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

CTS-9502971本项目研究了铂基电极上甲醇电催化的动力学和机理，以及该反应中表面氧的行为等基本问题。具体来说，它试图确定一氧化碳，被广泛认为是中毒的物种，是否是甲醇电氧化反应途径的必要中间体。已有足够的证据表明，存在另一种含氢中间体，这可能意味着有一条不含一氧化碳的单独反应途径。该项目试图分离和鉴定这种含氢中间体，并系统地量化其作为控制变量的功能：电位、温度、电解质性质和浓度以及表面形态。要特别注意电极电位，因为在技术燃料电池所需的低电位下的反应可能与氧化和去除一氧化碳中毒物质所需的高电位下的反应机制完全不同。通过预先吸附石墨或原子碳、乙炔、同位素标记的一氧化碳和硫的探针层来改变电极表面形态，有助于分别确定集合要求、共聚烃种类的影响、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，帮助区分竞争反应机制和确定特别有利的反应条件。表面氧的性质是通过预先吸附氧的电极的动力学测量和淬火电极表面的光谱分析来检验的。这些研究是根据非原位方法进行的，涉及一个直接耦合的电化学电池和超高真空表面分析系统。这种设备的组合允许制备干净的电极表面或用反应促进剂（钌或锡），上述探针涂层之一或预吸附氧精确修饰的电极表面。电化学研究采用循环伏安法和电位阶跃实验，而热解吸光谱、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解吸的相对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明确的电催化剂提供了必要的控制，记录了从电解液中去除后发生的任何变化，并在反应后对其表面进行了光谱表征。电化学燃料电池提供了最有效的能量转换手段之一，并允许大幅减少燃料消耗和排放与能源生产相关的污染物和温室气体。对于便携式发电、交通运输和休闲/家庭应用来说，直接甲醇燃料电池尤其具有吸引力，它将甲醇和氧气直接结合起来产生电能，但由于甲醇电氧化动力学差和有自中毒的倾向，其技术可行性有限。这在电催化中提出了一个问题，其目标是找到适当的电催化剂和操作条件，以最大限度地提高反应速率（从而最小化过电位），同时避免自中毒的条件。＊＊＊

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. ***