Contribution á l'étude de l'adsorption chimique réversible du soufre sur l'argent

The isotherms of reversible chemisorption of sulphur on the (111), (100) and (110) faces of silver have been measured at different temperatures in hydrogen-hydrogen sulphide mixtures. The heats of adsorption were determined for these three orientations; they are higher the less dense the surface. For coverages between 25 per cent and 80 per cent the heat of adsorption is constant for the (111) and (100) faces but it decreases with coverage for the (110) face. It can be deduced from the shape of the isotherms that there exists lateral attractive forces between the adsorbed atoms which are higher when the binding with the metallic substrate is weak. Over a wide range of pressures the surfaces fix a constant maximum quantity of sulphur, following the formation of a complete monolayer. The significance of this saturation is discussed in terms of the surface density of the metallic atoms and the size of the adsorbed atoms. For the experimental conditions corresponding to the beginning of the isotherm the effect of slight deviations from the low index orientation may be explained by a selective fixation on atomic steps.