Elise Berrier

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

A well-defined silica-supported dinuclear tungsten(III) amido species: synthesis, characterization and reactivity.

Grafting of [W(2)(NMe(2))(6)] onto dehydroxylated silica affords the well-defined surface species [([triple bond, length as m-dash]Si-O)W(2)(NMe(2))(5)], characterized by elemental analysis, and infrared, Raman and NMR spectroscopies, and the catalytic reactivity of this supported tungsten(III) d(3)-d(3) dimer and of its alkoxide derivatives towards alkynes has been probed.

Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO2‐Supported Molybdenum Oxide Catalysts

The properties of heterogeneous catalysts are directly correlated to the molecular structure of the active sites which often consists of nanometer-scale particles of transition metals (in a metallic, oxide or sulfide form) dispersed on an oxide support. The complexity of physico-chemical phenomena occurring during the catalyst synthesis/activation stages often leads to the formation of unknown supported phases featuring new ill-defined sites. Their identification requires an indepth characterization at the molecular scale of the catalyst with the use of various spectroscopic tools. Despite the valuable information so-obtained, these techniques sometimes are not able to provide the overall structure of the active species responsible for the catalytic activity. The use of theoretical tools to establish direct correlation between spectroscopic fingerprints and structural/electronic properties of catalysts is a powerful and quite new approach for unraveling the structure of catalysts. Thanks to its chemical and electronic (orbital) selectivity, X-ray absorption near-edge structure (XANES) spectroscopy is the technique of choice for molecular-scale characterization of catalysts. Furthermore, in the XANES spectra, the long mean free path of the photoelectron, induced by its small kinetic energy (Ec< 50–100 eV), provides a high contribution of multiple scattering events of the photoelectron allowing to probe the three-dimensional structure (3D) around the absorbing atom. XANES spectroscopy is however highly dependent on electronic parameters that make its fine interpretation difficult. This often leads to a restricted use of XANES spectra for the determination of the oxidation state of an absorber atom or/and basic consideration on its local symmetry. It is however possible to obtain a finer interpretation of spectra by calculation of XANES transitions. Indeed, the multiple scattering (MS) theory is well adapted to reproduce the XANES transitions observed at K edges of elements heavier than Li and at L2,3 edges of elements heavier than Cd. We propose herein to use MS simulations based on structural models predicted by DFT calculations for interpreting the MoK edge fingerprints observed for TiO2-supported molybdenum oxide catalysts which are widely used for olefin metathesis, selective oxidation reactions, and hydrotreatment. The structure of the oxomolybdate species formed during the catalyst activation is still unknown even though it has been extensively characterized by various spectroscopies. A three-step methodology was followed to unravel the catalyst structure: first simulations of XANES spectra of Mo reference compounds (ammonium heptamolybdate (hereafter noted AHM), (NH4)3[Al(OH)6Mo6O18] (noted AlMo6), a-(NH4)4[Mo8O26] (noted Mo8O26)) were performed to obtain direct correlation between spectroscopic fingerprints and structural properties of known structures. Then, these fingerprints are identified in the spectrum of the supported catalyst to generate catalyst structures for DFT optimization. Finally, the optimized geometry is used for modeling XANES spectra using MS theory. Figure 1A presents MoK edge experimental spectra of the activated catalyst (350 8C in oxygen), labeled hereafter 7.5 MoTi, together with four reference compounds in which

Applications in materials science of combining Raman and X-rays at the macro- and micrometric scale

Simultaneous combination of Raman with X-ray Absorption Spectroscopy (XAS) has become of great interest in Materials Science. Four applications are reviewed in this article that reflect the large range of possibilities offered by such coupling at the macro and micrometer scales. Special emphasis was laid on the macrometer scale on time-resolved studies of physico-chemical transformations, either for providing more complete analysis of structural changes or for looking at the sample integrity. The capability of Raman and XAS to provide complementary chemical and structural information was also pointed out for discriminating and characterizing chemical phases in heterogeneous materials at the micrometer scale.

Well-defined silica-supported molybdenum nitride species: silica grafting triggers alkyne metathesis activity

The grafting of [Mo(N)(NR2)(OR)2(pyr)] (R = SiMe3, pyr = pyridine) onto highly dehydroxylated silica proceeds to the formation of well-defined silica-supported species. Whereas the molecular precursor is inactive towards alkyne metathesis, its immobilized counterpart displays high activity, which can be significantly enhanced by the addition of 2 equivalents of B(C6F5)3.

