添加剂对配位储氢材料的性能及脱氢机理的影响：密度泛函理论研究

发布时间：2018-05-14 13:15

本文选题：第一性原理 + 配位储氢材料　；参考：《北京化工大学》2015年博士论文

【摘要】：近年来,配位氢化物因比储氢合金具有更低的形成能和更高的储氢量而备受关注。配位氢化物按组成的不同分为过渡金属配位氢化物和轻质配位氢化物。过渡金属配位氢化物因一般以混合物形式被合成出来,关于其性质的实验研究报道很少。过渡金属配位氢化物K_2ZnH_4实验上给出了两种不同的结晶结构,我们采用第一性原理计算的方法首先对两种不同的结构稳定性进行了判定,然后对K_2ZnH_4的结构、电子、热力学性质进行了分析。轻质配位氢化物虽然具有更高的储氢含量,但其放氢反应温度高、放氢速率缓慢、可逆吸氢性能差。目前,关于轻质配位氢化物的放氢机制及添加剂促进轻质配位氢化物放氢的机制研究尚不明确。我们通过向NaAlH_4中引入A1H3缺陷的方法,阐明了轻质配位氢化物NaAlH_4脱氢反应起始阶段的反应机制。然后,我们研究了Fe__2O_3团簇和钛原子修饰的石墨烯片对轻质配位氢化物LiBH_4的结构、电子性质及放氢性能的影响。1、我们通过计算研究发现实验上合成出来的过渡金属配位储氢材料K_2ZnH_4的稳定晶型为正交结构,而不是四方结构。我们计算的正交结构的K_2ZnH_4的结构参数与实验值相吻合。K_2ZnH_4的态密度和能带结构分析得出其能带带隙值为4.01 eV,证实了K_2ZnH_4的绝缘体特性。K_2ZnH_4中的Zn原子与H原子间存在着很强的共价键作用,Zn-H键强度大,不容易断裂。K_2ZnH_4在室温条件下是稳定存在的,其放氢反应通过两步反应完成,我们计算得出第一步反应温度为524 K,反应焓为49.16 kJ/mol,第二步反应温度为505 K,反应焓为62.52 kJ/mol。2、研究轻质配位储氢材料NaAlH_4的AlH3缺陷诱导放氢机制发现,A1H3在NaAlH_4 (001)面上比内部更容易生成。AlH3缺陷由NaAlH_4 (001)面的亚表面迁移到表面的过程是一放热反应,但需要跨越一能量为0.33eV的能垒。在本文中我们发现了AlH52-基团和AlH63-基团,分析并提出了A1H52-基团和AlH63-基团的形成机制。300 K条件下AlH63-基团不会发生分解；而当温度升高到400 K时A1H63-基团发生分解。在NaAlH_4内部游离的H-并不会相互结合成H2分子,而是配位到临近的Na+周围。3、我们研究了将Fe__2O_3团簇放置在LiBH_4(001)表面后,Fe__2O_3团簇对LiBH_4 (001)面结构、电子、及脱氢性质的影响。Fe__2O_3团簇吸附到LiBH_4(001)面上的结构是热力学稳定的。Li-O键的生成证实了Li-Fe二元氧化物的存在。升高温度使得Fe__2O_3团簇掺杂的LiBH_4(001)面结构变得松散和无序化,但是Li-Fe二元氧化物并不会发生分解。Fe__2O_3团簇的掺杂可以活化LiBH_4 (001)表面,降低B-H键断裂所需要的能量,进而提高LiBH_4的脱氢动力学。此外,Fe__2O_3团簇的掺杂还有利于H2分子在LiBH_4 (001)表面上的生成及解离。4、热力学和动力学研究结果得出LiBH_4团簇在含钛原子的石墨烯片上的稳定吸附结构为γ-LiBH_4@C_(31)Ti。在γ-LiBH_4@C_(31)Ti中,Li原子与H原子之间的配位数目由三变为二。LiBH_4团簇的电量转移到含钛原子的石墨烯片上,且Ti原子的d轨道对LiBH_4内部的轨道杂化影响大。受钛原子修饰的石墨烯片的限制性影响,LiBH_4第一步脱氢反应路径发生改变,改变后的脱氢反应具有更低的反应焓值。受含钛原子的石墨烯片的影响,LiBH_4中B-H键强度降低,使得LiBH_4团簇的放氢反应动力学增强。
[Abstract]:In recent years, coordination hydrogenated hydrogen storage alloys have attracted much attention for their lower formation energy and higher hydrogen storage capacity. The coordination hydrogenides are divided into transition metal coordination hydrides and light coordination hydrides according to their composition. The transition metal coordination hydrogenides are synthesized in the form of mixtures. Experimental studies on their properties are reported. Two different crystalline structures are given in the transition metal coordination hydride K_2ZnH_4 experiment. First, we first determine the stability of two different structures by the first principle calculation, and then analyze the structure, electron and thermodynamic properties of the K_2ZnH_4. Hydrogen content is high, the reaction temperature is high, the rate of hydrogen release is slow, and the reversible hydrogen absorption performance is poor. At present, the mechanism of the hydrogen release mechanism of the light coordination hydrides and the mechanism of the additive to promote the dehydrogenation of light coordination hydrogenides are not clear. By introducing the method of A1H3 defect into the NaAlH_4, we elucidate the dehydrogenation reaction of the light coordination hydride NaAlH_4. The reaction mechanism at the beginning stage. Then, we studied the effect of Fe__2O_3 clusters and titanium modified graphene tablets on the structure, electronic properties and hydrogen releasing properties of the light coordination hydride LiBH_4.1. We found that the stable crystalline form of the transition metal coordination hydrogen storage material K_2ZnH_4 is a positive junction. The structural parameters