Sb(III) as a surface site for water adsorption on Sn(Sb)O2, and its effect on catalytic activity and sensor behavior

Surface segregation of Sb in polycrystalline Sn(Sb)O-2 is known to affect the rate of surface-catalyzed combustion of hydrocarbons and carbon monoxide over this material. This combustion rate is also known to be affected by the presence of water vapor. We show that Sb segregation to the surface as Sb(III) in Sn1-xSbxO2 (x = 0.005, 0.05), controllable by thermal treatment, strongly alters the effect of water vapor both upon the surface-catalyzed combustion rate of carbon monoxide and upon the elevated-temperature electrical conductivity. We suggest that the surface defect states which mediate both the electrical behavior and the surface-catalyzed combustion are best formulated as an association complex of an oxygen vacancy with Sn(II) or Sb(III), and are then able to propose a simple model which unifies the interpretation of the behavior of SnO2 as both a gas sensor and a combustion catalyst. We postulate: that a correct formulation of the "adsorbed oxygen" (O2-) surface species mediating the electrical response is an oxygen molecule trapped in or on a surface oxygen vacancy; that the combustion reaction proceeds partly through these species and partly through lattice oxygen at the surface; that water competes with oxygen for the surface vacancies, blocking this route; and that the binding energy of water to the Sb(III). V-o surface defect complex is less than that to the Sn(II). V-o complex.