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International audience; Combined experimental infrared (IR) and theoretical approaches have been carried out in an attempt to specify the actual structure of the CO2 species adsorbed on the magnesium oxide surface. The interaction of CO2 with regular sites of the MgO(100), (111) and (110) surfaces as well as MgO(100) defect sites (steps, corners, kinks and di-vacancies) has been investigated by mean of Density Functional Theory study. Theoretical IR frequencies compared with IR experiments show distinguishable carbonate species, adsorbed on different planes and defects, vibrating in different IR-frequency ranges. In addition, by mean of thermodynamic model, the stability of carbonates as a function of temperature have been calculated and compared to the experiment. Analyzing the nature of basic sites, the results show that the most active site versus CO2, which is a Lewis acid, is not the same that the strongest site for the deprotonating adsorption of Brønsted acids. The present work revisits and improves the understanding of carbonate species that could exist on the magnesium oxide surface and gives a picture of the accessible planes of magnesium oxide as well as their surface Lewis basicity.

International audience; The location of the divalent nickel cation exchanged in low amount in dehydrated NaX is identified by a combination of experimental (XAS) and theoretical (DFT) approaches. The Ni-O and Ni-T distances as well as the coordination numbers evaluated from experiments are in good agreement with the predicted models obtained from calculations. After dehydration, the migration of Ni2+ towards the hexagonal prism is confirmed and three distinct possible nickel sites are predicted. The results show the tendency of Ni2+ to get closer to the most basic framework oxygen atoms

Times Cited: 0 Article English Cited References Count: 28 716na; Segregation energies of isolated Pd in PdAu(1 1 1) alloy are investigated by DFT calculations as a function of the oxygen coverage. Segregation of Pd is found to be oxygen-coverage dependent. While for 'clean' PdAu(1 1 1), Pd prefers to be in the bulk, in the presence of more than 1/3 ML of oxygen, Pd meanly segregates to the surface. The analysis of Pd d-band provides evidence of strong Pd-O affinity. The understanding of such behaviour is of interest in the field of heterogeneous catalysis because of the lack of information about the surface ordering in bimetallic alloys in the presence of reactive gas. (C) 2010 Elsevier B. V. All rights reserved.

Times Cited: 0 Article English Cited References Count: 79 741sw; The location and coordination of nickel cations inside the hexagonal prism of dehydrated sodium faujasites have been investigated as a function of the framework Al content and distribution by density functional theory (DFT) calculations. Three different Si/Al ratios have been considered in order to represent the Na-LSX (Si/Al = 1), Na-X (Si/Al = 1.4), and Na-Y (Si/Al = 2.0) faujasites that have the same framework topology but different basic properties. For each system, the most representative Al repartitions among the framework tetrahedral sites have been taken into account and their related preferred Ni2+ configurations have been established. Upon decrease of the Al content, the Ni2+ coordination changes from a perfect octahedral environment with Ni2+ located in the center of the hexagonal prisms (Ni/Na-LSX) to five 4-fold and four 3-fold coordinations in Na-X and Na-Y faujasites, respectively. The preferred locations, inside the hexagonal prism, are those where Ni2+ can bind the highest number (highest coordination) of basic framework oxygen atoms even though the related Ni-O distances simultaneously increase. All configurations are associated to a significant distortion of the hexagonal prism as compared to the host Na-faujasite cluster which illustrates the zeolite flexibility. While 4-fold coordinated nickel weakly interacts with the framework Y-faujasite, it induces a stronger framework distortion. The monitoring effect of lattice oxygen atoms toward Ni-framework interaction and framework deformation as well as Ni2+ coordination and location inside zeolites are rationalized.

International audience; The oxidative power toward CO of α-oxygen formed upon N2O dissociation over isolated and binuclear Fe/ZSM-5 zeolite is investigated by means of DFT calculations. The two α-sites [Fe-O]+ and [Fe-(μO)-(μOH)-Fe]+ exchanged in ZSM-5 were considered since their activity in the N2O decomposition was recently shown. Computed electronic properties, charge transfers and frequency analysis of α-oxygen and iron in [O-Fe-O]+ and [OFe-(μO)-(μOH)-FeO]+ suggest a FeII character for the isolated and FeIV for the binuclear Fe-ZSM-5 sites. Addition of CO on oxygen atoms reveals that along the oxidation reaction the valence state for the isolated iron is II and remains relatively constant while a clear change from IV to II is calculated for the binuclear iron. According to DFT calculations CO addition on the α-oxygen from the iron active sites induces a significant length increase of the Fe-α-oxygen bond. Whatever the α-sites, the addition of CO is strongly exothermic and leads to stable minima resembling an adsorbed CO2 on iron active site. This reactivity is in line with the well known high reactivity of α-oxygen and the rapid CO2 formation at low temperatures. Based on the calculated enthalpy values, the adsorption of CO is slightly more favourable on binuclear [OFe-(μO)-(μOH)-FeO]+ than over isolated [O-Fe-O]+ iron site. A comparison of the entropic parameters suggests the opposite with a stronger oxidative power of α-oxygen from isolated over those from binuclear iron site.

