M. Mangir Murshed

Kinetic Studies of Methane–Ethane Mixed Gas Hydrates by Neutron Diffraction and Raman Spectroscopy

In situ formations of CH(4)-C(2)H(6) mixed gas hydrates were made using high flux neutron diffraction at 270 K and 5 MPa. For this purpose, a feed gas composition of CH(4) and C(2)H(6) (95 mol% CH(4)) was employed. The rates of transformation of spherical grains of deuterated ice Ih into hydrates were measured by time-resolved neutron powder diffraction on D20 at ILL, Grenoble. Phase fractions of the crystalline constituents were obtained from Rietveld refinements. A concomitant formation of structure type I (sI) and structure type II (sII) hydrates were observed soon after the gas pressure was applied. The initial fast formation of sII hydrate reached its maximum volume and started declining very slowly. The formation of sI hydrate followed a sigmoid growth kinetics that slowed down due to diffusion limitation. This observation has been interpreted in terms of a kinetically favored nucleation of the sII hydrate along with a slow transformation into sI. Both powder diffraction and Raman spectroscopic results suggest that a C(2)H(6)-rich sII hydrate was formed at the early part of the clathration, which slowly decreased to approximately 3% after a reaction of 158 days as confirmed by synchrotron XRD. The final persistence of a small portion of sII hydrate points to a miscibility gap between CH(4)-rich sI and C(2)H(6)-rich sII hydrates.

Kinetics of methane-ethane gas replacement in clathrate-hydrates studied by time-resolved neutron diffraction and Raman spectroscopy.

The kinetics of CH(4)-C(2)H(6) replacement in gas hydrates has been studied by in situ neutron diffraction and Raman spectroscopy. Deuterated ethane structure type I (C(2)H(6) sI) hydrates were transformed in a closed volume into methane-ethane mixed structure type II (CH(4)-C(2)H(6) sII) hydrates at 5 MPa and various temperatures in the vicinity of 0 degrees C while followed by time-resolved neutron powder diffraction on D20 at ILL, Grenoble. The role of available surface area of the sI starting material on the formation kinetics of sII hydrates was studied. Ex situ Raman spectroscopic investigations were carried out to crosscheck the gas composition and the distribution of the gas species over the cages as a function of structure type and compared to the in situ neutron results. Raman micromapping on single hydrate grains showed compositional and structural gradients between the surface and core of the transformed hydrates. Moreover, the observed methane-ethane ratio is very far from the one expected for a formation from a constantly equilibrated gas phase. The results also prove that gas replacement in CH(4)-C(2)H(6) hydrates is a regrowth process involving the nucleation of new crystallites commencing at the surface of the parent C(2)H(6) sI hydrate with a progressively shrinking core of unreacted material. The time-resolved neutron diffraction results clearly indicate an increasing diffusion limitation of the exchange process. This diffusion limitation leads to a progressive slowing down of the exchange reaction and is likely to be responsible for the incomplete exchange of the gases.

Isomorphous gallium substitution in the alumosilicate sodalite framework: synthesis and structural studies of chloride and bromide containing phases

In the Na8[T1T2O4]6Y2 sodalite system (Y = Cl, Br, T2 = Si) the aluminium bearing T1 position was successively substituted with gallium. The synthesized products represent the new composition Na8[Al1–yGay-SiO4]6Y2 (0 ≤ y ≤ 1), where the Al/Ga ratio is calculated from the X-ray powder data Rietveld refinements. In each composition, the gallium concentration in the framework is observed lower than that in the initial feed. The linear expansion of the lattice parameters has been observed from [AlSiO4]6 to [GaSiO4]6 in increasing gallium concentration in the framework. The position of gallium and aluminium has been refined on a single crystallographic site. This result also indicates that both aluminium and gallium are indistinguishable and statistically distributed in the T1-sites. The T1-O distance, therefore, offers an averaged magnitude of Al/Ga—O distance which increases with increasing gallium concentration. In contrast, the Si—O distance remains almost constant at 162.8 pm (2σ) and 163.4 pm (3σ) for chloride and bromide sodalites, respectively. The increasing average tilt of the (Al,Ga)O4 and SiO4 tetrahedra leads to a decrease of the six-ring window dimension as a consequence of increasing gallium incorporation in the framework. The simultaneous shortening of Na—O and Na—Cl/Br distances, with increasing gallium content, shows a direct interaction between the framework host and the enclathrated guest. Additionally, the tetragonal tetrahedron distortion and the framework oxygen s-hybridization are discussed as a function of gallium concentration in the two new sodalite series.

