Thorsten M. Gesing

Crystal chemistry and properties of mullite-type Bi2M4O9: An overview

Abstract Bi2M4O9 (M = Al3+, Ga3+, Fe3+) belongs to the family of mullite-type crystal structures. The phases are orthorhombic with the space group Pbam. The backbones of the isostructural phases are edge-connected, mullite-type octahedral chains. The octahedral chains are linked by dimers of M2O7 tetrahedral groups and by BiO polyhedra. The Bi3+ cations in Bi2M4O9 contain stereo-chemically active 6s2 lone electron pairs (LEPs) which are essential for the stabilization of the structure. Although the octahedral chains of the closely related Bi2Mn4O10 are similar to those of Bi2M4O9, Bi2Mn4O10 contains dimers of edge-connected, five-fold coordinated pyramids instead of four-fold coordinated tetrahedra. Also the 6s2 LEPs of Bi3+ in Bi2Mn4O10 are not stereo-chemically active. Complete and continuous solid solutions exist for Bi2(Al1–xFex)4O9 and Bi2(Ga1–xFex)4O9 (x = 0–1). Things are more complex in the case of the Bi2(Fe1–xMnx)4O9+y mixed crystals, where a miscibility gap occurs between x = 0.25–0.75. In the Fe-rich mixed crystals most Mn atoms enter the octahedra as Mn4+, with part of the tetrahedral dimers being replaced by fivefold coordinated polyhedra, whereas in the Mn-rich compound Fe3+ favorably replaces Mn3+ in the pyramids. The crystal structure of Bi2M4O9 directly controls its mechanical properties. The stiffnesses of phases are highest parallel to the strongly bonded octahedral chains running parallel to the crystallographic c-axis. Perpendicular to the octahedral chains little anisotropy is observed. The temperature-induced expansion perpendicular to the octahedral chains is probably superimposed by contractions. As a result the c-axis expansion appears as relatively high and does not display its lowest value parallel to c, as could be inferred. Maximally 6% of Bi3+ is substituted by Sr2+ in Bi2Al4O9 corresponding to a composition of (Bi0.94Sr0.06)2Al4O8.94. Sr2+ for Bi3+ substitution is probably associated with formation of vacancies of oxygen atoms bridging the tetrahedral dimers. Hopping of oxygen atoms towards the vacancies should strongly enhance the oxygen conductivity. Actually the conductivity is rather low (σ = 7 · 10−2 S m−1 at 1073 K, 800°C). An explanation could be the low thermal stability of Sr-doped Bi2Al4O9, especially in coexistence with liquid Bi2O3. Therefore, Bi2Al4O9 single crystals and polycrystalline ceramics both with significant amounts of M2+ doping (M = Ca2+, Sr2+) have not been produced yet. Thus the question whether or not M2+-doped Bi2M4O9 is an oxygen conducting material is still open.

A versatile sol–gel coating for mixed oxides on nanoporous gold and their application in the water gas shift reaction

Based on a sol–gel coating method, a series of nanoporous gold (npAu) catalysts functionalized with titania–ceria mixed oxides were prepared. Metal-oxides with different composition were formed inside the mesoporous material (ligaments and pores ∼45 nm) after thermal treatment at over 200 °C for 2 h. The water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) was studied in a continuous flow reactor at ambient pressure using these Ce–TiOx/npAu catalytic materials. Formation of CO2 was observed at temperatures between 200 °C and 450 °C. The addition of CeO2 to TiO2 resulted in an strongly increased activity; the sample (with the molar ratio of Ceu2006:u2006Ti = 1u2006:u20062 abbreviated as Ce1Ti2Ox/npAu) shows the highest activity which was nearly twice as high as the activity of all other samples at 300 °C. The loss of activity after 2 catalytic runs was only about 10% at 450 °C for the Ce1Ti2Ox/npAu sample and no coarsening was observed. Raman spectroscopic characterization of the materials indicates a dynamic correlation between the crystallization (oxygen storage) of the metal-oxides under oxidizing and reducing conditions.

