Mechanical Properties and Processing Techniques of Bulk Metal-Organic Framework Glasses.

Melt quenched metal-organic framework (MOF) glasses define a new category of glass, distinct from metallic, organic, and inorganic glasses, owing to the dominant role of metal-ligand coordination bonding. The mechanical properties of glasses in general are important given their application in protective coatings and display technologies, though little is known about MOF glasses in this respect. The experimental elucidation of key properties such as their scratch resistance has been limited by the lack of processing methodologies capable of producing bulk glass samples. Here, nanoindentation was used to investigate the Young's modulus and hardness of four melt-quenched glasses formed from zeolitic imidazolate frameworks (ZIF): agZIF-4, agZIF-62, agZIF-76, and agZIF-76-mbIm. The creep resistance of the melt-quenched glasses was studied via strain-rate jump (SRJ) tests and through constant load and hold (CLH) indentation creep experiments. Values for the strain-rate sensitivity were found to be close to those for other glassy polymers and Se-rich GeSe chalcogenide glasses. Vacuum hot-pressing of agZIF-62 resulted in an inhomogeneous bulk sample containing the glass and amorphous non-melt-quenched aZIF-62. Remelting and annealing, however, resulted in the fabrication of a transparent, bubble-free bulk specimen, which allowed the first scratch testing experiments to be performed on an MOF glass.

Evaluation of Reoxidation Thresholds for γ-Al2O3-Supported Cobalt Catalysts under Fischer-Tropsch Synthesis Conditions.

Size-dependent phenomena at the nanoscale influence many applications, notably in the science of heterogeneous catalysis. In cobalt-based Fischer-Tropsch synthesis (FTS), the size of Co nanoparticles (NPs) dictates to a high degree catalyst's performance in terms of activity, selectivity, and stability. Here, a highly dispersed Re/Co/γ-Al2O3 catalyst with high Co surface area per gram of catalyst was exposed to industrially relevant FTS conditions and monitored in situ by synchrotron X-ray radiation. X-ray absorption near-edge structure spectra were obtained on the cobalt K edge and Re L3 edge of the working catalyst. The experimental results demonstrate development of tetrahedrally coordinated Co2+ forming at the expense of metallic Co(0). The structure of the oxide resembles CoAl2O4 and appears at the onset (first 5-10 h) of the reaction. Reoxidation of Co(0) is more pronounced close to the outlet of the reactor, where higher pH2O is anticipated. The state of the Re promoter does not change during the FT process. We propose that reoxidation of small Co NPs is followed by spreading of Co oxide that leads to the formation of CoxAlyOz phases. Hence, in order to avoid an irreversible loss of the active phase during process start-up, catalyst design should be restricted to Co NPs larger than 5.3 nm.

Boosted Supercapacitive Energy with High Rate Capability of aCarbon Framework with Hierarchical Pore Structure in an Ionic Liquid.

The specific energy of a supercapacitor (SC) with an ionic liquid (IL)-based electrolyte is larger than that using an aqueous electrolyte owing to the wide operating voltage window provided by the IL. However, the wide-scale application of high-energy SCs using ILs is limited owing to a serious reduction of the energy with increasing power. The introduction of macropores to the porous material can mitigate the reduction in the gravimetric capacitance at high rates, but this lowers the volumetric capacitance. Synthetic polymers can be used to obtain macroporous frameworks with high apparent densities, but the preservation of the frameworks during activation is challenging. To simultaneously achieve high gravimetric capacitance, volumetric capacitance, and rate capability, a systematic strategy was used to synthesize a densely knitted carbon framework with a hierarchical pore structure by using a polymer. The energy of the SC using the hierarchically porous carbon was 160 Wh kg-1 and 85 Wh L-1 on an active material base at a power of 100 W kg-1 in an IL electrolyte, and 60 % of the energy was still retained at a power larger than 5000 W kg-1 . To illustrate, a full-packaged SC with the material could store/release energy comparable to a Ni-metal hydride battery (gravimetrically) and one order of magnitude higher than a commercial carbon-based SC (volumetrically), within one minute.

Toward three-dimensional nanoengineering of heterogeneous catalysts.

Cobalt-based Fischer-Tropsch systems are widely used to convert synthesis gas to clean hydrocarbon fuel. However, surprisingly little is known about the morphology of the catalysts on the nanoscale. Here we show that scanning transmission electron tomography reveals their true 3-D morphology and provides direct evidence that the support controls the final morphology of the catalyst. Such direct local three-dimensional measurements provide unprecedented insight into catalysis, and can henceforth transform our understanding of these complex materials.

Analytical electron microscopy studies of lithium aluminum hydrides with Ti- and V-based additives.

The microstructure of LiAlD(4) with TiCl(3).1/3(AlCl(3)) and VCl(3) additives has been studied during different steps of the decomposition process using electron energy loss spectroscopy and energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope. Energy filtered transmission electron microscopy was used to show elemental distributions in the samples. The spatial distribution of the additives and the main elements within the alanate particles was examined with a resolution of a few nanometers. The analysis of the electron energy loss spectra reveals the chemical state of Al, O, and the additives. Ti and V do not appear to mix chemically with Al to a significant degree. V was found in high concentration in just a few particles, while Ti is more uniformly distributed. All the samples showed evidence of oxidation despite procedures being adopted to avoid exposing the material to air. The additives are oxidized in all the samples, and Al(2)O(3) forms a thin layer at the surface of the particles. This paper gives a comparison between samples at different stages of the decomposition process using different additives.