Eric Marceau

Influence of chlorine ions in Pt/A12O3 catalysts for methane total oxidation

Residual chlorine ions on a Pt/Al2O3 catalyst surface prepared from chlorine-containing precursors appear to inhibit the total oxidation of methane. At 450°C, as chlorine is eliminated with time on stream, the reaction rate increases despite the sintering of the platinum particles. The steady state reaction rate which is reached after 60 h is identical to that obtained with a catalyst prepared from a precursor containing no chlorine. Whether chlorine is present or not in the initial state of the catalyst does not appear to have an influence on the evolution of the platinum particle size.

Nickel(II) Nitrate vs. Acetate: Influence of the Precursor on the Structure and Reducibility of Ni/MCM‐41 and Ni/Al‐MCM‐41 Catalysts

Nickel(II) nitrate is the nickel precursor generally selected to prepare oxide‐supported catalysts, due to its low cost, high solubility in water and decomposition at moderate temperatures. However, its decomposition produces NOx, and it is known to melt and redistribute over the support upon heating, leading to poorly dispersed metal particles after reduction and to their partial expulsion from the porous support system. The benefits of using NiII acetate as an alternative to NiII nitrate are explored by investigating a series of model Ni catalysts (1–5 wt %) prepared from the mesoporous siliceous supports MCM‐41 and Al‐MCM‐41. The problems encountered with NiII nitrate, in terms of particle size and redistribution of nickel, are not observed with NiII acetate. In the case of MCM‐41‐supported systems, metal particles are well dispersed and remain located within the porous support. The higher pH of the impregnation solution, favoring electrostatic interactions between nickel ions and the support, may account for the higher metal dispersion. The formation of silicate‐like species in strong interaction with the support does not appear prominent on siliceous MCM‐41 when nickel acetate is the precursor, but seems to be enhanced for higher Ni contents. Finally, Al‐MCM‐41 supports are more sensitive to the exposure to nickel acetate than their siliceous analogues because acetate ions are able to extract Al ions from the support, resulting in a partial disruption of the support organization and to the formation of nickel silicate species.

Evolution of nickel speciation during preparation of Ni–SiO2 catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6−2x]2+ precursor complexes

The evolution of nickel speciation during the successive preparation steps of Ni-SiO(2) catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)(x)(H(2)O)((6-2x))](2+) (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(4)](2+) onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)(2)(SiO)(2)], resulting from the grafting of [Ni(en)(2)(H(2)O)(2)](2+) onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.

Evolution of nickel speciation from aqueous solution to the surface of Ni/SiO2: effect of the number of chelating ligands in [Ni(en)x(H2O)6-2x]2+

The evolution of nickel speciation during the successive preparation steps of Ni–SiO2 catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)x(H2O)(6 2x)]2+ (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H2O)4]2+ (64% of all Ni complexes), [Ni(en)2(H2O)2]2+ (81%) and [Ni(en)3]2+ (61%), respectively. Equilibrium adsorption of [Ni(en)x(H2O)(6 2x)]2+ on SiO2 results in the selective grafting of [Ni(en)(H2O)4]2+ and [Ni(en)2(H2O)2]2+, through the substitution of two labile H2O ligands by two surface SiO groups. The surface [Ni(en)(H2O)2(SiO)2] complex formed by the grafting of [Ni(en)(H2O)4]2+ onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)2(SiO)2], resulting from the grafting of [Ni(en)2(H2O)2]2+ onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni2+ ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.

Speciation of chromium on a straw lignin: adsorption isotherm, EPR, and XAS studies

The binding ability of a lignocellulosic substrate extracted from wheat straw with chromium(III) ion was investigated. The study was carried out at macroscopic and microscopic scales. The adsorbent used was first characterised using X-ray diffraction and electron microscopy. Then, macroscopic studies were conducted using batch adsorption experiments, at room temperature, as a function of time, pH, and metal concentration. The results obtained indicate that several successive phenomena take place at the substrate surface. Indeed, sorption, co-precipitation, and precipitation processes arise depending on the experimental conditions. The speciation of Cr was also investigated, at the atomic scale, by EPR, extended X-ray absorption fine structure (EXAFS), and X-ray absorption near edge structure (XANES). This study revealed that chromium(III) surface complexes have an octahedral geometry. Chromium ions are held in inner-sphere complexes, and are coordinated to six oxygen atoms at an average distance of 1.90 A.

A Time-Resolved In Situ Quick-XAS Investigation of Thermal Activation of Fischer–Tropsch Silica-Supported Cobalt Catalysts†

