The mechanisms of HCOOH/HCOO– oxidation on Pt electrodes: Implication from the pH effect and H/D kinetic isotope effect

Abstract The pH effect and the H/D kinetic isotope effect (KIE) for the oxidation of formic acid/formate anions (FOR) on Pt(111) have been studied. The pH-dependent FOR activity displays a volcano shape with a maximum at a pH close to the pK a of HCOOH. The H/D KIE factors for HCOOH/DCOOH or HCOO − /DCOO − are ca. 5, 2, and 1 in solutions with pH = 1.1, 3.6, and 13, respectively. These findings reveal that HCOOH → rds COO ad − + 2 H + + e is the rate-determining step (RDS) for FOR in acid. With increasing pH, the precursor to be discharged changes from HCOOH to HCOO − , and the latter goes through HCOO − → rds HCOO ad + e as the RDS. The origin of the appearance of the volcano-type pH dependent FOR activity is pH-induced change from excess HCOOH to excess HCOO – at pH = pKa and the change in overpotential for the RDS of FOR.