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This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8009
Cite this: Chem. Soc. Rev., 2012, 41, 8009–8020
Pd–Au bimetallic catalysts: understanding alloy effects from planar
models and (supported) nanoparticlesw
Feng Gao*aand D. Wayne Goodmanzb
Received 28th April 2012
DOI: 10.1039/c2cs35160a
Pd–Au bimetallic catalysts often display enhanced catalytic activities and selectivities compared
with Pd-alone catalysts. This enhancement is often caused by two alloy effects, i.e., ensemble and
ligand effects. The ensemble effect is a dilution of surface Pd by Au. With increasing surface
Au coverage, contiguous Pd ensembles disappear and isolated Pd ensembles form. For certain
reactions, for example vinyl acetate synthesis, this effect is responsible for reaction rate
enhancement via the formation of highly active surface sites, e.g., isolated Pd pairs. The
disappearance of contiguous Pd ensembles also switches off side reactions catalyzed by these sites.
This explains the selectivity increase of certain reactions, for example direct H2O2 synthesis. The
ligand effects are electronic perturbation of Pd by Au. Via direct charge transfer or by affecting
bond lengths, the ligand effects cause the Pd d band to be more filled, moving the d-band center
away from the Fermi level. Both changes make Pd more ‘‘atomic like’’ therefore binding reactants
and products more weakly. For certain reactions, this eliminates a so-called ‘‘self-poisoning’’
effect and enhances activity/selectivity.
1. Introduction
Alloys are a class of important heterogeneous catalysts as
they frequently exhibit much enhanced catalytic stabilities,
activities and selectivities, as compared with their single-metal
constituents.1 The Ponec and Bond definition of alloy is the
following: alloy is most conveniently defined as a metallic
system containing two or more components, irrespective of
their intimacy of mixing or, precise manner of mixing.2,3 When
alloys contain two metallic components, for example Pd–Au,
they are sometimes also called bimetallics. In this article, we
intend to call a Pd–Au catalyst ‘‘alloy’’ when Pd and Au are
intimately mixed otherwise ‘‘bimetallics’’ when Pd and Au are
segregated. Among alloy catalysts, Pd–Au has received a great
deal of attention because of its superior activity in a number of
reactions. Pd–Au catalysts are used in the industrial synthesis
of vinyl acetate (VA). In the United States alone, 4.8 million
tons of VA are produced over this catalyst annually.4 Pd–Au
catalysts also catalyze low-temperature CO oxidation,5–7
direct H2O2 synthesis from H2 and O2,8–11 hydrodechlorina-
tion of Cl-containing pollutants in underground water,12
hydrodesulfurization of S-containing organics,13 hydrogenation
of hydrocarbons,14–16 acetylene trimerization,17–20 and many
other reactions.
Alloying induces multiple changes in the physical and
chemical properties of the metallic components. Where
catalytic properties are concerned, two alloy effects are sig-
nificant: (1) ensemble effects, i.e., a finite number of atoms in a
particular geometric orientation that are required for facilitating
a particular catalytic process; and (2) ligand effects, i.e.,
electronic modifications resulting from hetero-nuclear metal–
metal bond formation. The latter could involve charge transfer
between the metals or orbital rehybridization of one or both
metallic components.1 It has to be noted that one cannot vary
the composition of the catalyst surface without affecting both
the distribution of the ensembles and changing the electronic
structure of the individual constituent atoms in the surface.21
Still, some suggest that ensemble effects play a more dominant
role than ligand effects.3,21
For the Pd–Au system in particular, an ensemble effect is
mainly a diluting effect where the catalytically more active
component (Pd) is diluted by the less active component (Au).
As the surface ratio of Au–Pd increases, sizes of contiguous Pd
ensembles decrease and eventually all Pd atoms are separated
by Au as isolated Pd monomers.1,5,6,22 With regard to the
possible ligand effects in this catalyst, one would intuitively
expect that charge is transferred from Pd to Au since Au has
higher electronegativity. This statement may be true but is
oversimplified. Among metals, Au has one of the largest
electron affinities. In bulk gold-containing intermetallic com-
pounds, Au usually gains s, p electrons, but this gain of charge
a Institute for Integrated Catalysis, Pacific Northwest NationalLaboratory, P.O. Box 999, Richland, WA 99352, USA.E-mail: [email protected]; Fax: +1 509 371 6066;Tel: +1 509-371-7164
bDepartment of Chemistry, Texas A&M University, P.O. Box 30012,College Station, TX 77842-3012, USA
w Part of the bimetallic nanocatalysts themed issue.z Dr Goodman deceased on February 27, 2012.
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8010 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012
is partially compensated by a depletion of Au 5d electrons.23
This situation also applies for the Pd–Au system. Indeed, there
appears to be general agreement that upon alloying, Au
gains s, p electrons and loses d electrons whereas Pd loses s,
p electrons but gains d electrons.1,13,14,23 Charge transfer
between Pd and Au also helps to explain why Au is able to
fully isolate Pd: notably, there exists some Coulomb Pd–Pd
repulsion in bulk Pd whereas Pd–Au attraction is realized as a
result of net charge transfer from Pd to Au. For late-transition
metals like Pd and Au, the d-character is much more impor-
tant than s, p-character in defining their chemisorption and
catalytic properties. For Pd, gaining d electrons shifts the d
band center away from the Fermi level, which leads to weaker
interaction between adsorbates and surface Pd atoms.21
Indeed, recent theoretical calculations demonstrate that the
Pd d band for Pd monomers surrounded by Au is much lower
in energy than that for Pd monolayer or bulk Pd surfaces.24
There is another reason that causes the Pd d band to narrow: a
lattice mismatch between Pd and Au, with Pd having a lattice
constant B5% smaller than Au.1 Upon alloying, however, Pd
has been found, in certain cases, to adopt the lattice constant
of Au.14,17 In this case, Pd–Pd bond length increases causing
the Fermi level within the Pd d band to rise. This also enhances
the atomic-like character of Pd atoms and correspondingly,
weakens binding toward reactants. From both charge transfer
and bond length arguments, we understand that Au is able to
weaken the binding strength of Pd by perturbing its d band.
However, this does not necessarily mean that Au weakens the
catalytic activity of Pd. In contrast, enhanced activity of Pd
within Pd–Au alloys is frequently found as compared to pure Pd.
Besides other factors, diminishing the so-called self-poisoning by
reactants/products is one reason that accounts for the activity
enhancement. Some examples are shown in the following sections.
