Metastable Corundum-Type In2O3: Phase Stability, Reduction Properties, and Catalytic Characterization

The phase stability, reduction, and catalytic properties of corundum-type rhombohedral In2O3 have been comparatively studied with respect to its thermodynamically more stable cubic In2O3 counterpart. Phase stability and transformation were observed to be strongly dependent on the gas environment and the reduction potential of the gas phase. As such, reduction in hydrogen caused both the efficient transformation into the cubic polymorph as well as the formation of metallic In especially at high reduction temperatures between 573 and 673 K. In contrast, reduction in CO suppresses the transformation into cubic In2O3 but leads to a larger quantity of In metal at comparable reduction temperatures. This difference is also directly reflected in temperature-dependent conductivity measurements. Catalytic characterization of rh-In2O3 reveals activity in both routes of the water-gas shift equilibrium, which gives rise to a diminished CO2-selectivity of 60% in methanol steam reforming. This is in strong contrast to its cubic counterpart where CO2 selectivities of close to 100% due to the suppressed inverse water-gas shift reaction, have been obtained. Most importantly, rh-In2O3 in fact is structurally stable during catalytic characterization and no unwanted phase transformations are triggered. Thus, the results directly reveal the application-relevant physicochemical properties of rh-In2O3 that might encourage subsequent studies on other less-common In2O3 polymorphs.(VLID)2581066Accepted versio

Structural, spectroscopic and computational studies on the monoclinic polymorph form I of potassium hydrogen disilicate KHSi2O5

Hydrothermal treatment of quartz with 2 M K2CO3 solutions at 623 K and 1 kbar resulted in the formation of single crystals of the monoclinic polymorph of potassium hydrogen disilicate KHSi2O5 or KSi2O4 OH . Basic crystallographic data of this so called phase I at room conditions are as follows space group C2 m, a 14.5895 10 , b 8.2992 3 , c 9.6866 7 , b 122.756 10 , V 986.36 10 3, Z 8. The structure was determined by direct methods and refined to a residual of R F 0.0224 for 892 independent observed reflections with I gt; 2s I . The compound belongs to the group of chain silicates. It is based on crankshaft like vierer double chains running parallel to [010]. The H atoms are associated with silanol groups. Hydrogen bonding between neighbouring double chains results in the formation of 5 A amp; 730; wide slabs. The three crystallographically independent K cations with six to eight O ligands provide linkage 1 between the chains of a single slab or 2 between adjacent slabs. Structural investigations have been supplemented by micro Raman spectroscopy. The interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species has been aided by DFT calculation

Rb2Ca2Si3O9 the first rubidium calcium silicate

The crystal structure of Rb2Ca2Si3O9 has been characterized by X ray diffraction techniques and Raman spectroscopy. Crystal growth was performed by the flux method in closed platinum capsules using a polycrystalline ceramic precursor as well as RbCl as a mineralizer. The crystal structure was solved from a single crystal diffraction data set acquired at 23 C and refined to a final residual of R F 0.022 for 1871 independent reflections. Basic crystallographic data are as follows monoclinic symmetry, space group type P1n1, a 6.5902 3 , b 7.3911 3 , c 10.5904 4 , amp; 946; 93.782 3 , V 514.72 3 3, Z 2. With respect to the silicate anions the compound can be classified as a sechser single chain silicate. The undulated chains run parallel to [ 101] and are connected by Rb and Ca cations, which are distributed among four crystallographically independent sites. In a first approximation the coordination polyhedra of the two different calcium ions in the asymmetric unit can be described by distorted trigonal prisms and tetragonal pyramids, respectively. The two rubidium sites exhibit more irregular coordination spheres with eight to nine next neighbors. Structural investigations on the new phase are completed by solid state micro Raman spectroscopy. DFT calculations were employed for the interpretation of the spectroscopic data including the allocation of the bands to certain vibrational specie

Temperature- and moisture-dependent studies on alunogen and the crystal structure of meta-alunogen determined from laboratory powder diffraction data

