Rose-Noëlle Vannier

Structural and computational studies of Bi2WO6 based oxygen ion conductors

A combination of neutron powder diffraction and computer simulation techniques was performed on undoped and doped Bi2WO6 Aurivillius type compounds to clarify some of the factors controlling oxygen transport in these materials. Oxygen vacancies in doped compounds are randomly distributed within the perovskite-like slab. The most favourable dopants are predicted to be NbVand TaV on WVI and LaIII on BiIII in accordance with the experimental results. The calculated migration energy of 0.63 eV is in agreement with the values deduced from impedance spectroscopy data for Ta and Nb doped Bi2WO6 at T>550 °C. At lower temperatures, pair clusters are predicted to form with a 0.25 eV mean binding energy, leading to a 0.88 eV activation energy for oxygen vacancy migration, in good correlation with experimental values. Finally, consideration of possible oxygen ion migration pathways in the structure showed that energy barriers to migration are lowest between adjacent apical and equatorial sites of WO6 oxygen octahedra.

Bi4V2O11 polymorph crystal structures related to their electrical properties

Abstract Combining X-ray single crystal and neutron powder diffraction data, the crystal structures of the three polymorphs of Bi4V2O11 were refined. These polymorphs can be described from a common orthorhombic mean cell with approximate parameters: am≈5.5, bm≈5.6 and cm≈15.3 A. They are all built upon well-defined Bi2O22+ layers spaced by perovskite-like VO3.5 slabs. A large disorder of the oxygen located in these slabs was observed for the γ phase in which the diffusion is very fast. The decrease in temperature leads to an ordering of the oxygen vacancies in the β and α-polymorphs. The oxygen packing in the β-polymorph remains close to that of a distorted perovskite with a vacancy located on one site. This perovskite lattice is not maintained in the α-polymorph. The β to α transition is reconstructive with the formation of typical trigonal bipyramids leading to a 6am superstructure. Because of the high parameter to data ratio, the structure of this α-polymorph was modelled in the smaller 3am superstructure. This superstructure can be explained by an ordering of some oxygen vacancies along [010]. However, this ordering could be more complex and some disorder may remain in this area, which would explain the low activation energy observed for this α-polymorph. The specific structural characteristics of the three polymorphs of Bi4V2O11 are correlated with their corresponding conductivity and activation energy.

Modelling the crystal structures of Aurivillius phases

Abstract Computer simulation techniques are used to model the structures of several Aurivillius phases (with the general formula (Bi 2 O 2 )[A m −1 (B) m O 3 m +1 ]) and related compounds. The methods are based upon interatomic potentials and efficient energy minimisation procedures. The results indicate general agreement between the simulated and experimental structures. From recent diffraction data of the Bi 2 GeO 5 system, a new refinement of the crystal structure has been obtained. A discussion of trends related to the Bi lone-pair parameters of these Aurivillius phases is also presented.

Nickel ferrite spinel as catalyst precursor in the dry reforming of methane：Synthesis,characterization and catalytic properties

Abstract Dry reforming of methane by CO2 using nickel ferrite as precursor of catalysts was investigated. Nickel ferrite crystalline particles were prepared by coprecipitation of nitrates with NaOH or ammonia followed by calcination, or by hydrothermal synthesis without calcination step. The textural and structural properties were determined by a number of analysis methods, including X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), among which X-ray diffraction (XRD) was at room and variable temperatures. All synthesized oxides showed the presence of micro or nanoparticles of NiFe2O4 inverse spinel, but Fe2O3 (hematite) was also present when ammonia was used for coprecipitation. The reducibility by hydrogen was studied by temperature-programmed reduction (TPR) and in situ XRD, which showed the influence of the preparation method. The surface area (BET), particle size (Rietveld refinement), as well as surface Ni/Fe atomic ratio (XPS) and the behavior upon reduction varied according to the synthesis method. The catalytic reactivity was investigated using isopropanol decomposition to determine the acid/base properties. The catalytic performance of methane reforming with CO2 was measured with and without the pre-treatment of catalysts under H2 in 650-800 °C range. The catalytic conversions of methane and CO2 were quite low but they increased when the catalysts were pre-reduced. A significant contribution of reverse water gas shift reaction accounted for the low values of H2/CO ratio. No coking was observed as shown by the reoxidation step performed after the catalytic reactions. The possible formation of nickel-iron alloy observed during the study of reducibility by hydrogen was invoked to account for the catalytic behavior.

