Kinetic Study of Heterogeneous Photocatalytic CO2 Reduction: Development of a General Formula for Relations between Activity and Reaction Conditions

Recently, the development of technologies for CO2 emission reduction has been extensively investigated to achieve the global 2 °C climate stabilization target. This study demonstrates that the photocatalytic activity and selectivity toward carbon monoxide (CO) evolution for the selective photocatalytic conversion of CO2 to CO, using H2O as an electron donor, are significantly dependent on the reaction conditions, such as additive concentration, CO2 partial pressure, and reaction temperature. The systematic experiments performed in this study revealed that the CO formation rate was proportional to the increase in the CO2 partial pressure and inversely proportional to the proton (H+) concentration in the reaction solution. Based on these results, a quantitative expression for the formation rate of CO with CO2 partial pressure and pH as variables was developed using kinetics, considering the equilibria of surface hydroxyl groups. This study confirmed that the derived mathematical expressions matched well with the experimental results of three different photocatalysts, whose activities and selectivities were completely different. Moreover, based on these expressions, a theoretical method is proposed to determine the kinetic constant and activation energy for the reduction of adsorbed CO2 molecules. The developed mathematical expression for photocatalytic CO2 conversation in water contributes to the development of CO2 recycling technologies.