Electro-oxidation of formic acid on Pt in acid is one of the most fundamental model reactions in electrocatalysis. However, its reaction mechanism is still a matter of strong debate. Two different mechanisms, bridge-bonded adsorbed formate mechanism and direct oxidation mechanism, have been proposed by assuming a priori that formic acid is the major reactant. Through systematic examination of the reaction over a wide pH range (0-12) by cyclic voltammetry and surface-enhanced infrared spectroscopy (SEIRAS), we will show that the formate ion is the major reactant over the whole pH range examined, even in strong acid. The performance of the reaction is maximal at a pH close to the pKa of formic acid. The experimental results are reasonably explained by a new mechanism in which formate ion is directly oxidized via a weakly adsorbed formate precursor. The reaction serves as a generic example illustrating the importance of pH variation in catalytic proton-coupled electron transfer reactions.Electrooxidation of formic acid (HCOOH) to CO2, the reaction taking place at the anode of direct formic acid fuel cells, is one of the most fundamental model electrocatalytic reactions and has been investigated intensively over the last four decades mostly in acidic media. [1][2][3][4] It is generally accepted that HCOOH is oxidized via a dual pathway mechanism; 1 a main pathway via a reactive intermediate and a pathway involving adsorbed CO (COads), a catalytic poison. COads is oxidized to CO2 at high potentials. The pathway involving COads has well been established in the 1980s, while the main pathway, non-CO pathway, is still matter of strong debate. Samjeské et al. 5 and others 6,7 proposed, on the basis of surface-enhanced infrared spectroscopy in an ATR geometry (ATR-SEIRAS) 8 and electrochemical measurements, that a formate species adsorbed on the electrode surface though its two O atoms (bridge-bonded formate) is the intermediate in the non-CO pathway and its decomposition to CO2 is the rate-determining step (bridge-bonded formate mechanism), while Chen et al. 9 argued that the adsorbed formate is a site-blocking spectator and that HCOOH is directly oxidized via a weakly adsorbed HCOOH precursor (direct HCOOH mechanism). A consensus has not been reached yet, either in theoretical studies of the reaction. [10][11][12] The aim of the present Communication is to make clear the real reaction mechanism through a systematic investigation of the reaction over a wide range of pH (0-12).Since HCOOH is a weak acid with a pKa of 3.75, 13 if the direct HCOOH pathway were the main reaction route, the oxidation current should decrease with increasing pH due to the decrease of HCOOH concentration. However, several earlier studies have reported that the oxidation current increases with pH. 14 Figure 1a shows representative cyclic voltammograms (CVs) for a rotating Pt disc electrode in 0.2 M

Please cite this article as: J. Joo, T. Uchida, A. Cuesta, M.T.M. Koper, M. Osawa , 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, Electrochimica Acta (2014), http://dx.doi.org/10. 1016/j.electacta.2014.02.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. AbstractThe 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

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Formic acid has great potential for use as fuel in direct formic acid fuel cells (DFAFC). Because the catalytic activity of Pt for formate oxidation is sluggish in alkaline media, researchers have focused on using acid media in DFAFC technology. Recently, Osawa’s group demonstrated that the best reaction performance can be achieved at pH ≈ pK a. A systematic investigation of the HCOOH/HCOO– oxidation in an actual single cell over a wide pH range has not yet been performed. Here, single-cell experiments confirm the direct formate pathway in HCOOH/HCOO– oxidation, which leads to better oxidation kinetics at the optimal pH.

Electro-catalytic oxidation of formic acid has a significant importance in fundamental research of small organic molecule oxidation, as well as practical application to fuel cells. Notwithstanding intensive research efforts for last couple of decades, there are still fundamental questions in debate on the mechanistic origin. This perspective presents underlying issues in the electro-catalytic oxidation of formic acid. Until now, the oxidation mechanism of formic acid is not fully understood in spite of its importance of this work, since the role of adsorbed formate is not clearly identified. In addition, we will discuss on the role of Bi on Pt that unambiguously enhances the activity of Pt in fuel cell systems but is in debate with single crystal Pt surface. We finally accentuate the causes of the deactivation of Pd catalysts, because the utilization of non-Pt electrocatalysts could be one of the key researches for cost reduction of fuel cell systems. Thus, we intend to introduce different views toward formic acid oxidation proposed by the other researchers and provide perspectives on the further research to close the gap between fundamental study and technical applications