J. Phys. Chem. C 2007, 111, 9761-9768
9761
Electronic and Geometric Structures of Supported Platinum, Gold, and Platinum-Gold Catalysts Eveline Bus and Jeroen A. van Bokhoven* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland ReceiVed: NoVember 9, 2006; In Final Form: April 18, 2007
X-ray absorption spectroscopy (XAS) on the Pt and Au L3 edges showed that the structure of platinum and gold nanoclusters differs from that of the bulk metals. The interatomic distances are contracted and the s, p, and d orbitals rehybridize, which results in less filled 5d valence bands. The XAS spectra of the monometallic clusters were used to interpret the effect of alloying platinum and gold. Preparation of SiO2- and TiO2supported PtAu catalysts from a Pt2Au4(CtCBut)8 precursor resulted in well-mixed bimetallic clusters. Gold was preferentially located on the surface of the small clusters. X-ray absorption near-edge spectroscopy (XANES) showed that the electronic structure of platinum and gold differed from that in the monometallic clusters. The Fermi levels of platinum and gold shifted in opposite directions: the position of the Pt L3 edge shifted to higher energy and the Au L3 edge shifted to lower energy. Furthermore, the white-line intensity increased for gold and decreased for platinum. These changes originate from interatomic charge transfer from the Au 5d to the Pt 5d band, accompanied by intra-atomic charge redistributions. Charge transfer affected the energy positions of the 5d valence bands. Analysis of the XANES spectra, aided by full multiple-scattering calculations, revealed that the changes in the electronic structures are due to alloying and distance effects.
1. Introduction Heterogeneous binary alloy catalysts often have different catalytic performances than either of the metal components. PtSn/SiO2, for example, selectively produces lactic acid from CO2 and ethylene in the presence of water, while Pt/SiO2 does not.1 Bimetallic supported platinum-palladium catalysts are more active in the hydrogenation of aromatics in the presence of sulfur2,3 and in the hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene4 than the combination of their monometallic constituents. Gold alloyed into the surface layer of a nickel catalyst improves the catalytic performance in steam reforming.5 Addition of gold to bulk platinum leads to a decrease in the activity for hydrocarbon isomerization and hydrogenolysis with increasing gold content, whereas the dehydrogenation activity increases.6-11 The same was found for PtAu sols on carbon.11 Other authors reported an increase in the skeletal isomerization selectivity in the methylcyclopentane conversion at the expense of hydrogenolysis.12 In the methylcyclohexane conversion, the addition of gold to platinum enhanced the overall activity and the selectivity toward toluene.13 Chandler et al. were the first to prepare bimetallic supported PtAu nanoclusters from a precursor containing both platinum and gold, namely, Pt2Au4(CtCBut)8.14 They observed enhanced cracking activity and a decrease in the rate of skeletal isomerization reactions in the conversion of hexane. The platinum-gold clusters were more resistant to deactivation than pure platinum clusters. In most studies on supported platinum-gold catalysts, gold is considered an inactive additive and the explanations of the (catalytic) behavior are focused on platinum. However, gold nanoclusters on porous supports are catalytically active.15,16 Thus, gold also requires attention. Moreover, many authors * To whom correspondence should be addressed. Phone: ++41-446325542; fax: ++41-44-6321162; e-mail:
[email protected].
