不同晶面的Pt-Ni催化剂表面甲醇的吸附及脱氢反应机理研究

发布时间：2018-05-14 21:38

本文选题：密度泛函理论 + 表面催化　；参考：《南京师范大学》2017年博士论文

【摘要】：催化剂表面的吸附和反应研究无论在催化科学领域还是工业应用及环境保护方面都有极其重要的意义,也是当前研究的热门课题之一。固体催化剂的不同晶面由于其原子排列、电子结构等特性不同,使得所具有的性质也不同。这些性质主要包括对吸附质的吸附性、稳定性,对化学键的选择性以及对化学反应的催化活性等。因此,从原子水平上研究催化剂不同晶面的特性十分重要,不仅有助于更好的理解催化剂表面上的化学反应,更有利于调控催化剂的性能。目前从电子结构入手从分子尺度上控制物质性能的方法不仅仅被用于催化剂领域,而是成为整个化学、物理和材料科学领域的一个共同的思路。本学位论文以甲醇的脱氢反应作为研究反应体系,使用密度泛函理论方法(DFT)研究了其在Pt-Ni合金表面的反应过程,构建反应势能面以及反应网络力求从微观层面深入解释反应机理,在已有实验研究的基础上,通过将实验现象和理论计算的结果加以比较分析,从而确定控制反应进行的关键步骤及影响因素,为设计更好的催化剂提供理论指导。通过DFT计算并结合周期性平板模型,进行了结构优化、过渡态搜索及反应势能面构建;采用态密度、功函数变化、Bader电荷分析等手段实现了对吸附构型及各基元反应过程的电子结构分析;进一步探讨了反应微观动力学分析对反应选择性的决定因素。本论文的主要内容及结果如下:1.研究了 CH3OH在Pt3Ni催化剂的(111)、(100)和(110)表面上的吸附行为。利用DFT方法以及范德华校正(vdW)优化了 CH3OH在Pt3Ni催化剂的(111)、(100)和(110)面上吸附的结构并且计算了相应的吸附能,分析结果表明CH3OH在Pt3Ni催化剂上的吸附强度遵循(110) (111) (100)顺序。同时从结构中Ni的d带中心相对于费米能级的移动、态密度、Bader电荷分析、功函变化以及CH3OH对不同表面的极化作用等方面讨论了 CH3OH吸附行为的机理。结果发现在吸附过程中Ni原子或者周围含Ni原子多的位点有利于CH3OH的吸附,即配体效应明显。同时极化作用也对吸附有一定的影响。本章结果为适用于非均相催化的纳米结构催化剂表面的合理设计和构建提供理论指导。2.研究了 Pt3Ni(111)上的甲醇分解反应机理。利用DFT方法以及范德华校正优化了 CH3OH及其在Pt3Ni(111)上分解的中间体的结构、吸附稳定性和最佳吸附位点,发现由于Pt3Ni(111)表面上的Pt和Ni原子分别带有少量的负电荷和正电荷,大多数中间体(如自由基中间体和具有孤对电子的物质)易于吸附在Ni位点周围。根据相关过渡态的空间位阻效应和电子结构以及Bronsted-Evans-Polanyi (BEP)关系讨论了通过O-H, C-H和C-O键的初始断裂的可能途径,研究发现,CH30H分解最初优先发生O-H键断裂而不是C-H和C-O键断裂。结合分解途径中基元反应步骤的热化学信息和反应能垒最终得到反应最佳路径。计算这些中间体吸附状态的振动频率,并利用简谐过渡态理论计算每个可能基元步骤的反应速率常数。构造势能面(PES),确定反应速控步,获得反应最佳途径是CH3OH → CH30 → CH20 → CHO →CO,其中O-H键的断裂是速控步。通过本工作与其他系统的结果比较时发现在DMFCs中Pt3Ni(111)可以有效地促进甲醇分解和减轻CO中毒问题。CH3OH及其分解中间体在Pt3Ni(111)吸附行为主要受配体效应影响,其脱氢过程主要受几何效应影响。本章结果将为更好的理解直接甲醇燃料电池(DMFC)中的PtNi阳极特性以及提高DMFC性能提供了理论指导。3.研究了 Pt3Ni(100)上的甲醇分解反应。利用DFT方法以及范德华校正(vdW)优化了 CH3OH及其在Pt3Ni(100)上分解的中间体的结构、吸附稳定性和最佳吸附位点,研究发现,与Pt3Ni(111)表面类似,在Pt3Ni(100)上大多数中间体(如自由基中间体和具有孤对电子的物质)易于吸附在Ni位点周围,其中Pt和Ni均对脱氢反应起协同催化作用。根据相关过渡态的空间位阻效应和电子结构以及BEP关系讨论了发生O-H, C-H和C-O键初始断裂的可能途径,结果发现CH3OH分解起始发生O-H断裂在热力学和动力学上最有利,C-H键断裂的活化能比O-H键断裂稍低,在常温下也可能发生,因此我们认为在Pt3Ni(100)表面上O-H断裂与C-H键断裂存在竞争机制。C-O键断裂在常温下最难发生。反应最佳途径是CH3OH → CH30 → CH20→CHO → CO,其中CHO中C-H键的断裂是速控步,与Pt3Ni(111)面上的速控步不同。此结果将为更好的理解PtNi合金的(100)面对合金总的催化性能的贡献以及提高DMFC性能提供了理论指导。
[Abstract]:The study of adsorption and reaction on the surface of the catalyst is of great significance in both the field of catalytic science, industrial application and environmental protection. It is also one of the hot topics in current research. The properties of different crystalline surfaces of solid catalysts are different because of their atomic arrangement and electronic structure. These properties are also different. It mainly includes adsorbability, stability, selectivity to chemical bonds and catalytic activity for chemical reactions. Therefore, it is very important to study the characteristics of different crystalline surfaces from the atomic level, which is not only helpful to better understand the chemical reaction on the surface of the catalyst, but also to control the performance of the catalyst. The method of controlling material performance from the molecular scale is not only used in the field of catalyst, but also a common thought in the field of chemistry, physics and materials science. This dissertation uses methanol dehydrogenation as the reaction system, and uses the density functional theory method (DFT) to study its Pt-Ni alloy table. In the reaction process, the reaction potential energy surface and the reaction network are constructed to explain the reaction mechanism deeply from the microscopic level. On the basis of the existing experimental research, the key steps and the influencing factors of the control reaction are determined by comparing the results of the experimental and theoretical calculations, so as to provide a better catalyst for the design of the catalyst. The structure optimization, the transition state search and the reaction potential surface construction are carried out by DFT calculation and the periodic plate model. The electronic structure analysis of the adsorption configuration and the reaction process of each basic element is realized by means of state density, work function change and Bader charge analysis, and the reaction microdynamics analysis is further discussed. The main determinants of this paper are as follows. The main contents and results of this paper are as follows: 1. the adsorption behavior of CH3OH on the surface of (111), (100) and (110) of Pt3Ni catalyst was studied. The structure of CH3OH adsorbed on Pt3Ni catalyst (111), (100) and (110) was optimized by DFT method and Fan Dehua correction (vdW), and the corresponding adsorption energy was calculated. The analysis results show