Unraveling the Oxidation Mechanism of Formic Acid on Pd(111) Electrode: Implication from pH Effect and H/D Kinetic Isotope Effect

The pH effect and H/D kinetic isotope effect (KIE) of formic acid oxidation reaction (FAOR) on Pd(111) have been systematically investigated by cyclic voltammetry. In addition, the pH effect of acetate adsorption on Pd(111) is also studied to have a deep understanding of the role of adsorbates on Pd(111) during the FAOR process. The results clearly show: (1) In solutions with a fixed concentration of acetic acid, the onset potential of acetate desorption is almost unchanged on the reversible hydrogen electrode (RHE) scale with the increase of pH when the pH is lower than the pK(a) of acetic acid. However, when the pH is higher than the pK(a) of acetic acid, the onset potential of acetate desorption shifts positively with a slope of ca. 20 mV per pH unit; (2) In solutions with pH less than 6, the maximum coverage of acetate adsorbed on the Pd(111) electrode is about 0.26 ML; (3) In acidic solutions, FAOR on Pd(111) has obvious pH effect in the potential range of current increasing on SHE scale. After correcting the pH(s)-induced shift, the onset potential of FAOR shifts negatively by ca. 90 mV per pH unit. However, the pH effect disappears in the potential range after the peak; (4) In alkaline solutions, the onset potential of FAOR is influenced by OHad; (5) The H/D KIE factor of FAOR on Pd(111) is ca. 5 in the pH range of 1-14. Two possible mechanisms trying to explain these results have been proposed. In one, COOad lambda- is the active intermediate and the formation of this intermediate is the rate-determining step (RDS), whereas, in the second one, monodentate adsorbed formate (HCOOm) is the active intermediate and its dissociation on the surface is the RDS. The simulated results with both mechanisms are compared with the experimental results and discussed critically.