Laurence Burel

Quasi All‐Silica Zeolite Obtained by Isomorphous Degermanation of an As‐Made Germanium‐Containing Precursor

Ge-containing ITQ-22 zeolites have been almost completely degermanated under strong acidic conditions without modifications of the framework topology. Simultaneous to Ge extraction, the framework was partially dissolved; mesopores were formed but the structure was maintained through the re-incorporation of some of silicon species at vacant sites. The presence of many defects in the degermanated framework enabled the incorporation of tetrahedral aluminum, opening the way to the preparation of new and stable acid catalysts with original topologies.

Recent progress in Chevrel phase syntheses: A new low temperature synthesis of the superconducting lead compound

This paper reviews recent progress in the synthesis of Chevrel phases. The objective was to produce better microscopic homogeneity of the elements and/or precursors required to form the phase. Among these new processes, the one dealing with insertion of lead into the binary Mo6S8 compound is particularly attractive for the development of Chevrel phase wire since the phase formation occurs at temperature as low as 440°C. To achieve good microscopic homogeneity of PbMo6S8 (PMS), PbS is used and the extra sulfur is removed under a hydrogen gas flow. Basic superconducting properties, like a.c. susceptibility transition and specific heat anomaly at Tc, demonstrate extremely fine super-conducting particles. Further annealing at the moderate temperature of 600°C improves the homogeneity and/or the crystallinity of the powder.

Ultrastable iridium–ceria nanopowders synthesized in one step by solution combustion for catalytic hydrogen production

Mesoporous ceria loaded with 0.06–0.93 wt% iridium was synthesized in one step by the ambient air combustion of an aqueous solution of ceric ammonium nitrate, ammonium hexachloroiridate, and glycine fuel. The structural properties of the powders, and the influence of such parameters as Ir loading and thermochemical post-treatments, were investigated combining aberration-corrected HRTEM, SEM, in situ XRD, XPS, DRIFTS, and Raman spectroscopy. The materials, which appeared spongy at the micrometer scale, exhibited ca. 30 nm-sized ceria crystallites with a layered structure at the nanoscale. After reducing treatments, Ir nanoparticles anchored at the surface of ceria grains were identified, and their size (ca. 2 nm) did not evolve upon further heating at up to 900 °C. A detailed picture of the Ir–CeO2 interface could be established, with the presence of Irx+–O2−–Ce3+ entities along with oxygen vacancies. The powders loaded with only 0.1 wt% Ir were successfully employed as catalysts for the production of hydrogen from methane and water in low-steam conditions at 750 °C. Due to their higher Ir dispersion and stronger Ir–CeO2 interaction, the combustion-synthesized materials outperformed their conventionally prepared counterparts in terms of activity and stability, making them promising as active catalytic layers for solid-oxide fuel cells integrating the gradual internal reforming concept.

Tc variation in PbMo6S8: A critical analysis and a comparison with pure phases

Abstract Different methods were employed to synthesize a series of high quality PbMo 6 S 8 Chevrel-phase compounds. All the samples were characterized by means of X-ray diffraction, Auger electron spectroscopy, specific heat, acsusceptibility, resistivity, scanning electron spectroscopy and microprobe analysis. A marked decrease in the superconducting transition temperature (T c ) and the hexagonal lattice constant ratio (c/a) were noticed on the samples prepared in quartz tubes. Based on a simple correlation between T c and c/a ratio, we present some of the significant structure-property relations and phase purity of the samples. This interdependence is discussed mainly in terms of oxygen content in the phase. For instance, oxygen-free samples have T c invariably above 14.5 K. Interestingly, the optimal T c is found in samples having rather different, from each other, crystallographic parameters, but with a same c/a ratio. On the other hand, the wide variation of T c and structural parameters of PbMo 6 S 8 observed in the present investigation and those of many earlier works are explained by the presence of varying amounts of oxygen in the ternary phase. These results are checked in parallel with the oxygen-doped (PbMo 6 S 8−x O x ; x ≥ 0.2) samples.

