过渡金属改善铝材料储氢性能的密度泛函理论研究
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摘要
催化改善轻金属配位氢化物的储氢性能是新能源领域的重要研究方向，广泛的实验探索对其催化机理的理论研究提出了迫切要求。我们主要采用第一性原理方法和改进的Nudged Elastic Band Method技术研究过渡金属掺杂对铝氢化的催化能力及机制，基于对氢分子在掺杂表面上的分解和氢原子扩散过程的最小能量路径、相应状态的几何结构和电子性质之上，在原子层次上阐述过渡金属的催化机理，为实验研究提供理论借鉴，促进新能源材料的设计开发和应用研究。本论文主要研究内容如下：
1、过渡金属Ti掺杂在Al(111)表面台阶下沿时，对材料的氢化过程有很好的催化作用，氢分子在台阶表面分解的能量势垒为0.37eV，分解后的氢原子只需要0.45eV能量就能脱离过渡金属的束缚。通过计算Ti原子在铝表面上的掺杂能，以及在Ti原子附近挖走一个铝原子的能耗，我们认为过渡金属Ti可以稳定存在于台阶下沿位置，在材料反复充放氢过程中保持良好的催化作用。文中也对氢分子分解过程过渡态的结构参数和电荷特性进行了详细研究，同时发现表面台阶提高了过渡金属Ti对材料氢化过程的催化能力。
2、基于系统研究的考虑，对比分析了过渡金属（Sc, Ti, V, Cr和Mn）替代掺杂在理想Al(111)模型的最上层和次上层、表面台阶模型的台阶边缘位置时对铝氢化的催化作用。过渡金属掺杂在Al(111)表面的最上层时，3d轨道和氢分子的H-H σ共价键之间存在电荷转移和反向转移的Kubas相互作用，能够明显降低氢分子的分解势垒。当过渡金属掺杂在光滑Al(111)表面的次上层时，由于表面上铝原子的屏蔽作用，过渡金属不能和吸附的氢分子之间产生Kubas相互作用。催化作用主要通过掺杂引起的结构形变、过渡金属对铝表面电荷的吸引能力、过渡金属的外层价电子的数目和3d轨道的劈裂以及参与反应的具体轨道等实现。为了实现过渡金属催化剂的循环利用，我们还对分解后的氢原子脱离过渡金属束缚的扩散过程进行了研究。当Sc和Ti掺杂在表面台阶的下沿时，在Kubas相互作用和活性增强的表面台阶的共同作用下，完成氢分子分解过程和氢原子的扩散过程只需要0.4eV的能量，是我们研究的所有构型中完成一个氢化循环需要能量最低的构型。对比过渡金属在不同体系中的催化性能，发现元素的原子半径和电负性是寻找和设计有利于提高轻金属储氢的新型催化材料选择的参考依据。据此，我们预测过渡金属Sc、Ti、Zr、Nb、Hf和Ta因为具有和铝原子相比更大的原子半径和更低的电负性，当它们形成近表面合金时将会对铝材料的氢化起到优良的催化性能。
3、尽管Al(100)表面的热稳定性低于Al(111)表面，但它也是实验研究中经常采用的指数面。同时Ti掺杂Al(100)表面的局域结构与实验报道的TiAl3催化活性材料具有相似的结构特征。因此，我们详细研究了过渡金属Sc、Ti、V、Cr和Mn在Al(100)台阶表面上掺杂时对氢分子分解过程的催化机制。通过计算掺杂体系的总能量，发现单个过渡金属原子掺杂在表面台阶的下沿位置具有最好的热力学稳定性。当Sc、Ti和V掺杂在Al(100)表面台阶下沿位置时，它们向周围的铝原子转移电荷，而Cr和Mn从周围的铝原子获得电荷。在Sc、Ti、V、Cr和Mn掺杂的LS构型表面上完成一个完整氢化反应过程，包括氢分子的分解过程和分解后的氢原子的扩散过程，所需要的总能量分别为0.37、0.43、0.54、0.56和0.61eV。氢化过程的能量势垒降低说明过渡金属掺杂在表面台阶下沿的位置时能有效提高铝的储氢性能。在Sc和Ti掺杂表面上，氢分子在过渡态中同时与过渡金属以及表面层上的铝原子成键，导致了双氢分子获得了0.8e的电荷，H-H被拉长了0.4，降低了氢分子的分解势垒。而在其它过渡金属掺杂情况时，氢分子仅与过渡金属之间存在弱相互作用，H2在过渡态中的作用是引起了表面电荷的再分布，双氢分子只获得不超过0.1e的电荷量。通过对结构参数和电荷特性的详细分析，理解过渡金属对铝材料氢化过程的催化机制，能够为实验科学家寻找适合的氢气储存介质提供帮助。
