Bimetallic Gold/Palladium Catalysts: Correlation between Nanostructure and Synergistic Effects Di Wang,��� Alberto Villa,���,�� Francesca Porta,��� Laura Prati,��� and Dangsheng Su*,��� Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany, and Dipartimento di Chimica Inorganica Metallorganica e Analitica e ISTM, Centre of Excellence CIMAINA UniVersita ` di Milano, Via Venezian 21, I-20133 Milano, Italy ReceiVed: January 28, 2008 ReVised Manuscript ReceiVed: March 19, 2008 Au nanoparticles are known to be a good catalyst or an effective promoter for a wide range of catalytic reactions. Bimetallic Au-Pd nanoparticles supported on activated carbon were synthesized following a two- step procedure: immobilization of Au sol onto activated carbon followed by immobilization of Pd(0). The catalysts showed superior activities compared to monometallic Pd or Au nanoparticles on the same support. A series of catalysts with Au:Pd ratios varying from 9.5:0.5 to 2:8 were prepared. These catalysts were characterized by TEM, HRTEM, EDX, and X-ray mapping techniques to obtain morphological information, particle size distributions, crystalline structure, and distribution of the two metals. Correlating with the result from catalytic tests of selective oxidation of glycerol to glyceric acid, we found that the surface configuration of Pd monomers isolated by Au atoms has a substantial effect on activity and stability. The Au:Pd ratio on the surface of the particles is the key parameter and can be finely tuned to achieve optimal catalytic performance. The segregation or inhomogeneity of Pd weakens the synergistic effect of the bimetallic catalyst. 1. Introduction The oxidation of alcohols to carbonyl compounds is an important task in fine chemical industry. Traditional methods for the oxidation of alcohols are based on the use of stoichio- metric amounts of either inorganic or organic oxidants.1���3 These methods lead to environmental and economical problems due to disposal of large amounts of byproduct. An alternative more environmentally friendly method is to use molecular oxygen or hydrogen peroxide4,5 in the presence of a catalyst, the main byproduct being water. It is of particular interest in catalytic oxidation of some polyalcohols like sorbitol and glycerol since they represent important chemical intermediates that can be derived from biomass resources. In the case of heterogeneous catalysts, Pt/C and Pd/C have been reported to be active in the liquid phase oxidation of alcohols,6���10 but they suffer from severe deactivation due to overoxidation and poisoning by (by)-products.11 Recently, supported gold nanoparticles have attracted more and more interest due to their special activity in catalytic reactions such as low-temperature oxidation of CO, hydrochlorination of alkyne, liquid phase oxidation of alcohols and polyols, etc.12 The addition of Au to Pd or Pt catalysts in the liquid phase oxidation of polyols (sorbitol and glycerol) in the presence of O2 under mild conditions ( 60 ��C and 4 atm) does not only improve catalytic activity and selectivity to the desired product, but also enhances the resistance to poisoning.13,14 However, most syntheses result in a mixture of monometallic and bimetallic particles, thus complicating the understanding of Au-metal synergistic effects. To avoid any segregation of metals and to obtain uniformly alloyed, single-phase homoge- neously dispersed bimetallic Au-Pd catalysts supported on activated carbon a two-step synthesis procedure was employed. The catalysts were tested in selective oxidation of glycerol to glyceric acid.15 The enhanced performance can directly be attributed to the synergistic effects of Au-Pd alloying. One goal of this study is to control the formation of alloyed particles thereby tuning the nanostructure of the particles to obtain the optimum catalytic properties. In this paper, we report further studies on a series of Au/Pd catalysts with different Au:Pd ratios (9.5:0.5, 9:1, 8:2, 6:4, and 2:8), tested in selective oxidation of glycerol to glyceric acid.16 While the catalytic performance has been reported in detail elsewhere, the present paper focuses on correlations between the nanostructure of the bimetallic catalysts and the synergistic effects. The series of catalysts are character- ized by various TEM techniques to obtain information on particle size and shape, spatial distribution of the metals, and the crystal structure of the particles. The difference in structures of the catalysts can be directly correlated with their catalytic behavior. 2. Experimental Section To prepare Au/Pd bimetallic particles supported on activated carbon from Camel (X40S SA ) 900-1100 m2/g PV ) 1.5 mL/g pH 9-10), a gold sol (obtained by NaBH4 reduction of a AuCl4- solution in the presence of PVA as protective agent) was immobilized on activated carbon. The preformed Au particles act as nucleation centers for the subsequent im- mobilization of Pd(0) (using H2 as reducing agent of a Pd(II) salt) thus forming a bimetallic system. The total metal loading is 1 wt % for all the catalysts. More details on the preparation method can be found in ref 15. The reactions were carried out in a thermostated glass reactor (30 mL) equipped with an electronically controlled magnetic stirrer connected to a large reservoir (5000 mL) containing * Corresponding author. Phone: +49-30-8413-4464. Fax: +49-30-8413- 4405. E-mail: dangsheng@fhi-berlin.mpg.de. ��� Fritz Haber Institute of the Max Planck Society. ��� Centre of Excellence CIMAINA Universita ` di Milano. �� Current address: Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany. J. Phys. Chem. C 2008, 112, 8617���8622 8617 10.1021/jp800805e CCC: $40.75 ��� 2008 American Chemical Society Published on Web 05/16/2008

