Chemisorption-induced structural changes and transition from chemisorption to physisorption in Au6(CO)n- (n=4-9)

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 Au-6(-) cluster in Au-6(CO)(n)(-) (n = 4-9). Previous studies have shown that the first three CO successively bind the three apex sites of the triangular Au-6(-). The current work reveals that the fourth 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 Au-6(CO)(6)(-), in which Au-6 has rearranged to accommodate the six CO adsorbates. The photoelectron spectra of larger clusters from Au-6(CO)(7)(-) to Au-6(CO)(9)(-) are observed to be almost identical to that of AU(6)(CO)(6)(-), suggesting that the additional CO units are simply physisorbed onto the Au-6(CO)(6)(-) core. Quasirelativistic density functional calculations are performed on both AU(6)(CO), and AU(6)(CO)(n)(-) (n = 4-6). The theoretical results are used to interpret the experimental observations and to provide insight into the nature of CO interactions with gold clusters. The Au-6 cluster is shown to be highly fluxional upon multiple CO adsorptions, stabilizing structures with more apex sites to accommodate the additional CO units. 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 may be important for understanding the mechanisms of CO oxidation on supported gold nanoparticles.