Comparison of electrosorption at activated polycrystalline and Pt(531) kinked platinum electrodes: surface voltammetry and charge displacement on potentiostatic CO adsorption

In this work we have tried to reproduce the hydrogen adsorption behaviour of electrochemically activated polycrystalline platinum electrodes by using a kinked platinum single crystal surface subjected only to flame cleaning before checking hydrogen electrosorption at its surface. The Pt(531) surface may be considered as a model for a platinum surface which has been extensively activated by cycles of oxygen adsorption-desorption. In the hard sphere model the surface unit cell for this orientation contains three sites, with four-fold, three-fold and two-fold symmetry respectively. The electrochemical behaviour of this surface, prepared from a platinum bead, was checked in 0.5 M H₂SO₄ solution by voltammetry after flame cleaning and various cooling conditions. It was compared with that of a rough platinum surface, cut from another bead in a direction chosen at random, either studied after flame cleaning or after electrochemical activation. Experiments on charge displacement by CO adsorption at constant potentials show, for both samples, that the voltammetric charge at the two limits of the hydrogen upd range is made up of two components of opposite sign. Only 0.69 and 0.77 of the voltammetric charge on Pt(531) and rough platinum respectively may be ascribed to adsorbed hydrogen; the remaining part of this charge is involved in a reversible adsorption state formed in an anodic process. These results are discussed in terms of the inadequacy of the conventional method of correcting, from double layer effects, the voltammetric charge of activated platinum electrodes in order to measure their hydrogen surface content.