Surface stoichiometry of ‘titanium suboxide’ Part I Volumetric and FTIR study

Abstract

Reduction of titania (TiO₂-P25) with dry hydrogen leads to the formation of oxygen vacancies on the surface. The number of vacancies increases with increasing treatment time and with increasing hydrogen flow rate. These effects are due to a shift in equilibrium which occurs because under the conditions of higher flow rates and longer treatment times less water vapour is present in the hydrogen gas. If titania is reduced in hydrogen containing a systematically adjusted water vapour pressure a constant number of oxygen vacancies is soon attained. At reduction temperatures of 400 °C or lower the equilibrium number of oxygen vacancies depends on the water vapour pressure according to the reaction equation: Ti⁴⁺ + O²⁻+H₂ ⇌ H₂O + V_O + Ti³⁺+e⁻. The oxygen vacancies and the Ti³⁺ ions are most likely located on the titania surface. The electrons are delocalised in the bulk and occupy the conduction band thus causing decreased IR transmittance and increased electrical conductivity. The increase in the number of oxygen vacancies with temperature corresponds to a reaction enthalpy of 183 kJ mol⁻¹ for the reaction given above.

Upon hydrogen treatment above 400 °C the density of surface vacancies seems to reach saturation and the vacancies are then formed in the bulk. In this range the total number of oxygen vacancies is less affected by either the water vapour pressure or by the pretreatment temperature.

The number of vacancies can be related to the IR absorbance. Therefore, it should also be possible to determine the number of vacancies in titania supported metal catalysts, where their measurement by volumetric methods is difficult because of oxygen adsorption on the metal.