Javier Fdez. Sanz

Gold, Copper, and Platinum Nanoparticles Dispersed on CeOx/TiO2(110) Surfaces: High Water-Gas Shift Activity and the Nature of the Mixed-Metal Oxide at the Nanometer Level

At small coverages of ceria on TiO(2)(110), the CeO(x) nanoparticles have an unusual coordination mode. Scanning tunneling microscopy and density-functional calculations point to the presence of Ce(2)O(3) dimers, which form diagonal arrays that have specific orientations of 0, 24, and 42 degrees with respect to the [1 -1 0] direction of the titania substrate. At high coverages of ceria on TiO(2)(110), the surface exhibits two types of terraces. In one type, the morphology is not very different from that observed at low ceria coverage. However, in the second type of terrace, there is a compact array of ceria particles with structures that do not match the structures of CeO(2)(111) or CeO(2)(110). The titania substrate imposes on the ceria nanoparticles nontypical coordination modes, enhancing their chemical reactivity. This phenomenon leads to a larger dispersion of supported metal nanoparticles (M = Au, Cu, Pt) and makes possible the direct participation of the oxide in catalytic reactions. The M/CeO(x)/TiO(2)(110) surfaces display an extremely high catalytic activity for the water-gas shift reaction that follows the sequence Au/CeO(x)/TiO(2)(110) < Cu/CeO(x)/TiO(2)(110) < Pt/CeO(x)/TiO(2)(110). For low coverages of Cu and CeO(x), Cu/CeO(x)/TiO(2)(110) is 8-12 times more active than Cu(111) or Cu/ZnO industrial catalysts. In the M/CeO(x)/TiO(2)(110) systems, there is a strong coupling of the chemical properties of the admetal and the mixed-metal oxide: The adsorption and dissociation of water probably take place on the oxide, CO adsorbs on the admetal nanoparticles, and all subsequent reaction steps occur at the oxide-admetal interface. The high catalytic activity of the M/CeO(x)/TiO(2)(110) surfaces reflects the unique properties of the mixed-metal oxide at the nanometer level.

Water‐Gas Shift Reaction on a Highly Active Inverse CeOx/Cu(111) Catalyst: Unique Role of Ceria Nanoparticles

=0.03 nA. Theheight image at the bottom right, showing the inside of a ceria island,was taken at imaging conditions of 2.7 V, 0.05 nA. The scheme (bot-tom left) was composed using the line profile indicated by the greenline shown near the middle of the top right image.[*] Dr. J. A. Rodriguez, Dr. J. Graciani, Dr. J. B. Park, Dr. F. Yang,Dr. D. Stacchiola, Dr. S. D. Senanayake, Dr. S. Ma, Dr. P. Liu,Dr. J. HrbekChemistry Department, Brookhaven National LaboratoryUpton, NY 11973 (USA)Fax: ( +1)631-344-5815E-mail: rodrigez@bnl.govProf. J. Evans, Prof. M. PrezFacultad de Ciencias, Universidad Central de VenezuelaCaracas 1020A (Venezuela)Prof. J. F. SanzDepartamento de Qumica Fsica, Universidad de Sevilla41012-Seville (Spain)[**] TheworkperformedatBNLwassupportedbytheUSDepartmentofEnergy, Office of Basic Energy Sciences, under contract DE-AC02-98CH10886. J.E. and M.P. are grateful to INTEVEP for partialsupport of the work carried out at the UCV. The work done at Sevillewas funded by MICINN, grant no MAT2008-04918 and theBarcelona Supercomputing Center—Centro Nacional de Super-computacin (Spain).Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/anie.200903918.

