γ-Al2O3 catalysts: Effect of composition of metallic particles

Journal of Catalysis 299 (2013) 171–180

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Vapour phase formic acid decomposition over PdAu/c-Al2O3 catalysts: Effect of composition of metallic particles Dmitri A. Bulushev a,b,⇑, Sergey Beloshapkin b, Pavel E. Plyusnin c, Yurii V. Shubin c, Valerii I. Bukhtiyarov d, Sergey V. Korenev c, Julian R.H. Ross a a

Chemical & Environmental Sciences Department, University of Limerick, Limerick, Ireland Materials & Surface Science Institute, University of Limerick, Limerick, Ireland Nikolaev Institute of Inorganic Chemistry, SB RAN, Novosibirsk 630090, Russia d Boreskov Institute of Catalysis, SB RAN, Novosibirsk 630090, Russia b c

a r t i c l e

i n f o

Article history: Received 7 September 2012 Revised 27 November 2012 Accepted 9 December 2012

Keywords: Formic acid Hydrogen Alloys Fuel cells Core–shell structure Inhibition by CO

a b s t r a c t A series of 1.8 wt.% PdAu/c-Al2O3 catalysts with a mean metal particle size of more than 10 nm and different Pd/Au ratios were prepared by impregnation followed by hydrazine reduction and tested in the decomposition of vapour phase formic acid (2.2 vol.%, 1 bar) with and without added CO to the gas mixture. The catalytic activity was correlated with the bulk (AAS, XRD, EDS/TEM) and surface (XPS) composition of PdAu particles. A novel result was obtained that the surface composition of the particles did not change despite a considerable variation of the Au atomic fraction and it was found to be close to that of the Pd2Au alloy. The activity of the Au/Al2O3 catalyst was negligible compared with that of the Pd/Al2O3 catalyst being a factor of 65 lower per gram of metal at 403 K. The surface electronic properties (XPS) and the catalytic activity of the Pd2Au layer calculated per one surface Pd atom (TOF) did not depend on the bulk composition of the metal particles. A lower activity of the Pd2Au layer as compared with the pure Pd layer (2–3 times) was observed, but the activation energies for the Pd and PdAu catalysts were similar (40 kJ mol1) and were lower than that for the Au/Al2O3 catalyst (62 kJ mol1). Addition of CO led to a considerable inhibition of the reaction for all PdAu catalysts, this being accompanied by an increase in the activation energy to 69 kJ mol1. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction It is well known that Au easily forms alloys with Pd at all Au–Pd compositions [1]. The formation of these alloys in bulk metals as well as in ultraclean monocrystals and unsupported and supported nanoparticles has been widely studied. Less is known about the surface composition of the alloy nanoparticles. The catalytic properties have often been correlated with the composition of the bulk alloys but not with the surface composition, in spite of the fact that the surface is the place where the catalytic reaction takes place. Some 40–50 years ago, Eley and Luetic [2] as well as Clarke and Rafter [3] studied formic acid decomposition over bulk PdAu catalysts in the form of either wires or films. However, the surface composition of these catalysts was not determined since the methods for surface analysis had not been developed at that time. During last few years, there has been a renewal of interest in formic acid decomposition over different catalysts because of the development of formic acid fuel cells [4] and because formic acid is a by-product ⇑ Corresponding author at: Chemical & Environmental Sciences Department, University of Limerick, Limerick, Ireland. Fax: +353 61202568. E-mail address: [email protected] (D.A. Bulushev). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.12.009

of biomass hydrolysis to produce levulinic acid [5,6]. The formic acid formed can be used for storage and production of hydrogen [7] or as a hydrogen donor in place of molecular hydrogen in some reactions [5,8]. The production of CO-free hydrogen is of great importance [9]. As compared to the pure metals, PdAu alloys often have improved catalytic properties (activity, selectivity, and stability) in a variety of different reactions: hydrogenation [10–12], oxidation [13–20], trimerization of acetylene to benzene [21], synthesis of vinylacetate from acetic acid and ethylene [22,23], hydrodechlorination [24,25] and synthesis of hydrogen peroxide from hydrogen and oxygen [26–28]. However, for some reactions such as N2O decomposition, alloying of Pd with Au may also decrease the activity [29]. Goodman and co-authors [22,23] found strongly increased turnover-frequencies of the synthesis of vinylacetate at low Pd coverages of gold. They concluded that two single Pd sites surrounded by Au atoms are active sites for this reaction. Rebelli et al. [11] observed a strongly enhanced rate of propylene hydrogenation over PdAu nanoparticles supported on silica at elevated Au coverage (>0.6) of Pd nanoparticles. Wang et al. [14] also found a 4–5 times increase in the activity in the selective oxidation of glycerol to glyceric acid at high Au fractions for PdAu/C catalysts. Solsona et al.

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[28] used PdAu/Al2O3 catalysts for the synthesis of hydrogen peroxide and observed a 2–3 times increase in the productivity of the catalysts at about 84% of Au as compared to the monometallic Pd catalyst; they also indicated that not only the surface composition but also the particle size of the alloy particles is important for the reaction. Recently, Lee et al. [30] reported that a surface fraction of Au about 0.4 was obtained by annealing an Au layer on Pd(1 1 1) and that this was optimal for crotyl alcohol oxidation to crotonaldehyde. In another paper [15], the same group reported the preparation of Pd3Au2 particles supported on TiO2 and demonstrated that this material had a high activity for the same reaction. Baddeley et al. [21,31] reported the formation of a surface Pd2Au alloy in the Pd/Au(1 1 1) system and demonstrated its improved catalytic properties in the trimerization of acetylene; they [21] found that the principal effect of Au atoms was to modify the surface ensembles and that this modification influenced the conversion of the reactant and enhanced the desorption of the product. Generally, the microstructure of alloy particles as well as their composition and size are determined by the method of preparation and pretreatment. Herzing et al. [27] studied a PdAu/Al2O3 catalyst with 2.5 wt.% Au and 2.5 wt.% Pd prepared by impregnation of Al2O3 with PdCl2 and HAuCl4 solutions and then pretreated in different ways. They reported that the as-synthesized PdAu particles represented homogeneous alloys; however, their surfaces were enriched in Pd after calcination in air, this forming a core–shell structure, and the resultant materials had lower activity in H2O2 formation from H2 and O2. Subsequent reduction at 773 K was found to be deleterious to the performance of the catalyst, this forming an Au-rich shell morphology that is more thermodynamically favoured [32]. The Pd/Au ratio as determined by XPS changed considerably with the method of pretreatment performed. The formation of a structure for PdAu catalysts based on an Aurich core and a Pd-rich shell has been confirmed by a number of different authors [16,18,20,27,33,34]. Tedsree et al. [35] have recently reported results for the production of hydrogen from formic acid using unsupported PdAg catalysts in the liquid phase and found that a thin Pd layer over the Ag particles significantly improved the rate of hydrogen production; however, they did not find an equivalent improvement for an unsupported PdAu catalyst. We have previously studied the vapour phase decomposition of formic acid [36] and the hydrogenation of olefins by formic acid [5] over single-metal-supported Pd and Au catalysts. The Pd catalysts had high activity and selectivity for hydrogen formation, but they were strongly inhibited by CO at low temperatures (<373 K). Several groups of authors [37–39] have recently reported that supported PdAu catalysts give good results for liquid-phase formic acid decomposition in the presence of sodium formate, this being thought to be a promoter for the reaction [40]. These catalysts showed high stability and demonstrated higher resistance to CO poisoning than did supported monometallic Pd catalysts. In the present work, we report the effect of the Pd/Au ratio on the microstructure of PdAu particles supported on Al2O3 and their catalytic properties in formic acid decomposition; we have studied these materials in both the absence and presence of added CO. The objectives of this investigation were to understand at what Pd/Au ratios the formation of the core–shell structures in the aforementioned PdAu system takes place and to correlate the catalytic activity with the bulk and surface composition of PdAu particles.

