No abstract

The effects of alkaline earth metal amides (Mg­(NH2)2, Ca­(NH2)2, and Ba­(NH2)2) on Ru in catalyzing NH3 decomposition were investigated. The catalytic activities rank in the order of Ru–Ba­(NH2)2 > Ru–Ca­(NH2)2 > Ru–Mg­(NH2)2, among which Ru–Ba­(NH2)2 and Ru–Ca­(NH2)2 catalysts have higher intrinsic activities (TOF) and lower apparent activation energies than those of Ru–Mg­(NH2)2 and Ru/MgO catalysts, indicating that Ca­(NH2)2 and Ba­(NH2)2 may have different roles from those of Mg­(NH2)2 and MgO. The TPR (temperature-programmed reaction) results show that Ca­(NH2)2 or Ba­(NH2)2 decomposes to N2 and H2 rather than NH3 in the presence of Ru. Ru may promote the NH x (x = 1, 2) coupling to H2 and N2 and change the decomposition pathways of Ca­(NH2)2 and Ba­(NH2)2. Kinetic analyses reveal that the Ru promoted NH x coupling to H2 and N2 may be the rate-determining step for catalytic ammonia decomposition. We suggest that the catalysis is very likely fulfilled via (1) Ru catalyzes the decomposition of amides to form H2, N2, and imides through an energy more favorable pathway and (2) imides react with NH3 to regenerate amides. The presence of Ca­(NH2)2 or Ba­(NH2)2 creates a NH x -rich environment, and Ru mediates the electron transfer from NH x to facilitate NH x coupling to N2 and H2.

Hydrazidotrihydridoborates of various alkali metals, i.e., NaN2H3BH3 and KN2H3BH3, were synthesized successfully via a liquid approach. The crystal structures of NaN2H3BH3 and KN2H3BH3 were determined, and their dehydrogenation properties were compared with LiN2H3BH3 and N2H4BH3. A clear correlation between sizes of metal cations in hydrazidotrihydridoborates and their corresponding melting and dehydrogenation temperatures was observed. The dehydrogenation temperature was found dependent on the melting temperature. Upon approaching the melting points, alkali metal hydrazidotrihydridoborates dehydrogenate rapidly in the first step, giving rise to the formation of intermediates that possess N2BH2, N2BH, and NBH3 species. Further increasing temperature leads to the release of additional H2 and the formation of N2BH species. Compared to pristine N2H4BH3, the alkali-metal-substituted hydrazidotrihydridoborates demonstrate significantly improved dehydrogenation behavior with no N2H4 emission and greatly suppressed NH3 release.