XANES study of rhenium oxide compounds at the L1 and L3 absorption edges

8 pagesInternational audienceWe report on the study of a set of rhenium oxide reference compounds (NH4ReO4, NaReO4, ReO3, ReO2, and Re2O7) using x-ray-absorption near-edge structure. The parallel use of Re L1 and L3 absorption edges permits a concomitant understanding of both the oxidation state and the local symmetry for these compounds. Experiments are compared with ab initio simulations. A good agreement is reached in most cases. The choice of the cluster size and the calculation method (full potential or not), which are mandatory ingredients allowing a satisfactory reproduction of the recorded spectra, is discussed in detail. In the meantime, these parameters give important pieces of information on the studied materials

Photon-in/photon-out spectroscopy to probe the electronic and geometric structure of heterogeneous catalysts

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Elaboration de catalyseurs MoOx/Al2O3 (une approche science des surfaces en phase aqueuse)

Ce travail avait pour but de développer une approche réductionniste de type science des surfaces pour la description et la compréhension de l élaboration de catalyseurs hétérogènes à l interface oxyde/phase aqueuse à partir du système modèle MoOx/a Al2O3. Grâce à une approche DFT périodique et un modèle thermodynamique nous avons pu déterminer la stabilité des différentes terminaisons potentielles des faces (0001) et (1 02) du support modèle utilisé (l alumine a) en fonction du taux de recouvrement en eau et ainsi identifier les sites OH de surface exposés par ce support. La caractérisation des systèmes modèles MoOx/a Al2O3 par spectroscopie EXAFS en incidence rasante (GI-EXAFS), XPS et AFM a montré que le mode d adsorption des oxoanions du Mo est fonction de la méthode de préparation, de la concentration de la solution d imprégnation et de l orientation de la face utilisée, révélant ainsi le rôle multiple joué par le support oxyde. Pour la face (0001), le support joue le rôle de simple réceptacle physique et on observe la précipitation de l espèce (NH4)2MoO4. Pour la face (1 02), le support est un véritable ligand supramoléculaire avec une interaction précurseur/support qui diffère selon la méthode de préparation utilisée. Dans le cas d une imprégnation, il y a reconnaissance chimique et complémentarité géométrique entre les sites de surface et l heptamolybdate (précurseur du Mo). Dans le cas d une adsorption à l équilibre, l espèce molybdique adsorbée est greffée par établissement de liaisons covalentes sur des sites Al4C- 1-OH. Les résultats obtenus sur alumine a monocristalline nous permettent de préciser la réactivité individuelle des faces exposées par l alumine g.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Adsorption of the Complex on the TiO2(110) surface in presence of water : Ab initio molecular dynamics studies

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Common intermediate species from reducing and activation of CoMo-based catalyst revealed via multivariate augmented system applied to time-resolved in situ XAS data

International audienceTime-resolved spectroscopy has reached great importance in studies of chemical transformations, particularly in heterogeneous catalysis, where in situ or operando conditions are suitable to correlate precursor transformation and/or genesis of active sites with catalytic efficiency. In this work we present advantages of using chemometric analysis to resolve multi-step chemical reactions in time-resolved XAS experiment. We followed complementary experiments (temperature programmed reduction and activation) of a Ti-supported CoMo HDS catalyst to unravel details in the evolution of the different species appearing during each process. A multivariate analysis uncovered that (i) at Mo K-edge, activation of oxidic precursor is a 3-step mechanism with one reduced-like and one oxysulfide species as intermediates and (ii) TPR is a 2-step process with in which the reduced intermediate is a common species between the two processes. This approach was fundamental to properly number the steps during the TPR and to correctly assign the first intermediate of the activation as a non-sulfide species. Further, when applied to low loading catalyst, remarkably structural differences on the kinetics was found, but a similar active phase was reach at the end. Thereby, using augmented analyses we demonstrated that a careful planning of the experiments is essential to resolve fine details in the kinetics of the reaction

Structure of clean and hydrated alpha-Al2O3 (1(1)over-bar02) surfaces: implication on surface charge

