Role of dissolved CO in the solution on the origin of CO pre-oxidation on Pt(1 1 1)-Type electrodes

Abstract

In voltammetric CO stripping experiments in acid media, CO pre-oxidation usually takes place on catalysts with a high density of surface defects, obeyed the conditions of full COads layer formation at suitable low potentials into the “hydrogen region” and, importantly, CO traces in the bulk of the solution is required. This study aims to interrogate that. The electro-oxidation of CO was voltammetrically conducted applying different scan rates, employing Pt(1 1 1) terraced surfaces and full CO coverage equilibrated with different content of dissolved CO in the bulk of an acid quiescent solution; also experiments were performed in which COads was present only on the (1 1 0)-type steps in a CO-free solution. It was found that: (1) slopes of peak potentials of CO pre-oxidation versus the logarithm of scan rates were smaller for higher CO contents in the solution; (2) charge density of CO pre-oxidation was higher for slower scan rate; (3) CO pre-oxidation takes place at low potentials where the (1 1 0)-type step sites were inactive in the catalysis of CO electro-oxidation. We conclude that, in such a quiescent solution, the content of dissolved CO in the bulk of the solution determines the magnitude and the shape of the curve of the CO pre-oxidation. This can be understandable in terms of prompt provision of CO molecules from the solution (diffusion layer) to surface active sites, that is, the most active sites, released during the CO pre-oxidation. In the CO pre-oxidation on Pt(1 1 1) terraced surfaces, the most active sites lie at the bottom of (1 1 1) terraces adjacent to the steps. The results provide evidence that, in presence of CO traces in the bulk of the solution, on one hand, the pre-oxidation of CO involves the diffusion-limited electro-oxidation of bulk CO at most active sites released during the CO pre-oxidation . On the other hand, the main CO electro-oxidation peak is a typical behavior of a surface-confined process. Our proposition applies to Pt electrodes in acidic medium.