Upload
buihanh
View
215
Download
1
Embed Size (px)
Citation preview
University of Southampton Research Repository
ePrints Soton
Copyright © and Moral Rights for this thesis are retained by the author and/or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder/s. The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders.
When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given e.g.
AUTHOR (year of submission) "Full thesis title", University of Southampton, name of the University School or Department, PhD Thesis, pagination
http://eprints.soton.ac.uk
University of Southampton
Faculty of Engineering, Science and Mathematics
School of Chemistry
High-Throughput Synthesis and Screening of Binary
Alloys for Hydrogen Evolution and Oxidation
Reactions
By
Faisal A. Al Odail
Thesis for the degree of Doctor of Philosophy
June 2010
I
Abstract
High throughput methods were employed for the physical vapour deposition (PVD) and
electrochemical screening of random and equilibrated (annealed at 300 ºC for 15
minutes) binary metal alloys for the hydrogen evolution reaction (HER) and the
hydrogen oxidation reaction (HOR). Results are presented for the Pd-Au, Pd-Bi and Ru-
Au alloy systems. Thin films of each alloy system were synthesized on a series of 10x10
array electrodes with a graded composition. A variety of analytical techniques including
Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS)
and powder X-ray Diffraction (XRD) were used for the analysis of bulk composition,
surface composition and structure respectively.
The compositional analysis reveals that alloy formation and synthesis of nearly the
whole compositional range of the alloy systems were achieved. Surface segregation of
Au in the Pd-Au alloy system and Ru in the Ru-Au alloy system was observed. The
surface segregation in the equilibrated Pd-Bi alloy system did not take place.
The HER and HOR activity on all the examined alloy systems exhibit a similar
compositional dependence. This suggests that the HER activity provides a good
descriptor for the HOR activity for systems with low overpotentials. An exception of
this occurs, however, at high concentrations of Au and Bi where the HER activity
decreases monotonically towards 100 at. %, while the HOR activity decreases more
rapidly at alloy compositions of ca. 90 at. %. An enhancement in the activity was found
on a variety of alloy compositions over the constituent components. The optimum Pd-Au
alloy composition for the HER and HOR was found to be at a composition of ca.
Pd50Au50 (more active than pure Pd). A comparable activity to pure Pd was found on Bi-
rich alloys (ca. Pd25Bi75) in the Pd-Bi alloy system. The activity on ca. Ru90Au10 and 60-
80 % Au was found to be higher than pure Ru.
The CO-tolerance in the HOR along the whole compositional range of each alloy system
was also assessed in the presence of a mixture of hydrogen and 500 ppm CO. The results
suggest that the Ru-Au alloy system is more CO tolerant than the other two systems.
II
Contents
Abstract I
Contents II
Declaration of Authorship VI
Acknowledgments VII
Table of Abbreviations VIII
Chapter 1: Introduction 1
1.1- Electrocatalysis: Principles and Applications 1
1.2- Fuel Cells 3
1.2.1- Fundamentals 3
1.2.2- Polymer Electrolyte Membrane Fuel Cell (PEMFC) 8
1.3- Electrocatalysis on Alloy Surfaces 10
1.3.1- Advantages of Electrocatalysis by Alloys 11
1.3.2- Enhancement of Catalytic Processes on Metal Alloys 15
1.3.3- Bulk & Surface Composition of Alloys 16
1.3.4- High Throughput Synthesis and Screening of Metal Alloys for
Electrocatalysis
17
1.4- The Hydrogen Evolution Reaction (HER)/Hydrogen Oxidation Reaction
(HOR)
19
1.4.1- Reaction Mechanisms 19
1.4.2- Alloy Catalysts for the HER/HOR in Acids 21
1.5- Aims of the Project 24
III
Chapter 2: Experimental 25
2.1- The High-Throughput Physical Vapor Deposition (HT-PVD) System 25
2.2- Sample Preparation 30
2.2.1- Sample Calibration 30
2.2.2- Electrochemical Array 31
2.3- Analytical Tools 32
2.3.1- Powder X-ray Diffraction (XRD) 32
2.3.2- Energy Dispersive X-ray Spectroscopy (EDS) 34
2.3.3- X-ray Photoelectron Spectroscopy (XPS) 35
2.4- Electrochemical Measurements 38
2.4.1- Electrochemical Cell 38
2.4.2- Array Measurements 40
2.4.2.1- Cyclic Voltammetry Measurements 40
2.4.2.2- Potential Step Measurements 42
Chapter 3: Palladium-Gold (Pd-Au) Alloy Surfaces 43
3.1- Introduction 43
3.1.1- Industrial Applications of Pd-Au Catalysts 45
3.1.2- Electrochemical Applications of Pd-Au Catalysts 46
3.2- Composition and Structure Analysis 48
3.2.1- EDS Analysis 48
3.2.2- XRD Analysis 49
3.2.3- XPS Analysis 54
3.3- Base Voltammetry and CO Stripping Measurements 58
3.4- The Catalytic Activity for the HER 73
3.5- The Catalytic Activity for the HOR 82
IV
3.6- The Carbon Monoxide Tolerance during the HOR 88
3.7- Conclusions 92
Chapter 4: Palladium-Bismuth (Pd-Bi) Alloy Surfaces 94
4.1- Introduction 94
4.1.1- Industrial Applications of Pd-Bi Catalysts 95
4.1.2- Applications of Pd-Bi Catalysts in Electrocatalysis 97
4.2- Composition and Structure Analysis 98
4.2.1- EDS Analysis 98
4.2.2- XRD Analysis 99
4.2.3- XPS Analysis 103
4.3- Base Voltammetry and CO stripping Measurements 109
4.4- The Catalytic Activity for the HER 116
4.5- The Catalytic Activity for the HOR 118
4.6- Conclusions 123
Chapter 5: Ruthenium-Gold (Ru-Au) Alloy Surfaces 125
5.1- Introduction 125
5.2- Composition and Structure Analysis 127
5.2.1- EDS Analysis 127
5.2.1- XRD Analysis 128
5.2.3- XPS Analysis 130
5.3- Base Voltammetry and CO stripping Measurements 135
5.4- The Catalytic Activity for the HER 140
5.5- The Catalytic Activity for the HOR 144
5.6- Conclusions 150
V
Chapter 6: Conclusions and General Discussions 153
6.1- Sample Characterization 153
6.2- The HER and HOR Activity 154
6.3- Suggestions for Further Studies 157
References 158
VI
Declaration of Authorship
I, Faisal A. Al Odail, declare that the thesis entitled:
High-Throughput Synthesis and Screening of Binary Alloys for Hydrogen
Evolution and Oxidation Reactions
and the work presented in the thesis are both my own, and have been generated by me as
the result of my original research. I confirm that:
This work was done wholly while in candidature for a research degree at this
university.
Where I consulted the published work of others, this is always clearly attributed.
Where I have quoted from the work of others, the source is always given. With the
exception of such quotations, this thesis is entirely my own work.
I have acknowledge all main sources of help.
Parts of this work have been prepared for publication as:
1- Faisal A. Al Odail, Alexandros Anastasopoulos, and Brian E. Hayden "The Hydrogen
Evolution Reaction and Hydrogen Oxidation Reaction on Thin Film PdAu Alloy
Surfaces", submitted to Physical Chemistry Chemical Physics.
2- Faisal A. Al Odail, Alexandros Anastasopoulos, and Brian E. Hayden "Hydrogen
Evolution and Oxidation Reactions on Palladium-Bismuth Alloys ", to be published.
3- GB Patent Application: Alexandros Anastasopoulos and Brian E. Hayden "Alloys for
Hydrogen Oxidation and Water Reduction", March 2010.
Signed:
Date:
VII
Acknowledgments
Thanks to all people who have helped me during my PhD study at the University of
Southampton. I would like to express my sincere thanks to Professor Brian E. Hayden
for giving me the opportunity to do my PhD research in his laboratory, helpful
discussions, lessons related to my area of research and his encouragement to carry on my
work. I would also like to express my sincere thanks to Professor John M. Dyke for his
advises and helpful discussions.
Thanks to the Government of Saudi Arabia for the PhD scholarship. I also thank the
European Union (EU) for funding this project.
A special thanks to Alexandros Anastasopoulos for training me on the instruments and
helping me in the experimental work, data analysis, computing, and also for helpful
discussions. Thanks to Robert Noble for his help especially with computing and writing
a script for surface atoms population. I also thank Mehdi Mirsaneh for helpful
discussions. Thank you to all other people that I have met in the surface science and
heterogeneous catalysis group: Laura W., Jens-Peter, Ben W, Duncan S., Rafael C-M.,
John B., Talal G., Louise H., Abdulrahman H. and Daniel C. I also thank Audrey, Naruo
Y., Piers, Claire M. for their help.
This work is for my parents, wife and children. I am very grateful to them for their
support in my various endeavours. I also thank and offer my regards to the rest of my
family.
VIII
Table of Abbreviations
High throughput Physical Vapor Deposition HT-PVD
Ultrahigh Vacuum UHV
Hydrogen Evolution Reaction HER
Hydrogen Oxidation Reaction HOR
Oxygen Reduction Reaction ORR
Oxygen Evolution Reaction OER
Proton Exchange Membrane Fuel Cell PEMFC
Polymer Electrolyte Membrane Fuel Cell PEMFC
Solid Polymer Electrolyte Fuel Cell SPEFC
Direct Methanol Fuel Cell DMFC
Alkaline Fuel Cell AFC
Phosphoric Acid Fuel Cell PAFC
Molten Carbonate Fuel Cell MCFC
Solid Oxide Fuel Cell SOFC
Membrane Electrode Assembly MEA
Polytetrafluoroethylene PTFE
Catalytic Preferential Oxidation CPOX
Underpotential Deposition UPD
Energy Dispersive X-ray Spectroscopy EDS
X-ray Energy Dispersive Spectrometer XEDS
X-ray Photoelectron Spectroscopy XPS
Electron Spectroscopy for Chemical Analysis ESCA
X-ray Diffraction XRD
IX
Low Temperature K-Cells LTKS
High Temperature K-Cells HTKS
Multichannel Analyzer MCA
Working Electrode WE
Reference Electrode RE
Counter Electrode CE
Reversible Hydrogen Electrode RHE
Rotating Disk Electrode RDE
Cyclic Voltammetry CV
Initial Potential Ein
Lower Potential Elo
Upper Potential Eup
Platinum Group Metals PGM
Hydrodesulfurization HDS
Vinyl Acetate Monomer VAM
Face Centered Cubic FCC
Hexagonal Close Packed HCP
Binding Energy BE
Monolayer ML
1
Chapter 1: Introduction
1.1- Electrocatalysis: Principles and Applications
Catalysis is an important phenomenon for industrial and economical considerations. An
enormous number of chemical and biochemical processes require a catalyst to be
completed. The use of catalysts is also essential to reduce air and water pollution such as
reducing emission of nitrogen monoxide (NO) from cars [1, 2]. Therefore, catalysis is an
important tool to approach ˝Green Chemistry˝ [3].
A catalyst is a material which can be added to a chemical process to accelerate the
reaction without being consumed during the process [4]. There are two common
categories of catalysis, homogenous and heterogeneous. In a homogeneous catalytic
process, the catalyst and the reactants as well as the products are in the same phase.
Examples of this type of catalyst are acids and bases, enzymes, transition metal ions and
alkyls. On the other hand, the phase of the catalyst in heterogeneous catalysis differs
from that of the reactants. Typically, a heterogeneous catalysis refers to a process in
which the catalyst is a solid (transition metal) and the reactants are gases [3, 4]. The role
of a catalyst in a catalyzed process has been argued along the history of catalysis. It has
been suggested that the presence of the catalyst may: (i) initiate the reaction, (ii) stabilize
the intermediates of the reaction, (iii) hold the reactants in close proximity and in the
right configuration to react (reducing the entropy of activation), (iv) facilitate bond
breaking by stretching them (reducing the energy of activation), (v) transfer energy into
reactants required to activate molecules, (vi) block undesired reactions, and (vii) donate
and accept electrons [4]. Nevertheless, the overall result is that the presence of the
catalyst accelerates the approach to equilibrium by providing an alternative pathway
with a lower activation barrier (energy) compared to that of the uncatalyzed reaction [2].
Electrocatalysis can be described as a combination between the principles of
electrochemistry and catalysis whereby the presence of an electrocatalyst enhances the
rate of an electrochemical reaction at the anode or the cathode [5, 6]. The increase in the
rate of the reaction can be realized by the increase in the exchange current density (jo) at
2
a fixed potential or by the decrease in the overpotential (η) of the electrochemical
reaction at a fixed current density (j) [5, 7]. The electrocatalyst can be either the
electrode material or an adsorbed species from the solution on the electrode surface. An
example of the latter type is in the oxidation of toluenes to benzaldehydes which takes
place at high overpotentials. It was found that the addition of a number of metal ions
Mn+ (such as Ag+, Ce3+, Mn2+, and Co2+) to the electrolyte lowers the overpotential of
the process [8].
The origin of electrocatalytic studies was as early as 1920s, when Bowden and Rideal
measured the rate of hydrogen evolution reaction (HER) on a series of metals [9, 10].
However, the first use of the term ˝electrocatalysis˝ has been traced back to Kobozev
and Monblanova in 1930s [9, 11]. The main application of electrocatalysis today is in
fuel cell technology, especially those incorporating a proton exchange membrane (PEM)
[12]. However, there are other possible applications which aim to reduce environmental
pollution. Examples of some applications of electrocatalysis are outlined in Figure 1.1
[12, 13]. More detail regarding these applications and others can be found in the latter
two references.
Figure 1.1: An outline of a number of applications of heterogeneous electrocatalysis [12, 13].
Applications of Electrocatalysis
Fuel Cell Technology Research Environmental Protection
Solid Oxide Fuel Cell
Phosphoric Acid Fuel Cell
Polymer Electrolyte Membrane Fuel Cell
Alkaline Fuel Cell
Molten Carbonate Fuel Cell
Electrosynthesis of H2O2 (safer than any other oxidizing agent)
Electrosynthesis of ClO2 as an alternative
to Cl2 (less hazard)
Oxidation of SO2 to H2SO4
Conversion of H2S into H2 and S
Destruction of organic pollutants from the aquatic environment
3
The nature of electrode material controls the kinetics and mechanism of the
electrochemical reaction [5]. There are several materials which can be utilized as
electrocatalysts including single metals, alloys, two component catalysts (such as using
Sn [14] or As [15] ad-atoms to enhance the electrocatalytic performance of Pt for CO
oxidation), oxides, or transition metal complexes [5, 8]. The choice of the electrode
material ˝electrocatalyst˝ is crucial and a number of factors should be considered in this
regard. An effective electrocatalyst should:
1- Improve the rate of the reaction under investigation and, equivalently, prevent the
undesired reactions [9]. An example of that is in a chlor-alkali cell where a proposed
catalyst should catalyze Cl2 evolution and, at the same time, inhibit the O2 evolution [5].
2- Resist cracking (physical stability) and corrosion (chemical stability) in the
experimental environment [5, 9, 16].
3- Be economically viable [9].
4- Be environmentally friendly (non-pollutant) [9].
5- Achieve a degree of CO tolerance in low temperature fuel cells [17].
This work is primarily related to electrocatalysis in fuel cell technology.
1.2- Fuel Cells
1.2.1- Fundamentals
The rapid progress in mobile communications requires the development of improved
power sources to solve the performance shortfall in the lithium ion and nickel metal
hydride batteries [18]. Also, the massive day-to-day use of electricity makes it important
to convert chemical energy into electrical energy. Furthermore, the production of
electricity and the propulsion of vehicles are, currently, based on the combustion of
fossil fuels (coal, oil, and natural gas). This process releases a number of pollutants into
the air including various metals, sulfur and nitrogen. The reaction of sulfur and nitrogen
with oxygen produces SO2 (the oxidation of SO2 could cause acid rain) and NOx (toxic
gas) respectively [13, 19]. The environmental problems associated with the use of fossil
4
fuels have dictated the search of alternatives. Fuel cell technology has been suggested to
meet all the above requirements [18-20].
A fuel cell is a device that converts the chemical energy released from reactions at the
anode and the cathode into electrical energy [21]. The discovery of fuel cell technology
is attributed to Professor Sir William Grove in 1839 who carried out measurements on
the dissociation of water into hydrogen and oxygen observing that the process is
reversible and that an electric current is produced with the recombination of the
components to form water [22].
Fuel cells are analogous to batteries in that the chemical energy is converted into
electrical energy through reactions at the anode and cathode (redox reactions). However,
the process of storage and energy conversion is different. In batteries, energy storage and
conversion occur internally (closed system) where the charge is produced by the
oxidation and reduction of the anode and cathode respectively which implies that the
anode and cathode are active masses in the redox process. On the other extreme, fuel
cells are open systems where the reactants ˝active masses˝ are supplied from external
sources (such as air or a tank) to the anode and cathode which means that the anode and
cathode are not active masses in the redox process. Thus, energy storage (in the tank) is
separated from energy conversion (in the cell) [23].
Following are the main advantages of fuel cell technology [20, 23-26]:
1- It is regarded as a preferable energy source to fossil fuel. This is because the use of
fuel cells could achieve zero-emission electricity generation and, therefore, reduces
environmental pollution.
2- High-efficient energy conversion.
3- Produces high power density.
3- Reduces noise pollution.
4- Requires low maintenance.
5- Offers a degree of flexibility, since it can be applied in various applications such as
transportations, electronics, or to supply electricity to a home or factory.
5
6- It may play an important role in establishing a hydrogen fuel economy.
7- Clean power generator.
There are, on the other hand, disadvantages of fuel cell technology such as [23]:
1- Limited availability.
2- Complexity in operation.
3- The presence of impurities in gas stream could influence the performance and life of
the cell.
4- High-costs of the units.
There are various types of fuel cells under research and development. Table 1.1
summarizes the common types and a number of their features [19, 20, 22, 27]. The
classification of fuel cells is based on either the type of the electrolyte used in the cell
(excluding the DMFC) or the operating temperature of the cell. Based on the operating
temperatures, the fuel cells can be classified into two categories: low temperature fuel
cells (PEMFC, DMFC, AFC and PAFC) and high temperature fuel cells (MCFC and
SOFC) [19].
6
Table 1.1: The common types of fuel cells [19, 20, 22, 27]
Type Common
abbreviation Fuel Electrolyte
Operating temperature, ºC
Polymer Electrolyte Membrane
PEMFC H2, CH3OH proton conducting polymer
50-125
Direct Methanol DMFC CH3OH sulphuric acid or solid polymer
50-120
Alkaline AFC H2 potassium hydroxide
50-90
Phosphoric Acid PAFC H2 orthophosphoric acid
190-210
Molten Carbonate MCFC hydrocarbons, CO
lithium/potassium carbonate mixture
630-650
Solid Oxide SOFC hydrocarbons, CO
stabilized zirconia 900-1000
Although the electrolyte is varied from a fuel cell to another, the fundamental principles
of how the electrical power is produced remain the same. A simple fuel cell system
consists of an anode, a cathode and an ion conducting electrolyte (Figure 1.2 [19, 22]).
The three components together are known as the membrane electrode assembly (MEA).
The anode compartment is fed with a fuel (typically hydrogen) and the cathode
compartment is fed with an oxidant (typically oxygen). Accordingly, catalyzed chemical
reactions take place at the anode and the cathode giving rise to ions which migrate
through the electrolyte carrying electric current to the other half cell. The anodic and
cathodic reactions for the different types of fuel cells are shown in Table 1.2. Typically,
hydrogen is oxidized at the anode and oxygen is reduced at the cathode. The electrolyte
allows ions to migrate through it and, simultaneously, acts as a barrier to gas diffusion.
The electrical power is generated by the flow of electrons from the anode compartment
to the cathode compartment through an external load (electrons are always produced at
the anode and consumed at the cathode) [19, 20, 22, 27].
7
Figure 1.2: A schematic representation of the components of a fuel cell [19, 22].
Table 1.2: The chemical reactions on the anode and cathode of the common fuel cells [19, 20, 27]
Fuel Cell Anode reaction Mobile ion Cathode reaction
PEMFC H2 → 2H+ + 2e- H+ → O2 + 4H+ + 4e- → 2H2O
DMFC CH3OH + H2O → CO2 + 6H+ + 6e-
H+ → O2 + 4H+ + 4e- → 2H2O
AFC H2 + 2OH- → 2H2O + 2e- ← OH- O2 + 2H2O + 4e- → 4OH-
PAFC H2 → 2H+ + 2e- H+ → O2 + 4H+ + 4e- → 2H2O
MCFC H2 + CO3
2- → H2O + CO2 + 2e-
CO + CO3 2- → 2CO2 + 2e-
← CO3 2- O2 + 2CO2 + 4e- → CO3
2-
SOFC
H2 + O2 2- → H2O + 2e-
CO + O2 2- → CO2 + 2e-
CH4 + O2 2- → 2H2O + CO2 + 8e-
← O2 2- O2 + 4e- → 2O2
2-
External load
Cat
hod
e- e-
Catalyst layers
An
ode
Electrolyte
8
Further detail regarding the PEMFC will be given in the following section due to its
relevant to this project.
1.2.2- Polymer Electrolyte Membrane Fuel Cell (PEMFC)
The Polymer Electrolyte Membrane Fuel cell (PEMFC) is also known as Proton
Exchange Membrane Fuel Cell or as Solid Polymer Electrolyte Fuel Cell (SPEFC) [19].
It was invented in 1960s by General Electric for a spacecraft and is, currently,
considered as the most promising fuel cell for the various applications owing to its lower
cost and ability to produce higher power density compared to the other types of fuel cells
(excluding AFC) [28]. The significant advantage of the PEMFC is the use of a solid
polymer electrolyte as proton exchange membrane [19]. This type of electrolyte
eliminates the concerns associated with the liquid electrolytes such as corrosion [28].
These membranes are, however, stable in a small temperature range which consequently
imposes the low operating temperature in the PEMFC [19, 28]. Typically, Nafion
produced by DuPont is used as an electrolyte in PEMFC [20]. The structure of this
electrolyte consists of a polytetrafluoroethylene (PTFE) which is chemically inert for
reduction and oxidation processes taking place in the cell (hydrophobic region), and
sulphate ions which participate in the proton (H+) exchange process (hydrophilic region)
[19]. The hydrophobic region provides chemical stability, while the hydrophilic region
allows proton conductivity [20]. However, the Nafion polymers are expensive and the
development of cheaper materials (such as hydrocarbon-based membranes) is a matter
under consideration in the recent PEMFC research [29]. A proposed electrolyte
membrane is supposed to be: (i) fast proton transporter; (ii) low gas permeable; and (iii)
mechanically, chemically and thermally stable [29].
The PEMFC uses hydrogen as fuel at the anode and oxygen or air as oxidant at the
cathode [30]. However, employing pure hydrogen is impractical due to its high cost
[31]. Methanol, gasoline, or natural gas is alternatively reformed to produce a mixture of
gases containing 40-70 % hydrogen (reformate) because of their lower cost [30, 31]. The
following equations exemplify the sequence of methanol reforming [31]:
9
CH3OH → CO + 2H2 (1.1)
CO + H2O → CO2 + H2 (1.2)
Besides hydrogen, the reformate contains 15-25% CO2, 1-2% CO as well as other
impurities such as H2S and NH3 [30, 32]. The high level of CO in the mixture influences
the catalyst performance in the cell and, therefore, must be lowered to below 100 ppm
[30, 31]. This can be achieved by passing the reformate through a catalytic preferential
oxidation (CPOX) reactor in order to oxidize CO to CO2 prior to entering the anode
compartment [30]. In recognition of this, the catalytic conversion of CO to CO2 has been
the subject of several investigations [33-46]. The mutual objective of them was to
develop proper and economically viable catalysts that enhance this process. Au nano-
particles supported on oxides, such as Au/TiO2, are reported as promising catalysts for
low temperature CO oxidation [47-49].
The anodic reaction in PEMFC is hydrogen oxidation (1.3), and the cathodic reaction is
oxygen reduction (1.4). The overall reaction (1.5) in the cell results in water [19]:
2H2 → 4H+ + 4e- (1.3)
O2 + 4H+ + 4e- → 2H2O (1.4)
2H2 + O2 → 2H2O (1.5)
The anodic hydrogen oxidation reaction (HOR) will be discussed later in more detail due
to its relevance to this work. Both reactions are catalyzed in practical PEMFC by Pt [50],
but its high cost and low abundance provide a major hurdle to commercial
implementation. Developing low-cost, high-efficiency alternatives to Pt is therefore an
ongoing area of research. One of the most researched alternatives to pure Pt is metal
alloys.
10
1.3- Electrocatalysis on Alloy Surfaces
The term ˝Alloy˝ should be distinguished from the term ˝Intermetallic Compound˝. An
alloy can be defined as ˝a metallic system containing two or more components,
irrespective of their intimacy of mixing or, precise manner of mixing˝ [51]. It can further
be expanded to contain non-metals as follows ˝a material consisting of two or more
metals (e.g. brass is an alloy of copper and zinc) or a metal and a nonmetal (e.g. steel is
an alloy of iron and carbon, sometimes with other metals included)˝ [52]. An alloy
system can thus be either a monophasic when the metallic components are completely
miscible forming a continuous series of solid solutions, or biphasic at the critical
temperature (when the components are not in complete miscibility and the system is
segregated into distinct phases) [51, 53]. On the other hand, an intermetallic compound
is ˝a chemical compound of two or more metallic elements and adopts an – at least
partly- ordered crystal structure that differs from those of the constituent metals˝ [54].
An intermetallic compound is thus a single phase system [54]. Based on the mixing
enthalpy change, alloys can be divided into a number of categories as revealed in Table
1.3 [51, 55-57].
Table 1.3: The behavior of metallic systems based on the mixing enthalpy changes.
∆H Category of the system Example Comment
Very near to zero Nearly ideal solid solutions Au-Ag The difference in atomic radii is negligible.
Small and negative Nearly ideal or regular solid solutions
Pt-Cu The difference of atomic radii in a regular solid solution is not negligible.
Large and negative Intermetallic compounds or ordered solutions
Pt-Sn -
Small and positive Mono or biphasic alloy Ni-Cu The type depends on the temperature of equilibration.
Large and positive Surface alloy can only be formed
Ru-Cu The solubility between the elements is very limited.
11
1.3.1- Advantages of Electrocatalysis by Alloys
Electrocatalysis by alloys can be beneficial in a number of aspects.
A- Economical Benefits
One of the main challenges in fuel cell technology is how to reduce the costs of
fabrication and the materials used in fuel cells [23]. The employment of Pt as catalysts
for the anode and cathode in fuel cells is costly. The price of Pt (5th October 2009,
www.platinum.matthey.com) is compared to a number of selected metals in Figure 1.3.
The development of low-cost non-Pt catalysts is, therefore, economically more favorable
[58]. However, the development of a cheaper electrocatalyst should be associated with a
good catalytic performance [59].
Figure 1.3: A column chart representing the prices ($) per kg of a number of metals. Data of Pt, Rh, Pd, Ru, and Ir were taken from www.platinum.matthey.com on the 5th of October 2009. The prices of Au and Ag were taken from www.goldprice.net on the 5th of October. The price of Bi ($20.944, 30th of September 2009) was taken from www.minormetals.com.
0
10000
20000
30000
40000
50000
60000
Pt Rh Pd Ru Ir Au Ag Bi
$ / kg
12
In relation to the issue of alloys, mixing a precious metal with a non-precious metal
implies that the content of the precious metal in the catalyst is reduced. One of the best
examples is Pt-Bi alloy system. It has been theoretically predicted and experimentally
proved that the HER activity on a Bi-Pt surface alloy (Bi as a host element and Pt as a
solute element, the solute coverage is 1/3 ML) is better than that on pure Pt [60]. From
an economical point of view, a Pt-Bi alloy catalyst costs less than that of a pure Pt
sample. Assuming that the price of Pt is $40000/kg and of Bi is $20/kg, and 100g of
both metals is required to prepare an alloy catalyst in the ratio of 50:50. This means that
the overall cost of the preparation of this catalyst is $4002. The same principle can be
applied when two or more precious metals, for instance Pd and Au, are alloyed.
B- Thermodynamic Stability
The thermodynamic stability under the experimental conditions (temperature, gas
atmosphere, pressure, pH, and electrochemical potential) is a fundamental requirement
of any proposed catalyst for the electrochemical applications [16, 59]. This feature of a
single metal can be improved by alloying. For instance, the Pourbaix diagram for Cu-
water system (Figure 1.4 [61]) shows that Cu undergoes anodic dissolution in acids and
very strong alkaline solutions. It has, however, recently been shown that the anodic
dissolution of Cu can be prevented by alloying Cu with Pd through a co-deposition
process of both elements from a 0.6 M HClO4 electrolyte containing CuSO4 and PdSO4
[62]. The resulting Cu-Pd alloy catalyst showed a significant activity for the nitrate
reduction in alkaline solutions.
13
Figure 1.4: The Pourbaix diagram for Cu-water system showing the domains of corrosion, immunity and passivation at 25 ºC [61].
C- Catalytic Activity
It has been shown in a number of electrocatalytic studies [63-68] that the catalytic
activity of a monocomponent catalyst was enhanced by alloying with another
component. For instance, the catalytic performance of a carbon supported Pt catalyst
(Pt/C) for the oxygen reduction reaction (ORR) was compared to that of a Pt-Pd/C
catalyst (Pt:Pd atomic ratio was 77:23) [66]. The ORR activity, in this case, was
observed to be higher on the binary catalyst than on the single component catalyst.
D- Reaction Selectivity
Electrocatalysis by alloys can be beneficial via increasing the selectivity of a desired
reaction. For instance, formic acid oxidation on Pt takes place through a direct
(dehydrogenation) or indirect (dehydration) mechanism [69]:
14
Pt + CO2 + 2H+ + 2e- (1.6)
Pt + HCOOH
Pt-COads + H2O (1.7)
Pt + H2O → Pt-OHads + H+ + e- (1.8)
Pt-COads + Pt-OHads → CO2 + 2Pt + H+ + e- (1.9)
It has been observed by Huang and colleagues [70] that formic acid oxidation activity on
a carbon nanofiber supported Pt-Au (Pt-Au/CNF) catalyst was better than on Pt/CNF
catalyst. The authors suggested that alloying Pt with Au enhances the selectivity of the
reaction through the direct pathway.
E- CO Tolerance in PEMFC
Besides its high cost, Pt exhibits low CO tolerance where a small amount of CO (>10
ppm) in the reformate stream poisons the Pt electrocatalysts at the anode compartment
[50, 71]. This is because CO adsorbs strongly on Pt surface blocking the active sites for
hydrogen adsorption and preventing, subsequently, the HOR [19, 30, 71]. Besides using
an effective catalyst to convert CO into CO2 before being fed to the anode, there are
other possible solutions to overcome the problem associated with the contamination of
the PEMFC anode catalyst by CO. For instance, an oxidizing agent (such as oxygen or
hydrogen peroxide) can be added to the reformate stream which facilitates the
conversion of CO to CO2. This, however, results in some fuel loss and an increase in
cost [29]. Increasing the operating temperatures of PEMFC has also been proposed. This
solution appears impractical as the development of high-temperature membranes is
consequently required which implies an increase in the cost of the system [19, 29].
Hence, the most appropriate way to solve the problem is possibly to develop new
electrocatalysts for the HOR that are more tolerant to CO poisoning and cheaper than Pt
[29, 58]. One of the best options in this regard is alloy catalysts. A number of Pt-based
binary alloy catalysts including Pt-Ru [72], Pt-Fe, Pt-Ni, Pt-Co and Pt-Mo [73] have
been reported to be more CO-resistant than pure Pt in the PEMFC.
dehydrogenation
dehydration
15
1.3.2- Enhancement of Catalytic Processes on Metal Alloys
The chemical and catalytic properties of a single-component catalyst can be improved
by alloying [74]. There are a number of proposed effects through which alloying
enhances the catalytic performance of a single-component catalyst.
A- Ensemble (Geometric) Effects
The "active sites" on a binary alloy catalyst consist of particular groups or ˝ensembles˝
of surface atoms arranged in a specific geometric orientation. Assume a binary alloy
catalyst consists of an active component and inactive component. The presence of the
inactive component facilitates the formation of these ensembles and enhances the overall
catalytic activity by blocking certain sites suppressing the formation of undesired
intermediates (improving selectivity) or inhibiting species (improving activity) [75-77].
For example, Ni-Cu alloyed Raney-type catalysts showed improved catalytic
performance for methane decomposition (CH4 C + 2H2) in comparison with
monometallic Raney-Ni catalysts [78]. Alloying Cu ˝inactive component˝ with Ni
˝active component˝ is believed to form ensembles of Ni surface atoms which are
responsible for the improvement in the catalytic performance on the alloy catalysts. The
addition of Cu reduces catalyst deactivation by minimizing the adsorption of carbon
species and, therefore, prevents the formation of encapsulating carbon.
B- Electronic (Ligand) Effects
The enhancement in the catalytic performance of a monocomponent catalyst by alloying
can be ascribed to a modification in its electronic properties (e.g. d-electron density)
resulting from the presence of the other component(s). This modification produces
heteronuclear metal-metal bonds (interactions) between the components of the alloy
leading to a system with improved chemical and catalytic properties [75, 77, 79]. The
electronic interaction between the components in the alloys is weak when the enthalpy
of formation (∆Hf) is positive, and is strong when ∆Hf is negative [80].
