COORDINATION CHEMISTRY OF NH3 ON ZNO(0001) AND CUCL(111) SURFACES - SIGMA-BONDING INTERACTIONS WITH D-10 METAL-ION SITES

NH3 adsorption on ZnO(0001) and CuCl(111) surfaces has been studied using a combination of ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and self-consistent-field-X-alpha-scattered wave (SCF-X-alpha-SW) electronic structure calcualtions. Both surfaces consist of coordinatively unsaturated tetrahedral d10 metal ion sites with their open coordination positions normal to the surface. Physisorption is dominant on both surface at high NH3 coverages. The NH3 3a1-1e orbital energy splitting at high ammonia coverages is 5.4 eV, which is the same as for gas-phase NH3. The physisorbed nitrogen 1s level was found at 405.0 eV for NH3/ZnO(0001) and at 405.8 eV for the NH3/CuCl(111) surface. At low coverages, chemisorbed molecular NH3 is found to dominate on both surfaces. The NH3 3a1, 1e and nitrogen 1s ionization energies were observed at 12.5, 16.5, and 403.0 eV below the vacuum level for ZnO(0001) and at 12.5, 15.5, and 403.5 eV for CuCl(111). The 3a1-1e splitting decreases to a greater extent for chemisorption to Cu(I) (2.4 eV) relative to Zn(II) (1.4 eV). Both surfaces show a decrease in the surface dipole moment upon NH3 absorption (-1.4 eV for ZnO(0001) versus -1.3 eV for CuCl(111), which indicates that charge is transferred from the NH3 3a1 molecular orbital to the surface. This is consistent with the net positive charge gained by the nitrogen atom as found from XPS core studies; the nitrogen atom donates 0.06 of a unit charge when coordinated to ZnO(0001) and 0.09 for CuCl(111). Heat of adsorption measurements for NH3 chemisorption find DELTA-H0 = 28 +/- 3 kcal/mol for ZnO(0001) and 24 +/- 3 kcal/mol for CuCl(111). The ionic contribution for NH3 binding was estimated to be 17 and 9 kcal/mole for ZnO and CuCl, respetively; thus, the covalent contribution to bonding is larger for Cu(I). SCF-X-alpha-SW calculations have been used to verify assignments in the UV photoelectron spectrum. The X-alpha calculations also show that ammonia has a greater covalent interaction with Cu(I) than with Zn(II) sites. Examination of the change in the atomic charge distribution summed over all valence levels induced upon NH3 binding shows that there is a net increase of 3.2% 4s and 13.2% 4p character for Cu(I), while Zn(II) has a net loss of 0.87% 4s and a small increase of 5.2% 4p character. The increased covalency for NH3 sigma bonding to Cu(I) relative to Zn(II) is discussed in terms of greater orbital overlap due to its reduced effective nuclear charge.