Photoreduction of CO2 with H-2 over ZrO2. A study on interaction of hydrogen with photoexcited CO2

By the analysis of the kinetic isotope effect and the effect of reaction temperature, and by EPR, the reaction between hydrogen and photoexcited CO2 species in the photoreduction of CO2 by H-2 over ZrO2 was investigated. As the yield of products decreased when D-2 was used as a reductant instead of H-2, the rate-determining step of the reaction was associated with hydrogen. The reaction was enhanced at elevated temperature, suggesting that the reaction rate was strongly influenced by a thermal process. The EPR study showed the formation of the CO2- anion radical on irradiation of ZrO2 with adsorbed CO2. The CO2- radical had a long life and was stable in the dark for at least 60 min. However, on contact with hydrogen, the CO2- radical was suppressed within 30 min, indicating the reaction of CO2 with hydrogen in the dark. It was concluded that hydrogen was activated in the dark to react with the photoexcited CO2- radical.