H_2在Pd_n（n=4，6，13，19，38）团簇上吸附解离的DFT研究
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摘要
原子团簇(简称团簇)是由几个至上千个原子组成相对稳定的聚集体,直径小于50nm,团簇是介于微观的原子和宏观的固体物质之间的一种新层状。Pd团簇由于其独特的物理和化学性质引起了人们极大的研究兴趣,Pd金属具有良好的催化活性、稳定性和选择性,广泛应用在催化加氢与脱氢反应中、汽车尾气催化转化、电催化氧化与还原过程中。在催化反应中,氢分子在催化剂表面的活化吸附与氢分子的吸附解离都是催化反应中的关键步骤。研究原子或分子氢在Pd金属团簇的吸附解离行为,有助于理解吸附物与催化剂相互作用的本质,为高效Pd金属催化剂的设计提供理论数据和依据。
本论文采用密度泛方法,在DMol3软件包中,选择GGA/PW91交换相关泛函对不同尺寸的Pdn(n=4,6,13,19,38)进行几何优化。在最稳定构型的基础上,我们分析了氢气在Pdn团簇上的一系列吸附性质,包括氢分子在不同吸附位稳定性,氢分子的解离吸附及迁移机理,解离过程的Hirshfeld电荷、HOMO-LUMO轨道和态密度等电子性质,探讨了氢气在钯团簇上吸附解离以及电子性质随团簇尺寸变化的规律。
全文共分三章。第一章主要陈述了团簇理论和表面吸附理论。第二章主要陈述了现代量子化学的相关理论。第三章介绍了氢分子在Pdn(n=4,6,13,19,38)金属团簇表面的吸附构型、表面解离和迁移机理,以及吸附解离过程中的电子性质变化。主要得到以下结论：
1.随着Pdn尺寸的增大,Pd团簇的平均结合能从1.660eV显著增大到2.785eV。Pd-Pd键长由2.661A逐渐增大到2.756A。H2分子吸附在Pd4H2-top, Pd6H2-top, Pd13H2-top, Pd19H2-5top, Pd38H2-5top的吸附能最大,最稳定吸附能分别为0.568,0.670,0.629,0.453,0.456eV。
2.解离的氢原子吸附在Pd4的Pd-Pd桥位更稳定,而吸附在Pd6, Pd13, Pd19, Pd38面位更稳定。从H2在Pd团簇的解离机制可以看出,氢气解离到最稳定吸附位的决速反应能垒是差别不大的,分别为0.448,0.401,0.314,0.304,0.782eV。随着团簇尺寸的增大(n=4,6,13,19)反应能垒减小,Pd38增大。氢气在Pd19上具有最小的吸附能和解离能。
3.Hirshfeld电荷分析表明,Pd团簇是电子给予体,氢原子是电子接受体,Pd转移电子越多,H-H键越长并解离。
Clusters are the agglomerates of a few to a few thousand atoms with relative stability and with a radius smaller than50nm. Clusters are the borderland between the microscopic single atom and the macroscopic solid state. The palladium-ydrogen system has attracted intensive studies in physical chemistry as a model system in view of its high storage capacity, its high sensitivity and selectivity to H2gas, and its ability to easily release hydrogen at room temperature. In particular, palladium has been widely utilized in many chemical processes such as hydrogenation, oxidation and reduction process. Hydrogen dissociative chemisorption on Pd transition metal catalysts is of great industrial importance in various hydrogenation or dehydrogenation processes. In these reactions, hydrogen molecule is dissociated with active hydrogen atoms adsorbed on the catalyst surfaces. Understanding the elementary processes of the chemical reactions is significant to design novel high efficiency catalysts and provide important theoretical knowledge to solve practical problems of industrial catalytic.
In this work, we use DFT calculations and transition state theory, and we optimize the different sizes'of Pdn(n=4,6,13,19,38) by GGA/PW91function in DMol3software. We investigate systematically a series of nature analysis on the base of optimized structures, including hydrogen molecular adsorption geometry and stability, mechanisms of dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38). We also examine electronic properties of palladium-hydrogen system by analyzing Hirshfeld charges, HOMO-LUMO gap and the density of states. Through the detailed system's theoretical calculation, we discuss hydrogen dissociative chemisorption on palladium clusters and the electronic properties with the cluster sizes increase.
In Chapter1, we summarized basic theory of the cluster and surface adsorption. In Chapter2, we introduced the theoretical basis of modern quantum chemistry. In Chapter3, we studied the mechanisms of hydrogen adsorption, dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38) and electronic and geometric properties of palladium-ydrogen system.
The main conclusions of this work include:
1. The average binding energies of the Pdn(n=4,6,13,19,38) clusters significantly increase from1.660to2.785eV with cluster size increased. Pd-Pd bond distances gradually change from2.661to2.756A. The stable adsorption sites for H2molecule are top sites of Pdn(n=4,6,13,19,38), with the respective adsorption energy of0.568,0.670,0.629,0.453and0.456eV. The stable adsorption sites of Pd19and Pd38are at top sites of five-coordinated Pd atoms.
2. Hydrogen atom adsorbed on the Pd4edge site is more stable while the adsorption on face site is more stable for Pd6, Pd13, Pd19and Pd38. From Pd4to Pd38. the rate-determining step of dissociating H2molecule is overcoming the energy barrier of0.448,0.401,0.314,0.304and0.782eV, respectively. The results show that Pd19has much lower chemisorption energy and the lowest energy barrier. Adsorptive H atom shifts on the Pdn(n=4,6,13,19,38) cluster are testified to be quite flexible.
