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MICROSCOPIE:RX+CARE:ECI2D:MEME+GFE:MAO:FBS:LBU:CGE:TEP:SLO:PVE; International audience; This study explores the dynamic properties of Pt/CeO2 catalysts to develop active catalytic converters for diesel exhausts. It is now well established that single Pt atoms can be stabilised on ceria surface defects under oxidising conditions. However, their catalytic activity is rather poor. A reducing treatment is required to build more active Pt nanoparticles. A Pt/CeO2 catalyst, containing mainly atomically dispersed Pt species, was exposed to reduction steps either at 250 or 500 °C to build Pt nanoparticles. Their redispersion after oxidising treatments in oxygen at three temperatures (room temperature, 250 °C and 500 °C) as well as in a simulated diesel exhaust gas was deeply investigated using different characterisation techniques such as high angle annular dark field scanning transmission electron microscopy, in situ Raman spectroscopy, in situ X-ray diffraction, X-ray photoelectron spectroscopy and H2-temperature-programmed reduction. The arrangement of close packed Pt clusters is a permanent event at low temperatures (

RX+ECI2D+OVO:FBS:JMI; International audience; Several M-modified iron oxides of the spinel family have proven to be effective electron and O2- vectors for the production of hydrogen in the chemical-loop reforming of bio-alcohols. The present work is specifically focused on investigation of ethanol anaerobic decomposition over spinel oxides, which results in significant structural changes of the oxygen carrier material itself and corresponds to a first step of the chemicalloop reforming process. In particular, a series of binary/ternary M-modified ferrospinels were prepared by a co-precipitation method and tested in terms of both redox properties and intrinsic catalytic activity in addition to a complex ex situ study that encompasses the solid-state chemistry investigations of the fresh and reduced oxygen carrier materials. It was found that Co/Cu incorporation facilitates total/partial oxidation of ethanol, giving rise to high yields of H-2, COx, and H2O; whereas Mn incorporation predominantly favored dehydrogenation and condensation reactions, leading to the formation of acetaldehyde and acetone. In addition, the incorporation of Mn contributed to significantly reduce the amount of coke formed; however, it caused a lower intrinsic reducibility, which was explained by the formation of a thermodynamically stable and hardly reducible layer of MnxFeyO solid solution.

RX+CARE+MTS:CZA:ABO:FBS:PVE; International audience; La0.6Sr0.4BO3 type-perovskites (with B = Fe, Mn or Ti) were synthesized by a complex route from the thermal decomposition of the chelated nitrate precursors, as soot oxidation catalysts under low oxygen partial pressures. The nature of the B-site cation modifies the morphology, the structural symmetry, the charge compensation mechanism and the redox properties. The generation of oxygen vacancies is predominant in LSF while an increase in the oxidation state of B cations occurs in LSM and LST, respectively. A direct link was established between the reducibility of the perovskites and their ability to oxidize soot. Fe and Mn-based perovskites exhibit similar catalytic behaviors, superior to LET, which are attributed to the presence of oxygen vacancies in LSF and Mn4+ cations in ISM. This latter redox property is particularly suitable to oxidize soot at low oxygen partial pressures, as encountered in gasoline direct injection engine exhausts. (C) 2014 Elsevier B.V. All rights reserved.

Magnetite samples were synthesized and studied as the cycling material of a chemical loop process for hydrogen production from ethanol and water used as reducing and oxidizing species, respectively. Surface and structural changes during the process were characterized by various techniques such as X-ray diffraction, X-ray photoelectron, and Mossbauer spectroscopy in order to evidence the real cycling process and understand the cause of the material deactivation so that it can be suppressed or minimized for an industrial application. We found that the complete recovery of the initial cycling material was possible, but that a slow accumulation of coke took place over time under cycling conditions. Indeed, this deposited coke corresponds to only a part of the coke formed, since water makes partial re-oxidation possible. A third step to burn the coke left over by the air will thus have to be periodically added for a sustainable industrial process, unless a cycling material and/or certain conditions capable of either totally preventing the formation of coke or leading to the formation of coke that is not oxidized by water are found.

MICROSCOPIE:RX+ECI2D:ING+FMO:MAO:FBS:JRO:LPI; International audience; PGM-CeO2 (PGM = Pt, Pd, Ir, Rh, Ru) catalysts were prepared by one-step solution combustion synthesis (SCS) and characterized by elemental analysis, N-2 volumetry, aberration-corrected HRTEM, X-ray diffraction, and CO-DRIFTS. The samples, consisting of 2-6 nm metal nanoparticles supported on mesoporous ceria, were tested in CO oxidation in absence and presence of hydrogen (PROX). To our knowledge, this work presents the first comparison of all the platinum-group metals (except Os) for these reactions in the same conditions. The as-prepared SCS catalysts are active in CO oxidation and a reducing treatment has no significant effect on their performances. While the best catalyst in H-2-free CO oxidation is Rh-CeO2, the addition of a high hydrogen excess decreases the Rh catalyst activity but enhances the CO oxidation rate on all other systems, including alumina-supported metals employed as reference catalysts. The resulting PROX turnover frequencies (Pt > Pd > Rh > Ir > Ru) follow the trends predicted by published density-functional-theory calculations considering the COad + O-ad elementary reaction as the rate-determining step. Pt-CeO2 is not only the most active but also the most selective catalyst, reaching near 100% CeO2 at low temperature (ca. 100 degrees C). The alumina-supported catalysts appeared less active than their ceria-supported counterparts in both reactions. The effect of heating/cooling cycles on the reaction kinetics was also investigated. Whereas the Pt, Pd and Ir ceria-supported catalysts were stable throughout PROX cycles, Rh and Ru ones exhibited apparently chaotic behaviors above ca. 200 degrees C, which are proposed to be induced by favorable CO dissociation and methanation pathways and/or variable metal oxidation states. (C) 2015 Elsevier B.V. All rights reserved.

MICROSCOPIE:RX:XPS+ATARI:ECI2D:ING+GPO:SLO:FBS:LBU:MAO:LMA:PGE:FMO:LPI; International audience; 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 degrees 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 degrees 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.

Seed-assisted solution synthesis of hollow IF-MoS2 nanoparticles allows independent control of particles size and MoS2 slabs crystallinity. Variations of the reaction mixture composition influence the particle size in the range 50–150 nm. As demonstrated by Rietvelt refinement of the X-ray diffraction patterns, the sulfide crystallinity depends only on the post-treatment temperature (350–750 °C) and not on the particle size. The tribological properties of new MoS2 nanoparticles prepared by seed-assisted solution technique were investigated and showed a strong decrease in the friction coefficient and wear compared with base oil. Small particles of 50–60-nm size showed the best results. The particle size above 100 nm is deleterious for the lubrication properties since it hinders particles penetration into the contact zone. MoS2 slabs crystallinity had lesser influence on the lubrication efficiency. However, less-crystallized samples treated at 350 °C showed better lubrication, apparently because of easier exfoliation of the individual MoS2 slabs, leading to more efficient formation of tribofilm.