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In this contribution, we would like to underline the peculiar chemical properties of nanome-ter scale metallic particles. To attain this goal, we select the case of DeNox catalysis (NOx reduction to nitrogen molecule) for which such nanomaterials play a crucial role. Experimental data as well as re-cent theoretical calculation through density functional theory are used to assess the relationship be-tween the adsorption mode of NO and the behaviour of nanometre scale metallic particles. Resume. Les oxydes d'azote NOx issus des gaz d'echappement des moteurs Diesel participent a la de-terioration de l'environnement, la moitie des emissions de NO leur etant imputable. Le monoxyde d'azote provient essentiellement des reactions chimiques entre l'azote de l'air et l'oxygene qui s'eta-blissent a tres haute temperature. Le defi consiste a reduire cette emission qui doit etre selective en azote (eviter la formation de N2O ou de NO2). En ce qui concerne les catalyseurs mis en & OELIG;uvre, on en distingue trois groupes : les oxydes de metaux, les metaux nobles et les zeolites. A partir de donnees ex-perimentales obtenues sur l'adsorption du NO sur des agregats monometalliques, une relation entre les modes d'adsorption de cette molzcule (adsorption molzculaire/chimisorption) et la rzponse de l'agrzgat mztallique face a cette adsorption (croissance/dissociation) a ztz proposze. Les ztudes thzo-riques baszes sur la thzorie de la fonctionnelle de la densitz confortent cette relation. Cette contri-bution przsente l'ensemble de ces rzsultats expzrimentaux et thzoriques et tache de montrer l'aspect przdictif d'une telle relation.

International audience; The development of high-performance catalytic nanomaterials is important to implement sustainable and electrochemical energy devices of alkaline fuel cells, metal–air batteries, and electrolyzers in the envisaged energy-transition scenarios. Synthesizing effective nanocatalysts as both anode materials for oxidation of glycerol (byproduct of the biodiesel industry) and cathode materials for the oxygen reduction reaction (ORR) is still a bottleneck. Herein, we report palladium-based nanomaterials whose physicochemical and electrochemical properties are tuned by the judicious choice of the support (rGO, Vulcan XC72R), the addition of a coelement (Fe), and the structure (alloy/core–shell). The bimetallic-based electrode shows a drastically enhanced electrocatalytic performance with a beneficial shifting of the onset potential, production of high currents, and good durability for both the ORR (kinetic current density jk = 2 mA cmPd–2 or 1 A mgPd–1) and glycerol oxidation (jp = 2.3 mA cmPd–2 or 1.11 A mgPd–1 at the peak), higher than those of commercial catalysts and existing literature values. The present results also provide new fundamental insights about the accurate measurement of the kinetic metrics of the ORR by employing a rotating-disk (-ring) electrode setup in alkaline electrolytes with metallic catalysts. Indeed, the anodic scanning of the electrode from a low potential to a higher one results in an ultrafast electrochemical kinetics with a positive shift of the half-wave potential of ΔE1/2,anodic/cathodic = 60 mV from the cathodic direction to the anodic one. The kinetic current density dramatically increases, jk,anodic = 19.0×, 6.9×, 3.4×, and 2.4× jk,cathodic at 950, 900, 870, and 850 mVRHE, respectively. The advantage of the synthesis methodology relies on the nonuse of organic molecules as capping agents and surfactants in order to produce bare (ligand-free) bimetallic PdFe electrocatalysts with a clean catalytic surface in a facile and straightforward way.

International audience; Single-atom alloys (SAAs) are emerging materials containing isolated metal atoms dispersed on host metal surfaces, exhibiting unique reactivity compared with the corresponding monometallic counterparts. However, the stability of the isolated atoms in the host metal has hardly been studied, although, metal segregation has been commonly observed in bimetallic nanoparticles under reaction conditions. In this work we focus on single-atom Pt anchored on various metallic support surfaces. Density Functional Theory (DFT) calculations coupled with environmental segregation energy analysis are performed to map the segregation trends of 22 different Pt-SAA surfaces under various hydrogen conditions. The results show the high stability of single-atom Pt in Ni, Co, Rh and Ir host metallic surfaces while no stability is predicted on Au and Ag surfaces. For Pd and Cu host supports, the single-atom Pt is found to be stable on specific surface facets and within definite temperature and pressure conditions. This work brings an important understanding of SAA systems through the prediction of of surface atomic ordering changes under operating conditions which related to the reactivity will ultimately allow the design of more efficient catalysts.

