An overview of the reaction conditions for an efficient photoconversion of CO₂

Carbon dioxide (CO₂) emission is one of the well-known causes of global warming. Photoconversion of CO₂ to useful chemical compounds using solar energy is an attractive approach as it reduces the major greenhouse gas and promises a sustainable energy source. This method involves radical-chain reactions that form cation and anion radicals generated as a result of the reaction with photogenerated electrons (e⁻) and holes (h⁺) between metal oxide photocatalyst and the reactants. Therefore, the product distribution of a modified photocatalyst even under specific reaction conditions is difficult to predict. The CO₂ photocatalytic reduction process is controlled by several conditions such as reactor configuration, photocatalyst type, and nature of the reducing agents. Here, we review the parameters such as temperature, pH, CO₂ pressure, type of reductant, role of co-catalysts, dopants, and type of photocatalysts that influence the end products of the photocatalytic reduction of CO₂. In this review, the different modifications recommended for the photocatalysts to improve CO₂ reduction and receive maximum valuable end product (methane, ethanol, methanol, hydrogen, and carbon monoxide) have been listed. The discussion also includes specific behaviors of photocatalysts which lead to different product distribution. It has been noted that different metal and nonmetal dopants improve the activity of a photocatalyst and influence the end product distribution by altering the active species. Similarly, the key factors, i.e. size, morphology and doping, which have been ruling the photocatalytic activity of CO₂ reduction under UV or visible light irradiation have been identified.