Temperature dependence of the photochemical reduction of CO2in the presence of H2Oat the solid/gas interface of TiO2

Abstract

The photochemical reduction of CO₂ at a TiO₂ surface in the presence of H₂O has been investigated. CO, CH₄, H₂, and some higher hydrocarbons C_nH_m, attributed to C₂H₆, could be detected as photoproducts by mass spectrometry. No oxidation products of H₂O could be found. The temperature dependence of the reaction was investigated between 25 and 200 °C. The rate of the reaction increases with increasing temperature indicating that a thermal step is rate limiting in this temperature range. A model comprising the heat of adsorption of reactants and products is used to calculate an apparent activation energy (E_a) for the CH₄ formation. At room temperature a value of ca. 2 kJ mol⁻¹ is found for E_a. This value decreases to almost zero at 200 °C. During the course of the reaction E_a tends to increase which indicates that product desorption is the rate limiting step.