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The electrooxidation of formic acid (HCOOH) on platinumgroupmetals has been widely studied for its great relevance to electrochemistry as a prototype reaction for the electrooxidation of small organics and its importance in understanding low-temperature fuel cells. 2] It is generally accepted that electrooxidation of HCOOH on Pt proceeds by a dual-path mechanism consisting of indirect and direct paths. In the indirect path, HCOOH is converted into adsorbed CO and then to CO2. In the direct path, HCOOH is converted into CO2 via a reactive intermediate, whose identity is still disputed. Unfortunately, the intermediates from both the indirect and direct paths compete with each another for adsorption sites and the opportunity to react with oxidizers (e.g. OH) on the surface. This competition couples these reaction paths kinetically, hampering the elucidation of their individual reaction mechanisms. In situ infrared reflection-adsorption spectroscopy (IRAS) identifies adsorbed CO, resulting from HCOOH dehydration, as the key reaction intermediate in the indirect path. However, a build-up of CO is observed to poison the system. In contrast, the identity of the reactive intermediate along the direct path is still controversial. Wilhelm and coworkers initially suggested either COH or CHO. Others have long assumed it to be COOH. Using IR spectroscopy, Osawa et al. and Feliu et al. found that formate (HCOO) is the reactive intermediate and that the oxidation of HCOO to CO2 is the rate-determining step for formic acid oxidation. In contrast, Behm et al. argue that weakly adsorbed HCOOH might be the key intermediate, leaving HCOO as a spectator. The electrochemical and spectral data obtained under both static and flow conditions, which provide the basis for these proposed reactive intermediates, are essentially identical. However, different interpretations of the non-linear relationship between the measured current and the formate coverage lead to different conclusions. Cyclic voltammograms (CVs) of HCOOH oxidation (Figure 1) show that the current (I) first peaks around 0.6 V as the potential (U) increases. The current then remains stable or decreases between 0.6 and 0.8 V in what is termed the negative differential resistance (NDR) region. A sharp, increasing around 0.95 V follows the NDR region. 10,12–14] During this process, IR spectroscopy measurements reveal that the polycrystalline Pt surface has a relatively constant coverage of CO below 0.8 V. Above 0.8 V, the coverage of adsorbed CO rapidly decreases due to oxidation, while HCOO quickly increases with increasing potential, until the surface is nearly saturated with HCOO at above 0.9 V. Once the CO coverage is almost completely depleted (ca. 0.95 V), the coverage of HCOO rapidly decreases with increasing potential up to 1.2 V. Thus, these IR measurements suggest that the CV curve can be understood in terms of CO adsorption, desorption, and oxidation by OH. Nevertheless, the detailed roles of adsorbed CO and OH have yet to be elucidated, substantially hindering our understanding of the mechanism of this fundamental reaction. First principles simulations have already been useful in studying HCOOH oxidation. Two density functional theory (DFT) studies indicate that HCOOH oxidation under electrochemical conditions either proceeds via intermediate COOH or initiates from a weakly adsorbed configuration of HCOOH, in which the C H bond is in a “down” configuration. However, several independent theoretical investigations report that the HCOOH adsorption models used in these studies do not correspond to the most energetically favorable structure. More recently, using a gas phase model (i.e. without treating solvation effects), we found that bidentate formate (HCOOB*) is the reactive intermediate in HCOOH oxidation. This result is fully consistent with Figure 1. The potential-dependent rate constants (R) and the experimental CV. The potential sweep rate is 50 mVs . |
