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

Abstract

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.

Benchmarking Catalysts for Formic Acid/Formate Electrooxidation

“…Formic acid/formate oxidation reaction on gold is influenced by pH [58,60,152]. Current density shows its maximum value around pH 3-3.5, close to the FA pKa value, similar to what was observed with Pt catalysts above [46,55,58,60,99,100], and this occurs regardless of the gold surface structure (Figure 10b). The onset potential is constant below pH 5, where it starts shifting towards more positive potentials, approximately 60mV per pH unit [60].…”

“…However, several researchers show a volcano-type behavior of the electrocatalytic activity with the pH. Optimum pH conditions are usually found at around FA pKa (i.e., 3.75) [46,55,58,60,99,100], although Haan et al did not reach the maximum electrocatalytic activity in the pH range 0-5 [101], and Ferre-Vilaplana et al obtained an optimum pH of 5.5 [81]. In any case, from the literature review, the convenience of operating at pH closer to neutral conditions rather than strongly acidic or basic conditions seems clear.…”

Cation Overcrowding Effect on the Oxygen Evolution Reaction

“…In turn, the EDL is influenced by interfacial reactions in that the surface charge density is a function of the coverages of reaction intermediates, which are the output of the microkinetic model. A consistent treatment of these models is a necessity for deciphering how electrolyte cations influence the OER and is also a unique feature of this work in comparison with similar models for the OER − and other electrocatalytic reactions. − …”

“…A consistent treatment of these models is a necessity for deciphering how electrolyte cations influence the OER and is also a unique feature of this work in comparison with similar models for the OER 30 − 32 and other electrocatalytic reactions. 33 − 35 …”

Influence of pH and Proton Donor/Acceptor Identity on Electrocatalysis in Aqueous Media

“…Electrocatalytic reactions involving the transfer of multiple protons and electrons at an aqueous electrolyte/electrode interface play a central role in electrochemical energy conversion. The hydrogen evolution reaction (HER), , oxygen reduction reaction (ORR), , CO/CO 2 reduction reactions, , ammonia oxidation reaction, , formic acid oxidation, , and oxygen evolution reaction (OER) , are examples of such processes. At a fixed reversible hydrogen electrode (RHE) potential ( U RHE ), that is, at a fixed overpotential, the rates and selectivity of such reactions often exhibit a complex dependence on electrolyte pH .…”