156
Z. Xiong et al. / Journal of Alloys and Compounds 441 (2007) 152–156
about 0.772 psi at this temperature. At other temperature points
of 230 and 240 ◦C isotherms were also measured with similar
shape(notshown)butdifferentplateaupressure. Van’tHoffplots
generated from those isotherms are shown in Fig. 9, from which
the heat-of-formation of one mole H2 in the Ca–Na–N–H sys-
tem is calculated at 92.1 kJ. Difference between this value and
the one derived from DSC measurement seems reasonable since
a fraction of imide species survived due to the slow kinetics of
hydrogenation and hence changed the reaction enthalpy.
In conclusion, post-milled Ca(NH2)2–NaH (1/1) sample can
release 1.1 wt% of hydrogen upon heating it to 270 ◦C, which
equals to 1H atom detached from one Ca(NH2)2–NaH (1 + 1)
mixture. NaNH2, together with a Ca–N–H solid solution were
identified to be the dehydrogenation product. 0.96 wt% of hydro-
gen was found to be recharged on above product.
Fig. 9. Van’t Hoff plot of Ca(NH2)2–NaH (1/1) sample.
Acknowledgement
The present work is financially supported by Agency for Sci-
ence, Technology and Research (A* STAR) Singapore.
aforementioned, Ca(NH2)2 can easily form solid solution with
its imide, i.e., 2CaNH + 1Ca(NH2)2. In the process of hydrogen
absorption/desorption over Ca–Na–N–H system solid solutions
with diverse compositions will be yielded at different reaction
stages, which makes them difficult to be investigated. In this
study, by summarizing the number of hydrogen detached from
the reactants and the phase transitions, the following reaction is
suggested to describe reversible hydrogen absorption and des-
orption over Ca(NH2)2–NaH system:
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