2. Experimental 2.1. Preparation of the catalysts PdAu/Al2O3 catalysts with different Pd/Au ratios were synthesized by incipient wetness impregnation of a c-Al2O3 support

(SBET = 250 m2 g1) with a mixture of [Pd(NH3)4](NO3)2 and HAuCl4 dissolved in a water–acetone solution. A change in the atomic Au fraction was achieved by varying the molar ratios of the precursors in the solution. The volume of the solution corresponded approximately to the volume of the support pores and the sum of Pd and Au concentrations were chosen to give 2 wt.% of the metals in the final material. The c-Al2O3 support was prepared from a commercial ‘‘active alumina’’ (AOA-2, Dneprodzerzhinsk) by calcination at 748 K as described by Moroz et al. [41]. The Na and Fe contents of the ‘‘active alumina’’ reported by the supplier were very low (<0.03 wt.%). [Pd(NH3)4](NO3)2 and HAuCl4 were prepared according to procedures used by Plyusnin et al. [42]. The water–acetone solution was used instead of water to prevent the formation of [Pd(NH3)4][AuCl4]2 crystals in the solution [42]. After deposition and overnight drying at 323 K, the samples were reduced in hydrazine hydrate at room temperature. Finally, they were washed in boiling water and dried in air overnight at 410 K. The high-temperature calcination step in air was omitted because formic acid decomposition takes place under reducing conditions and also because such calcination may lead to the deterioration of the properties of Pd-containing catalysts [27,28]. 2.2. Characterization Atomic adsorption spectroscopy (AAS) was used to determine the concentrations of Au and Pd in the catalysts. The total concentrations of the metals were similar for all the samples, being close to 1.8 wt.% (Table 1). CO chemisorption at room temperature was performed as described by Okhlopkova et al. [43] using a pulse method after reduction of each catalyst sample in hydrogen; a 1:1 CO/Pd ratio was assumed in order to determine the Pd surface concentrations in the catalysts. The mean particles size was estimated from these chemisorption values as described in Electronic supplementary information (ESI). X-ray diffraction measurements (XRD) were performed on a DRON SEIFERT RM4 diffractometer using Cu Ka radiation and a graphite monochromator in the reflected beam. Diffraction patterns were recorded in a step mode. The parameters of the face centred cubic structure of Pd, Au or PdAu particles were determined using the (3 1 1) peaks (2h = 77–82°) and PowderCell 2.4 software [44] after subtraction of the small contribution of the cAl2O3 support. Average crystallite sizes were estimated using the integral broadening of the peaks using the WINFIT 1.2.1 program [45] and the Scherrer formula. High-resolution and scanning transmission electron microscopy (TEM) images were taken with a JEOL JEM-2100F (200 kV) microscope. This instrument includes an EDAX X-ray energy dispersive spectrometer (EDX) and a JEOL high-angle annular dark-field (HAADF) detector. A mean particle size was calculated by averaging the diameters of 100–200 particles. X-ray photoelectron spectroscopy (XPS) studies were performed with a Kratos Axis 165 spectrometer using a monochromatic Al Ka radiation (hm = 1486.58 eV) and a fixed analyser pass energy of 20 eV. The binding energy scale was referenced against the C 1s line (284.8 eV). Neither the chlorine nor the nitrogen from the precursors used for synthesis was found on the surfaces of the catalysts. The Pd 3d5/2 and Au 4f7/2 lines were used to determine the electronic properties of the Pd and Au, respectively. It is known that the Au 3d5/2 line may contribute to the region close to Pd 3d5/2 and often the weaker Pd 3d3/2 line has been used [16,32,46,47]. However, for the samples studied, no noticeable contribution of the Au 3d5/2 line to the region mentioned was found as the ratios of the Pd 3d5/2 to Pd 3d3/2 areas were always the same (1.44–1.48) for all samples. With the Au/Al2O3 sample, the Au 3d5/2 line was almost invisible as it was very broad as

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D.A. Bulushev et al. / Journal of Catalysis 299 (2013) 171–180 Table 1 Characteristics of the PdAu/c-Al2O3 catalysts.

a b c d e

Catalysts

Atomic Au fraction

Total metal content, wt.%

Average crystallite sizea, nm

Concentration of surface Pdb (Pd dispersionc), lmol g1

Mean particle sized, nm

Selectivity to H2 at 403 K

Pd1 Pd0.9Au0.1 Pd0.67Au0.33 Pd0.53Au0.47 Pd0.33Au0.67 Pd0.13Au0.87 Pd0.03Au0.97 Au1

0 0.098 0.333 0.470 0.670 0.870 0.965 1

1.8 1.9 1.8 1.7 1.6 2.0 1.6 2.0

5.5 2.4 2 2 8.0 9.3 15 15

6.4 (0.038) 7.7 (0.051) 6.7 (0.076) 5.3 (0.088) 2.1 (0.069) 1.6 (0.113) <0.3 –

30 20 16 (21e) 16 36 (38e) 49 – –

0.97 0.91 0.92 0.95 0.89 0.97 – –

From XRD. From CO chemisorption. CO/Pdtotal. Based on CO chemisorption (see ESI). From TEM.

compared to its height. The best deconvolution of the Pd 3d5/2 and Pd 3d3/2 lines was achieved by assuming that there were two forms of Pd: one metallic (Pd 3d5/2 – 334.9 eV, Pd 3d3/2 – 340.2 eV) and the other cationic (Pd 3d5/2 – 336.8 eV, Pd 3d3/2 – 342.1 eV) formed by oxygen adsorption from air during the drying of the catalysts or during transferring them to the XPS chamber after the reaction. The contribution of the cationic form was 30% of the total surface Pd content for the Pd/Al2O3 sample and 10–20% for the PdAu samples. For the calculations of the Pd surface concentration, both forms of Pd were therefore taken into account.

catalytic experiments. A single-beam spectrum of the reduced catalyst after cooling to 333 K in He was taken as a background spectrum. Four spectra obtained by averaging of 64 scans with 2 cm1 resolution were measured during 10 min after introduction of a 2.5 vol.% CO/He mixture at 333 K. The flow was then switched back to He and desorption of CO was followed as a function of time.

