Methods to stabilize and destabilize ammonium borohydride.
Methods to stabilize and destabilize ammonium borohydride.
复制标题
稳定和不稳定硼氢化铵的方法。
作者：
T. K. Nielsen;A. Karkamkar;M. Bowden;F. Besenbacher;T. Jensen;T. Autrey
硼氢化铵NH（4）BH（4）具有高氢含量，ρ（m）= 24.5 wt% H（2），并且在T = 160 °C以下释放18 wt% H（2）。然而，在环境温度和压力下，本体NH（4）BH（4）的半衰期约为6小时，不足以用于实际应用。研究了NH（4）BH（4）（ABH（2））在不同氢气和氩气背压下的分解，以研究可能的压力介导的稳定化效应。当氢气背压从5巴增加到54巴时，在环境温度下从固体ABH（2）的氢气释放速率降低约16%。使用氩气压力获得了类似的结果，并且所观察到的稳定性可以通过正的活化体积来解释，约。73 ± 17 cc mol（-1），处于导致氢释放的过渡态。研究了在介孔二氧化硅MCM-41中的纳米限制作为稳定NH（4）BH（4）的替代手段。然而，其他因素似乎显着破坏NH（4）BH（4）的稳定性，并且根据本体反应方案，它在环境温度下迅速分解为[（NH（3））（2）BH（2）][BH（4）]（DADB）。从纳米限制的[（NH（3））（2）BH（2）][BH（4）]的氢解吸动力学适度增强，如通过与本体材料相比ΔT = -13 °C的DSC分解峰温度的降低所证明的。最后，我们注意到一个令人惊讶的结果，如XRD和固态MAS（11）B MAS NMR所揭示的，DADB在<-30 °C的温度下储存可逆地将[（NH（3））（2）BH（2）][BH（4）]转化为新的低温多晶型物。
Ammonium borohydride, NH(4)BH(4), has a high hydrogen content of ρ(m) = 24.5 wt% H(2) and releases 18 wt% H(2) below T = 160 °C. However, the half-life of bulk NH(4)BH(4) at ambient temperatures and pressures, ~6 h, is insufficient for practical applications. The decomposition of NH(4)BH(4) (ABH(2)) was studied at variable hydrogen and argon back pressures to investigate possible pressure mediated stabilization effects. The hydrogen release rate from solid ABH(2) at ambient temperatures is reduced by ~16% upon increasing the hydrogen back pressure from 5 to 54 bar. Similar results were obtained using argon pressure and the observed stabilization may be explained by a positive volume of activation, ca. 73 ± 17 cc mol(-1), in the transition state leading to hydrogen release. Nanoconfinement in mesoporous silica, MCM-41, was investigated as alternative means to stabilize NH(4)BH(4). However, other factors appear to significantly destabilize NH(4)BH(4) and it rapidly decomposes at ambient temperatures into [(NH(3))(2)BH(2)][BH(4)] (DADB) in accordance with the bulk reaction scheme. The hydrogen desorption kinetics from nanoconfined [(NH(3))(2)BH(2)][BH(4)] is moderately enhanced as evidenced by a reduction in the DSC decomposition peak temperature of ΔT = -13 °C as compared to the bulk material. Finally, we note a surprising result, storage of DADB at temperature <-30 °C transformed, reversibly, the [(NH(3))(2)BH(2)][BH(4)] into a new low temperature polymorph as revealed by both XRD and solid state MAS (11)B MAS NMR.