Hydrogen desorption properties of the Ca-N-H system

After synthesizing calcium amide (Ca(NH2)2) through the reaction of calcium hydride (CaH2) with ammonia (NH3), the thermal decomposition properties have been examined. The results indicated that Ca(NH2)2 decomposed to calcium imide (CaNH) and NH3 gas with increasing temperature, where the desorption of NH3 started from about 60 °C and peaked around 300 °C in the thermal desorption mass spectra (TDMS). From the X-ray diffraction profiles, the product after heat treatment up to 350 °C was CaNH, but an unknown phase appeared in further heating up to 500 °C. Hydrogen (H2) desorption properties of a mixture of Ca(NH2)2 and CaH2 with 1:3 molar ratio were also examined. The H2 gas of 3.5 wt.% in total was desorbed with two-peak structure in TDMS. The first peak corresponds to the reaction: Ca(NH2)2 + CaH2 → 2CaNH + 2H2, and the second one originates from the reaction: CaNH + CaH2 → Ca2NH + H2. Simultaneously performed differential thermal analysis indicated that the former and the latter reactions were, respectively, exothermic and endothermic, so that the H2 desorption corresponding to the first peak in TDMS disappeared after long-time-milling at room temperature.