Characterization of Ba2In2-xSnxO5+x/2 oxide ion conductors

International audienceThe Ba2In2-xSnxO5+x/2 solid solution was confirmed up to x = 1 by solid state reaction. X-ray diffraction at room and at elevated temperatures, Raman scattering and impedance spectroscopy were used to characterise the samples. Structure refinement of composition x = 0.1 from neutron diffraction data reveals that tin is preferentially located in the tetrahedral layers of the brownmillerite

Oxygen diffusion mechanisms in two series of oxide ion conductors: BIMEVOX and Brownmillerite materials

oral invitéInternational audienceThey are two ways to optimize the electrical properties of a material, 1) tailor its microstructure, 2) tailor its structure by partial substitution. The BIMEVOX family, which was evidenced in the end of the eighties by members of our group, was obtained by partial substitution for vanadium in the parent compound Bi4V2O11 with a metal. This led to the stabilization of the highly conductive gamma Bi4V2O11 form at room temperature. BIMEVOX exhibit the best oxide ion conduction at moderate temperature, 400-700°C. However, because of the sensitivity of bismuth toward reduction, they could not be directly used in Solid Oxide Fuel Cells. We attempted to develop them as electrolyte for the electrically driven separation of oxygen from air and as membrane for the oxidation of hydrocarbons. Their high oxide conduction is due to their layered structure. Another family of oxide ion conductors with a bidimensionnal structure is the Brownmillerite. In 1990, Goodenough evidenced high oxide conduction in the Ba2In2O5 Brownmillerite above 925°C. In order to stabilize these high properties at lower temperature, numerous partial substitutions were performed on either the Ba or the In site. Our group considered the partial substitution for Indium with cation with valence higher than 3, such as Sn, V, Ta, Nb, Mo and W. Solid solutions were obtained for all the dopants. High temperature neutron diffraction was performed on the two family of oxide ion conductors. Because of the high oxide ion disorder, anharmonic thermal parameters were used to take into account all the nuclear density. Preferential oxygen pathways were derived, energy barriers were calculated and compared to the experimental values obtained from impedance spectroscopy. In a second step, their microstructure were optimized to obtain the dense ceramic needed for the applications. This was easily achieved for BIMEVOX but not for Brownmillerite which hydrate around 200-400°C and exhibit proton conduction in this domain of temperature

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La2O3-Ta2O5 system was considered with a focus on the La3TaO7 phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La3−xSrxTaO7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La3TaO7 is a poor oxide ion conductor (σ700 °C = 2 × 10−5S.cm−1), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ700 °C = 2 × 10−4S.cm−1)

Caractérisation du transport de l'oxygène dans des électrolytes céramiques de l'échelle atomique à l'échelle macroscopique

oral invitéNational audienceDes outils tels que la spectroscopie d'absorption X (EXAFS/XANES), la diffraction des neutrons, la spectrométrie d'impédance, l'échange isotopique, la mesure de flux de semi-perméabilité permettent de sonder le transport de l'oxygène sur différentes échelles. Ces techniques ont été appliquées à deux familles de conducteurs par ions oxyde, les BIMEVOX, composés dérivés de Bi4V2O11, et les phases dérivées de la Brownmillerite Ba2In2O5. Les deux familles de composés présentent des structures cristallines bi-dimensionnelles. Leurs formes « haute température », conductrices par ions oxyde, sont facilement stabilisées à plus basse température par substitution partielle du vanadium ou de l'indium. La diffraction des neutrons à haute température a permis de déduire des chemins préférentiels pour la diffusion des ions oxyde dans la structure. Les valeurs des barrières énergétiques à la diffusion, déduites des cartes de densité de probabilité de présence des nucléons, ont été comparées aux valeurs des énergies d'activation mesurées par spectroscopie d'impédance. Cependant, les phénomènes de transport de l'oxygène sont plus complexes. Les applications comme électrolyte nécessitent des céramiques denses. Si les BIMEVOX sont faciles à densifier, les phases de type Brownmillerite le sont moins. Par ailleurs, comme le montre l'échange isotopique et les mesures de flux de semi-perméabilité, un autre paramètre doit être pris en compte dans le transport de l'oxygène, le transfert en surface de l'oxygène moléculaire

A neutron diffraction study of the oxygen diffusion in molybdenum doped Ba2InO5

International audienceThe structures of molybdenum doped Ba2In2O5 were refined using X-ray and neutron diffraction data at room and high temperature with the aim to derive preferred oxygen diffusion pathways. At room temperature, refinement of composition Ba2In2-xMoxO5+3x/2 with x=0.1 revealed molybdenum atoms are preferentially located in the tetrahedral layers of the brownmillerite. At 700°C, the structure can be viewed as the stacking of alternating In and In/Mo octahedral layers. The conduction process occurs preferentially in the later which is highly oxygen deficient. Preferred oxygen diffusion pathways were deduced from Joint Probability Density Function (JPDF) and energy barriers were derived. It was in good agreement with the activation energy deduced from impedance spectroscopy for composition x=0.1 at 950°C. However, calculation of energy barrier assumes a dynamic disorder of oxide ions which is unlikely to occur at lower temperature and for sample containing a larger amount of molybdenum. Composition x=0.5 is cubic on the whole range of temperature. At room temperature, JPDF revealed a static disorder of the oxygen atoms, which is likely due to the solution of molybdenum into the barium indium perovskite. When temperature increases the disorder becomes more and more dynamic

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

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