Direct and precursor dynamics in dissociative hydrogen chemisorption on Ni(100)

摘要

We report evidence for the coexistence of direct and precursor dynamics in the dissociative chemisorption of H2on Ni(l00). Hydrogen and deuterium uptakes on Ni(l00) were measured at various surface temperatures by following the secondary ion ratios, Ni2H+/Ni+and Ni2D+/Ni+, which are proportional to surface hydrogen and deuterium coverage on both clean and carbonhyphen;covered Ni(100). Between 100 and 200 K on clean Ni(l00), the initial sticking coefficient of hydrogen decreases, but only slightly, as the surface temperature increases. The decrease is more pronounced both in the presence of predosed carbon and for deuterium adsorption on Ni(100). This is interpreted as due to the involvement at low temperatures of a molecular precursor which mediates dissociative adsorption at low temperatures. The precursor probably involves surface defects and sites formed in the presence of carbon. In addition to the precursor channel, a direct dissociation channel also operates, and dominates forTge;200 K.