Starting from a synthetic sample with composition Al2(SO4)3·16.6H2O, the high-temperature- and moisture-dependent behavior of alunogen has been unraveled by TGA measurements, in situ powder X-ray diffraction as well as by gravimetric moisture sorption/desorption studies. Heating experiments using the different techniques show that alunogen undergoes a first dehydration process already starting at temperatures slightly above 40 C. The crystalline product of the temperature-induced dehydration corresponds to the synthetic equivalent of meta-alunogen and has the following chemical composition: Al2(SO4)3·13.8H2O or Al2(SO4)3(H2O)12·1.8H2O. At 90 C a further reaction can be monitored resulting in the formation of an X-ray amorphous material. The sequence of “amorphous humps” in the patterns persists up to 250 C, where a re-crystallization process is indicated by a sudden appearance of a larger number of sharp Bragg peaks. Phase analysis confirmed this compound to be anhydrous Al2(SO4)3. Furthermore, meta-alunogen can be also obtained from alunogen at room temperature when stored at relative humidities (RH) lower than 20 %. The transformation is reversible, however, water sorption of meta-alunogen to alunogen and the corresponding desorption reaction show considerable hysteresis. For RH values above 80 %, deliquescence of the material was observed. Structural investigations on meta-alunogen were performed using a sample that has been stored at dry conditions (0 % RH) over phosphorus pentoxide. Powder diffraction data were acquired on an in-house high-resolution diffractometer in transmission mode using a sealed glass capillary as sample holder. Indexing resulted in a triclinic unit cell with the following lattice parameters: a = 14.353(6) Å, b = 12.490(6) Å, c = 6.092(3) Å, = 92.656(1), = 96.654(1), = 100.831(1), V = 1062.8(8) Å3 and Z = 2. These data correct earlier findings suggesting an orthorhombic cell. Ab-initio structure solution in space group P 1~~~, using simulated annealing, provided a chemically meaningful structure model. The asymmetric unit of meta-alunogen contains three symmetry independent SO4-tetrahedra and two Al(H2O)6 octahedra. The polyhedra are isolated, however, linkage between them is provided by Coulomb interactions and hydrogen bonding. In addition to the water molecules which directly belong to the coordination environment of the aluminum cations there are two additional zeolitic water sites (Ow1 and Ow2). If both positions are fully occupied meta-alunogen corresponds to a 14-hydrate. Structural similarities and differences between the previously unknown structure of meta-alunogen and alunogen are discussed in detail. Since hydrous aluminum sulfates have been postulated to occur in Martian soils, our results may help identifying meta-alunogen by X-ray diffraction not only on the surface of the Earth but also using the Curiosity Rovers ChemMin instrument.(VLID)454130

Li2Ca2Si2O7 Structural, spectroscopic and computational studies on a sorosilicate

Synthesis experiments in the system Li2O CaO SiO2 resulted in the formation of single crystals of Li2Ca2Si2O7. Structural investigations were based on single crystal diffraction. At ambient conditions the compound has the following basic crystallographic data hexagonal symmetry, space group P6122, a 5.0961 2 , c 41.264 2 , V 928.07 6 3, Z 6. Structure solution was performed using direct methods. The final least squares refinement calculations converged at a residual of R F 0.0260. From a structural point the lithium calcium silicate belongs to the group of pyrosilicates containing [Si2O7] groups. Additional lithium and calcium cations are incorporated between the silicate dimers and are coordinated by four and six nearest oxygen neighbours, respectively. Each [LiO4] tetrahedron shares two common corners with directly neighboring tetrahedra forming zweier single chains which are running parallel to amp; 12296;1 0 0 amp; 12297; in z levels defined by the presence of the 61[0 0 1] screw axes. From the corner sharing [LiO4] and [SiO4] moieties a three dimensional framework can be constructed. An interesting feature of this framework is the presence of an O[3] type bridging oxygen linking three tetrahedra one [LiO4] and two [SiO4] units . Structural similarities with other silicates are discussed in detail. The high temperature behavior of the Si O, Ca O and Li O bond distances in Li2Ca2Si2O7 was investigated by in situ single crystal X ray diffraction in the range between 65 and 700 C. From the evolution of the lattice parameters, the thermal expansion tensor amp; 945;ij has been determined. The structural characterization has been supplemented by micro Raman spectroscopy. Interpretation of the spectroscopic data including the allocation of the bands to certain vibrational species has been aided by DFT calculations, which have been performed on the DIRAC cluster at the HZ

Temperature and moisture dependent powder X ray diffraction studies of kanemite NaSi2O4 OH x 3H 2 O

