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

Abstract

Size-selected niobium oxide nanoclusters (Nb 3 O 5 , Nb 3 O 7 , Nb 4 O 7 , and Nb 4 O 10 ) were deposited at room temperature onto a Cu(111) surface and a thin film of Cu 2 O 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; Nb 3 O 5 and Nb 4 O 7 are the reduced counterparts of Nb 3 O 7 and Nb 4 O 10 , respectively. From twophoton 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 Nb 3 O 7 and Nb 4 O 10 are primarily Nb 5+ on Cu(111), while for the reduced Nb 3 O 5 and Nb 4 O 7 clusters, a mixture of oxidation states was observed on Cu(111). Temperature-programmed desorption (TPD) experiments with D 2 O showed that water dissociation occurred on all systems except for the oxidized Nb 3 O 7 and Nb 4 O 10 clusters on the Cu 2 O film. A comparison of our XPS and TPD results suggests that Nb 5+ 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 Nb 3 O 7 support our TPD results, but the use of bulk Cu 2 O(111) as a model for the Cu 2 O 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.

Kinetic and spectroscopic evidence for reaction pathways and intermediates for olefin epoxidation on Nb in *BEA

“…S7). These features shift to lower binding energies when the Nb 5.0 -β sample has been treated with H 2 O 2 (details given in section 2.4).Figure 4bshows that XPS spectra of H 2 O 2 -activated Nb 5.0 -β contains overlapping doublet features with doublets at 210.6 and 208.0 eV as well as at 209.7 and 207.0 eV, which resemble reported values for Nb V and Nb IVspecies[72],[73], and is consistent with the presence of the proposed Nb V -(O 2 ) and Nb IV -(O 2 ) species, respectively. Under ultra-high vacuum conditions (i.e., 10 -9 Torr), the preferred state of the H 2 O 2 -activated Nb-β is Nb IV -(O 2 ), because the lack of free protons favors the formation of the superoxo species, as shown for H 2 O 2 -activated TS-, which explains why the magnitude of the features for Nb IV is much larger than for Nb V on H 2 O 2 -treated materials.…”

“…X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation state of Nb in both untreated and H 2 O 2 -activated (Section 2.4) Nb 5.0 -β samples in order to verify the presence of Nb IV species (i.e., Nb IV -(O 2 )). Figure 4a shows the XPS spectra for untreated Nb 5.0 -β in the Nb 3d region (200 -220 eV), which possesses a doublet with absorbance peaks centered at 210.6and 208.0 eV that correspond to Nb V 3d 3/2 and 3d 5/2 , respectively[72,73] (fitting that demonstrates the absence of Nb IV is provided in the SI,Fig. S7).…”

Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports

“…[23][24] We have recently shown that surface dipole moments derived from coverage-dependent work function shifts of metal oxide clusters deposited on a metal surface can be used to probe the electron transfer between the cluster and metal support. [25][26] The surfaces in these studies were prepared by depositing size-selected M x O y clusters (M = Mo, W, Ti, Nb) onto a Cu(111) surface as model "inverse" catalysts for the WGSR. Inverse models of WGSR catalysts have helped clarify the role of the metal oxide, and for Cu-ceria catalysts, the inverse CeO x /Cu(111) system is more reactive than the conventional ceria supported Cu catalyst.…”

“…13-14, 31, 36-39 The measured surface dipoles for the oxide clusters deposited on Cu(111) were found to be consistent with Cuoxide cluster electron transfer, the extent of which roughly correlated with the bulk work function of the metal oxide (stoichiometric and reduced) and also the metal-to-oxygen stoichiometry of the cluster. [25][26] Temperature programmed reaction studies show that electron transfer alone, however, is a poor predictor of water dissociation activity. For example, Mo 3 O 9 on Cu(111) exhibits a large surface dipole ( = -2.8 D) and no detectable water dissociation, whereas both substoichiometric Nb 3 O 5 ( = -0.4 D) and stoichiometric Nb 4 O 10 ( = -2.1 D) clusters on Cu(111) are active for water dissociation.…”

Self-organizing layers from complex molecular anions

“…Deposition of ions without their counterions from the gas phase is enabled by ion soft landing, a technique first introduced by Cooks and coworkers 11 , which enables immobilization of ions of one polarity on a support. Soft landing has been used previously for conformational enrichment of peptides 12 , understanding protein folding on surfaces 13 , 14 , deposition of intact protein assemblies for microscopic imaging 15 , generation of microarrays for biological screening 16 , preparation of well-defined model catalysts through deposition of ionic clusters, nanoparticles and organometallics 17 – 23 and in material sciences (e.g., thin composite materials 24 and processing of graphene 25 ). In these studies, relatively low coverage samples were prepared for characterization and surface science experiments 26 .…”