16
C- Bifunctional Effects
In this model, the promotion in the catalytic performance by alloying is achieved
by each individual component in the alloy activating a certain elementary reaction step
[75]. The catalytic behavior of various Pt-M alloys (M = Sn, Re, Mo and Ru) for
methanol oxidation [79] and Pt-Ru alloys for CO oxidation [72, 81] was reported to be
better than that of Pt. The enhancement on the binary alloy catalysts was explained by
both a modification in the electronic properties of Pt in the presence of the other
component and a bifunctional effect where Pt is believed to be active for CH3OH
adsorption (in the former process) or CO adsorption (in the later process) and the other
component on the surface is active for water or oxygen containing species (OH-)
adsorption.
1.3.3- Bulk & Surface Composition of Alloys
One of the fundamentals in electrocatalysis is that the nature and surface composition of
the catalyst dominates the electrocatalytic process including the interactions of reactants
with the surface, the strength of these interactions and the redox processes [82]. The
surface composition of an alloy catalyst can be different from the bulk composition. The
enrichment of an alloy surface by one of the component is known as ˝surface
segregation˝. This phenomenon could play a crucial role in changing the catalytic
performance of the alloy catalyst such as enhancing /inhibiting a desired reaction or an
undesired reaction [83].
The surface segregation in an alloy system can theoretically be predicted based on the
surface energies of the alloy components. An alloy component tends to segregate at the
surface if its surface energy is lower than that of the other component [83]. However,
there are a number of factors that could, in practice, result in a difference between the
surface and bulk composition and lead to surface enrichment with one of the alloy
components (sometimes different from the theoretical prediction) such as preparation
method of the alloy, chemical and thermal treatment, and applying a high
electrochemical potential [84]. Besides that, the difference could be ascribed to the
17
purity of the metallic systems as they always contain elements such as H, C, N, O and S.
The segregation of these elements with one of the alloy component could result in a co-
segregation effect changing the surface composition [83]. Furthermore, some reacting
molecules favor adsorption on a specific atom causing a segregation of this atom
towards the surface [84, 85]. For example, it was reported that Ag and Au in Pt-Ag and
Pd-Au alloy systems segregate under vacuum to the surface. In contrast, the presence of
CO on the surface acts as driving force of surface segregation of Pt and Pd in both
systems to form a strong metal carbonyl bond [86].
1.3.4- High Throughput Synthesis and Screening of Metal Alloys for Electrocatalysis
The development and examination of a catalyst material for a heterogeneous reaction
through traditional methods is inefficient and a time-consuming process. The use of high
throughput (or combinatorial) methods, in return, allows fast and parallel synthesis and
screening of arrays (libraries) of materials increasing the possibility of producing
efficient catalysts [87-89]. Advances in the discovery of pharmaceutical compounds
have been accomplished during the last two decades by employing combinatorial
methods. The success in this area has subsequently led to interest in employing these
methods for the discovery of inorganic and solid state materials [89]. There are a number
of examples where high throughput synthesis and/or screening methodology was applied
for the study of electrochemical reactions [60, 63, 90-95].
The synthesis of an alloy catalyst for electrochemical reactions can be achieved through
a number of preparation methods such as co-electrodeposition of both elements on a
substrate material [96], electroless deposition [97], co-melting followed by mixing of the
components [98], chemical synthesis using precursor salts [99], or by means of physical
vapor deposition (PVD) methodology [100, 101]. The employment of high throughput
methods in the synthesis of alloy electrocatalysts facilitates the exploration of new
catalyst compositions [102]. This ultimately improves knowledge of the composition-
activity relationship [63].
18
Among the various preparation methods of alloy catalysts, the PVD is believed to be a
promising method for high-throughput synthesis of libraries of thin film materials. The
high-throughput physical vapor deposition (HT-PVD) of thin film libraries can be
achieved either by sequential deposition of the material employing combinatorial
shadow masks, or by simultaneous deposition of the components employing multiple
evaporation sources [63, 89]. Following are a number of examples where the
employment of HT-PVD methods was shown to be beneficial in identifying new alloy
catalysts for electrochemical reactions.
Guerin and Hayden [89] have recently introduced a modified HT-PVD method through
which the fabrication of alloy catalysts is achieved by co-deposition of the elemental
components from multiple sources. This method was applied for the synthesis and
screening of a 100-electrode array of Pt-Pd-Au alloy catalysts for the oxygen reduction
reaction (ORR) showing that the ORR activity on various Pt-Pd alloy compositions are
better than on either of Pt or Pd alone [63].
Methanol oxidation activity was assessed on Pt-Ru-W and Pt-Ru-Co ternary alloy
systems in 0.5 M H2SO4 employing a combinatorial process [102]. Various
compositions of these two systems were prepared on 76 pads supported on a 2 inch
silicon wafer. The deposition of the alloy systems was achieved through a sputtering
system and computer controlled shutters. It was revealed through the electrochemical
screening at various temperatures ranging from room temperature to 60 ºC that
Pt25Ru0W75 and Pt17Ru17Co66 catalysts are superior to Pt-Ru catalyst at room
temperature. The optimum ternary composition at 60 ºC was found to be at Pt44Ru12W44
and Pt12Ru50Co38.
A combinatorial synthesis of a 64-electrode array of Pt-Co-Ru alloy catalysts was
carried out by Strasser [103] using an automated sputtering procedure with a movable
shutter to control the gradient of the deposited thin films. The high throughput
electrochemical screening of the resulting array sample for methanol oxidation in 0.5 M
19
H2SO4 has shown that the activity on a ternary Pt20Co60Ru20 catalyst is better than on
standard Pt-Ru catalysts.
A library of 63 discrete ternary Pt-Ni-Cr alloy catalysts was prepared through sequential
sputtering process on a silicon wafer employing a number of shadow masks and
examined for methanol oxidation reaction [104]. Among the examined catalysts,
Pt28Ni36Cr36 has been observed to have the highest activity for methanol oxidation and
also showed good corrosion resistance. Further examples are available in references [59,
105-108].
1.4- The Hydrogen Evolution Reaction (HER)/Hydrogen Oxidation Reaction
(HOR)
The HER is an important process in electrochemical technology. This is because of its
relevance to a wide range of electrochemical processes such as water electrolysis and
chlorine production. It is also a reaction in corrosion of metals in acid media and a
competing reaction in electroplating and organic reductions. Furthermore, it produces
hydrogen which can be employed as a fuel for a variety of industrial processes such as
fuel cell technology [5, 8, 60]. The HOR is also important in electrocatalysis as it is the
anodic reaction in PEMFC.
1.4.1- Reaction Mechanisms
The overall reaction of hydrogen evolution in acid media is expressed by the following
equation [5]:
2H+ + 2e- H2 (1.10)
, while in neutral and basic solutions is:
2H2O + 2e- H2 + 2OH- (1.11)
20
The detailed mechanism in acid media will be considered here as all the measurements
presented in this thesis were carried out in acid medium.
There are two proposed pathways through which the HER takes place on a metal surface
(M) in acid solutions: (i) Volmer-Heyrovsky mechanism or (ii) Volmer- Tafel
mechanism [5, 97, 109-111]. The first step in both cases is known as the charge transfer,
underpotential deposition (UPD) of hydrogen ions, or Volmer reaction (1.13). This step
involves the transfer of hydrogen ion (proton) from the bulk of the solution towards the
electrode surface and the formation of adsorbed hydrogen atom on the surface (M-Hads).
The discharge of hydronium atoms (H3O+) is the source of protons in acid media.
M + H+ + e- M-Hads (1.13)
This step can be followed by either an electrochemical reaction (Heyrovsky reaction
(1.14)) which involves desorption of hydrogen atom to combine with a proton producing
hydrogen gas (ion-atom recombination):
M-Hads + H+ + e- H2 + M (1.14)
or by a chemical reaction (Tafel Reaction (1.15)) which involves atom-atom
recombination:
2M-Hads H2 + 2M (1.15)
The rate determining step in both mechanisms can be either of the two steps. It is,
however, difficult to determine whether the reaction follows the first or second
mechanism when the first step is the rate determining step [5]. In the case that the
second step is the rate determining step, the HER pathway may be determined by the
catalyst activity. At low overpotentials (in the presence of a good catalyst), the reaction
follows the Volmer-Tafel mechanism and the Tafel reaction is the slowest of the two
steps (rate determining step). At higher overpotentials, on the other hand, the reaction
follows the Volmer-Heyrovsky mechanism and the Heyrovsky reaction is the rate
determining step [97].
21
The HOR is the reverse reaction of the HER. The overall reaction in acid media is
written as follows [19, 112]:
H2 → 2H+ + 2e- (1.16)
In reverse to the HER, the HOR follows either the Heyrovsky-Volmer or Tafel-Volmer
mechanism. In the former case, hydrogen adsorbs and dissociates on the catalyst surface
forming an adsorbed hydrogen atom on the surface (M-Hads) and a hydrogen ion (proton)
(1.17) followed by desorption of the adsorbed hydrogen atom to form another proton
(1.18).
Heyrovsky reaction: M + H2 → M-Hads + H+ + e- (1.17)
Volmer reaction: M-Hads → H+ + e- + M (1.18)
In the Tafel-Volmer mechanism, the HOR occurs through the adsorption and, then,
dissociation of hydrogen molecule on the surface into two adsorbed atoms (1.19)
followed by electrochemical desorption of the adsorbed atoms to give two protons
(1.20).
Tafel reaction: 2M + H2 → M-Hads + M-Hads (1.19)
Volmer reaction: 2M-Hads → 2H+ + 2e- + 2M (1.20)
1.4.2- Alloy Catalysts for the HER/HOR in Acids
The formation of the M-H bond (adsorbed hydrogen atom on the catalyst surface) is the
key process in the HER/HOR [109]. The strength of this bond can be determined from
the free energy of hydrogen adsorption (∆GH) on the catalyst surface. The ∆GH value on
a good catalyst for the HER is 0.0 eV [93]. The absolute values of ∆GH on several
binary surface alloys has been determined by Greeley and Nørskov [93]. Table 1.4
shows the measured values on the systems relevant to this work.
22
Table 1.4: Absolute free energies of hydrogen adsorption (∆GH, eV) on various surface alloys. The values were determined at T=298 K, θH = 1/3 ML. Coverage of solute elements in the surface layer of host elements is 1/3 ML [93].
Alloy system Host Solute |∆GH|
Pd-Au Pd Au 0.3 → 0.4
Au Pd 0 → 0.1
Pd-Bi Pd Bi > 0.5
Bi Pd 0 → 0.1
Ru-Au Ru Au 0.1 → 0.2
Au Ru 0.3 → 0.4
There is a relation between the free energy of hydrogen adsorption (or the strength of the
M-H bond) on a catalyst surface and its activity for the HER/HOR [31]. Figure 1.5
shows the HER exchange current densities as a function of the ∆GH values on several
pure metals and metal overlayers [60]. The relation occurs as a Volcano plot
demonstrating that the HER activity increases as the absolute value of ∆GH is closer to
0.0 eV. Similar plots are available in [31, 109, 113].
Figure 1.5: Volcano plot for the HER activity as a function of free energy of hydrogen adsorption on a series of pure metals and metal overlayers [60]. ∆GH values were calculated at 298 K , 1 bar of H2 and θH = 1/4 or 1/3 ML. α refers to the assumed transfer coefficient.
23
The Volcano plot shows that the ∆GH of an ideal catalyst for the HER is 0.0 eV. The
negative and positive values of ∆GH refer respectively to a strong (exothermic) and a
weak (endothermic) bonding. The M-H bond becomes stronger as the ∆GH is more
negative and weaker as the ∆GH is more positive. There is no HER activity when the
hydrogen adsorption is too weak or too strong. The very strong hydrogen adsorption
blocks the available sites on the catalyst surface poisoning the reaction. The intermediate
bonding is, therefore, the favorable case to permit the HER to take place on the catalyst
surface [31, 60, 113]. This is known as the Sabatier principle that suggests that an
optimum catalytic activity is achieved when the catalyst exhibits intermediate binding
energy or free energy of adsorption [60].
A variety of binary alloy catalysts have been employed for the HER [50, 96, 98, 114-
117] and HOR [99, 100, 118-120] in acid media. The selection of two elements to form
an alloy catalyst with high activity for the HER/HOR has been a matter of discussion in
a number of references [9, 97, 114, 121]. It is generally proposed that mixing a hypo-d-
electronic transition metal having empty or half-filled d orbitals with a hyper-d-
electronic transition metal having paired d electrons not available for bonding (i.e. hypo-
hyper-d-electronic combination) produces a favorable alloy catalyst for hydrogen
reactions [9, 114, 121]. This is because of a change in the electronic densities of pure
constituents upon alloying. For instance, an alloy catalyst consisting of Ni and Mo (Mo
as a hypo- and Ni as a hyper-d metal) has been proposed to be more active for the HER
than Ni or Mo alone. The enhancement in the catalytic activity by alloying was
explained by a change in the electronic densities of both elements and the strength of
proton bonding due to a spillover of electrons from Ni (3d8) to Mo (4d5) [97]. Other
examples of hypo-hyper-d-electronic alloy catalysts for the HER are Cu-Zr and Cu-Ti
[114], Mo-Co, Ni-Zr and Co-Zr [9].
The above assumption does not necessarily imply that any alloy catalyst for the
HER/HOR is supposed to consist of hypo-hyper d-metals. There are other cases where
alloying a transition metal with a non-transition metal or a non-transition metal alloy
24
system produced an active catalyst for the HER or HOR processes. Examples here are
Ni-P [96] and Pb-Bi [98] for the HER, and Pt-Pb, Pt-Sn and Pt-Sb [120] for the HOR.
1.5- Aims of the Project
The broad aim of the present work is to explore binary alloy catalysts as alternatives to
Pt for the HER and HOR through high throughput synthesis and electrochemical
screening methods. The choice of the elemental components that form the binary alloys
was based on: (i) mixing an element from Pt group metals (PGMs) with a non-Pt group
metal element, (ii) mixing an element from the right side in the Volcano plot [60] with
an element from the left side (Figure 1.5). The key objectives are to:
Employ a HT-PVD method for the synthesis of libraries of Pd-Au, Pd-Bi and Ru-Au
binary alloy systems.
Determine the bulk and surface composition of the prepared samples using Energy
Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS)
respectively.
Analyze the structure of the prepared alloys using powder X-ray diffraction (XRD)
technique.
Assess the HER and HOR activity on the whole compositional range of the studied
systems in order to identify the optimum composition of Pd-Au, Pd-Bi and Ru-Au alloy
systems for these electrochemical processes.
Investigate the effect of heat treatment on the compositions and catalytic performances
of the prepared alloy systems for the HER and HOR.
Assess the CO-tolerance in the HOR on the whole composition of the alloy systems.
25
Chapter 2: Experimental
2.1- The High-Throughput Physical Vapor Deposition (HT-PVD) System
Thin film arrays of binary catalysts composed of Pd-Au, Pd-Bi, and Ru-Au alloys were
deposited employing an ultrahigh vacuum (UHV) high-throughput physical vapor
deposition (HT-PVD) system (DCA Instruments) [89]. A schematic representation of the
system is illustrated in Figure 2.1. It consists of two physical vapor deposition chambers
(A and B). Both chambers were used for the HT-synthesis of the alloy samples. The
system is also provided with a single target sputtering chamber (this method of
deposition was not used in this work and this chamber was occasionally used for
annealing the samples instead). It is also equipped with an analytical chamber
incorporating imaging X-ray Photoelectron Spectroscopy (XPS, Resolve - 120 mm
Hemispherical Analyzer) which was used for analyzing the surface composition of the
deposited alloys. All chambers are linked by a transfer line composed of two trolleys
which were used to convey the sample holders to the desired chamber. A load lock,
which can separately be pumped down to the UHV and vented to atmospheric pressure,
was used to load/unload the samples in the system. The samples were easily transferred
into all chambers using a pick-up mechanism and transfer arms (TA). The thermal
deposition of the metal alloys was performed under an UHV environment with base
pressure between 1-5 x 10-9 mbar. Cryo (Helix Technology Corporation) and titanium
sublimation (Varian) pumps were employed in both chambers to achieve the UHV
conditions. The transfer line and the surface analysis chamber were pumped by ion
(Varian) and titanium sublimation pumps. The load lock was serviced by an oil free
rotary (Pfeiffer) and a turbo-molecular (Pfeiffer) pumps [122].
26
Figure 2.1: An over view of the High-Throughput Physical Vapor Deposition (HT-PVD) system used for depositing thin film arrays of Pd-Au, Pd-Bi and Ru-Au alloys. TA: transfer arm.
A top view of the source geometrics of chamber A and B is schematically depicted in
Figure 2.2. The substrate position in the deposition chambers is also indicated. Chamber
A consists of six off axis evaporation sources, three electron beam guns (e-guns,
Temescal) and three Knudsen cells (K-cells, DCA). Chamber B, on the other hand,
incorporates an e-gun and three K-cells. There are two types of K-cells which can be
used with the system, low and high temperature K-cells. The low-temperature K-cells
(LTKS, DCA) can be used to evaporate materials up to 1400 ºC, while the high-
temperature K-cells (HTKS, DCA) can be used to evaporate materials up to 2000 ºC.
The studied alloys were synthesized in both chambers (A or B) employing both e-guns
and K-cells. In each chamber a rotatable manipulator was used to hold the substrates
during the deposition. The E-guns were used to evaporate high melting point materials,
whereas the K-cells were used to evaporate low melting point materials. Palladium
(Unicore, 99.99 %), gold (Unicore, 99.99 %) and ruthenium (Alfa Aesar, 99.95 %) were
all deposited employing e-guns, whereas bismuth (Alfa Aesar, 99.999 %) was deposited
using a low temperature K-cell (LTKS). The deposition time of each sample was
typically about 30 minutes and film thicknesses were approximately around 100 nm.
27
Three types of substrates were used in the synthesis of the alloy samples. More detail
regarding these types is given later in this chapter.
Chamber A Chamber B
Figure 2.2: A schematic representation of the source positions in the growth chambers.
The HT-PVD methodology employed in this work for sample preparation has recently
been developed and described in detail by Guerin and Hayden [89]. In this method, a
combination between co-evaporation of pure elements from multiple finite-size sources
and movable wedge shutters (or apertures) is applied to achieve deposited materials with
controlled gradients. Employing this method, a simultaneous deposition with a
controlled gradient of up to six elements can be achieved on a substrate or an array of
pads [63, 90, 91]. The gradient of the deposited materials can be controlled by adjusting
the wedge shutters prior to the deposition as shown schematically in Figure 2.3 [89].
Using such a method for the preparation of alloy catalysts ensures mixing and alloy
formation and prevents the separation into bulk phases and surface segregation of one of
the alloy components as no heat treatment "annealing" is required to form the alloys
[63]. The bulk alloy composition is identical to the surface composition as no heat
treatment is required to form the alloy [63]. Alloys prepared without heat treatment are
K-cell 3 E-gun
Substrate
K-cell 1 K-cell 2
E-gun 1
K-cell 3
E-gun 3
K-cell 1
Buffer line
E-gun 2
K-cell 2
Buffer line
Substrate
28
described here as "random" or (non-equilibrated) alloys, and described "at equilibrium"
when the synthesis was followed by heat treatment.
Figure 2.3: A schematic representation of how the gradation of a material is achieved by using an aperture (a wedge shutter) [89].
In this scheme, (0, 0) refers to the center of the substrate face. A is the substrate size and
A1 and A2 express its two boundaries. B denotes the offset of the wedge shutter
(aperture) with respect to the axis center of the source. Bmin indicates the initial position
of the wedge shutter. B1, B2 and Bmax refer to different distances from Bmin. The source
size is given the symbol C and its two extremities are C1 and C2. D refers to the source
offset with respect to the substrate. The distance between the source and the aperture is
29
expressed by the symbol E, whereas the distance between the aperture and the substrate
is expressed by the symbol F. The flux direction of the evaporated materials from the
source C to the substrate A is shown by the lines A1C1, A1C2, A2C1 and A2C2. H refers
to the interaction point between lines A1C2 and A2C1 [89].
The wedge composition has been observed to be dependent on the offset of the wedge
shutter with respect to the axis center of the source B. At the position Bmin, a uniform
film can be obtained. The wedge composition can be accomplished by moving the
wedge shutter away from the position Bmin into the flow of evaporated materials. For
instance, moving the wedge shutter to the position B1 (which means that B is between
Bmin and B1) allows the point A1 at the substrate to be exposed to all the material from
the two extremities of the source C1 and C2. On the other hand, the point A2 is covered
from the material which comes from the region C2 and it will only be exposed to the
material near to the region C1. As a result of this, a partial gradient of the material from
point A1 to A2 can be achieved. A similar result (partial gradient) can be obtained by
moving the wedge shutter to the position between B2 and Bmax. A linear gradient across
the whole sample was accomplished at the position between B1 and B2. No deposition
was observed at the position Bmax. More detail regarding this method for the synthesis of
solid state materials is available in reference [89].
For the purpose of getting a uniform layer of a material, the wedge shutter was not used.
In this case, a motor drive associated with the sample holder (manipulator) in the
deposition chamber was employed to rotate the substrate. The wedge shutter was
employed in the synthesis of the alloy samples in order to obtain gradual concentrations
of the elemental components. Figure 2.4 shows an example of a wedge composition of a
binary alloy composed of elements A and B. It can be seen that the concentration of
element A increases gradually from 0 % on the right side to 100 % on the left side. In
contrast, element B increases gradually from 0 % on the left side to 100 % on the right
side. A unique mixture of both elements can be achieved in the middle. Thus, the
employment of the wedge shutter allowed the achievement of nearly the whole
compositional range of the studied alloy systems in this work.
30
Figure 2.4: A schematic representation of the wedge composition of an alloy consisted of two elements (A and B) which can be achieved by the HT-PVD methodology employed in the synthesis of the samples.
2.2- Sample Preparation
The alloy samples were initially calibrated on silicon or glass substrates and then
prepared on a number of electrochemical arrays. The calibration method and the
electrochemical array used in this work are described in what follows.
2.2.1- Sample Calibration
The alloy samples were calibrated prior to the synthesis on the electrochemical arrays in
order to determine an appropriate wedge composition and to observe the film thickness
as a function of time and evaporation temperature. A quartz microbalance (incorporated
in the deposition chamber) was used to determine the rate of deposition. The calibration
was carried out using either squares of silicon wafers (32 or 35 mm2 and thickness 0.5
mm, Nova Electronic Materials Ltd) or squares of glass (32 or 35 mm2 and thickness 1
mm, UQG Optics). The composition of the elemental components was, then, measured
by energy dispersive X-ray spectroscopy (EDS).
Thickness / nm
A
B
100% A 100% B
Position along the sample
31
2.2.2- Electrochemical Array
After calibration, the examined alloy systems were deposited on 100-element
electrochemical array electrodes. Figure 2.5 shows the components of an
electrochemical array (35 x 35 = 1225 mm2). It consists of a silicon nitride coated silicon
wafer as a substrate. On the top of the silicon nitride there are 100 gold pad electrodes
which are used as substrates for the deposited materials to fulfill the conductivity. The
area of each individual electrode is approximately 1.2 x 1.2 = 1.44 mm2. The electrical
conductivity between the 100 electrodes and the electrochemical screening instrument is
achieved by using gold tracks and 100 gold contact pads on the edges. The area of each
gold contact pad is approximately 0.8 x 0.8 = 0.64 mm2. The electrochemical arrays
were rinsed with ethanol and acetone before deposition to avoid the presence of
impurities on the surface [122, 123]. Pure Pd or Ru were deposited onto 10 electrodes in
the electrochemical array in order to compare their catalytic activities to that of the alloy
system for both the HER and HOR.
Figure 2.5: A schematic representation of the 100-element working electrode electrochemical array.
35 mm
35 mm
Silicon Nitride
Passivation
100-gold electrical
contact pads
Gold track
100 gold electrodes
The deposition of the alloys and pure constituents (Pd or Ru)
arrays was carried out using two types of contrary matrix contact masks (
The size of the contact masks was similar to that of the electrochemical array. These
specially designed contact masks were employed in order to match the number of
electrodes and to obtain the same compositions when another substrate (silicon) was
used for the analysis of the sample. A 10 x 10 contact mask with 10 blank squares
(Figure 2.6A) was initially employed to achieve the deposition of the alloy samples.
This contact mask was removed after the deposition and replaced by a blank contact
mask with only 10 squares (
Annealing of the array samples w
15 minutes.
Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample, (A) a contact mask with 10 blank squares which was used duringsamples, (B) a blank contact mask with only 10 squares which was used during the deposition of the active component (Pd or Ru) in the alloy sample.
2.3- Analytical Tools
2.3.1- Powder X-ray Diffraction (XRD)
XRD method is widely used in crystallography to identify the phase of a material
consisting of many crystals and to determine lattice type (structure) and parameters. X
rays are produced by bombarding a metal target with high energy electron beams (1
A
32
the alloys and pure constituents (Pd or Ru) on the electrochemical
arrays was carried out using two types of contrary matrix contact masks (
The size of the contact masks was similar to that of the electrochemical array. These
specially designed contact masks were employed in order to match the number of
rodes and to obtain the same compositions when another substrate (silicon) was
used for the analysis of the sample. A 10 x 10 contact mask with 10 blank squares
) was initially employed to achieve the deposition of the alloy samples.
ct mask was removed after the deposition and replaced by a blank contact
mask with only 10 squares (Figure 2.6B) available for the deposition of
Annealing of the array samples was carried out in the deposition chamber at 300 ºC for
Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample, (A) a contact mask with 10 blank squares which was used during the deposition of the alloy samples, (B) a blank contact mask with only 10 squares which was used during the deposition of the active component (Pd or Ru) in the alloy sample.
ray Diffraction (XRD)
XRD method is widely used in crystallography to identify the phase of a material
consisting of many crystals and to determine lattice type (structure) and parameters. X
rays are produced by bombarding a metal target with high energy electron beams (1
B
on the electrochemical
arrays was carried out using two types of contrary matrix contact masks (Figure 2.6).
The size of the contact masks was similar to that of the electrochemical array. These
specially designed contact masks were employed in order to match the number of
rodes and to obtain the same compositions when another substrate (silicon) was
used for the analysis of the sample. A 10 x 10 contact mask with 10 blank squares
) was initially employed to achieve the deposition of the alloy samples.
ct mask was removed after the deposition and replaced by a blank contact
) available for the deposition of pure Pd or Ru.
carried out in the deposition chamber at 300 ºC for
Figure 2.6: The 10 x 10 matrix contact masks used to match the number and compositions of the electrodes in the electrochemical array when a silicon substrate was used for analyzing the sample,
the deposition of the alloy samples, (B) a blank contact mask with only 10 squares which was used during the deposition of
XRD method is widely used in crystallography to identify the phase of a material
consisting of many crystals and to determine lattice type (structure) and parameters. X-
rays are produced by bombarding a metal target with high energy electron beams (1-100
33
keV). Their wavelengths are in the order of 10-10 m. Owing to their high energy, X-ray
photons are able to penetrate through the metal target in all directions causing multiple
collisions with the electrons of the atoms. The reflection of X-rays from the crystals
results in a diffraction pattern that reflects the structure of the target material. Atoms in a
crystal are aligned in planes and almost all planes participate in the diffraction of X-rays
[124-126].
The XRD measurements (Bruker D8 diffractometer with a 2 dimensional (C2) detector,
Cu Kα source with λ = 1.54184 Å) of the Pd-Au, Pd-Bi, and Ru-Au alloys were carried
out ex-situ. For these measurements, the alloy films were deposited on silicon wafers
and 10x10 matrix contact masks (Figure 2.6A-B) were employed to give 10x10 squares
with compositions similar to that obtained with the electrochemical arrays. The XRD
measurements were performed by Alexandros Anastasopoulos.
The lattice parameters of the alloys were calculated in some cases in order to assess the
lattice parameter-composition relationship and the extent of the obedience to Vegard’s
law in the alloy system. According to this law, it is proposed that the lattice parameters
change linearly with the composition of an alloy system consisting of two elements of
similar crystal structure and form solid solutions [127].
In a cubic crystal system, the lattice parameter (a) can be, simply, calculated using the
following equations [125, 128]:
a = d * (2.1)
d = λ / 2 sinθ (2.2)
where (d) is the distance between planes of a lattice which can be calculated according
to Bragg’s law as depicted in equation 2.2. (hkl) refer to miller indices of the studied
structure, whereas (λ) is the wavelength of X-ray photon (λ = 1.5418 Å).
34
2.3.2- Energy Dispersive X-ray Spectroscopy (EDS)
EDS, also known as X-ray Energy Dispersive Spectroscopy (XEDS), measurements
provide qualitative and quantitative analysis of chemical elements presented in a
specimen. The bombardment of a solid material with a high energy electron beam results
in ionization of an electron from an inner shell (K shell) in the atom and an electron
from a higher energy shell (L shell) falls into the vacancy emitting its excess energy as
an X-ray photon. The emitted X-rays are characteristic of atoms present in the specimen.
The measurement of the wavelength (or energy) of X-ray spectrum emitted by the
specimen provides information regarding the elements present in the specimen
(qualitative analysis), while the number of emitted X-rays per second allows the
measurement of the concentrations of the elements in the specimen (quantitative
analysis). A detector of incoming X-rays is used in the EDS technique to produce charge
pulses (signals) proportional to the detected energies of X-ray photons. The pulses are,
then, amplified and transferred to a multichannel analyzer (MCA) which collects all the
X-ray energies and displays them on a screen [125, 129, 130]. A schematic
representation of the process is shown in Figure 2.7 [129, 130].
Figure 2.7: A schematic representation of the EDS analysis of a target material [129, 130]. MCA is a multichannel analyzer.
35
The EDS (JEOL JSM5910 and Oxford Instruments INCA 300) analysis of the deposited
alloys was performed using silicon substrates to determine the bulk composition of the
100-element alloy samples. In this case, 10 x 10 matrix contact masks (Figure 2.6A-B)
were used to give 10 x 10 squares with compositions similar to that obtained with the
electrochemical arrays.
2.3.3- X-ray Photoelectron Spectroscopy (XPS)
XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), is a surface
characterization method that is used in chemical analysis to determine the composition
of elements present in a solid surface. It provides both qualitative and quantitative
analysis of all elements (except H and He) by measuring electrons ejected from the
surface atoms. The main feature in XPS is the escape depth of the measured electrons,
since these electrons come from layers which have thicknesses in the outermost 10 nm.
Hence, the contribution only comes from the surface species [125, 131].
In an XPS measurement, the target sample is placed in a vacuum environment and
irradiated with X-ray photons. The energies of X-ray photons are, then, transferred to
core-level electrons of the atoms present in the surface. As a result of that, the core-level
electrons will be ejected from these atoms as schematically illustrated in Figure 2.8.
These electrons are, then, separated according to their core ionization energies and
counted. The energy of the photoelectrons provides information regarding the nature of
the atoms in the sample (qualitative analysis) and the number of electrons provides
information regarding the concentration of these atoms in the surface (quantitative
analysis) [125, 131].
36
Figure 2.8: Ejection of a core-level electron by an X-ray photon [131].
The XPS (Resolve - 120 mm Hemispherical Analyzer) measurements were carried out
in situ in an ultrahigh vacuum environment with a base pressure of 1 x 10-9 mbar. The
source of X-rays was Al Kα with hν = 1486.6 eV. As in the cases of the XRD and EDS
measurements, the alloy films were deposited on silicon substrates. A 10 x 10 matrix
contact mask (Figure 2.9) was employed to give squares and compositions identical to
the 100-electrodes in the electrochemical arrays. This contact mask was removed inside
the masking station shown in Figure 2.1 prior to the XPS measurements. The XPS
measurements of an alloy system were performed on a number of alloy thin films along
the growth direction of the elemental components in the array sample to represent the
whole compositional range of the alloy system.
e-
photoejected electron
X-ray photon
2P
2S
1S
37
Figure 2.9: The 10 x 10 matrix contact mask used during the deposition of the alloy samples on silicon wafers for XPS measurements to give the same number of electrodes and compositions similar to that obtained with the electrodes in the electrochemical arrays.
The XPS spectra of the alloy systems were calibrated to: (i) the Pd (3d5/2) and Au (4f7/2)
peaks in the Pd-Au alloy system, (ii) the Pd (3d5/2) and Bi (4f7/2) peaks in the Pd-Bi alloy
system, and (iii) the Ru (3d5/2) and Au (4f7/2) in the Ru-Au alloy system. The binding
energies of these elements in XPS spectra are shown in Table 2.1 [132-134].
Table 2.1: The binding energies of the constituents of the alloy systems in XPS spectra [132-134].
Peak Pd (3d5/2) Au (4f7/2) Bi (4f7/2) Ru (3d5/2)
Binding Energy / eV 335 84 159 280
The surface compositions of the elements in the examined alloys were calculated using
the following equation [133]:
XA = (IA / I∞A) / ((IA / I∞
A) + (IB / I∞B)) (2.3)
Where A and B are the elemental components of an alloy AB, XA refers to the atomic
percentage of the element A in the alloy surface, I is the area under the peak (or the
intensity of the signal), and I∞ refers to the atomic sensitivity factor of a pure element
(Pd = 2.7, Au = 2.8, Bi = 4.25, and Ru = 2.15) [133]. The quantitative analysis of the
elemental surface composition by XPS has an experimental error of < ±10 % [131].
