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 exten ded X-ray absorption fine structure (EXAFS) spectroscopy and Au L-2,L-3 X-r ay absorption near-edge structure (XANES) spectroscopy. Pd-Au/SiO2 catalyst s with Pd/Au atomic ratios of approximately 1:1 were prepared by coadsorpti on of [Pd(NH3)(4)][NO3](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/Si O2 (1.3.1, pH 7) catalyst after in situ reduction at 300 OC indicates the f ormation 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 sp ectroscopy of the Pd-Au/SiO2 (1:1, pH 10) catalyst after in situ reduction at 350 degrees C indicates the formation of homogeneous Pd-Au alloy cluster s. 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 b ond distances are similar to 0.02 Angstrom shorter. Au L-2,L-3 XANES spectr oscopy of the supported Pd-Au clusters evidences a vanishingly small densit y of unoccupied Au 5d states. Excellent correspondence is observed between the XANES spectra of supported Pd-Au clusters and those of Pd-Au alloy film s having similar compositions.