氧化镁薄膜负载钨氧化物团簇的构型和电子结构的理论研究
发布时间：2018-07-24 16:41
【摘要】：负载型钨氧化物和双金属钨氧化物因其广泛应用于烃类的脱氢、加氢、氧化还原等各类催化反应而成为催化领域的研究热点。为了深入研究该类催化剂结构与催化性能之间的构效关系,本论文采用基于第一性原理的分子动力学与量子力学相结合的方法,分别对CrW_2O_9团簇负载在MgO(001)表面和MgO/Ag(001)薄膜以及W_3O_9团簇负载在MgO/Ag(001)缺陷薄膜上的吸附构型和电子结构进行了系统研究。对于MgO(001)表面和MgO/Ag(001)薄膜两种不同类型载体,负载后的CrW_2O_9团簇表现出完全不同的构型。当CrW_2O_9团簇负载在MgO(001)表面时,CrW_2O_9团簇通过形成两根W-O键和四根Mg-O键竖直吸附在MgO(001)表面。而在CrW_2O_9/MgO/Ag(001)体系中,CrW_2O_9团簇通过四根Mg-O键平行吸附在MgO/Ag(001)薄膜上。电子结构研究表明,CrW_2O_9团簇负载在MgO(001)表面时,只有少量电子(0.73 e)通过共价吸附键从表面转移到CrW_2O_9团簇上。而当CrW_2O_9团簇负载在MgO/Ag(001)薄膜时,团簇得到了较多电子(1.72 e),并且团簇所得的电子是由于薄膜的隧道效应,电子从Ag衬底穿过MgO薄膜转移到团簇上。进一步的研究还发现CrW_2O_9/MgO/Ag(001)体系的自旋电荷密度主要定域在Cr原子附近,表明Cr位是一个较强的路易斯酸位。可以预测,和单金属钨氧化物W_3O_9团簇相比,掺杂型双金属钨氧化物CrW_2O_9团簇可能具有更好的催化活性。当W_3O_9团簇沉积在MgO/Ag(001)缺陷薄膜时,团簇通过形成八根Mg-O键负载在MgO/Ag(001)缺陷薄膜,并且其中一个端氧原子填补到MgO薄膜的氧缺陷位。此时,与负载在MgO(001)单晶缺陷表面相比,W_3O_9团簇负载在MgO/Ag(001)缺陷薄膜时吸附能显著提高。进一步的电子结构研究结果表明,负载后W_3O_9团簇得到2.74 e。团簇得到的电子主要来源于金属Ag衬底,有两个电子的隧道电流自发地从金属Ag衬底穿过MgO薄膜转移到W_3O_9团簇,少量电子从薄膜缺陷位通过共价键转移到团簇。此外,扫描隧道显微镜(STM)图像和高分辨率电子能量损失谱(HREELS)的能谱模拟结果表明,W_3O_9团簇沉积在MgO/Ag(001)缺陷薄膜和负载在MgO单晶表面所得的STM图像以及HREELS谱图性质均显著不同。
[Abstract]:Supported tungsten oxide and bimetallic tungsten oxide are widely used in dehydrogenation, hydrogenation, redox and other catalytic reactions of hydrocarbons. In order to deeply study the structure-activity relationship between the structure and catalytic performance of this kind of catalysts, this paper adopts the method of combining molecular dynamics and quantum mechanics based on the first principle. The adsorption configurations and electronic structures of CrW_2O_9 clusters on MgO (001) and MgO/Ag (001) films and W_3O_9 clusters on MgO/Ag (001) defective films were studied. For MgO (001) surface and MgO/Ag (001) film, the loaded CrW_2O_9 clusters exhibit completely different configurations. When the CrW_2O_9 cluster is loaded on the surface of MgO (001), CrW2O9 clusters are vertically adsorbed on the MgO (001) surface by forming two W-O bonds and four Mg-O bonds. In CrW_2O_9/MgO/Ag (001) system, CrW2O9 clusters are adsorbed on MgO/Ag (001) thin films by four Mg-O bonds. The electronic structure study shows that only a small amount of electrons (0.73e) are transferred from the surface to the CrW_2O_9 cluster by covalent adsorption bond when the CrW _ 2O _ 9 cluster is loaded on the surface of MgO (001). When the CrW_2O_9 cluster is loaded on the MgO/Ag (001) film, a large number of electrons (1. 72 e),) are obtained in the cluster. The electrons obtained by the cluster are due to the tunneling effect of the film, and the electrons are transferred from Ag substrates through the MgO film to the cluster. It is also found that the spin charge density of CrW_2O_9/MgO/Ag (001) system is mainly located near Cr atom, indicating that Cr site is a strong Lewis acid site. It can be predicted that the doped bimetallic tungsten oxide W_3O_9 clusters may have better catalytic activity than monometallic tungsten oxide W_3O_9 clusters. When W_3O_9 clusters are deposited on MgO/Ag (001) defective films, the clusters are loaded on the MgO/Ag (001) defect films by forming eight Mg-O bonds, and one of the oxygen terminal atoms fills in the oxygen defect sites of the MgO films. At this time, compared with that loaded on the surface of MgO (001) single crystal defect, the adsorption energy of W _ (3) O _ (9) cluster on MgO/Ag (001) defect film is significantly increased. The results of further study on the electronic structure show that the W_3O_9 cluster is 2.74 e. after loading. The electrons obtained from the clusters mainly originate from the metal Ag substrates. The tunneling currents of two electrons spontaneously transfer from the metal Ag substrates through the MgO films to the W_3O_9 clusters, and a small amount of electrons are transferred from the defect sites of the films to the clusters by covalent bonds. In addition, the spectral simulation results of (STM) images of scanning tunneling microscope and (HREELS) of high resolution electron energy loss spectra show that the properties of STM images and HREELS spectra of MgO/Ag (001) defective films deposited on MgO/Ag (001) defect films and MgO single crystal surfaces are significantly different from those obtained from high resolution electron energy loss spectrum (HREELS).
