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) n 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.