International audience; The reaction mechanism for nitrous oxide (N2O) direct decomposition into molecular nitrogen and oxygen was studied on binuclear iron sites in Fe-ZSM-5 zeolite using the density functional theory (DFT). Starting from the hydroxylated bi-iron complex [HOFeIII(μ-O)(μ-OH)FeIIIOH]+, a reductive dehydroxylation pathway was proposed to justify the formation of the active site [FeII(μ-O)(μ-OH)FeII]+. The latter contains two FeII ions linked via oxo and hydroxo bridges, Z-[FeII(μ-O)(μ-OH)FeII]+, and for the first time was considered to catalyze the N2O decomposition. The DFT results show the activity of [FeII(μ-O)(μ-OH)FeII]+ complex for the N2O decomposition. The first step of the catalytic reaction corresponds to a spontaneous adsorption of N2O over FeII sites, followed by the surface atomic oxygen loading and the release of molecular nitrogen. The formation of molecular O2 occurs through the migration of the atomic oxygen from one iron site to another one followed by the recombination of two oxygen atoms and the desorption of molecular oxygen. The computed reactivity over the binuclear iron core complex [FeII(μ-O)(μ-OH)FeII]+ is consistent with experimental data reported in the literature. Although the dissociation steps of the N2O molecules, calculated with respect to adsorbed N2O intermediates, are highly energetic, the energy barrier associated with the atomic oxygen migration is the highest one. Up to 700 K, the oxygen migration step has the highest free energy barrier, suggesting that it is the rate-limiting step of the overall kinetics. This result explains the absence of O2 formation in experimental study of N2O decomposition at temperatures below 623 K.

Elementary reaction mechanisms for nitrous oxide (N 2 O) dissociation were studied on [Fen II (μ-O)(μ-OH)Fe II ] + exchanged in ZSM-5, using density functional theory (DFT). The effect of the cluster size on the energetics and on the reaction routes of N 2 O dissociation were investigated over di-iron core inserted inside two different Z cluster (Z − ) and (Z − OH ). The results show that while the relative stability changes with the cluster termination, the height of the energetic barriers are similar.

4 pages; International audience; Using reciprocal space formalism, we investigate the influence of elastic anisotropy on the ground state energy of square and rectangular domains on (001) faces of cubic crystals. On one hand, for numerous cubic metals of sufficiently high elastic anisotropy, the interaction energy between square domains exhibits attractive basins around 100 directions. These basins are responsible for spontaneous symmetry breaking and lead to the apparition of domain lines. On the other hand, elastic anisotropy has very little influence on the selection of isolated domain shape. For anisotropic microscopic boundary energy favoring boundaries running in 100 directions, the equilibrium shape of stressed domains is always infinite stripes whatever the elastic anisotropy.

9 pages; International audience; We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(0 0 1) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5 nm. When Au is evaporated onto this surface, it mainly grows at the intersections between the Cu lines. The islands organization reproduces then the substrate 2D ordering. However, if the substrate temperature is too low, islands form everywhere. On the contrary, if the substrate temperature is too high, some nucleation sites are empty. By following the intensity of the diffraction satellites during the growth, we have observed that the ordering of the Au particles is optimum when the substrate temperature is between 210 and 290 K. Using both an analytical treatment based on the rate equations and kinetic Monte-Carlo simulations, we have determined the activation energy for the diffusion process and the energy of the traps.

Mme. Marie-José CASANOVE (présidente)Mme. Sylvie ROUSSET (examinatrice)M. Stéphane ANDRIEU (rapporteur)M. Bernard LEGRAND (rapporteur)M. Pierre MÜLLER (directeur de thèse)M. Guy TRÉGLIA (directeur de thèse); Owing to its technological properties due to both its Si dopant effects and its use as surfactant there is a wide interest in the study of adsorption properties of Sb on Silicon surfaces. The work reported in this document corresponds to a theoretical and experimental study of the adsorption properties of Sb/Si(111).From an experimental viewpoint, a coupled study by Mass Spectrometry, Electron Diffraction and Scanning Tunnel Microscopy enables us to describe the structural, kinetics and thermodynamics properties of the Sb/Si(111) system. From a theoretical viewpoint, ab-initio methods have been used to study the evolution of the nature of the adsorption site with the deposited amount of Sb.We thus are able to propose a scenario in which the Sb adsorption site evolves from a ternary position (at weak coverage) towards an on-top position (at the layer completion) and in which the nature of the Sb-Sb effective interaction changes from repulsive (theory) or vanishing (experiment) to attractive (theory and experiment). This site transition allows us to reconcile our kinetic and thermodynamical results.; L'antimoine (Sb) est un élément qui a la particularité d'être utilisé à la fois comme dopant et comme surfactant du silicium (Si). L'étude du système Sb-Si est donc de première importance tant d'un point de vue fondamental que de celui de ses nombreuses applications en électronique. Le travail présenté dans ce mémoire de thèse correspond à une étude expérimentale et théorique des propriétés d'adsorption/désorption de Sb sur la surface nominale (111) de Si.Du point de vue expérimental, l'utilisation combinée de la spectrométrie de masse, de la diffraction des électrons et de la microscopie à effet tunnel nous a permis de décrire les propriétés structurales, cinétiques et thermodynamiques du système. Du point de vue théorique, l'utilisation des méthodes ab-initio nous a permis d'étudier l'évolution de la nature des sites d'adsorption avec le recouvrement de surface.Ce travail permet de dégager un scénario cohérent de l'adsorption, dans lequel la nature du site préférentiel change depuis un site ternaire (à faible recouvrement) vers un site apical (à complétion de la couche), permettant de passer d'un système à interactions répulsives (théorie) ou nulles (expérience) à un système à interactions attractives (théorie et expérience). Ce changement de site permet de réconcilier nos résultats expérimentaux cinétiques et thermodynamiques, qui semblaient contradictoires.

