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Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports Meng Xue, Miki Nakayama, Ping Liu, and Michael G White J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017
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Electronic Interactions of Size-Selected Oxide Clusters on Metallic and Thin Film Oxide Supports Meng Xue,2 Miki Nakayama,1§ Ping Liu1 and Michael G. White1,2* 1
Chemistry Department, Brookhaven National Laboratory, Upton NY 11973
2
Department of Chemistry, Stony Brook University, Stony Brook NY 11794
*Corresponding Author;
[email protected],
[email protected] Abstract The interfacial electronic structure of various size-selected metal oxide nanoclusters (M3Ox; M = Mo, Nb, Ti) on Cu(111) and a thin film of Cu2O 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 Cu2O/Cu(111), but those on the Cu2O surface are much larger in magnitude. In addition, sub-stoichiometric or “reduced” clusters exhibit smaller surface dipoles on both the Cu(111) and Cu2O 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 Cu2O/Cu(111), calculations of the surface electrostatic potentials show that the charge distributions associated with cluster adsorption structures or distortions at the cluster-Cu2O-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 Cu2O/Cu(111) are interpreted §
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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.
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1. Introduction Metal oxides play a prominent role in heterogeneous catalysis as both high surface area supports for small metallic particles and as primary catalysts for reactions, with the latter including mixed oxide systems in which one oxide is supported on another.1-7 As a support for metal-based catalysts, the metal oxide can either be a spectator to the chemistry or a direct participant through spill-over and/or by catalyzing specific transformations in a multistep reaction mechanism, e.g., bi-functional catalyst. Examples of bi-functional catalysts are oxide supported metals (Cu, Au, Pt) for the water-gas-shift reaction (WGSR), H2O + CO → CO2 + H2, where CO is adsorbed on the metal and water dissociation takes place on the oxide.8-11 In general, the oxophilic properties (e.g., Lewis acidity) and reducibility (i.e., propensity for oxygen vacancy formation) of the metal oxide cations largely determine the type of reactions that can be promoted. Reducible oxides are often most effective for systems involving water or oxygen dissociation, which are enhanced at surface O-vacancies,12-15 whereas Lewis acid properties of the metal cation are more important for reactions involving other kinds of transformations of oxygenates and alkenes, e.g., partial oxidation, ammoxidation and isomerization.5-6, 16-17 In all cases, the cation oxidation state and coordination at the active site are the key factors. These are strongly influenced by the local metal-to-oxygen stoichiometry, as well as electron transfer when in contact with metal nanoparticles or another oxide support. Information on electron transfer at metal-metal oxide interfaces and metal oxidation states is typically derived from core level shifts in x-ray photoelectron spectroscopy (XPS) and theoretical modeling using a combination of charge partitioning, e.g., Bader charge analyses, and charge density difference maps. Changes in oxidation states of metal oxides are usually clear in XPS as there are often known standard compounds for comparison; however,
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attributing absolute chemical shifts to electron transfer in the initial state can be problematic due to final state screening effects which can mask shifts induced by electron transfer.18-22 Changes in work function are also sensitive to electron transfer as a result of the surface dipole created by charge separation at the cluster-support interface.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 MxOy 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 CeOx/Cu(111) system is more reactive than the conventional ceria supported Cu catalyst.27-35 Electron transfer from Cu to the metal oxide is thought to stabilize O-vacancies, which are the likely active sites for water dissociation on reducible oxides such as TiO2 and CeO2. 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, Mo3O9 on Cu(111) exhibits a large surface dipole (µ = -2.8 D) and no detectable water dissociation, whereas both substoichiometric Nb3O5 (µ = -0.4 D) and stoichiometric Nb4O10 (µ = -2.1 D) clusters on Cu(111) are active for water dissociation.26 In this work, we extend our previous work to the deposition of size-selected metal oxide clusters (M3Ox; M≡ Ti, Nb, Mo) onto a Cu2O/Cu(111) metal oxide thin film to compare with the Cu(111) metallic surface. This extension to the Cu2O surface is relevant to catalysis as Cu+ cations have been implicated as the active sites in CO oxidation on ceria-
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copper catalysts,40-41 as well as in oxygen spill-over on inverse oxide/Cu(111) model catalysts.42 Moreover, mixed oxide surfaces where small nano-islands of a metal oxide are deposited on a Cu2O/Cu(111) oxide support have been found to promote both CO2 and CH4 activation.43-44 Here, a combination of XPS, coverage-dependent work function measurements and DFT calculations are used to investigate the role of cation charge state (oxophilicity), metal-to-oxygen stoichiometry and support (metal or oxide) on electron transfer at the oxide-Cu interface. These studies take advantage of the ability of size-selected deposition to vary the cluster stoichiometry, i.e., introduce oxygen “vacancies”, with atomic precision through mass selection of a specific cluster. This avoids the need for high temperature annealing or sputtering of the surface to introduce O-vacancies, which could lead to other unwanted structural or compositional changes in the cluster/support surface. The experimental results are interpreted with the help of DFT modeling, which provides information on the deposited cluster structures and the relative contributions of the cluster structure and electron transfer to the measured surface dipoles. The results show that electron transfer on both Cu(111) and Cu2O thin film is strongly influenced by cluster stoichiometry. The measured surface dipoles are also found to be much larger for the clusters deposited on the Cu2O thin film, which is shown to be a consequence of the formation of additional cation-oxygen bonds, and in some cases, large structural deformations of the cluster and oxide film.
2. Experimental The metal oxide clusters were generated by reactive DC magnetron sputtering of a metal target with a roughly 2% mixture of O2 in Ar. The magnetron (Oxford Applied Research, NC200U-B) was typically operated at powers of about 100 W for the Mo and Nb oxide clusters and about 230 W for the Ti oxide clusters. The generated cluster cations were
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mass-selected by a quadrupole mass filter to select only the cluster of interest for the experiment, and then deposited onto a Cu(111) crystal (Princeton Scientific; 11 mm diameter, 2 mm thickness) or a Cu2O thin film formed on the Cu(111) as described below. The Cu(111) crystal was cleaned with repeated cycles of Ar+ sputtering (1 keV beam energy, 30 min) and annealing (700–720 K, 20–60 min). The spatially averaged cluster coverages for our work function and XPS measurements were about 0.15 and 0.3 ML, respectively. Details of the cluster beam apparatus and the deposition parameters are provided elsewhere.26, 45 The Cu2O thin film was formed by exposing Cu(111) to 5 × 10–7 Torr of O2 at 650 K for 20 min and maintaining the oxygen environment until the substrate cooled down to 400 K. This procedure results in a single-layer Cu2O film that is primarily in the “44” structure, which resembles a distorted O-Cu-O layer of Cu2O(111) with a hexagonal motif.46-48 The local work function of the samples were measured by one-color 2PPE using the focused third-harmonic beam (~277 nm) generated by an ultrafast Ti:Sapphire laser (SpectraPhysics Tsunami; 750–900 nm tunable, 100-fs pulse width) as described in detail in our previous studies.25-26 The coverage-dependent work function shifts were obtained by taking measurements at the same points on the sample before and after cluster deposition. The local cluster coverage was determined by Auger electron spectroscopy (AES) using a focused electron gun (SPECS, EQ 22/35; beam energy 5 keV, beam size
