Transient behaviour of dense membranes of BIMEVOX (Me = Cu, Co) catalysts in the oxidation of propane

ME-doped Bi4V2O11 (BIMEVOX) oxides are highly oxide ion conducting materials and this property may be profitably used in selective oxidation of hydrocarbons. The catalytic properties of BICUVOX and BICOVOX when shaped as dense membranes displayed in catalytic dense membrane reactor are examined in the oxidation of propene and of propane. Mirror-polished BICUVOX and BICOVOX membranes studied previously were poorly active for propene oxidation because of a small number of active sites but showed an excellent stability and reproducibility (lasting more than one month) during which products of mild oxidation (acrolein, hexadiene) and CO were formed. Membranes with depolished surfaces exhibit high conversions of propene (up to 60 mol% ), and also of propane (up to 20 mol%) but – contrary to mirror polished membranes – a complex transient behaviour is observed during which syngas production occurs. The membrane polarization followed by in-situ Solid Electrolyte Potentiometry shows that the oxygen reservoir is far higher than expected on the reaction side which is separated (by the membrane) from the oxidising side where (diluted) oxygen is reduced to O2- specie. The influence of oxygen partial pressure on the catalytic performance suggests that the electronic conductivity of the material is limiting the oxygen flux through the membrane, and thus is determining the catalytic properties and transient behaviours of depolished membranes

Selective oxidation of hydrocarbons in a catalytic dense membrane reactor: Catalytic properties of BIMEVOX (Me = Ta)

Used as dense membranes in a catalytic dense membrane reactor, Bi2V0.8Ta0.2O5.5 (BITAVOX) is more or less active in the oxidation of propane, ethane and propene according to its surface roughness. When the surface roughness is low, membranes are poorly active in the oxidation of propane (Conversion < 2%). CO and propene are the main products and the stability is high over long experimental periods. BITAVOX membranes with increased surface roughness exhibit high conversions in the oxidation of propane, ethane and propene. Activities and selectivities evolve with time. In a first short period (<250s), products are similar to those of polished samples, then mainly H2 and CO are produced and subsequently H2 and coke. These evolutions are probably related to surface and sub-surface restructuring due to an insufficient oxygen supply through the membrane to face the high catalytic activity induced by the increase in surface area. The results suggest that the O2- diffusion is limited by the low electronic conductivity of BIMEVOX materials. Nevertheless the membrane is not reduced irreversibly, even in coking conditions. This is confirmed by ex-situ characterisations carried out on the materials (XRD, SEM, XPS). An improvement of the electronic conductivity of this material is necessary in order to stabilize the intermediate catalytic behaviour which is interesting for syngas or H2 production from light hydrocarbons

Bi2V1-xMexO5.5-y (Me = Ta, Ni) Membranes for Selective Oxidation of C1-C3 Alkanes in a Catalytic Dense Membrane Reactor

Pure and Me-doped Bi2V1-xMexO5.5-y (BIMEVOX) (Me = Ta, Ni) materials exhibit a transient behaviour during the oxidation of C1-C3 alkanes when shaped as dense membranes separating two compartments fed with diluted hydrocarbon and with air, respectively (catalytic dense membrane reactor). Alkenes are first produced at low conversion and CO and H2 follow at higher conversion. The H2/CO ratio increases up to values close to the stoichiometric ones and continues to increase after a peak of activity during which coke builds up. The same transient phenomena as initially observed happen after burning the coke by replacing diluted hydrocarbon by air. This in situ regeneration of the membrane means that the structure and integrity of the membranes have not been modified, as indeed shown by physicochemical analyses (XRD, SEM, XPS). The catalytic properties are mainly related to the presence of ME which affects the electronic conductivity of the materials. At variance with quite neutral BITAVOX, the behaviour of BINIVOX is close to that of BICOVOX which is also a p-type semiconductor. Only BINIVOX is able to oxidise methane (conversion ca. 22-25 mol%) to syngas at low temperature (650°C) in a quite steady manner

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in Γ-Bi4V2O11 by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO2 in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H2), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic vs. oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations

SOFC long term operation in pure methane by gradual internal reforming

International audienceA solid oxide fuel cell was designed to be operated in pure methane, without reforming or carrier gas. The fuel cell was built up from conventional NiO-YSZ anode supported cell with a specific Pt screen-printed anodic collecting system and a Ir-CGO catalytic layer. The operation principle is based on Gradual Internal Reforming. After an initiation in H2 for 30 minutes, the cell was operated for almost 2000 hours in pure and dry CH4 with a fuel utilization rate of 30 %. Intrinsic gradual degradation of 15 %/1000 h was observed, but no coking occurred at the anodic side

