Unraveling Photocatalytic CO2 Reduction Mechanisms via In Situ/Operando Characterization: From Dynamic Active Sites to Reaction Pathways.

Solar-driven photocatalytic CO2 reduction converts greenhouse gas into carbonaceous fuels and chemicals, offering a green route to address fossil energy shortages and advance carbon neutrality. However, the field faces core challenges of narrow visible-light response, low quantum efficiency, and poor product selectivity. The fundamental bottleneck lies in ambiguous multi-interface coupled reaction mechanisms. Conventional offline static characterization only captures pre- and post-reaction snapshots, failing to establish correlations among catalyst structure, process behavior, and catalytic performance, thus impeding rational catalyst design. Accurate dynamic information across the entire reaction chain is critical, including atomic-scale active-site evolution, cross-interface photogenerated carrier transport, and molecular-level reactant adsorption/activation along with key intermediate dynamics. Achieving this requires in-situ/operando characterization. This review systematically summarizes recent in-situ techniques applied to photocatalytic CO2 reduction, clarifying their principles, detection limits, and applicable scenarios. Their application value is highlighted across three dimensions: (1) real-time tracking of catalyst dynamic structural evolution (e.g., phase reconstruction, defect dynamics, valence transitions) under operating conditions; (2) quantitative elucidation of photogenerated carrier separation, migration, and recombination, identifying factors governing interfacial charge transfer; (3) accurate identification of adsorption configurations and transformation pathways of intermediates (*COOH, *CO, *CHO), providing direct experimental evidence for mechanistic deduction. Finally, common limitations of current in-situ techniques are outlined, and future directions, including multi-technique combinations and integration with theoretical calculations, are proposed.