Different behavior of adsorbed bridge-bonded formate from that of current in the oxidation of formic acid on platinum
Introduction
The electrochemical oxidation of formic acid on Pt in acidic media has been extensively investigated [1], [2], [3], [4], [5], [6] as a model electrocatalytic reaction because it is the simplest electrocatalytic reaction involving an organic molecule. The application of the reaction includes low temperature direct formic acid fuel cells (DFAFCs) [7], [8]. It has been widely accepted that the reaction proceeds via a dual path mechanism composed of a direct path and an indirect path [1], [2], [3]. The indirect path involves adsorbed CO [9], [10], [11], which is formed by dehydration of formic acid at potentials lower than approximately 0.6 V (vs. RHE). The adsorbed CO blocks the surface adsorption site at the stated potentials but is oxidized to CO2 at potentials higher than 0.6 V.
The direct path proceeds via a reactive intermediate, producing most current observed in the voltammogram. Sun et al. [12] reported that the reactive intermediate was a carboxylic acid species adsorbed on the surface through the carbon atom (–COOH) by infrared reflection–absorption spectroscopy (IRRAS). The species, however, was not detected by surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) [13], [14], [15], [16], [17], [18], [19], [20] developed by Osawa et al. [21], [22], [23], [24]. Because IRRAS involves a thin electrolyte layer confined between the electrode and an IR-transparent window, mass transport of reactants and products is suppressed and, hence, IRRAS is not pertinent to measure dynamical changes on the electrode. Such shortcomings have been overcome by SEIRAS, in which bulk solutions freely approach the electrode and, furthermore, a high surface selectivity and sensitivity is obtained [13], [14], [15], [16].
Osawa et al. [13], [14], [15], [16] have found that, other than adsorbed CO, the only formic acid-related surface species observed by SEIRAS is formate adsorbed on the surface in a bridging conformation, that is, bonded via both oxygen atoms to two surface Pt atoms. They argued that adsorbed bridge-bonded formate (HCOOB) is the reactive intermediate in the direct path, based on the experimental fact of the close similarity of potential dependence of the current and HCOOB band intensity [16]. The argument was also based on the ability to explain the mechanism of the appearance of a current peak at around 0.6 V (vs. RHE) during the positive-going potential sweep in the voltammogram [14], [15], [25], [26]. The current peak produces, at the positive potential side of the peak, a negative differential resistance necessary for the appearance of potential oscillation [27], [28], [29]. Grozovski et al. [30] have supported the idea of HCOOB as the reactive intermediate by use of Pt (111) and Cuesta et al. [31] have proposed that HCOOB is an intermediate common to the direct and indirect paths.
On the other hand, Chen et al. [17], [19], [20], [32] have thought differently that HCOOB is not a reactive intermediate in the direct path and is rather a site-blocking spectator, based on the experimental data obtained mainly with a thin-layer electrochemical flow cell coupled to SEIRAS. Peng et al. [33] have suggested that HCOOB is not a reactive intermediate in the main pathway on Sb-modified Pt electrodes, based on the fact that by electrodepositing Sb on Pt the formic oxidation current increased simultaneously with a decrease in the amount of HCOOB. Joo et al. [34] have reported that the reactive intermediate in the direct path is not HCOOB but is a weakly-adsorbed formate precursor, from experiments over a wide pH range.
The role of HCOOB is, thus, controversial at the moment. To determine whether it is the reactive intermediate in the direct path or not, we have studied in detail the potential dependent behavior of HCOOB on unmodified Pt, behavior which was a strong evidence for HCOOB being the reaction intermediate in the direct path. In this paper, we clarify a distinctly different behavior between the current and HCOOB, concluding that HCOOB is not the reactive intermediate in the direct path.
Section snippets
Experimental Section
Experiments were conducted in a three-electrode cell isolated from the surrounding air. The reference electrode was a reversible hydrogen electrode (RHE) in the supporting electrolyte solution (0.5 M H2SO4, M: mol dm−3). The counter one was a platinized Pt wire in a glass tube with bubbling nitrogen gas (Nippon Sanso Corp., Japan, over 99.9999%), a tube which was separated by a glass frit from the reaction solution. The working electrode was a thin (∼50 nm) Pt film deposited on one of the three
Behavior of current and HCOOB with 0.1 M formic acid during potential sweep.
Fig. 1a shows a cyclic voltammogram (CV) for the oxidation of 0.1 M formic acid between 0.05 and 1.4 V at a sweep rate of 0.1 V/s at room temperature. In the positive-going potential sweep, the current shows two peaks, peak I at around 0.6 V and peak II at around 0.9 V. In the reverse negative-going potential sweep, two other current peaks are observed, peak IV at around 0.7 V, and peak V (a shoulder) at around 0.5 V. The current peak nomenclature follows the literature notation [2], [37], [38].
Discussion
We have confirmed that adsorbed bridge-bonded formate is not the reactive intermediate in the direct path. Samjeské et al., however, have proposed the mechanism of appearance of peak I, based on the reaction of adsorbed bridge-bonded formate in the direct path [14], [15], [16], [26]. It is important to clarify the mechanism of peak I appearance to understand the oscillation mechanism. This is because the appearance of oscillation requires a negative differential resistance [27], [28], [29] in
Conclusions
To determine whether adsorbed bridge-bonded formate (HCOOB) is the reactive intermediate in the direct path or not, we have investigated the behavior of HCOOB during the CV measurements at a sweep rate from 0.1 mV/s to 1 V/s with a formic acid concentration from 1 mM to 1 M. The investigation has also been carried out at a fixed potential mainly of 0.7 V and during a potential sweep where adsorbed CO is absent. As a result, with a decrease in the CV sweep rate, the potential of the maximum HCOOB
Acknowledgements
The authors thank Professor Osawa of Hokkaido University for his kind introduction to SEIRAS. This work was partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grant Zc10-01 and by the Science Research Promotion Fund of the Promotion and Mutual Aid Corporation for Private Schools of Japan.
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