that the adsorption strength of CH3OH on the Pt3Ni catalyst follows (110) (111) (100) order, and the mechanism of CH3OH adsorption behavior is discussed from the movement of the d band center of Ni relative to the Fermi level, the density of state, the charge analysis of Bader, the change of the work function and the polarization of CH3OH on different surfaces. In the process, the Ni atom or the surrounding loci containing Ni atoms are beneficial to the adsorption of CH3OH, that is, the ligand effect is obvious. Meanwhile, the polarization effect also has a certain influence on the adsorption. This chapter provides a theoretical guidance for the rational design and construction of the surface of the nano structured catalyst for heterogeneous catalysis..2. has studied the methanol fraction on Pt3Ni (111). The reaction mechanism is solved by using the DFT method and the Fan Dehua correction to optimize the structure of CH3OH and its intermediates on Pt3Ni (111), the adsorption stability and the best adsorption site. It is found that the Pt and Ni atoms on the Pt3Ni (111) surface have a small amount of negative charge and positive charge respectively, and most intermediates (such as free radical intermediates and soliton pairs). The matter of electrons is easy to adsorb around the Ni site. According to the spatial steric effect of the associated transition state and the electron structure and the relationship between the Bronsted-Evans-Polanyi (BEP) relationship, the possible pathways through the initial fracture of the O-H, C-H and C-O bonds are discussed. It is found that the first priority of the CH30H decomposition is the fracture of the O-H bond instead of the C-H and C-O bond breakages. The thermo chemical information and the reaction energy barrier of the radical reaction step in the solution are finally obtained the best path of the reaction. The vibration frequency of the adsorption state of these intermediates is calculated and the reaction rate constant of each possible basic element is calculated by the simple harmonic transition state theory. The potential energy surface (PES) is constructed and the reaction speed control step is determined. The best way to obtain the reaction is CH3O H, CH30, CH20, CHO to CO, the fracture of the O-H bond is a speed control step. By comparing the results of this work with the results of other systems, it is found that Pt3Ni (111) in DMFCs can effectively promote methanol decomposition and reduce CO poisoning..CH3OH and its decomposition intermediate in Pt3Ni (111) adsorption is mainly influenced by ligand effect, and its dehydrogenation process is main. The results of this chapter will provide a theoretical guide for better understanding of the PtNi anode properties in direct methanol fuel cell (DMFC) and the improvement of DMFC performance..3. studies the methanol decomposition reaction on Pt3Ni (100). The DFT method and Fan Dehua correction (vdW) are used to optimize the intermediates of CH3OH and its decomposition on Pt3Ni (100). The structure, adsorption stability and the best adsorption site have been found to be similar to the Pt3Ni (111) surface. Most of the intermediates on Pt3Ni (100) (such as free radical intermediates and substances with isolated electrons) are easy to adsorb around the Ni site, and both Pt and Ni have synergistic catalytic action on dehydrogenation. The spatial steric effect of the related transition states The possible way to break the initial fracture of O-H, C-H and C-O bonds is discussed with the electronic structure and the BEP relationship. It is found that the initiation of O-H fracture at the beginning of CH3OH decomposition is most favorable in thermodynamics and kinetics. The activation energy of the C-H bond fracture is slightly lower than that of the O-H bond and may occur at the normal temperature. Therefore, we think the O-H fracture and C- on the Pt3Ni (100) surface are on the C-. The H bond fracture is the most difficult for the.C-O bond fracture at normal temperature. The best way to react is CH3OH to CH30, CH20 to CHO and CO, in which the fracture of the C-H bond in CHO is the speed control step, which is different from the speed control step on the Pt3Ni (111) surface. The result will be a better understanding of the contribution of the (100) to the overall catalytic performance of the PtNi alloy and the extraction of the overall catalytic properties of the PtNi alloy. High DMFC performance provides theoretical guidance.

【学位授予单位】：南京师范大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36

【参考文献】