Co-adsorption on metal-oxide crystal surfaces - Cases of CO/Cu/ZnO(0001) and CO2/Na/TiO2(110)

Coadsorption on metal-oxide crystal surfaces is discussed for two model catalyst examples in which a (CO2-) complex is involved: 1) Adsorption of CO onto Cu clusters and thin films supported by a thermally etched (0001) surface of a ZnO single crystal and 2) Adsorption of CO2 onto Na clusters and thin films supported by 1x1- and 1x2-terminated (110) surfaces of a TiO2 single crystal. In the former case we will apply electron spectroscopy results from adsorption of CO on clean and Cu-deposited ZnO(0001) surfaces to discuss evidence for formation of a (CO2-) chemisorption complex and for Cu oxidation during CO exposure. In the latter case we will apply synchrotron-radiation-induced core-level-, valence-band and resonant photoemission results from adsorption of CO2 on a 1x1 truncated and on a 1x2 reconstructed TiO2(110) surface to discuss the difference in Na adsorption behavior of the two surfaces (adsorption onto two different sites) and the strong reactivity towards CO2 when submonolayer quantities of Na are preadsorbed. Valence-band data shows formation of carbonate. At low coverage the presence of a (CO2-) complex suggests a 2CO(2)(-) --> CO(3)2 + CO reaction.