International audience; With the aim of rationalizing the reactivity of transition-metal (TM) clusters, the model reaction of NO dissociation on TM 13 clusters was investigated on a set of seven TMs. The adsorption energy and the activation energy for NOdissociation were calculated for each TM and discussed within the findings of Boudart in his article “model catalysts: reductionism for understanding”. In this work, we focused on selected TM cluster calculations instead of surface science slab-type calculations, which have been performed for some TMs and focus on the electronic structure to investigate and rationalize the TM reactivity. Besides the electronic and spin properties of the TM, we discuss the reactivity trends using also qualitatively the chemical hardness and softness. We conclude that the combination of these reactivity parameters will be necessary to fine- tune TM clusters’ reactivity with the scope of multimetallic TM clusters.

The electrocatalytic production of hydrogen from methanol dehydrogenation successfully uses platinum catalysts. However, they are expensive and Pt has the tendency to be poisoned from the intermediate compounds, formed during the methanol oxidation reaction (MOR). For these two reasons, there has been active research for alternative bi- and tri-component Pt-based catalysts. Herein, PtCu nanoparticles deposited on titania were studied and proposed to be efficient MOR catalysts. The catalyst was prepared by photo-deposition of Cu on a high-surface-area TiO2 powder support, followed by a partial galvanic displacement of the Cu deposit by platinum. The morphology and structure of the catalyst were characterized by physicochemical methods. The PtCu@TiO2 electro-catalyst has higher intrinsic catalytic activity and comparable mass specific activity for MOR in comparison with a commercial Pt/C catalyst. The experimental analyses were complemented by density functional theory-based computations. The theoretical results revealed that the most energetically favorable Pt and Cu arrangement in the supported PtCu nanoparticles was core (Cu)–shell (Pt) and/or phase-separated. The inter-atomic interactions responsible for the bimetallic cluster stabilization on titania were highlighted from the computed electronic charge distribution.

International audience; The stability of Ag-Pt systems: 13-atoms icosahedral cluster model (Ag13-nPtn) and 54-atoms (111) surface slab model (45Ag9Pt) have been systematically investigated by using Density Functional Theory within the generalized gradient approximation (DFT-GGA). Experimentally tested resistance of AgPt alloy nanoparticles supported on silica to decay upon temperature treatment (423 K-573 K in N2 flow) have shown to correlate with theoretical results. The stability of the clusters as a function of AgPt composition decreases with increasing content of Pt. The general stability trend of the AgPt alloy was maintained, in theory and experiment, for the Ag/Pt molar ratio of 1.6-5.1. Two particularly stable Ag/Pt compositions were revealed: 1.6 and 0.4. It was also found that surface location of platinum is responsible for stabilization of the Ag-Pt nanostructured alloy.

International audience; Despite intensive research efforts, the nature of the active sites for O2 and H2 adsorption/dissociation by supported gold nanoparticles (NPs) is still an unresolved issue in heterogeneous catalysis. This stems from the absence of a clear picture of the structural evolution of Au NPs at near reaction conditions, i.e., at high pressures and high temperatures. We hereby report real-space observations of the equilibrium shapes of titania-supported Au NPs under O2 and H2 at atmospheric pressure using gas transmission electron microscopy. In situ TEM observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O2 from 400 °C to room temperature. In comparison, no instant change in equilibrium shape is observed under a H2 environment. To interpret these experimental observations, the equilibrium shape of Au NPs under O2, atomic oxygen, and H2 is predicted using a multiscale structure reconstruction model. Excellent agreement between TEM observations and theoretical modeling of Au NPs under O2 provides strong evidence for the molecular adsorption of oxygen on the Au NPs below 120 °C on specific Au facets, which are identified in this work. In the case of H2, theoretical modeling predicts no interaction with gold atoms that explain their high morphological stability under this gas. This work provides atomic structural information for the fundamental understanding of the O2 and H2 adsorption properties of Au NPs under real working conditions and shows a way to identify the active sites of heterogeneous nanocatalysts under reaction conditions by monitoring the structure reconstruction.