4、为了理解过渡金属原子掺杂浓度对其催化性能的影响，我们也对过渡金属Ti掺杂Al(100)表面形成近表面合金时的催化效果作了详细研究。研究发现过渡金属Ti以单个孤立原子、位于次近邻(0,2)位置的Ti-Ti原子对，以及对应于较高浓度情形的Ti原子位于次近邻类型的[0,2]掺杂域有很好的催化性能。当它们掺杂在Al(100)表面的最上层时，能够促使氢分子以较低的能量势垒分解，但是Ti和H之间的强相互作用阻碍氢原子在表面上的扩散过程，体系完成一个氢化循环过程需要的能量势垒在0.6eV左右。当它们掺杂在铝表面的次上层时，能提高表面层的铝原子与氢原子之间的相互作用，促进氢分子在表面上的分解。在单个Ti原子、位于次近邻的Ti-Ti原子对，以及较高浓度的[0,2]掺杂域掺杂在次上层表面时，氢分子分解的能量势垒分别为0.80、0.68和0.48eV。由于表面铝原子的屏蔽作用，较强的H-Ti键没有形成，分解后的氢原子能非常容易地脱离过渡金属的束缚。适当升高过渡金属的掺杂浓度，能提高对材料氢化过程的催化作用。电荷分析表明过渡金属和双氢分子之间的电荷转移对氢分子的分解过程起重要作用。当过渡金属位于Al(100)表面的最上层时，Ti原子的3d轨道和氢分子的成键态之间产生Kubas类型的σ反馈键，导致氢分子从过渡金属获得电荷，H-H键被拉长，降低了分解势垒。当过渡金属Ti掺杂在次上层时，表面层的铝原子阻碍了Ti原子与H2之间产生直接相互作用。然而掺杂区域的结构形变、Ti原子较低的电负性、以及钛原子具有更多的价电子等因素共同作用，有利于其上方的铝原子向氢分子转移电荷，促进了氢分子的分解。
It has attracted intensive attention in the field of renewable energy materials to catalyticallyenhance the hydrogen storage properties of light metal complex hydrides. The extensiveexperimental investigations call for detailed theoretical studies on the catalytic mechanism. Weuse the first-principles method and the improved nudged elastic band technique in studying thecatalytic mechanism of the early transition metal doped aluminum. We have discussed in detailthe minimum energy path for the H2dissociation and the produced H atom diffusion onaluminum surface doped with the transition metals. Combining the analyses on structural andelectronic properties, we conclude the catalytic mechanism for improving the hydrogen storageperformance. Our theoretical studies can facilitate new materials design by providing predictionsand guidance to the future experimental studies. The main results of this thesis can besummarized below:
1．The Ti catalyst doped in the vicinity of surface step could split H2with0.37eVactivation energy and the dissociated hydrogen atom could diffuse by overcoming0.45eV barrier.Our analysis on the energies of the Ti doping and the surface etching phenomenon suggest thatTi could remain as recycling active catalyst during the aluminum hydrogenation. The electronicproperties of the intermediate state could account for the enhanced splitting properties. Thesestudies on the role of surface step could contribute to understanding the catalytic mechanism oftransition metal catalyzed hydrogen uptake in aluminum.
