Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H 2 SO 4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is h CO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E d ) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is h CO = 0.63, which corresponds to an E d = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

We present a detailed spectrokinetic study of the electrocatalytic oxidation of formic acid on Au and Pt electrodes using ATR-SEIRAS that has allowed us to unveil the mechanisms of both the direct (in which adsorbed CO is not involved) and the indirect (through adsorbed CO) paths of the reaction with unprecedented detail. Au electrodes were used to study the mechanism of the direct path without the interference of the indirect path, and the observed quadratic dependence of the reaction rate on the formate coverage was then shown to apply also to Pt. The direct path consists of three steps, namely, (i) the electroadsorption of formate (corresponding to the first electron transfer), (ii) the purely chemical bimolecular decomposition of adsorbed formate, and (iii) the second electron transfer. The dehydration of HCOOH to adsorbed CO, that is then oxidized to CO 2 in the indirect path, was studied on Pt at E < 0.4 V vs the reversible hydrogen electrode (RHE), at which potentials the dehydration reaction is the only one taking place on the Pt surface. Our results show that adsorbed formate is also the intermediate in the dehydration of formic acid to adsorbed CO and is, hence, the key intermediate in the electrocatalytic oxidation of formic acid on metals.

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As in previous work with Pt(1 1 1) electrodes, we have combined CO-stripping cyclic voltammetry, CO charge-displacement and FT-IR spectroscopy measurements to determine changes in the coverage and structure of CO adlayers on Pt(1 0 0) electrodes in 0.1 M H 2 SO 4 as a function of potential. In CO-free solutions the maximum coverage is h CO = 0.79, which can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E d ) is more negative than 0.25 V vs. RHE. Although the highest CO coverage of Pt(1 0 0) electrodes in contact with CO-saturated solutions could not be determined, our FT-IR results clearly show that, as in the case of Pt(1 1 1) electrodes, removing CO from the solution results in a partial, reversible desorption of the CO adlayer, and, hence, that the CO adlayer on the Pt(1 0 0) electrode is in equilibrium with the CO-containing solution. The lowest CO coverage at which hydrogen adsorption on the Pt(1 0 0) electrode is completely blocked is h CO = 0.75, which corresponds to E d = 0.40 V vs. RHE. The results reported here provide support to the hypothesis that the process at the pre-peak in CO-stripping voltammograms (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of CO by reaction with oxygenated species nucleating at steps, the main CO-oxidation peak appearing when nucleation of oxygenated species at the terraces also occurs.