In well-buffered solutions, the peak current density (j p) for formic acid oxidation (FAO) at Pt electrodes displays a bell-shaped pH dependence with the maximum at pH close to the pK a of FA (j p–pH plot), and the ratios of H/D kinetic isotope effect (KIE) factors for FAO (HCOOH/DCOOH) are ca. 5 ± 2 to 2 ± 1 and 1 in solutions with pH values of 1.1, 3.6, and 13, respectively. To rationalize such phenomena, the kinetic behaviors of possible pathways for FAO with HCOO– as a discharging precursor have been studied in detail by computational simulation. Our study reveals that (1) the bell-shaped j p–pH plot can be simulated well with a pathway involving the adsorption of either HCOOad or COOad – as the rate-determining step (RDS); (2) a dual-pathway mechanism with both HCOOad and COOad – pathways operating simultaneously is proposed and successfully applied in the simulation of the observed H/D KIE effect, and the H/D KIE effect mainly comes from the COOad – pathway in which the formation of COOad – by splitting of the C–H bond in HCOO– is the RDS; (3) in solutions with pH < 4, the COOad – pathway dominates, and with the increase in pH, the HCOOad pathway with the formation of HCOOad from HCOO– as the RDS gradually becomes the main pathway. Simulation based on such a mechanism reproduces well the j–E plot, the j p–pH plot, and the pH-dependent change of the H/D KIE. As the pH increases, the decrease in the contribution of the COOad – pathway to the overall FAO current can be rationalized by the corresponding positive shift of the potential of zero charge of Pt, which renders Pt(111) less favorable for COOad – formation.

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