The effect of sulfur on co adsorption/desorption on Pt(S)-[9(111) × (100)

Abstract

CO adsorption/desorption on clean and sulfur covered Pt(S)-[9(111) × (100)] surfaces was studied using AES, TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface — a low temperature state associated with the (111) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption significantly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was 1 × 10 15 s −1 exp ( −36.2 kcal/mole RT ) Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic of the step/defect sites. For the surface on which step/defect sites were blocked by sulfur the effective desorption rate constant was k eff = 1 × 10 13 s −1 exp (− 27.5 kcal/mole RT ) indicating an appreciable decrease in CO binding on the terraces, though sulfur-CO repulsive interactions had probably made k eff larger than the true rate constant for desorption from clean (111) planes. The results showed clearly a compensation effect in activation energy and preexponential factor.