38
2.4- Electrochemical Measurements
2.4.1- Electrochemical Cell
The electrochemical measurements were performed at room temperature in a specially
designed electrochemical cell [90]. A schematic representation of this cell and its size
are illustrated in Figure 2.9. The design of this cell allows a precise position of the array
and the electrical contacts. It consists of working electrode (WE), reference electrode
(RE) and counter electrode (CE) compartments. A Polytetrafluorethylene (PTFE) plate,
also called Teflon, (10 x 7 cm2) was used to make the body of the working electrode
compartment. The electrochemical array was held on a clamp socket (Figure 2.10) in
order to achieve a good solution seal. The clamp socket was provided with electrical
connections to make connectivity between the array and the electrochemical instrument.
Also, gaskets were positioned between the array and the clamp socket from one side,
and between the clamp socket and the PTFE plate on the other side. Pt gauze was
utilized as a counter electrode which is separated from the working electrode by a glass
sinter. The reference electrode was a commercial mercury/mercuric-sulfate electrode
(Hg/Hg2SO4, Sentec). The tip of the reference electrode is located close to the
electrochemical array (WE) in order to minimize IR drop effect [135]. The cell is also
provided with a water jacket, which is used for circulation of water from a water bath, to
keep the temperature during the measurements at 25 ºC. A gas inlet (GI) with a glass
sinter (GS) is utilized for purging the electrolyte with gases. The connection between the
electrical pads of the array wafer and the electrochemical instrument is achieved by an
integrated circuit (IC) socket. Before the electrochemical measurements, the cell, PTFE
plate and gasket were boiled in pure water, for about two hours, to remove contaminants.
Figure 2.9: The compartments of thethe alloy samples.
Figure 2.10: The clamp socket used with the electrochemical array to achieve a good solution sealand prevent electrolyte leaking.
3 cm
Front
The electrochemical array
Bolts used to attach PTFE plate and glass cell
39
compartments of the electrochemical cell used in the electrochemical screening of
2.10: The clamp socket used with the electrochemical array to achieve a good solution sealand prevent electrolyte leaking.
8 cm
17 cm
2 cm
Electrical connections
Front Back
The electrochemical array
Bolts used to attach PTFE plate and glass cell
Electrical connections to the electrochemical instrument
e electrochemical screening of
2.10: The clamp socket used with the electrochemical array to achieve a good solution seal
Electrical connections
Electrical connections to the electrochemical instrument
40
2.4.2- Array Measurements
The electrocatalytic assessment of the Pd-Au, Pd-Bi and Ru-Au alloy systems for the
HER and HOR were carried out at room temperature in a 0.5 M HClO4 electrolyte
synthesized using concentrated HClO4 (GFS Chemicals, 70% double distilled) and pure
water (Elga Option-Q 7BP, 18.2 mΩ cm). The reason of using HClO4 instead of H2SO4
is to avoid the inhibition of the studied reactions by the adsorption of sulfate or bisulfate
anions on the surface of the alloy catalyst [63].
An electrochemical instrument composed of a three-electrode potentiostat, two 64-
channel current followers (the current conversion sensitivity was 10 µA V-1) and data
acquisition cards (PCI-DAS6402/16, Talisman Electronic) monitored by a PC and a
software was employed to measure the electrochemical responses of the 100 electrodes
in the array sample [90]. The full description of this electrochemical instrument is
available elsewhere [92].
Both cyclic voltammetry and potential step methods were employed in the
electrochemical screening of the three alloy systems. All potentials stated here were
calibrated against the Reversible Hydrogen Electrode (RHE) in 0.5 M HClO4. A digital
voltmeter (Fluke, Model 83 Multimeter) was used for this calibration.
2.4.2.1- Cyclic Voltammetry Measurements
Cyclic voltammetry (CV) is a potential sweep technique (electrochemical spectroscopy)
that provides valuable information about processes taking place at the electrode /
electrolyte interface of a studied system (such as adsorption / desorption of species and
surface redox behavior) [8, 136].
The cyclic voltammetry measurements were carried out at room temperature with scan
rate either 50 or 20 mV s-1. The initial assessments of the three alloy systems by cyclic
voltammetry were performed using a sweep rate of 50 mV s-1 under the following
potential limits: cathodic limit (lower potential, Elo) = - 0.03 VRHE, anodic limit (upper
41
potential, Eup) = 0.5 VRHE, and initial potential (Ein) = 0.2 VRHE. In the case of the Pd-Au
system, the anodic limit was sometimes increased to 1.5 VRHE in order to observe the
surface redox processes of the elemental components on the alloy films. Prior to the
cyclic voltammetry measurements, the electrolyte was purged with Ar (Air Products,
99.997%) for 20 minutes to avoid the presence of the air. The voltammograms are
presented here with respect to current density values as a function of the alloy
composition. The current density values were determined by dividing the current values
obtained on each electrode in the electrochemical array by the geometric surface area of
the electrode (ca. 0.0144 cm2).
The CO stripping measurements were carried out using a scan rate of 20 mV s-1 and
under potential limits of Elo = - 0.03 VRHE, Eup = 1.1 VRHE and Ein = 0.2 VRHE. Before
these measurements, the electrolyte was saturated with CO (BOC, CP grade) for 20
minutes followed by purging with Ar for at least 20 minutes in order to remove the CO
molecules from the electrolyte. Figure 2.11 shows an example of a typical cyclic
voltammogram of Pd, recorded in a 0.5 M HClO4 electrolyte bubbled with 500 ppm CO,
and the potential region of various surface processes.
Figure 2.11: A typical cyclic voltammogram of Pd showing the potential region of various surface processes. The CV in this figure was recorded in 0.5 M HClO4 bubbled with 500 ppm CO, scan rate = 50 mV/s.
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
-2
-1
0
1
2
j / m
A c
m-2
(g
eo
)
Potential / VRHE
Oxidation region
Reduction region
HOR
HER
Double layer region
Removal of surface oxide
CO oxidation Surface oxide
formation
42
The anodic and cathodic features in Figure 2.11 are well-established in the literature
[84, 137-143] and the potentials at which they occur vary with the electrode type. A
good electrocatalyst provides a high rate of a reaction at low overpotential (i.e. closer to
the equilibrium potential) [5].
2.4.2.2- Potential Step Measurements
The idea behind the potential step method is that the current-time response of the
working electrode is recorded at a potential (E0) where no reaction of species takes
place. At time t, the applied potential to the working electrode is rapidly stepped to a
value (E1) where the reaction of interest occurs [5, 8, 136].
The electrocatalytic activities of the Pd-Au, Pd-Bi, and Ru-Au alloy systems for both the
HER and HOR were assessed employing the potential step measurements. In each
experiment, the current was initially recorded at 0 VRHE and then stepped to either
negative potentials (in the case of the HER) or positive potentials (in the case of the
HOR). The applied potential to the working electrode was held in each step for 90
second. The electrocatalytic assessment of the HER and HOR on the alloy systems by
the step potential method was carried out prior to characterizing the catalyst surface by
cyclic voltammetry at high potentials.
The HOR measurements were carried out in a H2 atmosphere with either pure H2 (BOC,
99.995 %) or a mixture of H2 and 500 ppm CO (BOC, 500 ppm CO / Hydrogen)
bubbling through the electrolyte. The aim of carrying out the HOR in the latter case was
to assess the CO tolerance of the examined alloy systems.
43
Chapter 3: Palladium-Gold (Pd-Au) Alloy Surfaces
3.1- Introduction
For a long time, gold was regarded to be catalytically inert because of its poor ability to
chemisorb reactants [144, 145]. The historical perception of gold as an inactive material
has been changed in the recent time due to the fact that nanocrystalline structures of gold
have exhibited unique catalytic properties in a variety of redox reactions [146]. The
unique catalytic properties of Au were initially realized in 1980s when supported Au
nanoparticles were proved to be active for the CO oxidation at low temperature [47].
Also, Au was predicted to be the best catalyst for ethyne hydrochlorination [48]. Since
then, catalysis by gold has become an attractive area for many researchers.
Au-based catalysts are used now in chemical industries, environmental protection and
maintenance, chemical sensors and electrochemical processes such as fuel cells,
batteries, and electrochemical sensors [146, 147]. One of the current widespread
applications of Au-based catalysts is in the low temperature oxidation of CO in
reformate gas for fuel cell technology, since CO poisons the catalyst surface [146, 148,
149]. The reason of the extensive interest in using Au-based catalysts is ascribed to their
abilities to catalyze reactions at low temperatures compared to Pt- or Pd-based catalysts
which are inactive below 200ºC [148, 150]. The particle size and support are important
influencing factors on the activity of Au for the CO electrochemical oxidation. It was
shown, in this respect, that reaction activity on titania supported gold nanoparticles (<
6.5nm) is higher than on carbon substrate. A maximum activity was observed with
particle size of ca. 3 nm [151].
The success of gold as a catalyst has led to an extensive interest in bimetallic catalysts
containing gold as one of the components. A particular concern has been directed
towards alloying Au with platinum group metals (pgm), since the Au-(pgm) alloy
catalysts have been observed to be superior to either Au or a pgm alone [152]. Examples
of the applications of the Au-(pgm) catalysts are in the synthesis of hydrogen peroxide
44
(H2O2), the methanol decomposition process, the hydrodesulfurisation of
dibenzothiophene, CO oxidation, toluene hydrogenation and the reduction of NO by
propene [152].
In particular, Pd-Au alloy system is used widely to catalyze a number of chemical
reactions important to industry, hydrogen fuel cells and pollution control. There are
many advantages of using a Pd-Au alloy as a catalyst instead of pure Pd. One of them is
to reduce the cost of the catalysts, since a less amount of the more precious metal is
contained in the alloy. Also, the catalytic activity, selectivity and stability of Pd can be
enhanced by alloying with Au [76, 148, 149].
The enhancement in the catalytic activity of Pd in the presence of Au has been attributed
to the formation of several active sites on the alloy surface that accelerate the reaction.
These sites are composed of ensembles of single Pd atoms (Pd monomers) surrounded
by Au atoms. The suggested role of Au in this case is to promote the formation of these
ensembles and, hence, enhances the catalytic activity of Pd [75, 76, 142, 153].
As pointed out by Goodman and colleagues [74], two model systems of the Pd-Au alloys
appear in the literature. The first is stable bulk alloys and the second involves the
deposition of one metal on a single crystal of another metal (such as the deposition of Pd
on Au (111) surface). The synthesis and the control of the compositions of the alloy
surfaces were difficult to achieve in these studies. Therefore, Goodman’s group has
proposed a system whereby stable alloy films of the Pd-Au system can be prepared by
the deposition of both metals on a substrate of a third metal, such as Mo (110). The
stabilization of the prepared films in this case was achieved by annealing. This method
of alloy synthesis is similar to the method applied here where the synthesis of alloys is
achieved through co-deposition of the elements on a substrate [89].
45
3.1.1- Industrial Applications of Pd-Au Catalysts
Pure noble metals, such as Pd, have a central role as catalysts for various reactions
which have environmental or industrial applications. The catalytic performance of Pd in
many catalyzed reactions was enhanced by alloying with Au. Hydrodesulfurization
(HDS) of petroleum feedstocks is an example of these reactions. The presence of sulfur
in this process results in catalyst poisoning. To overcome this problem, a number of
Aux-Pdy catalysts supported on silica were employed to examine the influence of
alloying Pd with Au on the resistance of the catalysts to poisoning by sulfur during the
HDS process. Au-Pd alloys were found to be more resistant to sulfur poisoning
compared to pure Pd due to the ensemble effect between the two elements in the alloy
[154].
One of the most important industrial processes catalyzed by Au-(pgm) catalysts, in
particular Au-Pd catalysts, is the production of vinyl acetate monomer (VAM) [152].
This process takes place according to the following chemical equation:
CH3COOH + C2H4 + 0.5 O2 → CH3COOCHCH2 + H2O (3.1)
Au was observed to have a promotional effect on the performance of Pd in this process
as alloying Au with Pd has lead to a better catalytic performance, selectivity and stability
compared to Pd alone. It was proposed that critical ensembles of several Pd atoms on the
alloy surface are the active sites in this reaction. The role of Au in the Pd-Au alloy
catalyst is to facilitate the formation of these critical ensembles by isolating single Pd
sites and, at the same time, it hinders any undesired reaction between products [76]. The
current production of vinyl acetate is based on the use of supported Pd-Au alloys with
low concentration of Au [146].
Hydrogen peroxide (H2O2) is used as a bleach and as a disinfectant. The production of
H2O2 by Pd-Au alloy catalysts has been found to produce higher yields than catalysts of
pure Pd or Au due to the ensemble effect in the alloy [146]. In most cases, supported Pd-
Au catalysts were used to enhance the rate of H2O2 production. For instance, the
46
catalytic activities of a range of supported Pd, Au, Pd-Au catalysts for the synthesis of
H2O2 from H2 and O2 have been discussed by a number of researchers [146, 155-157].
The general conclusion from these studies is that the supported Pd-Au catalysts, such as
Pd-Au/TiO2, were observed to perform better than the supported Pd or Au catalysts in
the formation of H2O2.
Au, Pd and Pd-Au alloy catalysts have also displayed remarkable activities in other
selective oxidation reactions such as alkene epoxidation, alcohol oxidation, sorbitol
oxidation and reducing sugars [48, 146, 158, 159]. For example, the catalytic activity
and selectivity of the Pd-Au / C catalyst in the selective oxidation of D-sorbitol was
greater than Pd/C or Au/C catalysts. The better performance of the bimetallic catalyst in
this case was again attributed to the ensemble effect between the components [158].
3.1.2- Electrochemical Applications of Pd-Au Alloy Catalysts
A major potential use of the Au-(pgm) catalysts is in fuel cell technology [152]. A
number of investigations have been concerned with the study of the electrocatalytic
behavior of Pd-Au alloys or Pd overlayers on a single crystal of Au in reactions
important to fuel cells such as the hydrogen oxidation reaction (HOR) and CO tolerance
[101, 139], the oxygen reduction reaction (ORR) [160, 161], carbon monoxide oxidation
[162, 163] and formic acid oxidation [29-31]. The hydrogen evolution reaction (HER)
has also been studied on Pd overlayers on Au single crystals [164, 165] and Au/Pd (111)
surface alloys [142]. Following are various examples where Pd-Au catalysts were
employed for studying electrochemical reactions.
The HER activity has been examined on Au/Pd (111) alloy films supported on a
Ru(0001) substrate in 0.1 M H2SO4 [142]. The Au/Pd (111) alloy catalysts were
synthesized by electrochemical deposition followed by heating up to 700 °C. The active
centers for the HER on Au-Pd alloys were suggested to consist of ensembles of Pd
atoms (a number of single Pd atoms, also called Pd monomers) surrounded by Au atoms.
The Au-Pd alloys containing 0.1-0.3 Pd surface fractions exhibited the maximum
47
activity for the HER and found to be catalytically superior to Pd(111) by a factor of ca.
20. The reason of this was ascribed to the presence of a high number of the Pd monomer
ensembles.
The electrooxidation of H2 has been examined by Schmidt et al [101] over two well
defined Au(111)-Pd surface alloys with Pd surface concentrations of ca. 38 and 65 %.
The catalysts were prepared by Pd vapor deposition in a UHV chamber. The authors
suggest that: (i) the HOR over Au(111)-Pd is slower by one order of magnitude in
comparison with Pt(111); (ii) the HOR kinetics on the Pd-rich surface is faster than the
other surface; (iii) the adsorption/dissociation of molecular H2 on the Pd sites of the
Au(111)-Pd surfaces may be the chemical rate-determining step in the HOR.
A comparative investigation of Pd (111), (100) and (110) overlayers of various
thicknesses on Au (hkl) and on Pt (hkl) [166] showed that formic acid activity on Pd / Pt
(hkl) catalysts is higher than on Pd / Au (hkl) catalysts. Kibler and colleagues [167]
examined the reaction kinetics on massive Pd (111) and Pd adlayers on Au (111). It was
concluded in their study that Pd film thickness on the metal overlayer influences the
adsorption behavior and catalyst reactivity due to electronic modifications. Formic acid
oxidation was also tested on both unsupported and carbon supported Pd-based catalysts
[168]. The reaction activity was found to be higher on carbon supported catalysts and
alloying Au with Pd on C (Pd-Au/C) has exhibited further improvement for the process.
A barrier in the electrocatalytic studies of the HER and HOR on alloy catalysts
containing Pd as one of the constituents is the absorption of hydrogen into the bulk of
the alloy forming bulk hydrides. This is because this process takes place in the under
potential deposition of hydrogen (UPD H) region influencing hydrogen
adsorption/desorption as well as the HER and HOR on the catalyst surface [169]. α-Pd/H
is formed when the hydrogen concentration is low, while the high concentration of
hydrogen forms β-Pd/H [170]. Hydrogen absorption/desorption into/from bulk Pd
electrodes may proceed through a dual mechanism as follows [171, 172]:
48
H+ +e- (3.2)
Habs = [Hβ ↔ Hα ↔ Hsubs] ↔ Hads
1/2 H2 (3.3)
where: Hα and Hβ denote hydrogen absorbed in the α- and β- phases respectively, while
Hsubs signifies the subsurface hydrogen (hydrogen atoms presented directly under the
catalyst surface in a layer of thickness between 20-50 nm). According to this scheme,
the removal of absorbed hydrogen from the bulk of a Pd electrode takes place through an
electrochemical process (the Volmer reaction, 3.2) or/and non-electrochemical
recombination process (the Tafel reaction, 3.3). The former process takes place under an
electrochemical potential and the latter process occur through the diffusion of hydrogen
from the bulk to the electrode surface to combine with an adsorbed hydrogen atom on
the surface producing a hydrogen molecule.
3.2- Composition and Structure Analysis
Several 100-element Pd-Au array samples were synthesized through simultaneous
deposition of the elements employing the HT-PVD method described in the previous
chapter. After the synthesis, a number of analytical tools were employed to determine
the composition and structure of the prepared samples. The bulk and surface
composition were determined by EDS and XPS respectively. The structural analysis of
the samples was achieved by XRD.
3.2.1- EDS Analysis
The use of the wedge shutter in the HT-PVD system facilitated the synthesis of non-
equilibrium (random) Pd-Au alloys with a compositional gradient through simultaneous
deposition of the elements. The atomic percentage of Pd and Au in a Pd-Au sample with
respect to the position (x, y) in the electrochemical array is illustrated by false color
maps in Figure 3.1. It can be clearly seen that Pd concentration in the alloy increases
gradually from the bottom left side to the top right side, whereas Au concentration
increases gradually from the bottom right side to the top left side. The latter Figure
49
shows that deposition of nearly the whole compositional range of the Pd-Au alloy
system was achieved. For instance, the atomic percentages of the elements in an Au-rich
position (electrode: 1, 1) is ca. Pd7Au93, while at a Pd-rich position (electrode: 1, 10) is
ca. Pd89Au11. The electrode (6, 5) has a bulk composition of ca. Pd49Au51. The
compositional gradient in this array sample is in agreement with the prediction of the
HT-PVD deposition method employed here for the synthesis of alloy systems [89].
Figure 3.1: A contour plot of the component elements in a sample of the Pd-Au alloy catalysts with respect to the position (x, y) in the array sample. The arrows refer to the growth direction of the elements in the array sample.
3.2.2- XRD Analysis
Pd and Au exhibit complete miscibility in the solid phase [77]. A continuous series of
solid solutions across the alloy compositional range is formed [140, 159, 173]. A Pd-Au
alloy has a face-centered cubic (fcc) crystal structure with Pd and Au atoms arbitrarily
located at all the faces of the cube [174, 175]. The formation of solid solutions in the Pd-
Au alloys means that Pd and Au atoms have almost the same size and structure. It also
denotes that Pd and Au are able to be completely soluble in each other and that atoms of
one component can be replaced by atoms of the other component. Thus, Pd and Au can
form substitutional alloys [53, 124].
Y
X
50
The phase diagram of the Pd-Au alloy system [53] is shown in Figure 3.2. The first
observation from this diagram is that the melting points of Pd-Au alloys increase
gradually from ca. 1064 to 1550 °C with the decrease of the Au concentration in the
alloy. It also confirms that Pd and Au are able to form solid solutions and completely
miscible in the solid phase at a wide range of temperatures (below 1064 °C in the Au-
rich alloys and below 1550 °C in the Pd-rich alloys). However, the phase diagram shows
that a miscibility gap arises at the Au rich side and disappears at alloy compositions ≤ 40
at. % Au. At concentrations of Au lower than ca. 40 at. %, Au and Pd are fully miscible
irrespective of the phase of the alloy. The Pd-Au alloys with concentrations of Au > 50
% are stable in the solid form below between ca. 1070-1500 °C and stable above this
range in the liquid form. The Pd-Au alloys with concentrations of Au < 50 % are stable
in the solid form below between ca. 1500-1555 °C and stable above this range in the
liquid form.
Figure 3.2: The phase diagram of the Pd-Au system [53]. L: refers to the liquid phase.
The structure of an array sample of Pd-Au alloys was characterized using thin film
powder XRD. Figure 3.3 illustrates typical X-ray diffractograms for a number of Pd-Au
alloys. The Au (111) Bragg peak appears at about 2θ = 38° and Au (200) Bragg peak
51
occurs at about 2θ = 44° [174]. Two peaks at about 2θ = 40° and 47° correspond to Pd
(111) and Pd (200) Bragg peaks respectively [176, 177]. The shift of 2θ values from that
of Au (111) peak to that of Pd (111) peak is an indication of the formation of solid
solutions of the alloys. Therefore, the peaks lie between 2θ = 38° and 40° are attributed
to the formation of a Pd-Au (111) alloy in each case. The peaks in the region between 2θ
= 44.5° - 47° similarly correspond to the Pd-Au (200) alloys. The results correspond
closely to the structure of Pd-Au alloys reported in the literature [174, 177, 178]
confirming the spontaneous formation of the alloy phase at ambient temperature.
Figure 3.3: XRD patterns for a number of Pd-Au alloys. The dashed lines indicate the 2θ values of the pure elements.
The Pd-Au array sample used in the latter measurement was also annealed at 300 °C for
15 minutes in order to assess the effect of annealing on the crystal structure or the
intensities of the peaks appearing in the latter result. Similar X-ray diffractograms and
intensities were obtained (Figure 3.4) suggesting that annealing Pd-Au alloys under
these conditions exhibits little impact on the bulk crystal structure (a similar phase
before and after annealing is obtained).
0
2
4
6
8
10
12
14
16
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Intn
en
sity
/ A
rbitra
ry U
nit
s
2θ Scattering Angle /
96 % Au
88 % Au
79 % Au
65 % Au
48 % Au
25 % Au
8 % Au
Pd (200) Au (200)
Au (111)
Pd (111)
52
Figure 3.4: XRD patterns for a number of Pd-Au alloys annealed at 300° C for 15 minutes. The dashed lines indicate the 2θ values of the pure elements.
Additional analysis of the (111) Bragg peaks in the region between 2θ = 38°-41° was
performed in order to investigate the variation of the position and the intensities of the
peaks as a function of the compositions of the Pd-Au alloy (Figure 3.5A-B). Increase in
the Au concentration decreases 2θ of the (111) diffraction peak in three stages (Figure
3.5A): (i) a steady decrease with concentrations of ca. at. % Au ≤ 20, (ii) an inflection of
the curve with concentrations of ca. 20-50 at. % Au and (iii) a steep decrease with
concentrations at. % Au ≥ 50. A plot of the peak intensity as a function of Au
composition (Figure 3.5 B) also shows three characteristic compositional regions: Au at.
% < 20, 20 < Au at. % < 75, and Au at. % > 75. The most intense peaks correspond to
the limiting pure compositions (alloys rich in Pd or Au) and the least intense peaks
correspond to the Pd-Au alloys with intermediate compositions.
0
2
4
6
8
10
12
14
16
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48
Inte
nsit
y /
Arb
itra
ry U
nit
s
2θ Scattering Angle /
96 % Au
88 % Au
79 % Au
65 % Au
48 % Au
25 % Au
8 % Au
Au (111)
Pd (111)
Au (200) Pd (200)
53
Figure 3.5: (A) A plot of the position of the (111) Bragg peaks in the region 2θ = 38° to 41° as a function of Au concentration, (B) The intensity of the peaks.
The lattice parameters (a) of pure crystalline Au and Pd are (4.08 Å) and (3.89 Å)
respectively [174]. According to Vegard’s law in crystallography, the lattice parameter
of a Pd-Au alloy should lie between these two values and follows a linear relation as a
function of the alloy composition [127, 140, 174]. Considering this, the XRD data were
used to calculate the lattice parameters of a number of Pd-Au alloys. The calculations
were performed on the (111) peak data. Figure 3.6 shows the lattice parameters of a
number of Pd-Au alloys as a function of Au composition. The dependency of the lattice
parameter on the alloy composition clearly obeys Vegard’s law, since a linear relation is
observed. This finding is consistent with the behavior one may expect upon alloying Pd
with Au [127, 140, 174].
0 10 20 30 40 50 60 70 80 90 10038.6
38.8
39
39.2
39.4
39.6
39.8
40
40.2
40.4
Atomic percent Au
FOMPeakPosition
111
0 10 20 30 40 50 60 70 80 90 1003
4
5
6
7
8
9
10
Atomic percent Au
FOMPeakMaxInt
111 B A
54
Figure 3.6: The lattice parameters (a) of a number of Pd-Au alloy catalysts as a function of Au composition. The red circles are the lattice parameters of pure elements (aAu = 4.08 Å, aPd = 3.89 Å) [174]. The dashed line represents the ideal relation according to Vegard’s law [127, 140, 174].
3.2.3- XPS Analysis
Annealing temperature and length influence surface morphology and composition of
alloys, since annealing at high temperatures smoothes the surface and makes it
dominated by the more stable surface component [74]. The surface composition and
surface segregation phenomenon in the Pd-Au alloys have been assessed in the literature
by a number of techniques [74, 77, 179-181]. It appears to be agreed that a strong
surface segregation of Au indeed takes place in this system at equilibrium (after
annealing). The surface free energy of Pd and Au are 2.043 Jm-2 and 1.626 Jm-2
respectively. The surface free energy of Au is lower than that of Pd. Therefore, Au tends
to segregate at equilibrium to the surface in order to minimize the surface free energy
[77].
A compositional analysis of a number of Pd-Au alloys was carried out by XPS in order
to determine the surface composition and compare it to the bulk composition measured
by EDS. The XPS measurements were performed on various fields in the 100-field Pd-
Au array. The selected fields were chosen to provide a range of alloy compositions
3.8
3.85
3.9
3.95
4
4.05
4.1
3.8
3.85
3.9
3.95
4
4.05
4.1
0 20 40 60 80 100
latt
ice p
ara
mete
r /
Å
latt
ice p
ara
mete
r /
Å
Au at. %
55
appearing in Figure 3.1. The main peak binding energies expected for metallic Au and
Pd are summarized in Table 3.1 [132].
Table 3.1: Binding energies of metallic Au and Pd [132].
Element Au(4f 7/2) Au(4f 5/2) Pd(3d 5/2) Pd(3d 3/2)
Binding Energy / eV 84 88 335 340
The Au (4f) XPS region of a number of unannealed and annealed (300 ºC for 15
minutes) Pd-Au alloys are shown in Figures 3.7 and 3.8 respectively. A shift, in both
cases, in the Au (4f) peak positions towards lower binding energies is observed with
decreasing Au concentration in the alloy. The Au (4f7/2) binding energy is shown in
Figure 3.9 as a function of bulk Au composition for various unannealed Pd-Au alloys.
There is clearly a chemical perturbation in the Au environment reflecting an interaction
between the Au and Pd components. The shift in the Au (4f) peak position upon mixing
with Pd can be considered as an indication of alloy formation [182].
Figure 3.7: The Au (4f) XPS region of various random Pd-Au alloys.
-50
0
50
100
150
200
80 82 84 86 88 90 92
Inte
nsit
y /
CP
S
Binding Energy (BE) / eV
91 % Au
82 % Au
58 % Au
32 % Au
12 % Au
Au (4f7/2)
Au (4f5/2)
56
Figure 3.8: The Au (4f) XPS region after annealing Pd-Au alloys at 300 °C for 15 minutes.
Figure 3.9: The Au (4f 7/2) peak positions as a function of Au composition for various unannealed Pd-Au alloys.
The surface Au composition (before and after annealing at 300 °C for 15 minutes) in a
number of Pd-Au alloys has been compared to the Au bulk composition (Figure 3.10).
The surface Au and surface Pd compositions were calculated using the area under the
Au (4f7/2) and the Pd (3d5/2) peaks respectively. As one may expect, similar
-50
0
50
100
150
200
80 82 84 86 88 90 92
Inte
nsit
y /
CP
S
Binding Energy (BE) / eV
91 % Au
82 % Au
58 % Au
40 % Au
12 % Au
83.3
83.5
83.7
83.9
84.1
84.3
0 20 40 60 80 100
Bin
din
g E
nerg
y (B
E)
/ eV
Au at. %
Au (4f5/2) Au (4f7/2)
57
compositions are observed before heat treatment indicating that the Au composition in
the surface is similar to that in the bulk of the alloy (no surface segregation). The surface
Au composition after annealing is higher than that in the bulk indicating a surface
segregation of Au upon annealing under these conditions. The dotted red line in Figure
3.10 represents a fitting to the data after annealing with a polynomial equation (y=1.85x-
0.0085x2). This equation was subsequently employed for the calculation of the surface
composition along the whole compositional range of the alloy system in order to assess
the HER and HOR activity as a function of surface composition. This will be shown
later in this chapter.
The surface segregation of Au observed after annealing is consistent with the behavior
reported in the literature as well as the theoretical prediction based on the surface free
energies of the elemental components [74, 77, 179-181]. It is also in accordance with the
observation by Yi and colleagues [77] that the surface composition of a 1:1 Pd-Au alloy
was changed to Au0.8Pd0.2 after annealing at 800 K.
Figure 3.10: Au surface composition as a function of bulk Au composition before heat treatment (blue diamonds) and after heat treatment at 300 °C for 15 minutes (red diamonds). The dotted blue line represents the relation one may expect before annealing based on that the bulk composition is similar to that in the surface (no surface segregation). The red dotted line represents a fitting to the data obtained after annealing with a polynomial equation: y=1.85x-0.0085x2.
0
20
40
60
80
100
0 20 40 60 80 100
Su
rfa
ce
Au
at.
%
bulk Au at. %
before heat treatment
after heat treatment
58
3.3- Base Voltammetry and CO Stripping Measurements
A preliminary understanding of an alloy system can be achieved by running a cyclic
voltammetry experiment due to its ability to provide a rapid qualitative analysis of the
system [5]. Therefore, the initial assessment of the Pd-Au system was carried out in a
0.5 M HClO4 electrolyte using this technique. An example of the cyclic voltammetry
responses of a 100 electrode array of the Pd-Au system with various compositions of Pd
and Au is shown in Figure 3.12. The direction of the arrow refers to the growth of Au
concentration in the array. The upper potential (Eup) in this case was restricted to 0.5
VRHE in order to assess the hydrogen underpotential deposition (Hupd), hydride formation
and hydrogen evolution/oxidation regions with minimal perturbation by surface
oxidation.
Figure 3.12: Cyclic voltammograms of a 100-electrode of Pd-Au alloys. The arrow refers to the growth direction of Au in the sample. X: refers to a dead electrode.
001
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 02
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
003
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
004
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
005
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
006
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
007
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
008
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
009
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
010
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
011
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 12
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
013
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
014
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
015
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
016
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
017
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
018
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
019
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
020
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
021
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 22
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
023
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
024
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
025
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
026
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
027
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
028
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
029
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
030
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
031
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 32
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
033
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
034
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
035
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
036
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
037
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
038
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
039
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
040
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
041
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 42
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
043
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
044
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
045
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
046
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
047
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
048
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
049
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
050
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
051
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 52
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
054
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
053
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
055
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
056
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
057
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
058
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
059
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
060
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
061
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 62
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
063
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
064
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
065
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
066
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
067
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
068
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
069
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
070
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
071
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 72
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
073
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
074
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-2e -5
0
2e -5
4e -5
075
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
076
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
077
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
078
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
079
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
080
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
081
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 82
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
083
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
084
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
085
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
086
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
087
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
088
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
089
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
090
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
091
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
0 92
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
093
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
094
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
095
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
096
0. 0 0 .1 0 .2 0. 3 0. 4 0. 5 0. 6
-4 e-5
-3 e-5
-2 e-5
-1 e-5
0
1e-5
2e-5
3e-5
4e-5
097
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
098
0 .0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
099
0. 0 0. 1 0. 2 0. 3 0 .4 0 .5 0. 6
-4e -5
-3e -5
-2e -5
-1e -5
0
1e -5
2e -5
3e -5
4e -5
100
0. 0 0. 1 0. 2 0 .3 0 .4 0. 5 0. 6
-4e-5
-3e-5
-2e-5
-1e-5
0
1e-5
2e-5
3e-5
4e-5
Cu
rren
t (A
)
Potential / VR H E
X X X
X
X X
X
X
59
Cyclic voltammetry responses of various electrodes are expanded in Figure 3.13A-F.