In this work first-principles density-functional-theory (DFT) calculations and Monte Carlo (MC) simulations are used to study the stability of the new surface phases formed under CO gas pressure on Au–Pd (100). The segregation isotherms reveal the apparition at θCO = 0.5 monolayer (ML) of two exotic ordered phase structures over the (100) facet, depending on the CO adsorption configurations. When CO is adsorbed on bridge sites, the ordered phase is formed by Pd-CO ensembles organized in linear chains, which are separated by Au atoms forming also linear chains free of adsorbed CO molecules. When CO is adsorbed on the on-top site, a checkerboard-like stable structure appears with CO adsorbed on Pd. The phase diagrams (temperature and CO pressure) of these large surface Pd ensembles for different bulk Pd concentrations [Pdbulk] are calculated. The effect of local density approximation (LDA) and generalized gradient corrected (GGA) exchange-correlation (XC) functionals are compared. The phase diagram of the c...

The reactivity of various Pd ensembles on the Au—Pd(100) alloy catalyst toward CO oxidation was investigated by using density functional theory (DFT). This study was prompted by the search for efficient catalysts operating at low temperature for the CO oxidation reaction that is of primary environmental importance. To this aim, we considered Pd modified Au(100) surfaces including Pd monomers, Pd dimers, second neighboring Pd atoms, and Pd chains in a comparative study of the minimum energy reaction pathways. The effect of dispersion interactions was included in the calculations of the O2 dissociation reaction pathway by using the DFT-D3 scheme. The addition of the dispersion interaction strongly improves the adsorption ability of O2 on the Au—Pd surface but does not affect the activation energy barriers of the Transitions States (TSs). As for O2 to dissociate, it is imperative that the TS has lower activation energy than the O2 desorption energy. DFT-D3 is found to favor, in some cases, O2 dissociation on...

Hydrogen interaction with bimetallic Au(Pd) and Au(Rh) systems are studied with the density functional theory (DFT)-based periodic approach. Several bimetallic configurations with varying concentrations of Pd and Rh atoms in the under layer of a gold surface(111) were considered. The reactivity of the doped Au(111) toward hydrogen adsorption and absorption was related to the property modifications induced by the presence of metal dopants. DFT-computed quantities, such as the energy stability, the inter-atomic and inter-slab binding energies between gold and dopants, and the charge density were used to infer the similarities and differences between both Pd and Rh dopants in these model alloys. The hydrogen penetration into the surface is favored in the bimetallic slab configurations. The underlayer dopants affect the reactivity of the surface gold toward hydrogen adsorption in the systems with a dopant underlayer, covered by absorbed hydrogen up to a monolayer. This indicates a possibility to tune the gold surface properties of bimetallic electrodes by modulating the degree of hydrogen coverage of the inner dopant layer(s) Fil: Juarez, Maria Fernanda. Universidad de Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Soldano, Germán. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina. Universidad de Ulm; Alemania Fil: Santos, Elizabeth del Carmen. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina Fil: Guesmi, Hazar. Institut Charles Gerhardt Montpellier; Francia Fil: Tielens, Frederik. Universite de Paris I Pantheon - Sorbonne; Francia Fil: Mineva, Tzonka. Institut Charles Gerhardt Montpellier; Francia

International audience; The surface composition of bimetallics can be strongly altered by adsorbing molecules where the metal with the strongest interaction with the adsorbate segregates into the surface. To investigate the effect of reactive gas on the surface composition of Au–Cu alloy, we examined by means of density functional theory to study the segregation behavior of copper in gold matrices. The adsorption mechanisms of CO, NO, and O2 gas molecules on gold, copper, and gold-copper low index (111), (100), and (110) surfaces were analyzed from energetic and electronic points of view. Our results show a strong segregation of Cu toward the (110) surface in the presence of all adsorbed molecules. Interestingly, the Cu segregation toward the (111) and (100) surface could occur only inthe presence of CO and at a lower extent in the presence of NO. The analysis of the electronic structure highlights the different binding characters of adsorbates inducing the Cu segregation.

International audience; A DFT based characterization of tungsten oxide supported on amorphous hydroxylated silica is presented. The different molecular organizations are investigated on the surface topology and tungsten oxygen coordination. The presence of mono- and di-grafted species is discussed and rationalized, using an atomistic thermodynamic approach. The presence of W[double bond, length as m-dash]O groups are preferred over W–OH groups and the grafting coordination is dominated by the degree of hydration of the silica surface. At room temperature di-oxo digrafted and mono-oxo-tetragrafted species are in competition regulated by the ambient degree of hydration which also affects the silanol density of the silica support. A comparison between tungsten and the other group VI elements confirms a greater chemical difference with Cr than with Mo.

