Improved de/hydrogenation properties and favorable reaction mechanism of CeH_2 + KH co-doped sodium aluminum hydride

摘要

Sodium aluminum hydride (NaAlH_4) was directly synthesized by ball milling NaH/Al co-doped with CeCl_3 + KH under a hydrogen pressure of 3 MPa at room temperature. Out of various samples corresponding to xNaH/Al + 0.02CeCl_3 + yKH (x + y = 1; y = 0, 0.02, 0.04 mol%) composites, the composite with y = 0.02 exhibits the optimum de/hydrogenation properties. It shows that the addition of KH can effectively improve the dehydrogenation properties of second step reaction of NaAlH_4 system. The composite with y = 0.02 starts to release hydrogen from 87 ℃ and completes dehydrogenation within 20 min at 170 ℃, with good cycling de/hydrogenation kinetics at relatively lower temperature (100-140 ℃). After ball milling, the CeCl_3 precursor can be changed into CeH_2 catalytic active component in the first several de/hydrogenation cycles. Apparent activation energy of the second decomposition step of NaAlH_4 system can be effectively decreased by addition of KH, resulting in the decrease of desorption temperatures. Based on the microstructure analyses combined with hydrogen storage performances, the improved dehydrogenation properties of sodium aluminum hydride system are ascribed to the lattice volume expansion of Na_3AlH_6 during the dehydrogenation process resulted from the addition of KH. Moreover, by analyzing the reaction kinetics of CeCl_3 + KH co-doped sample, both of the decomposition steps of composite with y = 0.02 were conformed to the two-dimension phase-boundary growth mechanism. The mechanistic investigations gained here could help to understand the de/rehydrogenation behaviors of catalyzed complex metal hydride systems.