Structural Evolution from 0D Units to 3D Frameworks in Pb Oxyhalides: Unexpected Strongly Corrugated Layers in Pb7O6Br2

Novel Pb7O6Br2 (1) lead oxybromide was prepared from Pb oxybromide melt by the rapid quenching route. Bonding scheme, thermal expansion, and structural properties were studied. The structural features of this unexpectedly complex phase are described on the basis of lone electron pair stereochemical activity and Pb-Br versus Pb-O bonding scheme. The structure of 1 contains a number of cavities, which can be assigned to the self-containments of the lone electron pairs on Pb(2+) cations. Empty □Pb4 chains are observed in between of the folding sides of the adjacent strongly corrugated oxocentered [Pb7O6](2+) layers. Highly isotropic thermal expansion of 1 appeared to be unexpected. The possible explanations of such a behavior in 1 are given. The structure of 1 is an interesting example of tetrahedral framework with mixed chemical bonding and is the densest known among Pb oxyhalides with the density of 18.4 tetrahedra/1000 Å(3). Current study shows that oxocentered layers derivatives from α-PbO can be very flexible and form rather dense three-dimensional structural topologies. The properties and structure are compared to other phases crystallizing in the anhydrous PbO-PbX2 (X = F, Cl, Br, I) systems, illustrate the complexity of lead oxyhalides, and reveal new and general pathways for the targeted synthesis of new phases with the Pb-O units of desired dimensionality. The indirect gap value of ∼ 2.04 eV obtained from generalized gradient approximation calculations demonstrates potentially good photocatalytic properties of 1.

Guanidinium tetra-bromidomercurate(II).

The Hg atoms in the crystal structure of the title compound, (CH6N3)2[HgBr4], are tetrahedrally coordinated by four Br atoms and the resulting [HgBr4]2− tetrahedral ions are linked to the [C(NH2)3]+ ions by bromine–hydrogen bonds, forming a three-dimensional network. In the structure, the anions are located on special positions. The two different Hg⋯Br distances of 2.664u2005(1) and 2.559u2005(1)u2005Å observed in the tetrabromidomercurate unit are due to the connection of Br atoms to different number of H atoms.

Highly active Co–Al2O3-based catalysts for CO2 methanation with very low platinum promotion prepared by double flame spray pyrolysis

Cobalt-based catalysts are often promoted with noble metals to improve the reducibility of the catalyst and provide a high number of metallic Co sites. The high cost of such noble metals requires new synthetic strategies enabling the use of such promoters at as low concentrations as possible. In this article, we present platinum-promoted Co–Al2O3 catalysts with very small concentrations of platinum (between 0.03 and 0.43 wt%) synthesized by double flame spray pyrolysis (DFSP) as a very versatile preparation technique. Catalysts with Pt contents as low as 0.03 wt% Pt lead to a significant improvement in the reducibility of Co3O4 and to high catalytic activity for the CO2 methanation reaction compared to non-promoted Co–Al2O3. Upon further increasing the Pt content up to 0.43 wt%, only a slight improvement in catalyst reduction and catalytic activity is observed. All prepared catalysts were characterised using XRD, BET, TPR, TEM and EDX followed by catalytic tests for CO2 methanation. Furthermore, two different preparation schemes were used for DFSP, where platinum was combusted either with Co or with the Al precursor solution in one flame, which results in catalysts with a tight chemical contact between Pt and Co3O4 or Pt and Al2O3, respectively. Based on TPR and catalytic tests it could be demonstrated that the deposition of platinum on one or the other oxidic phase has no influence on the reducibility and catalytic performance. The conversion and reducibility were similar for both preparation schemes, an observation which can be explained by H2 spillover during catalyst reduction and catalytic reaction.

Liebau density vector: a new approach to characterize lone electron pairs in mullite-type materials

Abstract The bismuth 6s2 lone electron pair (LEP) in mullite-type Bi2M4O9/10 (where M = Al, Fe, or Mn) was characterized by means of several parameters derived from experimental and theoretical calculations. The Wang-Liebau eccentricity (WLE) parameter proved to be very useful to quantify the stereochemical activity of the LEPs. Calculations of electronic distributions (three-dimensional charge density difference isosurfaces) were used as independent measurements, which validated the relevance of the WLE parameter for the characterization and quantification of LEPs. The distribution of the Bi 6s2 electrons around the nucleus was evaluated and the maximum of electron density calculated. The spatial orientation of this electron density with respect to the nucleus is expressed as “Liebau density vector”. Therefore, this vector is ascribed to be a key result of this work as a proof that the purely geometrically defined Wang-Liebau vector indeed points towards the maximum electron density of the LEP. The LEP stereochemical activity was studied in terms of the type of structure (Bi2M4O9 or Bi2M4O10) and the nature of M. The effect of exchanging bismuth by lanthanum as well as the relative stabilities of Bi2M4O9 or Bi2M4O10 structures were calculated and discussed.

