Mechanistic and Kinetic Studies of Electrooxidation of Methanol and Formic Acid on Well-Defined Platinum Electrodes

摘要CTS-9502971这个项目解决了关于甲醇在铂基电极上电催化的动力学和机理以及表面氧物种在同一反应中的行为的一些基本问题。具体地说，它试图确定被广泛接受为中毒物种的一氧化碳是否是甲醇电氧化反应途径中的必要中间体。已经有足够的证据表明另一种含氢中间体可能表示一条没有一氧化碳的单独反应途径。本项目试图分离和鉴定这种含氢中间体，并系统地量化它的存在作为控制变量的函数：电位、温度、电解液性质和浓度以及表面形态。特别注意电极电位，因为技术燃料电池所需的低电位下的反应可能与氧化和清除一氧化碳中毒物种所需的高电位下的反应具有完全不同的机制。通过预吸附碳或原子碳、乙炔、同位素标记的一氧化碳和硫的预吸附探针层，电极表面形态的变化有助于分别识别总体要求、受抑制的碳氢化合物物种的影响、一氧化碳的行为以及电氧化中电子相互作用的影响。动力学测量提供补充信息，以帮助区分相互竞争的反应机制，并确定特别有利的反应条件。表面氧的性质是通过用预吸附氧“播种”电极的动力学测量和淬火电极表面的光谱分析来检验的。这些研究是按照非原位方法进行的，包括直接耦合的电化学池和超高真空表面分析系统。这种表面活性剂的组合可以制备清洁的电极表面或用反应促进剂(Ru或TiN)、上述探头覆盖层之一或预吸附氧精确修饰的电极表面。循环伏安法和电势阶跃实验用于电化学研究，而热脱附谱、俄歇电子能谱、低能电子衍射、功函数测量、X射线光电子能谱和二次离子质谱仪则在真空系统中进行。电氧化后的热解吸测量通过氢和一氧化碳的相对脱附量来确定含氢中间体的性质和程度。其他表面分析方法为制备定义明确的电催化剂提供必要的控制，记录当它们从电解液中移除时发生的任何变化，并在反应后对其表面进行光谱表征。电化学燃料电池提供了可能的最有效的能源转换方法之一，并允许大幅减少燃料消耗和与能源生产相关的污染物和温室气体的排放。直接甲醇燃料电池直接结合甲醇和氧气产生电能，其技术可行性有限，因为甲醇电氧化动力学较差，而且有自毒的倾向，尤其对便携式发电、运输和休闲/家庭应用具有吸引力。这给电催化带来了一个问题，目标是找到合适的电催化剂和操作条件，使反应速度最大化(从而最小化过电位)，同时避免自毒条件。***

ABSTRACT CTS-9502971 This project addresses a number of fundamental issues regarding the kinetics and mechanism of methanol electrocatalysis on platinum-based electrodes as well as the behavior of surface oxygen species in this same reaction. Specifically, it seeks to identify whether carbon monoxide, widely accepted as the poisoning species, is a necessary intermediate in the reaction pathway of methanol electrooxidation. Sufficient evidence already exists for another, hydrogen containing intermediate that may signify a separate reaction pathway free of carbon monoxide. This project attempts to isolate and identify this hydrogen containing intermediate and systematically quantify its presence as a function of the controlling variables: potential, temperature, electrolyte nature and concentration, and surface morphology. Particular attention is paid to electrode potential as reaction at the low potentials required of a technical fuel cell may have an entirely different mechanism than at the high potential required for oxidation and removal of the carbon monoxide poisoning species. Variation of electrode surface morphology through preadsorbed probe adlayers of graphitic or atomic carbon, ethlidyne, isotopically labelled carbon monoxide, and sulfur helps to identify, respectively, ensemble requirements, the influence of coabsored hydrocarbon species, the behavior of carbon monoxide, and the influence of electronic interactions in electrooxidation. Kinetic measurements supply complementary information to help discrimination between competing reaction mechanisms and identify particularly favorable reaction conditions. The nature of surface oxygen is examined with kinetic measurements of electrodes "seeded" with preadsorbed oxygen and by spectroscopic analysis of the surface of quenched electrodes. These studies are conducted according to the ex-situ methodology involving a directly coupled electrochemical cell and ultrahigh vacuum surface analysis system. This combination of facil ities allows preparation of clean electrode surfaces or those precisely modified with a reaction promoter (ruthenium or tin), one of the probe adlayers mentioned above, or preadsorded oxygen. Cyclic voltammetry and potential step experiments are used for electrochemical studies, whereas thermal desorption spectroscopy, Auger electron spectroscopy, low energy electron diffraction, work function measurements, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry are conducted in the vacuum system. Thermal desorption measurements following electrooxidation identify the nature and extent of the hydrogen-containing intermediate through the relative amounts of hydrogen and carbon monoxide desorption. The other surface analysis methods provide the necessary controls for preparing well defined electrocatalysts, document any changes that occur upon their removal from the electrolyte, and spectroscopically characterize their surfaces following reaction. Electrochemical fuel cells offer one of the most efficient means of energy conversion possible, and allow substantial reductions in fuel consumption and emissions of the pollutants and greenhouse gases associated with energy production. Especially attractive for portable power generation, transportation, and leisure/domestic applications, the direct methanol fuel cell, which combines methanol and oxygen directly to produce electrical energy, has limited technical feasibility because of poor methanol electrooxidation kinetics and a tendency to self-poisoning. This presents a problem in electrocatalysis, where the goal is to find the proper electrocatalyst and operating conditions that maximize reaction rate (and hence minimize overpotential) while avoiding the conditions for self-poisoning. ***