The Dehydrogenation Reactions and Kinetics of 2LiBH4−Al Composite

Abstract

The dehydrogenation reactions and kinetics of the 2LiBH4−Al composite were investigated by means of thermogravimetry allied with mass spectroscopy, differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis, and isothermal dehydrogenation measurements. According to the analysis of the experimental results, the dehydrogenation of 2LiBH4−Al can be roughly divided into three stages that range from 553 to 648 K, 648 to 773 K, and 773 to 823 K. In the initial stage, it was observed that the decomposition of LiBH4 occurs simultaneously with the formation of AlB2. It was also found that the 3D diffusion mechanism in the form of the Jander equation dominates the kinetics of this stage. The second stage is the major dehydrogenation process. The well-fitted curves of the isothermal dehydrogenation by the Prout−Tompkins equation prove that this stage is mainly dominated by an autocatalysis reaction. The suggested reaction 2LiBH4 + AlB2 → 2LiH + Al + 4B + 3H2 takes place simultaneously with the reaction 2LiBH4 + Al → 2LiH + AlB2 + 3H2. AlB2 and Al serve not only as products but also as reagents for the decomposition of LiBH4. Within this stage, the activation energy is remarkably reduced to 94 ± 5 kJ/mol from that of bulk LiBH4. At the third stage, when the temperature is above 798 K, LiAl can be formed due to the decomposition of LiH. This stage was still characterized by the autocatalysis reaction.