Abstract: Hydrogen generated from clean and renewable energy sources has been considered as an alternate fuel to carbon based fossil fuels for several decades. Although many advances in hydrogen production and usage have been made, storing hydrogen remains a significant challenge. Many drawbacks including energy intensive processes, low volumetric densities, and safety concerns are associated with storing hydrogen as pressured or liquefied. Solid state hydrogen storage is considered to be the most promising method as a safe and effective storage option, but there is still no material or method that satisfies the requirements for a practical approach. A feasible hydrogen storage media should address several issues including targeted storage capacities, thermodynamics and hydrogen sorption kinetics, and safety. Nanostructured materials can provide tailor-made properties for storing and releasing hydrogen to fulfill, at least, the partial requirements. This short review, not a comprehensive review of all the materials or technologies in hydrogen storage, summarizes some of the recent developments in application of nanostructures for solid state hydrogen storage; particular attention has been devoted to the most recent development of nanocomposites with tuned dehydrogenation temperatures and kinetics through the control of pore size and surface chemistry.
Abstract: Ammonia borane (AB) with high gravimetric hydrogen capacity is of great interest for storing hydrogen in solid form which is an important issue in the growing field of hydrogen technology. In this work the effects of surface chemistry on dehydrogenation kinetics of carbon cryogel (CC) – ammonia borane nanocomposites have been studied. Boron-modified, nitrogen-modified, and boron-nitrogen- modified CCs were used as scaffold for AB and dehydrogenation kinetics of CC-ABs was studied by means of differential scanning calorimetry (DSC) at multiple heating rates. The results demonstrated that AB incorporated inside the mesopores of CC modified with nitrogen and boron possesses lower activation energy with enhanced kinetics of dehydrogenation due to catalytic effects as compared to AB in unmodified CC under otherwise the same or similar conditions. In addition, the lowest activation energy was observed for boron-modified CC-AB that could be attributed to the destabilization of AB by surface interactions with B2O3 that may accelerate the dehydrogenation process.
Abstract: This short paper reports the direct observation of the structure variation of mesoporous silica at temperatures higher than 600 oC by use of an in situ XRD technique. The mesostructure of SBA-15 or other mesoporous materials such as MCM-41 became almost invisible when the temperature rose to above 600 oC, but recovered or partially recovered once the temperature decreased. Contrarily, the characteristic XRD patterns of zeolites such as ZSM-5 kept unchangeable under the same conditions. On the basis of comparative experiments performed on various mesoporous samples, it is inferred that the reversible variation of XRD patterns probably originates from the thermal shock of the pore wall, not from the permanent collapse of the mesoscopic structure in these samples. This observation indicates the special features of SBA-15 at high temperature.
Abstract: Two types of mesoporous Si-MCM-41 materials were synthesized via a cationic surfactant template method using different Si-precursors. The materials obtained were characterized by FTIR, XRD, BET, TEM and 29Si MAS-NMR techniques. When fumed silica was used as Si precursor, a Si-MCM-41-I solid with wormhole-like pore topologies was obtained. However, when tetraethylorthosilicate (TEOS) was used as Si precursor, a mesoporous Si-MCM-41-II solid with hexagonal arranges and a long-range ordered structure could be obtained. These two kinds of mesoporous materials had a uniform pore size distribution with an average pore diameter within 2.3-2.8 nm. Rather weak Lewis acid sites were formed on both the Si-MCM-41 samples prepared by the two methods.
Abstract: Three CoMo supported catalysts with different supports, Al2O3, MgO and MgO-MgAl2O4, were prepared by a urea matrix combustion method. The physicochemical properties of the catalysts were characterized by N2 isothermal adsorption–desorption, powder X-ray diffraction (XRD) and temperature programmed reduction (TPR) techniques. The activity of these catalysts was evaluated in a fixed-bed high-pressure reactor using hydrodesulfurization of dibenzothiophene as a model reaction. The urea matrix combustion preparation method greatly favored the formation of highly dispersed Co- and Mo-oxo species on the support, which had significant influence on the hydrodesulfurization (HDS) activity. XRD analysis showed that MgO was more sensitive to the deposition of Co-O or Mo-O species than Al2O3 and MgAl2O4; the former might be potentially used as an indicator of the Co- and Mo-oxo species formation. Among these catalysts, CoMo/MgO-MgAl2O4 exhibited a high HDS activity.
Abstract: ZnAlFe mixed oxides with high surface area were obtained by the calcination (723 K) of ZnAlFe layered double hydroxides (LDHs). The calcined materials proved as basic catalysts in the gas phase acetone condensation exhibited high activity and high selectivity towards the formation of mesityl oxide.
Abstract: Perovskite-type compounds such as alkali tantalates, ATaO3 (A = Li, Na and K), prepared by the sol-gel method are reported as heterogeneous basic catalysts for the acetone aldol condensation. It has been proposed that the activity order and the selectivity patterns (LiTaO3 > > KTaO3 > NaTaO3) depend on the octahedral arrangements of the TaO6 tantalates.
Abstract: Mesoporous zirconia-Mn oxides were prepared by surfactant-assisted precipitation using different zirconia precursors and cetyl-trimethyl-ammonium bromide (CTAB) as a synthetic template. The objective of this work was to find out the influence of the zirconia precursors over the structural and textural characteristics of Mn-doped mesoporous zirconia solids. A series of syntheses were carried out by two methods using different zirconia precursors, modifying the Zr:surfactant ratio and the hydrolysis rate of the precipitate. After calcination at 500 °C, the samples were characterized by XRD, DTA, TEM and nitrogen adsorption-desorption isotherm. The use of the zirconium nitrate leads to materials having higher surface areas and narrow pore size distributions in the range of mesoporous materials; however, the preferential formation of the zirconia in the metaestable tetragonal phase was identified as the effect of the particle size allowed by the preparation method rather than the effect of the precursor. It was also found that the Mn and surfactant addition enhances the stabilization of the tetragonal crystalline phase and porosity.
Abstract: The Gd2O3 (gadolinia) modified CuO/AC catalysts for NO reduction by activated carbon were prepared and characterized by XRD, TPD-MS, EPR, XPS techniques. The results show that adding a small amount of Gd2O3 in the CuO catalyst can improve effectively its catalytic performance for NO reduction by activated carbon, and the appropriate molar ratio of Gd2O3/CuO is 0.03:1. The promotional effect of Gd2O3 stems from the cooperative effects between CuO and Gd2O3. The presence of Gd2O3 in the catalyst can alter the chemical state and environment of the CuO active sites and improve the catalytic activation of carbon by CuO to form more carbon reactive sites, resulting in the quicker transfer and release of oxygen decomposed from NO. The carboxylic groups on the surface of activated carbon play an important role in the catalytic reduction of NO by carbon at temperature below 300 °C.