Kinetic Isotope Effects as Mechanistic Probes in Heterogeneous Catalysis: From Thermo- and Electro- to Photocatalytic Reactions

Heterogeneous catalysis underpins modern chemical manufacturing and emerging energy technologies, yet rational catalyst design still hinges on identifying the elementary steps that control rate and selectivity. Kinetic isotope effects (KIEs), the changes in rate induced by isotopic substitution, provide a sensitive kinetic window into transition-state structure and isotope-sensitive motions along catalytic reaction coordinates. This review summarizes how experimentally measured KIEs have been used to interrogate mechanisms in thermocatalysis, electrocatalysis, and photocatalysis, with representative cases where isotope substitution helps differentiate parallel pathways, reveal kinetically relevant proton/electron-transfer steps, or diagnose condition-dependent shifts in kinetic bottlenecks. We further compare how KIE signatures differ across thermal, electrochemical, and photochemical environments and summarize best practices for robust experimental design, quantitative modeling, and corroborative mechanistic validation. Finally, we outline opportunities for integrating isotope-resolved operando spectroscopy, microkinetic modeling, first-principles calculations, and AI-assisted data mining to transform KIE analysis from qualitative mechanistic labeling into quantitative, mechanism-guided catalyst development.