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.

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.

Mullite-type Bi2Fe4O9 was synthesized using a polyol-mediated method. X-ray powder diffraction (XRD) revealed that the as-synthesized sample is nanocrystalline. It transformed into a rhombohedral perovskite-type BiFeO3 followed by a second transformation into mullite-type Bi2Fe4O9 during heating. Each structural feature, from as-synthesized into crystalline phase, was monitored through temperature-dependent XRD in situ. The locally resolved high resolution transmission electron micrographs revealed that the surface of some heated samples is covered by 4–13 nm sized particles which were identified from the lattice fringes as crystalline Bi2Fe4O9. XRD and Raman spectra demonstrate that the nucleation of both BiFeO3 and Bi2Fe4O9 might simultaneously commence; however, their growth and ratios are dependent on temperature. The diffuse UV/vis reflectance spectra showed fundamental absorption edges between 1.80(1) and 2.75(3) eV. A comparative study between the “derivation of absorption spectrum fitting method” (DASF) and the Tauc method suggests Bi2Fe4O9 to be a direct band gap semiconductor.

Inspired by model studies under ultrahigh vacuum (UHV) conditions, inverse monolithic gold/ceria catalysts are prepared using thermal decomposition of a cerium nitrate precursor on a nanoporous gold (npAu) substrate. Cerium oxide deposits throughout the porous gold material (pores and ligaments 30−40 nm) are formed. npAu disks and coatings were prepared with loadings of about 3 to 10 atom % of ceria. The composite material was tested for the water−gas shift (WGS) reaction (H 2 O + CO → H 2 + CO 2 ) in a continuous flow reactor at ambient pressure conditions. Formation of CO 2 was observed at temperatures as low as 135°C with excellent stability and reproducibility up to temperatures of 535°C. The considerably increased thermal stability of the material can be linked to the presence of metal oxide deposits on the nanosized gold ligaments. The loss of activity after about 15 h of catalytic conversion with heating to 535°C was only about 10%. Photoemission spectroscopy indicates a defect (Ce 3+ ) concentration of about 70% on the surface of the cerium oxide deposits, prior to and after WGS reaction. Raman spectroscopic characterization of the material revealed that the bulk of the oxide is reoxidized during reaction. ■ INTRODUCTIONDuring the last decades increasing demand for a novel type of water−gas shift (WGS) catalyst in the context of mobile and green energy harvesting such as in fuel cells surfaced. 1,2 For either low-temperature fuel cells (polymer electrolyte membrane fuel cells, PEMFC) or high-temperature fuel cells in mobile applications such as in cars a novel type of WGS catalyst is required. These catalysts need to be highly active at low temperatures, shifting CO almost quantitatively to hydrogen (<10 ppm for PEMFCs). They need to be safe, nonpyrophoric and oxidation resistant upon exposure to air (excluding, thus, traditional WGS catalysts), highly durable and long-lasting and have to withstand fast deactivation and shut down procedures. Last but not least they have to be readily applicable into small scale reactor designs, ideally as coatings or monolithic catalyst beds. The focus of this latest surge in research regarding WGS catalysts are precious metal based catalysts. 1,3Among possible candidates, gold is of particular interest as it is cheaper than, e.g., platinum and also a very selective catalyst for the oxidation of CO in the presence of H 2 , making it an ideal catalyst material for procession of hydrogen gas for fuel cells. 4 Recently, Au-CeO 2 nanomaterials have been reported to be very efficient catalysts for the WGS reaction. 2 The interplay between the oxide and the Au provides reactivity toward the dissociation of water. Similarly, as reported for oxidation reactions using molecular oxygen, the synergy between the two partners is critical for the catalytic activity. 5,6 Oxygen vacancies existing in the oxide nanoparticles are suggested to play a key role for the dissociation of water. 7 A blend of catalysts consisting of metal nanoparticles dispersed on an oxide support has been report...

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