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 CeOx species supported on the rutile TiO2 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 Ce3+ 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 Ce2O3 dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeOx nanoparticles NPs on the TiO2 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 TiO2. In particular the reactivity against CO/O2 indicates that on the surface the presence of Ce III is favored over Ce IV species. Our results also indicate that the CeOx /TiO2 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 /CeOx /TiO2 M =Au,Cu,Pt systems in the water-gas shift reaction, we have examined the dissociation of water on the CeOx /TiO2 surface and estimated a barrier as small as 0.04 eV, i.e. 8 times smaller than that computed for a TiO2 oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M /CeOx /TiO2 systems over M /TiO2.Gobierno de España. Ministerio Ciencia e Innovación (MICINN) MAT2005-01872 y CSD2008-0023Junta de Andalucía FQM-132Departamento de Energía de EE. UU. División de Subvención de Ciencias Químicas DE-AC02-98CH1088

Interaction of oxygen with TiN (001): N↔O exchange and oxidation process

This work presents a detailed experimental and theoretical study of the oxidation of TiN(001) using a combination of synchrotron-based photoemission and density functional theory (DFT). Experimentally, the adsorption of O2 on TiN(001) was investigated at temperatures between 250 and 450K. At the lowest temperature, there was chemisorption of oxygen (O2,gas→2Oads) without significant surface oxidation. In contrast, at 450K the amount of O2 adsorbed increased continuously, there was no evidence for an oxygen saturation coverage, a clear signal in the Ti 2p core level spectra denoted the presence of TiOx species, and desorption of both N2 and NO was detected. The DFT calculations show that the adsorption/dissociation of O2 is highly exothermic on a TiN(001) substrate and is carried out mainly by the Ti centers. A high oxygen coverage (larger than 0.5 ML) may induce some structural reconstructions of the surface. The exchange of a surface N atom by an O adatom is a highly endothermic process (ΔE=2.84eV). However, the overall oxidation of the surface layer is thermodynamically favored due to the energy released by the dissociative adsorption of O2 and the formation of N2 or NO. Both experimental and theoretical results lead to conclude that a TiN+mO2→TiOx+NO reaction is an important exit channel for nitrogen in the oxidation process.Ministerio de Educación y Ciencia de España, MEC. MAT2005-01872Junta de Andalucía. FQM-132División de Ciencias Químicas del Departamento de Energía de EE. UU. DE-AC02-98CH10086Fundación japonesa para la Ciencia de los Materiale

Interaction potentials from periodic density-functional theory calculations: Molecular-dynamics simulations of Au clusters deposited on the TiN (001) surface

Molecular-dynamics simulations of gold particles deposited on a TiN 001 surface have been accounted for through classical pair potentials describing the atom force field. The interaction between Ti–N, Ti–Ti, N–N, Au–Au, Au–Ti, and Au–N pairs was estimated by following a procedure in which the interaction energy between two sets of atoms is estimated from density-functional calculations performed with periodic boundary conditions using plane waves as basis set. The pair potentials were expressed as the sum of two contributions: long range in a Coulomb form and a short-range term, which included the rest of the energy contributions. Simulations of the TiN 001 isolated surface reproduced the already described surface relaxation, with a rippling parameter in agreement with that found from a purely first-principles approach. Simulations of gold deposition on such surfaces showed the formation of metal clusters with well-defined fcc structure and epitaxially grown.Ministerio de Ciencia y Tecnología de España MAT2005-01872Junta de Andalucía FQM-13

Effect of dispersion correction on the Au(1 1 1)-H2O interface: A first-principles study

A theoretical study of the H2O-Au(1 1 1) interface based on first principles density functional theory (DFT) calculations with and without inclusion of dispersion correction is reported. Three different computational approaches are considered. First, the standard generalized gradient approximation (GGA) functional PBE is employed. Second, an additional energy term is further included that adds a semi-empirically derived dispersion correction (PBE-D2), and, finally, a recently proposed functional that includes van der Waals (vdW) interactions directly in its functional form (optB86b-vdW) was used to represent the state-of-the art of DFT functionals. The monomeric water adsorption was first considered in order to explore the dependency of geometry on the details of the model slab used to represent it (size, thickness, coverage). When the dispersion corrections are included the Au-H2O interaction is stronger, as manifested by the smaller dAu-O and stronger adsorption energies. Additionally, the interfacial region between Au(1 1 1) slab surfaces and a liquid water layer was investigated with Born-Oppenheimer molecular dynamics (BOMD) using the same functionals. Two or three interfacial orientations can be determined, depending on the theoretical methodology applied. Closest to the surface, H2O is adsorbed O-down, whereas further away it is oriented with one OH bond pointing to the surface and the molecular plane parallel to the normal direction. For the optB86b-vdW functional a third orientation is found where one H atom points into the bulk water layer and the second OH bond is oriented parallel to the metal surface. As for the water density in the first adsorption layer we find a very small increase of roughly 8%. From the analysis of vibrational spectra a weakening of the H-bond network is observed upon the inclusion of the Au(1 1 1) slab, however, no disruption of H-bonds is observed. While the PBE and PBE-D2 spectra are very similar, the optB86b-vdW spectrum shows that the H-bonds are even more weakened.Ministerio de Ciencia e Innovación de España-MAT2012-31526 y CSD2008-002