The experimental study of the thermodynamic and kinetic properties of the Sb=Si111† interface reveals a surprising behavior: a 2D phase condensates when the Sb coverage increases, indicating strong attractive Sb-Sb interactions, whereas the isotherms present a quasi-Langmuir shape, suggesting that these interactions should be negligible. Ab initio calculations raise this contradiction: while the adsorption site evolves from ternary towards the on-top position with increasing coverage, the character of the Sb-Sb effective interactions changes from repulsive towards attractive, resulting in an almost constant average adsorption energy. A simple (Langmuir) thermodynamic behavior can then be the consequence of a surface phase transition. Although antimony adsorption on silicon has been extensively studied [1-16], only a few studies have been devoted to its thermodynamic and kinetic properties. These properties are studied here at the Sb=Si111† interface. Curiously, kinetic results put in evidence an attractive interaction between Sb adatoms leading to the formation, at equilibrium, of a 2D condensed phase whereas the adsorption or desorption isotherms are Langmuir-like, which a priori excludes any attractive lateral interaction between the adatoms. We will show here that a careful ab initio study of the energetics of the adsorption blows up this contradiction. Experimentally, the kinetic and thermodynamic properties of adsorption or desorption of Sb onto Si(111) surface are analyzed by means of mass spectroscopy [2]. Thus our previous kinetic study [3] revealed two different mechanisms as a function of coverage, involving either single Sb adatoms ( 0:7). This means that the lateral Sb-Sb interactions are strong enough to induce such a condensation beyond a given critical adatom density [3]. In this Letter, the kinetic study is completed by plotting the isotherms P† for various substrate temperatures [see Fig. 1(a)]. These isotherms present a sigmoi¨dsigmoi¨d trend specific of a Langmuir behavior. Let us recall that, in the Langmuir picture of adsorption, (i) the adsorption energy is the same for every adsorbed molecule independently of the other molecules, and (ii) every molecule that sticks on an already adsorbed molecule immediately returns to the gas phase. The Langmuir isotherm is thus based on the concept of noninteracting molecules adsorbed on definite sites on the surface so that the coverage varies from zero to one mono-layer without any accident. Therefore, at first sight, the quasi-Langmuir isotherms of Fig. 1(a) are the signature of weak or vanishing lateral interactions between Sb ada-toms. In order to be more quantitative, the experimental isotherms can be fitted by the more general mean-field expression: 1 y † ˆ P P 0 exp y E ads † kT ; (1) where E ads † is given by : E ads † ˆ E ads 0† ‡ ZV SbSb : (2) P 0 is a constant (P 0 10 9 Pa at the experimental temperature), E ads 0† is the adsorption energy of a single adatom, and V SbSb the effective interactions between Sb adatoms (either direct or mediated by the Si substrate) should be zero for perfect Langmuir isotherms. Z is the number of Sb-Sb neighbors (here Z ˆ 6). It is then easy to reverse the equation (1) to get the experimental variation of E ads †. Its average value, derived from the isotherms of Fig. 1(a), is plotted in Fig. 1(b). At first order, this curve can indeed be fitted linearly according to Eq. (2) with: E ads 0† ˆ y2:95‡= y 0:03† eV and V SbSb ˆ y0:02‡= y 0:006† eV (referred to as fit 1). This value of V SbSb seems to confirm the previous qualitative conclusion: the lateral interaction between antimony adatoms are small. Nevertheless, even though this result perfectly agrees with some old results on thermodesorption experiments [4-7], it completely contradicts kinetic experiments which can only be interpreted by means of the existence of significant lateral interaction between Sb adatoms [3]. Moreover, the quasi-Langmuir isotherms drawn using this linear fit of E ads † do not fit exactly the experimental ones [see Fig. 1(a)]. The weak deviations can be linked to variations of E ads † with respect to the linear fit. Indeed, in spite of the experimental error bars, one can see [Fig. 1(b)] that the experimental data should be better described by two linear regimes (referred to as fit 2): PRL 94, 076101 (2005) P H Y S I C A L