Kinetic Isotope Effects in CO 2 Reduction: Insights Into Reaction Mechanisms

Electrochemical and photochemical carbon dioxide reduction reactions (CO 2 RR) represent pivotal strategies for converting CO 2 into valuable chemicals, yet their multistep pathways and transient intermediates complicate the identification of the rate‐determining step (RDS). Kinetic isotope effects (KIEs) provide bond‐sensitive evidence by revealing how isotopic substitution changes reaction rates. This Perspective critically examines the application of KIEs to investigate CO 2 activation, C─H bond formation, C─C coupling, water activation, and interfacial hydrogen transfer. Across these processes, normal, inverse, and near‐unity KIEs, when combined with spectroscopy and theoretical calculations, help distinguish competing elementary steps and reaction pathways. Isotope‐dependent product abundances further differentiate Langmuir–Hinshelwood and Eley–Rideal hydrogen transfer pathways and reveal how catalyst structure and interfacial environments regulate reaction pathways and selectivity. Finally, we discuss advances in isotope‐resolved operando detection, broader application of isotope analysis, and closer integration with theoretical calculations as opportunities to strengthen mechanistic analysis of CO 2 RR.