There are two characteristic features which appear in some of these figures and refer to:
(1) the HER in the cathodic sweep at approximately -0.05 VRHE, (2) the HOR in the
anodic sweep at about 0.25 VRHE. Clearly, the addition of Au to Pd influences the extent
of the HER and HOR. The catalytic activity with respect to the HER is also reflected in
the subsequent HOR. A higher concentration of evolved hydrogen will subsequently
result in more HOR in the anodic sweep. The Pd-Au alloy with a small concentration of
Pd (ca. 16 at. % Pd, Figure 3.13A) exhibits no HER activity and it behaves almost as
pure Au. As the concentration of Pd increases, the geometric current densities and the
peaks intensities increased. The maximum activity for the HER in Figure 3.13 is
displayed by the alloy containing Pd76Au24 (higher than pure Pd sample). This result
suggests that alloying Pd with Au improves the HER activity.
60
Figure 3.13: Cyclic voltammograms of six electrodes with different compositions of Pd and Au (Pd:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4. CVs are from the 2nd Cycle.
Further voltammetric assessment of the Pd-Au system was carried out by cycling to an
upper limiting potential of 1.5 VRHE in order to observe the surface redox features which
occur at high electrode potentials. A number of features in the anodic and cathodic
sweeps could be identified as shown in Figures 3.14A-F. The voltammograms are a
revision of the HER (1) and HOR (2) observed at a lower potential (Figure 3.13). A
maximum HER activity is again observed to be higher on some of the alloy surfaces
than on pure Pd. No significant influence on the HER and HOR features is observed
suggesting a little or no perturbation of the surface for these processes upon cycling to
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4
-2
0
2
4
27 : 73
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4
-2
0
2
4
16 : 84
59 : 41
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4
-2
0
2
4
76 : 24
Potential (E ) / V
-0 .1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-4
-2
0
2
4
100 : 0
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-4
-2
0
2
4
41 : 59
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
j / m
A c
m-2
(g
eo
)
-4
-2
0
2
4
A
C
E F
D
B
2
1
61
this potential. The peak (3) in the anodic cycle above ca. 0.8 VRHE corresponds to the
oxidation of the surface and three peaks in the subsequent cathodic cycle correspond to
reduction of the surface oxide (4-6). The cathodic peaks at ca. 1.2 VRHE (4) and at ca.
0.65 VRHE can be ascribed to reduction of Au oxide and Pd oxide respectively [63, 110,
169, 183], while the cathodic peak at ca. 0.8 VRHE can be associated with the reduction
of Pd-Au ensemble oxide. As one may expect, the intensities of the peaks (4-6) vary
with the surface composition of the alloy. Similar compositional dependences of the
surface redox behavior on Pd-Au alloys were observed elsewhere [63, 183]. In
particular, the reduction peak of the Pd-Au oxide (peak 6) can be considered as further
evidence of the formation of the alloy. This is because it locates between the reduction
peaks of the pure Pd at ca. 0.65 VRHE and Au at ca. 1.2 VRHE. The broadness and position
of the alloy peak (6) vary with the alloy composition. It shifts towards more positive
potentials (above 0.8 VRHE) with increasing Au concentration and to more negative
potentials (below 0.8 VRHE) with decreasing Au concentration in the alloy. This variation
indicates a variation in the ensemble size on the alloy surface [75].
The key role in the HER and HOR is the adsorption of hydrogen on the catalyst surface.
Distinctively, Pd is a hydrogen absorbing metal which means that hydrogen can also
absorb into the bulk of both Pd and to perhaps a lesser extent its alloys. As a result of
this behavior, bulk Pd hydrides (Pd-H) can be formed in acidic electrolyte. Pd and
hydrogen can form two phases of hydrides (α-Pd/H with a low concentration of
hydrogen and β-Pd/H with a high concentration of hydrogen). The amount of hydrogen
that can absorb into an alloy of Pd-Au is governed by the bulk Pd content in the alloy.
Hence, there are also other peaks which may occur at the HER and HOR potential range
in the presence of the high Pd content alloys. These peaks could be ascribed to hydrogen
adsorption or hydride formation (7), and hydrogen desorption from the surface or
hydride decomposition (8) [84, 170]. In recognition of this, it is difficult sometimes to
distinguish the individual process due to the small range of potential where the responses
of all these processes may appear. A voltammetric examination on a number of Pd-Au
alloys [140] has shown that the absorption ratio of hydrogen into the bulk decreases
gradually with the increase of the Au content in the alloy composition and it becomes
62
zero at approximately 70 % Au. This indicates that alloying Pd with a high content of
Au suppresses the formation of Pd/H phases. Therefore, the peaks (7 and 8) disappeared
with the increase of Au concentrations in the alloys.
Figure 3.14: Cyclic voltammograms of various Pd-Au alloys (Pd:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4 with scan rate (γ) = 50 mVs-1. CVs are from the second cycle. The limiting potentials are: Ein = 0 VRHE, Elo = 0 VRHE and Eup = 1.5 VRHE.
16 : 84
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
23 : 77
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
70 : 30
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
j /
mA
cm
-2 (
ge
o)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
50 : 50
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
86 : 14
Potential / VRHE
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
100 : 0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
A
C D
E F
B
8
7
8
7
1
2 3
4 5 6
63
The potential region where the surface oxide reduction peaks of Pd, Au and Pd-Au alloy
occur is enlarged in Figure 3.15. The position of the alloy reduction peak is plotted in
the inset as a function of Au atomic composition. A linear increase is observed, i.e. shift
in the peak position towards higher potentials (away from that of pure Pd) with the
increase in the Au concentration in the alloy.
Figure 3.15: Enlarged surface reduction peaks on various Pd-Au alloys. The inset is the position of the surface reduction peak of the Pd-Au alloy as a function of Au composition in the alloy.
In order to assess the effect of heat treatment at 300° C for 15 minutes on the surface
redox processes on Pd-Au surfaces, voltammetric experiments were carried out at an
upper limiting potential of 1.5 VRHE using the same electrodes before and after heat
treatment under these conditions. Figure 3.16 shows window opening cyclic
voltammograms of a number of Pd-Au compositions before annealing (the solid lines)
and after annealing (the dashed lines). No remarkable difference in the features ascribed
to the HER and HOR (1-2) or the oxidation and reduction of the surfaces (3-6) in the
anodic and cathodic sweeps can be observed. The features ascribed to hydride formation
and decomposition (7-8) disappeared after heat treatment. This could be correlated with
the surface segregation of Au upon annealing at these experimental conditions.
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3
j / m
A c
m-2
(geo
)
Potential / VRHE
16 : 84
31 : 69
50 : 50
69 : 31
86 : 14
100 : 0
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0 20 40 60 80 100
Po
ten
tia
l / V
RH
E
Au at. %
64
Figure 3.16: Cyclic voltammograms of different Pd-Au compositions (Pd:Au ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 (γ = 50 mVs-1). The solid lines are before heat treatment and the dashed lines are after heat treatment. CVs are from the second cycles. The limiting potentials are: Ein = - 0.03 VRHE, Elo = 0.2 VRHE and Eup = 1.5 VRHE.
The surface composition of a metal alloy and its components can be determined from the
charges under surface oxide reduction peaks [84]. The charge associated with the surface
reduction peaks (4-6) on a single array of random Pd-Au alloys was determined by
integrating the peaks and are plotted as a function of Au composition in Figure 3.17.
The total charge associated with the three peaks is shown in Figure 3.18. As anticipated,
the charge associated with the reduction peak of Au increases steadily with the increase
in Au content in the alloy. It is also understandable that the charge associated with the
5 : 95
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
23 : 77
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
46 : 54
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
j /
mA
cm
-2 (
ge
o)
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
57 :43
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
76 : 24
Po tentia l / V RH E
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
A
E F
DC
B
1
2 3
4 6
5 7
8
65
reduction peak of Pd decreases with the increase in Au atomic concentration in the alloy.
This behavior could be clearly observed with compositions (≥ 10 % Au). A steep
decrease in the Pd peak charge occurs in the compositional range ≤ 10 at. % Au
followed by a steady decrease with the increase in the Au content in the alloy. This
suggests that a very strong electronic perturbation of Pd occurs by alloying with low
concentrations of Au (ca. 5-10 at. % Au). The charge associated with the Pd-Au
ensemble peak increases and decreases smoothly and symmetrically with a maximum at
ca. Pd50Au50. The total charge is not a linear function of composition, but exhibits a
minimum between (5-10 % Au) and a maximum at ca. 50:50. This behavior is an
indication of the electronic interaction between the components, since a linear decrease
in charge is expected for a non interacting system (the dotted black line in Figure 3.18).
Figure 3.17: Charge associated with reduction peaks of Au, Pd and the Pd-Au alloy as a function of Au composition. The dotted lines are guides to the eye.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
0 20 40 60 80 100
Ch
arg
e /
µC
Au at. %
Au
Pd
Pd-Au
66
Figure 3.18: The total charge (dashed line) associated with the reduction peaks of Au, Pd and Pd-Au alloy as a function of Au concentration. The dotted black line indicates the behavior for a non interacting system.
The surface redox behavior of Pd, Au and Pd-Au alloy can be interpreted based on the
ensembles of atoms as follows: (i) the surface redox behavior of Au ensembles is not
influenced by the neighbouring Pd atoms, (ii) the surface redox behavior of Pd
ensembles is strongly influenced by the neighbouring Au atoms, and (iii) the surface
redox behavior of the alloy is composition-dependent.
The adsorption and stripping of CO on and from the Pd-Au alloys could provide further
insights into the properties of the surfaces and the electronic behavior. Figure 3.19A-F
shows typical CO stripping voltammograms of a number of Pd-Au alloys. A sharp
anodic peak at around 0.95 VRHE was detected in the first scan (red lines). This peak can
be ascribed to the CO stripping from the surfaces [75, 184]. No peaks ascribed to the
HER or hydride formation could be observed in the first scan (red lines) due to a
blocking effect by adsorbed CO on the surface [162]. In the second cycle (black lines),
the CO stripping peaks are not observed which means that CO was completely removed
from the surface in the first scan. This allowed hydrogen to be adsorbed on the alloy
0.00
4.00
8.00
12.00
16.00
20.00
24.00
0 20 40 60 80 100
Ch
arg
e /
µC
Au at. %
Pd Au
Pd-Au
Total Charge
67
surfaces. The two features ascribed to the HER and hydride formation were, then,
detected in the second cycle.
Figure 3.19: Typical CO stripping voltammograms of a number of Pd-Au compositions (Pd:Au ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with 500 ppm CO. The CVs are from the first cycle (red lines) and the second cycle (black lines). The limiting potentials are: Ein = 0.2 VRHE, Elo = 0 VRHE and Eup = 1.1 VRHE. The scan rate = 50 mV/s.
As one may expect, the voltammograms indicate that CO adsorption on the alloy surface
increases with increasing Pd composition. This is consistent with the proposition that
CO adsorption on the Pd-Au surfaces takes place on Pd surface sites and not on Au
surface sites [75, 101]. The relative positions of the CO stripping peaks on the Pd-Au
alloys in Figure 3.19A-F are enlarged in Figure 3.20.
9 : 91
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2 21 : 79
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2
40 : 60
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
j /
mA
cm
-2 (
ge
o)
-2
-1
0
1
2 62 : 38
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2
88 : 12
Potential / VRHE
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2 100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-2
-1
0
1
2
A
E F
DC
B
68
Figure 3.20: The relative positions of the CO stripping peaks on various compositions of Pd-Au alloys. Pd:Au ratio, atomic %.
Clearly, the position and intensity of the CO stripping peak on a Pd-Au alloy vary with
the alloy composition. In order to understand the trend of this variation, the position of
the CO stripping peak is plotted as a function of bulk Au composition in Figure 3.21. A
symmetrical decrease and increase is observed with increasing the Au composition in
the alloy with a minimum potential at an alloy composition of Pd50Au50.
Figure 3.21: The positions of the CO stripping peaks as a function of Au composition in the alloy.
-0.2
0.2
0.6
1
1.4
1.8
0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02
j / m
A c
m-2
(geo
)
Potential / VRHE
9 : 91
21 : 79
40 : 60
62 : 38
88 : 12
100 : 0
0.92
0.925
0.93
0.935
0.94
0.945
0.95
0.955
0.96
0 20 40 60 80 100
Po
ten
tia
l / V
RH
E
Au at. %
69
The position of CO oxidation peak on a catalyst surface primarily depends on the
overpotential for water activation to produce adsorbed OH- species on the surface that is
required to react with adsorbed CO on the surface producing CO2 according to the
Langmuir-Hinshelwood mechanism [185]:
CO → COads (3.4)
H2O → OHads + H+ + e- (3.5)
COads + OHads → CO2 + H+ + e- (3.6)
The lowest peak potential in Figure 3.21 occurs on an alloy composition of ca. Pd50Au50
indicating that this alloy composition is possibly the optimum Pd-Au composition for
CO oxidation. Of relevance to this is the CO oxidation activity on Pt-Ru alloy catalysts
as the overpotential for CO oxidation on the alloy catalysts is lower than on pure Pt [81,
186]. A Pt-Ru alloy catalyst with a composition of Pt50Ru50 has been found to be the
optimum Pt-Ru composition for CO oxidation and is considered as one of the most
promising catalysts for this process [72, 187]. It is interesting to observe that the
optimum Pd-Au composition for CO oxidation (Pd50Au50, as indicated from Figure
3.21) is similar to the optimum Pt-Ru alloy composition.
The CO adsorption and stripping were also assessed on a number of annealed Pd-Au
alloy surfaces (300 °C for 15 minutes). Figure 3.22A-F shows the effect of annealing on
the CO stripping voltammograms of a number of Pd-Au alloys. It appears that annealing
the Pd-Au alloys under these conditions alters the properties of the surfaces towards the
adsorption of CO. The CO stripping peaks from the annealed samples (dashed lines) are
not observed with alloy compositions below 50 % Pd and can be (sometimes hardly)
seen with Pd concentrations > 50 %. Also, the intensities of the CO stripping peaks are
less after annealing which indicate that CO adsorbs weakly on the alloy surface at
equilibrium. This behavior suggests that annealing a Pd-Au alloy hinders the adsorption
of CO. The reason for that can be attributed to the concentration of Pd at the alloy
surface (the active sites for CO adsorption) being lower after annealing compared to the
case before annealing.
70
Figure 3.22: The CO stripping peaks of a number of Pd-Au compositions (Pd:Au ratio, atomic %) before annealing (the solid lines) and after annealing (the dashed lines).
The coverage of the adsorbed CO (θCO) on the Pd-Au alloys surfaces can be determined
by integrating the area under the CO stripping peaks. Figure 3.23 shows the charges
associated with the CO stripping peaks on a random (blue squares) and annealed (300° C
for 15 minutes, red circles) array samples as a function of the bulk Au composition in
the alloy. The dashed line represents the behavior one may predict based on that CO
adsorption occurs only on Pd sites [75, 101] and assuming that Au acts as an inert
diluent (no electronic interaction).
57 : 43
0.80 0.84 0.88 0.92 0.96 1.00 1.04
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
62 : 38
0.80 0.84 0.88 0.92 0.96 1.00 1.04
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
73 : 27
0.80 0.84 0.88 0.92 0.96 1.00 1.04
j /
mA
cm
-2 (
ge
o)
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
85 : 15
0.80 0.84 0.88 0.92 0.96 1.00 1.04
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
93 : 7
Potential / VRHE
0.80 0.84 0.88 0.92 0.96 1.00 1.04
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
100 : 0
0.80 0.84 0.88 0.92 0.96 1.00 1.04
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
C
FE
D
BA
71
The non-linear relationship demonstrates that alloying Pd with Au hinders the adsorption
of CO on the alloy surfaces. This indicates that the chemical and electronic properties of
Pd atoms in Pd-Au alloys are strongly perturbed by alloying with Au [75]. The charge
associated with the CO stripping peaks of the alloy surfaces in the annealed sample is
lower than that of the surfaces in the unannealed sample which implies that annealing
Pd-Au alloys with Au at. % < 80 % reduces CO adsorption on the surface. The reason
for that can be ascribed to that annealing the Pd-Au alloys under these conditions
enriches the surfaces with Au leading to less Pd atoms (the active sites for the CO
adsorption) on the alloy surface [75]. The effect of segregation phenomenon can be
clearly observed with alloy compositions (< 80 % Au).
Figure 3.23: Charges associated with the CO stripping peaks from the Pd-Au alloy surfaces before annealing (blue squares) and after annealing at 300° C for 15 minutes (red circles) as a function of the alloy composition. The dotted lines are guides to the eye. The dashed line reveals the behavior one may expect assuming a random dilution of surface Pd atoms by Au acting as an inert diluent.
The charge associated with the stripping peak of one CO monolayer (ML) on Pd (111) is
approximately 424 µC cm-2 [184]. This estimation was made assuming that each Pd
atom on the surface adsorbs one molecule of CO. Considering that, the real surface area
of the Pd-Au electrodes used in this study can be estimated by dividing the value of the
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0 20 40 60 80 100
Ch
arg
e /
µC
Au at. %
72
charge associated with the CO stripping peak of pure Pd (ca. 16 µC) by the estimated
charge of 1 ML of Pd (111). Hence, the real surface area is approximately equal to 0.038
cm-2. Thus, the roughness factor of the studied catalysts is ca. 2.71 considering the
geometric surface area of each electrode is equal to ca. 0.014 cm-2.
The population of Pd atoms as a function of Au composition is represented by a
statistical model in Figure 3.24 (with thanks to Robert Noble, the writer of the script).
This model was designed assuming a hexagonal surface layer surrounded by randomly
distributed alloy components. A representation of this assumption is shown in Figure
3.25 for a Pd atom surrounded by three active components (Pd atoms) and three inactive
components (Au atoms). The various ensembles on the Pd-Au alloy surface are shown in
this model (Figure 3.24) where the population of the Pd atoms varies with the alloy
composition. A Pd atom on the alloy surface can be surrounded by six Pd atoms (black),
five (grey), four (yellow), trimers (blue), dimers (green) or monomers of Pd atoms (red).
The second layer interaction has been ignored in this model.
Figure 3.24: A representative model of the population of Pd atoms on the alloy surface as a function of Au composition in the alloy assuming a hexagonal surface layer where the Pd atom on the surface can be surrounded by six (black), five (grey), four (yellow), three (blue), two (green) or one (red) neighbouring Pd atom.
Au at. %
0 20 40 60 80 100
Po
pu
lati
on
/ a
.u.
0
200
400
600
800
1000
1200
1400
1600
1 neighbour 2 neighbours 3 neighbours 4 neighbours 5 neighbours 6 neighbours
73
Figure 3.25: A Pd atom surrounded by hexagonally oriented three active components (Pd atoms) and three inactive components on the alloy surface.
The CO adsorption (coverage) on the Pd-Au alloy catalysts (Figure 3.23) can be
described according to the model shown above assuming that a CO molecule adsorbs on
a single Pd atom as follows: (i) CO adsorbs preferentially on the ensembles very rich
with Pd, (ii) the intermediate Pd-Au alloy ensembles incorporating even one or two Au
atoms exhibit low affinity towards CO adsorption as a steep decrease occurs in the
intermediate composition, (iii) CO adsorption is also preferential on the ensembles
having Pd monomers and dimers as Au-rich alloys remain active for CO adsorption.
These observations are consistent with the proposal that CO adsorption is favored on the
large Pd ensembles (when the alloy rich with Pd), and that CO adsorbs on Pd monomers
at poor-Pd alloys [75].
3.4- The Catalytic Activity for the HER
In order to assess the HER catalytic activity on the Pd-Au alloys, a series of potential
step experiments were performed at room temperature in 0.5 M HClO4 using a number
of 100-electrode electrochemical arrays with various compositions of both elements. In
each experiment, the potential was stepped from 0 VRHE where the HER does not occur
to more negative potentials where the HER takes place. The current was recorded at the
following potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007 → 0 VRHE.
At each step, the potential was held for 90s. The backward steps were performed in
Pd
Pd
Au
Au Pd
Au Pd
Active component
Inactive component
74
order to examine the reproducibility of the resulting currents from the forward steps.
Examples of the current densities (j) of various Pd-Au compositions at these potentials
as a function of time are shown in Figure 3.26. The non-zero current densities at E = 0
VRHE is attributed to the accuracy of the reference electrode used in the electrochemical
measurements so that there is a small difference between the precise potential in the
potentiostat and the real applied potential on the working electrode. The difference is
about 0.003 VRHE. Clearly, the responses are varied with the compositions of the alloys.
The Pd-Au alloy with high content of Au (Pd8Au92) shows the lowest activity among
these catalysts and a better activity is shown with decreasing the Au concentration in the
alloy to ca. 80 at. %. The highest activity is exhibited by the alloy with a composition of
Pd45Au55.
75
Figure 3.26: The current densities in the HER as a function of time recorded at room temperature on a number of random Pd-Au catalysts (Pd:Au ratio, atomic %) in 0.5 M HClO4.
The HER activity on a series of Pd-Au alloy catalysts were measured with the purpose
of providing data over the whole compositional range of the Pd-Au alloy system. Figure
3.27 shows the geometric HER activity at - 17.51 mVRHE as a function of the bulk alloy
composition obtained on a single array of Pd-Au alloy catalysts. This curve can be
divided into three regions. In the compositional range between ca. 1-25 at. % Au, the Pd-
Au alloys are observed to have almost similar activities to pure Pd. An interesting
catalytic behavior for the HER is exhibited in the range of Pd-Au alloys 30-70 at. % Au.
A maximum for the HER activity (ca. 0.35 mA cm-2) occurs at ca. Pd50Au50 and is ca.
three times larger than pure Pd. A sharp decrease in the catalytic activity is observed
8 : 92
0 100 200 300 400 500 600 700
-0.4
-0.3
-0.2
-0.1
0.020 : 80
0 100 200 300 400 500 600 700
-0.4
-0.3
-0.2
-0.1
0.0
45 : 55
0 100 200 300 400 500 600 700
j /
mA
cm
-2 (g
eo
)
-0.4
-0.3
-0.2
-0.1
0.0
71 : 29
0 100 200 300 400 500 600 700
-0.4
-0.3
-0.2
-0.1
0.0
86 : 14
0 100 200 300 400 500 600 700
-0.4
-0.3
-0.2
-0.1
0.0
100 : 0
Tim e / s
0 100 200 300 400 500 600 700
-0.4
-0.3
-0.2
-0.1
0.0
C
BA
E F
D
0
- 0 .00 7
0
- 0 .00 7
- 0 .0 14 - 0 .0 14
- 0 .0 21
76
with the increase in Au concentration on the alloys having compositions > 75 % Au. A
similar compositional dependence for the HER was obtained at - 10.38 mVRHE as shown
in Figure 3.28.
Figure 3.27: The geometric HER activity at - 17.51 mV as a function of the bulk composition of a number of Pd-Au catalysts measured at room temperature in a 0.5 M HClO4 electrolyte. The geometric current densities in this plot are the average of three experiments. The dotted line is a guide to the eye.
Figure 3.28: The geometric HER activity of a number of Pd-Au catalysts (the same alloys used in the latter plot) at - 10.38 mV measured at room temperature in 0.5 M HClO4.
0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
77
In order to determine the HER specific activity, the current values obtained on the Pd-
Au array sample used in the latter measurement were normalized to Pd composition in
the alloy assuming that the adsorption of hydrogen only takes place on Pd. Figure 3.29
illustrates the specific current densities at -17.51 mV as a function of the bulk alloy
composition. The maximum activity appears at a surface composition of ca. Pd30Au70
shifting towards higher compositions of Au compared to the case of the geometric
activity where the maximum activity was observed at ca. Pd50Au50 (Figure 3.27).
Figure 3.29: The specific HER activity at -17.51 mV of the Pd-Au catalysts measured at room temperature in 0.5 M HClO4.
The enhanced HER activity on the alloy surface compared to pure Pd could be
associated with the formation of low co-ordinated Pd ensembles (trimers, dimers and
monomers) as shown from the statistical model in Figure 3.24. It clearly indicates that
the optimum Pd-Au compositional range (at ca. Pd50Au50) for the HER is mainly
dominated by Pd trimers, dimers and monomers. The population of Pd monomers
increases as the Au concentration in the alloy increase. This is consistent with the
observations on Au/Pd(111) surface alloys [142] as a maximum activity was found on
the alloy with surface composition of ca. 80 % Au. The proposed role of Au at this range
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0 20 40 60 80 100
j / m
A c
m-2(specific)
Au at. %
78
of composition is to isolate Pd into monomers resulting in active sites for the HER
[142].
The HER on Pd monomers has been speculated to follow the Volmer-Heyrovsky
mechanism where the adsorption of a single hydrogen atom on a single Pd atom is more
likely than the adsorption of two atoms (Tafel step) [142]. This is however in contrast
with the proposal that Pd dimers are the smallest ensemble that is required for hydrogen
adsorption to take place [75].
The enhanced HER activity on the alloy surface can also be discussed in terms of
changes in the electronic structure upon alloying. The Au d-band is fully occupied and
contains no unpaired electrons. This electronic structure of Au is probably responsible
for the very low activity of Au on its own [188]. Upon alloying with Au, the position of
the Pd d-band centre is changed leading to altered adsorption properties of an adsorbate
[21]. According to the Sabatier principle, an optimum catalyst should exhibit an
intermediate hydrogen adsorption binding energy (intermediate interaction with the
catalyst surface) [60]. A continuous downward shift of the Pd d-band has been observed
with increasing Au concentration in the alloy [189]. The deep shift in the d-band center
indicates a small number of valence d electrons [21]. This may explain the decrease in
the activity on the Au-rich alloys as the hydrogen adsorption becomes weaker.
The HER activity was also assessed at room temperature on a series of annealed (300 °C
for 15 minutes) Pd-Au catalysts. Figure 3.30 shows the geometric current densities at
- 17.39 mVRHE on a single array of annealed Pd-Au alloys as a function of the bulk
composition. The catalytic activity curve differs from that obtained on the unannealed
sample (Figure 3.27) as the maximum activity occurs at a bulk composition of ca.
Pd35Au65. In other words, the optimum Pd-Au composition on the catalytic activity
curve has shifted to lower compositions of Au compared with the case before annealing.
This can be directly linked to XPS results which have shown that annealing Pd-Au
alloys results in surface segregation of Au atoms (Figure 3.10). Therefore, the
concentrations of Au in the Au-poor content alloy surfaces were increased by annealing.
79
This is consistent with the reported literature regarding the effect of annealing on the
surface composition of Pd-Au alloys [74, 77, 179-181].
Figure 3.30: the HER activity at -17.39 mV as a function of the Au bulk composition on several annealed Pd-Au catalysts (300º C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye.
The specific activity of the same catalysts is shown in Figure 3.31. The maximum
activity appears again at higher concentrations of Au (ca. Pd60Au40) compared to the
geometric activity (Figure 3.30). The difference between the two plots can also be
associated with the formation of low co-ordinated Pd ensembles in the presence of high
concentration of Au.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
80
Figure 3.31: The specific HER activity at - 17.39 mV on a number of annealed Pd-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4.
It should be emphasized here that the atomic percentages of Au in Figures 3.27 and 3.30
refer to the bulk composition measured by EDS assuming that the bulk composition is
identical to the surface composition before heat treatment. At equilibrium, the surface
concentration of the alloys differs from that of the bulk. Therefore, the HER activity at -
17.39 mV on the equilibrated Pd-Au alloys is plotted in Figure 3.32 as a function of the
surface composition of the alloys. The surface composition on the alloy surface was
determined from XPS data (Figure 3.10). A maximum activity is observed in the
compositional range of ca. Pd50Au50. This result is consistent with the behavior observed
with unannealed catalysts (Figure 3.27) where the optimum composition for the HER
was identified to be ca. Pd50Au50.
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0 20 40 60 80 100
j / m
A c
m-2
(specific
)
Au at. %
81
Figure 3.32: the HER activity at - 17.39 mV as a function of the surface composition (red diamonds) of several annealed Pd-Au catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye. The blue curve is the HER activity as a function of bulk Au composition shown in Figure 3.27 for comparison.
The increase in the HER activity on Pd-Au alloys could also be linked to a decrease in
the content of Pd hydride phase in the bulk with increasing Au concentration. This
would be in accordance with the conclusion by Kibler [190] that the formation of bulk
hydride reduces the HER activity on Pd/Au(100). It is also in agreement with the
observation by others [140] that increasing Au content decreases the Pd hydride
formation and completely suppresses it at ca. 70 at. % Au.
Another observation from the HER activity is that the geometric current densities
obtained on the annealed Pd-Au alloy samples were greater than the geometric current
densities obtained with the unannealed (random) samples. Similar behavior was
observed on Pd-Bi array samples. This point will be discussed later in this thesis.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
82
3.5- The Catalytic Activity for the HOR
A series of potential step experiments were performed on a number of Pd-Au
electrochemical arrays with different compositions of Pd and Au in order to assess their
catalytic activities for the HOR. The experiments were carried out at room temperature
in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution. The potential
was stepped from 0 VRHE where the HOR does not occur to more positive potentials
where this reaction takes place. The current was recorded at the following potentials: 0
→ + 0.007 → + 0.014 → + 0.021 → + 0.014 → + 0.007 → 0 VRHE and the potential was
held for 90s at each step. The direction of the steps was reversed in order to examine the
reproducibility of the currents. Figure 3.33 shows the geometric current densities
measured at various Pd-Au alloy surfaces. Again, the non-zero current densities at E = 0
VRHE is attributed to the accuracy of the reference electrode used in the electrochemical
measurements. The Pd-Au alloys with high and low Au concentrations (Pd11Au89 and
Pd81Au19) are observed to have activities similar to pure Pd. Alloys with compositions of
Pd23Au77 and Pd64Au33 resulted in similar responses. The highest current density is
observed on the Pd48Au52 (Pd:Au ratio of 1:1) alloy surface.
83
Figure 3.33: The current densities in the HOR as a function of time recorded at room temperature on a number of Pd-Au catalysts (Pd:Au ratio, atomic %) in 0.5 M HClO4.
The responses of the 100-electrodes were quantitatively analyzed to assess the catalytic
activity of a wide compositional range of Pd-Au alloy catalysts for the HOR. Figure
3.34 shows the geometric current densities measured on a number of Pd-Au alloy
catalysts as a function of the bulk alloy composition. Interestingly, the catalytic activity
curve for the HOR is similar to that for the HER (Figure 3.27). Alloys with low and
high concentrations of Au show the lowest activities for the HOR. Pure Pd displays a
low catalytic activity in this reaction and it behaves similarly to the Au-poor / rich
100 : 0
0 100 200 300 400 500 600 700
-0.2
-0.1
0.0
0.1
0.2
11 : 89
0 100 200 300 400 500 600 700
-0.2
-0.1
0.0
0.1
0.2
48 : 52
0 100 200 300 400 500 600 700
j /
mA
cm
-2 (
ge
o)
-0.2
-0.1
0.0
0.1
0.2
23 : 77
0 100 200 300 400 500 600 700
-0.2
-0.1
0.0
0.1
0.2
64 : 36
0 100 200 300 400 500 600 700
-0.2
-0.1
0.0
0.1
0.2
81: 19
Time / s
0 100 200 300 400 500 600 700
-0.2
-0.1
0.0
0.1
0.2
A
E F
DC
B
0 0
0.007
0.0070.014
0.014
0.021
84
alloys. The highest catalytic activity is observed in a wide compositional range of the
Pd-Au alloys with a maximum at around 50 % Au. The HOR specific activity as a
function of the bulk alloy composition is also shown in Figure 3.35. Similarly to the
HER activity curve (Figure 3.29), the highest activity appears in the compositional
range between ca. 60-80 at. % Au with a maximum at Pd30Au70.
Figure 3.34: The geometric HOR activity at 17.93 mV of a wide range of Pd-Au alloy catalysts measured at room temperature in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution. The j values are the average of two experiments. The j values at Pd electrodes are the average of two samples. The dotted line is a guide to the eye.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
85
Figure 3.35: The HOR specific activity at 17.93 mV on various Pd-Au alloys measured at room temperature in 0.5 M HClO4.
The HOR activity was also examined over a number of annealed Pd-Au alloys (300 °C
for 15 minutes). The result at 17.95 mV is plotted as a function of the bulk alloy
composition in Figure 3.36. A higher activity on the alloys than pure Pd is observed in
the compositional range of ca. ≤ 50 at. % Au with a maximum at a bulk composition of
ca. Pd35Au65. A steady decrease in the catalytic activity is observed with alloy
compositions above 50 at. % Au. This behavior was also observed in the HER (Figure
3.30) and could be due to the surface segregation of Au upon annealing.
Considering the surface composition (Figure 3.37), the results are also comparable to
those in the HER (Figure 3.32) as a maximum in the activity occurs in the surface
composition of ca. Pd50Au50. The HOR specific activity on an annealed array of Pd-Au
alloys (Figure 3.38) is slightly different as a small increase in the catalytic activity
appears at ca. Pd35Au65. This is also comparable to the behavior observed for the HER
(Figure 3.31) and can be again correlated with the formation of more low co-ordinated
Pd ensembles at compositions above 50 at. % Au.
