ELECTROCHEMICAL-BEHAVIOR OF PLATINUM SURFACES CONTAINING (110) SITES AND THE PROBLEM OF THE 3RD OXIDATION PEAK

The voltammetric behaviour of flame-treated Pt(110) electrodes has been studied for different cooling atmospheres. It can be stated that avoiding thermally adsorbed oxygen during the cooling step gives rise to samples with the lowest density of surface defects. Electrochemical criteria for checking the cleanliness of this orientation have been proposed. They involve the voltammetric characteristics of both hydrogen and oxygen adsorption-desorption processes. The monitoring of surface cleanliness by the two adspecies is due to the ambiguous interpretation of hydrogen adsorption-desorption only, whose voltammetric profile is not affected significantly by surface contaminants, while the change of the hydrogen electrical charge could be ascribed to two independent origins, either contamination or reconstruction lifting. Then, the problem of the so-called peak III is treated by considering Pt(110) prepared with different surface conditions and the stepped surfaces containing (110) sites on terraces or steps, respectively. It may be concluded that formation of peak III occurs if surface domains of (110) sites with a minimum size are present. Conversely, (110) sites far apart on the surface cannot give rise to peak III. A mechanism for the formation of a well defined type of site at high hydrogen coverage is discussed which accounts for the irreversibility of the adsorption desorption process ascribed to peak Ill where hydrogen is the desorbing species.