Importance of the Metal–Oxide Interface in Catalysis: In Situ Studies of the Water–Gas Shift Reaction by Ambient-Pressure X-ray Photoelectron Spectroscopy†

The traditional approach to the optimization of metal/oxide catalysts has focused on the properties of the metal and the selection of the proper oxide for its dispersion. The importance of metal–oxide interfaces has long been recognized, [1] but the molecular determination of their properties and role is only now emerging. [2] Atoms with properties ranging from metallic to ionic are available at the interface and create unique reaction sites. We show herein how sites associated with a metal–ceria interface can dramatically change the reaction mechanism of the water–gas shift reaction (WGSR; CO + H2O!H2 + CO2). The WGSR is critical in the production of hydrogen. Multiple reaction mechanisms have been proposed. [3] In the redox mechanism, CO reacts with oxygen derived from the dissociation of H2O. In the associative process, the formation of a carbonaceous COxHy intermediate must precede the production of H2 and CO2. In situ studies are essential for the detection of surface species and active phases only present under the reaction conditions. [4] We present a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP XPS), infrared reflection absorption spectroscopy (IRRAS), and density functional theory (DFT) calculations used to study the WGSR on CeOx nanoparticles deposited on Cu(111) and Au(111). Under WGSR conditions, adsorbed bent carboxylate (CO2 d� ) species were identified over both CeOx/Cu(111) and CeOx/ Au(111), with the ceria in a highly reduced state. By combining in situ experimental results with calculations, we

CO Oxidation on Inverse CeOx/Cu(111) Catalysts: High Catalytic Activity and Ceria-Promoted Dissociation of O2

A Cu(111) surface displays a low activity for the oxidation of carbon monoxide (2CO + O(2) → 2CO(2)). Depending on the temperature, background pressure of O(2), and the exposure time, one can get chemisorbed O on Cu(111) or a layer of Cu(2)O that may be deficient in oxygen. The addition of ceria nanoparticles (NPs) to Cu(111) substantially enhances interactions with the O(2) molecule and facilitates the oxidation of the copper substrate. In images of scanning tunneling microscopy, ceria NPs exhibit two overlapping honeycomb-type moiré structures, with the larger ones (H(1)) having a periodicity of 4.2 nm and the smaller ones (H(2)) having a periodicity of 1.20 nm. After annealing CeO(2)/Cu(111) in O(2) at elevated temperatures (600-700 K), a new phase of a Cu(2)O(1+x) surface oxide appears and propagates from the ceria NPs. The ceria is not only active for O(2) dissociation, but provides a much faster channel for oxidation than the step edges of Cu(111). Exposure to CO at 550-750 K led to a partial reduction of the ceria NPs and the removal of the copper oxide layer. The CeO(x)/Cu(111) systems have activities for the 2CO + O(2) → 2CO(2) reaction that are comparable or larger than those reported for surfaces of expensive noble metals such as Rh(111), Pd(110), and Pt(100). Density-functional calculations show that the supported ceria NPs are able to catalyze the oxidation of CO due to their special electronic and chemical properties. The configuration of the inverse oxide/metal catalyst opens new interesting routes for applications in catalysis.

Frequency dependent hyperpolarizabilities with application to formaldehyde and methyl fluoride

Methods for the determination of the frequency dependent hyperpolarizabilities, β(−2ω;ω,ω), β(−ω;ω,0), and β(0;ω,−ω) at the self‐consistent field (SCF) level of theory are discussed and compared. Two efficient alternatives are proposed; one involves determination of appropriate response vectors which arise in first‐order perturbation theory at frequency ω, and the other involves solving first‐order perturbation theory equations at frequency 0 (static), ω and 2ω. Neither approach involves solution of the second‐order perturbation theory equations. The method is illustrated with application to formaldehyde and methyl fluoride using large one‐particle basis sets. The basis set requirement for convergence of this property at the SCF level of theory is investigated. Basis sets including up to d functions on first row atoms and p functions on hydrogen incorporating diffuse functions appear to be adequate. The effect of electron correlation on the static hyperpolarizability is estimated using second‐order Mo/lle...

Real-Time TD-DFT Simulations in Dye Sensitized Solar Cells: The Electronic Absorption Spectrum of Alizarin Supported on TiO2 Nanoclusters

The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO2)n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient.