0
0.0005
0.001
0.0015
0.002
0.0025
0 20 40 60 80 100
j / m
A c
m-2
(specific
)
Au at. %
86
Figure 3.36: the HOR activity at 17.95 mV on several annealed Pd-Au catalysts (300 °C for 15 minutes) as a function of bulk composition measured at room temperature in 0.5 M HClO4. The value on pure Pd is the average of ten Pd samples. The dotted line is a guide to the eye.
Figure 3.37: the HOR activity at 17. 95 mV as a function of the surface composition of a number of annealed Pd-Au catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The value on pure Pd is the average of ten Pd samples. The dotted line is a guide to the eye.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Bulk Au at. %
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
87
Figure 3.38: The specific HOR activity at 17.95 mV on a number of annealed Pd-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in a 0.5 M HClO4.
The HER and HOR activities on random Pd-Au alloys (Figures 3.29 and 3.35) show a
similar compositional dependence as a composition that is active for the HER would
also be active for the HOR. This behavior, however, appears to be slightly different at
alloy compositions very rich with Au as the HER activity decreases monotonically
towards 100 at. % Au (Figure 3.29), while the HOR activity decreases more rapidly at
alloy composition of ca. 90 at. % Au (Figure 3.35). The HER and HOR activities at this
compositional range of the alloy is mainly dominated by Pd monomers. Thus, the
different behavior could be attributed to a higher selectivity towards the Tafel-Volmer
mechanism in the HOR that is suppressed on Pd monomers. This is consistent with the
proposition that the HER on Pd monomers proceeds according to the Volmer-Heyrovsky
mechanism and that a single hydrogen atom adsorbs on a Pd monomer [142].
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0 20 40 60 80 100
j / m
A c
m-2
(specific
)
Au at. %
88
3.6- The Carbon Monoxide Tolerance during the HOR
Another examination of the catalytic performance of Pd-Au alloys for the HOR was
carried out in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm carbon monoxide
bubbling through the solution during the experiment. The aim of this examination was to
investigate the influence of CO as a catalyst poison of Pd-Au alloys. To achieve that, a
series of potential step experiments were carried out. These experiments were performed
several times (after 1, 11, 22, 33 and 44 min. of bubbling with the H2/CO mixture) on
the same sample in order to examine the effect of CO with time on the catalytic activity
of the alloy system. The potentials used in these experiments were similar to the
potentials mentioned in the previous section. Figure 3.39 shows the geometric current
densities at 17.95 mVRHE on a 100-electrode array of Pd-Au alloys with various
compositions starting from ca. 8 % Au to approximately 96 % Au. Broadly, the HOR
activity curve in this Figure appears similar to the curve appearing in Figure 3.34 (with
only H2 bubbling through the solution) and it can be clearly observed even after
bubbling the solution with the mixture for 44 minutes. However, the HOR activity along
the whole Au compositional axis is reduced progressively with time. The HOR activity
on the optimum Pd-Au composition (Pd50Au50) is reduced for about 50% after 44
minutes of bubbling the solution with the H2/CO mixture.
89
Figure 3.39: The HOR activity at 17.95 mV on various Pd-Au alloys measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution. The values on pure Pd are the average of ten Pd samples. The black dashed curve is the HOR activity with only H2 bubbling through the solution (Figure 3.34) for comparison.
The HOR activity on a number of annealed Pd-Au alloys was also assessed in the
presence of the H2/CO mixture (Figure 3.40). Once more, a wide range of annealed Pd-
Au alloy catalysts (≤ 40 % Au) show similar activities to pure Pd and a steady decrease
in the catalytic activity is observed above this concentration. Similar result (with wider
compositional range, ≤ 60 % Au) is observed considering the surface composition
(Figure 3.41). Similar to the case before annealing, a progressive decrease in the Pd-Au
alloy activity for the HOR is observed with increase of bubbling time of the H2/CO
mixture.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
1 min.
22 min.
44 min.
90
Figure 3.40: : The HOR activity at 17.95 mV on various annealed Pd-Au alloys (300 °C for 15 minutes) as a function of the bulk composition. The measurement was performed at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution. The black dashed curve is the HOR activity on an annealed array sample with only H2 bubbling through the solution (Figure 3.36) for comparison.
Figure 3.41: : The HOR activity at 17.95 mV as a function of the surface composition of various annealed Pd-Au alloys (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the solution.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Bulk Au at. %
1 min.
22 min.
44 min.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100
j/ m
A c
m-2
(geo
)
Surface Au at. %
1 min
22 min.
44 min.
91
Pd-Au alloy catalysts are less active for the HOR than pure Pt [143]. However, Pd and
Pd-Au alloys appear to be superior to Pt and Pt-Ru alloys with respect to the CO
tolerance. It was shown in this regard that the presence of small concentrations of CO (≤
10 ppm) in the solution poisons the catalytic performance of Pt for the HOR [71], while
the HOR kinetic rates drop significantly on Pt-Ru alloys in the presence of CO with
concentrations above 100 ppm [191].
The results in Figures 3.39-3.41 indicate that the catalytic performance of Pd and Pd-Au
alloys for the HOR is not strongly poisoned by CO under these experimental conditions.
This is consistent with the strong reduction in CO adsorption with increasing Au
concentration on the alloy surface (Figure 3.23). It is also consistent with the high
resistivity observed by others for Pd overlayers on Au towards CO poisoning [166]. The
results also suggest that Pd-Au alloys offer a higher degree of CO tolerance than both Pt
and Pt-Ru alloys. This could be ascribed to a lower CO adsorption energy on the Pd-Au
alloy system [139].
The HOR activity on Pd-Au alloys in the presence of CO concentrations above 100 ppm
has been discussed in various studies in terms of their CO tolerance. It has been shown
that the HOR activity on various Pd-Au/C (Pd80Au20, Pd70Au30 and Pd50Au50) electrodes
is higher than on Pt50Ru50/C alloy in the presence of 1000 and 250 ppm of CO [139]. It
has also been observed that Pd38Au62 surface alloy is more active for 1000 ppm CO/H2
oxidation than Pd65Au35 above ca. 0.25 VRHE [101]. The higher HOR activity in both
cases has been attributed to a higher degree of CO tolerance.
The observations in Figures 3.39-3.41 eventually indicate that Pd-Au alloy catalysts
could offer alternatives to Pt for fuel cell reactions. A similar proposal has been put
forward by others [166].
92
3.7- Conclusions
The structure, bulk composition and surface composition of Pd-Au alloys were analyzed
by XRD, EDS and XPS respectively. The XRD data show a shift of 2θ values from that
of Au (111) peak to that of Pd (111) peak which indicate the formation of solid solutions
in the alloys. The quantitative analysis of the unannealed Pd-Au alloys by EDS and XPS
illustrated that the bulk composition is similar to the surface composition. The
comparison between the surface composition before and after annealing at 300 °C for 15
minutes showed a surface segregation of Au in agreement with the reported literature.
Cyclic voltammetry experiments have identified a number of features in the anodic and
cathodic sweeps correspond to the HER, HOR, formation of surface oxides, reduction of
Pd, Au and Pd-Au ensembles as well as hydrogen adsorption and desorption. The CO
stripping voltammograms has indicated that CO adsorption on the alloy surface
increases with increasing Pd concentration in the alloy. The analysis of charges
associated with the CO stripping peaks has shown that alloying Pd with Au strongly
hinders CO adsorption on the alloy surfaces. Further analysis of CO stripping data has
revealed that the real surface area of the studied catalysts is about 0.038 cm-2 and the
roughness factor is ca. 2.71.
The catalytic assessment of the HER has suggested that the optimum Pd-Au bulk and
surface composition for this reaction is in the compositional range of ca. Pd50Au50 which
was found to be about three times more active than pure Pd. The enhanced HER activity
on the alloy surface compared to Pd was associated with the formation of Pd ensembles
having more trimers, dimers and/or monomers. The HER activity on the alloys very rich
with Au is most likely dominated by Pd monomers (single Pd atoms surrounded by Au
atoms). The enhanced activity could also be ascribed to a modification in the electronic
structure upon alloying that results in higher activity. It could also be linked to a
decrease in the hydride formation in the bulk with increasing Au content.
93
The HER and HOR activities on random Pd-Au alloys have shown similar activity-
composition relationships as an alloy catalyst that is active for the HER would also be
active for the HOR. However, a slight deviation from this relation occurs on the very
rich Au alloys where a monotonic decrease in the HER activity towards 100 at. % Au
and a more rapid decrease in the HOR activity at alloy composition of ca. 90 at. % Au
was observed. This behavior is attributed to that the Tafel-Volmer mechanism being
favoured in the HOR that is suppressed on Pd monomers (the most available form of Pd
ensembles at this compositional range of the alloy). The HER activity on the annealed
Pd-Au array samples were greater than on the unannealed (random) samples. This point
will be discussed later in this thesis.
The catalytic performance of Pd and Pd-Au alloys for the HOR has been observed not to
be strongly poisoned in the presence of a mixture of 500 ppm CO and H2. This indicates
that Pd-Au alloys offer a higher degree of CO tolerance in comparison with both Pt (≤
10 ppm) and Pt-Ru alloys (≤ 100 ppm). A possible interpretation of this is that CO
adsorption energy on Pd-Au alloys is lower than on Pt or Pt-Ru alloys. This behavior
would suggest that Pd-Au alloys could offer alternative to Pt for fuel cell reactions.
94
Chapter 4: Palladium-Bismuth (Pd-Bi) Alloy Surfaces
4.1- Introduction
Bi is an inactive material for heterogeneous catalysis, but it can be an effective promoter
for the overall catalytic performances of Pd or Pt based catalysts. Incorporation of Bi
with Pd or Pt may improve the activity, selectivity and/or the lifetime of the catalyst
[192, 193]. There are various interpretations in the literature of the promoting role of Bi
on the catalytic performances of Pd or Pt. A number of them are summarized in the
following:
(i) The presence of Bi adatoms with Pt or Pd atoms on the surface may improve the
adsorption properties of the precious metal by forming new active sites for the reactant
species [192, 193].
(ii) The promoter may cause a geometric blocking of active sites on the noble metal
surface leading to a controlled surface orientation of the reactants and consequently
improve the selectivity [192, 193].
(iii) The promotion could possibly be achieved by minimizing the size of the active site
ensembles on the catalyst surface which suppress the formation and strong adsorption of
poisoning intermediates [193].
(iv) The Bi adatoms could improve the CO tolerance of the precious metal catalyst
through a so called "third-body effect" in which the presence of Bi as a third-body (in
addition to the precious metal and the adsorbed species) blocks a number of active sites
available for CO adsorption resulting in a decrease in the amount of adsorbed CO on the
surface (geometrical hindrance) [194-196].
(v) It has also been proposed that the role of Bi as a promoter is to improve the precious
metal catalyst resistance towards over-oxidation and hence deactivation during the
catalytic process [193, 197].
95
(vi) The role of the promoter is to inhibit the corrosion of the precious metal in acidic
media [192, 193].
(vii) The electrocatalytic activity of Pt or Pd in the presence of Bi as promoter can also
be enhanced through an electronic effect in which Bi influences the nearest Pt or Pd
neighbor atoms leading to a better catalytic performance [198].
4.1.1- Industrial Applications of Pd-Bi Catalysts
Pd-Bi catalysts are mostly used in the industrial synthesis of a number of organic
compounds. For instance, Bi promoted Pd supported on C (Bi-Pd/C) catalysts has been
used to catalyze the oxidation of glucose to gluconic acid at 313 K [199]. The catalyst
was synthesized at room temperature by the deposition of Bi using a surface redox
reaction on Pd supported on carbon. The presence of Bi adatoms in this process has led
to better activity, selectivity, and stability of Pd. High yields of gluconate (99.3 %) was
achieved. Oxygen acts as a poison of the metal catalysts during the catalytic oxidation of
carbohydrates or alcohol. Bi adatoms have been suggested to prevent the deactivation,
caused by oxygen poisoning, by acting as a co-catalyst in the process.
The catalytic oxidation of glucose to gluconic acid was also examined using a number of
Pd catalysts supported on SiO2 [200]. The activity and selectivity of those catalysts in
this reaction were greatly improved by the addition of Bi as a catalyst modifier. The
reason for that was attributed to the formation of intermetallic compounds BiPd and
Bi2Pd in the bimetallic catalyst due to a strong interaction between Pd and Bi (ligands
effect).
Similar investigation on the selective oxidation of glucose to gluconic acid was carried
out using bimetallic Pd-Bi, Pd-Tl, Pd-Sn, and Pd-Co catalysts supported on C and SiO2
[201]. The best activity and selectivity were obtained using catalysts modified with Bi.
XRD analysis of the Pd-Bi/SO2 catalyst proved the formation of intermetallic
compounds BiPd and Bi2Pd responsible for the increase in the selectivity of the catalyst.
96
Bi was also used as a promoter of Pd/C catalyst in the selective oxidation of glucose to
gluconate [192]. The addition of adequate amounts of Bi enhanced the overall catalytic
performance of the catalyst by forming Bi-glucose or Bi-gluconate complexes in
solution or at the catalyst surface which facilitates glucose dehydrogenation and the
subsequent steps in the proposed mechanism. The active sites on the catalyst surface
were suggested to compose of an assortment of one Bi atom and two or three Pd atoms.
Another industrial application of Pd/C catalyst promoted by Bi is for the selective
oxidation of lactose by molecular oxygen to sodium lactobionate in a pH range 7-10 and
at temperatures up to 333 K [202]. The Bi-Pd/C catalyst has uniquely enhanced the
selectivity for sodium lactobionate to 100 % resulting in yields of 95 % in about one
hour.
The synthesis of sodium gluconate from glucose oxidation has also been examined on Bi
promoted Pd/C catalyst prepared from a chemical solution containing PdCl2 and
Bi(NO)3.5H2O [203]. The ratio of Bi and Pd in the catalyst was observed to influence
the reaction rate. The best promotion effect was observed when the PdCl2 to
Bi(NO)3.5H2O weight ratio was 1 to 3 in the solution for catalyst preparation. The Bi-
Pd/C catalyst has exhibited a better catalytic performance for the synthesis of sodium
gluconate compared to Pd/C catalyst alone as the incorporation of Bi prevents Pd/C
deactivation in this process. The lifetime of the Bi-Pd/C catalyst was also found to be
longer than that of Pd/C catalyst.
Biella and colleagues have examined the selective oxidation of D-glucose to D-gluconic
acid using a number of Au, Pd and Pt on carbon catalysts [204]. The best result was
given by the Au/C catalyst. The authors suggested that adding Bi to Pt or Pd on C
catalysts may enhance their performance for the oxidation process.
The catalytic efficiency of Bi-Pd/C catalyst was also observed to be superior to that of
Pd/C catalyst (prepared by the same procedure) in the selective oxidation of glyoxal into
glyoxalic acid (OHC-COOH) [197]. The catalyst composition was observed to influence
97
the catalytic activities of Bi-Pd/C catalysts. The best results were obtained using
catalysts with molar ratios Bi/Pd between 0.5 and 1. The catalytic activity of Bi-Pd/C
catalyst was also compared to that of a commercial trimetallic Pd-Pt-Bi/C catalyst in the
same reaction. The bimetallic catalyst exhibited a constant catalytic activity with time,
while the trimetallic catalyst showed deactivation after 10 h operation.
The benzylacetate synthesis by oxyacetoxylation of toluene has been assessed by
Miyake and colleagues [205] using pure Pd, and two Pd-Bi binary catalysts (molar ratios
Pd/Bi = 3 and 1). One of the concerns in such a process is the deactivation of the catalyst
resulting from the dissolution of Pd. Incorporation of Bi with Pd (especially Pd/Bi = 3)
led to better activity, selectivity and stability. The reason of this behavior was ascribed
to the formation of an intermetallic compound in the binary catalyst. The authors
suggested that the Pd-Bi (Pd/Bi = 3) catalyst is likely to be a promising catalyst for the
industrial synthesis of benzylacetate.
The catalytic activity of Pd/Al2O3 in the hydrogenation of 1-hexyne and 2-hexyne has
been improved by modification with Bi or Pb [206]. Better selectivity in the process was
achieved using either Bi or Pb as a catalyst modifier. However, Bi has been suggested to
be a better catalyst modifier than Pb. The improvement of the selectivity in this process
was attributed to that the addition of the catalyst modifiers (Bi or Pb) suppresses
undesired isomerisation reactions of the primary products.
4.1.2- Applications of Pd-Bi Catalysts in Electrocatalysis
Pd-Bi alloys are not commonly used in electrocatalysts. To the extent of the author’s
knowledge, there is no reported data for the anodic reactions in fuel cells on Pd-Bi alloy
catalysts. Most of the research efforts in this field were directed towards the employment
of Bi to promote the performance of Pt-based catalysts. It has been theoretically
predicted that the HER activity on a surface alloy of Pt-Bi is comparable or even higher
than pure Pt [60]. The experimental data has proved this theoretical prediction [60]. A
98
further theoretical prediction has proposed that Pd3Bi bulk alloy may offer a promising
activity for the HER [93].
Another application of Pt-Bi catalysts is for the electrochemical oxidation of formic acid
where the catalytic activity of the binary catalyst was found to be better than pure Pt
[141, 198, 207-209]. The superior catalytic performance of Pt-Bi catalyst for this process
was ascribed to that the presence of Bi in the catalyst surface: (i) improves formic acid
adsorption and produces surface oxides at low potential, and (ii) minimizes the CO
poisoning effect, since pure Pt can be easily affected by poisoning species [141].
CO electro-oxidation has also been assessed on Bi modified Pt(110)-(1x2) and Pt (111)
surfaces [194, 196]. The authors concluded that the adsorption of Bi on the Pt surfaces
reduces the active sites for CO adsorption and, therefore, improves their CO tolerance. It
has also been concluded that the incorporation of Bi results in an increase and a decrease
in the CO oxidation potential on Pt(110)-(1x2) and Pt (111) respectively.
4.2- Composition and Structure Analysis
Thin film libraries of random Pd-Bi alloys have been synthesized on a number of 100-
field Pd-Bi arrays employing the HT-PVD method described previously. The
preparation method allowed the deposition of a wide compositional range of Pd-Bi alloy
catalysts. The bulk and surface compositions of the alloys have quantitatively been
determined using EDS and XPS respectively.
4.2.1- EDS Analysis
Figure 4.1 shows false color maps of the bulk Pd and Bi compositions in a single array
of Pd-Bi alloy thin films with respect to the (x, y) position in the array. A diagonal
increase in the Pd concentration from the bottom left side to the top right side and an
opposite increase in the Bi concentration are observed. The EDS analysis of the bulk
alloy composition clearly shows that the deposition method employed for synthesis of
99
alloys allowed the achievement of approximately the whole compositional range of Pd-
Bi alloy system.
Figure 4.1: False color maps of Pd and Bi concentrations in a single array of Pd-Bi alloys with respect to the position in the array sample. The dashed arrows denote the growth direction of the elemental components in the array.
4.2.2- XRD Analysis
Pure Pd has a fcc crystal structure with a lattice parameter = 3.89 Å [174], while pure Bi
crystallizes in the rhombohedral structure with lattice parameters a = 4.55 Å and c =
11.86 Å [210, 211]. The powder XRD patterns of a number of random and annealed
(300 °C for 15 minutes) Pd-Bi thin films are shown in Figures 4.2 and 4.3 respectively.
The Bragg peaks appearing in these figures could be ascribed to a number of Bi or Pd
planes [176, 177, 210, 212]. The intensities of some of these peaks are observed to be
less after heat treatment which possibly signifies that annealing a Pd-Bi alloy catalyst
produces preferential orientation of the crystallites. A Bragg peak can be identified at ca.
2θ = 32.28° indicating the formation of an intermetallic compound Bi2Pd with a
monoclinic structure [Froodite, card number: 11-251]. This peak is more pronounced
X
Y
100
with alloy composition of ca. Pd25Bi75. The formation of the intermetallic compound
Bi2Pd in this range of composition has an effective influence on the catalytic activity of
the Pd-Bi alloys for the HER and HOR. This influence will be shown in detail later in
this chapter.
The XRD measurements of a number of PdBi/SiO2 surfaces containing 2% Bi 5% Pd,
4% Bi 5% Pd and 5% Bi 5% Pd have also proved the presence of intermetallics with
compositions Bi2Pd and BiPd [200]. The presence of two intermetallic compounds
including BiPd and BiPd3 has also been reported by Alardin and colleagues [197]. A
thermodynamic assessment of the Pd-Bi system [213] showed that the energy of
formation of PdBi ( ) and PdBi2 (
) are -33.6 and -25.1 kJ mol-1
respectively, while no calculations were performed of the composition Pd3Bi due to its
complexity. The formation of an intermetallic compound in the Pd-Bi alloys could be
attributed to a strong interaction between the elemental components. This chemical
property may play an essential role in the enhancement of the catalytic performance of
Pd-Bi binary catalysts [200].
Figure 4.2: Typical XRD patterns of various Pd-Bi alloys.
0
3
6
9
12
15
20 24 28 32 36 40 44 48
Inte
ns
ity
/ A
rbit
rary
Un
its
2θ Scattering Angle / °
6 at. % Bi
26 at. % Bi
48 at. % Bi
73 at. % Bi
88 at. % Bi
98 at. % Bi
Bi
(003)
Bi
(012)
Pd
(111)
Bi
(104)
Bi
(015)
Bi
(006)
Bi2Pd
101
Figure 4.3: Typical XRD patterns of various Pd-Bi alloys annealed at 300 °C for 15 min.
The phase diagram of the Bi-Pd system (Figure 4.4) [53] shows that the formation of
the intermetallic compound α(β)-Bi2Pd takes place in a narrow compositional range (ca.
≥ 70 at. % Bi). The phases α(β) BiPd and α(β) Bi2Pd5 could be formed in the
compositional ranges between 50-70 and 30-50 at. % Bi respectively, while the
formation of the composition α(β) BiPd3 occurs in a very narrow region (ca. 25-30 at. %
Bi). The α-Bi2Pd could be formed at temperatures below 271 ºC and the β-Bi2Pd could
be formed after annealing at a temperature higher than the melting point of pure Bi
(above 271 ºC).
The phase diagram of this system also shows that the formation of solid solution alloys
between the elemental components may occur in a narrow compositional region (ca. ≤
25 at. % Bi). Considering this, the lattice parameter of a number of Pd-Bi alloys in the
compositional region between 0-25 at. % Bi was calculated. The obtained values are
plotted in Figure 4.5 as a function of Bi bulk composition. A linear relation is observed
which is in accordance with the relation one may predict based on Vegard’s low [127].
Sakamoto and colleagues [214] have shown that alloying Pd with up to 10 at. % Bi
forms fcc solid solutions. The lattice parameter (a), in this case, has also been observed
to expand with the increase in the Bi concentration in the alloy from 0.3890 nm of pure
0
2
4
6
8
10
12
14
20 24 28 32 36 40 44 48
Inte
ns
ity
/ A
rbit
rary
Un
its
2θ Scattering Angle / °
7 at. % Bi
31 at. % Bi
52 at. % Bi
67 at. % Bi
82 at. % Bi
92 at. % Bi
Bi2Pd
102
Pd to 0.3947 nm of Pd-Bi alloy containing 10 at. % Bi. A similar composition
dependence of the lattice parameter (a) was observed for the bulk Pd-Bi alloys with
composition range of 0 < xBi ≤ 0.2 [215].
Figure 4.4: The phase diagram of the Pd-Bi system [53]
Figure 4.5: The calculated lattice parameter of pure Pd and Pd-Bi alloys as a function of bulk Bi composition (up to 25 at. %). The dotted line is a guide to the eye. The Pure Pd (3.89 Å) value was taken from [174].
3.85
3.9
3.95
4
4.05
0 5 10 15 20 25
latt
ice p
ara
mete
r /
Å
Bi at. %
103
Further analysis of the Bi2Pd peak appearing in Figures 4.2 and 4.3 was carried out in
order to assess the intensity-composition relation for the random and annealed Pd-Bi
array samples. Figure 4.6 shows the intensity of this peak (before and after annealing)
as a function of Bi composition. A similar trend is observed in both cases. The peak
intensity remains unchanged over a wide compositional range (up to 60 at. % Bi). The
maximum intensity occurs in the compositional range of ca. Pd25Bi75. A steady decrease
in the peak intensity is observed with alloy compositions above 80 at. % Bi.
Figure 4.6: The intensity of the Bi2Pd peak as a function of Bi at. %, (a): before annealing and (b) after annealing at 300 °C for 15 minutes.
4.2.3- XPS Analysis
The surface composition of 10-fields in an array of Pd-Bi alloys has been determined by
XPS. The measurements have been performed diagonally along the growth direction of
Bi composition in the array in order to represent nearly the whole compositional range
of Pd-Bi alloys. In the Bi (4f) XPS region, two doublets ascribed to 4f7/2 and 4f5/2 can be
detected at about 159 and 164 eV respectively [133]. In the Pd (3d) XPS region, on the
other hand, two doublets correspond to 3d5/2 and 3d3/2 can be detected at 335 and 340 eV
respectively [132]. Figure 4.7 show the Bi (4f) spectra as a function of the bulk Bi
B A
104
composition in various Pd-Bi alloys. A steady shift towards lower binding energies
(away from that of pure Bi) is observed with the decrease in the Bi concentration in the
alloy. Similar behavior is observed upon annealing at 300 ºC for 15 minutes (Figure
4.8).
Figure 4.7: The Bi (4f) XPS spectra as a function of the bulk composition of various Pd-Bi alloys.
Figure 4.8: The Bi 4f XPS spectra as a function of the bulk composition of the Pd-Bi alloys after annealing at 300 °C for 15 minutes.
-100
0
100
200
300
400
500
152 154 156 158 160 162 164 166 168 170
Inte
nsit
y /
CP
S
Binding Energy (BE) / eV
11 at. % Bi
26 at. % Bi
48 at. % Bi
62 at. % Bi
85 at. % Bi
96 at. % Bi
-100
0
100
200
300
400
500
150 152 154 156 158 160 162 164 166 168 170
Inte
nsit
y /
CP
S
Binding Energy /eV
2 at. % Bi
16 at. % Bi
34 at. % Bi
67 at. % Bi
85 at. % Bi
98 at. % Bi
Bi4f7/2 Bi4f5/2
Bi4f7/2
Bi4f5/2
105
The Bi (4f7/2) peak position of the 10 Pd-Bi alloys before and after annealing is plotted
as function of the bulk Bi composition in Figure 4.9. A shift in the Bi (4f) peak position
towards lower binding energy with the decrease in the bulk Bi concentration in the alloy
is observed. The shift in the peak position is not linear, since the alloys having bulk
composition of ca. 80-100 at. % Bi show similar peak positions.
Figure 4.9: The Bi (4f7/2) peak position as a function of the bulk Bi composition.
In the Pd (3d) region, a shift towards higher binding energies (away from that of pure
Pd) is observed with the increase in the bulk Bi composition before and after annealing
at 300 ºC for 15 minutes (Figurers 4.10 and 4.11). The Pd (3d5/2) peak position is
plotted as a function of bulk Bi composition in Figure 4.12 showing similar behavior to
that observed in the Bi (4f7/2) region (Figure 4.9).
156.5
157
157.5
158
158.5
159
159.5
160
0 20 40 60 80 100
Bin
din
g E
ne
rgy
(BE
) /
eV
Bulk Bi at. %
Before annealing
After annealing
106
Figure 4.10: The Pd (3d) XPS spectra as a function of the bulk composition of various Pd-Bi alloys.
Figure 4.11: The Pd (3d) XPS spectra as a function of the bulk composition of the Pd-Bi alloys after annealing at 300 ºC for 15 minutes.
-50
0
50
100
150
200
328 330 332 334 336 338 340 342 344 346 348
Inte
nsit
y /
CP
S
Binding Energy (BE) / eV
6 at. % Bi
26 at. % Bi
48 at. % Bi
62 at. % Bi
85 at. % Bi
-50
0
50
100
150
200
250
300
350
400
450
328 330 332 334 336 338 340 342 344 346 348
Inte
nsit
y /
CP
S
Binding Energy (BE) / eV
2 at. % Bi
16 at. % Bi
34 at. % Bi
67 at. % Bi
98 at. % Bi
Pd3d5/2
Pd3d3/2
Pd3d5/2
Pd3d3/2
107
Figure 4.12: The Pd (3d5/2) peak position as a function of the bulk Bi composition.
The shift in the peak positions of Pd and Bi is a clear evidence of alloy formation [77].
Commonly, shifts in the binding energy of elements forming an alloy system are
discussed in terms of electronegativity differences where a charge flow from the less
electronegative constituent towards the more electronegative one is predicted [216].
According to this, a charge flow from Bi to Pd is predicted in the Pd-Bi alloy system due
to its relatively lower electronegativity (about 2.2 eV of Pd and 2.0 eV of Bi according
to Pauling electronegativity scale). This means that the Pd (3d5/2) peak would be
predicted to shift to lower binding energy (as a Pd atom in the alloy acquire a negative
charge) and the Bi (4f7/2) peak would be predicted to shift to higher binding energy (as a
Bi atom acquires a positive charge) upon alloying [182].
The shift in the Bi (4f7/2) peak position towards lower binding energy (Figure 4.9) and
Pd (3d5/2) peak position towards higher binding energy (Figure 4.12) upon alloying is
not consistent with the prediction based on the electronegativity differences. A shift
towards higher binding energy (compared to the values of pure metals) has been
observed for Pd in the Pd3Bi alloy [205] and for Bi in Pt-Bi alloys [141, 217].
334.5
335
335.5
336
336.5
337
337.5
338
338.5
0 20 40 60 80 100
Bin
din
g E
nerg
y (B
E)
/ eV
Bulk Bi at. %
Before annealing
After annealing
108
The shift in the Pd (3d5/2) peak position to higher binding energy could be due to a
modification in the electronic density of Pd caused by the interaction with Bi to form the
alloy (intra-atomic charge transfer could result in a shift to larger binding energy) [182].
The surface compositions of the measured alloys have been compared to that in the bulk
before and after annealing at 300 °C for 15 minutes. Similar compositions have been
obtained in both cases as shown in Figure 4.13. The similarity between the bulk and
surface compositions observed at equilibrium indicates no surface segregation in the Pd-
Bi alloys (under these conditions, 300 ° for 15 minutes). No reported data for surface
segregation phenomenon in the Pd-Bi alloy system could be found. The only available
reference has referred to that a surface segregation of Bi is predicted to occur in the Pd-
Bi alloys due to its lower surface energy [197] (the surface free energies of Pd and Bi are
2.043 Jm-2 [77] and 0.55 Jm-2 [218] respectively).
There are a number of possible reasons for not observing any surface segregation of Bi
in this study. It is likely that the concentration of surface Bi is slightly higher at
equilibrium compared to that before heat treatment, but could not be measured due to the
accuracy of XPS (error < ±10 % [131]). The purity of the sample could also influence
the measurements of the surface composition. For instance, the surface composition of
an alloy may be changed due to the so-called "co-segregation effects" caused by the
surface segregation of H, O, N, C, and S present in the metallic system as well as the
segregation of the predicted metal (Bi in the case of the Pd-Bi alloy) [83]. Another
possibility is that the annealing conditions employed here are not sufficient to observe
this phenomenon in the Pd-Bi alloys. Therefore, annealing at higher temperatures than
300 °C or for time longer than 15 minutes may be required in order to observe a clear
surface segregation of Bi in this system. Nevertheless, the similarity between the bulk
and surface compositions before and after annealing in the measured Pd-Bi alloys
correlates well with the electrocatalytic activity results which will be shown later in this
chapter.
109
Figure 4.13: The Bulk and surface Bi compositions in a number of Pd-Bi alloys before annealing (blue squares) and after annealing at 300 °C for 15 minutes (red squares).
4.3- Base Voltammetry and CO Stripping Measurements
A first voltammetric assessment of the Pd-Bi alloy catalysts was carried out at room
temperature in 0.5 M HClO4 and at 50 mV/s under the following limiting potentials: Ein
= 0.2, Elo = - 0.03, Eup = 0.5 VRHE. Figure 4.14 shows voltammetric profiles of a number
of Pd-Bi alloys. Two features in the cathodic sweep can be ascribed to the adsorption /
hydride formation (1) and the HER (2) [219], while a feature due to the oxidation of
adsorbed / absorbed hydrogen (3) can be identified in the anodic sweep [84]. Clearly, the
hydrogen adsorption and desorption (hydrogen under potential deposition, Hupd) region
is dominated by the concentration of Pd in the alloy. The latter surface processes are
more pronounced in the presence of high concentration of Pd in the alloy.
0
20
40
60
80
100
0 20 40 60 80 100
Su
rfa
ce
Bi a
t. %
(fr
om
XP
S)
Bulk Bi at. % (from EDS)
Before Annealing
After Annealing
110
Figure 4.14: Cyclic voltammograms of a number of Pd-Bi alloys (Pd : Bi ratio, atomic %) recorded at room temperature in 0.5 M HClO4, scan rate of 50 mV/s, and limiting potentials: Elo = -0.03 VRHE, Ein = 0.2 VRHE and Eup = 0.5 VRHE. CVs are from the second cycle.
