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

Abstract

The interfacial electronic structure of various size-selected metal oxide nanoclusters (M 3 O x ; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu 2 O supports were investigated by a combination of experimental methods and density functional theory (DFT). These systems explore electron transfer at the metal-metal oxide interface which can modify surface structure, metal oxidation states and catalytic activity. Electron transfer was probed by measurements of surface dipoles derived from coverage dependent work function measurements using two-photon photoemission (2PPE) and metal core level binding energy spectra from x-ray photoelectron spectroscopy (XPS). The measured surface dipoles are negative for all clusters on Cu(111) and Cu 2 O/Cu(111), but those on the Cu 2 O surface are much larger in magnitude. In addition, sub-stoichiometric or "reduced" clusters exhibit smaller surface dipoles on both the Cu(111) and Cu 2 O surfaces. Negative surface dipoles for clusters on Cu(111) suggest Cucluster electron transfer, which is generally supported by DFT-calculated Bader charge distributions. For Cu 2 O/Cu(111), calculations of the surface electrostatic potentials show that the charge distributions associated with cluster adsorption structures or distortions at the cluster-Cu 2 O-Cu(111) interface are largely responsible for the observed negative surface dipoles. Changes observed in the XPS spectra for the Mo 3d, Nb 3d and Ti 2p core levels of the clusters on Cu(111) and Cu 2 O/Cu(111) are interpreted with help from the calculated Bader charges and cluster adsorption structures, the latter providing information about the presence of inequivalent cation sites. The results presented in this work illustrate how the combined use of different experimental probes of along with theoretical calculations can result in a more realistic picture of cluster-support interactions and bonding.

Preparation and Structural Characterization of ZrO2/CuOx/Cu(111) Inverse Model Catalysts

“…However, according to the O 1s spectra in Figure (right), the growth of O peak from CuO x (∼529.4 eV) proves that the decrease in the binding energy of Zr 3d spectra cannot be interpreted by only considering the change of oxidation state because Zr has a much higher formation enthalpy with oxygen (−1097.5 kJ/mol) compared to that of Cu (−170.7 kJ/mol) . A similar phenomenon was also observed in other studies, in which Ti 3 O 5 clusters were deposited on Cu(111) and CuO x /Cu­(111) . It was found that, for Ti 3 O 5 clusters on CuO x /Cu­(111), the Ti 2p shifted to lower energies compared to Ti 3 O 5 on the bare Cu(111) surface, which was explained by the uncompensated changes in the local work functions after the shift from Cu(111) to CuO x /Cu­(111) and also by the possible final state effects.…”

“…37 A similar phenomenon was also observed in other studies, in which Ti 3 O 5 clusters were deposited on Cu(111) and CuO x / Cu(111). 38 It was found that, for Ti 3 O 5 clusters on CuO x / Cu(111), the Ti 2p shifted to lower energies compared to Ti 3 O 5 on the bare Cu(111) surface, which was explained by the uncompensated changes in the local work functions after the shift from Cu(111) to CuO x /Cu(111) and also by the possible final state effects. In addition, an XPS study on ZrO 2 / Pt(111) also observed that under an oxygen ambient, the Zr 3d peaks shifted to slightly lower binding energies because of the presence of adsorbed oxygen on Pt(111).…”

Morphology and reactivity of size-selected titanium oxide nanoclusters on Au(111)

“…48 Calculations for single clusters on the Au(111) surface were performed with U values of 2.5 eV and 4.5 eV, which span the range typically used to describe Ti 3d levels. 29,31,49,50 The resulting optimized cluster structures on the Au(111) surface were essentially identical for both U values, and only the results for U = 2.5 eV are reported here.…”

“…We have recently examined the correlations of electron transfer measured by laser photoemission with reactivity for water dissociation for a number of metal oxide nanoclusters MxOy (M = Ti, Nb, Mo, and W) deposited on Cu(111) as model inverse catalysts for the WGSR. [29][30][31] In those studies, massselected cluster deposition was used to prepare MxOy/Cu(111) surfaces, which allows independent control of cluster size, coverage, and stoichiometry, i.e., metal-to-oxygen ratio. The latter made it possible to investigate the role of cation coordination and the role of O-atom vacancies on the electron transfer with the Cu support and water dissociation reactivity.…”

Ensemble representation of catalytic interfaces: soloists, orchestras, and everything in-between

“…For example, highly electronegative surfaces like Au (111), thought too costly to use for anything other than a model catalyst, will draw electron density out of oxide clusters, 75 whereas other metals such as copper will donate electrons to the cluster instead. 76 Additionally, much like reducible oxides, metal supports may be prone to local oxidation, leading to surface reconstruction and sharp increase in chemical space.…”