Adsorption-desorption kinetics of H 2 from clean and sulfur covered Ru(001)

Abstract

The adsorption and desorption kinetics of hydrogen from clean and partially sulflded surfaces of the (001) face of ruthenium were studied. Adsorption is dissociative and can be described by (1 − θ) 2 kinetics. Desorption rate isothermal analysis of flash desorption spectra of hydrogen from the clean surface show that the desorption energy for hydrogen decreases linearly with increasing H 2 coverage. The low coverage value of the desorption energy is 26 kcal/mole, decreasing to ≈11 kcal/mole at a coverage θ H ≈ 0.8. The pre-exponential factor varies in sympathy with the desorption energy and thus the apparent second-order rate constant demonstrates a compensation effect. No hydrogen could adsorb on the first ordered overlayer of sulfur—the Ru(001)−(2 × 2)S. The LEED pattern was preserved which indicated that hydrogen does not displace sulfur from this surface. The effect of increasing amounts of sulfur on hydrogen adsorption is to rapidly suppress the saturation amount of hydrogen that can be adsorbed; sulfur acts to block dissociation sites for hydrogen adsorption and recombination sites for hydrogen desorption. The desorption rate parameters for hydrogen at fixed hydrogen coverage and increasing sulfur coverage decreases. The decrease parallels the decrease found for the clean surface with increasing hydrogen coverage.