NH3 chemisorption on stoichiometric and oxygen-deficient SnO2(110) surfaces

Ammonia chemisorption has been used as a probe of the electronic and acidic properties of Sn cations on well-defined SnO2(110) surfaces. Thermal desorption spectroscopy and ultraviolet photoelectron spectroscopy are used to characterize the interaction of NH3 at cation sites of different coordination. NH3 heats of adsorption indicate that four-coordinate Sn2+ cations at bridging oxygen vacancies are "more acidic" than five-coordinate Sn4+ cations. The larger heat of adsorption is attributed to a greater covalent contribution to the chemisorption bond. The introduction of in-plane oxygen vacancies reduces the apparent Lewis acidity of neighboring cations. This phenomenon is attributed to an inductive effect from the electronic charge around the in-plane oxygen vacancies. The apparent Lewis acidity of SnO2(110) goes through a maximum as the surface condition changes from stoichiometric to oxygen deficient. This trend matches one observed for the activity of methanol dissociation on SnO2(110) under similar conditions. (C) 2002 Published by Elsevier Science B.V.