Hydrogen on Pd(100)-S: the effect of sulfur on precursor mediated adsorption and desorption

Abstract

The influence of adsorbed sulfur on the adsorption and desorption of H2 on Pd(100) was studied using temperature programmed desorption (TPD) for sulfur coverages (θS) from 0.00 to 0.35 ML. The saturation coverage (θH,sat) drops linearly with sulfur coverage, and above 0.28 ML of sulfur no hydrogen adsorbs. Direct site blocking is implied by the linear fall of θH,sat with θS, with each sulfur atom effectively blocking 3.7 ± 0.5 sites for hydrogen adsorption. The adsorption of H2 on the sulfur free Pd(100) surface is best described by a second-order precursor model. When sulfur is added to the surface, however, the effect of the precursor state is diminished, and for θS ⩾ 0.15 hydrogen uptake is adequately modeled by secondrder direct adsorption. For low hydrogen coverages values for the activation energy and the preexponential factor for hydrogen adatom recombination drop in compensatory fashion from 85 kJmol and 10−2.5cm2s on the sulfur free surface to 49 kJmol and 10−6.8cm2s at θS = 0.15, respectively. The effect of sulfur on the desorption kinetics of hydrogen suggests that the influence of adsorbed sulfur is more complex than simple site blocking. The compensation effect between the preexponential factor and the activation energy from surfaces with sulfur adlayers may arise from a more constrained transition state for desorption on sulfur covered surfaces or from a distribution of activation energies for desorption.