Methanol conversion over TiO2-anatase supported oxomolybdate catalysts: an integrated operando – DFT modeling approach

TiO2-supported molybdenum oxide has been studied through a combination of X-ray absorption near edge structure (XANES) and Raman operando spectroscopy. XANES gives insights on the electronic structure of the molybdenum embedded in the active phase, while Raman spectroscopy probes the structure of the oxomolybdate phase at the molecular level. These paired experiments have also been complemented by density functional theory calculations to better refine the resulting structure and make it possible to further understand the structure–activity correlations. In line with the approach, we show that the reactivity of the supported oxomolybdate phase is strongly dependent on the molybdenum loading of the catalyst. Indeed, low molybdenum content catalysts exhibit isolated tetrahedrally coordinated molybdenum centers that are not reducible and show a very low activity in methanol conversion. On the other hand, high molybdenum loading catalysts present reducible molybdenum centers that are rather active in methanol conversion. This change in the behavior of the supported phase is found to be related to a change in the oxygen coordination spheres: high loading induces molybdenum oxide clusters of higher nuclearity where the molybdenum atoms are surrounded by five or six oxygen atoms.

One‐Pot Sol–Gel Preparation for Efficient Cobalt–Molybdenum–Titania Hydrotreating Catalysts

Titania‐based hydrodesulfurization catalysts were prepared by using a one‐pot sol–gel method with the titanium peroxo complex as titanium precursor and molybdenum and cobalt precursors dissolved in the aqueous hydrogen peroxide solution used during synthesis. Catalysts with MoO3 loadings varying from 5 to 40 wt % and a Co/Mo atomic ratio of 0.5 were prepared. Solids with molybdenum loadings below 20 wt % MoO3 demonstrated poor hydrodesulfurization activity for the model compound thiophene. This was attributed to the presence of a large amount of embedded cobalt and molybdenum species in the titania matrix. A significant increase in the catalytic activity was observed for solids containing 20 and 25 wt % of MoO3 on which active species appeared to be accessible on the titania surface. Furthermore, the highest conversion obtained on these sol–gel solids was superior to the highest conversion obtained on reference catalysts prepared through the impregnation of ammonium heptamolybdate and cobalt nitrate on commercial titania.

First-Principles Investigation of the Relevant Surfaces Exposed by Polycrystalline LaFeO3

A systematic comparison of surface energy values computed over a series of possible surfaces and terminations exposed by lanthanum orthoferrite (LaFeO3) was performed for elucidating the nature of the exposed surfaces of LaFeO3‐based three way catalysts. Surface reconstruction of polar surfaces was attempted, which did change the ranking of the most stable surfaces. Indeed, the non‐polar (1 2 1) and (1 0 0) surfaces were found to remain the most stable ones in vacuum. The relative stability of surfaces was also questioned when a partial pressure of water was applied, as is expected under the operating conditions of three‐way catalysis. Our results show that the predominance of (1 2 1) and (1 0 0) surfaces is even strengthened upon water exposure.

Combining sorption experiments and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to study the adsorption of propranolol onto environmental solid matrices – Influence of copper(II)

The bioavailability of pharmaceuticals is governed by their sorption in soils/sediments, as the retention processes determine their concentration in surface- and ground-water. The adsorption of these contaminants can involve various solid components such as organic matter, clays and metallic oxides, and their distribution among these solid components depends on contaminant and solid properties. In this paper we studied the adsorption of the pharmaceutical propranolol - a beta-blocker - on eight different solids (six soils, one sediment and one kaolinite-based sample) by batch experiments. The influence of contact time, propranolol concentration and pH was considered, as well as the presence of copper(II). The investigated solids displayed a wide variability in terms of CEC (cationic exchange capacity) and organic carbon and carbonates contents. The influence of pH was negligible in the pH range from 5.5 to 8.6. The adsorbed amounts were greatly dependent on the solid and two groups of solids were evidenced: three soils of high CEC and organic carbon contents which retained high amounts of propranolol, and three soils, the sediment and the kaolinite-based sample (low CEC and organic carbon content) displaying a low adsorption capacity for the beta-blocker. A linear model enabling the determination of the sorption parameters Kd and Koc was pertinent to describe the adsorption isotherms but the Koc values showed a great variability. It was shown that organic carbon content alone could not explain propranolol adsorption. The CEC value was identified as influent parameter and a simple empirical model was proposed to describe propranolol adsorption. At microscopic and molecular scales, ToF-SIMS experiments indicated (i) a decrease of potassium on the surface upon propranolol adsorption with a distribution of the beta-blocker similarly to alumino-silicates, iron and organic carbon on the surface confirming a cation exchange mechanism and (ii) the absence of degradation products and copper-propranolol complexes.

Oxidative dehydrogenation of propane under steady-state and transient regimes over alumina-supported catalysts prepared from mixed V2W4O4−19 hexametalate precursors

Abstract An integrated approach combining the development of an innovative catalyst and the research of a set of adequate operating conditions for the propane oxidative dehydrogenation (ODH) is described. The experimental set-up, specially designed for steady-state and transient studies is presented. The preparation method, the characterization and the performances in steady-state and transient regimes of catalysts based on V2W4O4−19 Lindqvist isopolyanion used as a precursor and supported on alumina are reported. The influence of the preparation method of the catalyst and the role of water in the feed gas are more particularly discussed.