Unique CO Chemisorption Properties of Gold Hexamer: Au₆(CO)n⁻(n = 0-3)
Elucidating the chemisorption properties of CO on gold clusters is essential to understanding the catalytic mechanisms of gold nanoparticles. The gold hexamer Au? is a highly stable cluster, which is known to possess a D?h 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 Au?-CO complexes, Au?(CO)n? and Au?(CO)n (n = 0-3). CO chemisorption on Au? 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 Au? was observed to have very little effect on the electron binding energies of the first PES band of Au?(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 Au? to 1.72, 1.45, and 1.17 eV for Au?(CO)n (n = 1-3), respectively. Extensive DFT calculations showed that the first three CO successively chemisorb to the three apex atoms of the D?h Au?. It is shown that the LUMO (6a??) and LUMO+1 (8e?) of Au? are energetically near-degenerate, which are well separated from the HOMO (7e?), giving rise to the unusually large HOMO-LUMO gap in Au?. Upon CO chemisorption, the degeneracy of the HOMO and LUMO orbitals are both lifted, leading to a?+b? components under C?v symmetry. In the Au?(CO)n complexes, one of the a?+ b? components of the LUMO+1 orbital, which mainly involves the inner triangle of the Au? 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 Au?(CO)n? (n = 0-3). On the other hand, the a?+b? components of the HOMO of Au? are significantly destabilized through HOMO-LUMO mixing in Au? and electron donation from the 5? orbital of CO, resulting in the smaller HOMO-LUMO gaps observed in the Au?(CO)n complexes.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15020826
- Report Number(s):
- PNNL-SA-45651; 3227a; KP1301030; TRN: US200521%%274
- Journal Information:
- Journal of the American Chemical Society, Vol. 127, Issue 34
- Country of Publication:
- United States
- Language:
- English
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