oxygen at 1.5 atm. The oxygen uptake was followed by a mass flow controller connected to a PC through an A/D board, plotting flow vs time. Alcohol, 0.3 M, and the catalyst (substrate/metal ) 1000 mol/mol) were mixed in distilled water (total volume 10 mL). The reactor was pressurized at the desired pressure of O2 and thermostated at 50 ��C. The reaction was initiated by stirring. The identification and analysis of the products were performed on a Varian 9010 HPLC equipped with a Varian 9050 UV (210 nm) and a Waters R.I. detector in series. An Alltech OA-10308 column (300 mm �� 7.8 mm) was used with aqueous H3PO4 0.1% wt/wt M (0.5 mL/min) as the eluent. Morphology and microstructures of the catalysts were char- acterized in a Philips CM200 FEG electron microscope, operating at 200 kV and equipped with a Gatan imaging filter, GIF100. Powder samples of the catalysts were ultrasonicated in ethanol and dispersed on copper grids covered with a holey carbon film. Metal distribution analysis with a resolution of a few nanometers was performed in the STEM mode in combina- tion with energy dispersive X-ray spectroscopy (EDX) using a DX4 analyzer system (EDAX) in the same microscope. 3. Results 3.1. Catalytic Activity. The activities and the selectivities of the catalysts are shown in Figure 1, compared to the pure Pd or Au reference catalysts.16 With an increase of the Au fraction, the activity increases distinctly and reaches a maximum for Au: Pd ) 9:1. For Au:Pd ) 9.5:0.5 and pure Au, it drops drastically. The highest selectivity was observed for pure Pd catalysts, though it deactivated quickly and the conversion only reached 50%. The behavior was similar for Au:Pd ) 2:8, showing high selectivity but rapid deactivation. For the other catalysts, the selectivities were reported at 90% conversion and only decreased slightly with increasing Au content. This means that Au-Pd bimetallic catalysts with a certain range of Au:Pd ratio (from about 1:1 to 9:1 molar ratio) exhibit high activity as well as selectivity. To exclude any additional AC effect, a blank experiment has been carried out with pure AC instead of M/AC catalyst under the same conditions. After 2 h of reaction no conversion of glycerol was detected. 3.2. Morphological Characterization. The microstructure of the five catalysts was studied by SEM, TEM, STEM, and EDX. From overview TEM images, the metal particles for all five bimetallic catalysts are homogeneously immobilized on the activated carbon support. The particle size distribution for each catalyst was determined by measuring the mean diameter of over 300 particles from different areas (Figure 2). Each size distribution can be fitted by a log-normal function, indicating that coalescence to some extent takes place during synthesis processes.17 The obtained statistical median value of particle size and the standard deviation for all the catalysts are listed in Table 1. For Au:Pd equal to 9.5:0.5, 9.1, and 8:2, the catalysts show a similar size distribution while for Au:Pd equal to 6:4 and 2:8, the mean particle size is larger and the size distribution is broader than those for the catalysts with higher Au content. Some areas of the Au:Pd equal to 2:8 catalyst show a particle morphology (Figure 3a) differing from the typically observed morphology (Figure 3b) of all the catalysts. These areas were omitted for particle size analysis. The particles in Figure 3a are more irregularly shaped. The corresponding EDX spectrum indicates that the area is Pd-rich. From the analysis of morphol- ogy and particle size, it is clear that Au seeds formed in the first synthesis step play an important role in controlling the final particle morphology and size. With an increase in the Pd amount, there are not enough Au seeds so that some Pd segregation may take place, resulting in large, irregularly shaped Pd-rich particles. 3.3. Structural Characterization. For each catalyst, high- resolution transmission electron microscopy (HRTEM) images were taken from several tens of particles of different size. The representative particle structure is determined from particles with diameters ranging from 2 to 4 nm, which was most frequently observed. Except for some extremely small ( 1 nm) clusters, which show complicated structures, most of the particles are multiply twinned in the form of decahedra or truncated decahedra. In this configuration, the exposed surfaces are mainly low surface energy {111} facets and to a lesser extent {200} facets. The boundary energy introduced by multiply twinning is negligible so that the system energy is a minimum. For the catalysts with Au:Pd ratios of 9:1, 8:2, and 6:4, the HRTEM images reveal uniform lattice spacings between the Pd (111) plane (2.25 ��) and the Au (111) plane (2.35 ��), which implies the alloying state, in good agreement with Vegard���s law.18,19 A representative image is shown in Figure 4a. A twin boundary can be seen through the middle of the particle. The fast Fourier transforms (FFT) of the parts at both sides of the twin boundary are also inserted in Figure 4a. Two sets of (111) reflections are visible in both FFT maps. The angle between the two sets of (111) planes in both FFT maps is ca. 56�� and not ca. 71�� as observed in an fcc single crystal. Such a configuration of two sets of (111) planes is produced by two overlapping fcc lattices forming a twin boundary in between. On the basis of the HRTEM images, a multiply twinned structure taking the shape of truncated decahedra is suggested. A model of the structure along the 5-fold symmetric axis is shown in Figure 4b (top left). In Figure 4b (top right), the same particle is tilted to a specific orientation to give the projection analogous to the experimental HRTEM image. HRTEM images are simulated for a series of focus values in this projection. The simulation with a defocus value of -130 nm shown in Figure 4b (bottom) is in good agreement with the contrast variations observed in the experimental image. The catalyst with Au:Pd ) 9.5:0.5 consists of mainly small particles as well as some big irregularly shaped particles ( 10 nm). Usually the small particles are Au-rich and the lattice spacings are also close to that of Au (111) planes. An irregularly shaped large particle is shown in Figure 5a. In this HRTEM image, the lattice spacing Figure 1. Activity and selectivity of the series bimetallic catalysts with different Au:Pd ratios in comparison with pure Au and Pd catalysts. 8618 J. Phys. Chem. C, Vol. 112, No. 23, 2008 Wang et al.