On the mechanism of the direct pathway for formic acid oxidation at a Pt(111) electrode

Abstract

In order determine whether formate is a reaction intermediate of the direct pathway for formic acid oxidation at a Pt electrode, formic acid (HCOOH) oxidation at a Pt(111) electrode has been studied by normal and fast scan voltammetry in 0.1 M HClO4 solutions with different HCOOH concentrations. The relationship between the HCOOH oxidation current density (j(ox)) and formate coverage (θ(formate)) is quantitatively analyzed. The kinetic simulation reveals that the previously proposed formate pathway, with decomposition of the bridge-bonded formate (HCOO(B)) as a rate determining step (rds), cannot be the main pathway responsible for the majority of the current for HCOOH oxidation. Instead, a kinetic model based on a mechanism with formic acid adsorption [structure: see text], along with simultaneous C-H bond activation as the rds for the direct pathway, explains the measured data well. It was found for the relatively slow rate of formic acid oxidation, that adsorption-desorption of the formate is faster, which competes for the surface sites for formic acid oxidation.

Revealing the Active Intermediates in the Oxidation of Formic Acid on Au and Pt(111)

“…[29] In contrast, Behm et al argue that on Pt, weakly adsorbed HCOOH might be the key intermediate, leaving HCOO as a spectator. [20][21][22][23] Importantly, the experimental findings on Pt, which provide the basis for the two proposed reactive intermediates, are essentially identical. [14][15][16][17][18][19][20][21][22][23][24] Here different interpretations of the nonlinear relationship between the measured current and the formate coverage lead to different conclusions.…”

Formic Acid Electrooxidation on Noble‐Metal Electrodes: Role and Mechanistic Implications of pH, Surface Structure, and Anion Adsorption

“…[3;4] ), both mechanisms would lead to a quadratic relation between oxidation current and adsorbed formate coverage (at constant potential) as it was observed experimentally [3;4] . Here, it is important to point out that based on a coulometric analysis Xu et al [12] recently derived that the oxidation current on a Pt(111) electrode is not linearly proportional to the square of the adsorbed formate coverage. Nevertheless, also the mechanism proposed by these authors [12] , which involves the formation of adsorbed carboxyl species as r.d.s., followed by their oxidative deprotonation, would not match the pH effects observed in the present work.…”

“…Nonetheless, the nature of the reactive intermediate for the direct pathway is still strongly debated in the literature, especially since adsorbed formate (HCOOads) has been detected by Osawa's group employing in situ Infrared (IR) spectro-electrochemistry [2] . This latter species has been proposed as candidate for the reactive intermediate species in numerous contributions [3][4][5][6][7][8] , while, oppositely, it has been considered as a site-blocking spectator species in other ones [9][10][11][12] . It should be noted that the conclusions were based on essentially equivalent experimental observations.…”

Electroanalysis of oxygen reduction and formic acid oxidation using reduced graphene oxide/gold nanostructures modified electrode

“…Generally, two types of pathway are proposed for the electrochemical oxidation of formic acid depending on the electrode material. One pathway involves direct oxidation of formic acid to carbon dioxide (CO 2 ) (direct or dehydrogenation pathway) and the other involves the adsorption of carbon monoxide (CO) that can be oxidized at higher potentials (indirect or dehydration pathway) [36]. It is previously reported that the oxidation of formic acid on Au surface is proceeded through a two-step mechanism which involves a slow oxidation of adsorbed HCOO − anions to adsorbed HCOO · radicals in the first step and their further oxidation to CO 2 in the second step [37,38].…”