Heterogeneous photocatalysis and electrocatalysis have attracted tremendous interest in energy chemistry including CO2 reduction, N2 fixation, water oxidation, oxygen reduction, organic transformations, etc. The molecular-level understanding of reaction mechanisms in photocatalysis and electrocatalysis is pivotal to facilitate these processes, and the quantitative isotope measurement provides such a toolbox. Herein, we summarize the theory of two kinds of important isotope measurements (i.e., kinetic isotope effect and isotope labeling methods) and then review their important applications in photocatalytic/electrocatalytic CO2 reduction, N2 fixation, water oxidation, oxygen reduction and organic transformations on heterogeneous catalysts. Future prospects for quantitative isotope measurements are also presented.