The high temperature and moisture dependent behaviour of synthetic kanemite NaSi2O4 OH 3H2O or SKS 10 has been studied by in situ powder X ray diffraction. Heating experiments in the range between ambient temperatures and 250 C confirm earlier investigations that the dehydration of kanemite occurs in two steps. According to our results the two different reactions start at amp; 8764;30 and 75 C. The dehydration products have the following compositions NaSi2O4 OH H2O monohydrate and NaSi2O4 OH , respectively. The crystal structures of both phases have been solved at ambient conditions ab initio from laboratory powder diffraction data using samples that have been carefully dehydrated at 60 and 150 C, respectively, and refined subsequently by the Rietveld method. Both compounds belong to the group of single layer silicates based on Si2O4 OH sheets. The sodium cations are located between the tetrahedral sheets and are surrounded by oxygen atoms from silicate anions and or water molecules. Depending on the dehydration step the coordination numbers of the alkali ions vary between six kanemite and five NaSi2O4 OH . Kanemite and its two dehydration products show structural similarities which are discussed in detail. Moisture dependent diffraction studies at ambient temperatures indicate that kanemite is stable between 10 and at least 90 relative humidity. Below the lower threshold a transformation to the monohydrate phase was observed. Dehydration and rehydration as a function of humidity is reversible. However, this process is combined with a significant loss of crystallinity of the sample

On the ambient pressure polymorph of K2Ca3Si3O10 An unusual mixed anion silicate and its structural and spectroscopic characterization

An ambient pressure polymorph of K2Ca3Si3O10 has been synthesized via solid state reactions. Single crystal X ray diffraction experiments show, that this new modification crystallizes in the triclinic space group P 1. The structure was solved by direct methods and subsequently refined. A special feature of the crystal structure is the coexistence of two different types of silicate anions. Isolated [SiO4] tetrahedra as well as [Si4O12] vierer single rings occur in the ratio 2 1, resulting in the crystallochemical formula K4Ca6[SiO4]2[Si4O12]. To the best of our knowledge, this is the first example of an oxo silicate where insular and cyclic silicate anions appear concomitantly. Further characterization of this new compound was carried out by electron microprobe analysis and Raman spectroscopy. DFT calculations were employed i to assign Raman bands to certain vibrational modes and ii to determine the relative stabilities of the monoclinic high pressure and the triclinic ambient pressure polymorph of K2Ca3Si3O1

Enhanced Kinetic Stability of Pure and Y-Doped Tetragonal ZrO2

The kinetic stability of pure and yttrium-doped tetragonal zirconia (ZrO2) polymorphs prepared via a pathway involving decomposition of pure zirconium and zirconium + yttrium isopropoxide is reported. Following this preparation routine, high surface area, pure, and structurally stable polymorphic modifications of pure and Y-doped tetragonal zirconia are obtained in a fast and reproducible way. Combined analytical high-resolution in situ transmission electron microscopy, high-temperature X-ray diffraction, and chemical and thermogravimetric analyses reveals that the thermal stability of the pure tetragonal ZrO2 structure is very much dominated by kinetic effects. Tetragonal ZrO2 crystallizes at 400 C from an amorphous ZrO2 precursor state and persists in the further substantial transformation into the thermodynamically more stable monoclinic modification at higher temperatures at fast heating rates. Lower heating rates favor the formation of an increasing amount of monoclinic phase in the product mixture, especially in the temperature region near 600 C and during/after recooling. If the heat treatment is restricted to 400 C even under moist conditions, the tetragonal phase is permanently stable, regardless of the heating or cooling rate and, as such, can be used as pure catalyst support. In contrast, the corresponding Y-doped tetragonal ZrO2 phase retains its structure independent of the heating or cooling rate or reaction environment. Pure tetragonal ZrO2 can now be obtained in a structurally stable form, allowing its structural, chemical, or catalytic characterization without in-parallel triggering of unwanted phase transformations, at least if the annealing or reaction temperature is restricted to T 400 C.(VLID)2852398Accepted versio

The effects of contact patterns and genetic specificity on host and parasite evolution

<p>Many bacteria, viruses and other parasites cause severe morbidity or mortality in their host populations, creating strong selection for physiological or behavioural mechanisms to avoid disease. Likewise, changes in host susceptibility and contact patterns can dramatically influence the spread of infectious diseases, and hence selection for traits such as virulence and infectivity range in parasites. Understanding how ecological and evolutionary changes in one population affect selection in another represents a key challenge for theoreticians and empiricists alike, and is essential for gaining further insights into host-parasite relationships.</p> <p>This thesis contains theoretical models that explore how genetic and environmental factors shape the evolutionary and coevolutionary dynamics of hosts and parasites. In particular, the roles of genetic specificity (i.e. genotype-by-genotype interactions) and population mixing patterns are investigated, using both mathematical models and computer simulations. A broad range of scenarios are covered, including the coevolution of broad resistance and infectivity ranges (generalism), the persistence of coevolutionary cycling and the maintenance of sex, the effects of mating behaviour on disease prevalence and evolution, and the evolution of sexual and social behaviour. The models presented herein develop our understanding of host-parasite relationships and highlight the importance of genetic interactions and ecological feedbacks.</p>This thesis is not currently available on ORA