Influence of Cluster–Support Interactions on Reactivity of Size-Selected Nb_xO_y Clusters

Size-selected niobium oxide nanoclusters (Nb₃O₅, Nb₃O₇, Nb₄O₇, and Nb₄O₁₀) were deposited at room temperature onto a Cu(111) surface and a thin film of Cu₂O on Cu(111), and their interfacial electronic interactions and reactivity toward water dissociation were examined. These clusters were specifically chosen to elucidate the effects of the oxidation state of the metal centers; Nb₃O₅ and Nb₄O₇ are the reduced counterparts of Nb₃O₇ and Nb₄O₁₀, respectively. From two-photon photoemission spectroscopy (2PPE) measurements, we found that the work function increases upon cluster adsorption in all cases, indicating a negative interfacial dipole moment with the positive end pointing into the surface. The amount of increase was greater for the clusters with more metal centers and higher oxidation state. Further analysis with DFT calculations of the clusters on Cu(111) indicated that the reduced clusters donate electrons to the substrate, indicating that the intrinsic cluster dipole moment makes a larger contribution to the overall interfacial dipole moment than charge transfer. X-ray photoelectron spectroscopy (XPS) measurements showed that the Nb atoms of Nb₃O₇ and Nb₄O₁₀ are primarily Nb⁵⁺ on Cu(111), while for the reduced Nb₃O₅ and Nb₄O₇ clusters, a mixture of oxidation states was observed on Cu(111). Temperature-programmed desorption (TPD) experiments with D₂O showed that water dissociation occurred on all systems except for the oxidized Nb₃O₇ and Nb₄O₁₀ clusters on the Cu₂O film. A comparison of our XPS and TPD results suggests that Nb⁵⁺ cations associated with NbO terminal groups act as Lewis acid sites which are key for water binding and subsequent dissociation. TPD measurements of 2-propanol dehydration also show that the clusters active toward water dissociation are indeed acidic. DFT calculations of water dissociation on Nb₃O₇ support our TPD results, but the use of bulk Cu₂O­(111) as a model for the Cu₂O film merits future scrutiny in terms of interfacial charge transfer. The combination of our experimental and theoretical results suggests that both Lewis acidity and metal reducibility are important for water dissociation.