In order to study how adsorption of CO molecules changes the surface composition of AuPd alloys, we develop a theoretical methodology which is able to take this effect into account. An Ising model based on density functional theory calculations is derived to define interatomic potentials that describe metal–metal, metal–CO, and CO–CO interactions. Then, through the use of Monte Carlo simulations within the semi-grand canonical ensemble, the effect of adsorption-induced segregation for the AuPd(100) surface is well-reproduced for different temperatures and CO pressures. Segregation isotherms identify a Pd surface enrichment for low CO pressures, and CO surface saturation is reached at an intermediate coverage of θ = 0.5 ML. Furthermore, Pd chains induced by an ordering of adsorbed CO molecules appear at low temperature and intermediate CO pressures. These chains are the result of a competitive effect between CO–CO repulsions and metal–CO interactions.

International audience; In order to determine the energetic driving forces for surface segregation in AuPd alloys, we use a combined approach coupling ab initio calculations and an analysis via an effective Ising model. Previously, this approach has been used successfully in the framework of N-body interatomic potentials to study the surface segregation in both semi-infinite alloys and nanoparticles, allowing one to determine the relative contributions of the different elementary driving forces. Here, we go beyond the use of N-body interatomic potentials by using ab initio calculations to evaluate the segregation enthalpy and the effective pair interactions, the contribution due to the size difference between the constituants being still obtained by N-body interatomic potentials. We show that the decomposition of the segregation enthalpy into its different elementary contributions is still valid at this level of description. This allows us to analyze the segregation driving forces in the two infinite dilute limits of the Au–Pd system, for both (100) and (111) surfaces. Finally, between the two infinite dilute limits, we find that our results are consistent with existing experimental data.

International audience; Gas or liquid phase transesterification reactions are used in the field of biomass valorization to transform some platform molecules into valuable products. Basic heterogeneous catalysts are often claimed for these applications but the role of basicity in the reaction mechanism depending on the operating conditions is still under debate. In order to compare the catalyst properties necessary to perform a transesterification reaction both in liquid and gas phases, ethyl acetate and methanol, which can be easily processed both in these two phases, were chosen as reactants. The catalyst studied is MgO, known for its basic properties and its ability to perform the reaction. By means of appropriate thermal treatments, different kinds of MgO surfaces, with different coverages of natural adsorbates (carbonates and hydroxyls groups), can be prepared and characterized by means of CO2 adsorption followed by IR spectroscopy and hept-1-ene isomerization model reaction. New results on the basicity of the natural MgO surface (covered by carbonate and hydroxyl groups) are first given and discussed. The catalytic behavior in the transesterification reaction is then determined as a function of the adsorbate coverage. It is shown that the transesterification activity in the liquid phase is directly correlated with the kinetic basicity of the surface in agreement with the mechanism already proposed in the literature. On the reverse, no direct correlation with the basicity of the surface was established with the transesterification activity in the gas phase. A very high activity, in the gas phase, was observed and discussed for the natural surface pre-treated at 623 K. Preliminary DFT modeling of ester adsorption and methanol adsorption capacity determination were performed to investigate plausible reaction routes.

A systematic study on the magnetic property variations of Pd13 particles as a function of particle sizes and structures, and of Pd13 interactions with the (100) TiO2 anatase surface, was carried out with standard density functional theory (DFT) based method with periodic boundary conditions. The biplanar Pd13 structure with the spin state quintet is obtained to be the ground state in the gas-phase and on the anatase surface, being more stable than the compact icosahedral configuration by 0.067 and 0.865 eV, respectively. The preferred particle nucleation on the anatase surface is that with the largest possible contacts between Pd13 and the oxide surface oxygens. The supported Pd13 particles are better stabilized by 0.3 eV compared to the isolated ones. The profiles of the relative energies as a function of the spin state reveal that the Pd13 magnetic isomers, belonging to the same cluster topology, become closer in energy and nearly degenerate on the anatase surface compared to their gas-phase counterpart...

