PNNL

Abstract

The interactions of CO with gold clusters are essential to understanding the catalytic mechanisms of CO oxidation on supported gold nanoparticles. Here we report a photoelectron spectroscopy and theoretical study of CO adsorption on a well-defined Au6- cluster in Au6(CO)n- (n = 4-9). Previous studies have shown that the first three CO successively bind the three apex sites of the triangular Au6-. The current work reveals that the forth CO induces a major structural change to create more apex sites to accommodate the additional CO. Definitive spectroscopic evidence is obtained for the chemisorption saturation at Au6(CO)6 -, in which Au6 has rearranged to accommodate the six CO adsorbates. The photoelectron spectra of larger clusters from Au6(CO)7 - to Au6(CO)9 - are observed to be almost identical to that of Au6(CO)6 -, suggesting that the additional CO’s are simply physisorbed onto the Au6(CO)6 - core. Quasirelativistic density functional calculations are performed on both Au6(CO)n and Au6(CO)n - (n = 4-6). The theoretical results are used to interpret the experimental observations and provide insight into the nature of CO interactions with gold clusters. The Au6 cluster is shown to be highly fluxional upon multiple CO adsorptions, stabilizing structures with more apex sites to accommodate the additional CO’s. The CO-induced structural transformation is analogous to structural flexibility and mobility in heterogeneous catalysis. The observations of the propensity of CO toward apex sites and CO-induced structural changes in small gold clusters m