AB-INITIO AND SEMIEMPIRICAL STUDIES OF THE ADSORPTION AND DISSOCIATION OF WATER ON PURE, DEFECTIVE, AND DOPED MGO(001) SURFACES

Ab initio and semiempirical calculations of large cluster models have been performed in order to study water adsorption and dissociation on pure, defective (vacancies) and doped (Li, Na, K, Ca, Fe) MgO (001) su rfaces. The geometries of the adsorbed and dissociated molecules have been optimized preparatory to analysis of binding energies, stretching frequencies, charge transfers, preferential sites of interaction, and bond distances. We have used Mulliken, natural bond order, and electr ostatic-derived atomic and overlap populations to analyze charge distr ibutions in the clusters. We have also investigated transition structu res, activation energies, energy gaps, HOMO, density of states, SCF or bital energies as well as the acid-base properties of our cluster mode l. Numerical results are compared, where possible, with experiment, in terpreted in the framework of various analytical models, and correlate d with site coordination numbers, corner and edge site preferential lo cations, and direction of charge transfer. A thorough charge analysis indicates substantial charge redistribution in the magnesium oxide cry stal as a result of water adsorption and dissociation in pure, defecti ve, and doped MgO crystals. The introduction of heavier impurities and vacancies could produce substantial changes in the physical and chemi cal properties of the catalyst and increase the binding and dissociati on energies. Some of the largest changes originate from the introducti on of vacancies. Two and three-dimensional potential energy surfaces a re used to investigate activation energies of hydroxylation on the MgO surface. Stretching frequencies are correlated with magnesium and oxy gen coordination numbers. (C) 1998 American Institute of Physics.