Bimetallic catalysts have manifold technological applications; their reactivity can greatly exceed that of the original single metals. In this work, we investigate an ideal model-system consisting of a complete monolayer of Pd underneath the surface of Au(111). First, we investigate the stability of this system, and then its interaction with hydrogen. We analyze in detail the energetics and the electronic interactions for the Volmer step of the hydrogen evolution reaction (HER), and the further absorption below the surface layer of gold. We combine Density Functional Theory based on computational techniques with the theory of electrocatalysis. Fil: Juarez, Maria Fernanda. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Soldano, Germán. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Guesmi, Hazar. Institut Charles Gerhardt Montpellier; Francia Fil: Tielens, Frederik. Universite Pierre et Marie Curie; Francia Fil: Santos, Elizabeth del Carmen. Universitat Ulm; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Física Enrique Gaviola. Universidad Nacional de Córdoba. Instituto de Física Enrique Gaviola; Argentina

International audience; This paper reports a systematic study of the effect of CO gas on the chemical composition at the surface of gold-based alloys. Using DFT periodic calculations in presence of adsorbed CO the segregation behavior of group 9-10-11 transition metals (Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co) substituted in semi-infinite gold surfaces is investigated. Although, CO is found to be more strongly adsorbed on (100) than on the (111) surface, the segregation of M impurities is found to be more pronounced on the (111) surface. The results reveal two competitive effects: the effect of M on CO and the effect of CO on M. Thus, on one hand, if M exists on the (100) gold facet, CO would be strongly adsorbed on it. But if M is initially located in the bulk, it would segregate to the (111) facet instead of the (100) in order to bind to CO.

International audience; This combined experimental and theoretical study aims at understanding why surface hydroxyl groups may enhance catalytic reactivity of MgO surfaces in basic catalysis, whereas hydroxyls are weakly deprotonating groups. We investigated that reactivity enhancement in the catalytic conversion of 2-methyl-but-3-yn-2-ol (MBOH). Reaction kinetics was experimentally determined on partially hydroxylated MgO: active sites were saturated with the reactant and catalyzed its conversion with an activation energy of 85 kJ·mol–1. Reaction pathways were calculated over fully hydroxylated, partially hydroxylated, and dehydroxylated MgO surfaces by means of first-principles simulations. To highlight the effect of reactant coverage, we also calculated the reaction pathway on MgO precovered with MBOH molecules. The results show that the surface OH groups generated by dissociative adsorption of water induce a lowering in the activation energy barriers when they keep a bare Mg2+-O2– pair available in the vicinity for MBOH to adsorb and react. Interestingly, OH groups do not directly interact with MBOH converting on the surface, but they modify the basic properties of the vicinal bare Mg2+-O2– pair on which MBOH adsorbs and converts. A similar effect is predicted when MBOH converts on a bare Mg2+-O2– pair in the vicinity of a second adsorbed MBOH molecule. The beneficial effect of coadsorbates on the reactivity of active sites is analyzed by means of a thermodynamic model and an electronic structure analysis.

International audience; Combined Density Functional Theory (DFT) calculations and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) were performed to study the distribution of Pd atoms in bimetallic AuPd nanoparticles in the presence of adsorbed CO. Compared to vacuum condition, the results showed evidence of Pd surface enrichment where both Pd monomers and Pd dimers could exist. The energetic stability calculated for several alloy configurations evidenced the preference of Pd to occupy under-coordinated edge sites in the presence of CO gas. Moreover, the calculation of the vibrational frequencies of adsorbed CO for the first time allowed the fine assignment of the complex experimental DRIFTS bands of CO interacting with the bimetallic nanoparticles and their evolution with time exposure. Electronic structure analysis shows preponderant π-back-donation from under-coordinated Pd to CO inducing strong bonding on edge sites.

International audience; Reliable experimental IR and theoretical approaches, both investigating CO adsorption on NaY faujasites, are supporting that CO capture occurs through the completion of the vacant coordination of Na+ cations located in the accessible SII sites. As a result, carbonyl adsorbed species are formed by the capture of one, two or three CO molecules and are experimentally discernable by their respective IR positions that are down-shifted by an average 11-12 cm−1 value for each captured CO molecule. DFT analysis is proposed for comparison and reproduces well the observed experimental shift of the νCO positions of the different polycarbonyls of interest. In addition, the effect of Si or Al composition surrounding the SII Na+ cation is investigated and results suggest that polycarbonyls that are formed might be in connection with the acidic strength of the cationic sites. This combined study completes and improves the understanding of the complex issue of CO adsorption at 80 K widely used as a model to explain how physical adsorption takes place in NaY faujasites working as an efficient industrial adsorbent in gas separation or gas purification processes.

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.