Classification of reactive sites on the surface of polycrystalline tin dioxide

Abstract

A new method is introduced for study of the nature of reactive sites on oxide surfaces, which is derived from the study of gas sensors and is based on the correlation between combustion catalysis and the effects of gases on electrical conductivity at elevated temperature. Chemical modification of the surface, using a gaseous siloxane to react progressively with the surface sites, is employed to discriminate and classify different types of site. Both the electrical response and the combustion rate are altered by the titration of the surface sites against the siloxane. The method comprises the sequential application of test gas (hexamethyldisiloxane, HMDS, at low concentration in air) and probe gas (methane, water vapour), monitoring methane combustion rate, resistivity in pure, dry air and electrical response to methane and water vapour, as a function of HMDS dose. The measurements require the material to be fabricated into a gas sensor device having an array of electrodes designed to probe conductivity variations throughout the porous ceramic body, which allows the combustion rate and the electrical response to be obtained simultaneously. Four different types of reactive surface site have thereby been distinguished on polycrystalline SnO₂: an electrically neutral oxygen site which catalyses methane and HMDS combustion, two electrically charged oxygen sites which respectively mediate methane response and dissociative chemisorption of water, and a site for molecular chemisorption of water. By comparison with other literature, the neutral site which catalyses combustion could be attributed to a coordinatively unsaturated lattice oxygen. The two different electrically charged sites could be associated with oxygen adsorbed on or at two different types of surface oxygen vacancy associated with reduced (Sn²⁺) surface cations.