Pd–Au catalysts are divided into two categories in this
article: (1) planar models used by surface scientists, including
single crystals and thin films; (2) nanoparticles for practical
applications. The latter include traditional high-surface-area
carrier supported metallic particles where the sizes of the
particles typically fall in the nano-dimensional range; and, in
the past two decades, the rapidly developing unsupported
nanoparticles. Each category has its advantages and dis-
advantages. Planar model catalysts are generally referred to
as ‘‘well-defined’’ catalysts as these are synthesized under well-
controlled conditions and characterized with a wide array of
surface-sensitive techniques such that they are often under-
stood at an atomic level.25,26 However these model catalysts
are typically used under non-practical ultrahigh vacuum
(UHV) conditions. To overcome the ‘‘pressure gap’’, some
researchers use coupled high-pressure reactor/UHV systems to
study catalysis on planar models at elevated pressures.27 On
the other hand, nano-particle catalysts are more practical and
can be used under various realistic reaction conditions. How-
ever, these catalysts are generally too complex to understand at
an atomic level. Indeed, as stated by Crooks and co-workers,28
unambiguous structure determination for particles in the size
range of o2 nm remains a major analytical challenge. In this
article we will cover both categories of catalysts. However to
address the alloy effects more explicitly, more attention is paid
to planar model catalysts since both ensemble and ligand
effects can be probed at an atomic level in this case.
Feng Gao
Feng Gao received under-graduate and graduate educa-tion at Tianjin University,China, in the 1990s inChemical Engineering. Hejoined the University ofWisconsin-Milwaukee in 2000as a graduate student andreceived a PhD in PhysicalChemistry in 2004 under Prof.Wilfred T. Tysoe. From 2007to 2009, he was a postdoc atTexas A&M University underProf. D. Wayne Goodman. Hehad a brief stay at WashingtonState University as a research
faculty member and is currently a staff scientist at PacificNorthwest National Laboratory (PNNL), conducting researchin basic and environmental heterogeneous catalysis. He is acoauthor of 60 publications.
D. Wayne Goodman
Wayne Goodman joined thefaculty of the ChemistryDepartment at Texas A&Min 1988 where he is currentlya Distinguished Professor andthe Robert A. Welch Chair.Previously he was the Headof the Surface ScienceDivision at Sandia NationalLaboratories. He was therecipient of the Ipatieff Awardof the American ChemicalSociety in 1983, the Colloidand Surface Chemistry Awardof the American ChemicalSociety in 1993, the Yarwood
Medal of the British Vacuum Society in 1994, a HumboldtResearch Award in 1995, a Distinguished Research Award ofTexas A&M University in 1997, the Giuseppe Parravano Awardin 2001, the Adamson Award for Distinguished Service in theAdvancement of Surface Chemistry of the American ChemicalSociety in 2002, the Gabor A. Somorjai Award of the AmericanChemical Society in 2005 and elected Fellow of the AmericanChemical Society in 2009. He is the author of over 540 publica-tions/book chapters and an active member/officer of a number ofprofessional societies. He has served as an Associate Editor forthe Journal of Catalysis, and served on the Advisory Boards ofSurface Science, Langmuir, Catalysis Letters, and The Journalof Physics: Condensed Matter.
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8011
2. Planar model catalysts
2.1. Model catalyst formation and characterization
Three types of planar model Pd–Au alloy catalysts have been
developed: (1) Pd–Au single crystals, e.g. AuPd(100)5,6,22
and Au3Pd(100) alloys;29 (2) Pd–Au thin films generated by
depositing Au on Pd single crystals,19,30–34 or depositing Pd on
Au single crystals,18,35,36 or co-depositing Au and Pd on Au37
or refractory metal substrates;1,38 and (3) Pd–Au clusters
generated by co-depositing Au and Pd on planar oxide thin
films.39,40 For the latter cases, annealing of the thin films after
deposition is required to facilitate alloying.
Since the reactions occur on surfaces of heterogeneous
catalysts, knowledge of the surface composition and structure,
preferably at an atomic level, is of vital importance. Three
important surface science techniques used to characterize
Pd–Au alloy surfaces are briefly described below. First, since
Pd and Au have different electronic density of states (DOS)
near the Fermi level and alloying does not eliminate this
difference, scanning tunneling microscopy (STM) can be used
to image a Pd–Au alloy surface which gives a contrast between
surface Pd and Au atoms. Fig. 1 presents an example of this
for a clean AuPd(100) surface.22 In this case, as shown in
Fig. 1(A), some surface atoms appear to be brighter than the
others. Following a simple data treatment to enhance the
contrast (shown in Fig. 1(B)), coupled with techniques capable
of chemical identification, one is able to construct the surface
structure shown in Fig. 1(C) (Pd atoms are displayed as darker
spots) where all Pd ensembles (isolated monomers, isolated
monomer pairs, dimers, trimers, etc.) can be identified.
A second technique that deserves mentioning is low energy
ion scattering spectroscopy (LEISS). In this technique, a beam
of noble gas ions with energy between 0.1 to 10 keV impinges
on a solid surface and are scattered. The energy of outgoing
ions is determined by the laws of energy and momentum
conservation; therefore, surface atoms with different atomic
masses are distinguished. Note that this technique is essentially
exclusively sensitive to the topmost layer of a surface since
more than 99% of the ions penetrating the first layer will be
neutralized and won’t be detected by an ion energy analyzer.
Fig. 2(A) presents a LEISS example of thin Pd–Au alloy films
deposited on Mo(110) in ultrahigh vacuum. In this experi-
ment, 5 monolayers (ML) of Au are first deposited on
Mo(110) followed by 5 ML of Pd. The thin film is then
annealed to various temperatures to allow different levels of
intermixing. A stable Pd–Au alloy, with apparent Au surface
segregation, is achieved between 600 and 1000 K. At higher
temperatures, Au first sublimes followed by Pd.1 Using the
scattering intensities of Pd and Au, corrected with a sensitivity
factor (i.e., the signal intensity ratio of pure Pd and Au), the
surface coverage of Pd and Au for any Pd–Au alloy surface
can be precisely determined. By varying the bulk Pd–Au ratio,
the composition of the stable surface layer varies accordingly
and a phase diagram is thus constructed as shown in Fig. 2(B).
Clearly, strong Au segregation to the surface occurs for all
bulk Pd–Au ratios in UHV. This is rationalized by the fact
that Au has smaller surface free energy than Pd.1 In general,
the surface segregation of an alloy component depends on the
enthalpy of mixing, the atomic sizes of the metals and the
surface free energies (which are proportional to the heats of
sublimation).23 As will be shown below, segregation can also
be adsorption/reaction induced.
