STRUCTURE OF SUPPORTED PDAU CLUSTERS DETERMINED BY X-RAY-ABSORPTION SPECTROSCOPY

The extended X-ray absorption fine structure (EXAFS) above the Pd K and Au L(III) edges and the X-ray absorption near edge structure (XANES) associated with the Au L(III) edge were used to study the structure and local electronic environment of PdAu bimetallic clusters supported on silica. Four samples with total metal loadings of approximately 100 mu mol g(-1) and varying PdAu compositions were investigated. Transmission electron microscopy performed on a bimetallic sample (Pd:Au mole ratio equal to 60:40) exposed to air failed to detect any metal particles, indicating that their average diameter was less than 1 nm, which corresponds to similar to 100% metal exposed. The EXAFS for bimetallic clusters (Pd:Au = 60:40) in vacuum at 77 K shows that Pd atoms decorate a core of Au atoms. The Pd-Pd interatomic distance in the bimetallic sample is the same as in bulk Pd, whereas the Au-Au interatomic distance is 3% shorter than that in bulk Au. The XANES associated with the Au L(III) edge indicates that Au has an atomic-like electronic structure when alloyed with Pd, which is due to Au atoms having fewer like nearest neighbors in the clusters compared to bulk Au.