2.3. Catalytic activity measurements and DRIFTS of CO adsorbed

3.1.1. Catalytic activity and selectivity The conversions of formic acid over Pd, PdAu and Au catalysts determined at 403 K are shown in Fig. 1a. The Pd/Al2O3 catalyst had the highest activity. The conversion decreased with an increase in the Au fraction and was almost negligible for the Au/Al2O3 catalyst, the activity of this sample being a factor of 65 lower per gram of metal than that of the Pd/Al2O3 material. This low activity of Au is in line with the earlier data for formic acid decomposition over bulk Au films published by Clarke and Rafter [3]; however, it is in contrast to the data of Ojeda and Iglesia [48] who have recently demonstrated that a highly dispersed gold on alumina catalyst was extremely active in this reaction; they explained their results by suggesting that TEM-invisible Au species determine the rates of reaction measured. Bi et al. [49] have since shown that such TEM-invisible Au species with a mean particle size of 0.8 nm are more active than those with a particle size of a few nm when used in the liquid phase decomposition of formic acid in the presence of sodium formate, triethylamine and water. It is often considered that the water–gas shift reaction requires similar Au active sites to those for the formic acid decomposition to hydrogen and carbon dioxide [48]. Since it is known that highly dispersed Au and TEM-invisible Au species are very active in this reaction [36,48–51] giving sufficient conversion at temperatures lower 473 K, we have performed a few tests of this reaction in order to confirm additionally that we do not have highly dispersed Au in the studied samples. The results of the experiments for the water–gas shift reaction showed that the activities per gram of metal of some of the catalysts (Pd1, Pd0.33Au0.67 and Pd0.67Au0.33) were at least 20 times lower at 583 K than that obtained by Ojeda and Iglesia [48] at close reactants’ concentrations and much lower temperature – 523 K. We therefore conclude that the Au is not present on the surface of the studied catalysts in a highly dispersed or TEM-invisible form and that it behaves like a bulk, almost inert, metal. The main products of the decomposition of formic acid over the PdAu catalysts were CO2 and H2, and the selectivities towards

Details of the experiments with formic acid and the relevant calculations are given elsewhere [5,36]. The catalysts (0.036 g) were placed in a quartz tube reactor with internal diameter 4 mm. The catalytic activities for formic acid decomposition were measured after reduction of the catalysts in a 1 vol.% H2/Ar mixture for 1 h at 573 K and cooling in He to the temperature of the reaction. The reduction temperature of 573 K corresponded to the highest temperature used for formic acid decomposition. Using of the reduction temperature 200 K lower (373 K) led only to a slightly higher conversion (<20%, Fig. S1). Significant difference was observed if the reduction was performed at a temperature 200 K higher (773 K). In this case, the activity decreased by a factor of 2.6. Formic acid (Riedel-de Haan, 98–100% purity) was introduced into an evaporation volume using a syringe-pump (Sage); the system lines were heated to 340 K to prevent condensation. The concentration of formic acid in the gas phase was 2.2 vol.%, and helium was used as a carrier gas. The effect of CO on the rate of formic acid decomposition was studied by using a 2.5 vol.% CO/He mixture instead of He. The water–gas shift reaction was tested over some of the samples using a gas mixture of the following composition: 2.5 vol.% CO, 2.3 vol.% H2O, balance He. The total flow rate of the gas mixtures for all experiments was 51 ml min1. A gas chromatograph (HP-5890) with a TCD detector was used for gas analysis. The only products determined were H2, CO, CO2 and H2O. Activation energies for the reaction were determined at low conversions from Arrhenius plots; in the absence of added CO, the typical range of temperature was 358–433 K and, in the presence of CO, it was 403–498 K. DRIFTS of adsorbed CO was performed using the same gas supply system as that used in the catalytic set-up. A SpectraTech-0030 DRIFTS ‘‘in situ’’ cell was used in conjunction with a Nicolet Magna 560 FT-IR spectrometer with an MCT detector; the same reduction of the catalyst samples was performed in hydrogen as in the

3. Results and discussion 3.1. Catalytic activity of metal particles

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the TOFs for the Pd/Al2O3 sample were 2–3 times higher than those for the PdAu/Al2O3 catalysts. Possible explanations for this difference are proposed below (Section 3.5). It was shown earlier that H2 and especially CO, the products of the decomposition of formic acid, inhibit its decomposition over Pd catalysts [36,37]. It would not be expected that the inhibition by CO is stronger for alloys since the formation of alloys normally facilitates desorption of CO [22,23,52,53]. The concentration of CO in the performed experiments (Fig. 1) was low (60.015 vol.%) and it decreased with conversion. This decrease probably explains a small increase in the TOFs as a function of the Au fraction for the PdAu catalysts.

(a) 0.2 0.15

Conversion

these products being in the range 0.89–0.97 (Table 1). Thus, all the catalysts were relatively selective in H2 formation at least at low conversions (<20%). It should be mentioned that the level of CO corresponding to these selectivities was too high for the use of the catalysts studied here in fuel cell applications; however, it should be recognized that the selectivities can be improved by adding steam to the formic acid flow [5,9,36]. The much higher activity of the Pd/Al2O3 sample than of the other materials leads to the conclusion that only the Pd has significant activity in the PdAu/Al2O3 samples. CO chemisorption was used to determine the Pd surface concentrations since it is well established that Au does not adsorb CO to any measurable extent at room temperature. The CO chemisorption data are shown in Table 1. The results of Table 1 show that the Pd0.9Au0.1 catalyst adsorbed slightly more CO as compared to the Pd/Al2O3 catalyst. The concentration of adsorbed CO further decreased with an increase in the Au fraction. Generally, there are two factors which determine the concentrations of surface Pd sites, a surface Pd/Au ratio and a mean metal particle size; the smaller the metal particle size, the larger the number of surface metal atoms. The higher value for the Pd0.9Au0.1 catalyst than for the Pd1 catalyst can be explained by a higher Pd dispersion in that catalyst (Table 1). Determination of the surface concentration of the Pd in the Pd0.03 Au0.97 sample was not possible because of insufficient sensitivity of the adsorption measurement; the low surface Pd concentration was also confirmed by XPS which was unable to detect any Pd in that sample. As the activity of the Pd surface sites in the PdAu catalysts was much higher than that of the Au surface sites, the Au could be considered as an inert additive. Reaction rates were therefore calculated from the conversion data (Fig. 1a) and related to the surface concentration of Pd determined by CO chemisorption (Table 1) to obtain turnover frequencies (TOFs). Fig. 1b shows that the TOFs for the PdAu catalysts did not depend to any significant extent on the Au bulk fraction in the range of 0.1–0.87. However,

0.1

0.05

0 0

0.2

0.4

0.6

0.8

1

Au atomic fraction

(b) 0.8 0.6

TOF, s-1

Fig. 1. Conversion of formic acid (a) and turnover frequencies (based on Pd surface concentration) (b) at 403 K as a function of the Au atomic fraction in the PdAu/Al2O3 catalysts (catalyst weight – 0.036 g, 2.2 vol.% HCOOH/He, 51 ml min1).

3.1.2. Catalytic activity in the presence of added CO To understand the effect of CO at different Au contents, the reaction was performed in the presence of CO. Strong inhibition by CO was observed for almost all the PdAu samples as no conversion was found in the presence of CO at 403 K (Fig. S2). Conversions close to those obtained without CO at 403 K were obtained with CO only at 463 K (Fig. 2a). Their dependence on the Au content was quite similar to the dependence obtained without the addition of CO, there being a higher formic acid conversion over the undoped Pd catalyst. A slightly higher conversion was also observed at high Au content in the presence of CO. This could be explained by a higher contribution of Au to the reaction at 463 K than at 403 K. It is also known that Au catalysts are much more resistant to CO adsorption than are Pd catalysts. The dependence of TOFs on Au content (Fig. 2b) also resembles the dependence obtained in the absence of CO (Fig. 1b): the TOF was 2–3 times higher for the undoped Pd catalyst, and the TOFs values for the PdAu catalysts were independent of the Au content.

0.4

0.2

0 0

0.2

0.4

0.6

0.8

1

Au atomic fraction Fig. 2. Conversion of formic acid (a) and turnover frequencies (based on Pd surface concentration) (b) in the presence of 2.5 vol.% CO at 463 K as a function of the Au atomic fraction in the PdAu/Al2O3 catalysts (other conditions are in Fig. 1).

