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 (RhO*) 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 RhO* 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 RhO* clusters. O∗ on RhO* 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 RhO* clusters but exothermic on Rh(111)O*, OH∗ produced on RhO* clusters showed a thermal stability superior to that on the Rh(111)O* surface - thermally stable up to 400 K.