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

The extended X-ray absorption fine structure (EXAFS) above the Pd K an d 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 a nd 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. Transmi ssion electron microscopy performed on a bimetallic sample (Pd:Au mole ratio equal to 60:40) exposed to air failed to detect any metal parti cles, indicating that their average diameter was less than 1 nm, which corresponds to similar to 100% metal exposed. The EXAFS for bimetalli c clusters (Pd:Au = 60:40) in vacuum at 77 K shows that Pd atoms decor ate a core of Au atoms. The Pd-Pd interatomic distance in the bimetall ic sample is the same as in bulk Pd, whereas the Au-Au interatomic dis tance is 3% shorter than that in bulk Au. The XANES associated with th e Au L(III) edge indicates that Au has an atomic-like electronic struc ture when alloyed with Pd, which is due to Au atoms having fewer like nearest neighbors in the clusters compared to bulk Au.