Bridge-Bonded Formate:  Active Intermediate or Spectator Species in Formic Acid Oxidation on a Pt Film Electrode?

Abstract

We present and discuss the results of an in situ IR study on the mechanism and kinetics of formic acid oxidation on a Pt film/Si electrode, performed in an attenuated total reflection (ATR) flow cell configuration under controlled mass transport conditions, which specifically aimed at elucidating the role of the adsorbed bridge-bonded formates in this reaction. Potentiodynamic measurements show a complex interplay between formation and desorption/oxidation of COad and formate species and the total Faradaic current. The notably faster increase of the Faradaic current compared to the coverage of bridge-bonded formate in transient measurements at constant potential, but with different formic acid concentrations, reveals that adsorbed formate decomposition is not rate-limiting in the dominant reaction pathway. If being reactive intermediate at all, the contribution of formate adsorption/decomposition to the reaction current decreases with increasing formic acid concentration, accounting for at most 15% for 0.2 M DCOOH at 0.7 VRHE. The rapid build-up/removal of the formate adlayer and its similarity with acetate or (bi-)sulfate adsorption/desorption indicate that the formate adlayer coverage is dominated by a fast dynamic adsorption-desorption equilibrium with the electrolyte, and that formate desorption is much faster than its decomposition. The results corroborate the proposal of a triple pathway reaction mechanism including an indirect pathway, a formate pathway, and a dominant direct pathway, as presented previously (Chen, Y. X.; et al. Angew. Chem. Int. Ed. 2006, 45, 981), in which adsorbed formates act as a site-blocking spectator in the dominant pathway rather than as an active intermediate.

Oscillatory instabilities during the electrocatalytic oxidation of methanol on platinum

“…Very importantly, they reported that CO ad acts not only as a poison or site-blocking species but it also inhibits the formate decomposition step. However, using a similar experimental approach, Behm and co-workers 52 have recently demonstrated that adsorbed bridge-bonded formate species cannot be considered as a reaction intermediate in the main reaction pathway during formic acid electrooxidation on platinum. The authors proposed a triple pathway mechanism including the direct pathway, the indirect pathway, and the formate pathway.…”

Oxidation Mechanism of Formic Acid on the Bismuth Adatom-Modified Pt(111) Surface

“…[8][9][10][11][12][13] Electrocatalytic reactions rates depend on the surface structure of the electrodes. So, Pt(111) electrodes exhibit low activity through the active intermediate and negligible CO formation, whereas Pt(100) surfaces show the highest activity for both routes.…”

“…20 It has been pointed out that, in the oxidation process of formic acid on pure platinum electrodes, adsorbed formate plays an important role. This specie has been detected by FTIR [9][10][11][12][13] and voltammetry, 21 coinciding the onset for the oxidation process with that for the adsorption of formate. 21 Also, DFT calculations indicate that adsorbed formate is a key element in the considered oxidation process.…”

On the activation energy of the formic acid oxidation reaction on platinum electrodes

“…On the other hand, the reaction route which gives directly CO 2 is called the direct oxidation route. For this route, the nature of the intermediate is still subject of discussion [4][5][6][7][8][9][10][11].…”