NH 3 chemisorption on stoichiometric and oxygen-deficient SnO 2(1 1 0) surfaces

Abstract

Ammonia chemisorption has been used as a probe of the electronic and acidic properties of Sn cations on well-defined SnO 2(1 1 0) surfaces. Thermal desorption spectroscopy and ultraviolet photoelectron spectroscopy are used to characterize the interaction of NH 3 at cation sites of different coordination. NH 3 heats of adsorption indicate that four-coordinate Sn 2+ cations at bridging oxygen vacancies are “more acidic” than five-coordinate Sn 4+ 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 SnO 2(1 1 0) 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 SnO 2(1 1 0) under similar conditions.