Bridge-bonded formate: active intermediate or spectator species in formic acid oxidation on a Pt film electrode?
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.
Chen
Y-X
YX
Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany. yan-xia.chen@uni-ulm.de
Heinen
M
M
Jusys
Z
Z
Behm
R J
RJ
D000327:Adsorption; D004566:Electrodes; D005561:Formates / Q000737:chemistry*; D007700:Kinetics; D008567:Membranes, Artificial; D010084:Oxidation-Reduction; D010984:Platinum / Q000737:chemistry*; D012680:Sensitivity and Specificity; D012825:Silicon / Q000737:chemistry; D013055:Spectrophotometry, Infrared / Q000379:methods; D013499:Surface Properties; D013997:Time Factors
D005561:Formates; D008567:Membranes, Artificial; C030544:formic acid; D010984:Platinum; D012825:Silicon