Theoretical study of hydrogen adsorption on Ru(0001): Possible surface and subsurface occupation sites

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Abstract
We have performed a first-principles calculation of the adsorption of hydrogen on the Ru(0001) surface. The adsorption energies associated with various hydrogen sites above and below the ruthenium surface layer are evaluated using ab initio pseudopotentials within the local-density-functional approximation. It is found that the surface threefold hollow site has the lowest energy and is likely to be the adsorption site for one monolayer coverage. The equilibrium position and vibrational frequency are also calculated. On the basis of energy consideration, the subsurface octahedral site is another likely adsorption site, which is separated by an energy barrier from the surface. The occupation of this site may explain the finding of two hydrogen binding states in the measurements of the work-function changes and sticking coefficients. Other electronic properties such as the hydrogen-induced states and work-function changes are also studied and comparisons with experiment are made.