In this paper, geometric bulk parameters, bulk moduli, energy gaps and relative stabilities of the TiO 2 anatase and rutile phases were determined from periodic DFT calculations. Then, for the rutile phase, structures, relaxations and surface energies of the (110), (100), (101) and (001) faces were computed. The calculated surface energies are consistent with the natural rutile powder composition, even if a dependence on the number of layers of the slab used to model the surface was identified. Internal constraints, consisting in freezing some internal 2 layers of the slab to atomic bulk positions, were thus added to mimic the bulk hardness in order to stabilize the computed surface energies for thinner systems. In parallel, the influence of pseudopotentials was studied and it appears that four valence electrons for titanium atoms are sufficient. The aim of this study was to optimise accurate rutile TiO 2 surfaces models that will be used in further calculations to investigate water and uranyl ion sorption mechanisms.

XPS and periodic DFT calculations have been used to investigate water sorption on the TiO 2 rutile (110) face. Two sets of XPS spectra were collected on the TiO 2 (110) single crystal 2 clean and previously exposed to water: the first set with photoelectrons collected in a direction parallel to the normal to the surface; and the second set with the sample tilted by 70°, respectively. This tilting procedure promotes the signals from surface species and reveals that the first hydration layer is strongly coordinated to the surface and also that, despite the fact that the spectra were recorded under ultra-high vacuum, water molecules subsist in upper hydration layers. In addition, periodic DFT calculations were performed to investigate the water adsorption process to determine if molecular and/or dissociative adsorption takes place.The first step of the theoretical part was the optimisation of a dry surface model and then the investigation of water adsorption. The calculated molecular water adsorption energies are consistent with previously published experimental data and it appears that even though it is slightly less stable, the dissociative water sorption can also take place. This assumption was considered, in a second step, on a larger surface model where molecular and dissociated water molecules were adsorbed together with different ratio. It was found that, due to hydrogen bonding stabilisation, molecular and dissociated water molecules can coexist on the surface if the ratio of dissociated water molecules is less than ≈ 33 %. These results are consistent with previous experimental works giving a 10-25 % range.

A combined theoretical and experimental analysis of the electrode potential dependencies of activation energies is presented for the first step in oxygen reduction over platinum and platinum alloy catalysts in both polycrystalline and carbon supported form. Tafel data for several of the catalysts are used to predict potential-dependent activation energies for oxygen reduction over the 0.6-0.9 V range in strong and weak acid. Comparisons with the theoretical curve show good agreement above 0.8 V, suggesting a fairly constant preexponential factor. Arrhenius determinations of activation energies over the 0.7-0.9 V range yield little trend for weak acid, possibly because of the larger uncertainties in the Arrhenius fits, but the strong acid results have smaller uncertainties and for them the measured activation energies trend up with potential.

Dipole polarizabilities of a series of ions in aqueous solutions are computed from first-principles. The procedure is based on the study of the linear response of the maximally localized Wannier functions to an applied external field, within density functional theory. For most monoatomic cations (Li(+), Na(+), K(+), Rb(+), Mg(2+), Ca(2+) and Sr(2+)) the computed polarizabilities are the same as in the gas phase. For Cs(+) and a series of anions (F(-), Cl(-), Br(-) and I(-)), environmental effects are observed, which reduce the polarizabilities in aqueous solutions with respect to their gas phase values. The polarizabilities of H((aq)) (+), OH((aq)) (-) have also been determined along an ab initio molecular dynamics simulation. We observe that the polarizability of a molecule instantaneously switches upon proton transfer events. Finally, we also computed the polarizability tensor in the case of a strongly anisotropic molecular ion, UO(2) (2+). The results of these calculations will be useful in building interaction potentials that include polarization effects.

Periodic density functional theory (DFT) calculations using plane-wave basis sets were performed in order to study the bulk of nickel ferrite NiFe2O4. The local spin density approximation (LSDA) and the generalized gradient approximation (GGA) formalism were used, and it appeared that the LSDA failed to describe the magnetic structure of this compound. However, the GGA formalism gave reliable results in good agreement with experimental data for the lattice parameters, the electronic properties and the bulk modulus. In addition, the calculated density of states of the metallic species d block as well as their local magnetic moments were correlated to the crystal-field theory. Then, a charge deformation map was computed and, as expected from the electronegativity scale, the electron excess is localized around oxygen atoms along the bond axes. The formation energies of metallic vacancies are in good agreement with the inverse spinel structure experimentally observed.

