Dissociation of water on atomic oxygen-covered Rh nanoclusters supported on graphene/Ru(0001)

We studied the dissociation of water (H₂O*, with * denoting adspecies) on atomic oxygen (O*)-covered Rh nanoclusters (Rh_O ) supported on a graphene film grown on a Ru(0001) surface [G/Ru(0001)] under ultrahigh-vacuum conditions and with varied surface-probe techniques and calculations based on density-functional theory. The graphene had a single rotational domain; its lattice expanded by about 5.7% to match the Ru substrate structurally better. The Rh clusters were grown by depositing Rh vapors onto G/Ru(0001); they had an fcc phase and grew in (111) orientation. Water adsorbed on the Rh clusters was dissociated exclusively in the presence of O*, like that on a Rh(111) single-crystal surface. Contrary to the case on Rh(111)_O , excess O* (even at a saturation level) on small Rh_O clusters (diameter of 30-34 Å) continued to promote, instead of inhibiting, the dissociation of water; the produced hydroxyl (OH*) increased generally with the concentration of O* on the clusters. The difference results from more reactive O* on the Rh_O clusters. O* on Rh_O clusters activated the dissociation via both the formation of hydrogen bonds with H₂O* and abstraction of H directly from H₂O*, whereas O* on Rh(111)_O assisted the dissociation largely via the formation of hydrogen bonds, which was readily obstructed with an increased O* coverage. As the disproportionation (2 OH* → H₂O* + O*) is endothermic on the Rh_O clusters but exothermic on Rh(111)_O , OH* produced on Rh_O clusters showed a thermal stability superior to that on the Rh(111)_O surface-thermally stable up to 400 K.