The Comparison in Dehydrogenation Properties and Mechanism between MgCl2(NH3)/ LiBH4 and MgCl2(NH3)/NaBH4 Systems

摘要

The dehydrogenation properties and mechanism of MgCl2(NH3)/MBH4 (here, M is Li or Na) were investigated by thermogravimetric analysis and mass spectrometry, X-ray diffraction (XRD), solid-state ~(11)B NMR, Fourier transform infrared, and differential scanning calorimetry (DSC). As for the MgCl2(NH3)/LiBH4 system, it was found that a new phase, namely, MgCl2(NH3)·LiBH4, to which the following dehydrogenation relates, is formed after ball milling. Judging from the reaction products, it is confirmed that MgCl2 is inclined to work as an ammonia carrier, and the ligand NH3, transferring from MgCl2, is able to combine with the LiBH4 to release H2 with a trace of ammonia at ca. 240 °C. With the increase of LiBH4 content in the mixture, the emission of ammonia was totally suppressed, and Mg(BH4)2 was produced by the decomposition reaction of MgCl2 with the excessive LiBH4 after the ligand NH3 was exhausted, resulting in an improved dehydrogenation in the whole system. As for the MgCl2(NH3)/NaBH4 system, no new phases are detected by XRD after ball milling. The MgCl2 works as a BH_4~- acceptor, and the ligand NH3 stays with Mg~(2+) to combine with the BH_4~-, which transfers from NaBH4 to Mg~(2+), resulting in a totally different decomposition route and thermal effects as compared with the MgCl2(NH3)/LiBH4 system. DSC results revealed that the decomposition of MgCl2(NH3)/LiBH4 presented an exothermic reaction with an enthalpy of —3.8 kJ mol~(-1) H2, while the MgCl2(NH3)/NaBH4 showed two apparent endothermic peaks associated with its two-step dehydrogenation with enthalpies of 8.6 and 2.2 kJ mol~(-1) H2, respectively. Moreover, the MS profiles of the MgCl2(NH3)/ 2NaBH4, with excessive BH_4~-, still released a trace of NH3, indicating that the NaBH4 is not so effective in suppressing the emission of NH3 as LiBH4 did.