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.

The pseudo-catalytic properties of lithium (Li)-based alloys with group 14 elements were investigated for ammonia (NH3) synthesis under ambient conditions. The reaction between the Li alloys and nitrogen (N2) proceeds below 500 °C to form lithium nitride (Li3N). The peak temperature of nitridation was lower in the order of Li4.4Si < Li4.4Ge ≈ Li4.4Pb < Li4.4Sn. The N2 dissociation activity is related to the value of Knight shift observed in the 7Li solid-state magic angle spinning nuclear magnetic resonance (7Li MAS NMR) spectra, indicating that the metallic feature of Li species is an important factor for low-temperature N2 dissociation. Although the reaction yields for NH3 synthesis were depending on the alloys, NH3 was generated at the same temperature around 240 °C because the NH3 formation proceeded by the same reaction between Li3N and hydrogen (H2), generating lithium hydride (LiH) as a by-product. For all the alloys, LiH desorbs H2 to form Li alloys with higher Li composition at lower temperature than that of thermal decomposition of pure LiH. In addition, the conventional catalytic process was also investigated under a mixed gas of H2 and N2. NH3 was synthesized at 150 and 200 °C by using Li4.4Si and Li4.4Ge, respectively.

In this work, the reaction properties of ammonia (NH3) synthesis via the chemical looping process of lithium hydride (LiH) are investigated, and kinetic improvement is carried out. During the heating process up to 500 °C under 0.1 MPa nitrogen flow conditions, LiH reacts with N2 and changes to lithium imide (Li2NH) with hydrogen desorption. However, the kinetics of the reaction between LiH and N2 is slow due to agglomeration of the products. Lithium oxide (Li2O) as a scaffold is effective to drastically improve the reaction kinetics because Li2O suppresses the agglomeration. In this case, the reaction of LiH and N2 is completed within 20 min, which is drastically short compared with that of LiH (more than 1000 min). NH3 can be generated by reaction between Li2NH as the product and 0.1 MPa H2 from about 350 °C. Crushing the agglomerated particles and addition of Li2O can improve the reaction kinetics of NH3 synthesis, and then, the reaction completely proceeds at a lower temperature and shorter time. It is expected from the experimentally obtained reaction products and thermodynamic database that the N2 dissociation and NH3 generation are exothermic reactions. From the abovementioned results, it is concluded that NH3 can be produced at ambient pressure via successive reactions of LiH with N2 and H2 by exothermic processes, and the kinetics can be controlled using scaffolds.

As catalysts to improve the kinetics of the reaction between Mg and H 2 , Nb 2 O 5 , Ta 2 O 5 , and Nb 2 O 5 Ta 2 O 5 mixture gels with and without heat treatment were synthesized using simple sol-gel methods. Furthermore, MgH 2 was ball milled to superficially disperse 1 mol% of each oxide for 2 h, which was ten times shorter than that of previous works. All the oxides show catalytic effects on using easier and simpler synthesis processes. The catalysis of Nb oxides is better than that of Ta oxides and mixtures. The as-synthesized Nb 2 O 5 gel without heat treatment is the best catalyst and improves hydrogenation kinetics at room temperature. The Nb 2 O 5 gel with higher catalysis is further reduced compared to the heat-treated one because the gel oxide is more unstable owing to the lesser network between Nb and O atoms due to the existence of OH groups. By using gel oxides, highly activated Mg can be synthesized under milder conditions compared with previous cases.

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