of the orthogonal structure of the K_2ZnH_4 we calculate coincide with the experimental values of the experimental values. The state density of.K_2ZnH_4 and the band structure analysis show that the band gap value is 4.01 eV, which confirms that the Zn atom in the.K_2ZnH_4 insulator of K_2ZnH_4 has a strong covalent bond between the H atom and the H atom, and the Zn-H bond strength is large. It is not easy to break.K_2ZnH_4 to be stable at room temperature. Its hydrogen release reaction is completed by two steps. The first step reaction temperature is 524 K, the reaction enthalpy is 49.16 kJ/mol, the second step reaction temperature is 505 K, and the reaction enthalpy is 62.52 kJ/mol.2. The mechanism of AlH3 defect induced hydrogen releasing mechanism of the light mass coordination hydrogen storage material NaAlH_4 is studied. It is found that the process of A1H3 on the NaAlH_4 (001) surface is more likely to generate.AlH3 defects from the subsurface of the NaAlH_4 (001) surface to the surface, which is a exothermic reaction, but needs to cross a energy barrier of 0.33eV. In this paper, we found the AlH52- group and the AlH63- group, and analyzed and proposed the forming machine of A1H52- and AlH63- groups. Under the condition of.300 K, the AlH63- group does not break down; and when the temperature rises to 400 K, the A1H63- group breaks down. The H- free in the NaAlH_4 does not bind to each other to H2 molecules, but is a coordination to the adjacent Na+ around.3. We studied the Fe__2O_3 clusters on the LiBH_4 (001) surface, and the clusters are to the 001 surface junction. The structure, electron, and dehydrogenation properties of the.Fe__2O_3 cluster adsorbed on the LiBH_4 (001) surface is a thermodynamically stable.Li-O bond that confirms the existence of Li-Fe two element oxides. The elevated temperature makes the LiBH_4 (001) surface structure of Fe__2O_3 clusters loose and unordered, but the Li-Fe two element oxide does not break down.Fe_ The doping of _2O_3 clusters can activate the LiBH_4 (001) surface, reduce the energy required for the B-H bond fracture and improve the dehydrogenation kinetics of LiBH_4. Furthermore, the doping of Fe__2O_3 clusters is also beneficial to the formation and dissociation of H2 molecules on the surface of LiBH_4 (001) and the dissociation of.4. The results of thermodynamic and kinetic studies have shown that the LiBH_4 clusters are on the graphene containing titanium atoms. The stable adsorption structure on the chip is gamma -LiBH_4@C_ (31) Ti. in gamma -LiBH_4@C_ (31) Ti. The number of coordination between the Li and H atoms from three to two.LiBH_4 clusters is transferred to the graphene sheet containing the titanium atom, and the D orbit of Ti atoms has a great influence on the orbital hybridization in LiBH_4. The limiting shadow of the graphene film modified by the titanium atom The reaction path of dehydrogenation in the first step of LiBH_4 changes, and the dehydrogenation reaction after the change has a lower enthalpy value. The B-H bond strength in LiBH_4 decreases with the influence of the graphene sheet containing titanium atoms, which makes the reaction kinetics of the LiBH_4 clusters increase.

【学位授予单位】：北京化工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：O641.4;TB34

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