| Hydrogen, a kind of clean, cost-effective and renewable energy resource, is one of the most prospective alternative energy resources and attracts lots of attentions. However, finding suitable hydrogen storage materials is still one of the roadblocks on the way to a "hydrogen economy". It is an essential requirement to elucidate hydrogen storage mechanism for improving and developing new hydrogen storage materials. In this thesis, the reaction mechanisms of hydrogen storage materials, belonging to ammonia borane and ammonia, are investigated, with the aims to theoretically determine the reason why a single molecule is difficult to release hydrogen and to search for effective catalysts to accelerate the H2-loss process. The following three systems are studied:1. The reaction mechanisms of hydrogen release from phosphine alane and phosphine borane with or without alane and borane have been explored by using the molecular orbital theory. At the MP2/aug-cc-pVDZ level, geometries of all stationary points are optimized, and at the CCSD(T)/aug-cc-pVTZ level, the energy profiles for all the reaction are refined. The theoretical results show that H2 elimination from both phosphine alane and phosphine borane monomer can not be comparable with the Al-P and B=P bonds dissociation. Both alane and borane can accelerate H2 dissociation from phosphine alane. However, it is difficult for phosphine alane to release H2 once the strong adduct is formed. Similarly, alane and borane can have an important catalytic influence on H2 loss from phosphine borane and substantially reduce the energy barrier for H2 release. The calculated results show that borane is a better catalyst than alane for H2 elimination from phosphine borane. Moreover, phosphine borane is more favorable potential hydrogen storage material than phosphine alane in the presence of alane or borane. So a catalyst is essential for the generation of H2 from phosphorous systems since the barrier height of H2-loss is higher than the X-P (X=A1 and B) bond energy. Borane may be the appropriate catalyst for potential phosphine borane with a low energy barrier, which is even lower than B-P bond dissociation.2. The mechanisms of hydrogen release from methylamine with or without borane, alane, diborane, dialane, and borane-alane are theoretically explored. At the MP2/aug-cc-pVDZ level, geometries of all stationary points are optimized, and at the CCSD(T)/aug-cc-pVTZ level, the energy profiles for all the reaction are refined. The results show that it is impossible for H2 elimination from the isolated CH3NH2 because of a high energy barrier. All studied catalysts can facilitate H2 loss from CH3NH2. However, borane or alane has no real catalytic effect because the H2-release is not preferred as compared with the B-N or Al-N bond cleavage once a corresponding adduct is formed. The diborane, dialane, and borane-alane will lead to a substantial reduction of energy barrier as a bifunctional catalyst and borane-alane is the best choisce with the lowest enery barrier and the least reaction step. Moreover, hydrogen bond and six-member ring formation are two crucial factors to decrease the energy barriers in line with the similar and distinct points among various catalysts compared.3. The mechanisms of hydrogen release from methylamine with or without boranes and alanes are theoretically studied at the CCSD(T)/aVTZ//MP2/aVDZ level. The results demonstrate that the hydrogen elimination from an isolated methylamine is almost impossible. All the catalysts studied can attack the N atom of amino group or imido group to decrease the energy barrier of H2 dissociation from methylamine. However, the H2 production process is still difficult once the strong adduct is formed in the presence of borane and alane with an excessive energy barrier. The diborane, dialane, and borane-alane may exert a more important effect on H2-loss from methylamine than borane and alane. The borane-alane is the best catalyst in-this-work.Moreover,the N atom of imido is easier to be attacked by-the catalysts, but H2 release is apt to happen with the N atom of amino attacked by catalysts with a lower energy barrier. |
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