3. In the charge transfer process, the Pd cluster behaves as a donor and H2behaves as an acceptor. There is charge being transferred from the Pd atoms to the antibonding orbital of H2, associated with the breaking of the H-H bond.
本论文采用密度泛方法,在DMol3软件包中,选择GGA/PW91交换相关泛函对不同尺寸的Pdn(n=4,6,13,19,38)进行几何优化。在最稳定构型的基础上,我们分析了氢气在Pdn团簇上的一系列吸附性质,包括氢分子在不同吸附位稳定性,氢分子的解离吸附及迁移机理,解离过程的Hirshfeld电荷、HOMO-LUMO轨道和态密度等电子性质,探讨了氢气在钯团簇上吸附解离以及电子性质随团簇尺寸变化的规律。
全文共分三章。第一章主要陈述了团簇理论和表面吸附理论。第二章主要陈述了现代量子化学的相关理论。第三章介绍了氢分子在Pdn(n=4,6,13,19,38)金属团簇表面的吸附构型、表面解离和迁移机理,以及吸附解离过程中的电子性质变化。主要得到以下结论：
1.随着Pdn尺寸的增大,Pd团簇的平均结合能从1.660eV显著增大到2.785eV。Pd-Pd键长由2.661A逐渐增大到2.756A。H2分子吸附在Pd4H2-top, Pd6H2-top, Pd13H2-top, Pd19H2-5top, Pd38H2-5top的吸附能最大,最稳定吸附能分别为0.568,0.670,0.629,0.453,0.456eV。
2.解离的氢原子吸附在Pd4的Pd-Pd桥位更稳定,而吸附在Pd6, Pd13, Pd19, Pd38面位更稳定。从H2在Pd团簇的解离机制可以看出,氢气解离到最稳定吸附位的决速反应能垒是差别不大的,分别为0.448,0.401,0.314,0.304,0.782eV。随着团簇尺寸的增大(n=4,6,13,19)反应能垒减小,Pd38增大。氢气在Pd19上具有最小的吸附能和解离能。
3.Hirshfeld电荷分析表明,Pd团簇是电子给予体,氢原子是电子接受体,Pd转移电子越多,H-H键越长并解离。
Clusters are the agglomerates of a few to a few thousand atoms with relative stability and with a radius smaller than50nm. Clusters are the borderland between the microscopic single atom and the macroscopic solid state. The palladium-ydrogen system has attracted intensive studies in physical chemistry as a model system in view of its high storage capacity, its high sensitivity and selectivity to H2gas, and its ability to easily release hydrogen at room temperature. In particular, palladium has been widely utilized in many chemical processes such as hydrogenation, oxidation and reduction process. Hydrogen dissociative chemisorption on Pd transition metal catalysts is of great industrial importance in various hydrogenation or dehydrogenation processes. In these reactions, hydrogen molecule is dissociated with active hydrogen atoms adsorbed on the catalyst surfaces. Understanding the elementary processes of the chemical reactions is significant to design novel high efficiency catalysts and provide important theoretical knowledge to solve practical problems of industrial catalytic.
In this work, we use DFT calculations and transition state theory, and we optimize the different sizes'of Pdn(n=4,6,13,19,38) by GGA/PW91function in DMol3software. We investigate systematically a series of nature analysis on the base of optimized structures, including hydrogen molecular adsorption geometry and stability, mechanisms of dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38). We also examine electronic properties of palladium-hydrogen system by analyzing Hirshfeld charges, HOMO-LUMO gap and the density of states. Through the detailed system's theoretical calculation, we discuss hydrogen dissociative chemisorption on palladium clusters and the electronic properties with the cluster sizes increase.
In Chapter1, we summarized basic theory of the cluster and surface adsorption. In Chapter2, we introduced the theoretical basis of modern quantum chemistry. In Chapter3, we studied the mechanisms of hydrogen adsorption, dissociative chemisorption and diffusion on Pdn(n=4,6,13,19,38) and electronic and geometric properties of palladium-ydrogen system.
The main conclusions of this work include:
1. The average binding energies of the Pdn(n=4,6,13,19,38) clusters significantly increase from1.660to2.785eV with cluster size increased. Pd-Pd bond distances gradually change from2.661to2.756A. The stable adsorption sites for H2molecule are top sites of Pdn(n=4,6,13,19,38), with the respective adsorption energy of0.568,0.670,0.629,0.453and0.456eV. The stable adsorption sites of Pd19and Pd38are at top sites of five-coordinated Pd atoms.
2. Hydrogen atom adsorbed on the Pd4edge site is more stable while the adsorption on face site is more stable for Pd6, Pd13, Pd19and Pd38. From Pd4to Pd38. the rate-determining step of dissociating H2molecule is overcoming the energy barrier of0.448,0.401,0.314,0.304and0.782eV, respectively. The results show that Pd19has much lower chemisorption energy and the lowest energy barrier. Adsorptive H atom shifts on the Pdn(n=4,6,13,19,38) cluster are testified to be quite flexible.
3. In the charge transfer process, the Pd cluster behaves as a donor and H2behaves as an acceptor. There is charge being transferred from the Pd atoms to the antibonding orbital of H2, associated with the breaking of the H-H bond.
引文
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