The Fischer–Tropsch (FT) process transforms coal-, natural gasor biomass-derived syngas (a CO/H2 mixture) into liquid hydrocarbons, which can be used as valuable petroleum substitutes because of their high cetane number and low content of sulfur and aromatics. Supported cobalt catalysts are suitable for low temperature FT process. Their catalytic performance is strongly affected by cobalt dispersion: higher metal dispersion supposes higher proportion of reduced metal in the catalyst and lower average nanoparticle size. This can be achieved by optimizing catalyst texture, adding organic compounds, or promoters during catalyst preparation, decomposing cobalt nitrate in a glow discharge or by controlling the catalyst thermal activation. 11] In particular, de Jong et al. prepared smaller Co3O4 and Co 0 particles on SBA-15 silicas by activating the catalyst in a NO-containing atmosphere instead of air prior to the reduction. In the present paper, in situ quick X-ray absorption spectroscopy (QXAS) has been used as a unique tool to accurately monitor the transformations of dispersed phases in supported catalysts under different atmospheres (air, helium and 5 % NO/He), both from the structural, quantitative, and kinetic standpoints. The time-resolved QXAS spectra were continuously collected at the Co K edge in the transmission mode during the catalyst activation. The experimental setup at SAMBA beamline (SOLEIL synchrotron) also allowed simultaneous in situ recording Raman spectra. The details of preparation, activation and characterisation of CoACHTUNGTRENNUNG(10 wt %)/SiO2 catalysts are given in Experimental Section. Figure S1 (see the Supporting Information) displays timeresolved in situ X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) Fourier transform moduli obtained during the activation of SiO2-supported hexahydrated cobalt(II) nitrate [Co ACHTUNGTRENNUNG(H2O)6] ACHTUNGTRENNUNG(NO3)2 in air or helium. The shift of the edge position towards higher energies and changes in the white line intensity and shape above 160 8C are consistent with the transformation of the cobalt salt into Co3O4, characterised by a typical triangular XANES white line. 13] Actually, the presence of two successive series of isobestic points in the XANES spectra, below and above 140 8C, respectively, shows that decomposition of hydrated cobalt(II) nitrate takes place in two distinct steps, each involving two phases: first, dehydration to anhydrous Co ACHTUNGTRENNUNG(NO3)2 (with progressive replacement of aqua ligands by NO3 ions in Co ions coordination sphere), followed by decomposition of CoACHTUNGTRENNUNG(NO3)2 into cobalt oxide ([Eq. (1) and (2)]).

Preparation of Co2+/ZSM5 catalysts by solid-state reaction : Influence of the precursor on cobalt speciation

Abstract Co2+/ZSM5 catalysts (Co/Al = 1, overstoichiometric ratio) have been prepared by solid-state reaction at 500 °C in argon from H-ZSM5 and four Co(II) precursors: formate, nitrate, chloride and acetate. Co3O4 particles supported by the zeolite grains are the only cobalt-containing phases detected when formate is used. When cobalt nitrate is the precursor, Co3O4 particles appear to be independent from the grains. A fraction of cobalt has nevertheless migrated into the zeolite; this is linked with [Co(H2O)6](NO3)2 low melting point and wetting of the zeolite grains by the molten salt at the beginning of the thermal treatment. Catalysts prepared from cobalt chloride contain both nanometric Co3O4 particles on the support grains and cobalt ions inside the zeolitic channels. Ion exchange is possible, at least partly, due to the fact that unlike nitrate and formate, cobalt chloride has not decomposed before reaching the temperature at which the exchange process is activated. This process is chemically driven by the release of HCl in the gas phase. Finally, the major phase detected when cobalt acetate is the precursor is a poorly organized cobalt phyllosilicate surrounding zeolite grains. Acetate ions attack the zeolitic framework from outside during the solid-state reaction; cobalt ions are inserted in the silicate matrix and progressively migrate deeper in the grains upon thermal activation.

Amphiphilic Polyoxometalates for the Controlled Synthesis of Hybrid Polystyrene Particles with Surface Reactivity

Amphiphilic organo-polyoxometalates (POMs) used in the radical emulsion polymerization of styrene allowed the preparation in aqueous medium of stable 50-100 nm polystyrene-POM composite latexes. Thanks to the presence of a trithiocarbonate group in the POM amphiphile, POMs could be covalently linked to the polymer particle surface. The chemical and catalytic integrity of the POMs was confirmed, and the POM-mediated surface photoactivity of the latexes was demonstrated by the spatially controlled nucleation of silver nanoparticles at the periphery of the composites.

Relationship between soil composition and retention capacity of terbumeton onto chalky soils

Environmental context. The wide use of pesticides for pest and weed control contributes to their presence in underground and surface waters, which has led to a continuously growing interest in their environmental fate. Soils play a key role in the transfer of these compounds from the sprayer to the water as a result of their capacity to retain pesticides depending on the soil components. The knowledge of soil composition should enable one to predict pesticide behaviour in the environment. Abstract. Eight calcareous soils of Champagne vineyards (France) were studied to investigate the adsorption of the herbicide terbumeton (TER). A preliminary characterisation of the soil samples using X-ray diffraction (XRD), elemental and textural analyses, revealed a wide range of soil properties for the selected samples. The adsorption isotherms of TER were plotted for all samples. The determination of soil properties, which significantly correlated with the Kd distribution coefficient, allowed identification of organic matter and CaCO3 as the two main soil components that govern the retention of the herbicide. Organic matter was the predominant phase involved in the retention but its role was limited by the presence of calcite. Finally, the ratio of CaCO3 content to organic matter content was proposed as a useful parameter to predict the adsorption of terbumeton in chalky soils. The evolution of Kd as a function of this ratio was successfully described using an empirical model.

Isopropanol Dehydration on Amorphous Silica–Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo‐Bridging Silanols

The mechanism of isopropanol dehydration on amorphous silica-alumina (ASA) was unraveled by a combination of experimental kinetic measurements and periodic density functional theory (DFT) calculations. We show that pseudo-bridging silanols (PBS-Al) are the most likely active sites owing to the synergy between the Brønsted and Lewis acidic properties of these sites, which facilitates the activation of alcohol hydroxy groups as leaving groups. Isopropanol dehydration was used to specifically investigate these PBS-Al sites, whose density was estimated to be about 10-1  site nm-2 on the silica-doped alumina surface under investigation, by combining information from experiments and theoretical calculations.