Photoreduction of CO2 with H2 over ZrO2. A study on interaction of hydrogen with photoexcited CO2

Abstract

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 H₂ over ZrO₂ was investigated. As the yield of products decreased when D₂ was used as a reductant instead of H₂, 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.