D.A. Bulushev et al. / Journal of Catalysis 299 (2013) 171–180

Hence, an increase in the Au fraction from 0.1 to 0.67 did not lead to an increased resistance of the catalyst to CO when the Au content in the sample was increased. This appears to indicate that there is a similar surface Pd/Au ratio in the surface layer formed on the PdAu particles. 3.1.3. Activation energies Apparent activation energies for the formic acid decomposition were calculated from the temperature dependences of the steadystate rates of the reaction and are shown in Fig. 3 as a function of the Au fraction. It is seen that the activation energies are similar for the Pd and PdAu catalysts. They are also similar over a wide range of Au atomic fractions (up to 0.87). However, a difference was observed when the Au1 or Pd0.03Au0.97 catalysts were used. Thus, the values of the activation energies for the Pd and the PdAu catalysts are 40 ± 2 kJ mol1 but are about 62 kJ mol1 for the Au/Al2O3 catalyst. The value for Au is consistent with the data that have been reported in the literature for supported Au/C, Au/TiO2 [36] and highly dispersed Au/Al2O3 catalysts [48], these having activation energies in the range 53–63 kJ mol1. The literature values for bulk Au catalysts in the form of films [2] and wires [3] were about 60 kJ mol1 and these also correspond well to the data obtained here. The activation energy for the Pd/Al2O3 catalyst (40 ± 2 kJ mol1, Fig. 3) was, however, lower than that for highly dispersed Pd/C catalysts (65–67 kJ mol1) [36]. Different values have previously been obtained for bulk Pd films and wires. For example, Eley and Luetic [2] reported a value of 33 kJ mol1 for a pure Pd wire at low formic acid pressures (6  104 Torr); they also found that the activation energy increased with an increase in the Au atomic fraction for values above 0.3. Clarke and Rafter [3] found that if a Pd film was pretreated in vacuum at high temperature (673–773 K), this giving the densest (1 1 1) face, the activation energy was about 66 kJ mol1; with an increase in the Au fraction, the activation energy passed through a minimum of about 42 kJ mol1. However, if the film was prepared and pretreated only at low temperatures, the Pd gave activation energy of about 40 kJ mol1, a value similar to the results of the present work. CO present in small concentration in the reaction may affect the values of activation energy. Thus, apparent activation energies of the reaction were also measured in the presence of added CO (Fig. 3). The values fell in the range 69 ± 7 kJ mol1, a value about 30 kJ mol1 higher than that obtained in the experiments without addition of CO. Evidently, CO adsorbs on the PdAu catalysts, decreasing the concentration of active Pd sites and leading to a strong inhibition of the reaction. The increase in the activation energy with the addition of CO could be related to the temperature dependent coverage of the Pd sites with adsorbed CO. Adsorption of CO is dependent on the structure of the surface layer and its composition.

Fig. 3. Activation energies in the formic acid decomposition as a function of the Au atomic fraction (conditions are in Fig. 1).

175

The decomposition of formic acid over Pd and Au catalysts normally occurs with a reaction order of close to zero [36,48], this indicating that the decomposition of a surface formate with the breakage of the C–H bond is probably the rate-determining step. The presence of the formates on the noble metal surfaces under the conditions of formic acid decomposition (383–473 K) have been confirmed recently by infrared spectroscopy [9]. The 2.5-fold difference in the TOFs for the Pd and PdAu catalysts found in this work (Figs. 1 and 2b) is too small to allow us to conclude whether the difference is due to a change in the activation energy or in the pre-exponential factor. This difference could be caused by a change in the activation energy of only 3 kJ mol1, this being only slightly higher than the possible error in the activation energy determination (Fig. 3). Alternatively, it could correspond to a difference of 7.6 J K1 mol1 in the entropy of activation. Owens et al. [54] have found that the reactivity of acetic acid on a Pd(1 1 1) surface is strongly modified by the presence of Au; the latter acts both to stabilize adsorbed acetate species and to decrease the ability of acetic acid to decompose on adsorption to produce adsorbed carbon. To the best of our knowledge, formic acid does not give adsorbed carbon on a Pd surface. The results of Fig. 1 show that the TOFs in the formic acid decomposition over the PdAu catalysts are almost independent of the Au bulk mole fraction in the range 0.1–0.87, while the activation energies do not also change over the composition range from 0 to 0.87 (Fig. 3). A similar picture is observed in the presence of added CO (Figs. 2 and 3). In order to understand these observations, different methods of characterization of the bulk and surfaces of the metal particles have been applied. The mean particle size and bulk composition have been studied using XRD, TEM and chemisorption data. 3.2. Bulk composition and mean size of metal particles 3.2.1. X-ray diffraction study XRD was used to determine the bulk composition of the catalysts and the average crystallite size of the metal particles; these measurements were performed on samples reduced in hydrazine before the reaction. However, we showed that pre-reduction of the Pd0.67Au0.33 sample in hydrogen at 573 K did not lead to any difference in the XRD peak position or its width (Fig. S3). Typical XRD patterns of the samples are shown in Fig. 4a. The peaks observed are rather symmetrical and are very different from those obtained with the PdAu/C catalysts studied earlier for which a distinct peak asymmetry was distinguished [25]. This allows one to conclude that the bulk composition of PdAu crystallites is relatively homogeneous. It is known that the composition of the equilibrated PdAu alloys can be determined from the lattice parameter according to the Vegard law, assuming a linear dependence of the lattice parameter on the Au fraction [15,47,55]. Fig. 4b clearly demonstrates the formation of bulk alloys at Au atomic fractions up to 0.5 as the lattice parameter was close to that of the homogeneous alloy. However, there is a strong deviation from the linear dependence in the Au atomic fraction range of 0.67–0.87 and this appears to indicate that the mixture of metals is not equilibrated. At the Au fractions equal or higher than 0.67, the main peaks in the diffraction pattern were close to those of pure Au. As Pd was not seen by XRD, it is probable that the Pd is located as a thin layer over the metal particles. This will be confirmed below by reference to the XPS results (Section 3.3). The average metal crystallite sizes in the Pd/Al2O3, PdAu/Al2O3 and Au/Al2O3 catalysts obtained using XRD are shown in Table 1. In the Pd catalyst without Au, the size was close to 5.5 nm. Smaller crystallite sizes were determined in the catalysts with the Au content up to 0.5. However, some non-uniformity in the composition of the metal particles may also contribute to the peak broadening

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(a)

Intensity

Pd0.13Au0.87 Pd 0.53Au0.47

75

Pd 222

Au 222

Au 311

Pd 311

Pd 0.67 Au0.33

80

85

90

Angle 2Θ, degree

(b)

Fig. 4. XRD patterns (a) and lattice parameter as a function of the Au atomic fraction (b) before (squares) and after reduction in H2 (triangle). Straight diagonal line corresponds to the homogeneous alloy composition.