Further assessment of the surface has been achieved by running CO stripping
voltammetry at room temperature in 0.5 M HClO4 and at scan rate of 20 mV/s. The
electrolyte was initially saturated with CO for 20 minutes by bubbling, and then the CO
was removed from the electrolyte by purging with Ar for at least 20 minutes. The
measurements were performed under the following limiting potentials: Ein = 0.4, Elo = -
0.03, Eup = 1.1 VRHE. The cyclic voltammograms of a number of Pd-Bi alloys are shown
in Figure 4.15. The voltammetric profiles can be defined by a number of potential
10 : 90
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
27 : 73
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
49 : 51
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
j /
mA
cm
-2 (
ge
o)
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
71 : 29
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
89 : 11
Potentia l / V RHE
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
100 : 0
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
A
E
DC
B
F
1 2
3
111
regions: (i) hydrogen evolution at E < 0.0 VRHE (ii) the reversible hydrogen adsorption
and desorption (Hupd region) at E < 0.4 VRHE, (iii) a large double layer in the potential
range 0.4-0.6 VRHE, and (iv) the CO stripping peak as well as the irreversible surface
oxidation / reduction "surface redox" at E > 0.6 VRHE [137, 138, 141].
Figure 4.15: The voltammetric profiles of a number of random Pd-Bi alloys (Pd : Bi ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with CO for 20 minutes, scan rate of 20 mV /s, and limiting potentials: Elo = -0.03 VRHE, Ein = 0.4 VRHE and Eup = 1.1 VRHE. CVs are from the 1st (red line) and 2nd (black line) cycles.
13 : 87
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
27 : 73
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
48 : 52
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
j /
mA
cm
-2 (
ge
o)
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
71 : 29
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
86 : 14
Potential / V RHE
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
A
E
DC
B
F
112
Features in the Hupd region are very sensitive to the surface structure of the catalyst
[196]. This possibly explains the variety in these features on the alloy surface (Figures
4.15A-E) compared to that of pure Pd (Figure 4.15F). The magnitude of the cathodic
feature below 0.0 VRHE is significantly increased at around 75 at. % Bi (Figure 4.15B)
indicating a better hydrogen evolution in this compositional range compared to the other
alloy compositions. This could be correlated to the formation of the intermetallic
compound Bi2Pd in the compositional range 75 at. % Bi discussed early in this chapter.
The CO stripping from the surface is represented by a broad anodic feature in the first
cycle (red lines in Figures 4.15A-F) at around 0.9 VRHE. The CO stripping feature is
more pronounced with increasing the Pd concentration in the alloy (Figure 4.16)
indicating that alloying Pd with Bi reduces CO adsorption and perhaps hinders it at high
concentrations of Bi. This is consistent with the behavior one may predict as CO
adsorption only takes place on Pd [75, 101]. It is also in accordance with the conclusion
that the deposition of Bi layers on Pt(110)-(1x2) reduces the amount of adsorbed CO
[196]. The CO stripping peak on the Bi-rich alloys occurs at potentials higher than on
pure Pd, while alloys having low and intermediate concentrations of Bi exhibit CO
stripping peaks at potentials relatively lower than that on pure Pd. This indicates a better
activity for CO oxidation on the latter alloy composition than pure Pd. The deposition of
a sub-monolayer of Bi (0.1ML) has been shown to improve CO oxidation on Pt(111)
surface [194].
113
Figure 4.16: The position of the CO stripping peak exhibited by Pd and a various compositions of Pd-Bi alloy catalysts. Pd:Bi ratio, atomic %.
The removal of CO from the surface of the catalyst allows the surface redox processes to
take place. The identification of the individual process in the surface redox region on the
Pd-Bi alloy catalysts appears to be difficult due to the fact that the oxidation / reduction
processes of Pd and Bi take place in the same potential region (0.739 - 0.789 VRHE on Pd
[219] and 0.77 - 0.88 VRHE on Bi [196]). The anodic feature at around 0.95 - 1.0 VRHE
(in Figures 4.15A-F) could therefore be ascribed to the oxidation of both Pd and Bi as
well as Pd-Bi intermetallic surface [217], while the feature in the cathodic sweep at
around 0.8 VRHE is possibly due to the reduction of the formed oxides. The feature
ascribed to the reduction of surface oxides appears in a very narrow potential region
(between ca. 0.75 - 0.8 VRHE) as shown in Figure 4.17. The relative position of this peak
is plotted as a function of the Bi concentration in the alloy (Figure 4.18) showing a
steady and smooth decrease and increase with a minimum in the compositional range
between ca. 30-50 at. % Bi. The variation in the reduction peak position is a clear
evidence of alloy formation and an electronic interaction between Pd and Bi atoms.
-0.02
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.75 0.8 0.85 0.9 0.95 1 1.05 1.1
j / m
Acm
-2(g
eo
)
Potential / VRHE
13:87
27:73
48:52
71:29
86:14
100:0
114
Figure 4.17: The oxide reduction peak on various random Pd-Bi alloys (Pd:Bi ratio, atomic %).
Figure 4.18: The relative position of the oxide reduction peak as a function of Bi concentration in the Pd-Bi alloys.
The effect of annealing an array of Pd-Bi alloy catalysts at 300 °C for 15 minutes on the
voltammetric features observed with random alloys (Figure 4.15A-F) has also been
assessed. Figure 4.19A-F shows the voltammetric profiles of a number of equilibrated
Pd-Bi alloys. Broadly, the anodic and cathodic processes on the annealed Pd-Bi alloys
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.5 0.6 0.7 0.8 0.9 1 1.1
j / m
A c
m-2
(geo
)
Potential / VRHE
15:85
29:71
52:48
76:24
89:11
100:0
0.758
0.762
0.766
0.77
0.774
0.778
0.782
0 20 40 60 80 100
Po
ten
tia
l / V
RH
E
Bi at. %
115
take place at potentials similar to the case before annealing. A remarkable difference
appears in the HER, Hupd and hydride formation regions in the cathodic sweep, since that
the magnitude of the features ascribed to these processes on Pd and Pd-rich alloys
(Figures 4.19D-F) is extremely enhanced by annealing under these conditions (the
geometric current density is higher by more than twofold after annealing). Similar
behavior was observed on Pd-Au alloys where higher geometric current densities were
obtained upon annealing under the same conditions. This point will be discussed in
chapter six.
Figure 4.19: The voltammetric profiles of a number of annealed Pd-Bi alloys (300 °C for 15 minutes, Pd : Bi ratio, atomic %) recorded at room temperature in a 0.5 M HClO4 electrolyte bubbled with CO for 20 minutes, scan rate = 20 mV /s, CVs are from the 1st (red line) and 2nd (black line) cycles. Limiting potentials are: Elo = -0.03 VRHE, Ein = 0.4 VRHE and Eup = 1.1 VRHE.
22 : 78
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
35 : 65
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
52 : 48
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
j /
mA
cm
-2 (
ge
o)
-1.5
-1.0
-0.5
0.0
0.5
1.0
76 : 24
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
89 : 11
Poten tia l / V RH E
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
A
D
FE
C
B
116
4.4- The Catalytic Activity for the HER
The catalytic activity of nearly the whole compositional range of the Pd-Bi alloy
catalysts for the HER and HOR has been assessed at room temperature in 0.5 M HClO4
using a number of 100-field Pd-Bi arrays. The HER measurements have been carried out
using the potential step experiment. The current was recorded at the following
potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007 → 0 VRHE. The
potential was held for 90s at each step. Figure 4.20 shows the geometric HER activity at
- 16.44 mVRHE of a number of random Pd-Bi alloy catalysts. The catalytic activity curve
could be divided in a number of regions: (i) a similar activity to pure Pd in the
compositional range below ca. 10 at. % Bi, (ii) a sharp decrease in the activity between
10 - 20 at. % Bi, (iii) a minimum activity at around 25 at. % Bi, (iv) a steady increase in
the catalytic activity in the compositional range between about 30 - 70 at. % Bi, (v) a
catalytic activity at around Pd25Bi75 comparable to that of pure Pd, and (vi) a sharp
decrease in the catalytic activity above 80 at. % Bi.
Figure 4.20: The HER activity at - 16.44 mVRHE on a wide range of random Pd-Bi alloy catalysts measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.
0
0.04
0.08
0.12
0.16
0.2
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bi at. %
117
The current values obtained on the array sample shown in the latter measurement were
normalized to Pd composition in the Pd-Bi alloys in order to present the specific HER
activity (Figure 4.21). The behavior in this case appears similar to the geometric activity
in terms of that a maximum is observed at ca. Pd25Bi75 alloy composition. The activity in
this compositional region is superior to pure Pd.
Figure 4.21: The specific HER activity normalized to Pd composition in the Pd-Bi alloys.
A similar HER catalytic behavior is observed using an annealed array (300 °C for 15
minutes) of Pd-Bi alloy catalysts (Figure 4.22). One can distinguish that the current
density value on pure Pd is much higher after annealing (ca. 0.5 mA cm-2) than before
annealing (ca. 0.14 mA cm-2), while on the Pd-Bi alloy catalysts (10 at. % Bi and above)
is almost the same (in the range 0.1 – 0.2 mA cm-2) before and after annealing.
0
0.002
0.004
0.006
0.008
0 20 40 60 80 100
j / m
A c
m-2
(sp
ec
ific
)
Bi at. %
118
Figure 4.22: The HER activity at - 17.48 mVRHE on a wide range of annealed Pd-Bi alloy catalysts (300 ºC for 15 minutes) measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.
4.5- The Catalytic Activity for the HOR
The catalytic activity of the Pd-Bi alloy catalysts for the HOR has been assessed using
the potential step experiment with hydrogen bubbling through the electrolyte. The
current has been recorded at the following potentials: 0 → 0.007 → 0.014 → 0.021 →
0.014 → 0.007 → 0 VRHE. The potential was held for 90s at each step. The HOR activity
at 18.95 mVRHE on a number of random Pd-Bi alloy catalysts is shown in Figure 4.23.
Clearly, the HOR activity curve is similar to that of the HER (Figure 4.20) with lower
current density values under the same compositions. An interesting catalytic behavior
could also be observed in the compositional range of Pd25Bi75. The HOR specific
activity is shown in Figure 4.24 demonstrating a similar compositional dependence to
the HER specific activity (Figure 4.21). A similar result was also obtained after
annealing at 300 °C for 15 minutes (Figure 4.25). Once more, annealing influences the
properties of pure Pd giving a rise to the current density values, while no obvious change
take place on the alloys after annealing under these conditions.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bi at. %
119
Figure 4.23: The HOR activity at 18.95 mVRHE on a wide range of random Pd-Bi alloy catalysts measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.
Figure 4.24: The Specific HOR activity normalized to Pd composition in the Pd-Bi alloys.
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bi at. %
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0 20 40 60 80 100
j / m
A c
m-2
(sp
ec
ific
)
Bi at. %
120
Figure 4.25: The HOR activity at 17.75 mVRHE on a wide range of annealed Pd-Bi alloy catalysts (300 ºC for 15 minutes) measured at room temperature in 0.5 M HClO4. The current density values are the average of three experiments. The dashed line is a guide to the eye.
The HOR catalytic activity on Pd-Bi alloy catalysts has also been assessed with a
mixture of hydrogen and 500 ppm CO bubbling through the electrolyte. The
measurements have been carried out after 1 minute of bubbling with the mixture, and
then repeated after bubbling with the mixture for 11 minutes. Figure 4.26 shows the
HOR activity at 18.67 mVRHE on a number of random Pd-Bi alloy catalysts (the same
array sample used in the former assessment for the HOR). Apparently, the presence of
CO in the electrolyte influences the catalytic performance of the Pd-Bi alloy catalysts for
the HOR. However, poisoning effect is strongly dependent on the alloy composition. A
small effect could be observed on the catalytic performance of Pd and Pd-rich alloys (≤
20 at. % Bi) even after bubbling with the H2/CO mixture for 11 minutes. In contrast, the
HOR activity on the Pd-Bi alloys with compositions above 20 at. % Bi is significantly
reduced after 1 minute of bubbling with the H2/CO mixture and completely suppressed
after 11 minutes indicating a strong poisoning of the alloy catalysts by CO. This implies
a low-level of CO tolerance in this compositional range of the Pd-Bi alloy catalysts. It
has been shown that the deposition of Bi on a carbon supported Pt electrode reduces the
CO tolerance of the Pt catalyst [194]. This has been ascribed to Bi offering low
effectiveness towards facilitating oxygen transfer during CO oxidation [194].
0
0.02
0.04
0.06
0.08
0.1
0.12
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bi at. %
121
Figure 4.26: The HOR activity at 18.67 mV on various random Pd-Bi alloys measured at room temperature in 0.5 M HClO4 with a mixture of hydrogen and 500 ppm CO bubbling through the electrolyte. The j values on pure Pd are the average on ten Pd electrodes.
The variation in the HER and HOR catalytic activity along the alloy composition
suggests a composition-activity relation which may be correlated to the formation of a
number of intermetallic compounds shown in the phase diagram of this system (Figure
4.4). The catalytic behavior in the compositional range between 20-80 at. % Bi
(particularly at ca. Pd25Bi75) is extremely interesting and valuable. This is because of the
contrast between the catalytic activity of Pd (highly active) and Bi (totally inactive) as
shown from the volcano plot for the HER [220]. In view of its lower cost, this alloy
system provides an alternative to Pt for the HER and HOR.
The surface processes in the Hupd region and the kinetics of a reaction are sensitive to the
catalyst surface structure and morphology [196, 221]. The enhancement of the HER and
HOR activities on the Pd-Bi alloy catalysts with compositions around Pd25Bi75 could be
ascribed to the formation of a large number of isolated Pd atoms (monomers) which
results in improved activity for HER at this alloy composition. This would be in
accordance with the conclusion that Pd monomers on Au/Pd (111) surface alloys
prepared on Ru(0001) are more active for the HER than Pd atoms in a Pd(111) overlayer
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bi at. %
Pure H2
1 min.
11 min.
122
[142]. Those monomers may enhance the HER through: (i) improving the interaction
with incoming hydrogen atoms or molecules, and (ii) providing selectivity towards the
Heyrovsky step in the HER [142].
It is also possible that the formation of the intermetallic compound Bi2Pd with a
maximum at ca. Pd25Bi75 (Figure 4.6) is responsible for the improvement of the overall
catalytic performance of the Pd-Bi alloy catalysts for the HER and HOR "electronic
effect". This would be in accordance with the conclusion that formation of intermetallic
Bi2Pd results in higher selectivity and activity than pure Pd for glucose oxidation [200].
The XPS data has shown a shift in the Pd (3d5/2) peak position upon alloying with Bi
(Figure 4.12) indicating an electronic perturbation of Pd atoms. It is likely that the inter-
metallic interaction produces a change in the d-band centre [21]. This change could
directly influence the catalytic activity of a transition metal surface [222]. A negative
shift in the d-band centre of Pd would probably be accompanied with a reduction in the
hydrogen adsorption energy that results in higher activity for the HOR [21].
Similarly to the Pd-Au alloy system, the increase in the activity on the Bi-rich alloys
could be associated with a decrease in the formation of Pd hydride phase in the bulk
with increasing Bi content. It has been deduced by Kibler [190] that increasing Pd
coverage on Au(100) reduces the HER activity.
The HER activity and HOR activity on Pd-Bi alloy surfaces show a similar
compositional dependence. An exception of this dependence occurs, however, at high
concentration of Bi (≥ 75 at. % Bi) where the HER activity decreases monotonically
towards 100 at. % Bi (Figure 4.20), while the HOR activity decreases more rapidly at
ca. 90 at. % Bi (Figure 4.23). This behavior is similar to that observed in the Pd-Au
alloy system (Figures 3.29 and 3.35) and was ascribed to a higher selectivity in the
HOR towards the Tafel-Volmer mechanism that is hindered on Pd monomers (most
popular ensembles in this compositional region).
123
4.6- Conclusions
A series of Pd-Bi alloys were synthesized employing a HT-PVD methodology on 10x10
arrays. The bulk and surface compositions of the Pd-Bi alloy samples were determined
by the EDS and XPS respectively. The EDS analysis showed that the synthesis of nearly
the whole compositional range of the Pd-Bi alloy system was achieved. The XPS
measurements were performed on 10-fields in an array of Pd-Bi alloys showing that the
surface composition is similar to the bulk composition before and after annealing at 300
°C for 15 minutes. This similarity suggests no surface segregation of one of the
constituents in the alloy.
The XRD analysis of the Pd-Bi alloy catalysts (unannealed and annealed at 300 °C for
15 minutes) showed a peak at ca. 2θ = 32.28º which was more pronounced with alloy
composition of ca. Pd25Bi75. This peak was attributed to the formation of intermetallic
compound Bi2Pd with a monoclinic structure. The phase diagram of the Pd-Bi system
indicates that α-Bi2Pd is formed at temperatures < 271 °C, while β-Bi2Pd is formed at
temperatures > 271 °C in this compositional range of the alloy. The phase diagram of
this alloy system also shows that solid solutions are formed in a narrow compositional
region (< 25 at. % Bi). The calculation of lattice parameter in a number of alloys in this
compositional range showed a consistency with Vegard’s law.
The voltammetric measurements on Pd-Bi alloys in 0.5 M HClO4 at room temperature,
an upper potential limit of 0.5 VRHE, and scan rate of 50 mV/s showed that the Hupd
region is mainly dominated by the Pd composition in the alloy (the high concentration of
Pd produces more pronounced surface processes). The voltammetric scan (room
temperature and scan rate of 20 mV/s, and an upper potential of 1.1 VRHE) with CO
bubbling through the solution showed a number of features correspond to: (i) the HER at
E < 0.0 VRHE (ii) the reversible hydrogen adsorption and desorption at E < 0.4 VRHE, (iii)
a large double layer in the potential range 0.4 - 0.6 VRHE, and (iv) the CO stripping peak
as well as the irreversible surface oxidation and reduction at E > 0.6 VRHE. The
voltammetric profiles of a number of equilibrated Pd-Bi alloys exhibited features at
124
potentials similar to the unannealed alloys with a remarkable effect on the Hupd region on
Pd and the Pd-rich alloys.
The catalytic assessment of the Pd-Bi alloy system for the HER and HOR has shown
very interesting and valuable results. A compositional dependence was observed with an
optimum alloy composition at ca. Pd25Bi75 (comparable to pure Pd). A similar
compositional dependence was found after annealing Pd-Bi alloys at 300 °C for 15
minutes. The compositional dependence of the HOR is similar to that of the HER except
at Bi-rich alloys where the HER activity decreases monotonically towards 100 at. % Bi,
while the HOR activity decreases more rapidly at ca. 90 at. % Bi. This is possibly due to
that the Tafel-Volmer mechanism is favoured in the HOR that is suppressed on Pd
monomers (most popular ensembles in this compositional region).
The CO tolerance of Pd-Bi alloy system in the HOR was assessed in 0.5 M HClO4 with
a mixture of hydrogen and 500 ppm CO bubbling through the solution. The strength of
CO poisoning was found to be strongly composition-dependent. Bi-poor alloys have
shown a higher level of CO tolerance than Bi-rich alloys. This is possibly due to that Bi
provides low effectiveness towards facilitating oxygen transfer in CO oxidation [194].
The enhancement of the HER and HOR activities on the Pd-Bi alloy catalysts with
compositions around Pd25Bi75 could be ascribed to the formation of a large number of Pd
monomers at this compositional range "ensemble effect" that enhances the interaction
with hydrogen atoms and molecules, and/or provides selectivity towards the Heyrovsky
step in the HER [142]. Another possibility is that the formation of intermetallic Bi2Pd
with a maximum at ca. Pd25Bi75 results in higher activity for the HER and HOR
"electronic effect". The increase in the activity could also be linked to a decrease in the
hydride phase content in bulk with increasing Bi concentration.
In view of its lower cost, this alloy system may provide a valuable alternative to Pt for
the HER and HOR.
125
Chapter 5: Ruthenium-Gold (Ru-Au) Alloy Surfaces
5.1- Introduction
Ru is used as a catalyst to promote the production of hydrocarbons from CO and
hydrogen [223]. The employment of bimetallic catalysts containing Ru as one of the
components in the industrial catalysis could lead to better activities. For instance, Ba-
Ru/MgO catalyst (the molar ratio Ba:Ru is 1:1) has been utilized for the ammonia
synthesis yielding very high activity and stability [224]. The components of this material
are recyclable with high purity. Therefore, the authors suggested that this catalyst could
be an alternative to Ru/C and iron-based catalysts in the industrial synthesis of ammonia.
Various studies have considered either deposition of Au films on a single crystal of Ru
[225, 226] or deposition of Ru films on a single crystal of Au [227-229] suggesting that
the substrate morphology and temperature dominate the behavior of the coverage. The
deposition process results in a redistribution of charge and localization of electrons in
the interface region as a result of electronic interactions between Au and Ru. This
behavior has been suggested to be responsible for producing a largely stable Au-Ru
bond [225]. Another research effort has been directed towards the deposition of Au, Pd
and co-deposited Pd-Au ultrathin films on Ru (001) [230, 231]. The bimetallic (Pdx-Auy)
monolayer films with x = 0.4 exhibited higher activity than Pd/Ru (001) film in the
catalytic conversion of acetylene to benzene due to ensemble effects in the bimetallic
catalyst [231].
CO poisoning anode catalysts remains a challenge in fuel cell technology [232]. Ru is
considered as a constituent element of the anode in fuel cell technology due to its ability
to prevent CO poisoning [233, 234]. A number of Pt-based alloy catalysts have been
investigated in order to develop catalysts with a satisfactory degree of CO tolerance
[232]. Among those catalysts, Pt-Ru alloy has exhibited distinctive properties towards
CO tolerance and found to be the most active catalyst for the CO oxidation [72, 187] as
well as for the electrochemical oxidation of methanol or formic acid [232, 235-237].
126
Ru/Pd and Ru/Au heterobimetallic complexes have also been observed to promote the
electrochemical oxidation of methanol [236].
The catalytic performance of the Pt-Ru alloy catalyst in the latter processes has been
reported to be superior to that of pure Pt [186, 187]. A Pt-Ru alloy with a 50:50 surface
composition has been suggested to be the best catalyst for the CO oxidation, while a
90:10 Pt/Ru ratio is the best alloy composition for the methanol oxidation [72, 187]. A
study by Iwasita and colleagues [238] has shown that the preparation method of the Pt-
Ru catalysts influences their catalytic performance for the methanol oxidation. The total
number of Pt-Ru pair sites has also been reported to dominate this catalytic process
[239].
The enhancement of the CO oxidation on the Pt-Ru alloy could be ascribed to the fact
that Ru is more active than Pt for the adsorption process of dissociative water and the
subsequent production of OHads (5.1) which promotes the conversion of CO adsorbed on
Pt into CO2 (5.2) [81].
Ru + H2O → Ru-OHads + H+ + e- (5.1)
Pt-COads + Ru-OHads → Pt + Ru + CO2 + H+ + e- (5.2)
Similar mechanistic interpretation has been proposed for the electrocatalytic oxidation of
methanol [134, 239] or formic acid [232] on Pt-Ru alloy surfaces.
A further application of Ru and Pt-Ru alloy catalysts in electrocatalysis is in the HOR. A
comparative study of this reaction on Ru (0001) and on Ru (10 – 10) surfaces employing
a rotating disk electrode (RDE) in H2SO4 and HClO4 solutions showed that the rates on
the Ru (10 – 10) surface is higher than that on the Ru (0001) surface [240]. It was also
concluded from this investigation that the formation of Ru oxide inhibits the HOR. The
authors suggested that the formation of Ru oxide in H2SO4 is slower than in HClO4
solution which, subsequently, leads to faster reaction kinetics in the former acid. The
HOR has also been measured on Pt, Ru and Pt-Ru alloy (Ru surface composition of ≈ 50
127
atomic % and ≈ 90 atomic %) surfaces using RDE technique in a 0.5 M H2SO4 solution
saturated with H2 [241] and mixtures of CO/H2 [242]. The HOR activity on both Pt and
Pt-rich alloy surfaces has been observed, in the former case, to be better than on pure Ru
at room temperature. The Ru-rich alloy exhibited lower HOR activity in the presence of
CO/H2 mixtures, while pure Ru observed to be totally inactive for the oxidation of
CO/H2 mixtures.
Another interest has been paid to the employment of ruthenium oxide catalysts in fuel
cell technology due to the fact that Ru possesses the characteristic of oxophilicity
resulting in oxides with specific catalytic properties [240]. Ruthenium oxides are also
stable over a wide range of operating potentials which make them applicable in a
number of electrochemical applications such as energy storage and capacitors [243].
One of the electrocatalytic applications of ruthenium dioxide (RuO2) catalyst is in
hydrogen oxidation [244]. It is also used as a catalyst for the oxygen evolution reaction
(OER) [223]. A reduction in the electrochemical activity of RuO2 for the OER has been
observed with the increase of calcining temperature from 350 to 550 °C during the
preparation of the catalyst [245].
5.2- Composition and Structure Analysis
A series of 100-field arrays of Ru-Au alloys have been synthesized employing the HT-
PVD methodology. As in the case with the Pd-Au and Pd-Bi alloy systems, the bulk and
surface compositions of Ru-Au alloy samples have been measured by EDS and XPS
respectively.
5.2.1- EDS Analysis
The bulk Ru and Au composition in an array of the Ru-Au alloy system with respect of
(x, y) position in the array is shown by false color maps in Figure 5.1. The dashed
arrows in this Figure refer to the growth direction of the elemental components in the
array sample proving the achievement of the deposition of nearly the whole
compositional range of the Ru-Au alloy system.
128
Figure 5.1: False color maps of the bulk atomic percentage of Ru and Au in an array of Ru-Au alloy system. The dashed arrows refer to the growth direction of the constituents in the array sample.
5.2.2- XRD Analysis
Figure 5.2 shows the XRD patterns of a number of Ru-Au samples. The dotted lines
belong to 2θ values of various Au and Ru planes from the literature [174, 234, 246]. A
steady shift towards higher 2θ values from that of Au (111) is observed with decreasing
Au concentration in the alloy indicative of the formation of solid solutions between Ru
and Au atoms. However, the diffraction peak ascribed to the highest composition of Ru
(Ru74Au26) is almost flat (no strong peak) which may indicate that the Ru-rich alloys are
amorphous. It appears from those two observations that the formation of solid solutions
in the Ru-Au alloy system is composition-dependent as no formation of solid solutions
occurs at alloys very rich with Ru. This is consistent with the behavior predicted of the
Ru-Au alloy system due to the fact that its elemental components have different crystal
structures (the crystal structure of pure Ru is hexagonal close packed (h.c.p.) with lattice
constants a = 2.71 Å and c = 4.28 Å, while that of pure Au is face centered cubic (f.c.c.)
with a = 4.08 Å) [247].
X
y
129
Figure 5.2: Typical XRD patterns of a number of Ru-Au alloys.
The phase diagram of the Ru-Au system (Figure 5.3) [248] is a monotectic type [249]. It
broadly shows that Ru and Au can dissolve in each other forming solid solutions. The
very large difference in the melting points between Ru (2334 °C) and Au (1064.43 °C)
prevents the formation of intermetallic compounds between Ru and Au [249]. The Ru-
Au alloys with high concentrations of Au have been suggested to form solid solutions in
the fcc structure (a replacement of a number of Au atoms on the lattice points by Ru
atoms), while the Ru-rich alloys may form solid solutions in the hcp structure (a
replacement of a number of Ru atoms on the lattice points by Au atoms) [248]. In a Pt-
Ru catalyst, a solid solution in the fcc structure could be formed if the Ru atomic
fraction is about 0.7 or less, while above 0.7 solid solutions in the hcp structure could be
formed [250]. The absence of the diffraction peak ascribed to hexagonal Ru in Figure
5.2 suggests that the Ru-Au alloys are in the fcc structure. Parallel XRD findings and
interpretations have been reported of a number of Pt-Ru catalysts (Pt:Ru atomic ratio 1:1
and 1:3) [237, 250], since no hcp Pt-Ru reflections could be observed indicative of the
formation of fcc structures only.
0
2
4
6
8
10
32 34 36 38 40 42 44 46 48
Inte
ns
ity
/ A
rbitra
ry U
nit
s
2θ Scattering Angle / °
95 at. % Au
77 at. % Au
65 at. % Au
47 at. % Au
31 at. % Au
26 at. % Au
Ru (100)
Au (111)
Ru (101)
Au (200)
Ru (002)
130
Figure 5.3: The phase diagram of the Ru-Au system [248].
5.2.3- XPS Analysis
The surface composition of a number of Ru-Au alloy catalysts have been measured by
XPS. Figures 5.4-5.5 show the Au (4f) XPS spectra of various alloys before and after
annealing at 300 ºC for 15 minutes respectively. The dotted lines represent the binding
energy values of metallic Au (84 and 88 eV [132]). The Au (4f7/2) peak position is
shown in both cases as a function of bulk Au composition in Figure 5.6. A shift in the
peak position from that of metallic Au towards lower binding energy is observed with
decreasing the Au concentration in the alloy.
131
Figure 5.4: The Au (4f) XPS spectra of various random Ru-Au alloys. The dotted lines represent the binding energies of metallic Au in the (4f) region [132].
Figure 5.5: The Au (4f) XPS spectra after annealing the Ru-Au alloys at 300 ºC for 15 minutes. The dotted lines represent the binding energies of metallic Au in the (4f) region [132].
-20
0
20
40
60
80
100
120
140
160
76 78 80 82 84 86 88 90 92
Inte
ns
ity
/ C
PS
Binding Energy / eV
17 at. % Au
28 at. % Au
41 at. % Au
61 at. % Au
86 at. % Au
91 at. % Au
-40
-20
0
20
40
60
80
100
120
140
160
78 80 82 84 86 88 90 92
Inte
nsit
y /
CP
S
Binding Energy /eV
17 at. % Au
41 at. % Au
61 at. % Au
72 at. % Au
91 at. % Au
Au (4f 7/2)
Au (4f 5/2)
Au (4f 7/2)
Au (4f 5/2)
132
Figure 5.6: The Au (4f7/2) peak position as a function of bulk Au composition in the alloy.
The Ru (3d) XPS spectra of various alloys before and after annealing are also shown in
Figures 5.7-5.8 respectively. The dotted lines at 280 and 284 eV correspond to metallic
Ru [134]. The Ru (3d5/2) peak position shifts towards higher binding energy from that of
metallic Ru with increasing Au concentration in the alloy (Figure 5.9).
The shift in the peak positions upon alloying indicates an electronic perturbation of
metallic constituents. The shift towards lower binding energy (in the case of Au) and
towards higher binding energy (in the case of Ru) is consistent with the simple charge
transfer idea based on electronegativity differences. It suggests that Au atom acquire a
negative charge and Ru atom acquire a positive charge because Au has relatively higher
Pauling electronegativity value (2.4) than Ru (2.2) [182].
81
81.5
82
82.5
83
83.5
84
0 20 40 60 80 100
Bin
din
g E
nerg
y /
eV
Au at. %
Before annealing
After annealing
133
Figure 5.6: The Ru (3d) XPS spectra of various random Ru-Au alloys. The dotted lines represent the binding energy of metallic Ru in the (3d) region [134].
Figure 5.7: The Ru (3d) XPS spectra after annealing the Ru-Au alloys at 300 ºC for 15 minutes. The dotted lines represent the binding energy of metallic Ru in the (3d) region [134].
-20
0
20
40
60
80
276 278 280 282 284 286 288 290
Inte
ns
ity /
CP
S
Binding Energy / eV
17 at. % Au
41 at. % Au
61 at. % Au
80 at. % Au
90 at. % Au
-20
-10
0
10
20
30
40
50
60
70
274 276 278 280 282 284 286 288 290
Inte
nsit
y /
CP
S
Binding Energy / eV
17 at. % Au
41 at. % Au
61 at. % Au
72 at. % Au
91 at. % Au
Ru (3d 5/2)
Ru (3d 3/2)
Ru (3d 3/2) Ru (3d 5/2)
134
Figure 5.9: The XPS Ru (3d5/2) peak position as a function of bulk Au composition in the alloy.
The surface composition of the measured Ru-Au alloy catalysts have been compared to
their bulk composition before and after annealing at 300 °C for 15 minutes (Figure
5.10). A surface segregation of Ru is observed in both cases (i.e. random and annealed
alloys). This would be in contrast with the theoretical prediction that a surface
segregation of Au takes place in this alloy system at equilibrium due to its lower surface
energy (1.626 Jm-2 [77]) compared to that of Ru (3.4 Jm-2 [231]).
The surface segregation of Ru could be directly correlated with the well known behavior
of Ru as an oxophilic metal, since it could easily form a number of oxide phases at the
solid/gas interface in the presence of oxygen [58, 233, 240]. Thus, the high
concentration of Ru at the surface before and after annealing could be ascribed to the
presence of both metallic Ru as well as Ru oxides (formed by the residual oxygen
species in the deposition chamber). This would be consistent with the proposition that an
alloy component tends to segregate to the surface in the presence of an adsorbate to form
a chemical bond with this adsorbate [84, 86]. It is also in accordance with the behavior
predicted in a Pt-Ru alloy in the presence of adsorbed oxygen as Ru tends to segregate to
the surface to form a strong bond with oxygen [251].
