Catalytic Activities of Various Niobium Oxides for Hydrogen Absorption/Desorption Reactions of Magnesium.

Five types of niobium(V) oxides (Nb2O5) were synthesized by hydrothermal and heat treatment processes, and their structural properties and catalytic activities for the hydrogen absorption/desorption reactions of magnesium were characterized. The synthesized Nb oxides were dispersed on magnesium hydride (MgH2), a typical hydrogen storage material, using the ball-milling method. All the synthesized Nb oxides improved the reaction kinetics of the hydrogen desorption/absorption reactions. The catalytic activities for the hydrogen desorption were comparable, while the hydrogen absorption rates were significantly different for each synthesized Nb oxide. This difference can be explained by the structural stability of Nb2O5, which is related to the formation of a catalytically active state by the reduction of Nb2O5 during the ball-milling process. Notably, the highest catalytic effect was observed for Nb2O5 with a highly crystalline pyrochlore structure and a low specific surface area, suggesting that pyrochlore Nb2O5 is a metastable phase. However, only the amorphous Nb oxide was out of order, even though there is a report on the high catalytic activity of amorphous Nb oxide. This is attributed to the initial condensed state of amorphous Nb oxide, because particle size affects the dispersion state on the MgH2 surface, which is also important for obtaining high catalytic activity. Thus, it is concluded that Nb2O5 with lower stability of the crystal structure and smaller particle size shows better catalysis for both hydrogen desorption and absorption reactions.

Effective Factor on Catalysis of Niobium Oxide for Magnesium.

Magnesium is a promising hydrogen storage material but requires an efficient catalyst to enhance the sluggish kinetics of its hydrogen desorption/absorption reactions. Niobium catalysts have been shown to accomplish this, but the effective factors for catalysis on hydrogen desorption/absorption of Mg are not well understood. In order to investigate these aspects, various types of Nb oxides were synthesized and mixed with Mg, and their catalytic properties were investigated. The spray pyrolysis synthesis of Nb oxides at different temperatures produced homogeneous spherical particles with different degrees of crystallinity, while Nb oxide particles synthesized by simple calcination of ammonium niobium oxalate were nonuniform. These Nb oxides show significant catalytic activities for the hydrogen desorption/absorption of Mg, with amorphous oxides being more effective catalyst precursors than crystalline precursors. Metastable, amorphous Nb oxide is more easily converted to the reduced state, which is the catalytically active state for the reactions. In addition, Nb in the deactivated sample is in the oxidized state compared with the initially activated sample, and the catalytically active (reduced) state is recovered by reactions with hydrogen. Based on these findings, it is concluded that the chemical state of Nb is an important factor in catalyzing the desorption/absorption of hydrogen by Mg, and the catalytically active state can be preserved without further treatments.

Surface modification effects of graphite for selective hydrogen absorption by titanium at room temperature.

Surface modification effects of graphite and organic solvents on Ti were investigated by thermogravimetry (TG), Raman spectroscopy, and transmission electron microscopy (TEM) observations to improve its hydrogen absorption properties. As a result, Ti ball-milled with graphite showed high reactivity and selectivity for hydrogen with high durability.

Metallurgical Synthesis of Mg2FexSi1-x Hydride: Destabilization of Mg2FeH6 Nanostructured in Templated Mg2Si.

Magnesium-based transition-metal hydrides are attractive hydrogen energy materials because of their relatively high gravimetric and volumetric hydrogen storage capacities combined with low material costs. However, most of them are too stable to release the hydrogen under moderate conditions. Here we synthesize the hydride of Mg2FexSi1-x, which consists of Mg2FeH6 and Mg2Si with the same cubic structure. For silicon-rich hydrides (x < 0.5), mostly the Mg2Si phase is observed by X-ray diffraction, and Mössbauer spectroscopy indicates the formation of an octahedral FeH6 unit. Transmission electron microscopy measurements indicate that Mg2FeH6 domains are nanometer-sized and embedded in a Mg2Si matrix. This synthesized metallographic structure leads to distortion of the Mg2FeH6 lattice, resulting in thermal destabilization. Our results indicate that nanometer-sized magnesium-based transition-metal hydrides can be formed into a matrix-forced organization induced by the hydrogenation of nonequilibrium Mg-Fe-Si composites. In this way, the thermodynamics of hydrogen absorption and desorption can be tuned, which allows for the development of lightweight and inexpensive hydrogen storage materials.