and the mean crystallite size determination. Hence, this determination cannot be precise for the Au fractions 0.1–0.5 and the obtained values can be used mainly for comparison of the catalysts. At higher Au content, the composition was close to pure Au and the mean crystallite size increased, reaching 15 nm. 3.2.2. TEM studies The XRD results described above do not exclude the presence of larger agglomerates of metal crystallites in addition to very small metal particles. In accordance, TEM measurements of the Pd0.67Au0.33 (Fig. 5a) and Pd0.33Au0.67 (Fig. 5b) catalysts were carried out and these showed that the mean particle size for these samples was several times larger than the average crystallite size

determined by XRD (Table 1). The shape of the particles indicates that they are agglomerates of smaller particles and crystallites. A comparison of different TEM images for the Pd0.67Au0.33 and Pd0.33Au0.67 catalysts shows that the typical size of the particle agglomerates is greater for the sample with the larger content of Au: about 38 nm as compared to about 21 nm (Table 1). The particle size distribution for the Pd0.33Au0.67 catalyst was broader than for the Pd0.67Au0.33 catalyst (Fig. S4). Fig. 6 shows a HRTEM image of a particle of about 24 nm size present in the Pd0.67Au0.33 catalyst. The inter-crystalline boundaries are clearly seen, this confirming that the particle concerned consists of several smaller crystallites. Similar particles have been observed also for the monometallic Pd/Al2O3 catalyst. Additionally, the TEM measurements of the same catalysts before and after the reaction showed that the particle size distribution became narrower after the reaction as a result of a decrease in the contribution of smaller particles, probably as a result of sintering. However, it is not possible to be categorical on this since only a very minor part of the sample is presented in the TEM images. The mean particle size estimated on the basis of the chemisorption data (ESI) is also shown in Table 1, there being a minimum size of about 16 nm. Such a value is in agreement with the mean size of the metal particles determined by TEM. We conclude that the XRD, TEM, and chemisorption data all indicate that the metal particles in the catalysts studied consist of agglomerates of smaller particles or crystallites. It is important that these agglomerates are relatively large (>10 nm). Thus, particle size effect appearing normally at the sizes of less than 5 nm cannot be used to explain the TOFs data (Fig. 1b). Solsona et al. [28] have concluded that PdAu particles of similarly relatively large sizes with a core–shell structure might be the most effective in H2O2 production. To understand why there were no changes of the values of TOFs or activation energy over a wide range of bulk composition, the surface composition of the catalysts was studied with XPS.

3.3. Surface composition of metal particles Fig. 7 shows the surface composition results obtained by XPS for the various catalyst samples after reduction in hydrogen at 573 K followed by the reaction with formic acid and cooling in helium; results were also obtained for two catalysts before reaction (triangles, Fig. 7). This was done in order to elucidate the effects of reaction and of reduction in hydrogen at higher temperature on the surface composition.

Fig. 5. TEM images of the Pd0.67Au0.33 (a) and Pd0.33Au0.67 (b) catalysts after the reaction.

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Fig. 6. HRTEM image of a metal particle in the Pd0.67Au0.33 catalyst.

The results of Fig. 7 show that while the Au fraction changes about 9 times (from 0.1 to 0.87), the surface Au composition changes only slightly – 1.5 times (from 0.2 to 0.33). The surface composition is close to the composition of homogeneous (equilibrated) alloy only at an Au fraction of about 0.33. At lower fractions, the surface is enriched in Au while at higher fractions, it is enriched in Pd. Thus, the surface is enriched with the metal that is taken in a smaller concentration during the synthesis process. The surface Au fraction hardly changes with an increase in the bulk Au fraction in the range 0.33–0.87, having a value in the range 0.28–0.36. This may indicate that a relatively stable surface alloy is formed. The atomic ratio of Pd to Au at this surface composition is close to 2, this corresponding to a Pd2Au alloy. It is interesting to note that Baddeley et al. [21,31], using low energy electron diffraction (LEED), found that an ordered Pd2Au alloy of at least two layers thickness was formed after deposition of Pd on an Au(1 1 1) surface followed by annealing at 500 K under UHV conditions. These researchers reported that this layer behaved as a more effective catalyst for acetylene trimerization [21] than did pure Pd. The formation of a surface layer with a composition of Pd2Au was also found using photoemission spectroscopy for an Au/Pd(1 1 1) system by Weissman–Wenocur and co-authors [56]. The XPS data clearly indicate that the surface of the metal particles with high Au content is enriched with Pd, forming an alloy with an average composition of Pd2Au. We cannot be certain that the surface structure of that alloy is exactly the same as that

Fig. 7. Surface Au atomic fraction (XPS) as a function of the Au atomic fraction for the PdAu/Al2O3 catalysts before (triangles) and after (squares) the reaction. Straight diagonal line corresponds to the homogeneous alloy composition.

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reported by Baddeley et al. [31]. For example, we cannot see this thin shell by XRD. Thus, when we talk about a surface alloy, we mean a layer with an average composition of Pd2Au. On the basis of the average metal crystallite size determined by XRD (Table 1) and assuming that there is only pure Au in the cores of the crystallites, it can be estimated that the thickness of such a shell corresponds to 1–2 layers for the Pd0.13Au0.87 sample and to 3–4 layers for the Pd0.33Au0.67 sample. Such layers are too thin to allow them to be detected by XRD. A small difference in the surface composition for the catalysts before and after the reaction was observed (Fig. 7). Thus, the Pd atomic fraction increased slightly (10–23%) after the reaction indicating that some Pd segregation had occurred. This segregation may take place because of higher temperatures used than those applied in the drying of the catalysts, this leading to a more rapid diffusion of metal atoms as well as to a stronger interaction of the components of the reaction mixture with Pd than with Au. Recently, Owens et al. [54] reported that the adsorption of acetic acid leads to some segregation of Pd from a PdAu system. CO is also known to provide such effect [57]. It is important to note that the differences in the composition of the surface layer before and after the reaction are not significant and will not affect seriously the behaviour of the TOFs (Fig. 1b). Generally, the surfaces of equilibrated PdAu alloys tend to be enriched with Au because of the higher surface free energy of Pd (2.043 J m2) as compared to Au (1.626 J m2) [32]. Such enrichment probably occurs in the case of the low Au fractions (0.1) for the studied samples, these showing a content of Au on the surface twice that of the bulk (Fig. 7). At higher Au fractions (>0.3), an enrichment of the surface with Pd is observed. It is important that the same surface composition corresponding to a Pd2Au layer is formed over a wide range of  Pd/Au ratios. As the redox potential of AuCl4 =Au0 (1.00 V vs. standard hydrogen electrode) is considerably higher than the redox po0 tential of PdðNH3 Þ2þ (0.0 V), the reduction of the Au4 =Pd containing precursors takes place faster than that of the Pd precursors during the preparation of the catalysts [33,58,59]. This leads to a core–shell structure of metal particles. Hence, the nuclei of Au are formed earlier and are the sites at which the surface Pd2Au layer is formed. To confirm the presence of the Pd2Au layer in a wide range of Au fractions, the surface electronic properties of Au and Pd were also determined using XPS. 3.4. Surface electronic properties of metal particles The performed XPS study showed that cationic Au was not present in the samples, and the concentration of cationic Pd obtained by oxidation of the surface Pd by oxygen in air at room temperature or the drying temperature was low (Section 2). The low concentration of cationic Pd observed is reasonable as no calcination of the catalysts in air at high temperatures was performed, and the XPS measurements were made on the catalysts after reductive treatments. Thus, both Pd and Au are present on the surface mainly in the metallic state. Fig. 8 shows the binding energies of the metallic Pd 3d5/2 and Au 4f7/2 XPS signals as a function of the Au fraction. Measurements of the samples before and after reaction did not show any significant differences in the electronic properties of surface Pd and Au, this confirming that the surface layer is mainly formed during the preparation of the catalysts rather than during the reduction in hydrogen followed by the reaction. This is in accordance with the catalytic activity data on the effect of the reduction temperature (Fig. S1) and with the XPS data on the surface composition (Fig. 7). The binding energy of Pd 3d5/2 for the Pd/Al2O3 catalyst was higher than the values for the PdAu catalysts (Fig. 8a). As discussed above, the average surface composition over a wide range of the Au

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Fig. 8. Binding energies for metallic Pd 3d5/2 (a) and Au 4f7/2 (b) as a function of the Au atomic fraction for the PdAu/Al2O3 catalysts before (triangles) and after (squares) the reaction.