Times Cited: 0 Article English Cited References Count: 68 738yqPeriodic DFT calculations coupled to a first-principle thermodynamic approach have allowed us to establish a surface phase diagram for the different terminations of the alpha-Al2O3 (1 (1) over bar 02) surface in various temperature and water pressure conditions. Theoretical results are compared with previous experimental data from the literature. Under a wide range of temperature and water pressure (including ambient conditions) the most stable surface (denoted C2_1H(2)O in this work) is terminated with singly coordinated hydroxyls on four-fold coordinated aluminium (Al-4C-mu(1)-OH) while most existing surface models are only considering six-fold coordinated surface Al atoms as in the bulk structure of alumina. The presence of more acidic Al-4C-mu(1)-OH sites helps explain the low Point of Zero Charge (PZC) (between 5 and 6) determined from the onset of Mo oxoanions adsorption on (1 (1) over bar 02) single crystal wafers. It is also postulated that another termination (corresponding to the hydration of the non-polar, stoichiometric surface, stable in dehydrated conditions) may be observed in aqueous solution depending on the surface preparation conditions

Time-resolved characterization of the embryonic state formation of nuclei during gold nanoparticles synthesis in a microfluidic device

Gold nanoparticles (Au-NPs) are among the most intensely studied nanoscale materials. [1-2] The molecular mechanisms of Au-NPs formation remain unclear due first to the quantity of material analyzed (~mM), second to the relative short time scale for the nucleation and growth steps and third to the limited accessibility to in situ time-resolved characterization. To overcome those issues and regarding the experimental aspects, Synchrotron radiation related techniques are useful to characterize such rapid NPs formation. [3] In order to access to a direct in situ time-resolved characterization, we build and adapt a micro-fluidic system under continuous flow conditions, whereby we follow, via in situ XANES spectroscopy, the Au-NPS formation from a gold precursor (HAuCl4) with sodium citrate as reducing and stabilizer agent to final. The setup is constituted by two glass syringes, a syringe pump, and one tee-mixer for combining the reagents and a 1.5 m 7 200 \u3bcm internal diameter fused silica capillary as micro-reactor heated using a heating plate. By focusing the X-rays (analysis spot of 50*50 \ub5m), the reaction was followed by monitoring the gold LIII-edge XAS spectra at different locations along the micro-tube, temperature and flow rates. Thanks to this set up it is possible to convert a "time scale" reaction in a "length scale" reaction. We describe the embryonic state NPs formation by characterizing the oxidation state of gold, geometry modifications of the precursor and ligand substitutions. [1] M. C. Daniel, D. Astruc, Chem. Rev., 104 (2004) 293. [2] A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed., 45, (2006), 7896. [3] J. Polte, , T. T. Ahner, F. Delissen, , S. Sokolov, F. Emmerling, A. F.Th\ufcnemann, R. Kraehnert, J. Am. Chem. Soc, 132, (2010), 1296

DFT study of the adsorption of microsolvated glycine on a hydrophilic amorphous silica surface

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Efficient and Robust Reforming Catalyst in Severe Reaction Conditions by Nanoprecursor Reduction in Confined Space

International audienceThe in situ autocombustion synthesis route is shown to be an easy and efficient way to produce nanoscaled nickel oxide containing lanthanum-doped mesoporous silica composite. Through this approach, similar to 3 nm NiO particles homogeneously dispersed in the pores of silica are obtained, while lanthanum is observed to cover the surface of the silica pore wall. Subsequent reduction of such composite precursors under hydrogen generates Ni-0 nanoparticles of a comparable size. Control over the size and size distribution of metallic nanoparticles clearly improved catalytic activity in the methane dry reforming reaction. In addition, these composite materials exhibit excellent stability under severe reaction conditions. This was achieved through the presence of LaOx species, which reduced active-site carbon poisoning, and the confinement effect of the mesoporous support, which reduced metallic particle sinterin

Study of the genesis of HDS catalysts by in situ photon in photon out spectroscopy

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