2. With the purpose to present a systematic investigation on the catalytic mechanism, wehave also carefully analyzed the H2splitting processes catalyzed by the early transition metals(Sc, Ti, V, Cr and Mn), which are substitutionally doped in the top layer and the subsurface of anideal flat Al surface and at the edge site of surface step. The transition metal doped in the topsurface can provide3d orbitals to develop the well-known donation and back-donation Kubasinteraction with σ-type H-H bond, which could significantly reduce the activation energy of H2splitting. The catalyst doped in the subsurface could not develop Kubas interaction with H2because of the screening from the charge distributed on the top surface, whose role could beunderstood by combining the structural deformation induced by the doping, the attraction of thedopant to the electrons distributed around Al atoms in the top layer, and the d orbital attendancein the reaction. For the sake of recycling perspectives of the doped catalyst, the diffusion of thedissociated H atoms has also been studied. Thus, the Sc and Ti doping at the lower edge site ofthe stepped surface are better for their low activation energies. The atomic size andelectronegativity could be used to aid new catalyst design for enhancing the hydrogen recharge properties of alanate hydrides. Accordingly, the near-surface alloying of Sc, Ti, Zr, Nb, Hf, andTa in the aluminum surface could be expected to have superior catalytic properties.
3. Though the thermodynamics stability of Al(100) is lower than Al(111), it is usuallyinvestigated in experimental studies. Also, the local structure of Ti-doped Al(100) has analogousstructural characteristics of the TiAl-terminated TiAl3(001) surface (the TiAl3is found inexperiment to be catalytically active for aluminum hydrogenation). So, we have also studied indetail the enhanced hydrogen interaction with transition-metal (Sc, Ti, V, Cr and Mn) dopedAl(100) stepped surface. Judged from the calculated total energies, the early transition metalsprefer to dope at the lower edge sites of surface step. The Sc, Ti, and V donate electrons whilethe Cr and Mn gain electrons. The low energy costs for activating both the H2splitting and the Hatomic diffusion show improved catalytic performances. In the transition states, hydrogen wouldbond to both transition metal and Al atoms for H2splitting on Sc-and Ti-doped surfaces, while itwould only develop rather weak interaction with the metals in the other studied materials. Thecharge transfer results in0.8e charge gain and0.4increase in bond length of H2, facilitatingH2dissociation on Sc-and Ti-doped surfaces. However, in the other studied materials, thepresence of hydrogen only induces charge re-distribution, resulting in a rather small charge gainof H2(<0.1e). The insights into catalytic mechanism, on the basis of our detailed analysis onstructural and electronic properties, could benefit the experimental investigations in pursuingmoderate hydrogen storage medium.
4. Based on the detailed first-principles studies, we have investigated the catalyticperformances of near-surface alloy of Ti in Al(100) along with the analysis on catalyticmechanism. The doping of single Ti atom, Ti-Ti pair in next-nearest neighbor configuration (0,2),and the local Ti domain [0,2] in the next-nearest neighbor arrangement have better catalyticperformances. In top surface, they need to cost~0.6eV energy to complete a whole catalyticcycle. The main obstacle comes from the strong Ti-H bond hindering the dissociated H atoms todiffuse. They, when doped in subsurface, can also enhance hydrogen interaction with aluminumsurface to catalyze H2splitting. The calculated activation energies are0.80,0.68, and0.48eV forsingle Ti atom,(0,2) pair, and [0,2] domain, respectively. Due to less of the strong Ti-H bond, thedissociated H atoms could diffuse quickly with small activation energies. The charge transferbetween metal and dihydrogen plays crucial role. In the top surface, the Ti could provide3doribitals to develop the Kubas type σ back-bonding interaction with H2, resulting in the chargegain and bond elongation of dihydrogen. However, the Ti doped in subsurface could not bedirectly approached by H2molecule. The slight structural expansion in doping domain, the lowerelectronegativity of Ti, and the fact of more valence electrons of Ti could cooperatively facilitate the charge transfer from the above Al atoms to H2molecule, accounting for the enhancedsplitting properties.
