Theoretical Investigations On The Structure And Hydrogen Storage Of Ti-doped Pyracylene

In recent years, human hopes that the hydrogen's universal use will solve the increasingly serious energy crisis and environmental problems because of its richer raw materials, higher chemical energy, less environmental pollution and so on. However, due to the status of hydrogen and the difficulty in its compression, hydrogen's storage and transportation bottleneck the development of hydrogen energy. Therefore, many researchers have been looking for a suitable hydrogen storage materials in experimental and theoretical investigation to solve the problem. So far, they have found a lot of hydrogen storage materials with the different adsorption mechanisms.Based on density functional theory (DFT) with Gaussian03, the structures of Ti-substitute-doped pyracylene and the hydrogen absorption abilities of substitution products are systematically studied. In view of the symmetry of pyracylene, there are four different positions where C atoms can be replaced, and four substitution products with different structures can be obtained. Molecular structure, stability and the corresponding hydrogen adsorption ability of these four kinds of are analyzed. Finally, the mechanism of these absorptions are discussed.The thesis covers the following aspects:Firstly, we study four different kinds of Ti substituted pyracylene, and make theoretical calculations of the molecular structure, NBO charge and molecular orbital of substitution products. The results show that: 1. Except that Ti substitution in the position IV has a great influence on host material, the substitutions in the other positions have less influence that Ti atoms (the H atoms connected to Ti atoms) protrude out of host material in position I and II while there is few changes in position III. 2. C atom in the position III is most easily substituted by Ti atoms, positionIthe second, position II the third, substitution in the position IV occurs most difficultly. 3. Ti atoms in all substitution products have a certain amount of positive charge, Ti atom in the position III has the largest number of electric charge, position II the second, positionIthe third, Ti atom in the position IV has the fewest number, this makes the Ti atoms are not easy to bond with each other, because they get close to each other and Coulomb repulsion happens, so as to ensure the stability of organometallic molecules.Secondly, we study a single hydrogen molecule adsorption of four different kinds of substitution products and the hydrogen complexs'molecular structures, NBO charges and molecular orbitals are calculated. The results show that: 1. Hydrogen molecule getting close to Ti atom in the position III is divided into two hydrogen atoms, which are combined with the Ti atom and the adjacent C atom respectively, and this chemical adsorption has great influence on the host material's structure (position III) while hydrogen molecule in the other positions are molecularly (but the corresponding bond lengths are longer than the natural ) adsorbed near Ti, which have less influence on the host materials'structures (position I, II and IV). 2. Hydrogen adsorption have certain influence on Ti atom's NBO charge and the charges have different degrees of reduction, the charges in the position I, II and IV change greatly while the one in the position III change little. 3. Hydrogen molecule in all hydrogen complexs ( except in the position III ) as a whole show a certain amount of positive charge. 4. the molecular orbital images show molecular orbitals between the hydrogen adsorbed molecularly and host materials (Ti atoms and their attached C atoms) in some extent overlap in all hydrogen complexs ( except in the position III ).Finally, we study a successive hydrogen molecule adsorption of four different kinds of substitution products and the hydrogen complexs'molecular structures, NBO charges and molecular orbitals are calculated. The results show that: 1. the successive hydrogen molecules adsorption is not chemical hydrogen adsorption but molecular, these hydrogen molecules can be divided into two groups, hydrogen molecules'bond length increase obviously in one group while increasing little in the other group. 2. With the successive introduction of hydrogen molecules, Ti atoms'NBO charges further reduce, but when the number of adsorbed reachs a certain number, that is to say,after that hydrogen molecules are adsorbed not by Tis but host materials, Ti atoms'charges does not change any more, and in these hydrogen adsorbed by host materials, the hydrogen atoms near host materials are positively charged, the other far from host materials are negatively charged. 3. In the position III, when attacking Ti atom from the two opposite sides, two hydrogen molecules do not break and almost symmetrically absorbed by Ti atom on opposite sides, which leads into isomerism. 4. The molecular orbital images show molecular orbitals between the hydrogen molecules adsorbed near Ti atoms and host materials (Ti atoms and their attached C atoms) in some extent overlap in all hydrogen complexs while there are no overlap between the hydrogen molecules adsorbed by host materials and host materials (Ti atoms and their attached C atoms) except the molecular orbit HOMO-19 of Ti(III-b).Results of this study show that, four different substitution products of Ti- substitute-doped pyracylene can adsorb hydrogen molecules; these kinds of adsorption are weaker than strong chemical interaction, while stronger than a weak physical interaction, which is consistent with international requirements for hydrogen storage materials; this is a hydrogen storage which is worthy of study.