Temperature dependence of the photochemical reduction of CO2 in the presence of H2O at the solid/gas interface of TiO2

The photochemical reduction of CO2 at a TiO2 surface in the presence of H2O has been investigated. CO, CH4, H-2, and some higher hydrocarbons CnHm, 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 degrees 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 degrees C. During the course of the reaction E-a tends to increase which indicates that product desorption is the rate limiting step.