Both STM and LEISS, and most other surface-sensitive
techniques require ultrahigh vacuum to operate; therefore,
in situ applications at elevated temperature and pressure are
exceedingly difficult. Fortunately, infrared reflection absorp-
tion spectroscopy (IRAS), coupled with CO titration, can be
used as a powerful tool to probe surface Pd and Au ensembles
under such conditions. Especially, when a photoelastic modu-
lator is added to the IR beam to perform polarization
modulation (PM) to eliminate gas-phase signals, the so-called
PM-IRAS technique is very useful in probing ensembles on
planar Pd–Au alloy surfaces at elevated temperature and
pressures.5,6,40 Fig. 3 presents temperature-dependent
PM-IRAS spectra of 1 � 10�3 Torr (A) and 10 Torr (B) of
CO exposed to an AuPd(100) surface initially enriched with
Au (B90%). CO vibrational features are assigned as follows:
nCO at >2100 cm�1 corresponds to atop CO on Au sites; nCOat 2060–2085 cm�1, to atop CO on isolated Pd sites; and nCObetween 1900 and 2000 cm�1, to bridging CO on contiguous
Pd sites. As is clearly displayed in Fig. 3(A), due to the strong
interaction between Pd and CO, subsurface Pd atoms are
‘‘pulled out’’ to the surface at temperatures higher than
B240 K. However the Pd segregation is insufficient to form
contiguous Pd sites at this CO pressure. Higher CO pressure is
needed to segregate a sufficient amount of Pd to the surface to
Fig. 1 (A) STM image of an AuPd(100) bulk alloy (10 nm � 10 nm, Vs = �15 mV, It = 6.3 nA). The large white features are impurities thought
to be carbon. (B) The same STM image as that in (A) excluding all data points below the cutoff height, which is set to 5 pm below the highest point
of the image. The color bar scale spans from 0 to 5 pm. The red circles denote the features designated to be Pd atoms. These red circles are set to
have an area of B0.15 nm2. (C) Schematic representation of (A) for clarity. Figure adapted with permission from ref. 22. Copyright (2007) by
American Chemical Society.
8012 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012
form contiguous Pd sites (Fig. 3(B)). This chemisorption
induced segregation appears to be easy to understand: as
stated by Haire et al., on thermodynamic grounds, it is to be
expected that the surface composition will adjust so that the
surface becomes enriched in the element which interacts more
strongly with the adsorbate.41 Importantly, it is found from
Fig. 3(B) that at temperatures aboveB475 K, the bridging CO
band disappears while the atop CO band is still present up to
B650 K. It is expected, however, that the binding energy of
bridging CO species is higher than atop CO. This is best
rationalized by the fact that Au starts to diffuse back to the
surface to isolate Pd as the temperature rises. This is because
the sticking probability of CO decreases at high temperatures
and the ‘‘pulling force’’ added to surface Pd therefore weakens.
This simple chemisorption experiment, nevertheless, repre-
sents a good example of dynamic catalyst restructuring at
elevated temperature and pressures. Spectroscopic methods
applicable at such conditions, for example PM-IRAS and sum
frequency generation (SFG), are highly desirable in these cases
to capture the dynamic changes. Ligand effects have also been
found to influence CO adsorption on Pd–Au alloy surfaces. It
has been well-documented that on pure Pd(111), CO occupies
threefold hollow sites at low coverage.42 Recent XPS measure-
ments on an Au/Pd(111) system has revealed that even 10%
surface Au is sufficient to switch off binding at threefold
hollow sites.43 Clearly, 10% surface Au is insufficient to
remove all Pd threefold hollow sites (ensemble effect) but
apparently does sufficiently destabilize them (ligand effect).
This example also demonstrates a synergy between ensembles
and ligand effects in affecting chemisorption.
2.2. Examples of catalytic reactions over planar models
2.2.1. Acetylene trimerization. Acetylene trimerization
(3C2H2 - C6H6) is an interesting model reaction as it occurs
both in UHV on Pd single crystal surfaces and at elevated
pressures on supported Pd catalysts. Surface science studies
have revealed that this reaction proceeds via a C4H4 metalla-
cycle intermediate; the coupling of this intermediate with
another acetylene molecule gives rise to benzene. The rate
limiting step has been found to be benzene desorption that
occurs from two states of adsorbed benzene: tilted and flat-lying.
The former state occurs at much lower temperatures than the
latter; the latter state also accompanies certain levels of product
dissociation.44 The study of acetylene trimerization over Pd–Au
alloys is interesting since (1) this reaction is sensitive to both the
structure and composition of the metal surface, and (2) pure Pd is
very active, however, pure Au is totally inert.17
Lambert and co-workers systematically studied acetylene
trimerization over Pd–Au thin films formed on Au(111)18 and
Pd(111).19 Due presumably to the mobility difference of Au in
Fig. 2 (A) LEISS spectra of 5 ML Pd/5 ML Au/Mo(110) as a
function of annealing temperature. LEISS spectra were collected at
300 K after the sample was annealed to the specified temperature. (B)
Surface concentration of various Pd–Au alloys on Mo(110) measured
by LEISS compared to the corresponding bulk concentration. The
sample was annealed at 800 K for 20 min. Figure adapted with permis-
sion from ref. 1. Copyright (2005) by American Chemical Society.
Fig. 3 (A) Temperature-dependent PM-IRAS spectra of 1 �10�3 Torr of CO on an AuPd(100) surface well-annealed at 800 K for
30 min. (B) Temperature-dependent PM-IRAS spectra of 10 Torr of
CO. Figure adapted with permission from ref. 5. Copyright (2009) by
American Chemical Society.
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8013
Pd and Pd in Au, a (O3 � O3)R301 (composition Pd2Au)
surface alloy forms following Pd deposition onto Au(111) and
annealing.18 However, after Au deposition onto Pd(111), Pd
and Au distribute quasi-randomly.19 In this case, higher
annealing temperatures induce more Au diffusion into the
Pd bulk; therefore, simply by varying the annealing tempera-
tures, surface alloys with different Pd–Au ratios are
formed.19,33 In terms of reactivity, benzene forms and desorbs
at much lower temperatures over the alloy surfaces than
Pd(111) or pure Pd overlayers on Au(111). Lambert and
co-workers gave two reasons why the alloy is more effective
than pure Pd. First, the degree of rehybridization of the
adsorbed acetylene decreases on the alloy surface. This results
in more efficient conversion of acetylene to benzene than
acetylene decomposition. Second, the strength of adsorption
of the benzene product is also decreased on the alloy surface.
