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

- The H/D KIE factor for HCOOH/DCOOH or HCOO-/HCOO- is ca. 5, 2 and 1 in solutions with pH = 1.1, 3.6 and 13, respectively.
- HCOOH→rdsCOOad−+2H++e is the RDS for FOR in acid, with increase of pH the RDS goes through HCOO−→rdsHCOOad+e.
- The origin of the pH dependent FOR activity is pH induced opposite change of reactant concentration and overpotential for the RDS.

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 pKa 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→rdsCOOad−+2H++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−→rdsHCOOad+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.