279.8
280
280.2
280.4
280.6
280.8
281
281.2
281.4
281.6
0 20 40 60 80 100
Bin
din
g E
nerg
y /
eV
Au at. %
Before annealing
After annealing
135
Figure 5.10: The bulk and surface Au compositions before (blue diamonds) and after annealing (red diamonds) at 300 °C for 15 minutes. The dotted line represents the relation one may expect for no surface segregation of one of the alloy constituents. The dashed line is a fitting to the data with a polynomial equation: y = 0.35x + 0.0065x2, where y is the surface composition and x is the bulk composition determined by EDS.
5.3- Base Voltammetry and CO Stripping Measurements
A series of voltammetric measurements were carried out on a number of random and
annealed Ru-Au alloys at room temperature in 0.5 M HClO4. Figure 5.11A-F shows the
voltammetric curves of various Ru-Au alloys cycled to an upper limiting potential of 0.5
VRHE. A main observation in this restricted range of potential is the increase in the
double layer current (1) with increasing Ru content in the alloy. This has also been
observed on Ru/Au(111) surface and associated with the presence of Ru [186]. The
voltammograms also show a variety in the hydrogen adsorption and evolution features
(2) between ca. - 0.05 to 0.05 VRHE with varying the alloy composition.
0
20
40
60
80
100
0 20 40 60 80 100
Su
rface A
u a
t. %
(fr
om
XP
S)
Bulk Au at. % (from EDS)
Before Annealing
After Annealing
136
Figure 5.11: The cyclic voltammograms of various random Ru-Au alloys recorded at room temperature in 0.5 M HClO4 using a scan rate of 50 mV/s. The limiting potentials are: Ein = 0.2 VRHE, Elo = 0.0 VRHE and Eup = 0.5 VRHE. The CVs are from the third cycle.
Further voltammetric assessment of the Ru-Au alloy surfaces were carried out by
increasing the limiting upper potential to 1.6 VRHE in order to observe the surface
reduction peak and to assess the effect of the alloy composition on its position. Figure
5.12A-F shows window opening cyclic voltammograms of a number of random (solid
lines) and annealed (300 ºC for 15 minutes, dashed lines) Ru-Au alloy surfaces.
15 : 85
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
30 : 70
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
46 : 54
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
j /
mA
cm
-2 (
ge
o)
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
63 : 37
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
79 : 21
Potential / VRHE
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
100 : 0
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
A
E F
DC
B
1
2
137
Figure 5.12: Window opening cyclic voltammograms of various Ru-Au electrodes (ratio, Ru:Au) before (solid lines) and after (dashed lines) annealing at 300 ºC for 15 minutes measured at room temperature in 0.5 M HClO4 with scan rate of 50 mV/s. Limiting potentials are: Ein = 0.2 VRHE, Elo = - 0.03 VRHE and Eup = 1.6 VRHE. The CVs are from the third cycles.
There is no significant effect on the voltammograms can be observed after annealing
under these conditions. The voltammograms clearly show two features (1 and 2)
ascribed to the surface redox processes. It also shows a shoulder at around 0.2 VRHE that
possibly corresponds to reduction of oxidized Ru [252]. The redox processes on pure Ru
occur at a large potential range starting from ca. 0.2 VRHE (Figure 5.12F). This could
produce overlap with other processes preventing the observation of their features [243].
The position of the reduction feature in the potential range between 1.0-1.2 VRHE varies
7 : 93
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-4
-2
0
2
4
19 : 81
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-4
-2
0
2
4
34 : 66
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
j /
mA
cm
-2 (
ge
o)
-4
-2
0
2
4
54 : 46
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-4
-2
0
2
4
77 : 23
Potential / VRHE
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-4
-2
0
2
4
100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
-4
-2
0
2
4
A
FE
C D
B
1
2 3
138
with varying the alloy composition (Figure 5.13). This variation is a clear evidence of
alloy formation. The oxide reduction peak becomes broader with increasing Ru content
in the alloy. This could be associated with the redox behavior of Ru.
Figure 5.13: The relative position of the oxide reduction peak on various random Ru-Au alloys (Ru:Au ratio, atomic %).
The CO stripping was subsequently assessed on a series of Ru-Au alloy surfaces at an
upper limiting potential of 1.1 VRHE (Figure 5.14). Prior to these measurements, the
electrolyte was saturated with CO for 20 minutes, and then CO was removed from the
bulk by bubbling with Ar. In the first cycle, a peak arises at around 0.7 VRHE (1) in the
anodic sweep which corresponds to the stripping of adsorbed CO from the surface. The
presence of the CO molecules on the surface prevents the observation of other surface
processes in the first cycle. The removal of CO allows the observation of the features in
the Hupd as well as the HER and HOR (2, 3) features, and surface redox features (4, 5) in
the second cycle. The voltammograms are also a revision to the significant current in the
double layer region.
-2
-1.5
-1
-0.5
0
0.5
0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5
j / m
A c
m-2
(g
eo
)
Potential / VRHE
0 at. % Au
23 at. % Au
46 at. % Au
66 at. % Au
81 at. % Au
93 at. % Au
139
Figure 5.14: The Cyclic voltammograms of a number of random Ru-Au alloy catalysts (Ru:Au ratio, atomic %) recorded at room temperature in 0.5 M HClO4 saturated with CO for 20 minutes, and a scan rate of 20 mV/s. The limiting potentials are: Ein = 0.2 VRHE, Elo = 0 VRHE and Eup = 1.1 VRHE. The dotted lines are from the first cycle and the solid lines are from the second cycle.
The CO stripping peak overlaps in most cases with the surface oxide formation peak (4).
This prevents the analysis of the charge associated with the CO stripping peak. The latter
peak is more pronounced on pure Ru (Figure 5.14F) than on the alloy surfaces. This is
consistent with the behavior one may predict for the CO adsorption on an alloy of Ru-
Au, since Au is inactive for this process and it only takes place on Ru sites [186, 253-
255]. This would also be in accordance with the observations by Jänsch et al. [254]
using temperature programmed desorption (TPD) where the intensity of CO desorption
13 :87
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
30 : 70
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
48 : 52
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
j / m
A c
m-2
(g
eo
)
-1.5
-1.0
-0.5
0.0
0.5
66 : 34
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
81 : 19
Potential / VRHE
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.5
-1.0
-0.5
0.0
0.5
100 : 0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2-1.5
-1.0
-0.5
0.0
0.5
A
FE
DC
B
1 3
2
4
5
140
peak has been found to decrease simultaneously with increasing Au precoverage on Ru
(001).
The CO stripping peak on Ru and Ru-Au alloy catalysts (Figure 5.14) is shifted towards
lower (more negative) potentials compared to the case on the Pd-Au (Figure 3.19) or
Pd-Bi (Figure 4.15) alloy catalysts. The enhancement of CO oxidation on Ru alloy
catalysts can be directly correlated with the oxophilicity of Ru [58, 233, 240]. A number
of publications have revealed that the CO stripping and oxidation on an alloy of Pt-Ru
catalyst occurs at overpotentials lower than that on pure Pt [72, 186, 187, 241, 252, 256].
The reason for this behavior has been attributed to the incorporation of Ru with Pt
catalysts which facilitates the adsorption of oxygen containing species (OHads) at lower
electrode potentials compared to the case on pure Pt [72, 186, 241, 252]. This is known
as a bifunctional effect where CO adsorb on Pt sites, while Ru provides a site for water
discharge reaction producing OHads at lower potential [72, 81].
5.4- The Catalytic Activity for the HER
The HER activity was examined on a number of unannealed and annealed (300 °C for
15 minutes) electrochemical arrays of Ru-Au alloy catalysts at room temperature in 0.5
M HClO4 using the potential step measurements. The current was recorded at the
following potentials: 0 → - 0.007 → - 0.014 → - 0.021 → - 0.014 → - 0.007→ 0 VRHE.
The potential was held for 90s at each step. Figure 5.15 shows the geometric current
density on pure Ru and a series of random Ru-Au alloy catalysts. An increase in the
catalytic activity for the HOR, in comparison to Ru, appears on alloy compositions < 20
at. % Au with a maximum at ca. Ru90Au10. Similar catalytic activity to that of pure Ru is
exhibited by a broad range of Ru-Au alloys (ca. 20-75 at. % Au). A steady decrease in
the activity is observed with concentrations of Au above ca. 75 at. %.
A similar HER compositional dependence was obtained after annealing the Ru-Au alloy
catalysts at 300 ºC for 15 minutes (Figure 5.16). The HER activity on Ru-Au alloys
after annealing under these conditions is slightly higher than before annealing (Figure
141
5.15). A similar behavior was observed with the Pd-Au and Pd-Bi alloy systems. A
discussion regarding this point is given latter in this thesis.
Figure 5.15: The HER activity at - 17.33 mV on several random Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4. The j values are the average of three experiments. The dotted line is a guide to the eye.
Figure 5.16: The HER activity at - 17.30 mV on a series of annealed Ru-Au alloy catalysts (300 °C for 15 minutes) measured at room temperature in 0.5 M HClO4. The dotted line is a guide to the eye.
0
0.1
0.2
0.3
0.4
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bulk Au at. %
0
0.1
0.2
0.3
0.4
0.5
0.6
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Bulk Au at. %
142
Figures 5.17-5.18 show the HER activity on the random and annealed Ru-Au alloy
surfaces as a function of Au surface composition. The surface composition along the
whole compositional range of the Ru-Au alloy system was determined using the
polynomial equation (y = 0.35x + 0.0065x2 where: y is the surface composition and x is
the bulk composition determined by EDS) used for the fitting of XPS data (Figure
5.10). The HER specific activity is also plotted as a function of Au surface composition
before and after annealing in Figures 5.19 and 5.20 respectively. The latter was
determined by normalizing the current values to surface Ru composition in the alloy.
Figure 5.17: The HER at - 17.33 mV on a series of random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.
0
0.1
0.2
0.3
0.4
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
143
Figure 5.18: The HER activity at -17.30 mV on a series of annealed Ru-Au alloys (300 ºC for 15 minutes) as a function of surface composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.
Figure 5.19: The HER specific activity at -17.33 mV on several random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.
0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
0
0.001
0.002
0.003
0.004
0.005
0 20 40 60 80 100
j / m
A c
m-2
(sp
eci
fic)
Surface Au at. %
144
Figure 5.20: The HER specific activity at -17.30 mV on a series of annealed (300 ºC for 15 minutes) Ru-Au alloys measured at room temperature in 0.5 M HClO4.
5.5- The Catalytic Activity for the HOR
The HOR catalytic measurements on a number of unannealed and annealed (300 °C for
15 minutes) electrochemical arrays of Ru-Au alloy catalysts were carried out at room
temperature in a 0.5 M HClO4 electrolyte with hydrogen bubbling through the solution
using the potential step technique. The current was recorded at: 0 → 0.007 → 0.014 →
0.021 → 0.014 → 0.007→ 0 VRHE. The potential was held for 90s at each step. Figures
5.21-5.22 show the HOR catalytic activity on various random and heat-treated (300 ºC
for 15 minutes) Ru-Au alloy catalysts as a function of bulk Au composition. The HOR
activity shows a similar compositional dependence to the HER activity on Ru-Au alloys
(Figures 5.15-5.16) with a maximum at around Ru90Au10.
0
0.003
0.006
0.009
0.012
0.015
0 20 40 60 80 100
j / m
A c
m-2
(sp
ecif
ic)
Surface Au at. %
145
Figure 5.21: The HOR at 18.00 mV on a several random Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4 with hydrogen bubbling through the solution. The j values are the average of two experiments. The dotted line is a guide to the eye.
Figure 5.22: The HOR activity at 18.11 mV on a number of annealed (300 ºC for 15 minutes) Ru-Au alloy catalysts measured at room temperature in 0.5 M HClO4 with hydrogen bubbling through the solution. The line is a guide to the eye.
0
0.01
0.02
0.03
0.04
0.05
0.06
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Au at. %
0
0.01
0.02
0.03
0.04
0.05
0.06
0 20 40 60 80 100
j / m
A c
m-2
(ge
o)
Au at. %
146
The HOR activity on the random and annealed Ru-Au alloys is plotted in Figures 5.23-
5.24 respectively as a function of Au surface composition. The HOR specific activity is
also plotted before and after annealing as a function of surface composition in Figures
5.25 and 5.26 respectively. In all cases, the HOR activity shows a similar compositional
dependence to the HER activity.
Figure 5.23: The HOR activity at 18 mV on a series of random Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.
0
0.01
0.02
0.03
0.04
0.05
0.06
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
147
Figure 5.24: The HOR activity at 18.11 mV on a series of annealed Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4. The surface composition was determined using a polynomial equation y = 0.35x + 0.0065x2.
Figure 5.25: The HOR specific activity at 18.00 mV on various Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.
0
0.01
0.02
0.03
0.04
0.05
0.06
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Surface Au at. %
0
0.00001
0.00002
0.00003
0.00004
0.00005
0.00006
0 20 40 60 80 100
j / m
A c
m-2
(sp
ec
ific
)
Surface Au at. %
148
Figure 5.26: The HOR specific activity at 18.11 mV on various annealed Ru-Au alloys as a function of surface Au composition measured at room temperature in 0.5 M HClO4.
The HER and HOR data on the random and annealed Ru-Au alloys (Figures 5.15-5.20
and 5.21-5.26) indicate two compositional regions where the activity of the alloy is
higher than pure Ru, at ca. Ru90Au10 and 60-80 at. % Au. The increase in the activity for
the HER and HOR in these compositional regions could be due to a modification in the
electronic properties of Ru upon alloying with Au. It has been theoretically predicted
that a Ru overlayer on Au produces a positive shift in the d-band centre of Ru, while a
Au overlayer on Ru gives a negative shift in the d-band centre of Au [21]. A negative
shift in the d-band centre of Ru may lower the hydrogen adsorption energy giving a
better HOR activity [21]. The application of Sabatier principle to the HER [60] suggests
that |∆GH| value on the alloy is closer to zero than pure Ru.
The HER and HOR on Ru-Au alloys show a similar compositional dependence. This
behavior, however, is different at high concentrations of Au where the HER activity
decreases monotonically towards 100 at. % Au (Figure 5.15), while the HOR activity
decreases more rapidly at ca. 90 at. % Au (Figure 5.21). This observation is in
agreement with the data obtained on Pd-Au (Figures 3.29 and 3.35) and Pd-Bi (Figures
4.20 and 4.23) alloys.
0
0.0003
0.0006
0.0009
0.0012
0.0015
0 20 40 60 80 100
j / m
A c
m-2
(sp
ec
ific
)
Surface Au at. %
149
The oxophilicity of Ru [58, 233, 240] could provide a barrier to the HER and HOR
activity on Ru-containing surfaces. This is because hydrogen adsorption can be blocked
by a preferential adsorption of a hydroxide anion [257]. The increase in the HER and
HOR activity at high concentration of Au (ca. 60-80 % Au) could be associated with a
decrease in the oxophilicity of Ru that results in a suppression of hydroxide adsorption
and improves hydrogen adsorption. This is consistent with the observation by Jansch
et.al. [254] that increasing Au precoverage on Ru(001) results in a simultaneous
decrease in CO adsorption.
The HOR activity on Ru-Au alloy surfaces was also examined in 0.5 M HClO4 with a
mixture of hydrogen and 500 ppm CO bubbling through the electrolyte in order to assess
CO tolerance of the alloy system. The results after bubbling the electrolyte with the
mixture for 1 minute and 44 minutes are presented in Figure 5.27. It appears that this
alloy system offers a high degree of CO tolerance under these conditions, since the
presence of CO in the electrolyte does not significantly affect the activity for the HOR
even after 44 minutes of bubbling. This behavior is not surprising as Ru is well-
documented [72, 81, 258-260] to effectively enhance CO tolerance of Pt through the
bifunctional mechanism. Au, on the other hand, has been reported to prevent CO
adsorption on Ru(001) surface [254]. Au was also shown here to hinder CO adsorption
on Pd (Figure 3.23).
150
Figure 5.27: The HOR activity at 18.12 mV on several random Ru-Au alloy surfaces measured at room temperature in 0.5 M HClO4 in the presence of a mixture of hydrogen and 500 ppm CO. The line is a guide to the eye.
5.6- Conclusions
It was shown from the EDS analysis that the deposition of nearly the whole
compositional range of Ru-Au alloy system was achieved. Upon alloying, a shift in the
Ru (3d) XPS spectra towards higher binding energy compared with metallic Ru
accompanied by a shift in the Au (4f) XPS spectra towards lower binding energy
compared to metallic Au was observed. This result was attributed to electronegativity
differences between the constituents which suggest a charge transfer from Ru to Au due
to the higher electronegativity value of Au.
Comparing the bulk composition (measured by EDS) to the surface composition
(measured by XPS) has shown a surface segregation of Ru before and after annealing at
300 ºC for 15 minutes. This behavior was attributed to that Ru tends to segregate at the
surface to form oxides with residual oxygen species in the deposition chamber due to its
oxophilicity.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 20 40 60 80 100
j / m
A c
m-2
(geo
)
Au at. %
1 min.
44 min.
151
The XRD analysis of a number of the Ru-Au alloy catalysts has shown a consistent shift
from the 2θ value of pure Au towards higher values accompanied by a decrease in the
peak intensity with decreasing Au concentration in the alloy. The shift in the peak
position indicates the formation of solid solutions between the constituents. The
decrease in the peak intensity was observed to be associated with the increase in the Ru
concentration in the alloy suggesting that Ru-rich alloys are amorphous. The
observations from XRD analysis suggest that the formation of solid solution in the Ru-
Au alloy system is composition-dependent. The latter behavior is predicted for this alloy
system due to the fact that its elemental components have different crystal structure.
The voltammetric assessment of various random and annealed Ru-Au alloy surfaces has
shown a significant current in the double layer region which was associated with the Ru
content in the alloy. A surface reduction feature was observed in the potential range
between 1.0-1.2 VRHE upon cycling to a limiting potential of 1.6 VRHE. The position of
this feature varies with the alloy composition confirming the alloy formation. There was
no significant effect of annealing Ru-Au alloy catalyst at 300 ºC for 15 minutes on the
voltammetric features.
The CO stripping voltammetry on Ru and Ru-Au alloy surfaces has shown a peak at ca.
0.7 VRHE that corresponds to CO stripping process. In most cases, this peak was
overlapped with the surface oxide formation peak preventing the determination of the
charge associated with it. The CO stripping peak on Ru-Au alloy catalysts occurs at
potentials lower than on Pd-Au or Pd-Bi alloy catalysts. This behavior was attributed to
the presence of Ru facilitating the adsorption of OHads species at lower potentials due to
its oxophilicity leading to improved CO oxidation.
The HER activity on Ru-Au alloy surfaces was found to be composition-dependent. The
HOR activity shows a similar compositional dependence to the HER activity on the Ru-
Au alloy surfaces. An exception of this occurs at high compositions of Au where the
HER decreases monotonically towards 100 at. % Au, while the HOR activity decreases
more rapidly at ca. 90 at. % Au. A higher reaction activity than pure Ru was found at ca.
152
Ru90Au10 and 60-80 at. % Au. The increase in the activity was ascribed to a modification
in the electronic properties upon alloying that results in a better activity for the HER and
HOR. It was also suggested that the increase in the activity at ca. 60-80 at. % Au could
be due to a decrease in the oxophilicity of Ru that enhances hydrogen adsorption and
reduces hydroxide adsorption. The presence of a mixture of hydrogen and 500 ppm CO
in the electrolyte does not significantly affect the HOR activity on the Ru-Au alloy
surfaces which indicates that this alloy system offers a high degree of CO tolerance
under these experimental conditions.
153
Chapter 6: Conclusions and General Discussion
The broad aim of this project was to assess the catalytic behaviour of various binary
alloy systems in order to identify alternatives to Pt for electrochemical reactions. A high
throughput physical vapour deposition method [89] was employed for the synthesis of
libraries of thin films of Pd-Au, Pd-Bi and Ru-Au alloys. A high throughput screening
method was subsequently employed for the electrocatalytic assessment of these
materials for the HER and HOR. It is the first time that high throughput methods have
been applied for the assessment of the HER and HOR activity on these alloy systems.
The high throughput methods employed here allowed the identification of the optimum
alloy composition (in each alloy system) for the HER and HOR.
6.1- Sample Characterization
Various analytical tools were employed for the characterization of Pd-Au, Pd-Bi and Ru-
Au alloys including EDS for bulk composition analysis, XPS for surface composition
analysis and XRD for structure analysis. The key findings are:
The EDS analysis has shown that the deposition of nearly the whole compositional
range of the alloy systems was achieved.
The XPS analysis has shown shifts in the peak positions correspond to metallic
constituents of alloy systems proving the formation of the alloy in each case.
Comparing bulk composition with surface composition has shown a surface
segregation of Au in the Pd-Au alloys (Figure 3.10), no measurable surface segregation
in the Pd-Bi alloys (Figure 4.13) and a surface segregation of Ru in the Ru-Au alloys
(Figure 5.10).
The XRD analysis has shown that a continuous series of solid solutions is formed in
the Pd-Au alloy system, while the formation of solid solutions in both the Pd-Bi and Ru-
Au alloy systems is composition-dependent.
154
6.2- The HER and HOR Activity
The electrocatalytic assessment of the Pd-Au [261], Pd-Bi [262] and Ru-Au alloy
surfaces for the HER and HOR suggests that:
The HER and HOR activity on these alloy systems are composition-dependant.
There appears to be a high correlation between the HER and HOR activity on an alloy
catalyst. A similar compositional dependence was observed on Pd-Au (Figures 3.27 and
3.35), Pd-Bi (Figures 4.20 and 4.23) and Ru-Au (Figures 5.15 and 5.21) alloy surfaces.
This suggests that the HER activity provides a good descriptor for the HOR activity for
systems with low overpotentials. An exception of this occurs, however, at high
concentrations of Au and Bi where the HER activity decreases monotonically towards
100 at. %, while the HOR activity decreases more rapidly at alloy compositions of ca. 90
at. %. The activity in this compositional range would be expected to be dominated by Pd
or Ru monomers. Therefore, this behaviour could be associated with a higher selectivity
towards the Tafel-Volmer mechanism in the HOR that is suppressed on monomers. This
would be in accordance with the proposition that the Volmer-Heyrovsky mechanism is
favoured in the HER on Pd monomers, since the adsorption of two hydrogen atoms on a
single Pd atom is less likely to occur [142] .
A number of Pd-Au alloys are superior to pure Pd with respect to activity for the HER
(Figure 3.27) and HOR (Figure 3.34). The optimum Pd-Au alloy composition for the
HER and HOR is in the range of ca. Pd50Au50. The Pd-Au alloys offer a higher degree of
CO tolerance in the HOR than Pt, since they were less strongly poisoned in the presence
of a mixture of hydrogen and 500 ppm CO (Figure 3.39) than one would expect for Pt.
This suggests that the Pd-Au alloys may provide an alternative to Pt for fuel cell
reactions which could only tolerate a concentration of ca. ≤ 10 ppm CO [71]. The
improved CO tolerance of the Pd-Au alloys could be attributed to a weaker interaction
of CO with the surface resulting from the ligand effect [21, 263]. This is consistent with
the data obtained here from CO stripping measurements (Figure 3.23) which indicates
that alloying Pd with Au results in a decrease of CO adsorption.
155
The HER (Figure 4.20) and HOR (Figure 4.23) activity on ca. Pd25Bi75 is comparable
to pure Pd. The CO tolerance of Pd-Bi alloys is composition-dependent. The HOR
activity on the Pd-Bi alloys with concentrations of Bi above 20 at. % is strongly
poisoned by CO in the presence of a mixture of hydrogen and 500 ppm CO, since that
the activity was found to be significantly decreased after 1 minute of bubbling the
electrolyte with the mixture and completely suppressed after 11 minutes (Figure 4.26).
This is in accordance with the conclusion that the deposition of Bi on carbon supported
Pt lowers CO tolerance of Pt, since Bi offers low effectiveness towards facilitating
oxygen transfer in CO oxidation [194]. The Pd-Bi alloys with compositions below 20 at.
% Bi, however, offer a higher degree of CO tolerance.
Ru-Au alloys with compositions of Ru90Au10 and 60-80 at. % Au are more active than
Ru for the HER (Figure 5.19) and HOR (Figure 5.25). The Ru-Au alloy system exhibits
a high degree of CO tolerance during the HOR, since no significant influence on the
activity was found even after bubbling the electrolyte with a mixture of hydrogen and
500 ppm CO for 44 minutes (Figure 5.27).
The Ru-Au alloy system appears to be superior to the other two systems with respect
to CO tolerance. This is because the presence of CO does not significantly influence the
HOR activity on Ru-Au alloys (Figure 5.27), while a decay in the activity was found on
both Pd-Au (Figure 3.39) and Pd-Bi (Figure 4.26) alloys. This could be correlated with
well-documented role of Ru as a promoter of CO tolerance of Pt through the
bifunctional mechanism [72, 81, 258-260]. It could also be correlated with the
conclusion that the presence of Au on Ru(001) surface prevents CO adsorption [254].
Au was also found through this study to hinder CO adsorption on Pd (Figure 3.23).
Based on the results obtained on Pd-Au and Pd-Bi alloy systems, the Pd-Au and Pd-Bi
optimum compositions could be incorporated in the Volcano plot for the HER (Figure
6.1 [60]) assuming that a superior catalyst would have a |∆G| value closer to zero [60].
With respect to their lower cost, these alloys may provide alternatives to Pt for the HER
and HOR. The absence of Ru from the Volcano plot prevents prediction of the position
of the optimum Ru-Au alloy composition. Nevertheless, the application of Sabatier
156
principle [60] suggests that the |∆G| on the Ru-Au alloys that offer a higher activity for
the HER is closer to zero than Ru.
Figure 6.1: The Volcano plot for the HER on several pure metals and metal overlayers [60]. The colored circels represents the predicted position of the Pd50Au50 (red) and Pd25Bi75 (green) based on their activity for this reaction obseved throughout this work.
The increase in the HER and HOR activities observed here on the Pd-Au and Pd-Bi
alloys compared to pure Pd could be linked to a decrease in the bulk hydride content as
the formation of Pd hydrides becomes suppressed by adding higher concentrations of the
other constituent (Au or Bi). This is consistent with the conclusion that hydride
formation in the bulk lowers the activity for the HER [190]. It is also in accordance with
the observation by others that hydrogen absorption capacity decreases monotonically as
a function of bulk Au content producing zero (no hydrogen absorption) at ca. 70 at. %
Au [140]. Hydrogen absorption into bulk Pd-M (M = Cr, Mo and W) alloy and hydride
formation have also been shown to be monotonically suppressed by increasing the
content of the other constituent [264].
-8
-7
-6
-5
-4
-3
-2
-1
0
-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
Lo
g(i
0(A
cm
-2))
∆GH (eV)
Pd
Pd25Bi75
Au
Bi
Pd50Au50
157
The HER and HOR activity on the annealed array samples were observed to be greater
than on unannealed samples. It is generally proposed that annealing could improve the
performance of a catalyst material through: (i) removing impurities formed on the
catalyst surface during the preparation of the catalyst, (ii) forming more active sites on
the catalyst surface, and/or (iii) producing a uniform dispersion of metal particles in the
case of supported catalysts [58]. The presence of impurities on the array samples used in
this study before annealing is to a considerable extent unlikely as the synthesis of the
alloy catalysts was performed under UHV conditions. It is therefore likely that there was
a contribution from the substrate (Au) after annealing as an alloy can be formed
resulting in more activity. Another possibility is that the surface is reconstructed upon
annealing resulting in more active crystal faces for the HER and HOR [265, 266].
6.3- Suggestions for Further Studies
While more interest is given for research on Pd-Au alloy system, there is a clear lack of
information regarding surface chemistry and electrocatalytic studies on Pd-Bi and Ru-
Au alloys. The data obtained here provide valuable insights about those alloy systems.
There are many prospects for widening research on the Pd-Au, Pd-Bi and Ru-Au alloy
catalysts. Following are suggestions for further studies:
1. The high throughput methods employed for the synthesis and screening of alloy
systems examined here are powerful in terms of the study of support and particle size
effects on the electrocatalytic activity [90]. Therefore, it is interesting to investigate
these effects on the catalytic performance for the HER and HOR.
2. Valuable kinetic data of an electrochemical reaction such as transfer coefficient (α)
and limiting current density (jL) can be obtained from rotating disc electrode (RDE)
measurements [5]. Therefore, a further work may involve deposition of the optimum
alloy compositions on rotating disk electrodes (RDE) in order to determine kinetic
parameters for the HER and HOR on these catalysts.
158
References
[1] A.T. Bell, Science. 299 (2003) 1688-1691.
[2] M. Bowker, ''The Basis and Applications of Heterogeneous Catalysis'', Oxford
University Press, 1998.
[3] G. Rothenberg, '' Catalysis: Concepts and Green Applications'', WILEY-VCH
Verlag GmbH & Co. KGaA, 2008.
[4] R.I. Masel, ''Chemical Kinetics and Catalysis'', John Wiley & Sons, Inc., 2001.
[5] D. Pletcher, ''A First Course in Electrode Processes'', The Electrochemical
Consultancy, England, 1991.
[6] J. Lipkowski, P.N. Ross (Eds.), ''Electrocatalysis'', Wiley-VCH, Inc., 1998.
[7] D. Pletcher, F.C. Walsh, ''Industrial Electrochemistry'', Second ed., Chapman and
Hall Ltd, 1990.
[8] Southampton Electrochemistry Group, ''Instrumental Methods in
Electrochemistry'', Horwood Publishing Limited, 2001.
[9] S.H. Jordanov, P. Paunovic, O. Popovski, A. Dimitrov, S. Dragan, Bulletin of the
Chemists and Technologists of Macedonia. 23 (2004) 101-112.
[10] J.O.M. Bockris, Journal of the Serbian Chemical Society. 70 (2005) 475-487.
[11] N.M. Markovic, P.N. Ross, Surface Science Reports. 45 (2002) 117-229.
[12] G.-Q. Lu, A. Wieckowski, Current Opinion in Colloid & Interface Science. 5
(2000) 95-100.
[13] S. Trasatti, International Journal of Hydrogen Energy. 20 (1995) 835-844.
[14] S. Motoo, M. Shibata, M. Watanabe, Journal of Electroanalytical Chemistry. 110
(1980) 103-109.
[15] S. Motoo, M. Watanabe, Journal of Electroanalytical Chemistry. 111 (1980) 261-
268.
[16] A.M. Couper, D. Pletcher, F.C. Walsh, Chemical Reviews. 90 (1990) 837-865.
[17] G.J.K. Acres, G.A. Hards, Philosophical Transactions: Mathematical, Physical
and Engineering Sciences. 354 (1996) 1671-1680.
[18] K. Green, J.C. Wilson, Electronics & Communications Engineering Journal. 13
(2001) 43-47.
159
[19] L. Carrette, K.A. Friedrich, U. Stimming, ChemPhysChem. 1 (2000) 162-193.
[20] S.M. Haile, Acta Materialia. 51 (2003) 5981-6000.
[21] E. Christoffersen, P. Liu, A. Ruban, H.L. Skriver, J.K. Nørskov, Journal of
Catalysis. 199 (2001) 123-131.
[22] D.S. Cameron, Platinum Metals Review. 34 (1990) 26-35.
[23] M. Winter, R.J. Brodd, Chemical Reviews. 104 (2004) 4245-4270.
[24] M.A.J. Cropper, S. Geiger, D.M. Jollie, Journal of Power Sources. 131 (2004)
57-61.
[25] A. Bauen, D. Hart, A. Chase, International Journal of Hydrogen Energy. 28
(2003) 695-701.
[26] G.D. Berry, A.D. Pasternak, G.D. Rambach, J. Ray Smith, R.N. Schock, Energy.
21 (1996) 289-303.
[27] G.J.K. Acres, J.C. Frost, G.A. Hards, R.J. Potter, T.R. Ralph, D. Thompsett, G.T.
Burstein, G.J. Hutchings, Catalysis Today. 38 (1997) 393-400.
[28] C. Song, Catalysis Today. 77 (2002) 17-49.
[29] N.P. Brandon, S. Skinner, B.C.H. Steele, Annual Review of Materials Research.
33 (2003) 183-213.
[30] T.R. Ralph, M.P. Hogarth, Platinum Metals Review. 46 (2002) 117-135.
[31] H. Wendt, E.V. Spinace, A.O.N.e.M. Linardi, Quimica Nova. 28 (2005) 1066-
1075.
[32] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J.
Shen, Journal of Power Sources. 165 (2007) 739-756.
[33] K. Sirichaiprasert, S. Pongstabodee, A. Luengnaruemitchai, Journal of the
Chinese Institute of Chemical Engineers. 39 (2008) 597-607.
[34] J. Won Park, J. Hyeok Jeong, W.L. Yoon, C.S. Kim, D.K. Lee, Y.-K. Park, Y.W.
Rhee, International Journal of Hydrogen Energy. 30 (2005) 209-220.
[35] H.C. Lee, D.H. Kim, Catalysis Today. 132 (2008) 109-116.
[36] Y.-Z. Chen, B.-J. Liaw, H.-C. Chen, International Journal of Hydrogen Energy.
31 (2006) 427-435.
[37] G. Zhou, Y. Jiang, H. Xie, F. Qiu, Chemical Engineering Journal. 109 (2005)
141-145.
