Nitrous Oxide Decomposition on the Binuclear <Fe(μ−O)(μ−OH)Fe>\left<Fe\left(\mu -O \right)\left(\mu -OH \right)Fe \right> Center in Fe-ZSM-5 Zeolite

The reaction mechanism for nitrous oxide (${N}_{2}O$) 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 {\left}^{+}, a reductive dehydroxylation pathway was proposed to justify the formation of the active site {\left}^{+}. The latter contains two FeII ions linked via oxo and hydroxo bridges, {{Z}^{-}\left}^{+}, and for the first time was considered to catalyze the ${N}_{2}O$ ecomposition. The DFT results show the activity of {\left}^{+} complex for the ${N}_{2}O$ decomposition. The first step of the catalytic reaction corresponds to a spontaneous adsorption of ${N}_{2}O$ over FeII sites, followed by the surface atomic oxygen loading and the release of molecular nitrogen. The formation of molecular ${O}_{2}$ 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 {\left}^{+} is consistent with experimental data reported in the literature. Although the dissociation steps of the ${N}_{2}O$ molecules, calculated with respect to adsorbed ${N}_{2}O$ 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 te-limiting step of the overall kinetics. This result explains the absence of ${O}_{2}$ formation in experimental study of ${N}_{2}O$ decomposition at temperatures below 623 K

Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5

The characterization of Fe/ZSM5 zeolite materials, the nature of Fe-sites active in N2O direct decomposition, as well as the rate limiting step are still a matter of debate. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(m-O)(m-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches. The overall calculated path of N2O decomposition involves the oxidation of binuclear FeII core sites by N2O (atomic a-oxygen formation) and the recombination of two surface a-oxygen atoms leading to the formation of molecular oxygen. Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was compared to the experimentally observed steady state kinetics of the N2O decomposition and temperature-programmed desorption (TPD) experiments. A switch of the reaction order with respect to N2O pressure from zero to one occurs at around 800 K suggesting a change of the rate determining step from the a-oxygen recombination to a-oxygen formation. The TPD results on the molecular oxygen desorption confirmed the mechanism proposed

Etude expérimentale et théorique des mécanismes d'adsorption/désorption de l'antimoine sur une surface de silicium (111)

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

Theoretical insights on the effect of reactive gas on the chemical ordering of gold-based alloys

International audienceAlloy catalysts typically operate under high pressure and high-temperature conditions, and these reactive environments may substantially influence the alloy surface composition. Theoretical studies of catalytic properties are often investigated on model systems where no account is taken for the possibility that the surface composition can be modified after the gas exposure. This is a serious drawback that may prevent reliable description of the catalyst reactivity that mainly depends on the configuration of the surface. Nowadays, modelling the equilibrium structure of metal surfaces and alloys in a reactive environment is still a barely studied subject and remains an extremely challenging task. Recent methodological advances and their applications, mainly on gold-based alloy systems, are presented and discussed in this brief overview

How does CO capture process on microporous NaY zeolites? A FTIR and DFT combined study

International audienceReliable 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

DFT study of the M segregation on MAu alloys (M = Ni, Pd, Pt) in presence of adsorbed oxygen O and O2

International audienceSegregation phenomena of group 10 (M = Ni, Pd, Pt) transition-metals substituted in Au (1 1 1) surface and sub-surface layers are investigated by DFT periodic calculations in presence of adsorbed atomic and molecular oxygen. In contrast with vacuum conditions, where the metal impurities M prefer to be in the bulk of gold, in the presence of adsorbed O or O2, the impurities mainly segregate to the surface. The analysis of oxygen adsorption trends and electronic surface structures explain the change in the local atomic arrangement which is expected to occur on the surface of alloys during reaction conditions

Welcome to the world of nanometric gold!,Bienvenue dans le monde de l'or nanométrique !

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Chromium Oxide Species Supported on Silica: A Representative Periodic DFT Model

International audienceThe Cr/SiO2 system is investigated using periodic DFT. The model represents the amorphous character of the silica surface and allows the investigation of the effect of hydration on the Cr(VI) monomers. First, the geometry and energetics are discussed and compared with experimental data. The phase diagram plotted from an atomistic thermodynamics model confirms the higher stability of mono-oxo and dioxo chromium, in comparison with species containing Cr―OH groups. In addition, the effect of the siloxane ring size on the spectroscopic signature of chromium is analyzed. A preliminary study is presented on the surface doping effect by Ti on the structure and stability of chromium species. The results reveal that the charge transfer process between Ti and Cr can explain the observed change in the reactivity of chromium species