Characterization of Silica-Supported Pd−Au Clusters by X-ray Absorption Spectroscopy

Silica-supported Pd−Au clusters were characterized by Pd K and Au L 3 extended X-ray absorption fine structure (EXAFS) spectroscopy and Au L 2,3 X-ray absorption near-edge structure (XANES) spectroscopy. Pd−Au/SiO 2 catalysts with Pd/Au atomic ratios of approximately 1:1 were prepared by coadsorption of [Pd(NH 3 ) 4 ][NO 3 ] 2 and [Au(en) 2 ]Cl 3 (en = ethylenediamine) onto silica gel from pH 7 and pH 10 slurries. EXAFS spectroscopy of the Pd−Au/SiO 2 (1.3:1, pH 7) catalyst after in situ reduction at 300 °C indicates the formation of bimetallic clusters exhibiting Pd surface enrichment. The EXAFS results can be approximated using a cluster “decoration” model in which a Au-rich core is covered by a partial monolayer of Pd. In contrast, EXAFS spectroscopy of the Pd−Au/SiO 2 (1:1, pH 10) catalyst after in situ reduction at 350 °C indicates the formation of homogeneous Pd−Au alloy clusters. The homonuclear bond distances in these clusters agree closely with the Vegard's Law prediction for a 1:1 Pd−Au alloy; however, the heteronuclear bond distances are ∼0.02 Å shorter. Au L 2,3 XANES spectroscopy of the supported Pd−Au clusters evidences a vanishingly small density of unoccupied Au 5d states. Excellent correspondence is observed between the XANES spectra of supported Pd−Au clusters and those of Pd−Au alloy films having similar compositions.