Quantitative analysis of the distribution of the hydrogen adsorption states at platinum surfaces: Part I. Application to Pt(100) in sulphuric acid medium

Abstract

The adsorption of hydrogen on platinum electrodes is a very sensitive probe for the surface structures and their change after electrochemical adsorption-desorption of oxygen alone or combined with hydrogen which generally induces a redistribution of the hydrogen adsorption states. In many cases when the transformation takes place the redistribution of the states occurs with a constant total electrical charge. As the voltammetry of adsorbed hydrogen on platinum surfaces is similar to spectroscopy of the adspecies, a quantitative numerical treatment of the voltammograms has been developed in order to characterize in situ the various crystallographic surface structures by using the detailed distribution of the adsorption states at these surfaces. The multiplicity of peaks on the voltammograms has been ascribed to the presence of several hydrogen adsorption states, where the term “state” has a general significance linked to the intrinsic and induced heterogeneity of the surface. For the quantitative description of this adsorption equilibrium, as in the gas phase on solids, an adsorption isotherm has to be chosen. Because the peaks are symmetrical with various half-height widths for a one-electron transfer process, the Frumkin-Fowler isotherm has been selected which takes into account the interactions between adsorbed species. Three or four adsorption states have been obtained in the deconvolution of the voltammograms of Pt (100), depending on the surface order as a result of pretreatment applied to the surface. Significant relations between the proportions of hydrogen relative to each state have been found. The results of the calculation show that all the states in sulphuric medium are characterized by an attractive interaction constant.