Density functional study of the structural and electronic properties of RuS2(111): II. Hydrogenated surfaces

Hydrogen chemisorption on RuS2(111) surfaces with distinct sulfur coverages has been investigated by a periodic density functional approach. Consistent with experimental and previous theoretical studies, dissociative hydrogen chemisorption is an exothermic process. A sulfur enriched termination (without anionic vacancies) with a hydrogen coverage of four hydrogen atoms per unit cell is found to be energetically favored compared to the stoichiometric and highly reduced surfaces. This structure involves exclusively protonic SH species at the surface. Additional reduction with hydrogen is endothermic. Generation of hydridic RuH species requires an under-stoichiometric surface termination and enhances the potential reactivity of the ruthenium sites. Hydrogen adsorption is observed to compensate the relaxation of RuS2(111) bare surfaces, restoring a bulk-like structure. The calculated relaxation trends on hydrogen adsorption are discussed in terms of the changes in hybridization between sulfur 3p-π and ruthenium 4d bands, induced by mixing with the hydrogen s-states.