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. ***
