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摘要：直接甲醇燃料电池（DEFCs）在能量存储和转换中发挥着非常重要的作用，其重要特点是甲醇在阳极（MOR）上氧化，以提供清洁，充足和可靠的能源（CH3OH + H2O → CO2+6H+ + 6e-)。人们在过去几十年中对MOR反应的反应机理及动力学过程做了大量研究。虽然实验仪器探测水平有了很大提高，电化学原位谱学方法 (IR、Raman 等) 的建立和表面科学研究技术 (AES、LEED、XPS 和 STM 等) 的运用能够在分子水平观测到一些实验现象，但在微观结构方面实验结果急需理论予以解释，并对于电势有关的界面现象 (如分子结构、化学吸附、水的活性以及表面重组等) 提供全面彻底的信息。但遗憾的是，由于电化学界面的复杂性 (包括电极、电解质和电势)，人们对MOR电催化反应的微观过程了解甚少。因此，详细研究电极条件下固/液界面的微观结构和电化学反应的机理，对于设计制备高效的MOR电极催化剂无疑具有重大的意义。因此本论文的主要工作是建立研究金属/溶液界面的模型，发展快速、可靠的密度泛函理论 (DFT) 并行计算方法求解带电金属/溶液体系的电子结构。应用所建立的模型计算模拟双电层的一些基本性质 (如微分电容)，并阐释其本质和影响因素。在此基础上，通过结合密度泛函理论（DFT）计算和最近发展的基于修正的泊松-玻尔兹曼周期性连续介质模型（continuum solvation model based on modified-Poisson-Boltzmann equation, CM-MPB）对Pt/水界面的甲醇氧化(MOR)反应展开了研究，尝试构建一个能够完整理解MOR 反应动力学的理论框架，为合理设计MOR 催化材料提供依据。我们围绕甲醇初步脱氢过程等关键问题展开了研究，建立了C-H 和O-H 反应的过程的自由能图像，计算了该初步过程的电荷转移系数。通过研究发现，甲醇解离中第一步反应为C-H 断键，并且该断键过程为决速步(CH3OH(aq)→CH2OH*+H*)。由于C-H 过程中甲醇需要从体相溶液中扩散至表面并取代表面吸附的水分子，该反应过程需要克服很大的熵变。C-H 断键过程无明显的电荷传递过程，其电荷转移系数值为0.36。计算结果表明金属/吸附物间的相互作用和溶剂化在电化学动力学过程中发挥重要作用。我们希望从甲醇氧化过程出发，运用该理论模型和方法进一步理解固/液界面有机电化学反应的详细过程。20390
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毕业论文关键词: 甲醇氧化；塔菲尔动力学；第一性原理；连续介质模型
Potential-Dependent Reaction Kinetics at Solid-Liquid Interface
Abstarct: The direct methanol fuel cells (DEFCs) are regarded as a key technology for energy storage and conversion, which features methanol oxidation (MOR) on an anode to deliver clean, abundant and reliable energy (CH3OH + H2O → CO2 +6H+ + 6e-). Extensive studies on MOR kinetics have been carried out in the past decades with the aim to reduce Pt usages (e.g. alloys or dispersing into nanoparticles) while improving CO tolerance. While well-defined spectroscopic characterization of surfaces via sum frequency generation, surface enhanced Raman spectroscopy, and diffuse reflectance infrared spectroscopy, as well as scanning tunneling microscopy, are beginning to provide atomic scale resolution. There is a strong need for theory to complement these experimental efforts and help provide a fundamental understanding of potential-dependent interfacial phenomena including changes in molecular structure, chemisorption, water activation, and surface reconstruction. To resolve the kinetics at the atomic level, this work investigates the potential-dependent reaction kinetics of methanol oxidation on Pt(111) using the periodic first principles continuum solvation model based on Modified-Poisson-Boltzmann equation (CM-MPB), focusing on the initial dehydrogenation elementary steps. A theoretical model to predict Tafel kinetics (current vs potential) is established by considering that the rate-determining step of methanol oxidation (to CO) is the first C-H bond breaking (CH3OH(aq) → CH2OH* + H*) according to the computed free energy profile. The first C-H bond breaking reaction needs to overcome a large entropy loss during methanol approaching to the surface and replacing the adsorbed water molecules. While no apparent charge transfer involves in this elementary step, the charge transfer coefficient of the reaction is calculated to be 0.36, an unconventional value for charge transfer reactions, and the Tafel slope is deduced to be 166 mV. The results show that the metal/adsorbates interaction and the solvation environment play important roles on influencing the Tafel kinetics. The knowledge learned from the potential-dependent kinetics of methanol oxidation can be applied in general for understanding the electrocatalytic reactions of organic molecules at the solid-liquid interface.