very different from
the activity of the Co3O4 nanoplates which exhibit
880 870 860 850
(b)
Binding Energy (eV)
Inte
nsity
(a.
u.) Ni(OH)2(2p1/2 ) NiO(2p1/2) Ni(OH)2(2p3/2 )
NiO(2p3/2)
345 340 335
(c)
Inte
nsity
(a.
u.)
Binding Energy (eV)
Pd+2 (3d3/2)Pd+2 (3d5/2)
Pd0 (3d3/2)
Pd0 (3d5/2)
69.48% Pd0
30.52% Pd +2
10 20 30 40 50 60 70 80 90
(a)
2θ (degree)
Inte
nsity
(a.
u)
001
100 10
1
102
11111
0
201
103
22231
1
22020
011
1
Pd nanoparticlesNi(OH)2 nanoplates
70 wt % Pd / Ni(OH)2
50 wt % Pd / Ni(OH)2
30 wt % Pd / Ni(OH)2
Fig. 6 a XRD patterns of Pd nanoparticles, Ni(OH)2 nanoplates, and Pd supported on Ni(OH)2 nanoplates. b XPS (Ni 2p) and c XPS
(Pd 3d) of 50 wt% Pd/Ni(OH)2 catalyst
2477 Page 8 of 11 J Nanopart Res (2014) 16:2477
123
higher activities for CO oxidation with T50 and T100
occurring at 117 and 140 �C, respectively. The
activity of Pd/Ni(OH)2 nanoparticles does not show
much improvement over that of the unsupported Pd
nanoparticles. For example, the best Pd/(NiO)2 cata-
lyst (50 wt% Pd) has T50 and T100 occurring at 162 and
168 �C, respectively, which are not significantly
different from the T50 and T100 of unsupported Pd
nanoparticles (150 and 179 �C, respectively). This
indicates a lack of favorable interactions between the
Pd nanoparticles and the Ni(OH)2 support. This is
consistent with the similar activity shown for the
50 wt% Pd/Ni(OH)2 catalyst and the physical mixture
of Pd and Ni(OH)2 nanoparticles as shown in Fig. 7b,
indicating that the simultaneous reduction of Pd and
Ni nitrates under MWI produces weakly interacting Pd
nanoparticles and Ni(OH)2 nanoplates without strong
catalyst-support interactions. This is also consistent
with the stronger interactions between the Pd nano-
particles, and the iron or cobalt oxide nanoparticles
which can directly enhance the activity of the
supported Pd/Fe3O4 and Pd/Co3O4 catalysts for the
catalytic oxidation of CO.
Table 1 summarizes the catalytic activities of the
Pd nanoparticle catalysts supported on Fe3O4, Co3O4,
and Ni(OH)2 nanoplates, and Fig. 8 illustrates the
effect of the Pd loading on the oxidation activities of
the three catalyst systems. Both the Pd/Fe3O4 and Pd/
Co3O4 catalysts show high activity mainly due to
stronger interactions between the Pd nanoparticles and
the Fe3O4 and Co3O4 nanoplates. The Pd/Co3O4
catalyst shows improved activity over the Pd/Fe3O4
catalyst as indicated by achieving the same T100 of
Table 1 Temperature at which CO conversion reaches 50 %
(T50) and 100 % (T100) for Pd, Fe3O4, Co3O4, and Ni(OH)2
nanoplates as well as the supported catalysts Pd/Fe3O4, Pd/
Co3O4, Pd/Ni(OH)2
T50 % (�C) T100 % (�C)
wt% Pd/Fe3O4
5 200 204
10 175 180
20 153 157
40 125 130
50 116 127
80 160 168
wt% Pd/Co3O4
5 170 173
10 127 164
20 105 151
30 113 127
40 125 135
50 148 165
wt% Pd/Ni(OH)2
5 210 240
10 190 206
30 181 188
50 162 168
70 174 184
Pd nanoparticles 150 179
Fe3O4 nanoplates 264 273
Co3O4 nanoplates 117 140
Ni(OH)2 nanoplates 213 244
40 80 120 160 200 2400
20
40
60
80
100 (b)
% C
O C
onve
rsio
n
In Situ MWI synthesis 50 wt% Pd + Ni(OH)2 mixture
50 wt% Pd/Ni(OH)2
0 40 80 120 160 200 2400
10
20
30
40
50
60
70
80
90
100
Catalyst Temperature (°C)
Catalyst Temperature (°C)
% C
O C
onve
rsio
n(a)
50 wt% Pd/Ni(OH)2
70 wt% Pd/Ni(OH)2
30 wt% Pd/Ni(OH)2
Ni(OH)2 nanoplates
Pd nanoparticles
Fig. 7 a CO catalytic oxidation by Pd nanoparticles supported
on Ni(OH)2 with different compositions. b Comparison between
the activity of the 50 wt% Ni(OH)2 catalyst and a 50 wt%
mixture of Pd and Ni(OH)2 nanoparticles
J Nanopart Res (2014) 16:2477 Page 9 of 11 2477
123
127 �C for the 50 wt% Pd/Fe3O4 catalyst but with
only 30 wt% Pd in the Pd/Co3O4 catalyst. This can be
attributed to the favorable oxidation activity of the
Co3O4 support where 30 wt% Pd may represent the
optimum composition needed to maximize the activ-
ities of both Pd and Co3O4 nanoparticles. It appears
that when the Pd loading is higher than 40 %, the
agglomeration of the Pd nanoparticles decreases the
adsorption of CO and O2 on the active sites of the
Co3O4 nanoplates, and this leads to lower activity. The
lower activity of the Pd/Ni(OH)2 catalyst could be due
to a weaker catalyst-support interaction that may result
from the partial formation of Pd–Ni alloy with a few
active sites for CO oxidation as compared to Pd
nanoparticles.
It should also be noted that the Pd catalysts
supported on Fe3O4, Co3O4, and Ni(OH)2 nanoplates
were not pre-reduced prior to the CO oxidation
measurements and as indicated by the XPS results,
the Pd2? species vary from 18, 14 to 31 % in 50 wt%
Pd catalysts supported on Fe3O4, Co3O4, and Ni(OH)2
nanoplates, respectively. This shows that the pre-
reduction step is not necessary in these catalysts which
could simplify their practical uses.
Conclusions
In conclusion, a simple, versatile, and rapid method
has been developed for the synthesis of Pd
nanoparticle catalysts supported on Fe3O4, Co3O4,
and Ni(OH)2 nanoplates via MWI. The important
advantage of microwave dielectric heating over con-
vective heating is that the reactants can be added at
room temperature (or slightly higher temperatures)
without the need for high-temperature injection.
Furthermore, the same method can be used to synthe-
size bimetallic nanoalloys supported on metal oxide
nanoparticles as nanocatalysts for CO oxidation. The
current results reveal that the Pd/Co3O4 catalyst has
particularly high activity for CO oxidation as a result
of the strong interaction between the Pd nanoparticles
and the Co3O4 nanoplates. Optimizations of the size,
composition, and shape of these catalysts could
provide a new family of efficient nanocatalysts for
the low temperature oxidation of CO.
Acknowledgments We thank the National Science
Foundation (CHE-0911146 and OISE-1002970) for the
support of this work.
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