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

Abstract

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.