钯催化燃料脱氢氧化反应机理研究

发布时间：2018-10-17 17:57

【摘要】：催化反应机理是世界上一个非常重要的研究课题,特别是金属表面的固/液交汇处的反应更是研究的热门领域。这个研究课题在燃料电池等方向上有着广泛的运用。本论文从理论出发,运用从头算理论研究了甲酸和甲醇在催化剂表面覆盖不同介质情况下的氧化反应机理,这个研究方向在燃料电池的运用上有着巨大的潜力。本工作运用密度泛函理论广义梯度近似(DFT-GGA)计算了金属/水表面体系,通过研究不同有机物在催化剂表面反应的活化能,研究了不同反应路径的可行性。具体研究内容如下:(1)本文运用vasp计算软件包,建立了详细的水分子在金属表面的吸附模型以研究对甲酸催化分解反应的影响,优化了反应过程中的反应物、中间体、过渡态以及产物的构型。具体研究了甲酸存在于H2O/Pd(111)体系中的吸附能以及不同的反应通道。本文在相同基组的水平上对反应的反应物和产物进行了优化,计算了在3层Pd原子表面下甲酸的吸附能,确定了各个反应体系的过渡态和中间体。得出了以下结论:研究结果发现氢键对甲酸在H2O/Pd(111)体系的吸附构型有很大影响,,H2O分子和甲酸中的O和H原子形成氢键相互作用改变了甲酸在Pd表面的稳定吸附形态。通过对甲酸催化裂解反应过程过渡态的搜索,结果表明在水溶液和Pd金属界面上甲酸C-OH键很难解离,只有C-H键解离这一条可行的反应路径,这个反应路径是甲酸生成CO2的决定性步奏。(2)本文运用DFT方法第一次计算了质子和质子、甲酸根共同存在的情况下甲酸在H2O/Pd(111)体系中的反应过程。研究表明,在质子存在的情况下,C-OH键解离和C-H键解离都变成了结构敏感反应,C-OH键解离所需的能垒相较于质子不存在时降低了1.6eV,C-OH键解离后C-H键中的H吸附到Pd的表面,反应最终生成CO和H2O;在甲酸根和质子共同存在的情况下C-OH键的解离更加容易,C-H键解离所需的活化能却升高了,通过电荷密度分析甲酸单独存在和质子、甲酸根存在的情况,发现甲酸根的电子云影响了甲酸的电荷密度使C-OH键裂解。由此说明CO和CO2的生成形成了竞争关系。(3)本文研究了甲醇在Pd金属表面的催化反应机理,以及可能的裂解反应步骤,计算了相关反应的反应物、过渡态、中间体以及产物,探索了甲醇在中性条件和碱性条件下在Pd金属表面裂解的反应通道。研究表明:甲醇吸附在Pd的表面上时容易被羟基负离子进攻得到甲二醇离子OCH2OH-,OCH2OH-→OCH2O2-→HCOO-和OCH2OH-→HCOOH→HCOO-两种反应通道都是可行的,在反应过程中他们的能垒都非常低。我们对HCOO-离子的脱氢过程进行了研究,结果表明甲醛能通过这个反应通道变成CO:H2CO→CHO→CO。甲醇在碱性环境下Pd电极表面生成CO和HCOO-的反应机理是合理的。
[Abstract]:Catalytic reaction mechanism is a very important research topic in the world, especially the reaction of solid / liquid interface on metal surface is a hot field. This research topic has been widely used in fuel cell and so on. In this paper, the mechanism of oxidation of formic acid and methanol in different media was studied by ab initio theory. This research direction has great potential in the application of fuel cells. In this paper, the generalized gradient approximation (DFT-GGA) of density functional theory is used to calculate the metal / water surface system. The feasibility of different reaction paths is studied by studying the activation energy of different organic compounds on the surface of the catalyst. The specific research contents are as follows: (1) in this paper, a detailed adsorption model of water molecules on metal surface was established by using vasp software package to study the effect on the catalytic decomposition of formic acid, and the reactants and intermediates in the reaction process were optimized. Transition state and the configuration of the product. The adsorption energy and different reaction channels of formic acid in H2O/Pd (111) system were studied. In this paper, the reactants and products of the reaction were optimized at the same base group level. The adsorption energy of formic acid on the surface of three layers of Pd atoms was calculated, and the transition states and intermediates of each reaction system were determined. The results show that hydrogen bond has a great influence on the adsorption configuration of formic acid in H2O/Pd (111) system. The hydrogen bond interaction between H 2O molecule and O and H atoms in formic acid changes the stable adsorption of formic acid on Pd surface. By searching the transition states in the catalytic cracking of formic acid, the results show that the formic acid C-OH bond is difficult to dissociate at the interface between aqueous solution and Pd metal, and only C-H bond dissociates this feasible reaction path. This reaction path is the decisive step in the formation of CO2 from formic acid. (2) the reaction process of formic acid in H2O/Pd (111) system in the presence of proton, proton and formate is calculated for the first time by using DFT method. The results show that in the presence of proton, both C-OH bond dissociation and C-H bond dissociation become structure-sensitive reactions, and the energy barrier required for C-OH bond dissociation decreases the H adsorption of C-H bond to the surface of Pd after the dissociation of 1.6eV C-OH bond compared with the absence of proton. In the presence of formate and proton, the dissociation of C-OH bond is easier, but the activation energy required for C-H bond dissociation is increased, and the existence of formic acid alone and protons is analyzed by charge density analysis. It was found that the electron cloud of formic acid affected the charge density of formic acid and the C-OH bond was cracked. It is concluded that the formation of CO and CO2 forms a competitive relationship. (3) the catalytic reaction mechanism of methanol on the surface of Pd and the possible pyrolysis steps are studied. The reactants, transition states, intermediates and products of the related reactions are calculated. The reaction channels of methanol cracking on the surface of Pd metal under neutral and alkaline conditions were investigated. The results show that methanol adsorbed on the surface of Pd is easy to be attacked by hydroxyl anion to get OCH2OH-,OCH2OH- OCH2O2- HCOO- and OCH2OH- HCOOH HCOO-, both of which are feasible, and their energy barriers are very low during the reaction. We have studied the dehydrogenation process of HCOO- ion. The results show that formaldehyde can be transformed into CO:H2CO CHO CO. through this reaction channel. The reaction mechanism of methanol to form CO and HCOO- on the surface of Pd electrode in alkaline environment is reasonable.
【学位授予单位】：武汉理工大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：O643.31;TM911.4

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