ELECTROCHEMICAL-BEHAVIOR OF CO LAYERS FORMED BY SOLUTION DOSING AT OPEN CIRCUIT ON PT(111) - VOLTAMMETRIC DETERMINATION OF CO COVERAGES AT FULL HYDROGEN ADSORPTION BLOCKING IN VARIOUS ACID-MEDIA

The electrochemical oxidation of CO on Pt(111) in sulphuric, perchloric and chloride-containing acid solutions has been investigated by cyclic voltammetry. In all cases the CO coverages correspond to a full blocking of the hydrogen adsorption sites in order to estimate from coulometry the maximum amount of CO which could be adsorbed on Pt(111) from solution dosing. A comprehensive discussion of the correction of the double-layer contribution is given for voltammetric experiments using ultra-clean systems with the lowest amount of surface defects. Two different models for the double-layer correction have been discussed. They differ essentially in the inclusion or omission of the charge corresponding to the unusual adsorption states of Pt(111) in the double-layer correction. The results show that in both cases the CO coverages reached from solution dosing are higher than those obtained in the gas phase in the same temperature range. If equal maximum coverage in the gas or liquid phase were achieved, it would lead to improbably high double-layer corrections for the latter system because some of the usual hydrogen adsorption states should be involved in this correction in a range of potentials where their presence is incompatible. The full blocking of hydrogen adsorption sites on Pt(111) was observed for different CO stripping charges (differing by 105-mu-C cm-2). This quite large difference, which is independent of the model used for corrections, indicates that 0.22 CO monolayers can be adsorbed additionally on the Pt(111) electrode at the threshold of hydrogen adsorption. In the cases of highest coverages, a CO stripping charge contribution of around 30-mu-C cm-2 has been observed at lower potentials. A tentative interpretation of this early oxidation step is proposed. After this latter stripping process the electrode surface always remains fully blocked for hydrogen adsorption.