Oxygen permeation in bismuth-based materials part I: Sintering and oxygen permeation fluxes

Oxygen permeation measurements were performed on two layered bismuth based oxide ceramics: a rhombohedral phase belonging to the Bi2O3-CaO system, (Bi2O3)0.73- (CaO)0.27 (BICAO) and a BICOVOX phase. Oxygen permeability for these systems was compared to permeability of the cubic fluorite type structure with composition (Bi2O3)0.75(Er2O3)0.25 (BE25). Low oxygen permeability was observed for the pure ceramic. As for BE25, permeability was considerably increased if 40 vol% of silver was added to BICAO. In contrast, permeability was not improved by addition of gold to BICOVOX. For this latter phase, the oxygen molecular exchange at the surface is clearly the limiting step in the oxygen transfer

Sociedad y Religión, 30 años: Interrogantes, historia y poder en la producción de conocimiento sobre el fenómeno religioso en América Latina

Con motivo de la celebración de los 30 años de la Revista Sociedad y Religión se convocó a un grupo de especialistas a responder cuatro preguntas orientadoras sobre el estado del arte de  de las ciencias sociales de la religión en América Latina. A saber: 1) ¿Cuáles considera son los núcleos problemáticos delas ciencias sociales de la religión en la actualidad?; 2) ¿Cuáleshan sido los principales ejes o momentos de debate en las ciencias sociales dela religión en América Latina durante los últimos 30 años? ¿Y cuáles piensa quese instalarán o persistirán en el futuro próximo?; 3) ¿Cómo definiría la relación de la academiaLatinoamericana con la academia Europea y la anglosajona? En este sentido,¿Cuáles considera que son las ventajas y las desventajas de las cienciassociales de la religión en América Latina?; 4) ¿Considera que existen actualmente conceptos oteorías obsoletas en las ciencias sociales de la religión? ¿Cuáles? y ¿Por qué?Fil: Wright, Pablo Gerardo. Universidad de Buenos Aires. Facultad de Filosofía y Letras. Instituto de Ciencias Antropológicas. Sección de Etnología y Etnografía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: de la Torre, Reneé. Centro de investigaciones y Estudios Superiores en Antropología Social; MéxicoFil: Steil, Carlos Alberto. Universidade Federal do Rio Grande do Sul; BrasilFil: Mansilla, Miguel Ángel. Universidad Arturo Prat; ChileFil: Garma Navarro, Carlos. Universidad Autónoma Metropolitana Iztapalapa; MéxicoFil: Giumbelli, Emerson. Universidade Federal do Rio Grande do Sul; BrasilFil: Ceriani Cernadas, César Roberto. Facultad Latinoamericana de Ciencias Sociales. Sede Académica Argentina Buenos Aires. Programa de Antropología Social y Política; ArgentinaFil: Suárez, Hugo José. Universidad Nacional Autónoma de México; MéxicoFil: Burity, Joanildo. Fundação Joaquim Nabuco; BrasilFil: Ludueña, Gustavo Andres. Universidad Nacional de San Martín. Instituto de Altos Estudios Sociales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

On the role of the pore morphology on the electrical conductivity of porous yttria-stabilized zirconia

International audienceYttria Stabilized Zirconia ceramics with well-controlled porosity, pore size and shape were prepared using well-calibrated poly-methyl-methacrylate (PMMA) micro-beads (MB) as a pore-forming agent. The microstructure was observed by Scanning Electron Microscopy. Impedance spectroscopy was used to evaluate the effect of pore morphology (pore size, pore size distribution, pore shape and interconnectivity) on the electrical properties of YSZ ceramics. Archie's law based analyzes to express the dependence of conductivity on porosity have shown that Archie's law is independent of pore size for a pore diameter of between 1 and 7 μm. The Bruggeman model could be used to predict the bulk conductivity if the porosity was less than 25%, thus showing that the impedance response included the effect of sinuousness and constriction induced by pores. Therefore, the tortuosity factor calculated from the bulk conductivity was higher than that predicted by the Bruggeman model for porosities greater than 25% and spherical pores wide (>20 μm). Another point relates to the comparison between tortuosity factors obtained for pore samples fabricated with pore-forming PMMA or by sub-sintering