“Forbidden” reflections in neutron diffraction on bismuth metal oxides: symmetry reduction, λ/2 effect or Umweganregung?

Abstract Large crystals of Bi2M4O9 (M = Fe, Ga) were examined by single-crystal neutron diffraction at the Institut Laue Langevin (ILL) in Grenoble. The crystal structures of these bismuth metal oxides are commonly described as mullite-type compounds in space group Pbam. However, up to several hundreds of reflections were observed in both compounds with significant intensity violating the reflection conditions in this space group. Psi-scans with a short wavelength (0.837 Å at D9) were quasi-continuous and thus provided ambiguous results, hampering the identification of the source of these intensities. With a much longer wavelength (2.364 Å at D10) the Psi scans displayed strong variations of the intensities with small changes of the scan angle Psi. These Psi scan intensities could be well matched by an appropriate calculation with the program UMWEG (Rossmanith, 2003) and thus multiple diffraction turned out to be the source of these extra reflections. As a result the space group Pbam proved to be the correct one. Due to widespread misconceptions the geometrical background of Umweganregung is discussed to considerable detail.

Symmetry reduction due to gallium substitution in the garnet Li6.43(2)Ga0.52(3)La2.67(4)Zr2O12.

Gallium-substituted lithium lanthanum zirconate (LLZO; Li6.62La2.65Ga0.49Zr2O12) belongs to the family of garnets and shows a reduction of the symmetry to space group I 3d compared to Ia d typically observed for these structures.

Czochralski growth and characterization of TbxGd1−xScO3 and TbxDy1−xScO3 solid-solution single crystals

TbxGd1−xScO3 and TbxDy1−xScO3 solid-solution single crystals were grown by the Czochralski method to validate the practicality of solid solutions between neighboring RE scandates (REScO3). Within this material family, it is demonstrated that fine-tuning the pseudo-cubic lattice parameters between nearly all endmembers is now possible by adjusting the chemical composition of the mixed crystals. Film-tailored substrate lattice spacings enable fine adjustments of epitaxial strain or the growth of nearly strain-free heteroepitaxial films. Investigations of the chemical composition of the grown crystals revealed low segregation, which qualifies the solid-solutions as suitable substrate materials. The melting behavior was studied by differential thermal analysis. To calculate the thermal conductivity, λ(T), the heat capacity and thermal diffusivity were measured by differential scanning calorimetry and the laser flash technique, respectively. It was found that the thermal conductivity shows a minimum near 900 K and increases untypically at higher temperatures.

The Influence of the Pyrolysis Temperature on the Material Properties of Cobalt and Nickel Containing Precursor Derived Ceramics and their Catalytic Use for CO2 Methanation and Fischer–Tropsch Synthesis

Ni and Co containing precursor derived ceramics (ceramers) were prepared from a polysiloxane based preparation route. All catalysts were characterised by BET, XRD and TEM as well as by water and heptane adsorption and tested for CO2 methanation and Fischer–Tropsch synthesis. Different pyrolysis temperatures between 400 and 600u2009°C were used to get catalysts with different surface hydrophilicities. With increasing synthesis temperature less organic groups remain on the surface, resulting in a more hydrophilic catalyst. For all Ni containing ceramers, well dispersed particles in the range of 3xa0nm were formed and comparable surface areas were found. The catalysts with the lowest tendency towards water adsorption showed the highest activity for CO2 methanation. In contrast to the Ni catalysts, for the Co containing ceramers particle formation was dependent on the pyrolysis temperature. While no metallic particles were formed at 400u2009°C, small particles in the range of ~5xa0nm were obtained, using a pyrolysis temperature of 500u2009°C. Increasing the pyrolysis temperature to 600u2009°C, the particle size increased to ~10xa0nm. First tests for CO2 methanation and Fischer–Tropsch reaction were successfully carried out and the catalysts with the less hydrophilic surface showed higher activity and a higher selectivity towards C5+-products.Graphical Abstract