New synthesis route of PbMo6S8 superconducting Chevrel phase from ultrafine precursor mixtures : II. Pbs, Mo6S8, and Mo powders

Fine powders of PbMo{sub 6}S{sub 8} Chevrel phase ({approximately} 0.5 {micro}m) obtained from new ultrafine precursor powders present excellent intrinsic superconducting properties, such as a critical temperature, {Tc}, of about 14 K. These results are due to ultrafine quasi-spherical PbS, MoS{sub 2}, and Mo precursor powders (0.05--0.5 {micro}m) of lead Chevrel phase synthesized by soft chemistry methods (precipitation, co-precipitation). Thus, a better reactivity between the grains of the mixture allows a decrease in the synthesis temperature of the resulting PbMo{sub 6}S{sub 8} phase (800 C instead of 950--1,000 C) required for the PbMo{sub 6}S{sub 8} synthesis from the same precursor powders made by the classical ceramic route. These new Chevrel phase powders have very fine homogeneous grains with high potential surface reactivity, giving improved grain boundaries that are able to carry high current densities.

Self-molybdenum intercalation: stabilization of metastable pseudo-binary Chevrel phases in Mo6Se8–Mo6S8 system

Abstract Three different types of pseudo-binary Mo 6 Se 8−x S x Chevrel phases series have been reported in the literature. Confusion among them has existed, although two of them are metastable series. We present a new study of these series, based on their structural differences found mainly through single crystal investigations, and compare one of their more interesting physical properties, the superconducting critical temperature. This allows us to provide for the first time a complete understanding of the metastability and stabilization mechanisms through self-molybdenum intercalation into pseudo-binary Chevrel phases and to access new metastable Chevrel phases by soft chemistry.

New synthesis route of PbMo{sub 6}S{sub 8} superconducting Chevrel phase from ultrafine precursor mixtures. 2: PbS, Mo{sub 6}S{sub 8}, and Mo powders

The authors have used a new ultrafine precursor, the Mo{sub 6}S{sub 8} binary sulfide, and ultrafine PbS and Mo powders ({approximately} 0.5--0.5 {micro}m) to prepare an original mixture for PbMo{sub 6}S{sub 8} synthesis. The reaction of PbMo{sub 6}S{sub 8} formation occurs in a two-step process, at 600 and 800 C, leading to a {approximately} 0.5 {micro}m grain size. {chi}{sub ac} measurements exhibit good superconducting quality ({Tc} {approx} 14 K) of the resulting PbMo{sub 6}S{sub 8} phase. Additionally, the authors have perfected new synthesis processes of the Ni{sub {approximately}2}Mo{sub 6}S{sub 8} and Li{sub {approximately}3}Mo{sub 6}S{sub 8} ternary Chevrel phases, precursors of the Mo{sub 6}S{sub 8} compound. These syntheses are carried out by H{sub 2}S gas sulfurization of the liquid or solid route and then reduction by H{sub 2} gas over solid. The Li{sub {approximately}3}Mo{sub 6}S{sub 8} phase was chosen because its cation deintercalation yields the smallest Mo{sub 6}S{sub 8} grain size ({approximately} 0.3 {micro}m).

Hot pressing sintered CuxMo6S8 targets for laser ablation thin films deposition

Abstract Sintering conditions of Cu 4 Mo 6 S 8 pellets, 20 mm in diameter and about 4 mm thick have been determined. The expulsion of copper out of the pellet for an annealing temperature of 1100 ° C has been observed, in agreement with the evolution of the phase diagram when the temperature increases. The appearance of Cu 2 S cristallites on the surface of the films deposited from highly densified targets associated with a high excess of copper and sulfur can be correlated with the behavior observed on the pellets. In fact the increase of the temperature of the target under the laser beam impact is likely responsible for a copper migration on its surface, which leads to the observed composition of the film.

CO Hydrogenation on Cobalt‐Based Catalysts: Tin Poisoning Unravels CO in Hollow Sites as a Main Surface Intermediate

Site poisoning is a powerful method to unravel the nature of active sites or reaction intermediates. The nature of the intermediates involved in the hydrogenation of CO was unraveled by poisoning alumina-supported cobalt catalysts with various concentrations of tin. The rate of formation of the main reaction products (methane and propylene) was found to be proportional to the concentration of multi-bonded CO, likely located in hollow sites. The specific rate of decomposition of these species was sufficient to account for the formation of the main products. These hollow-CO are proposed to be main reaction intermediates in the hydrogenation of CO under the reaction conditions used here, while linear CO are mostly spectators.

Jc investigations in Chevrel phase wires

Abstract Two-step heat treatments have been carried on ceramic-route precursor wires and lead to the optimization of critical current density. J c curves as a function of heat treatment time show a maximum value at any temperature, corresponding to a compromise between the fabrication and the degradation of the superconducting Chevrel phase.