Nature of the Dissociation Sites of Hydrogen Molecules on Ru(001)

Scanning tunneling microscopy (STM) was used to study the dissociative adsorption of H 2 on Ru(001) near saturation coverage, when the number of residual hydrogen vacancies (i.e., unoccupied Ru sites) is small. We found that H 2 dissociation takes place only on Ru sites where the metal atom is not bound to any H atom. Such active sites are formed when at least 3 H-vacancies aggregate by thermal diffusion. Sites formed by single H-vacancies or pairs of adjoining vacancies were found to be unreactive toward H 2 . As a similar phenomenon was found previously on Pd(111); the present results indicate that the active sites for H 2 dissociation share a common characteristic among catalytically active transition metals.