Photocatalytic reduction of CO₂: A brief review on product analysis and systematic methods

Photocatalytic reduction of CO₂ for fuels production by using solar energy is an attractive process to address both energy and environmental issues. Currently, it is still a great challenge to control the CO₂ reduction pathways. As a result, many possible chemicals could be present in the reaction products, ranging from CO, CH₄ to higher alkanes in the gas phase, and oxygenates in the liquid phase such as alcohols, aldehydes and carboxylic acids. A thorough and accurate determination of products is of critical importance for the evaluation of photocatalyst performance. However to date, only very few studies have paid attention to the screening of reduction products, and more comprehensive and standard analysis methods are not yet reported in the literature. In this review, different analysis methods reported so far are summarized. At the same time, we also developed one set of techniques involving (1) the design of a two-channel-three-detector gas chromatography system for gaseous product analysis in the presence of a high concentration of CO₂ and alcohols analysis in the liquid phase, and (2) a high performance liquid chromatography system for the analysis of aldehydes and carboxylic acids in the liquid phase. The effect of commonly used organic additives as well as alkaline conditions on liquid phase analysis was also investigated for the first time.