1、过渡金属Ti掺杂在Al(111)表面台阶下沿时，对材料的氢化过程有很好的催化作用，氢分子在台阶表面分解的能量势垒为0.37eV，分解后的氢原子只需要0.45eV能量就能脱离过渡金属的束缚。通过计算Ti原子在铝表面上的掺杂能，以及在Ti原子附近挖走一个铝原子的能耗，我们认为过渡金属Ti可以稳定存在于台阶下沿位置，在材料反复充放氢过程中保持良好的催化作用。文中也对氢分子分解过程过渡态的结构参数和电荷特性进行了详细研究，同时发现表面台阶提高了过渡金属Ti对材料氢化过程的催化能力。
2、基于系统研究的考虑，对比分析了过渡金属（Sc, Ti, V, Cr和Mn）替代掺杂在理想Al(111)模型的最上层和次上层、表面台阶模型的台阶边缘位置时对铝氢化的催化作用。过渡金属掺杂在Al(111)表面的最上层时，3d轨道和氢分子的H-H σ共价键之间存在电荷转移和反向转移的Kubas相互作用，能够明显降低氢分子的分解势垒。当过渡金属掺杂在光滑Al(111)表面的次上层时，由于表面上铝原子的屏蔽作用，过渡金属不能和吸附的氢分子之间产生Kubas相互作用。催化作用主要通过掺杂引起的结构形变、过渡金属对铝表面电荷的吸引能力、过渡金属的外层价电子的数目和3d轨道的劈裂以及参与反应的具体轨道等实现。为了实现过渡金属催化剂的循环利用，我们还对分解后的氢原子脱离过渡金属束缚的扩散过程进行了研究。当Sc和Ti掺杂在表面台阶的下沿时，在Kubas相互作用和活性增强的表面台阶的共同作用下，完成氢分子分解过程和氢原子的扩散过程只需要0.4eV的能量，是我们研究的所有构型中完成一个氢化循环需要能量最低的构型。对比过渡金属在不同体系中的催化性能，发现元素的原子半径和电负性是寻找和设计有利于提高轻金属储氢的新型催化材料选择的参考依据。据此，我们预测过渡金属Sc、Ti、Zr、Nb、Hf和Ta因为具有和铝原子相比更大的原子半径和更低的电负性，当它们形成近表面合金时将会对铝材料的氢化起到优良的催化性能。
3、尽管Al(100)表面的热稳定性低于Al(111)表面，但它也是实验研究中经常采用的指数面。同时Ti掺杂Al(100)表面的局域结构与实验报道的TiAl3催化活性材料具有相似的结构特征。因此，我们详细研究了过渡金属Sc、Ti、V、Cr和Mn在Al(100)台阶表面上掺杂时对氢分子分解过程的催化机制。通过计算掺杂体系的总能量，发现单个过渡金属原子掺杂在表面台阶的下沿位置具有最好的热力学稳定性。当Sc、Ti和V掺杂在Al(100)表面台阶下沿位置时，它们向周围的铝原子转移电荷，而Cr和Mn从周围的铝原子获得电荷。在Sc、Ti、V、Cr和Mn掺杂的LS构型表面上完成一个完整氢化反应过程，包括氢分子的分解过程和分解后的氢原子的扩散过程，所需要的总能量分别为0.37、0.43、0.54、0.56和0.61eV。氢化过程的能量势垒降低说明过渡金属掺杂在表面台阶下沿的位置时能有效提高铝的储氢性能。在Sc和Ti掺杂表面上，氢分子在过渡态中同时与过渡金属以及表面层上的铝原子成键，导致了双氢分子获得了0.8e的电荷，H-H被拉长了0.4，降低了氢分子的分解势垒。而在其它过渡金属掺杂情况时，氢分子仅与过渡金属之间存在弱相互作用，H2在过渡态中的作用是引起了表面电荷的再分布，双氢分子只获得不超过0.1e的电荷量。通过对结构参数和电荷特性的详细分析，理解过渡金属对铝材料氢化过程的催化机制，能够为实验科学家寻找适合的氢气储存介质提供帮助。
4、为了理解过渡金属原子掺杂浓度对其催化性能的影响，我们也对过渡金属Ti掺杂Al(100)表面形成近表面合金时的催化效果作了详细研究。研究发现过渡金属Ti以单个孤立原子、位于次近邻(0,2)位置的Ti-Ti原子对，以及对应于较高浓度情形的Ti原子位于次近邻类型的[0,2]掺杂域有很好的催化性能。当它们掺杂在Al(100)表面的最上层时，能够促使氢分子以较低的能量势垒分解，但是Ti和H之间的强相互作用阻碍氢原子在表面上的扩散过程，体系完成一个氢化循环过程需要的能量势垒在0.6eV左右。当它们掺杂在铝表面的次上层时，能提高表面层的铝原子与氢原子之间的相互作用，促进氢分子在表面上的分解。在单个Ti原子、位于次近邻的Ti-Ti原子对，以及较高浓度的[0,2]掺杂域掺杂在次上层表面时，氢分子分解的能量势垒分别为0.80、0.68和0.48eV。