Again, the weaker interaction with the alloy surface increases
the probability of product desorption without decomposition.18
Both ensemble and ligand effects seem to give satisfactory
qualitative explanations for the reduced binding of both
the reactant and product with the alloy surfaces. On the
Au/Pd(111) alloy surfaces, an AuPd6 ensemble was found to
give rise to the highest benzene yield. This suggests that a
7-atom ensemble is needed for benzene formation.19 However
the high activity of AuPd6 is obtained under highly idealized
conditions during temperature-programmed desorption
following adsorption of a saturation coverage of acetylene in
UHV. This does not necessarily mean that an AuPd6 ensemble
is highly active, or even needed under other reaction condi-
tions. The (O3 � O3)R301 (composition Pd2Au) surface alloy
formed on Au(111) is also very active yet AuPd6 ensembles
should not be present at high concentrations on this surface.
Nevertheless, the planar model catalysts can provide impor-
tant information for the acetylene trimerization reaction over
supported Pd–Au catalysts under realistic conditions. For
example, supported Pd–Au alloy catalysts show higher activity
and selectivity than supported Pd-shell–Au-core bimetallic
catalysts,17 fully consistent with the surface science studies.
2.2.2. Vinyl acetate synthesis. Acetoxylation of ethylene on
silica-supported bimetallic Pd–Au catalysts promoted with
potassium acetate is a well-established commercial route for
synthesis of vinyl acetate (VA), given by the following reac-
tion: CH3COOH+ C2H4 + 0.5O2 - CH3COOCHQCH2 +
H2O.4,35,45,46 Compared with pure Pd, the addition of Au has
been shown to significantly improve reaction rates whilst also
moderately improving the reaction selectivity.46 The side
reactions are combustion of the reactants acetic acid and
ethylene, and the product VA; ethylene combustion has been
found to be the dominant one under typical reaction condi-
tions.4 Using temperature-programmed desorption, the much
weaker binding of ethylene with Pd–Au alloys (as compared to
pure Pd) was confirmed.45 Since ethylene combustion must
involve C–C and C–H cleavage of chemisorbed ethylene, the
fact that ethylene binds much weaker on Pd–Au alloys makes
it easy to understand why VA selectivity is higher on a Pd–Au
alloy catalyst. Using silica-supported Pd (1.0 wt%) and Pd–Au
alloy (1.0 wt% Pd and 0.4 wt% Au) catalysts at reaction
conditions mimicking those used in the industrial process, VA
formation rates were found to be more than 10 times faster on
Pd–Au catalysts, consistent with findings for the industrial
process.46 This promotion was initially assigned to a ligand
effect where the weaker binding between reactants with the
alloy surfaces (for example weakly bound monodentate acetate
rather than strongly bound bidentate acetate, weakly bound
p-ethylene rather than di-s-ethylene) is expected to yield faster
coupling; and the weaker binding between the product VA
with the alloy surfaces facilitates its desorption.46
While it is clear that a ligand effect must play a role, studies
by Chen et al. on Pd/Au(100) and Pd/Au(111) model catalysts
revealed that ensemble effects play a more significant role in
affecting VA formation rates.35 Since VA formation requires
coupling between chemisorbed acetate and ethylene, a corre-
lated pair of Pd sites is needed. Considering the bond lengths
of adsorbed ethylene and acetate species, the optimized dis-
tance between two active sites is 3.3 A. Separation between
pairs of Pd monomers on Au(100) will be 4.08 A, while on
Au(111) this distance will be 4.99 A, prohibitively long for
coupling of these two reactive intermediates. The reaction
rate on Pd/Au(111) is indeed much lower than that on Pd/
Au(100) as evidenced in Fig. 4(A). The bonding and relative
distances involved between reacting species are shown
schematically in Fig. 4(B). The pair of isolated Pd sites, while
aiding in the formation of VA by providing the optimum
required spacing for coupling of the surface acetate and
ethylene species, was also proposed to suppress the formation
of reaction by-products, such as CO and CO2, thus improving
the overall selectivity.45
2.2.3. CO oxidation. Both planar models and high surface
area supported Pd are excellent catalysts for CO oxidation
(CO + 0.5O2 - CO2). Similarity in rates on both these
catalyst systems is generally regarded as strong evidence for
the structure insensitivity of this reaction over late transition
metals.26 The situation is more complex for Au. While Au
nanoparticles supported on certain oxides (especially the
reducible ones, e.g., TiO2) are specifically active for low-
temperature CO oxidation, bulk gold is inert. The reason
appears easy to understand: as a Langmuir–Hinshelwood type
of reaction, chemisorbed CO must react with a chemisorbed
active oxygen species to form CO2. In most cases this active
oxygen is atomic oxygen. The inertness of bulk gold is due to
its inability to activate di-oxygen. Indeed, if atomic oxygen is
pre-adsorbed onto planar Au, low-temperature CO oxidation
does occur facilely.47 It is therefore quite interesting to
investigate CO oxidation over bulk Pd–Au alloys. Presumably
by adding Pd (which is capable of activating di-oxygen) to Au,
one is able to form an active catalyst.
This hypothesis was tested on AuPd(100),5,6 Pd–Au alloy
thin films, and supported Pd–Au particles.40 Fig. 5(A) shows
reaction data at 10�7 Torr of CO pressure over planar Pd and
Pd–Au nanoparticles grown on a thin TiO2 layer deposited on
Mo(110).40 Clearly, while Pd alone is active, adding Au into
Pd greatly inhibits CO oxidation in vacuum. When the Au–Pd
ratio reaches 1, the alloy is totally inert. An AuPd(100) single
crystal also shows no activity under identical conditions.5,6
However, when kinetic measurements were carried out at
elevated CO pressures, the reaction data shown in Fig. 5(B)
8014 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012
reveal that AuPd(100) becomes orders of magnitude more
active than pure Pd at temperatures below B500 K. The
reaction kinetics shown in Fig. 5 raise three questions: (1)
what causes CO oxidation reaction to ‘‘turn on’’ at elevated
CO pressures on Pd–Au? (2) why is the alloy surface much
more active than pure Pd at elevated CO pressures and
relatively low temperatures? (3) why does CO2 formation ‘‘roll
over’’ at temperatures higher than B450 K over AuPd(100)?
The answer to the first question is twofold. (i) O2 does not
dissociate on isolated Pd sites. This is easily proven by an O2
temperature-programmed desorption experiment: on a Pd–Au
alloy surface with only isolated Pd surface sites, O2 desorption
due to recombination of chemisorbed atomic oxygen does not
occur suggesting that O2 does not dissociate during adsorption
to this surface.48 For the Pd–Au alloy model catalysts used to
acquire reaction data shown in Fig. 5(A), the lack of O2
dissociation precludes the subsequent CO2 formation reaction.