【学位授予单位】：江西师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O641.1;O643.36
[Abstract]:Supported tungsten oxide and bimetallic tungsten oxide are widely used in dehydrogenation, hydrogenation, redox and other catalytic reactions of hydrocarbons. In order to deeply study the structure-activity relationship between the structure and catalytic performance of this kind of catalysts, this paper adopts the method of combining molecular dynamics and quantum mechanics based on the first principle. The adsorption configurations and electronic structures of CrW_2O_9 clusters on MgO (001) and MgO/Ag (001) films and W_3O_9 clusters on MgO/Ag (001) defective films were studied. For MgO (001) surface and MgO/Ag (001) film, the loaded CrW_2O_9 clusters exhibit completely different configurations. When the CrW_2O_9 cluster is loaded on the surface of MgO (001), CrW2O9 clusters are vertically adsorbed on the MgO (001) surface by forming two W-O bonds and four Mg-O bonds. In CrW_2O_9/MgO/Ag (001) system, CrW2O9 clusters are adsorbed on MgO/Ag (001) thin films by four Mg-O bonds. The electronic structure study shows that only a small amount of electrons (0.73e) are transferred from the surface to the CrW_2O_9 cluster by covalent adsorption bond when the CrW _ 2O _ 9 cluster is loaded on the surface of MgO (001). When the CrW_2O_9 cluster is loaded on the MgO/Ag (001) film, a large number of electrons (1. 72 e),) are obtained in the cluster. The electrons obtained by the cluster are due to the tunneling effect of the film, and the electrons are transferred from Ag substrates through the MgO film to the cluster. It is also found that the spin charge density of CrW_2O_9/MgO/Ag (001) system is mainly located near Cr atom, indicating that Cr site is a strong Lewis acid site. It can be predicted that the doped bimetallic tungsten oxide W_3O_9 clusters may have better catalytic activity than monometallic tungsten oxide W_3O_9 clusters. When W_3O_9 clusters are deposited on MgO/Ag (001) defective films, the clusters are loaded on the MgO/Ag (001) defect films by forming eight Mg-O bonds, and one of the oxygen terminal atoms fills in the oxygen defect sites of the MgO films. At this time, compared with that loaded on the surface of MgO (001) single crystal defect, the adsorption energy of W _ (3) O _ (9) cluster on MgO/Ag (001) defect film is significantly increased. The results of further study on the electronic structure show that the W_3O_9 cluster is 2.74 e. after loading. The electrons obtained from the clusters mainly originate from the metal Ag substrates. The tunneling currents of two electrons spontaneously transfer from the metal Ag substrates through the MgO films to the W_3O_9 clusters, and a small amount of electrons are transferred from the defect sites of the films to the clusters by covalent bonds. In addition, the spectral simulation results of (STM) images of scanning tunneling microscope and (HREELS) of high resolution electron energy loss spectra show that the properties of STM images and HREELS spectra of MgO/Ag (001) defective films deposited on MgO/Ag (001) defect films and MgO single crystal surfaces are significantly different from those obtained from high resolution electron energy loss spectrum (HREELS).
【学位授予单位】：江西师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O641.1;O643.36
【参考文献】
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1 徐艺军,李俊{，
本文编号：2141996
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