Nitrogen Dissociation via Reaction with Lithium Alloys.

Lithium alloys are synthesized by reactions between lithium metal and group 14 elements, such as carbon, silicon, germanium, and tin. The nitrogenation and denitrogenation properties are investigated by thermal and structural analyses. All alloys dissociate the nitrogen triple bond of gaseous molecules to form atomic state as nitrides below 500 °C, which is lower than those required for conventional thermochemical and catalytic processes on nitride syntheses. For all alloys except for germanium, it is indicated that nanosized lithium nitride is formed as the product. The denitrogenation (nitrogen desorption) reaction by lithium nitride and metals, which is an ideal opposite reaction of nitrogenation, occurs by heating up to 600 °C to form lithium alloys. Among them, the lithium-tin alloy is a potential material to control the dissociation and recombination of nitrogen below 500 °C by the reversible reaction with the largest amount of utilizable lithium in the alloy phase. The nitrogenation and denitrogenation reactions of the lithium alloys at lower temperature are realized by the high reactivity with nitrogen and mobility of lithium. The above reactions based on lithium alloys are adapted to the ammonia synthesis. As a result, ammonia can be synthesized below 500 °C under 0.5 MPa of pressure. Therefore, the reaction using lithium alloys is recognized as a pseudocatalyst for the ammonia synthesis.

Enhanced hydrogen desorption properties of LiAlH4 by doping lithium metatitanate.

High efficiency catalysts are needed to improve the kinetics of complex hydrides for practical applications. In this study, lithium metatitanate (Li2TiO3) is introduced in lithium alanate (LiAlH4), and the catalytic effect for notable complex/metal hydrides, such as LiAlH4, is investigated. Experiment results indicate that Li2TiO3 improves the kinetics of LiAlH4. In particular, Li2TiO3 dramatically improves the onset temperature of LiAlH4, which decreases to 75 °C and is within the temperature range for use in proton exchange membrane fuel cells. Transmission electron microscopy (TEM) observations help understand the catalytic effect of Li2TiO3 in the nanoscale. First principles calculations also show the improvement of H- and Li+ mobility by doping Li2TiO3, where calculations indicate that the physical origin of the catalytic effect is due to two factors: charge transfer and minor surface relaxation. Thus, experimental and theoretical evidence reveals the catalytic mechanism of Li2TiO3 in LiAlH4.

Hydrogen Absorption of Palladium Thin Films Observed by in Situ Transmission Electron Microscopy with an Environmental Cell.

A window type of the environmental cell system for a high-voltage electron microscope was developed and applied to in situ observation of a palladium (Pd) thin film. For in situ hydrogenation of Pd thin films, the distances of the lattice fringes were 0.20 and 0.23 nm, which correspond to the lattice d spacings of ß-phase (200) and (111) planes. Expansion of the Pd lattice happened as a result of phase transformation from the α phase to the ß phase. In particular, the lattice fringes were clearly distinguished, and the dislocation behavior during Pd hydrogenation was easily recognized according to the corresponding inverse fast fourier transform images. Furthermore, significant growth in the number of dislocations was observed at the grain boundary during increasing hydrogen pressure in the cell.

Gas-generated thermal oxidation of a coordination cluster for an anion-doped mesoporous metal oxide.

Central in material design of metal oxides is the increase of surface area and control of intrinsic electronic and optical properties, because of potential applications for energy storage, photocatalysis and photovoltaics. Here, we disclose a facile method, inspired by geochemical process, which gives rise to mesoporous anion-doped metal oxides. As a model system, we demonstrate that simple calcination of a multinuclear coordination cluster results in synchronic chemical reactions: thermal oxidation of Ti8O10(4-aminobenzoate)12 and generation of gases including amino-group fragments. The gas generation during the thermal oxidation of Ti8O10(4-aminobenzoate)12 creates mesoporosity in TiO2. Concurrently, nitrogen atoms contained in the gases are doped into TiO2, thus leading to the formation of mesoporous N-doped TiO2. The mesoporous N-doped TiO2 can be easily synthesized by calcination of the multinuclear coordination cluster, but shows better photocatalytic activity than the one prepared by a conventional sol-gel method. Owing to an intrinsic designability of coordination compounds, this facile synthetic will be applicable to a wide range of metal oxides and anion dopants.

Kinetic Modification on Hydrogen Desorption of Lithium Hydride and Magnesium Amide System.

Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH2)2) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH and Mg(NH2)2 lead to the fast hydrogen desorption. To preserve the potential hydrogen desorption properties, thermochemical and mechanochemical rehydrogenation processes are investigated. Although the only thermochemical process results in slowing the reaction rate due to the crystallization, the ball-milling can recover the original hydrogen desorption properties. Furthermore, the mechanochemical process at 150 °C is useful as the rehydrogenation technique to preserve the suitable crystalline size and mixing state of the reactants. As a result, it is demonstrated that the 8LiH and 3Mg(NH2)2 system is recognized as the potential hydrogen storage material to desorb more than 5.5 mass% of H2 at 150 °C.

Interaction of electrons with light metal hydrides in the transmission electron microscope.

Transmission electron microscope (TEM) observation of light metal hydrides is complicated by the instability of these materials under electron irradiation. In this study, the electron kinetic energy dependences of the interactions of incident electrons with lithium, sodium and magnesium hydrides, as well as the constituting element effect on the interactions, were theoretically discussed, and electron irradiation damage to these hydrides was examined using in situ TEM. The results indicate that high incident electron kinetic energy helps alleviate the irradiation damage resulting from inelastic or elastic scattering of the incident electrons in the TEM. Therefore, observations and characterizations of these materials would benefit from increased, instead decreased, TEM operating voltage.

Enhancing the hydrogen storage capacity of TiFe by utilizing clusters.

The titanium iron (TiFe) alloy is a notable hydrogen storage material which can operate at ambient temperature. However, low hydrogen storage capacity is a major drawback that is needed to be overcome. Enhancement of the hydrogen capacity of TiFe is considered by utilizing TiFe clusters within the density functional theory. Calculations reveal that TiFe clusters can absorb large amounts of hydrogen. Furthermore, the desorption energies of Ti1Fe1H6 are lower than that of bulk TiFeH where the physical origins of low desorption energies are considered to be due to the closed shell structure of Ti1Fe1. This indicates that the Ti1Fe1H6 has the possibility to operate at near ambient temperature; therefore, only hydrogen gas pressures are required to control the hydrogen storage and release.

Low temperature hydrogenation of iron nanoparticles on graphene.

Hydrogenation of iron nanoparticles was performed both computationally and experimentally where previously chemically-bonded iron hydride is considered to be unachievable under ordinary conditions. Density functional theory (DFT) calculations predict that hydrogenated iron nanoparticles are stabilized on a single-layer graphene/Cu substrate. Experimentally, iron nanoparticles were deposited onto a graphene/Cu substrate by vacuum deposition. Hydrogenation was done at 1atm of hydrogen gas and under liquid nitrogen. Mass spectrometry peak confirmed the hydrogen release from hydrogenated iron nanoparticles while a scanning transmission electron microscopy is used in order to link a geometrical shape of iron hydride nanoparticles between experimental and theoretical treatments. The hydrogenated iron nanoparticles were successfully synthesized where hydrogenated iron nanoparticles are stable under ordinary conditions.

H2 dissociation over NbO: the first step toward hydrogenation of Mg.

Niobium-based oxide nanoparticles have proven to be catalytically effective toward hydrogenation of Mg where H2 dissociation over the niobium-oxides is considered to be a crucial reaction step. However, the role of niobium oxides toward H2 dissociation still remains unclear as to what atomic configurations are responsible for the catalytic activity. H2 dissociation over different surface planes of Nb, NbO, and Nb2O5 as well as small NbO clusters is performed by using a density functional theory. The calculations reveal that H2 dissociation, adsorption energy, and the bond type between H and surfaces (clusters) depend on the atomic configurations of Nb and O. In particular, H2 adsorption on NbO(111) is enhanced by O atoms without forming O-H bond where the bond type of H and surface is found to be an electron pairing. Thus, NbO(111) could not only be a effective catalyst but also potentially prevent the formation of MgO during the hydrogenation of Mg. The results should be helpful in developing and tailoring the efficient catalyst toward H2 dissociation and hydrogenation of Mg.

Plastic bag method for active sample loading into transmission electron microscope.

A plastic bag method was developed to observe air-sensitive samples on microstructure and phase distribution without exposure to air during the holder transfer process into the transmission electron microscope (TEM). As an example, a type of lithium aluminum hydride (Li(3)AlH(6)) was observed in the TEM to demonstrate the effectiveness of this method. Results show that the plastic bag method is a simple and practical TEM transfer method utilized to reduce air contact for a series of air-sensitive materials.

Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique.

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.