The oxidation of adsorbed CO on Pt(100) electrodes in the pre-peak region

The oxidation of CO adsorbed on Pt(100) electrodes in the pre-peak region (between 0.30 and 0.60 V) was studied using cyclic voltammetry, current–time transients and Fourier transform infrared spectroscopy. According to our results, the pre-peak corresponds to the oxidation of a small fraction (<15%) of the CO adlayer, adsorbed on the terraces and adjacent to the step sites, the rate of CO oxidation being limited, within this potential region and at these high CO coverages, by the diffusion of CO to the reactive sites. The picture emerging from the results reported is as follows: the reaction is initiated by a one-dimensional nucleation-and-growth oxidation of COads along the steps, immediately followed by surface diffusion-controlled oxidation of COads diffusing from the terraces to the steps and by a two-dimensional nucleation-and-growth process. The analysis of the Tafel slopes, as obtained from cyclic voltammograms at different scan rates and from plots of the logarithm of the current at the maximum, the time at the current maximum and the current at the minimum vs. the potential, suggests that the two-dimensional nucleation-and-growth process occurs more slowly than the other two processes. Only the process responsible for the two-dimensional nucleation and growth has not been clearly identified, although two possibilities are given.