In situ mechanistic studies of the oxidation of carbon monoxide, ethylene, and acetylene over platinum surfaces.

Abstract

Carbon monoxide, ethylene, and acetylene oxidation reactions play an important role in catalytic combustion, removal of unburned hydrocarbons from exhaust streams, and calorimetric hydrocarbon sensing. Fundamental, molecular-level understanding of reaction mechanisms and catalytic sites promises to advance the development of catalysts and sensors. This dissertation focuses on developing an improved understanding of CO and C₂ hydrocarbon oxidation mechanisms over a series of platinum surfaces of increasing complexity. These reactions were studied using a combination of classical, low-pressure, surface science techniques coupled with in-situ soft x-ray methods capable of monitoring reactions at elevated pressures. This combination of techniques provides unique information regarding the concentrations, stoichiometries, and bonding of carbon containing species over a broad temperature and pressure range. In CO oxidation on platinum surfaces, concentrations and mixing of the adsorbed oxygen and CO play a vital role in the oxidation mechanism. Rate-limiting processes are dictated by surface stoichiometries of CO and oxygen. The three platinum surfaces studied in order of increasing complexity are: Pt(111), a 100 A Pt/Al₂O₃ thin film, and a 4.8 wt % Pt/Al₂O₃ catalyst. CO oxidation mechanisms are dominated by defect reactivity on the 100 A Pt film and the alumina supported platinum catalyst. For ethylene and acetylene oxidation, C-H bond strength proves critical in oxidation mechanisms on Pt(111). Oxydehydrogenation precedes skeletal oxidation for monolayer ethylene oxidation. A stable tri-sigma vinyl intermediate (C₂H₃) is formed over a broad temperature range. In monolayer acetylene oxidation, oxydehydrogenation and skeletal oxidation occur simultaneously, indicating an increased C-H bond strength. Adsorbed acetylene maintains a constant C₂H₂ stoichiometry throughout the oxidation process. These results provide significant new insights into CO, ethylene, and acetylene oxidation processes and illustrate the power of in-situ soft x-ray methods.