attribute the influence of gold on the catalytic performance of platinum solely to a geometric, or ensemble-size, effect.17-19 Since it has been proposed that the strength of the adsorbate bonding and thus the catalytic activity depends on the position of the d band with respect to the Fermi level,20,21 electronic and ligand effects cannot be ruled out. In-situ X-ray absorption spectroscopy (XAS) can be used to study the structure of materials under various conditions. The first intense feature in the L edges is called the white line and is part of the X-ray absorption near-edge structure (XANES), which gives information on the electronic structure. The white line in the XANES spectra of the Pt and Au L2 and L3 edges probes the shape and electron density of the 5d valence band. Charge redistribution occurs when alloying metals.22 Electrons are transferred from the metal with the more occupied valence band to the metal with the less filled band, in this way stabilizing the bimetallic bond. Intra-atomic charge redistribution between the s, p, and d levels also occurs. Because of the charge redistribution, the position of the valence bands with respect to the Fermi level shifts and the bands broaden or narrow.22 XANES spectroscopy is useful to study the charge redistribution in supported bimetallic clusters.23-25 Probing the L edges directly detects the number of holes in the d band.26-30 The extended X-ray absorption fine structure (EXAFS) part of the XAS spectrum provides information on the geometric structures such as coordination numbers and interatomic distances. We studied PtAu clusters with a size between 1 and 3 nm supported on SiO2 and TiO2 with electron microscopy and XAS and compared the physical properties to those of monometallic platinum and gold clusters of similar size on the same supports. To obtain a better understanding of the XANES spectra, the L3 near-edge structures and electron configurations of platinum and gold atoms in mono- and bimetallic clusters were simulated using the FEFF8 code, which reproduces the L3 XAS spectra
10.1021/jp067414k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/14/2007
9762 J. Phys. Chem. C, Vol. 111, No. 27, 2007 of small metal clusters well.31 The aim of this work is to establish the geometric and electronic structures of the supported platinum-gold clusters. 2. Experimental Methods PtAu/SiO2 and PtAu/TiO2 were prepared by wet impregnation using Pt2Au4(CtCBut)8 as the metal precursor. This organometallic complex was synthesized using a multistep procedure32 starting with tetrahydrothiophen (Merck, 821095.0005), HAuCl4 (abcr, S79-0500, 99.9%, 49% Au), 3,3-dimethyl-1-butyne (Aldrich, 244392), and PtCl2 (Johnson Matthey Chemicals Ltd.). The 0.60 g of Pt2Au4(CtCBut)8 in 100 mL hexane was added to a slurry of 13 g SiO2 (Merck 60, pore volume 0.74 mL/g, specific surface area 460 m2/g) in 40 mL hexane. The PtAu complex adsorbed directly on the SiO2, which was subsequently dried under vacuum at 298 K. The metal loading was 2 wt % Au and 1 wt % Pt. For PtAu/TiO2, 0.31 g Pt2Au4(CtCBut)8 in 135 mL hexane was added to 6.7 g TiO2 (Degussa P25, 0.22 mL/g, 50 m2/g). The entire amount of metal complex was not adsorbed on the TiO2. The metal loading was 1.4 wt % Au and 0.7 wt % Pt. Metallic PtAu clusters were obtained after calcination in air at 573 K and reduction in H2 at 473 K. Prior to use in the XAS experiments, the PtAu catalysts were reduced again in situ at 473 K. The 1 wt % Pt/SiO2 was prepared by adding an aqueous solution of tetraammine platinum nitrate (PTA) to a slurry of SiO2 (Davison 644, pore volume 1 mL/g, specific surface area 290 m2/g) in an NH4OH solution with a pH of 10. After adsorption of the PTA, the powder was filtered, was washed, was dried at 373 K, and was prereduced at 423 K. The 1.4 wt % Pt/TiO2 was prepared by adding TiO2 to an aqueous solution of H2PtCl6 (Fluka, 81080, 38% Pt) with pH 4.5 and by stirring the slurry for 2 h at 343 K. The resulting paste was filtered and washed with warm water, was dried under vacuum at 298 K, and subsequently was prereduced at 473 K. The supported platinum catalysts were reduced in situ at 473 K prior to use. The 3 wt % Au/SiO2 was prepared by incipient-wetness impregnation of SiO2 (Aerosil 300, 300 m2/ g) with an aqueous solution of HAuCl4 (pH ) 1). After impregnation, the sample was washed using aqueous ammonia (pH ) 11) and deionized water and was dried under vacuum at 298 K for 2 h. Prior to use, Au/SiO2 was reduced in situ at 473 K. The 2 wt % Au/TiO2 was prepared by depositionprecipitation.33 TiO2 (P25, Degussa) was added to an aqueous solution of HAuCl4 with a pH of 7. The slurry was stirred for 1 h at 343 K. The resulting paste was filtered, was washed with warm water, and was dried under vacuum at 298 K. Prior to use, Au/TiO2 was reduced in situ at 373 K. Elemental analysis reported a chlorine content in the gold catalysts below the detection limit (