Fig. 9. STEM image of the Pd0.13Au0.87 sample after the reaction and line-scanning EDS of the metal particle indicated with a line on the STEM image.

content was quite similar (Fig. 7) and it corresponded to the Pd2Au surface alloy. The difference between the binding energies of Pd 3d5/2 for the Pd/Al2O3 catalyst and the Pd2Au layer was not high and consisted of 0.27 eV. The change in the Au atomic fraction in the samples from 0.1 to 0.87 does not affect noticeably the binding energy, this indicating the same surface electronic properties. As with the Pd component, the binding energy of Au 4f7/2 did not change much as a function of the bulk Au fraction when the surface Pd2Au layer was formed. The difference between the binding energies of the Au 4f7/2 signal for the Au/Al2O3 catalyst compared with the Pd2Au layer was 0.33 eV. The widths of the Au 4f7/2 and Pd 3d5/2 peaks were almost independent of the Au content of the PdAu catalysts. The change in the surface electronic properties with an increase in the Au fraction is consistent with the suggestion that a surface alloy of the same average composition (Pd2Au) is formed and that its electronic properties differ from the properties of the individual pure supported metals. Such changes of the electronic properties of Pd and Au upon alloying have been reported earlier [16,32,46,60]. Quite similar shifts for the Au 4f7/2 and Pd 3d5/2 peaks were predicted from calculations from first-principles within densityfunctional theory [61]. EDS/STEM was used to confirm the presence of the core–shell structure in the supported metal particles. Infrared spectroscopy of adsorbed CO was also applied as a method to discriminate between the different Pd sites on Pd and PdAu surfaces [11,23,32,52,53,62].

Measurement of the metal concentrations in the particle shown demonstrates that the concentration of Au increased from the edges to the centre while the concentration of Pd did not change strongly. Hence, the edges of the particles are enriched with Pd, this again supporting the idea of a core–shell structure with the gold predominantly in the core of the particles and the Pd2Au alloy in the shell. This result is consistent with the features observed by XRD and XPS.

3.5. Surface structure of metal particles and catalytic activity 3.5.1. EDS/STEM studies Line-scanning EDS [16,63] of some of the metal particles in the Pd0.13Au0.87 sample after the reaction was performed to gain a better understanding of the microstructure of the PdAu particles and the results are shown in Fig. 9. These results indicate that the particles with a diameter about 10 nm consist of both Au and Pd.

3.5.2. DRIFTS of adsorbed CO Fig. 10 shows the IR spectra of adsorbed CO measured on the catalysts with different Au content. The bands at about 1930 and 2050 cm1 are seen. They are very weak since the concentration of the Pd surface sites in the studied samples is relatively small (Table 1). The band with a maximum at 1930 cm1 has a broad shoulder to the lower wavenumbers indicating the presence of another form of CO adsorption. The shape of this band seems does not change with the change in the Au content in the samples. The intensity decreases with the Au content till 0.67. This band is not seen at higher Au contents. Generally, the region of 1780– 1980 cm1 corresponds to CO adsorption on contiguous Pd sites. The band at 1920–1950 cm1 is normally assigned to bridged CO adsorbed on terrace sites of (1 1 1) Pd facets [64]. The shoulder at the lower wavenumbers corresponds to CO adsorption on threefold hollow Pd sites [62,64]. A weak band at about 2050 cm1 is seen, for example, for the Pd0.67Au0.33 sample (Fig. 10). This band is not observed for the lower dispersed Pd1 sample. It was assigned to linear adsorbed CO on (1 1 1)/(1 1 1) and (1 1 1)/(1 0 0) edge sites [62]. The concentration of these sites decreases with the increase in Pd particle size. Thus, it was expected to be low for the used catalysts as they contain relatively large particles (Table 1). According to the surface science work of Goodman et al. [22,23,57,65], single Pd sites surrounded by Au sites are formed at high Au contents. These sites should give CO adsorption provid-

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Fig. 10. DRIFT spectra of CO region of the PdAu/Al2O3 catalysts taken after 10 min contact with a 2.5 vol.% CO/He mixture at 333 K.

ing a relatively sharp peak at about 2060–2085 cm1. Such peak was not observed for the samples studied in the presence of gas phase CO (Fig. 10) or in its absence (not shown). The difference between the results of surface science and conventional catalytic measurements could be explained by the presence of some dissolved carbon impurities in the conventional catalysts [66,67] or to a difference in the structures of the metal surfaces. Hence, the DRIFTS of adsorbed CO study indicated the presence of contiguous sites on the surface of the samples up to the high Au content. This result could be related to the high degree of enrichment of the surface layer with Pd, corresponding to the surface alloy with an average composition of Pd2Au, this following from the XPS measurements (Figs. 7 and 8). Relatively low resistance of the studied catalysts to CO inhibition (Fig. S2) should be also related to this high enrichment of the surface layer with Pd.

3.5.3. Discussion A simplified schematic representation of the surface and bulk compositions of the metal particles studied in the present work is shown in Fig. 11. This picture also shows the change in the particle size with the Au content (Table 1), but it does not attempt to represent the agglomeration of particles that was observed by TEM (Figs. 5 and 6). At low Au atomic fractions, the surface is enriched

Fig. 11. A simplified vision on the surface and bulk composition of the PdAu particles.

