Surface diffusion of hydrogen on sulfur-covered Ru(001) surfaces studied using laser-induced thermal desorption

The effects of surface sulfur coverage on the surface diffusion of hydrogen on Ru(001) were studied using laser-induced thermal desorption techniques. Measurements at T = 270 and 300 K revealed that the surface mobility of hydrogen decreased rapidly as a function of increasing surface sulfur coverage. At T = 300 K the hydrogen surface diffusion coefficient dropped approximately a factor of 30 from 8.5 × 10−7 cm2/s to less than 3 × 10−8 cm2/s as the surface sulfur coverage increased from θs = 0 to θs = 0.25 monolayer. The reduction of hydrogen surface mobility versus surface sulfur coverage was compared to predictions from a site-blocking model. In order to reproduce the rapid decrease in hydrogen surface mobility as a function of sulfur coverage, the site-blocking model demonstrated that each sulfur adatom must block ten hydrogen adsorption sites. The effect of sulfur on hydrogen surface mobility was attributed to both steric and long-range electronic effects. Reduced surface mobility versus surface sulfur coverage may help to explain the effect of sulfur as a poison of catalytic reactions.