160
[38] Y.-Z. Chen, B.-J. Liaw, W.-C. Chang, C.-T. Huang, International Journal of
Hydrogen Energy. 32 (2007) 4550-4558.
[39] G. Sedmak, S. Hocevar, J. Levec, Journal of Catalysis. 213 (2003) 135-150.
[40] K. Sirichaiprasert, A. Luengnaruemitchai, S. Pongstabodee, International Journal
of Hydrogen Energy. 32 (2007) 915-926.
[41] Y.-Z. Chen, B.-J. Liaw, C.-W. Huang, Applied Catalysis A: General. 302 (2006)
168-176.
[42] G. Avgouropoulos, T. Ioannides, H. Matralis, Applied Catalysis B:
Environmental. 56 (2005) 87-93.
[43] J.-W. Park, J.-H. Jeong, W.-L. Yoon, H. Jung, H.-T. Lee, D.-K. Lee, Y.-K. Park,
Y.-W. Rhee, Applied Catalysis A: General. 274 (2004) 25-32.
[44] G. Avgouropoulos, T. Ioannides, Applied Catalysis A: General. 244 (2003) 155-
167.
[45] F. BalIkçI Derekaya, C. Kutar, Ç. Güldür, Materials Chemistry and Physics. 115
(2009) 496-501.
[46] H. Zou, S. Chen, W. Lin, Journal of Natural Gas Chemistry. 17 (2008) 208-211.
[47] M. Haruta, T. Kobayashi, H. Sano, N. Yamada, Chemistry Letters. 16 (1987)
405-408.
[48] G.J. Hutchings, Gold Bulletin. 37 (2004) 3-11.
[49] B.E. Hayden, D. Pletcher, J.-P. Suchsland, Angewandte Chemie International
Edition. 46 (2007) 3530-3532.
[50] J.M. Jaksic, L. Vracar, S.G. Neophytides, S. Zafeiratos, G. Papakonstantinou,
N.V. Krstajic, M.M. Jaksic, Surface Science. 598 (2005) 156-173.
[51] V. Ponec, Applied Catalysis A: General. 222 (2001) 31-45.
[52] J. Daintith (Ed.), ''A Dictionary of Chemistry'', Fourth ed., Oxford University
Press, 2000.
[53] H. Baker, H. Okamoto (Eds.), ASM Handbook: volume 3 ''Alloy Phase
Diagrams''. First ed., ASM International, 1992.
[54] K. Kovnir, M. Armbrüster, D. Teschner, T.V. Venkov, F.C. Jentoft, A. Knop-
Gericke, Y. Grin, R. Schlögl, Science and Technology of Advanced Materials. 8 (2007)
420-427.
161
[55] J.H. Hodak, A. Henglein, M. Giersig, G.V. Hartland, The Journal of Physical
Chemistry B. 104 (2000) 11708-11718.
[56] Y. Sun, C. Lei, Angewandte Chemie. 121 (2009) 6956-6959.
[57] J.C. Bauer, X. Chen, Q. Liu, T.-H. Phan, R.E. Schaak, Journal of Materials
Chemistry. 18 (2008) 275-282.
[58] C.W.B. Bezerra, L. Zhang, H. Liu, K. Lee, A.L.B. Marques, E.P. Marques, H.
Wang, J. Zhang, Journal of Power Sources. 173 (2007) 891-908.
[59] T. He, E. Kreidler, L. Xiong, E. Ding, Journal of Power Sources. 165 (2007) 87-
91.
[60] J. Greeley, T.F. Jaramillo, J. Bonde, I. Chorkendorff, J.K. Norskov, Nature
Materials. 5 (2006) 909-913.
[61] M. Pourbaix, ''Atlas of Electrochemical Equilibria in Aqueous Solutions'',
Second ed., National Association of Corrosion Engineers 1974.
[62] C. Milhano, D. Pletcher, Journal of Electroanalytical Chemistry. 614 (2008) 24-
30.
[63] S. Guerin, B.E. Hayden, C.E. Lee, C. Mormiche, A.E. Russell, The Journal of
Physical Chemistry B. 110 (2006) 14355-14362.
[64] X. Wang, N. Kariuki, J.T. Vaughey, J. Goodpaster, R. Kumar, D.J. Myers,
Journal of The Electrochemical Society. 155 (2008) B602-B609.
[65] F.J.R. Varela, O. Savadogo, Journal of The Electrochemical Society. 155 (2008)
B618-B624.
[66] T. Lopes, E. Antolini, E.R. Gonzalez, International Journal of Hydrogen Energy.
33 (2008) 5563-5570.
[67] T. Lopes, E. Antolini, F. Colmati, E.R. Gonzalez, Journal of Power Sources. 164
(2007) 111-114.
[68] K. Endo, K. Nakamura, Y. Katayama, T. Miura, Electrochimica Acta. 49 (2004)
2503-2509.
[69] Y. Suo, I.M. Hsing, Electrochimica Acta. 55 (2009) 210-217.
[70] J. Huang, H. Hou, T. You, Electrochemistry Communications. 11 (2009) 1281-
1284.
162
[71] G. Avgouropoulos, T. Ioannides, Applied Catalysis B: Environmental. 56 (2005)
77-86.
[72] C. Roth, N. Benker, R. Theissmann, R.J. Nichols, D.J. Schiffrin, Langmuir. 24
(2008) 2191-2199.
[73] H. Igarashi, T. Fujino, Y. Zhu, H. Uchida, M. Watanabe, Physical Chemistry
Chemical Physics. 3 (2001) 306-314.
[74] T. Wei, J. Wang, D.W. Goodman, The Journal of Physical Chemistry C. 111
(2007) 8781-8788.
[75] F. Maroun, F. Ozanam, O.M. Magnussen, R.J. Behm, Science. 293 (2001) 1811-
1814.
[76] M. Chen, D. Kumar, C.-W. Yi, D.W. Goodman, Science. 310 (2005) 291-293.
[77] C.-W. Yi, K. Luo, T. Wei, D.W. Goodman, The Journal of Physical Chemistry
B. 109 (2005) 18535-18540.
[78] A.F. Cunha, J.J.M. Órfão, J.L. Figueiredo, International Journal of Hydrogen
Energy. 34 (2009) 4763-4772.
[79] B. Gurau, R. Viswanathan, R. Liu, T.J. Lafrenz, K.L. Ley, E.S. Smotkin, E.
Reddington, A. Sapienza, B.C. Chan, T.E. Mallouk, S. Sarangapani, The Journal of
Physical Chemistry B. 102 (1998) 9997-10003.
[80] B. Coq, F. Figueras, Journal of Molecular Catalysis A: Chemical. 173 (2001)
117-134.
[81] L. Zhuang, J. Jin, H.D. Abruna, Journal of the American Chemical Society. 129
(2007) 11033-11035.
[82] T. Zawodzinski, S. Minteer, G. Brisard, The Electrochemical Society Interface.
15 (2006) 62-65.
[83] A.V. Ruban, H.L. Skriver, J.K. Nørskov, Physical Review B. 59 (1999) 15990-
16000.
[84] M. Lukaszewski, A. Zurowski, M. Grden, A. Czerwinski, Electrochemistry
Communications. 9 (2007) 671-676.
[85] W.M.H. Sachtler, Applications of Surface Science. 19 (1984) 167-180.
[86] D. Kumar, A. Mookerjee, V. Kumar, Journal of Physics F: Metal Physics. 6
(1976) 725-738.
163
[87] A. Hagemeyer, P. Strasser, A.F.V. Jr. (Eds.), ''High-Throughput Screening in
Chemical Catalysis'', WILEY-VCH Verlag GmbH & Co. KGaA, 2004.
[88] E.S. Smotkin, R.R. Diaz-Morales, Annual Review of Materials Research. 33
(2003) 557-579.
[89] S. Guerin, B.E. Hayden, Journal of Combinatorial Chemistry. 8 (2006) 66-73.
[90] S. Guerin, B.E. Hayden, D. Pletcher, M.E. Rendall, J.-P. Suchsland, Journal of
Combinatorial Chemistry. 8 (2006) 679-686.
[91] S. Guerin, B.E. Hayden, D. Pletcher, M.E. Rendall, J.-P. Suchsland, L.J.
Williams, Journal of Combinatorial Chemistry. 8 (2006) 791-798.
[92] S. Guerin, B.E. Hayden, C.E. Lee, C. Mormiche, J.R. Owen, A.E. Russell, B.
Theobald, D. Thompsett, Journal of Combinatorial Chemistry. 6 (2004) 149-158.
[93] J. Greeley, J.K. Nørskov, Surface Science. 601 (2007) 1590-1598.
[94] R. Liu, E.S. Smotkin, Journal of Electroanalytical Chemistry. 535 (2002) 49-55.
[95] T.F. Jaramillo, A. Ivanovskaya, E.W. McFarland, Journal of Combinatorial
Chemistry. 4 (2002) 17-22.
[96] A. Królikowski, A. Wiecko, Electrochimica Acta. 47 (2002) 2065-2069.
[97] G. Lu, P. Evans, G. Zangari, Journal of The Electrochemical Society. 150 (2003)
A551-A557.
[98] Y.M. Wu, W.S. Li, X.M. Long, F.H. Wu, H.Y. Chen, J.H. Yan, C.R. Zhang,
Journal of Power Sources. 144 (2005) 338-345.
[99] L.D. Santos, E. Gonzalez, ECS Meeting Abstracts. 702 (2007) 1496.
[100] Sung J. Yoo, H.-Y. Park, T.-Y. Jeon, I.-S. Park, Y.-H. Cho, Y.-E. Sung,
Angewandte Chemie International Edition. 47 (2008) 9307-9310.
[101] T.J. Schmidt, V. Stamenkovic, N.M. Markovic, P.N. Ross, Electrochimica Acta.
48 (2003) 3823-3828.
[102] J.S. Cooper, P.J. McGinn, Journal of Power Sources. 163 (2006) 330-338.
[103] P. Strasser, Journal of Combinatorial Chemistry. 10 (2008) 216-224.
[104] J.S. Cooper, M.K. Jeon, P.J. McGinn, Electrochemistry Communications. 10
(2008) 1545-1547.
[105] J.S. Cooper, P.J. McGinn, Applied Surface Science. 254 (2007) 662-668.
164
[106] M.K. Jeon, J.S. Cooper, P.J. McGinn, Journal of Power Sources. 185 (2008) 913-
916.
[107] J.F. Whitacre, T.I. Valdez, S.R. Narayanan, Electrochimica Acta. 53 (2008)
3680-3689.
[108] M.K. Jeon, J.S. Cooper, P.J. McGinn, Journal of Power Sources. 192 (2009) 391-
395.
[109] B.E. Conway, B.V. Tilak, Electrochimica Acta. 47 (2002) 3571-3594.
[110] Y. Xu, International Journal of Hydrogen Energy. 34 (2009) 77-83.
[111] D.R. Gabe, Journal of Applied Electrochemistry. 27 (1997) 908-915.
[112] J.J.T.T. Vermeijlen, L.J.J. Janssen, G.J. Visser, Journal of Applied
Electrochemistry. 27 (1997) 497-506.
[113] J. Greeley, J.K. Nørskov, L.A. Kibler, A.M. El-Aziz, D.M. Kolb,
ChemPhysChem. 7 (2006) 1032-1035.
[114] K. Brunelli, M. Dabalà, R. Frattini, G. Sandonà, I. Calliari, Journal of Alloys and
Compounds. 317-318 (2001) 595-602.
[115] A. Altube, A.R. Pierna, F.F. Marzo, Journal of Non-Crystalline Solids. 287
(2001) 297-301.
[116] P. Fricoteaux, C. Rousse, Journal of Electroanalytical Chemistry. 612 (2008) 9-
14.
[117] J.H. Chun, S.K. Jeon, International Journal of Hydrogen Energy. 28 (2003)
1333-1343.
[118] H. Uchida, K. Izumi, M. Watanabe, The Journal of Physical Chemistry B. 110
(2006) 21924-21930.
[119] A. Velázquez, F. Centellas, J.A. Garrido, C. Arias, R.M. Rodríguez, E. Brillas,
P.-L. Cabot, Journal of Power Sources. 195 (2010) 710-719.
[120] A.F. Innocente, A.C.D. Ângelo, Journal of Power Sources. 162 (2006) 151-159.
[121] M.M. Jaksic, International Journal of Hydrogen Energy. 26 (2001) 559-578.
[122] J.-P. Suchsland, PhD Thesis, School of Chemistry, University of Southampton,
Southampton, 2007.
[123] L.J. Williams, PhD Thesis, School of Chemistry, University of Southampton,
Southampton, 2007.
165
[124] K.J. Laidler, J.H. Meiser, ''Physical Chemistry'', Third ed., Houghton Mifflin
1999.
[125] P. Atkins, J.d. Paula, ''Atkins' Physical Chemistry'', eighth ed., Oxford University
Press, 2006.
[126] P. Atkins, T. Overton, J. Rourke, M. Weller, F. Armstrong, ''Inorganic
Chemistry'', Fourth ed., Oxford University Press, 2006.
[127] W.H. Rothery, G.V. Raynor, ''The Structure of Metals and Alloys'', Fourth ed.,
The institute of Metals, 1962.
[128] E.J.W. Whittaker, ''Crystallography: An Introduction for Earth Science (and
other solid state) Students'', Pergamon, 1981.
[129] D.B. Williams, C.B. Carter, ''Transmission Electron Microscopy: IV
Spectrometry'', , First ed., Springer Science + Business Media, Inc., 1996.
[130] P.J. Goodhew, J. Humphreys, R. Beanland, ''Electron Microscopy and Analysis'',
Third ed., Taylor & Francis, 2001.
[131] J.C. Vickerman (Ed.), ''Surface Analysis: The Principlal Techniques'', John
Wiley & Sons Ltd, 1997.
[132] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg,
''Handbook of X-ray Photoelectron Spectroscopy'', Physical Electronics Division,
Perkin-Elmer Corp., 1979.
[133] D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray
Photoelectron Spectroscopy, John Wiley & Sons, 1983.
[134] A.L. Zhu, M.Y. Teo, S.A. Kulinich, Applied Catalysis A: General. 352 (2009)
17-26.
[135] D. Pletcher, "A First Course in Electrode Processes", Second ed., The Royal
Society of Chemistry, Cambridge, 2009.
[136] C.H. Hamann, A. Hamnett, W. Vielstich, ''Electrochemistry'', Second ed., Wiley-
VCH Verlag GmbH & Co. KGaA, 2007.
[137] T.J. Schmidt, B.N. Grgur, R.J. Behm, N.M. Markovic, P.N. Ross, Jr., Physical
Chemistry Chemical Physics. 2 (2000) 4379-4386.
[138] T.J. Schmidt, V. Stamenkovic, G.A. Attard, N.M. Markovic, P.N. Ross,
Langmuir. 17 (2001) 7613-7619.
166
[139] T.J. Schmidt, Z. Jusys, H.A. Gasteiger, R.J. Behm, U. Endruschat, H.
Boennemann, Journal of Electroanalytical Chemistry. 501 (2001) 132-140.
[140] M. Łukaszewski, K. Kuśmierczyk, J. Kotowski, H. Siwek, A. Czerwiński,
Journal of Solid State Electrochemistry. 7 (2003) 69-76.
[141] A.V. Tripkovic, K.D. Popovic, R.M. Stevanovic, R. Socha, A. Kowal,
Electrochemistry Communications. 8 (2006) 1492-1498.
[142] Y. Pluntke, L.A. Kibler, D.M. Kolb, Physical Chemistry Chemical Physics. 10
(2008) 3684-3688.
[143] P.S. Ruvinsky, S.N. Pronkin, V.I. Zaikovskii, P. Bernhardt, E.R. Savinova,
Physical Chemistry Chemical Physics. 10 (2008) 6665-6676.
[144] R. Grisel, K.-J. Weststrate, A. Gluhoi, B.E. Nieuwenhuys, Gold Bulletin. 35
(2002) 39-45.
[145] M. Haruta, Gold Bulletin. 37 (2004) 27-36.
[146] G.J. Hutchings, Chemical Communications (2008) 1148-1164.
[147] M. Haruta, Catalysis Today. 36 (1997) 153-166.
[148] G.J. Hutchings, Gold Bulletin. 29 (1996) 123-130.
[149] C.W. Corti, R.J. Holliday, D.T. Thompson, Applied Catalysis A: General. 291
(2005) 253-261.
[150] C.T. Campbell, Science. 306 (2004) 234-235.
[151] B.E. Hayden, D. Pletcher, M.E. Rendall, J.-P. Suchsland, The Journal of Physical
Chemistry C. 111 (2007) 17044-17051.
[152] D.T. Thompson, Platinum Metals Review. 48 (2004) 169-172.
[153] M. Neurock, Journal of Catalysis. 216 (2003) 73-88.
[154] A.M. Venezia, V. La Parola, G. Deganello, B. Pawelec, J.L.G. Fierro, Journal of
Catalysis. 215 (2003) 317-325.
[155] P. Landon, P.J. Collier, A.J. Papworth, C.J. Kiely, G.J. Hutchings, Chemical
Communications (2002) 2058-2059.
[156] P. Landon, P.J. Collier, A.F. Carley, D. Chadwick, A.J. Papworth, A. Burrows,
C.J. Kiely, G.J. Hutchings, Physical Chemistry Chemical Physics. 5 (2003) 1917-1923.
[157] J.K. Edwards, B.E. Solsona, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely, G.J.
Hutchings, Journal of Catalysis. 236 (2005) 69-79.
167
[158] N. Dimitratos, F. Porta, L. Prati, A. Villa, Catalysis Letters. 99 (2005) 181-185.
[159] G.C. Bond, Platinum Metals Review. 51 (2007) 63-68.
[160] S. Devarajan, P. Bera, S. Sampath, Journal of Colloid and Interface Science. 290
(2005) 117-129.
[161] M. Nie, P.K. Shen, Z. Wei, Journal of Power Sources. 167 (2007) 69-73.
[162] A.M. El-Aziz, L.A. Kibler, Journal of Electroanalytical Chemistry. 534 (2002)
107-114.
[163] Z. Suo, C. Ma, M. Jin, T. He, L. An, Catalysis Communications. 9 (2008) 2187-
2190.
[164] A. Roudgar, A. Groß, Journal of Electroanalytical Chemistry. 548 (2003) 121-
130.
[165] L.A. Kibler, Electrochimica Acta. 53 (2008) 6824-6828.
[166] M. Baldauf, D.M. Kolb, The Journal of Physical Chemistry. 100 (1996) 11375-
11381.
[167] L.A. Kibler, A.M. El-Aziz, D.M. Kolb, Journal of Molecular Catalysis A:
Chemical. 199 (2003) 57-63.
[168] R. Larsen, S. Ha, J. Zakzeski, R.I. Masel, Journal of Power Sources. 157 (2006)
78-84.
[169] M. Grden, M. Lukaszewski, G. Jerkiewicz, A. Czerwinski, Electrochimica Acta.
53 (2008) 7583-7598.
[170] M.S. Rau, P.M. Quaino, M.R. Gennero de Chialvo, A.C. Chialvo,
Electrochemistry Communications. 10 (2008) 208-212.
[171] A. Czerwiński, M. Grdeń, M. Łukaszewski, Journal of Solid State
Electrochemistry. 8 (2004) 411-415.
[172] A. Czerwinski, I. Kiersztyn, M. Grden, J. Czapla, Journal of Electroanalytical
Chemistry. 471 (1999) 190-195.
[173] M. Conte, A.F. Carley, G. Attard, A.A. Herzing, C.J. Kiely, G.J. Hutchings,
Journal of Catalysis. 257 (2008) 190-198.
[174] S. Yalcin, R. Avci, Applied Surface Science. 214 (2003) 319-337.
[175] J. William D. Callister, ''Materials Science and Engineering : An Introduction'',
seventh ed., John Wiley & Sons, 2007.
168
[176] S. Ikeda, M. Nagano, Japanese Journal of Applied Physics. 38 (1999) L882-
L884.
[177] G. Bernardotto, F. Menegazzo, F. Pinna, M. Signoretto, G. Cruciani, G. Strukul,
Applied Catalysis A: General. 358 (2009) 129-135.
[178] T. Nakagawa, H. Nitani, S. Tanabe, K. Okitsu, S. Seino, Y. Mizukoshi, T.A.
Yamamoto, Ultrasonics Sonochemistry. 12 (2005) 249-254.
[179] J. Kuntze, S. Speller, W. Heiland, P. Deurinck, C. Creemers, A. Atrei, U. Bardi,
Physical Review B. 60 (1999) 9010-9018.
[180] L. Piccolo, A. Piednoir, J.-C. Bertolini, Surface Science. 592 (2005) 169-181.
[181] V. Soto-Verdugo, H. Metiu, Surface Science. 601 (2007) 5332-5339.
[182] W.F. Egelhoff, Surface Science Reports. 6 (1987) 253-415.
[183] M. Lukaszewski, A. Czerwinski, Electrochimica Acta. 48 (2003) 2435-2445.
[184] B. Losiewicz, L. Birry, A. Lasia, Journal of Electroanalytical Chemistry. 611
(2007) 26-34.
[185] B.E. Hayden, in: A. Wieckowski, E.R. Savinova, C.G. Vayenas (Eds.),
''Catalysis and Electrocatalysis at Nanoparticle Surfaces'', Marcel Dekker, Inc., 2003, pp.
171-210.
[186] S. Strbac, F. Maroun, O.M. Magnussen, R.J. Behm, Journal of Electroanalytical
Chemistry. 500 (2001) 479-490.
[187] W.F. Lin, T. Iwasita, W. Vielstich, The Journal of Physical Chemistry B. 103
(1999) 3250-3257.
[188] F.H.B. Lima, J. Zhang, M.H. Shao, K. Sasaki, M.B. Vukmirovic, E.A. Ticianelli,
R.R. Adzic, The Journal of Physical Chemistry C. 111 (2007) 404-410.
[189] W. Olovsson, C. Göransson, L.V. Pourovskii, B. Johansson, I.A. Abrikosov,
Physical Review B. 72 (2005) 064203.
[190] L.A. Kibler, ChemPhysChem. 7 (2006) 985-991.
[191] S. Zhou, K. McIlwrath, G. Jackson, B. Eichhorn, Journal of the American
Chemical Society. 128 (2006) 1780-1781.
[192] M. Wenkin, P. Ruiz, B. Delmon, M. Devillers, Journal of Molecular Catalysis A:
Chemical. 180 (2002) 141-159.
169
[193] T. Mallat, Z. Bodnar, P. Hug, A. Baiker, Journal of Catalysis. 153 (1995) 131-
143.
[194] B.E. Hayden, Catalysis Today. 38 (1997) 473-481.
[195] X. Yu, P.G. Pickup, Journal of Power Sources. 182 (2008) 124-132.
[196] B.E. Hayden, A.J. Murray, R. Parsons, D.J. Pegg, Journal of Electroanalytical
Chemistry. 409 (1996) 51-63.
[197] F. Alardin, P. Ruiz, B. Delmon, M. Devillers, Applied Catalysis A: General. 215
(2001) 125-136.
[198] M.D. Macia, E. Herrero, J.M. Feliu, Journal of Electroanalytical Chemistry. 554-
555 (2003) 25-34.
[199] M. Besson, F. Lahmer, P. Gallezot, P. Fuertes, G. Fleche, Journal of Catalysis.
152 (1995) 116-121.
[200] S. Karski, I. Witonska, Journal of Molecular Catalysis A: Chemical. 191 (2003)
87-92.
[201] S. Karski, T. Paryjczak, I. Witoñska, Kinetics and Catalysis. 44 (2003) 618-622.
[202] H.E.J. Hendriks, B.F.M. Kuster, G.B. Marin, Carbohydrate Research. 204 (1990)
121-129.
[203] R.B. Bian, J. Shen, Scientific Research and Essay. 1 (3) (2006) 057-060.
[204] S. Biella, L. Prati, M. Rossi, Journal of Catalysis. 206 (2002) 242-247.
[205] T. Miyake, A. Hattori, M. Hanaya, S. Tokumaru, H. Hamaji, T. Okada, Topics in
Catalysis. 13 (2000) 243-248.
[206] J.A. Anderson, J. Mellor, R.P.K. Wells, Journal of Catalysis. 261 (2009) 208-
216.
[207] B.-J. Kim, K. Kwon, C.K. Rhee, J. Han, T.-H. Lim, Electrochimica Acta. 53
(2008) 7744-7750.
[208] D. Volpe, E. Casado-Rivera, L. Alden, C. Lind, K. Hagerdon, C. Downie, C.
Korzeniewski, F.J. DiSalvo, H.D. Abruna, Journal of Electrochemical Society. 151 (7)
(2004) A971-A977.
[209] J. Lee, P. Strasser, M. Eiswirth, G. Ertl, Electrochimica Acta. 47 (2001) 501-508.
[210] E. Sandnes, M.E. Williams, U. Bertocci, M.D. Vaudin, G.R. Stafford,
Electrochimica Acta. 52 (2007) 6221-6228.
170
[211] X. Gonze, J.-P. Michenaud, J.-P. Vigneron, Physica Scripta. 37 (1988) 785-789.
[212] X. Duan, J. Yang, W. Zhu, X.a. Fan, C. Xiao, Materials Letters. 61 (2007) 4341-
4343.
[213] J. Vrest'al, J. Pinkas, A. Watson, A. Scott, J. Houserova, A. Kroupa, Calphad. 30
(2006) 14-17.
[214] Y. Sakamoto, M. Ura, T. Hisamoto, T.B. Flanagan, International Journal of
Hydrogen Energy. 21 (1996) 1009-1015.
[215] M. Ellner, Journal of Alloys and Compounds. 436 (2007) 78-81.
[216] I. Coulthard, T.K. Sham, Physical Review Letters. 77 (1996) 4824.
[217] D.R. Blasini, D. Rochefort, E. Fachini, L.R. Alden, F.J. DiSalvo, C.R. Cabrera,
H.D. Abruna, Surface Science. 600 (2006) 2670-2680.
[218] W.A. Jesser, G.J. Shiflet, G.L. Allen, J.L. Crawford, Materials Research
Innovations. 2 (1999) 211-216.
[219] P.N. Bartlett, B. Gollas, S. Guerin, J. Marwan, Physical Chemistry Chemical
Physics. 4 (2002) 3835-3842.
[220] J. Greeley, T.F. Jaramillo, J. Bonde, I. Chorkendorff, J.K. Norskov, Nat Mater. 5
(2006) 909-913.
[221] S. Abbet, A. Sanchez, U. Heiz, W.-D. Schneider, A.M. Ferrari, G. Pacchioni, N.
Rosch, Journal of the American Chemical Society. 122 (2000) 3453-3457.
[222] A. Ruban, B. Hammer, P. Stoltze, H.L. Skriver, J.K. Nørskov, Journal of
Molecular Catalysis A: Chemical. 115 (1997) 421-429.
[223] W.B. Wang, M.S. Zei, G. Ertl, Physical Chemistry Chemical Physics. 3 (2001)
3307-3311.
[224] H. Bielawa, O. Hinrichsen, A. Birkner, M. Muhler, Angewandte Chemie
International Edition. 40 (2001) 1061-1063.
[225] M. Kuhn, J.A. Rodriguez, J. Hrbek, A. Bzowski, T.K. Sham, Surface Science.
341 (1995) L1011-L1018.
[226] S. Poulston, M. Tikhov, R.M. Lambert, Surface Science. 331-333 (1995) 818-
823.
[227] S. Strbac, R.J. Behm, A. Crown, A. Wieckowski, Surface Science. 517 (2002)
207-218.
171
[228] T. Cai, Z. Song, Z. Chang, G. Liu, J.A. Rodriguez, J. Hrbek, Surface Science.
538 (2003) 76-88.
[229] S. Strbac, C.M. Johnston, G.Q. Lu, A. Crown, A. Wieckowski, Surface Science.
573 (2004) 80-99.
[230] A. Steltenpohl, N. Memmel, E. Taglauer, T. Fauster, J. Onsgaard, Surface
Science. 382 (1997) 300-309.
[231] J. Storm, R.M. Lambert, N. Memmel, J. Onsgaard, E. Taglauer, Surface Science.
436 (1999) 259-268.
[232] W. Chen, L.-P. Xu, S. Chen, Journal of Electroanalytical Chemistry. 631 (2009)
36-42.
[233] M.S. Zei, G. Ertl, Physical Chemistry Chemical Physics. 2 (2000) 3855-3859.
[234] C. Thambidurai, Y.-G. Kim, J.L. Stickney, Electrochimica Acta. 53 (2008) 6157-
6164.
[235] H.A. Gasteiger, N. Markovic, P.N. Ross, E.J. Cairns, The Journal of Physical
Chemistry. 97 (1993) 12020-12029.
[236] G.J. Matare, M.E. Tess, Y. Yang, K.A. Abboud, L. McElwee-White,
Organometallics. 21 (2002) 711-716.
[237] Sheng-yang Huang, Chia-ming Chang, Kuan-wen Wang, Chuin-tih Yeh,
ChemPhysChem. 8 (2007) 1774-1777.
[238] T. Iwasita, H. Hoster, A. John-Anacker, W.F. Lin, W. Vielstich, Langmuir. 16
(2000) 522-529.
[239] H. Hoster, T. Iwasita, H. Baumgartner, W. Vielstich, Physical Chemistry
Chemical Physics. 3 (2001) 337-346.
[240] H. Inoue, J.X. Wang, K. Sasaki, R.R. Adzic, Journal of Electroanalytical
Chemistry. 554-555 (2003) 77-85.
[241] H.A. Gasteiger, N.M. Markovic, P.N. Ross, The Journal of Physical Chemistry.
99 (1995) 8290-8301.
[242] H.A. Gasteiger, N.M. Markovic, P.N. Ross, The Journal of Physical Chemistry.
99 (1995) 16757-16767.
[243] V. Horvat-Radosevic, K. Kvastek, M. Vukovic, D. Cukman, Journal of
Electroanalytical Chemistry. 482 (2000) 188-201.
172
[244] G.J. Hutchings, M. Haruta, Applied Catalysis A: General. 291 (2005) 2-5.
[245] H. Ma, C. Liu, J. Liao, Y. Su, X. Xue, W. Xing, Journal of Molecular Catalysis
A: Chemical. 247 (2006) 7-13.
[246] D.-J. Lee, S.-W. Kang, S.-W. Rhee, Thin Solid Films. 413 (2002) 237-242.
[247] S. Ikeda, M. Kiguchi, K. Saiki, Philosophical Magazine. 84 (2004) 1671 - 1682.
[248] H. Okamoto, T.B. Massalski, Bulletin of Alloy Phase Diagrams. 5 (1984) 388-
390.
[249] L. Qubo, Z. Xinming, Gold Bulletin. 31 (1) (1998) 30-32.
[250] E. Antolini, F. Cardellini, Journal of Alloys and Compounds. 315 (2001) 118-
122.
[251] B.-J. Hwang, Loka S. Sarma, G.-R. Wang, C.-H. Chen, D.-G. Liu, H.-S. Sheu, J.-
F. Lee, Chemistry - A European Journal. 13 (2007) 6255-6264.
[252] S.R. Brankovic, J.X. Wang, Y. Zhu, R. Sabatini, J. McBreen, R.R. Adzic,
Journal of Electroanalytical Chemistry. 524-525 (2002) 231-241.
[253] I. Paseka, Journal of Solid State Electrochemistry. 11 (2007) 52-58.
[254] H.J. Jansch, C. Polenz, C. Bromberger, M. Detje, H.D. Ebinger, B. Polivka, W.
Preyβ, R. Veith, D. Fick, Surface Science. 495 (2001) 120-128.
[255] S. Strbac, C. Johnston, A. Wieckowski, Russian Journal of Electrochemistry. 42
(2006) 1244-1250.
[256] A.L. Ocampo, M. Miranda-Hernández, J. Morgado, J.A. Montoya, P.J.
Sebastian, Journal of Power Sources. 160 (2006) 915-924.
[257] J.C. Davies, B.E. Hayden, D.J. Pegg, Electrochimica Acta. 44 (1998) 1181-1190.
[258] Y. Tong, H.S. Kim, P.K. Babu, P. Waszczuk, A. Wieckowski, E. Oldfield,
Journal of the American Chemical Society. 124 (2002) 468-473.
[259] R. Venkataraman, H.R. Kunz, J.M. Fenton, Journal of The Electrochemical
Society. 150 (2003) A278-A284.
[260] E.I. Santiago, V.A. Paganin, M. do Carmo, E.R. Gonzalez, E.A. Ticianelli,
Journal of Electroanalytical Chemistry. 575 (2005) 53-60.
[261] F.A. Al Odail, A. Anastasopoulos, B.E. Hayden, To be published (2010).
[262] F.A. Al Odail, A. Anastasopoulos, B.E. Hayden, To be published (2010).
173
[263] J.C. Davies, J. Bonde, Á. Logadóttir, J.K. Nørskov, I. Chorkendorff, Fuel Cells.
5 (2005) 429-435.
[264] D. Wang, J.D. Clewley, S. Luo, T.B. Flanagan, Journal of Alloys and
Compounds. 325 (2001) 151-159.
[265] E.M. McCash, ''Surface Chemistry'', Oxford University Press, 2001.
[266] G.A. Somorjai, Annual Review of Physical Chemistry. 45 (1994) 721-751.