We present results of a periodic spin-density-functional theory study of the electronic structure and the local adsorption properties of the annealed Pt 3 Cr alloy surface in comparison with the Pt͑111͒ surface. Each is modeled as a four-layer slab where the two topmost layers and the adsorbates are allowed to relax. The annealed alloy has Pt segregated to the surface and is modeled by a Pt͑111͒ monolayer covering the Pt 3 Cr(111) L1 2 face bulk phase alloy. We have calculated OH and H 2 O structures and adsorption energies at low coverage. The top adsorption sites are predicted to be the most stable for these adsorbates on both surfaces, but adsorption energies for both decrease on the Pt skin, with a larger decrease for OH. An empirical model based on reaction energy calculations in acid is used to estimate the reversible potential of OH ads formation from H 2 O ads on the Pt skin and on Pt͑111͒ in acid. A positive shift (⌬U ‫ؠ‬ ϭ 0.11 V͒ is predicted for the surface of the Pt skin relative to Pt͑111͒. This result is in qualitative agreement with the 40-60 mV reduction in overpotential observed experimentally for oxygen reduction on the Pt 3 Cr alloy compared to Pt surfaces, which is attributed to OH ads being a surface poison.This last decade, considerable attention has been given to developing an understanding of the oxygen reduction reaction ͑ORR͒ on Pt and Pt-bimetallic alloy surfaces. The ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. An often-quoted scheme for the pathways by which O 2 is reduced at metal surfaces is due to Wroblowa et al. 1 ͓1͔Here it is assumed that O 2 can be electrochemically reduced either directly to water without intermediate formation of H 2 O 2ads ͑called direct 4e Ϫ reduction͒, or to H 2 O 2ads with assumed equilibria with O 2ads and gas-phase H 2 O 2 ͑series 2e Ϫ reduction͒, or the H 2 O 2ads intermediate is further reduced to water ͑series 4e Ϫ pathway͒. Recent theoretical studies from our laboratory suggested that a series pathway via an H 2 O 2ads intermediate may be operative on Pt catalyst when only onefold surface sites are available 2When twofold sites are available a novel mechanism not involving adsorbed H 2 O 2 was predicted 3The standard reversible potential for O 2 reduction in water is 1.23 V on the standard hydrogen scale ͑SHE͒, but due to kinetic effects, an oxygen cathode in a fuel cell has a working potential of around 0.8 V, corresponding to an overpotential of 400 mV. The overpotential is believed to be caused by the electrosorption of molecules at around 0.8 V that block the bonding of O 2 to the surface sites needed for reduction. 4-6 It is presently thought that in acid solution, OH ads is formed on Pt͑111͒ at this potential from oxidation of water 4,7 H 2 O ads → OH ads ϩ H aq ϩ ϩ e Ϫ ͑ U͒ ͓10͔In voltammograms the current associated with OH ads formation begins to flow at 0.6 V and the current density increases slowly to 0.7 V, then rises to a peak at 0.8 V, and finally drops to a ...

A study combining theoretical predictions and experimental measurements was made to gain an understanding of the beneficial effect of alloying cobalt into platinum for electroreduction of oxygen. Carbon-supported Pt 3 Co catalyst particles were characterized by X-ray diffraction spectroscopy and X-ray absorption near-edge structure, which gave evidence for a surface layer composed of Pt, called the Pt skin. Electrochemical measurements were made in 1 M trifluoromethane sulfonic acid with a rotating ring disk setup. Cyclic voltammetry showed significantly less oxide formation in the Ͼ0.8 V range over the skin on the alloy compared to nonalloyed Pt. Tafel plots showed a 50-70 mV reduction in overpotential for O 2 reduction over the Pt skin. The Vienna Ab Initio Simulation Program was used for calculating H 2 O and OH adsorption bond strengths on the Pt skin on Pt 3 Co͑111͒ for comparison with prior work with the Pt͑111͒ surface. The bond strength variations were used to estimate the shift in reversible potential for OH ads formation from H 2 O ads oxidation. A shift of 80 mV was found, which indicates that an increase in the reversible potential for OH ads formation correlates with the decrease in overpotential for O 2 reduction over the Pt skin on Pt 3 Co nanoparticles.

The quantification of He and Ne diffusion behavior in crystals rich in U and Th such as zircon is key for the interpretation of (U-Th)/ 4 He and (U-Th)/ 21 Ne thermochronometric ages.Multiple parameters such as chemical substitution, channel obstruction and damage can modify the diffusivity compared to a pristine structure. To investigate the impact of these parameters, we have conducted a theoretical diffusion study combining a series of methods and approaches to address the problem across the necessary range of scales (atomic to crystal size). First, using quantum calculation, we determine the different He and Ne insertion sites, insertion energies and diffusion pathways at the atomic scale for an ideal pristine zircon structure (i.e. damage free). These results serve as input for a 3D random walk simulation of atomic trajectories that provides diffusion coefficients for damage-free zircon crystals. Second, as natural zircon crystals are not perfect, we model the impact of different types of damage and diffusion pathway obstruction at the atomic level on He and Ne diffusion in 3D. The calculated He and Ne diffusion coefficients for pure ZrSiO4 exhibit strongly anisotropic behavior and very high diffusivity along the c-axis, and with 3D, closure temperatures of -197°C and -202 °C respectively. The results for He are comparable to previous DFT studies but strongly different from experimental diffusion results; results for Ne are similar in this respect. Modelling the impact of different types of damage (vacancies, recoil, fission, voids or fluid inclusions) and obstruction on He and Ne diffusion reveals important implications for the (U-Th)/He and (U-Th)/Ne thermochronometers. First, obstruction alone does not significantly modify He and Ne diffusion except to reduce anisotropy. Second, trapping is the primary mechanism altering He and Ne diffusion even at low dose, and we predict the maximal trapping energies for He and Ne to be 164 and 320 kJ/mol, similar to values inferred from experimental data. We also propose that the closure temperature increases non-linearly with damage, with effective trapping energy increasing with dose until a threshold, possibly corresponding to a percolation 3 transition, after which retentivity decreases. Based on field data sets we also anticipate a value for this threshold of around ~2-5×10 /g, lower than previously proposed. We show Ne to be highly blocked by damage and predict similar diffusion behavior to He, but with higher retentivity. We demonstrate the importance of investigating rare gas diffusion at the atomic level for comparison with experimental data, in order to build a predictive diffusion law at different scales.