PNNL

Abstract

Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. The gold hexamer Au6 is a highly stable cluster, which is known to possess a D3h triangular ground state structure with an extremely large HOMO-LUMO gap. Here we report a photoelectron spectroscopy (PES) and quasi-relativistic density functional theory (DFT) study of Au6-CO complexes, Au6(CO)n? and Au6(CO)n (n = 0-3). CO chemisorption on Au6 was observed to be highly unusual. While the electron donor capability of CO is known to decrease the electron binding energies of Aum(CO)n? complexes, CO chemisorption on Au6 was observed to have very little effect on the electron binding energies of the first PES band of Au6(CO)n? (n = 1-3). However, the second PES band is significantly red-shifted upon successive CO chemisorption, resulting in a rapid closing of the HOMO-LUMO energy gap from 2.30 eV in Au6 to 1.72, 1.45, and 1.17 eV for Au6(CO)n (n = 1-3), respectively. Extensive DFT calculations showed that the first three CO successively chemisorb to the three apex atoms of the D3h Au6. It is shown that the LUMO (6a1’) and LUMO+1 (8e’) of Au6 are energetically near-degenerate, which are well separated from the HOMO (7e’), giving rise to the unusually large HOMO-LUMO gap in Au6. Upon CO chemisorption, the degeneracy of the HOMO and LUMO orbitals are both lifted, leading to a1+b2 components under C2v symmetry. In the Au6(CO)n complexes, one of the a1+ b2 components of the LUMO+1 orbital, which mainly involves the inner triangle of the Au6 motif, becomes the LUMO. Thus CO chemisorption on the apex Au sites (outer triangle) has little effect on this orbital, resulting in the roughly constant electron binding energies for the first PES band in Au6(CO)n? (n = 0-3). On the other hand, the a1+b2 components of the HOMO of Au6 are significantly destabilized through HOMO-LUMO mixing in Au6 and electron donation from the 5s orbital of CO, resulting in the smaller HOMO-LUMO gaps observed in the Au6(CO)n complexes.