Au ↔ N Synergy and N-Doping of Metal Oxide-Based Photocatalysts

N-doping of titania makes photocatalytic activity possible for the splitting of water, and other reactions, under visible light. Here, we show from both theory and experiment that Au preadsorption on TiO2 surfaces significantly increases the reachable amount of N implanted in the oxide. The stabilization of the embedded N is due to an electron transfer from the Au 6s levels toward the N 2p levels, which also increases the Au-surface adhesion energy. Theoretical calculations predict that Au can also stabilize embedded N in other metal oxides with photocatalytic activity, such as SrTiO3 and ZnO, producing new states above the valence band or below the conduction band of the oxide. In experiments, the Au/TiN(x)O(2-y) system was found to be more active for the dissociation of water than TiO2, Au/TiO2, or TiO(2-y). Furthermore, the Au/TiN(x)O(2-y) surfaces were able to catalyze the production of hydrogen through the water-gas shift reaction (WGS) at elevated temperatures (575-625 K), displaying a catalytic activity superior to that of pure copper (the most active metal catalysts for the WGS) or Cu nanoparticles supported on ZnO.

Comparative Study on the Performance of Hybrid DFT Functionals in Highly Correlated Oxides: The Case of CeO2 and Ce2O3

The outstanding catalytic properties of cerium oxides rely on the easy Ce(3+) ↔ Ce(4+) redox conversion, which however constitutes a challenge in density functional based theoretical chemistry due to the strongly correlated nature of the 4f electrons present in the reduced materials. In this work, we report an analysis of the performance of five exchange-correlation functionals (HH, HHLYP, PBE0, B3LYP, and B1-WC) implemented in the CRYSTAL06 code to describe three properties of ceria: crystal structure, band gaps, and reaction energies of the CeO2 → Ce2O3 process. All five functionals give values for cell parameters that are in fairly good agreement with experiment, although the PBE0 hybrid functional is found to be the most accurate. Band gaps, 2p-4f-5d in the case of CeO2 and 4f-5d in the case of Ce2O3, are found to be, in general, overestimated and drop off when the amount of Hartree-Fock exchange in the exchange-correlation functional decreases. In contrast, the reaction energies are found to be underestimated, and increase when the amount of HF exchange lowers. Overall, at its standard formulation, the B1-WC functional seems to be the best choice as it provides good band gaps and reaction energies, and very reasonable crystal parameters.

A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts

The structural and electronic properties of CeO(x) species supported on the rutile TiO(2)(110) surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce(3+) ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce(2)O(3) dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeO(x) nanoparticles (NPs) on the TiO(2) surface is highly exothermic and our calculations show that the redox properties of the Ce(III)-Ce(IV) couple are significantly altered when it is supported on TiO(2). In particular the reactivity against CO/O(2) indicates that on the surface the presence of Ce(III) is favored over Ce(IV) species. Our results also indicate that the CeO(x)/TiO(2) interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M/CeO(x)/TiO(2) (M=Au,Cu,Pt) systems in the water-gas shift reaction, we have examined the dissociation of water on the CeO(x)/TiO(2) surface and estimated a barrier as small as 0.04 eV, i.e. approximately 8 times smaller than that computed for a TiO(2) oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M/CeO(x)/TiO(2) systems over M/TiO(2).

Accurate ab initio determination of magnetic interactions and hopping integrals in La2−xSrxCuO4 systems

The nature of magnetic interactions and electron transfer processes in La2−xSrxCuO4 systems are studied, by means of an ab initio embedded cluster model approach, using a difference dedicated configuration interaction (DDCI) procedure. For the undoped system, the crucial role played by the ligand to metal charge transfer (LMCT) configurations in the magnetic process makes necessary the use of an enlarged DDCI space, which explicitly takes account of the relaxation of these LMCT configurations. This procedure allows us to approach the experimental magnetic coupling constant value. In hole-doped systems, the value obtained for the electron transfer integral, t, is of 0.54–0.57 eV. The extra hole, characterized from the nature and occupation of differential natural orbitals, has a strong p character (approximately 50%) and is essentially localized in CuO2 planes. These results are in agreement with the experimental evidence about these kinds of compounds. Neither the value of t nor the nature of the extra ho...