Kinetic Isotope Effects in Complex Reaction Networks: Formic Acid Electro-Oxidation

The determination of kinetic isotope effects (KIEs) for different reaction pathways and steps in a complex reaction network, where KIEs may affect the overall reaction in various different ways including dominant and minority pathways or the buildup of a reaction‐inhibiting adlayer, is demonstrated for formic acid electro‐oxidation on a Pt film electrode by quantitative electrochemical in situ IR spectroscopic measurements under controlled mass‐transport conditions. The ability to separate effects resulting from different contributions—which is not possible using purely electrochemical kinetic measurements—allows conclusions on the nature of the rate‐limiting steps and their transition state in the individual reaction pathways. The potential‐independent values of ≈1.9 for the KIE of formic acid dehydration (COad formation) in the indirect pathway and ≈3 for the COad coverage‐normalized KIE of formic acid oxidation to CO2 (direct pathway) indicate that 1) CH bond breaking is rate‐limiting in both reaction steps, 2) the transition states for these reactions are different, and 3) the configurations of the transition states involve rather strong bonds to the transferred D/H species, either in the initial or in the final state, for the direct pathway and—even more pronounced—for formic acid dehydration (COad formation). Which way? Information obtained from kinetic measurements is the result of a complex combination of different reaction pathways. For formic acid electro‐oxidation on Pt, the kinetic isotope effect (KIE) in a single reaction step (the formation of COad by CH bond breaking) can be determined by in situ IR spectroscopy to be 1.9, independent of the potential, while the KIE in the dominant direct pathway (direct oxidation to CO2) is significantly higher (≈3, see figure).