EVALUATION OF ABSOLUTE SATURATION COVERAGES OF CARBON-MONOXIDE ON ORDERED LOW-INDEX PLATINUM AND RHODIUM ELECTRODES

The limitations on evaluating surface concentrations, and hence fractional coverages theta(CO)sat, for saturated irreversibly adsorbed CO adlayers on low-index platinum and rhodium electrodes by anodic voltammetry are discussed with particular reference to Pt(111). In agreement with Feliu et al., substantially higher theta(CO)sat estimates (about 0.9) are obtained by this means in aqueous sulfuric acid than in perchloric acid (0.6-0.7). Additional evidence from nonelectrochemical methods, however, indicates that the latter estimate approximates the true theta(CO)sat values. In particular. thin-layer IR spectrophotometry enables theta(CO)sat values to be extracted from the absorbance of the O-C-O stretch for the CO2 product. This procedure yields theta(CO)sat values for all six platinum and rhodium low-index faces that are close to (within about 10% of) the values obtained at the corresponding metal-ultrahigh vacuum interfaces at low temperatures. Some likely systematic errors in the conventional voltammetric analysis are discussed in the light of the present results.