| Cu(I) and Zn(II) sites have different electronic structures, and often show different activity in adsorption and catalysis. Chemisorption on these metal ion sites which in higher oxidation state are bound to anionic ligands can be fundamentally different from that which occurs on metals. Studies of the adsorption of catalytically relevant molecules to Cu(I) and Zn(II) on single crystal CuCl, Cu{dollar}sb2{dollar}O and ZnO surfaces are hence important. A variety of surface science techniques including variable energy PES, XPS, UPS, AES, LEED, CIS and NEXAFS are applied in these studies. It is found that {dollar}sigma{dollar} bonding is stronger for CO on CuCl(111) than ZnO(1010), and {dollar}pi{dollar} backbonding is negligible for CO/ZnO, but of intermediate strength for CO/CuCl. This decrease in the {dollar}sigma{dollar} and {dollar}pi{dollar} bonding of CO to Zn(II) vs. Cu(I) is ascribed to orbital contraction and d band stabilization associated with the higher effective nuclear charge on Zn(II). Decrease in the surface dipole moment and in the C-O bond length is found upon CO adsorption to both Cu(I) and Zn(II), demonstrating that {dollar}sigma{dollar} bonding dominates over {dollar}pi{dollar} backbonding for these ions. The experimentally determined positive charge on carbon is larger for Zn(II) than Cu(I), indicating that the additional promotional effect of Cu(I) is not electrostatic activation but related to the Cu(I)'s ability to {dollar}pi{dollar} backbond. The bonding between NH{dollar}sb3{dollar} and metal ions on CuCl(111) and ZnO(0001) is found to be derived from {dollar}sigma{dollar} covalent and electrostatic interactions, with the former stronger in NH{dollar}sb3{dollar}/CuCl(111) while the latter stronger in NH{dollar}sb3{dollar}/ZnO(0001). Studies of H{dollar}sb2{dollar}S adsorption on Cu{dollar}sb2{dollar}O(111), ZnO(0001) and ZnO(1010) reveal that the higher sensitivity of Cu-containing catalysts towards sulfur poisoning correlates to the higher reactivity of H{dollar}sb2{dollar}S to Cu(I) than Zn(II), which derives from a stronger bonding interaction which lowers the activation barrier for dissociative chemisorption. CO and O{dollar}sb2{dollar} adsorptions on Cu{dollar}sb2{dollar}O(111) are also studied. |