(ii) At near-atmospheric CO pressures, a sufficient amount of
Pd segregates to the surface and generates contiguous Pd sites
which, in contrast to isolated Pd sites, are capable of dissociat-
ing O2 to allow the CO2 formation reaction to proceed
(Fig. 5(B)). The driving force for this ‘‘chemisorption induced
segregation’’ is the stronger binding of CO with Pd than Au.
This phenomenon has been shown clearly by the PM-IRAS
data displayed in Fig. 3.5,6
Fig. 4 (A) Vinyl acetate (VA) formation rates (turnover frequencies,
TOFs) as a function of Pd coverage on Au(100) and Au(111). The VA
synthesis was carried out at 453 K, with acetic acid, ethylene, and O2
pressures of 4, 8, and 2 Torr, respectively. The total reaction time was
3 hours. The error bars are standard deviations, based on background
rate data. The two insets show Pd monomers and monomer pairs on
the Au(100) and Au(111) surfaces. (B) Schematic for VA synthesis
from acetic acid and ethylene. The optimized distance between the two
active centers for the coupling of surface ethylenic and acetate species
to form VA is estimated to be 3.3 A. With lateral displacement,
coupling of an ethylenic and acetate species on a Pd monomer pair
is possible on Au(100) but implausible on Au(111). Figure adapted
with permission from ref. 35. Copyright (2005) by the American
Association for the Advancement of Science.
Fig. 5 (A) CO conversion as a function of reaction temperature over
TiO2/Mo(110)-supported Pd and Pd–Au alloy particles. Reaction was
carried out at steady-state using a stoichiometric CO–O2 mixture at
PCO = 1 � 10�7 Torr. Note that kinetic data with different surfaces
are shown with different symbols. Figure adapted with permission
from ref. 40. Copyright (2010) by American Chemical Society. (B)
Arrhenius plots of the CO2 formation rate (in TOF) over AuPd(100) and
Pd(110) with 16 Torr CO and 8 Torr O2. Figure adapted with permission
from ref. 6. Copyright (2009) by American Chemical Society.
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8015
Some knowledge of the CO oxidation reaction mechanism
over Pt-group metals at near-atmospheric pressures is needed
to answer the second question. Briefly, for the majority of late
transition metals at relatively low temperatures, the metal
surfaces are covered with a full layer of chemisorbed CO
under typical reaction conditions. As such, the rate-limiting
step is CO desorption which creates empty surface sites for O2
chemisorption and dissociation. This reaction mechanism is
fully supported by (1) reaction rates displaying +1 order
dependence in O2 pressures and �1 order dependence in CO
pressures; and (2) the measured reaction activation energies
close to the CO desorption activation energies.25,26 The
reaction kinetic data shown in Fig. 5(B) indicate much weaker
binding energies for CO on a Pd–Au alloy catalyst as
compared to pure Pd. This is indeed the case proved by
calculating CO binding energies on Pd and Au sites on
AuPd(100) using the Clausius–Clapeyron relation. At zero
CO coverage, the CO heat of adsorption was found to be
69 kJ mol�1 on Au sites and 84 kJ mol�1 on Pd sites. The CO
heat of adsorption on Au single crystal surfaces is typically
very low, i.e., B50 kJ mol�1 on step sites and less than
40 kJ mol�1 on terraces. In contrast, the CO binding energy
on pure Pd is as high as 150 kJ mol�1.6 These CO binding
energies not only are consistent with reaction kinetics shown in
Fig. 5(B), but also provide strong evidence for charge transfer
from Pd to Au. Specifically, the CO binding energy of
69 kJ mol�1 on Au sites on the AuPd(100) surface is substan-
tially larger than CO binding energies on any pure Au
surfaces. This is best explained as a ligand effect where charge
transfer from Pd to Au enhances back-donation of electrons
from Au to CO, thus increasing CO binding energies.
Finally, we note that the surface composition of AuPd(100)
surfaces vary dynamically under reaction conditions where
higher temperatures cause more Au segregation to the surface
(Fig. 3). At the same time, the low binding energies of CO on
the alloy surfaces cause decreased CO residence times on the
surface at high temperatures. The combination of these two
factors explains the ‘‘rollover’’ of CO2 formation over
AuPd(100) at elevated temperatures.
3. (Supported) Nanoparticle catalysts
3.1. Synthesis
Supported Pd–Au catalysts can be easily synthesized using
traditional impregnation or deposition–precipitation of Pd
and Au salts, either concurrently or sequentially, followed
by calcination and reduction.4,7–11,13,49,50 The major drawback
is the lack of homogeneity of the formed catalysts in terms of
particle size, composition and shape. For example, it is not
uncommon to find Au alone particles, Pd alone particles and
Pd–Au alloy particles with different compositions on the same
support.7–11,13 Such catalysts may be useful for various appli-
cations but these are not ideal for fundamental catalytic
chemistry and catalyst design type of studies. In the latter
cases, metal nanoparticles with uniform composition and
monodispersion are highly desirable.51 Fortunately, during
the past two decades or so, much progress has been made in
nanomaterial synthesis such that the formation of size, shape
and orientation specific monometallic nanoparticles for use in
catalysis has become rather mature.51 The most commonly
used method is a colloid technique where nanoparticles are
synthesized by the reduction of a metal precursor with a
reducing agent in the presence of a protective agent to prevent
aggregation. Unlike the monometallic case, the formation of
bimetallic nanoparticles is more complex. By varying the
formation methods, either core–shell bimetallics or quasi-
homogeneous alloys are formed. Different techniques have
been developed in the past for bimetallic nanoparticle synth-
esis including alcohol reduction, citrate reduction, polyol
process, solvent extraction reduction, sonochemical method,
photolytic reduction, decomposition of organometallic pre-
cursors, and electrolysis of bulk metal.52 These nanoparticles
can be used directly as catalysts for certain reactions on
condition that the capping layer is porous and stable; or they
can be grafted onto high-surface-area supports, followed by a
capping layer removal step, as ‘‘regular’’ heterogeneous cata-
lysts. Two points are addressed for this process: (1) colloid
Pd–Au bimetallic nanoparticles, as these are usually synthe-
sized from Pd and Au chloride precursors, are negatively
charged due to residual Cl�. Adjusting the solution pH
to make the support surfaces positively charged helps the
grafting process.12 (2) Removal of the capping layer while
maintaining the nanoparticle dispersion, composition and size
is a great challenge. Decomposing the capping layer at the
lowest possible temperature appears to greatly inhibit sintering.53
In the following, examples are given on size and structure
control of Pd–Au nanoparticles.
