MECHANISM OF HYDROGEN DESORPTION IN Li-N-H HYDROGEN STORAGE SYSTEM CATALYZED BY TITANIUM COMPOUNDS

摘要

The Li-N-H system composed of LiH and LiNH_2 catalyzed by titanium compound desorbs ～6 mass% hydrogen and a quite small amount of ammonia in temperature range from 150 to 250℃ under He flow atmosphere. In this work, hydrogen desorption (H-desorption) mechanism in the reaction from LiH+LiNH_2 to Li_2NH+H_2 in a non-equilibrium condition was clarified by TDMS, TG and FT-IR analyses for the products replaced by LiD or LiND_2 for LiH or LiNH_2, respectively. The results suggested that H-desorption reaction progresses through two-step elementary reactions mediated by ammonia. As a next step, we carefully determined the amount of the ammonia containing in desorbed hydrogen for the Li-N-H system in a closed system from the practical point of view. The results indicated that a quite small of ammonia thermodynamically coexisted in the desorbed hydrogen in closed system, although ammonia could not be almost detected in emitted hydrogen gas by TDMS under some non-equilibrium conditions. Moreover, for understanding the role of titanium, the chemical bonding state of the titanium compounds doped as catalyst in the mixture was examined by XAFS measurement. The results indicate that the Ti atoms, which can induce catalytic effect, are in a particular chemical bonding state.