Coadsorption of sulphur and hydrogen on platinum (110) studied by radiotracer and electrochemical techniques

Abstract

The influence of chemisorbed sulphur on the adsorption of hydrogen on the (110) face of platinum in acid medium was quantitatively studied. Sulphur was pre-adsorbed in a gaseous phase (H 2S/H 2) and the sulphur coverage measured by using the 35S radioisotope. The hydrogen was underpotentially deposited and its coverage measured by cyclic voltametry. The relation between the coverages of sulphur ( θ S) and hydrogen ( θ H) coadsorbed on the platinum surface was established. On the sulphur-free surface the hydrogen coverage at saturation is 1.5, consistent with a reconstructed surface (110)−(1×2). The adsorption of hydrogen is totally inhibited when the surface is saturated by sulphur ( θ S = 0.8). The θ S versus θ H curve is not linear. At low θ S, θ H decreases sharply when increasing θ S. The number of adsorption sites blocked by one sulphur atom is 12±3. This indicates that all hydrogen adsorption sites containing at least one platinum atom in nearest-neighbour position with a sulphur atom are deactivated. With increasing θ S, the influence of each sulphur atom decreases. A model is suggested for the adsorption of hydrogen in sulphur overlayer structures, which accounts for the observed poisoning effect. The shape of the isotherms for hydrogen adsorption on surfaces partially covered by sulphur indicates that sulphur weakens the PtH bond at θ H → 0 and has a screening effect on the interactions between H atoms at higher θ H.