VARIABLE PHOTON ENERGY PHOTOELECTRON SPECTROSCOPIC STUDY OF CO ADSORPTION TO COORDINATIVELY UNSATURATED TETRAHEDRON CU(I) AND ZN(II) SITES ON CUCL(111) AND ZNO(1010) SURFACES - D10 CONTRIBUTIONS TO CO BONDING AND ACTIVATION

Cu is an intracrystalline promoter for methanol synthesis catalyzed by ZnO. High-affinity CO bonding to Cu/ZnO has been shown (Didziulis, S. V.; Butcher, K. D.; Cohen, S. L.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 7110) to involve a coordinatively unsaturated tetrahedral Cu(I) site which is also present on the CuCl(111) model surface. CO is found to bind to the CuCl(111) surface nondissociatively with its molecular axis normal to the surface and with a high heat of adsorption of 23 +/- 2 kcal/mol, similar to that of CO on Cu/ZnO. Variable photon energy photoelectron spectroscopy (PES) has been used to investigate the nature of this bonding and to compare this to CO bonding to the coordinatively unsaturated Zn(II) site on ZnO(1010BAR). CO binding to CuCl is found to induce a shift of Cu 3d-pi levels to deeper binding energy and to result in the presence of intense CO core level satellites, both results indicating the presence of pi back-bonding of intermediate strength, which is not found to be present for CO bonding to ZnO. Differences in the sigma bonding interaction are indicated by the 2.2 vs 1.3 eV shift of the CO 5-sigma peak to deeper binding energy upon adsorption on CuCl vs ZnO. This however is complicated by the CO 5-sigma interaction with the fully occupied Cu 3d band, which is found to be present from the 5-sigma photoemission cross section and shifts the 5-sigma peak energy but does not contribute to the net bonding interaction. A more direct probe of sigma bonding is given by constant initial state (CIS) resonance PES studies at the metal 3p --> 4s transition edge which gives a 2.0 vs 0.8 eV shift in the M4s energy for CO adsorption on CuCl vs ZnO, demonstrating that sigma bonding is stronger for CO/CuCl than CO/ZnO. This decrease in the sigma and pi bonding of CO to Zn(II) as compared to Cu(I) sites is ascribed to orbital contraction and d band energy stabilization associated with the higher effective nuclear charge on Zn(II). From He I PES work function studies the surface dipole moment is found to decrease and from NEXAFS studies the C-O bond length shortens upon CO adsorption to both d10 metal ion surfaces, demonstrating that in contrast to CO adsorption on transition-metal surfaces sigma bonding plays a dominate role over pi back-bonding for these two d10 ions. Analysis of XPS and Auger results shows the presence of positive effective atomic charge on the carbon end of the adsorbed CO molecule on both CuCl and ZnO. This positive charge on the carbon results from the dominance of sigma donation and would activate the CO for nucleophilic attack by hydride. However the positive charge on carbon is found to be larger for Zn(II) than Cu(I), demonstrating that the additional promotional effect of Cu(I) is not electrostatic activation of CO but would appear to relate to the ability of Cu(I) to pi back-bond which would stabilize later reaction intermediates in the catalytic mechanism of methanol synthesis.