Mechanistic Implication of the pH Effect and H/D Kinetic Isotope Effect on HCOOH/HCOO- Oxidation at Pt Electrodes: A Study by Computer Simulation

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.

Automated summary

The kinetic behaviors of formic acid oxidation on Pt electrodes were found to vary with pH, showing a dominant COOad- pathway at low pH ( <4 ) and gradually shifting to a HCOOad pathway at higher pH. Computational simulations successfully explained the observed phenomena and changes in kinetic parameters at different pH values.