Generally, for the most commonly used colloid techniques,
particle sizes are affected by multiple factors including type
and concentration of the metal precursors, reducing agents
and protective agents (soluble polymers, surfactants, and
organic ligands), formation temperature, etc. Two interesting
size control methods are briefly introduced: (1) Pd–Au can be
synthesized as dendrimer-encapsulated nanoparticles (DENs).
In this method, Pd and Au ions are first extracted from
solution into the dendrimer interior via complexation with
internal tertiary amines. Second, the metal ions are reduced
with BH4�, and the resultant atoms subsequently coalesce to
form zero-valent nanoparticles within the dendrimer templates.
In this case, the dendrimer framework not only controls the
sizes of the nanoparticles but also stabilizes them.28,54 (2)
Another interesting size managing approach is to synthesize
Pd–Au particles in reverse micelles. First, nanosized water
droplets are dispersed in a continuous oil phase. Second, metal
precursors and reductants are introduced into these water
droplets to react. The sizes of the alloy particles formed are
confined by the sizes of the water droplets.52
As to composition/structure control, the two approaches
typically used are simultaneous or sequential reduction of
appropriate precursors. In the former, weaker reducing agents,
for example polyol, lead to the formation of Au-core–Pd-shell
structures. This is because Au reduces more easily and provides
a seed for the reduction of the Pd shell. The size and thickness
of the core–shell can be controlled by the ratio of Pd–Au in the
precursor solutions. Strong reducing agents (e.g., BH4�) result
in the formation of a quasi-random distribution of Pd–Au
alloys. In sequential reduction, a monometallic core is synthesized
8016 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012
first and in the second step, the second metal is reduced onto
the core surface. This method is used to generate Au-core–
Pd-shell and Pd-core–Au-shell structures where the former are
more common. However, if synthesis is carried out at higher
temperatures, the enhanced mobility of atoms causes alloy
formation instead.54 Note that Au-core–Pd-shell structures are
also most often formed for supported Pd–Au bimetallics.
Besides the easier formation of Au0 as nucleation seeds, the
calcination–reduction treatments, often used during the synth-
esis of such catalysts, also promote formation of a Pd-shell.55
This is due to Pd diffusion to the surface during calcination
since it is more readily oxidized than Au. Upon reduction, Pd
can still remain enriched in the shell. It is to be noted, however,
that any stable Au-core–Pd-shell structure should be consid-
ered as kinetically stabilized because of (1) the lower surface
free energy of Au and (2) the complete miscibility of the two
metals.1
3.2. Characterization
Numerous techniques are used to characterize supported
Pd–Au bimetallic catalysts. General characterization methods,
e.g., specific surface area, pore size and distribution, metal
loading and dispersion, etc., are not discussed here. Instead,
some of those used to acquire detailed structural information
are briefly described in the following.
3.2.1 X-Ray diffraction (XRD). XRD is a routinely avail-
able technique that can be used to study Pd–Au alloys. As fcc
metals, Pd and Au have strong diffractions along the (111) and
(200) directions between 2y of 30 to 501. The diffractions of
PdxAuy alloy phases fall in between the corresponding diffrac-
tions of the pure metals. This offers straightforward identifi-
cation of alloy formation. Quantitative analysis of the XRD
patterns allows the composition of the alloy phases to be
determined. This can be done using Vegard’s law,13,15 or more
accurately, the Rietveld refinement method.7 Fig. 6 presents
XRD patterns of SiO2 supported pure Pd, Au and bimetallic
Pd–Au catalysts at various Pd–Au ratios prepared by impreg-
nation of the silica support with an alcohol solution of
colloidal dispersion of the two metal particles, followed by
air calcination.13 Several important points are worth mention-
ing: (1) the heterogeneity of the nanoparticles formed during
Pd–Au bimetallic synthesis including pure Pd phases (oxidized
to PdO during calcination), pure Au phases and alloy phases.
This is a good example demonstrating that the formation of
uniform alloy nanoparticles remains a major challenge for
supported catalysts. (2) The PdxAuy alloy phases are more
resistant to oxidation than pure Pd. This is evidenced by the
fact that during calcination in air, pure Pd oxidizes to PdO
while the alloy phases maintain metallic. This indicates that,
for catalytic reactions in heavily oxidizing environments,
Pd–Au alloys might be a good option for preventing catalyst
deactivation due to Pd oxidation. (3) The precursor Pd–Au
ratio clearly has profound effects on alloy formation where
substantially more alloy forms for Au75Pd25 compared to
Au25Pd75 precursor weight ratios.
3.2.2 X-Ray photoelectron spectroscopy (XPS). The surface-
sensitive nature of XPS allows for determination of the
near-surface composition of supported Pd–Au bimetallic
catalysts.15,7–13,49 This includes (1) near-surface Pd–Au ratios
and (2) oxidation states of Pd and Au. Also XPS analysis of
catalysts before and after catalytic reactions often yields rich
indirect information regarding changes of the catalysts during
reactions. Note that in ratioing Pd–Au XPS signals (Pd 3d3/2and Au 4f7/2 core-level features are generally used), atomic
sensitivity factors of both elements must be included.7 For
oxide supported catalysts, charging is always a significant issue
affecting accurate oxidation state determination. To circum-
vent this problem, generally binding energies (BE) are cali-
brated using internal standards (e.g., adventitious carbon C 1s
at B285.0 eV).7
In principle, the core-level BE change (as compared to BE of
pure metals) can be used to verify alloy formation since, as
discussed earlier, charge transfer does occur upon Pd–Au alloy
formation. However, for metal particles in the nano size
ranges, surface metal atoms are substantially more under-
coordinated than bulk atoms. In this case, final state effects
(e.g. screening) can be more pronounced than initial state
effects in determining binding energies in core level spectro-
scopy. Note that even when valence band spectroscopy fails to
reveal any charge transfer between the alloy components, one
can see changes in the core level BE.3 Therefore one must be
extremely careful in interpreting Pd–Au alloy formation and
charge transfer using core level XPS analysis. Finally, we note
that traditional XPS is measured at pressures lower than
B10�7 Torr; therefore, in situ applications at elevated pressures
are not possible. In recent years an ambient pressure XPS
technique (AP-XPS) has been developed. Although the highest
pressure allowable at present is only B1 Torr, this technique
has already shown the ability to monitor in situ dynamic
changes of Pd–Au alloy catalysts during CO oxidation.56
3.2.3 X-Ray absorption spectroscopy (XAS). XAS requires
synchrotron X-ray beamlines; therefore, widespread applica-
tion of the technique is limited. Still, the technique offers
unparalleled advantages for in situ applications under realistic
high temperature and pressure reaction conditions due to the
high energy and flux of synchrotron beams. XAS has two
Fig. 6 X-Ray diffractograms of monometallic and bimetallic catalysts
after calcination at 673 K. The numbers in the sample notation refer to
the relative weight percentages of the metals. Figure adapted with
permission from ref. 13. Copyright (2003) by Elsevier.