From Bi4V2O11 to Bi4V2O10.66: the VV–VIV transformation in the aurivillius-type framework

Bi4V2O11, the parent compound of the BIMEVOX family, has been studied by TEM and HREM, at room temperature and at high temperatures. Under the experimental conditions, Bi4V2O11 undergoes a VV–VIV reduction, ultimately leading to the formation of Bi6V3O16(Bi4V2O10.66) with VIV/VV= 1/2. This transformation is confirmed by in situ X-ray diffraction studies of Bi4V2O11 under reducing atmospheres up to 330 °C. The symmetry of the α-Bi4V2O11 polymorph, stable at ambient temperature, is dependent on the chemical purity of V2O5 used for the synthesis.

BICOVOX family of oxide anion conductors: chemical, electrical and structural studies

The BICOVOX family of compounds (cobalt-doped Bi4V2O11) has been characterised combining chemical, structural and electrical approaches. The compositional extent of the single-phase BICOVOX solid solution, stable at ambient temperature, was determined. Single-crystal and powder X-ray diffraction data, combined with density measurements, enabled us to formulate the solid solution as Bi2(V1–x–yCoxBiy)Oz. The maximum x value (0.25) is reached when y= 0.04, and the maximum y value (0.06), when x= 0.10. The evolution of the Arrhenius conductivity plots vs. x and y has been studied within the tetragonal domain. The maximum σ value is roughly independent of composition for 0.10 ⩽x⩽ 0.15 and 0 ⩽y⩽ 0.04.

Structural, textural and acid–base properties of carbonate-containing hydroxyapatites

Carbonate-containing hydroxyapatites with different Ca/P ratios and optionally containing Na+ cations were successfully synthesized using a precipitation method. The solids were extensively characterized by XRD, LEIS, XPS, IR, TGA and NMR. Further, their acid–base properties were determined by NH3-TPD, PEA-XPS, CO2-TPD and by pulsed liquid chromatography using benzoic acid as a probe. The so-obtained structural, textural and acid–base properties could be finely correlated to give a clear picture of the system. The acidic properties of hydroxyapatites were attributed to Ca2+, surface HPO42− and OH− vacancies and the basic properties were attributed to PO43−, OH− and CaO species. The fine-tuning of the amount, of the nature and the strength of acid–base properties derived by varying the carbonate content in hydroxyapatites can find applications in catalysis, which was illustrated by isopropanol reactivity.

Reactivity of ethanol over hydroxyapatite-based Ca-enriched catalysts with various carbonate contents

The Guerbet reaction of ethanol to produce heavier products was performed over a series of extensively characterized carbonate-containing hydroxyapatites (HAPs) with different Ca/P ratios and thus different densities, strengths and natures of acid and basic sites. These properties were correlated with the reactivity of the solids and an optimal ratio between the amount of acid and basic sites was evidenced (ca. 5). The best performance was accordingly obtained over the Hap-CO3 catalyst, which gave a yield of 30% of heavier alcohols at 40% ethanol conversion.

Electrical and structural investigations on a new bismuth lead vanadium oxide solid electrolyte

Three types of solid solutions derived from Bi4V2O11 have been prepared and characterized: Bi2V1−xPbxO (11−3x)2 with x up to 0.10, Bi2−yPbyVO(11−y2with y up to 0.20 and Bi9Pb0.1+xV1−xO(10.9−3x)/ 2 with x up to 0.12. A single-crystal X-ray structure determination was performed on a sample corresponding to the formula Bi1.9Pb0.2V0.9O5.3. The material was shown to be isostructural with BICUVOX.10 (γ-Bi4V2O11). The ability of substitution of V by Pb was confirmed. Conductivity measurements led to values of σ close to that of BICUVOX. 10 at high temperature, but to significantly lower performances at low temperature. This was related to a decrease of the stability of the γ form in the lead-substituted phases.

A new class of mono-dimensional bismuth-based oxide anion conductors with a structure based on [Bi12O14]columns

Abstract Bi26Mo10O69, i.e. [Bi12O14]2[Bi2Mo10O41] is a pure oxide anion conductor. It can be considered as the first member of the bismuth-based mono-dimensional oxide anion conductors with a skeleton built upon [Bi12O14]∞ columns. In order to modulate its electrical performances, attempts of substitutions in bismuth sites and molybdenum sites were performed. Several dopants have been checked: (i) PbII, CaII, SrII, BaII for bismuth sites, (ii) PV, WVI for molybdenum sites. Six solid solutions, formulated Bi26−xMexMo10O69−0.5x with Me=Pb, Ca, Sr, Ba, Bi26Mo10−xPxO69−0.5x and Bi26Mo10−xWxO69, were characterized. The substitution ratio in bismuth site is systematically limited to two, indicating that the bismuth atoms located in the columns were not affected. In a general way, introduction of oxygen vacancy in the parent compound led to a small decrease of its performances. However the electrical performances of these materials were considerably enhanced by partial substitution for molybdenum with tungsten, giving rise to very attractive properties.