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with Au until the composition of the surface layer becomes similar to the bulk composition corresponding to the Pd2Au alloy. A further increase in the Au content leads to the retention of the same surface composition with the same stable Pd2Au shell and with an increase in the Au content in the core. These core–shell structures are non-equilibrated as diffusion of Au and Pd is slow at temperatures lower than 600 K [22,30,32,52]. This was also confirmed by the decrease in the catalytic activity with an increase in the reduction temperature (Fig. S1) being in accordance with the data of Herzing et al. [27]. It is important to emphasize that in spite of the difference of the cores in the particles with the Au fractions of 0.33 and 0.87, their surface compositions (Fig. 7) and their electronic (Fig. 8) and catalytic (TOFs) (Figs. 1b and 2b) properties are all similar. However, the TOFs for the pure Pd are 2–3 times higher than those for the Pd2Au alloy. It could be proposed as an explanation that Au substitutes very active Pd sites on the surface of metal particles. These could be edge and corner sites or alternatively contiguous (threefold hollow and bridged) sites. Thus, Yudanov and Neyman [68] as well as Abbot et al. [53] using DFT calculations predicted a very stable structure on the surface of PdAu nanoparticles when Au occupies the edge sites. As we use relatively large particles, the concentration of these edge sites should be very low. Hence, the explanation via the substitution of edge and corner sites hardly can be valid for the studied samples. Lee et al. [30] reported that about 10% of Au on the surface of Pd(1 1 1) is sufficient to switch off the contiguous threefold hollow Pd sites for CO adsorption. The concentration of bridged sites decreased more slowly with Au concentration. An increase in the Au content in the studied samples did not lead to a change in the shape of the spectra for CO adsorbed on bridged and threefold hollow sites (1780–1980 cm1, Fig. 10). Thus, Au does not discriminate significantly the contiguous sites in the studied samples at least till the Au content of 0.67. The behaviour of the TOFs obtained in this work (Fig. 1b) also argues against the idea that new single Pd sites [22,23,57,65] surrounded by Au atoms are formed and are active for the formic acid decomposition. The high enrichment of the surface layer with Pd is probably unfavourable for the formation of these single sites. In agreement with this, Tedsree et al. [35] concluded from 13C NMR studies that bidentate (bridged) formate species prevail on terrace Pd sites as precursors for H2 and CO2 while other forms of formate (linear and multilinear) on surface-unsaturated sites (adatoms, corners, steps, kinks) give H2O and CO. Zhou et al. [69] using DFT calculations for the formation of formates on Pd(1 1 1) also indicated the importance of the bridged formate species attached to two Pd surface atoms for hydrogen and carbon dioxide formation. The presence of these formates on Pd(1 1 1) was confirmed by LEED and reflection-absorption infrared spectroscopy studies performed by Zheng et al. [70]. Thus, the difference in the TOFs for Pd and Pd2Au layer (Figs. 1b and 2b) can be explained using the approach of ‘‘ligand effect’’ as proposed by Tedsree et al. [35]. The effect is provided by a change in the d-band surface centres by alloying with Au [71]. This change is indicated by the observed shift of the Pd 3d5/2 binding energy by 0.3 eV (Fig. 8a). Generally, in this work, no improvement in catalytic properties has been found by doping of alumina supported Pd with Au when the relatively large particle sizes (>10 nm) were used. An improvement might be expected if particle sizes (<5 nm) are present for which the catalytic properties of gold change drastically [41,48– 51,72]. An additional increase in the catalytic activity might be attained by doping the catalysts with alkali metal ions. We have recently demonstrated that doping a Pd/C catalyst with potassium ions improved the rate of hydrogen production from formic acid by 1–2 orders of magnitude [40].

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4. Conclusions A study of PdAu/Al2O3 catalysts prepared by impregnation followed by hydrazine reduction with a mean metal particle size of more than 10 nm and different Pd/Au ratios showed that the activity of Au for formic acid decomposition was negligible as compared to the activity of Pd being a factor of 65 lower per gram of metal. The main novel result of the research performed here is that the same surface alloy of the average composition Pd2Au is formed over a wide range of Au fractions, from 0.33 to 0.87, and that this surface alloy determines the surface electronic and catalytic properties. The TOFs of this alloy (based on Pd surface concentration) were 2–3 times lower than those of a single-metal Pd/Al2O3 catalyst with and without added CO. This could be explained by a ‘‘ligand effect’’ which manifested itself by a shift of the Pd 3d5/2 binding energies by 0.3 eV. The results obtained can be helpful for the development of catalysts for reactions such as oxidation, hydrodechlorination or hydrogenation where the PdAu alloy activity is higher than those for the single metals. Acknowledgments This publication has emanated from research conducted with the financial support of Science Foundation Ireland under Grant Number 06/CP/E007. This work was also conducted under the framework of the INSPIRE programme, funded by the Irish Government’s Programme for Research in Third Level Institutions, Cycle 4, National Development Plan 2007–2013. The research was also supported by the RFBR Grant 11-03-00668, the interdisciplinary project No 112 from the Presidium SB RAS and the Russian Federation state contract P960 of the Federal target program ‘‘Scientific, Research and Teaching Specialists in Russia’’. The authors gratefully thank Prof. A.S. Lisitsyn for measurements of the Pd surface concentration and Dr. B.L. Moroz and Dr. P.A. Pyrjaev for assistance and valuable advices. We also thank the referees for very constructive and helpful remarks which have enabled us to greatly improve the manuscript. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcat.2012.12.009. References [1] [2] [3] [4] [5] [6]

[7] [8] [9] [10] [11] [12]

[13] [14] [15] [16] [17] [18]

E.G. Allison, G.C. Bond, Catal. Rev.-Sci. Eng. 7 (1972) 233. D.D. Eley, P. Luetic, Trans. Faraday Soc. 53 (1957) 1483. J.K. Clarke, E.A. Rafter, Z. Phys, Chem. Chem. Neue Folge 67 (1969) 169. S. Ha, R. Larsen, R.I. Masel, J. Power Sour. 144 (2005) 28. D.A. Bulushev, J.R.H. Ross, Catal. Today 163 (2011) 42. D.J. Hayes, S. Fitzpatrick, M.H.B. Hayes, J.R.H. Ross, in: B. Kamm, P.R. Gruber, M. Kamm (Eds.), Biorefineries-Industrial Processes and Products, vol. 1, WileyVCH, Weinheim, 2006, p. 139. M. Grasemann, G. Laurenczy, Energy Environ. Sci. 5 (2012) 8171. R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129. F. Solymosi, A. Koos, N. Liliom, I. Ugrai, J. Catal. 279 (2011) 213. A. Sarkany, O. Geszti, G. Safran, Appl. Catal. A 350 (2008) 157. J. Rebelli, M. Detwiler, S.G. Ma, C.T. Williams, J.R. Monnier, J. Catal. 270 (2010) 224. J. Kaiser, L. Leppert, H. Welz, F. Polzer, S. Wunder, N. Wanderka, M. Albrecht, T. Lunkenbein, J. Breu, S. Kummel, Y. Lu, M. Ballauff, Phys. Chem. Chem. Phys. 14 (2012) 6487. Y.L. Lam, J. Criado, M. Boudart, J. Nouveau, Chimie-New J. Chem. 1 (1977) 461. D. Wang, A. Villa, F. Porta, L. Prati, D.S. Su, J. Phys. Chem. C 112 (2008) 8617. A.F. Lee, C.V. Ellis, K. Wilson, N.S. Hondow, Catal. Today 157 (2010) 243. J. Xu, T. White, P. Li, C.H. He, J.G. Yu, W.K. Yuan, Y.F. Han, J. Am. Chem. Soc. 132 (2010) 10398. E. Smolentseva, B.T. Kusema, S. Beloshapkin, M. Estrada, E. Vargas, D.Y. Murzin, F. Castillon, S. Fuentes, A. Simakov, Appl. Catal. A 392 (2011) 69. S. Alayoglu, F. Tao, V. Altoe, C. Specht, Z.W. Zhu, F. Aksoy, D.R. Butcher, R.J. Renzas, Z. Liu, G.A. Somorjai, Catal. Lett. 141 (2011) 633.