由于表面铝原子的屏蔽作用，较强的H-Ti键没有形成，分解后的氢原子能非常容易地脱离过渡金属的束缚。适当升高过渡金属的掺杂浓度，能提高对材料氢化过程的催化作用。电荷分析表明过渡金属和双氢分子之间的电荷转移对氢分子的分解过程起重要作用。当过渡金属位于Al(100)表面的最上层时，Ti原子的3d轨道和氢分子的成键态之间产生Kubas类型的σ反馈键，导致氢分子从过渡金属获得电荷，H-H键被拉长，降低了分解势垒。当过渡金属Ti掺杂在次上层时，表面层的铝原子阻碍了Ti原子与H2之间产生直接相互作用。然而掺杂区域的结构形变、Ti原子较低的电负性、以及钛原子具有更多的价电子等因素共同作用，有利于其上方的铝原子向氢分子转移电荷，促进了氢分子的分解。
It has attracted intensive attention in the field of renewable energy materials to catalyticallyenhance the hydrogen storage properties of light metal complex hydrides. The extensiveexperimental investigations call for detailed theoretical studies on the catalytic mechanism. Weuse the first-principles method and the improved nudged elastic band technique in studying thecatalytic mechanism of the early transition metal doped aluminum. We have discussed in detailthe minimum energy path for the H2dissociation and the produced H atom diffusion onaluminum surface doped with the transition metals. Combining the analyses on structural andelectronic properties, we conclude the catalytic mechanism for improving the hydrogen storageperformance. Our theoretical studies can facilitate new materials design by providing predictionsand guidance to the future experimental studies. The main results of this thesis can besummarized below:
1．The Ti catalyst doped in the vicinity of surface step could split H2with0.37eVactivation energy and the dissociated hydrogen atom could diffuse by overcoming0.45eV barrier.Our analysis on the energies of the Ti doping and the surface etching phenomenon suggest thatTi could remain as recycling active catalyst during the aluminum hydrogenation. The electronicproperties of the intermediate state could account for the enhanced splitting properties. Thesestudies on the role of surface step could contribute to understanding the catalytic mechanism oftransition metal catalyzed hydrogen uptake in aluminum.
