A 5 8 1

Surface Science 223 (1989) 119-130 North-Holland, Amsterdam

C O O X I D A T I O N M E C H A N I S M S O V E R Z n O :

M O L E C U L A R O R B I T A L T H E O R Y

S . F . J E N a n d A l f r e d B. A N D E R S O N

Chemistry Department, Case Western Reserve University, Cleoeland, OH 44106, USA

Received 13 April 1989; accepted for publication 12 July 1989

119

An atom superposition and electron delocalization molecular orbital s tudy has been made of CO oxidation to CO z and the bonding of CO 2 to form surface carbonate on ZnO. The results provide interpretation for a number of experimental studies. CO reacts readily with O at the surface to form CO 2 which can immediately bind to 0 2- to form surface carbonate. The reaction with isolated O - has a higher barrier because when the O - - C O bond forms the electron is promoted to the surface conduction band. In this case CO 2 will dissociate from the surface, which stabilizes the promoted electron. When 2 O - are available, the formation of surface carbonate can proceed with the lower barrier. CO 2 can also bind strongly to 2-fold coordinated Zn ÷ and weakly to 3-fold coordinated Zn + sites as bent CO~-.

Surface Science 223 (1989) 131-150 North-Holland, Amste rdam

T H E A D S O R P T I O N O F W A T E R A N D H Y D R O X Y L O N N i ( l l l )

H o n g Y A N G a n d J e r r y L . W H I T T E N

Department of Chemistry, State University of New York at Stony Brook, Stony Brook, N Y 11794-3400, USA Received 11 April 1989; accepted for publication 20 July 1989

131

The adsorption of water and hydroxyl on the (111) surface of Ni is treated using a many-electron embedding theory to describe the electronic bonding, modelling the lattice as a 28-atom, three layer cluster. Ab initio valence orbital configuration interaction (multiple parent) calculations carried out on a local surface region permit an accurate description of bonding at the surface, Molecular H 2 0 adsorbed on the N i ( l l l ) surface is found to prefer an atop atom site with an adsorption energy of 12 kca l /mo l and a N i - O equilibrium distance of 2.06 ,~. The equilibrium geometry of H 2 0 is calculated to lie in a plane inclined by about 25 ° to the normal to the surface, but tilting the plane of the molecule from 0 ° to 50 o or rotating the molecule about the N i - O axis changes the energy only slightly. The OH radical binds strongly to the N i ( l l l ) surface at both three-fold and bridge sites with adsorption energies of 87 kca l /mo l and N i - O bond lengths from 2.02-2.08 ,~. The OH axis of adsorbed OH is inclined about 10 ° from the surface normal at a three-fold site. Dissociation of H 2 0 to OH and H adsorbed at nearby three-fold sites is exothermic, and for OH and H at a large distance of separation, the reaction H20(ads ) OH(ads) + H(ads) is 52 kca l /mo l exothermic. A high energy barrier is found at the initial stage of dissociation. The work function decreases by - 0.5 eV on H 2 0 adsorption and increases by - 0,2 eV on OH adsorption.