Effects of chemisorbed sulphur on the hydrogen adsorption and evolution on metal single crystal surfaces

The effects of chemisorbed sulphur on the hydrogen adsorption and evolution in aqueous acid solution are reviewed for well-controlled surfaces of platinum, nickel, iron and silver. The hydrogen underpotential adsorption (HUA) on platinum is poisoned by adsorbed sulphur. The hydrogen evolution reaction (HER) on Pt, Ni, Fe and Ag may be impeded or promoted, depending on the nature of the metal substrate. Adsorbed sulphur always blocks sites for H adsorption, whereas the M-Hads bond energy in the sites not blocked may either decrease or increase. The results are analysed in the light of a model for the effects of adsorption site blocking elements on the H surface reactions, where the site-blocking effects are decoupled from the energetic effects. The effects of site blocking by a preadsorbed strongly-bonded element (X) are modelized, taking into account the structure and orientation of the substrate, the nature of the adsorption sites, the poisoning range of one X atom and the degree of long-range order in the X overlayer. Literature data concerning the effects of Pb, Cd and T1 adatom s on the HER arc re-examined in the light of this model. Our experimental data on sulphur effects are well fitted by the model. Evidence is given that on Pt the sites of the HER are different, both energetically and structurally, from the sites of the HUA. The site-blocking range of sulphur for the HER is smaller than for the HUA. Concerning the energetic effects, it is shown that on sulphur-covered Pt(111) the M-Hads bond in the available sites of the HUA is weakened, whereas the M-Hads bond in the available sites of the HER is strengthened . The correlation with gas-phase dissociative adsorption of H2 is presented. Finally the implications of the sulphur effects on the H surface reactions on the H entry into metals are discussed