[19] A.N. Simonov, P.A. Pyrjaev, B.L. Moroz, V.I. Bukhtiyarov, V.N. Parmon, Electrocatal. 3 (2012) 119. [20] M. Hosseini, T. Barakat, R. Cousin, A. Aboukais, B.L. Su, G. De Weireld, S. Siffert, Appl. Catal. B 111 (2012) 218. [21] C.J. Baddeley, R.M. Ormerod, A.W. Stephenson, R.M. Lambert, J. Phys. Chem. 99 (1995) 5146. [22] D. Kumar, M.S. Chen, D.W. Goodman, Catal. Today 123 (2007) 77. [23] M.S. Chen, K. Luo, T. Wei, Z. Yan, D. Kumar, C.W. Yi, D.W. Goodman, Catal. Today 117 (2006) 37. [24] M.O. Nutt, K.N. Heck, P. Alvarez, M.S. Wong, Appl. Catal. B 69 (2006) 115. [25] M. Bonarowska, J. Pielaszek, V.A. Semikolenov, Z. Karpinski, J. Catal. 209 (2002) 528. [26] J.K. Edwards, A.F. Carley, A.A. Herzing, C.J. Kiely, G.J. Hutchings, Faraday Discus. 138 (2008) 225. [27] A.A. Herzing, A.F. Carley, J.K. Edwards, G.J. Hutchings, C.J. Kiely, Chem. Mater. 20 (2008) 1492. [28] B.E. Solsona, J.K. Edwards, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely, G.J. Hutchings, Chem. Mater. 18 (2006) 2689. [29] X. Wei, X.F. Yang, A.Q. Wang, L. Li, X.Y. Liu, T. Zhang, C.Y. Mou, J. Li, J. Phys. Chem. C 116 (2012) 6222. [30] A.F. Lee, S.F.J. Hackett, G.J. Hutchings, S. Lizzit, J. Naughton, K. Wilson, Catal. Today 145 (2009) 251. [31] C.J. Baddeley, C.J. Barnes, A. Wander, R.M. Ormerod, D.A. King, R.M. Lambert, Surf. Sci. 314 (1994) 1. [32] C.W. Yi, K. Luo, T. Wei, D.W. Goodman, J. Phys. Chem. B 109 (2005) 18535. [33] M.L. Wu, D.H. Chen, T.C. Huang, Langmuir 17 (2001) 3877. [34] Y.L. Fang, J.T. Miller, N. Guo, K.N. Heck, P.J.J. Alvarez, M.S. Wong, Catal. Today 160 (2011) 96. [35] K. Tedsree, T. Li, S. Jones, C.W.A. Chan, K.M.K. Yu, P.A.J. Bagot, E.A. Marquis, G.D.W. Smith, S.C.E. Tsang, Nature Nanotech. 6 (2011) 302. [36] D.A. Bulushev, S. Beloshapkin, J.R.H. Ross, Catal. Today 154 (2010) 7. [37] X.C. Zhou, Y.J. Huang, W. Xing, C.P. Liu, J.H. Liao, T.H. Lu, Chem. Commun. (2008) 3540. [38] X.J. Gu, Z.H. Lu, H.L. Jiang, T. Akita, Q. Xu, J. Am. Chem. Soc. 133 (2011) 11822. [39] Y.J. Huang, X.C. Zhou, M. Yin, C.P. Liu, W. Xing, Chem. Mater. 22 (2010) 5122. [40] D.A. Bulushev, L.J. Jia, S. Beloshapkin, J.R.H. Ross, Chem. Commun. 48 (2012) 4184. [41] B.L. Moroz, P.A. Pyrjaev, V.I. Zaikovskii, V.I. Bukhtiyarov, Catal. Today 144 (2009) 292. [42] P.E. Plyusnin, I.A. Baidina, Y.V. Shubin, S.V. Korenev, Russ. J. Inorg. Chem. 52 (2007) 371. [43] L.B. Okhlopkova, A.S. Lisitsyn, V.A. Likholobov, M. Gurrath, H.P. Boehm, Appl. Catal. A 204 (2000) 229. [44] W. Kraus, G. Nolze, PowderCell 2.4, Program for the Representation and Manipulation of Crystal Structures and Calculation of the Resulting X-Ray Powder Patterns, Berlin, 2000. [45] S. Krumm, Mater. Sci. Forum 183 (1996) 228. [46] Z.J. Li, F. Gao, Y.L. Wang, F. Calaza, L. Burkholder, W.T. Tysoe, Surf. Sci. 601 (2007) 1898. [47] F. Liu, D. Wechsler, P. Zhang, Chem. Phys. Lett. 461 (2008) 254. [48] M. Ojeda, E. Iglesia, Angew. Chem. Int. Edit. 48 (2009) 4800. [49] Q.Y. Bi, X.L. Du, Y.M. Liu, Y. Cao, H.Y. He, K.N. Fan, J. Am. Chem. Soc. 134 (2012) 8926. [50] Q. Fu, H. Saltsburg, M. Flytzani-Stephanopoulos, Science 301 (2003) 935. [51] M. Shekhar, J. Wang, W.S. Lee, W.D. Williams, S.M. Kim, E.A. Stach, J.T. Miller, W.N. Delgass, F.H. Ribeiro, J. Am. Chem. Soc. 134 (2012) 4700. [52] Z.J. Li, F. Gao, O. Furlong, W.T. Tysoe, Surf. Sci. 604 (2010) 136. [53] H.L. Abbott, A. Aumer, Y. Lei, C. Asokan, R.J. Meyer, M. Sterrer, S. Shaikhutdinov, H.J. Freund, J. Phys. Chem. C 114 (2010) 17099. [54] T.G. Owens, T.E. Jones, T.C.Q. Noakes, P. Bailey, C.J. Baddeley, J. Phys. Chem. B 110 (2006) 21152. [55] Y. Shubin, P. Plyusnin, M. Sharafutdinov, Nanotechnology 23 (2012) 405302. [56] D.L. Weissman-Wenocur, P.M. Stefan, B.B. Pate, M.L. Shek, I. Lindau, W.E. Spicer, Phys. Rev. B 27 (1983) 3308. [57] F. Gao, Y.L. Wang, D.W. Goodman, J. Phys. Chem. C 113 (2009) 14993. [58] D.V. Goia, J. Mater. Chem. 14 (2004) 451. [59] D.V. Goia, E. Matijevic, New J. Chem. 22 (1998) 1203. [60] B. Gleich, M. Ruff, R.J. Behm, Surf. Sci. 386 (1997) 48. [61] W. Olovsson, C. Goransson, L.V. Pourovskii, B. Johansson, I.A. Abrikosov, Phys. Rev. B 72 (2005). [62] T. Lear, R. Marshall, J.A. Lopez-Sanchez, S.D. Jackson, T.M. Klapotke, M. Baumer, G. Rupprechter, H.J. Freund, D. Lennon, J. Chem. Phys. 123 (2005) 174706. [63] D. Ferrer, A. Torres-Castro, X. Gao, S. Sepulveda-Guzman, U. Ortiz-Mendez, M. Jose-Yacaman, Nano Lett. 7 (2007) 1701. [64] M. Baumer, H.J. Freund, Prog. Surf. Sci. 61 (1999) 127. [65] F. Gao, Y.L. Wang, D.W. Goodman, J. Am. Chem. Soc. 131 (2009) 5734. [66] D. Teschner, A. Pestryakov, E. Kleimenov, M. Havecker, H. Bluhm, H. Sauer, A. Knop-Gericke, R. Schlogl, J. Catal. 230 (2005) 195. [67] G. Rupprechter, C. Weilach, Nano Today 2 (2007) 20. [68] I.V. Yudanov, K.M. Neyman, Phys. Chem. Chem. Phys. 12 (2010) 5094. [69] S. Zhou, C. Qian, X. Chen, Catal. Lett. 141 (2011) 726. [70] T. Zheng, D. Stacchiola, D.K. Saldin, J. James, D.S. Sholl, W.T. Tysoe, Surf. Sci. 574 (2005) 166. [71] P. Liu, J.K. Norskov, Phys. Chem. Chem. Phys. 3 (2001) 3814. [72] D.A. Bulushev, I. Yuranov, E.I. Suvorova, P.A. Buffat, L. Kiwi-Minsker, J. Catal. 224 (2004) 8.