The constrained space orbital variation analysis for periodic ab initio calculations

The constrained space orbital variation (CSOV) method for the analysis of the interaction energy has been implemented in the periodic ab initio CRYSTAL03 code. The method allows for the partition of the energy of two interacting chemical entities, represented in turn by periodic models, into contributions which account for electrostatic effects, mutual polarization and charge transfer. The implementation permits one to carry out the analysis both at the Hartree-Fock and density functional theory levels, where in the latter the most popular exchange-correlation functionals can be used. As an illustrating example, the analysis of the interaction between CO and the MgO (001) surface has been considered. As expected by the almost fully ionic character of the support, our periodic CSOV results, in general agree with those previously obtained using the embedded cluster approach, showing the reliability of the present implementation.Ministerio de Educación y Ciencia de España MAT2005-1872Secretaría de la Educación Pública (SEP)-Consejo Nacional de Ciencia y Tecnología (CONACYT). Gobierno de México SEP-2004-CO1-4698

Mechanism of Cu deposition on the α−Al2O3 (0001) surface

The growth mechanism of the Cu/α−Al2O3 (0001) interface is studied by first-principles molecular-dynamics simulations as a function of the transition-metal coverage (θ) and the temperature of the system. On the anhydrous surface growth of Cu(0) 3D clusters is predicted. On the partially hydroxylated surface, a Cu(I) monolayer, relatively stable upon the temperature rising, is first observed (θ<1/3  ML). Increasing Cu loading leads to Cu(I)/Cu(0) mixed phases that when heated aggregate into 3D particles increasing the number of Cu(0) atoms, in agreement with the Auger spectra of Kelber et al.Ministerio de Ciencia y Tecnología MAT2002-057

Molecular-dynamics simulations of (NaO2)x(SiO2)1-x glasses: Relation between distribution and diffusive behavior of Na atoms

Molecular-dynamics simulations of sodium silicate glasses in a range of alkali concentration going from 1.8% to 33.33% molar of Na2O are reported. Our simulations show that there is a tight relationship between Na atoms and nonbridging oxygens which are mainly located in the first Na coordination shell. In the whole range of composition, Na and nonbridging oxygen atoms appear to segregate giving rise to a heterogeneous distribution. For the higher alkali concentrations, formation of microchannels is observed. The activation energies for alkali diffusion have been computed and found in agreement with the experiment. The mechanism for the diffusion has also been investigated and found to occur through the microchannels, if present, or across the network for low compositions, but the Na motions always appear to be assisted by nonbridging oxygen atoms.Dirección General de Investigación Científica y Técnica PB95-1247Unión Europea ERBCT1-CT94-006

Ab initio group model potentials including electron correlation effects

A method for determination of ab initio group model potentials, with the intention of describing the effects of a whole molecule or a chemical group within a density functional theory framework, is reported. The one-electron part of the Kohn–Sham equations is modified by incorporation of a Coulomb operator, which accounts for the classical electron interaction arising from the group. Exchange and correlation effects are introduced by a suitable modification of the exchange-correlation functionals. The strong orthogonality condition, usually required by the theory of separability of many electron systems, is written in terms of first order reduced density matrices. In order to check the method a group model potential for H2O (environment) was obtained and employed in the calculation of HF⋯H2O and H2O⋯H2O complexes using several functionals. Equilibrium intergroup distances and binding energies are compared with all-electron calculations.Dirección General de Enseñanza Superior (DGES). España PB98-1125Junta de Andalucía FQM13

N doping of TiO 2(110) Photoemission and density-functional studies

The electronic properties of N-doped rutile TiO2(110) have been investigated using synchrotron-based photoemission and density-functional calculations. The doping via N+2 ion bombardment leads to the implantation of N atoms (∼5% saturation concentration) that coexist with O vacancies. Ti 2p core level spectra show the formation of Ti3+ and a second partially reduced Ti species with oxidation states between +4 and +3. The valence region of the TiO2−xNy(110) systems exhibits a broad peak for Ti3+ near the Fermi level and N-induced features above the O 2p valence band that shift the edge up by ∼0.5eV. The magnitude of this shift is consistent with the “redshift” observed in the ultraviolet spectrum of N-doped TiO2. The experimental and theoretical results show the existence of attractive interactions between the dopant and O vacancies. First, the presence of N embedded in the surface layer reduces the formation energy of O vacancies. Second, the existence of O vacancies stabilizes the N impurities with respect to N2(g) formation. When oxygen vacancies and N impurities are together there is an electron transfer from the higher energy 3d band of Ti3+ to the lower energy 2p band of the N2− impurities.Ministerio de Ciencia y Tecnología español MAT2005-01872Junta de Andalucía FQM-13