The effect of pH on the electrocatalytic oxidation of formic acid/formate on platinum: A mechanistic study by surface-enhanced infrared spectroscopy coupled with cyclic voltammetry

摘要

The electrocatalytic oxidation of formic acid (HCOOH) and formate (HCOO~-) to CO_2 on platinum has been studied over a wide range of pH (0-12) by surface-enhanced infrared absorption spectroscopy (SEIRAS) coupled with cyclic voltammetry. The peak current of HCOOH/HCOO~- oxidation exhibits a volcano-shaped pH dependence peaked at a pH close to the pK_a of HCOOH (3.75). The experimental result is reasonably explained by a simple kinetic model that HCOO~- oxidation is the dominant reaction route over the whole pH range. HCOOH is oxidized after being converted to HCOO~- via the acid-base equilibrium. The ascending part of the volcano plot at pH <4 is ascribed mostly to the increase of the molar ratio of HCOO~-, while the descending part at pH > 4 is ascribed to the suppression of HCOO~- oxidation by adsorbed OH or oxidation of the electrode surface. In acidic media, HCOOH is adsorbed on the electrode as formate with a bridge-bonded configuration. The bridge-bonded adsorbed formate is stable and suppresses HCOO~- oxidation by blocking active site. However, the suppression is not fatal because bridge-bonded adsorbed formate enhances the oxidation of HCOO~- at high potential by suppressing the adsorption of OH or surface oxidation. The complex cyclic voltammograms for HCOOH/HCOO~- oxidation also can be well interpreted in terms of the simple kinetic model. The experimental results presented here serve as a generic example illustrating the importance of pH variations in catalytic proton-coupled electron transfer reactions.