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8017
major uses: X-ray absorption near edge structure (XANES) is
used to determine local coordination geometries and metal
oxidation states; and extended X-ray absorption fine structure
(EXAFS) is used to identify neighboring atoms, interatomic
distances, and coordination numbers.
Lambert and co-workers17 were among the first to use EXAFS
and XANES to study colloidal Pd–Au nanoparticles. More work
has been done more recently by other researchers.53,54,57,58
Perhaps the best use of EXAFS in Pd–Au nanoparticles
characterization is the measurement of partial nearest-neighbor
coordination numbers (CN) nPdPd, nPdAu, nAuPd and nAuAu
obtained concurrently for data collected at Pd and Au
absorbing atom edges. The total CN for Pd is given by nPdM =
nPdPd + nPdAu, whereas the total CN for Au is nAuM =
nAuPd + nAuAu. These values are important in distinguishing
core–shell or random alloy structures. For example, if Pd
segregates to the surface while Au stays in the core of the
nanoparticles, nPdM will be smaller than nAuM. This is because
atoms at the surface have fewer neighbors than those in the
core. On the other hand, if nPdM is very close to nAuM, a quasi-
random alloy structure is likely. One might expect that
EXAFS could be a powerful technique for in situ monitoring
of dynamic composition changes of Pd–Au particles by mea-
suring the CNs described above, although not much work has
been done so far in this regard. Note also that the total
coordination numbers also provide a good estimate of particle
sizes. The other significant use of EXAFS is the determination
of Pd–Pd, Pd–Au and Au–Au bond lengths. This also yields
important information regarding the short-range environ-
ments around Pd and Au atoms. Pd–Pd bond lengths for bulk
Pd are 2.743 A and Au–Au bond lengths for bulk Au are 2.861 A.
For monometallic Pd and Au nanoparticles, these bond
lengths are slightly smaller. In Pd–Au alloys, the Pd–Au bond
lengths are rather close to Au–Au bond lengths and elongation
of Pd–Pd bond lengths also occurs. As has been discussed
earlier, this enhances the atomic-like character of Pd atoms and
weakens binding with reactants. The situation is more complex
for core–shell structures. For Pd-core–Au-shell structures,
Pd–Pd bond lengths stay rather close to pure Pd53 while for
Au-core–Pd-shell structures, the Pd shell adopts the bond
length of the Au core up to a shell thickness of B1 nm.14,54
Summarily, Pd–Pd bond lengths are more informative than
Pd–Au and Au–Au bond lengths for structure determinations.
Generally speaking, Pd–Pd bond lengths close to pure Pd
suggest Pd core structures or a high percentage of Pd-only
nanoparticles, whereas Pd–Pd bond lengths close to pure Au
suggest alloy or Pd-shell bimetallic structures.
In XANES studies, even simple comparisons of the spectra
for the nanoparticles with those for pure Pd and Au can yield
useful qualitative information regarding alloying and charge
transfer. For example, normalized derivatives of Pd K-edge
XANES spectra give rise to a broad maximum at the edge
energy, E0, attributed to weakly allowed 1s–4d transitions that
also reflect the density of unoccupied states in the Pd d band.
Comparing the spectra between pure Pd and Pd–Au colloid
samples led Lambert and co-workers to conclude that Pd
atoms go from a Pd-like environment in the core–shell struc-
ture to an alloy phase upon annealing.17 Scott and co-workers
compared XANES spectra at the Au LIII-edge between pure
Au and Pd–Au nanoparticles and observed a reduction of the
white line (the first feature after the edge jump) with increasing
Pd content. Since the intensity of the white line depends on the
number of d-holes in the Au atoms, this finding can be
rationalized by the enhanced filling of the Au d-band in
the PdAu alloys via charge transfer from the Pd s-band
(and perhaps p-band) to the Au d-band.53
3.2.4 Transmission electron microscopy (TEM). TEM is a
powerful and mature technique for studying bare and sup-
ported metallic nanoparticles. The most common low magni-
fication mode (bright field) is used to sample many particles
simultaneously to obtain particle size distributions. In the high
magnification mode (i.e., high-resolution TEM, HRTEM), single
particles can be analyzed at an atomic level to obtain information
such as faceting and lattice spacing of the nanoparticles. Com-
monly, TEM instruments are equipped with an energy dispersive
spectrometer (EDS) detector for qualitative elemental analysis.
TEM has been used extensively for studies of Pd–Au nano-
particles.4,7–11,14,15,46,53–56,60 Two examples are given below.
Fig. 7 presents scanning TEM-EDS of large particles of
Au–Pd (2.5 wt% Au–2.5 wt% Pd) catalysts supported on
carbon, TiO2 and Al2O3 calcined at 400 1C.10,55 Interestingly
for Au–Pd/C, Au and Pd maps cover identical areas and the
RGB reconstructed map is homogeneous indicating formation
of a homogeneous Pd–Au alloy. In contrast, for Au–Pd/TiO2
and Au–Pd/Al2O3, Pd maps occupy larger areas than Au maps
and the RGB maps are apparently non-uniform indicating
formation of Pd-shell–Au-core structures. In another study,
Ferrer et al. used energy-filtered TEM and scanning TEM-EDS
line-scanning techniques to image three-layer core–shell struc-
tures in Pd–Au nanoparticles.59 It is emphasized that such
structural details cannot be obtained using the other techni-
ques described above.
Other commonly used techniques to characterize Pd–Au
nanoparticles are UV-Vis spectroscopy12,14,28,52,54,59 and
Fig. 7 Montage of high-angle dark-field (HAADF) imaging (column 1),
Au map (column 2), Pd map (column 3) and RGB reconstructed overlap
map (column 4) [Au – blue: Pd – green] for calcined AuPd/C (row 1),
calcined AuPd/TiO2 (row 2) and calcined AuPd/Al2O3 (row 3). Note that
the calcined AuPd particles on TiO2 and Al2O3 supports show a Au rich-
core–Pd-rich shell morphology, whereas calcined AuPd particles on
activated C are homogeneous alloys. Figure adapted with permission
from ref. 10. Copyright (2008) by the Royal Society of Chemistry.
8018 Chem. Soc. Rev., 2012, 41, 8009–8020 This journal is c The Royal Society of Chemistry 2012
FTIR (especially DRIFTS).13,16,46,49 These, however, are not
covered in this article.