2. With the purpose to present a systematic investigation on the catalytic mechanism, wehave also carefully analyzed the H2splitting processes catalyzed by the early transition metals(Sc, Ti, V, Cr and Mn), which are substitutionally doped in the top layer and the subsurface of anideal flat Al surface and at the edge site of surface step. The transition metal doped in the topsurface can provide3d orbitals to develop the well-known donation and back-donation Kubasinteraction with σ-type H-H bond, which could significantly reduce the activation energy of H2splitting. The catalyst doped in the subsurface could not develop Kubas interaction with H2because of the screening from the charge distributed on the top surface, whose role could beunderstood by combining the structural deformation induced by the doping, the attraction of thedopant to the electrons distributed around Al atoms in the top layer, and the d orbital attendancein the reaction. For the sake of recycling perspectives of the doped catalyst, the diffusion of thedissociated H atoms has also been studied. Thus, the Sc and Ti doping at the lower edge site ofthe stepped surface are better for their low activation energies. The atomic size andelectronegativity could be used to aid new catalyst design for enhancing the hydrogen recharge properties of alanate hydrides. Accordingly, the near-surface alloying of Sc, Ti, Zr, Nb, Hf, andTa in the aluminum surface could be expected to have superior catalytic properties.
3. Though the thermodynamics stability of Al(100) is lower than Al(111), it is usuallyinvestigated in experimental studies. Also, the local structure of Ti-doped Al(100) has analogousstructural characteristics of the TiAl-terminated TiAl3(001) surface (the TiAl3is found inexperiment to be catalytically active for aluminum hydrogenation). So, we have also studied indetail the enhanced hydrogen interaction with transition-metal (Sc, Ti, V, Cr and Mn) dopedAl(100) stepped surface. Judged from the calculated total energies, the early transition metalsprefer to dope at the lower edge sites of surface step. The Sc, Ti, and V donate electrons whilethe Cr and Mn gain electrons. The low energy costs for activating both the H2splitting and the Hatomic diffusion show improved catalytic performances. In the transition states, hydrogen wouldbond to both transition metal and Al atoms for H2splitting on Sc-and Ti-doped surfaces, while itwould only develop rather weak interaction with the metals in the other studied materials. Thecharge transfer results in0.8e charge gain and0.4increase in bond length of H2, facilitatingH2dissociation on Sc-and Ti-doped surfaces. However, in the other studied materials, thepresence of hydrogen only induces charge re-distribution, resulting in a rather small charge gainof H2(<0.1e). The insights into catalytic mechanism, on the basis of our detailed analysis onstructural and electronic properties, could benefit the experimental investigations in pursuingmoderate hydrogen storage medium.
4. Based on the detailed first-principles studies, we have investigated the catalyticperformances of near-surface alloy of Ti in Al(100) along with the analysis on catalyticmechanism. The doping of single Ti atom, Ti-Ti pair in next-nearest neighbor configuration (0,2),and the local Ti domain [0,2] in the next-nearest neighbor arrangement have better catalyticperformances. In top surface, they need to cost~0.6eV energy to complete a whole catalyticcycle. The main obstacle comes from the strong Ti-H bond hindering the dissociated H atoms todiffuse. They, when doped in subsurface, can also enhance hydrogen interaction with aluminumsurface to catalyze H2splitting. The calculated activation energies are0.80,0.68, and0.48eV forsingle Ti atom,(0,2) pair, and [0,2] domain, respectively. Due to less of the strong Ti-H bond, thedissociated H atoms could diffuse quickly with small activation energies. The charge transferbetween metal and dihydrogen plays crucial role. In the top surface, the Ti could provide3doribitals to develop the Kubas type σ back-bonding interaction with H2, resulting in the chargegain and bond elongation of dihydrogen. However, the Ti doped in subsurface could not bedirectly approached by H2molecule. The slight structural expansion in doping domain, the lowerelectronegativity of Ti, and the fact of more valence electrons of Ti could cooperatively facilitate the charge transfer from the above Al atoms to H2molecule, accounting for the enhancedsplitting properties.
引文
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