Methanol oxidation on stepped Pt[n(111) x (110)] electrodes:: A chronoamperometric study

The methanol oxidation reaction has been studied on Pt[n(111) x (110)]-type electrodes in a 0.5 M sulfuric acid and 0.025 M methanol solution, using cyclic voltammetry and chronoamperometry. The voltammetric behavior of methanol on the three electrodes under investigation [Pt(I 11), Pt(554), and Pt(553)] shows that the overall oxidation rate increases with an increasing step density and that the defects are affected more by the presence of methanol than terraces. The latter implies that either the decomposition products of methanol or the methanol itself preferably sit at the steps. Investigation of the chronoamperometric data showed that the steady-state current, recorded at 900 s after the start of the experiment, increases with an increasing step density. Moreover, surfaces with a higher step density display a faster dropping current, which suggests that the decomposition of methanol into CO poisoning species also preferentially takes place on the steps and defects. Unlike the stepped electrodes, most transients recorded on Pt(111) showed an initial current increase, which may be explained by the CO oxidation being faster than the methanol decomposition. This low decomposition rate is probably the result of a sufficiently low defect density and the low methanol concentration used in our experiments. Fitting the chronoamperometric data with a mathematical model, which includes the methanol decomposition reaction, the CO oxidation reaction, and the direct methanol oxidation reaction, suggests that steps and defects catalyze all these reactions. Furthermore, the model indeed predicts that when the CO, oxidation rate is faster than the decomposition rate, a rising current transient can be expected, as was seen for Pt(111).