3.3 Examples of catalytic reactions
3.3.1. Direct H2O2 synthesis from H2 and O2. Hydrogen
peroxide is an important green oxidant that is used in many
large scale processes such as bleaching and as a disinfectant.
Current industrial synthesis of H2O2 utilizes a sequential
hydrogenation–oxidation route of an alkyl anthraquinone.
The process is only economical at large scale and at high
product concentrations. However when used, H2O2 is often
required on a much smaller scale and at lower concentrations.
Direct small scale production of H2O2 at the site where it is
used is highly desirable in these areas. Due to its superior
activity in hydrogenation, Pd was the catalyst of choice for
many initial investigations. In recent years, Hutchings and
co-workers have made major contributions in this area by
discovering that supported Pd–Au catalysts have higher
activity and selectivity for this reaction.8–11,55,60 In the follow-
ing the atomic origin of this promoting effect is addressed.
First of all, one has to realize that direct H2O2 formation
from H2 and O2 is not an oxidation reaction, but rather a
reduction reaction. This is understood by the fact that an H–H
bond breaks while an O–O bond is maintained during the
reaction. The fundamental difficulty of the direct route is that
H2O2 is unstable with respect to both hydrogenation and
decomposition while the non-selective combustion product,
H2O, is thermodynamically much more stable. This complexity
is shown schematically in Fig. 8.55 Clearly, switching off the
combustion path (that is, preventing O–O bond cleavage) is of
vital importance in enhancing H2O2 selectivity. As is well-
known, Pd is an excellent hydrogenation catalyst; unfortu-
nately for this reaction, it is also an excellent oxidation
catalyst. As such, a Pd-alone catalyst cannot achieve very
high H2O2 selectivity. The situation is drastically different for
Pd–Au catalysts. As has been discussed in detail in Section 2,
adding Au to Pd dilutes surface Pd concentrations such that
contiguous Pd sites disappear and only isolated Pd sites exist
at sufficiently high Au coverage. Significantly, isolated Pd sites
do not catalyze O2 dissociation.5,6,48 On the other hand, the
structure insensitivity for hydrogenation reactions over
Pd-based catalysts indicates that isolated Pd sites are still able
to activate H2.2,3 Therefore, by taking advantage of this
ensemble effect, one is able to tune the catalytic properties of
Pd–Au catalysts to dramatically enhance H2O2 selectivity. The
reaction results obtained by the Hutchings group are fully
consistent with this ensemble effect argument. For Pd–Au
catalysts supported on Al2O3, TiO2 and carbon, the Pd–Au/C
catalyst shows much higher selectivity than the other two. This
is because homogeneous alloys form on C while Pd-shell–
Au-core bimetallics form on Al2O3 and TiO2 (Fig. 7).10,55
Clearly, on the alloy surface Pd atoms are better isolated than
those on Pd-shell–Au-core surfaces. By using acids to treat the
Pd–Au/C catalysts, these authors found enhanced gold disper-
sion by generating smaller Pd–Au nanoparticles. Again, better
Pd isolation is fully expected upon enhanced Au dispersion.
Indeed, side reactions can be almost completely switched off
following acid treatments.60
3.3.2. Hydrodesulfurization (HDS) reaction. Noble metal
catalysts are widely used in hydrodesulfurization of petroleum
feed stocks. Pd is a good HDS catalyst although it tends to be
poisoned by the sulfur present in the feed. Venezia et al.
studied HDS of a model compound, dibenzothiophene
(DBT), over Pd/SiO2 and AuPd/SiO2 catalysts.13 The reaction
results shown in Fig. 9(A) clearly demonstrate the beneficial
effects of alloying, with Pd–Au bimetallics showing higher
Fig. 8 Schematic of the reactions and the corresponding reaction
heats during the direct synthesis of H2O2. Note that H2O2 is unstable
with respect to both hydrogenation and decomposition, and the non-
selective combustion of hydrogen is a facile competing reaction.
Figure adapted with permission from ref. 55. Copyright (2008) by
the Royal Society of Chemistry.
Fig. 9 (A) Total DBT conversion obtained at 593 K as a function of
gold content. (B) X-ray diffractograms of monometallic Pd and
bimetallic Au50Pd50 catalyst: (a) after calcination at 673 K; and (b)
after HDS of thiophene. Figure adapted with permission from ref. 13.
Copyright (2003) by Elsevier.
This journal is c The Royal Society of Chemistry 2012 Chem. Soc. Rev., 2012, 41, 8009–8020 8019
activity than either Pd or Au alone. Fig. 9(B) displays XRD
patterns of the catalysts after HDS. For Pd/SiO2, the metallic
phase converts to Pd4S after reaction while for Au50Pd50/SiO2,
no sulfide formation was found after reaction. The resistance
to sulfur poisoning of the alloy phase, therefore, nicely
explains the activity data. These results can again be under-
stood via an ensemble effect. First, rather large contiguous Pd
ensembles are required to form Pd4S which are lacking on the
alloy surface. Second, sulfur-containing substrate molecules
selectively adsorb on gold sites due to the strong affinity of
gold for sulfur, leaving surface Pd sites available for activating
H2 and facilitating the HDS reaction.13
Summary
Pd–Au bimetallic catalysts are technically important for vinyl
acetate synthesis. They also have great potential of being used
as catalysts for other industrial processes, including direct
H2O2 synthesis. From a basic catalytic science point of view,
the complete miscibility, small lattice mismatch, as well as
vastly different catalytic properties of Pd and Au make Pd–Au
a unique system for study. Both ensemble and ligand alloy
effects are used to describe the catalytic modification of Pd by
Au. To study these effects at an atomic scale, surface scientists
use single crystals, thin films and planar clusters as model
catalysts to study Pd–Au interactions, chemisorption and
catalytic reactions at well-defined conditions. Concurrently,
researchers in heterogeneous catalysis and materials sciences
use bare and supported Pd–Au nanoparticles to study their
catalytic properties under technically relevant conditions.
Overall, an ensemble effect is shown to be responsible for
the generation of specific surface sites that are highly active for
certain reactions. Also modification of surface ensembles
switches off side reactions and, therefore, enhances reaction
selectivities. A ligand effect is responsible for changing the d
character of Pd. This causes weaker interaction between
surface Pd and reactants/products. For certain reactions, this
also enhances activity and selectivity.
Acknowledgements
F.G. and D.W.G. gratefully acknowledge the support for this
work by the US Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences, and
Biosciences, and the Robert A. Welch foundation (A-300).
F.G. also thanks Dr C.H.F. Peden (PNNL) for fruitful
suggestions. The Pacific Northwest National Laboratory is
operated by Battelle for the U.S. Department of Energy.
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