Gas-Phase Photocatalytic CO₂ Methanation over Ru/TiO₂: Effects of Pressure, Temperature, and Illumination

This work examines the effects of temperature (100–150 °C), pressure (1–25 bar), and UV illumination on gas-phase photocatalytic CO2 methanation over Ru/TiO2. Irradiating the catalyst enables an ∼20% decrease in operating pressure to maintain the same production rate. Furthermore, the relative light enhancement is greater at 100 °C than 150 °C─a phenomenon attributed to increased carrier recombination rates at higher temperatures. Arrhenius experiments indicate that the global activation energy (Ea) of the reaction is similar in the dark and light, suggesting that rate enhancements are determined by changes in surface coverage. To test this hypothesis, the CO2 reduction mechanism is probed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The spectra show an increase in the relative surface coverage of the intermediate formyl group under irradiation. These findings illustrate that, within the experimental conditions explored in this study, the photoexcitation of charge carriers in the semiconductor primarily influences the surface coverage of intermediates to drive enhanced methane formation at low temperatures.