Thermodynamics of hydrogen adsorption on ruthenium fcc surfaces: a density functional theory study

Within the framework of the application of liquid organic hydrogen carriers (LOHC) to store, transport and re-generate hydrogen, ruthenium (Ru) is by far the most widely used catalyst. In its natural bulk state, the most abundant phase observed is the hexagonal close-packed (hcp) phase, but experimental studies on nanoparticles have shown that the face-centred cubic (fcc) phases are also present and are highly active in catalytic reactions. In this study, we have carried out calculations based on the density functional theory, with the generalized gradient approximation and long-range dispersion corrections, to investigate the behaviour of hydrogen adsorption at the fcc Ru (001), (011) and (111) surfaces. The Ru surfaces have been covered systematically with hydrogen (H), with a focus on the geometries, stabilities and adsorption energies. A detailed analysis has been performed of the energetic and electronic properties of a hydrogen monolayer on the Ru surfaces, combined with a thermodynamic analysis of the effect of temperature and pressure on the surface coverage, where the highest surface coverage observed was on the Ru (001) and (011) surfaces. The results indicate that the dissociation of H₂ occurs readily and that the adsorption energies of single H atoms are between 0.4 and 0.6 eV. Neither recombination of H atoms to form molecular hydrogen (H₂) or surface poisoning was observed.