Gallium substitution in the alumosilicate framework : synthesis and structural studies of hydro sodalites

Abstract A series of hydro sodalites of composition Na6[Al1−yGaySiO4]6(H2O)8 (0 ≤ y ≤ 1) have been prepared from Na6+x[Al1−yGaySiO4]6(OH · H2O)x(H2O)8−4x sodalites by NaOH/H2O exchange methods carried out in acidic aqueous media. One of the end members (y = 1) Na6[GaSiO4]6(H2O)8 is obtained from Na8[GaSiO4]6I2 by a hydrothermal transformation process. The polycrystalline samples are characterized by Rietveld refinements using X-ray powder diffraction data. The cell parameter fluctuates slightly from the [AlSiO4]6 to the [GaSiO4]6 framework with an average value of 884.8 pm. The occurrence of an almost constant cell parameter has been explained as a combined effect of hydrogen bonding between the oxygen atoms of the entrapped water (or the occluded [Na3□(H2O)4]3+ complex in the β-cage as a whole), and the framework together with six-ring window deformation. During XRD refinements both aluminium and gallium atoms have been converged to a single crystallographic site. The Al/Ga—O bond distance therefore indicates the mean of Al—O and Ga—O distances, which increases with increasing gallium content in the framework. However, the Si—O distance remains almost constant at 162.9 pm at any Ga/Al ratio. The coupled twisting of the (Al,Ga)O4 and SiO4 tetrahedra confirms the increase of tilt angles and decrease of six-ring pore dimension upon aluminium replacement by gallium in the framework. Results of 1H and 29Si MAS NMR as well as FTIR spectroscopy are also discussed in terms of successive gallium incorporation in the alumosilicate hydro sodalite frameworks.

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 Ce : Ti = 1 : 2 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 properties of mullite-type Pb(Al1–xMnx)BO4

Abstract We report on the structural characterization of the mullite-type PbAl1-xMnxBO4 series using neutron, synchrotron and in-house X-ray powder diffraction, Raman spectroscopy and density functional theory (DFT) calculations. The planar geometry of the BO3 group changes only slightly over the whole composition range. The rigid BO3 group plays the dominant roles in the thermal contraction in the a-direction followed by expansion in the b- and c-directions, leading to a correlation a · b/c ~ unity. The unit-cell volume at zero-pressure and 0 K was obtained, as well evaluated as the isothermal bulk-modulus from pressure dependent synchrotron X-ray diffraction using a diamond anvil cell as well as DFT calculations. Thermal expansion of the metric parameters was modeled using a first-order Grüneisen approximation for the zero-pressure equation of state. We used the double-Debye-double-Einstein- Anharmonicity model to calculate the temperature-dependent internal energy of the crystalline end members. The simulation helped to understand the anisotropic thermal expansion and together with the experimental and calculated bulk moduli to approximate the thermodynamic Grüneisen parameters.

Isomorphous framework cation substitution in the alumosilicatesodalites: synthesis, structural and spectroscopic studies of nitritecontaining phases

Abstract Partial substitution of aluminium with gallium in the Na8[Al1–yGaySiO4]6(NO2)2 sodalite system (y = 0–1) revealed an extensive isomorphous miscibility. Each composition (y = 0–1) is isotypic to either the alumosilicate or gallosilicate end members (y = 0 or 1, respectively). The Al/Ga ratio was controlled by selective initial stoichiometries in the direct hydrothermal syntheses. The samples were investigated by X-ray powder diffraction, SEM, FTIR and 29Si MAS NMR spectroscopies. The gallium concentration in the framework was calculated using the X-ray data Rietveld refinement, which was always lower than the feed concentration in each composition. The cell parameter and tilt angle increased, while the T—O—T angle and six-ring window dimension decreased with increasing gallium content in the framework. The Al/Ga—O distance is the mean of Al—O and Ga—O distances and increases with increasing gallium content. In contrast, the Si—O distance fluctuates close to 163 pm with increasing gallium insertion. The broadening of the XRD peaks in the patterns with mid-range gallium concentrations (y = 0.35–0.70) is associated with the formation of Al-rich and Ga-rich domains, which is explained in terms of Al/Ga—O distances and average crystal size with respect to Al/Ga ratio in the framework. The domains are assumed to be smaller than the coherence length of the X-ray radiation but large enough to detect by MAS NMR techniques.

Synthesis, characterization and time dependent phase transformation of Li0.4WO3 bronze

Abstract Lithium tungsten bronzes with a nominal composition of Li0.4WO3 were synthesized by solid state reactions in silica tubes at 973 K and various low pressures. Samples were characterized using X-ray single crystal and powder diffraction, and neutron powder diffraction. The air pressure inside the sealed quartz tubes played roles for the formation of Li0.4WO3 phases with different symmetries. Whereas using a pressure of 10–7 MPa a pure body centered cubic (Im 3¯

Synthesis and Characterization of Niobium Substituted Hexagonal Tungsten Bronzes

\bar 3

Synthesis and characterization of mullite-type (Al1-xGax)4B2O9

) tungsten bronze was formed, at higher pressures (>10–7 MPa) a mixture of Im 3¯