Elementary-step modeling and transient FTIR studies of carbon-monoxide and ethylene oxidation on silica-supported platinum and rhodium catalysts

This dissertation contains results from transient Fourier transform infrared (FTIR) experiments and elementary-step kinetic modeling of the oxidation of CO and ethylene on silica-supported Pt and Rh catalysts. Ignition, extinction and oscillatory behavior during CO oxidation on Pt/SiO{dollar}sb2{dollar} are demonstrated to occur in a spatially non-uniform manner. Furthermore, these spatial differences in dynamic behavior are linked to variations in the distribution and size of the metal crystallites.; Interactions between CO and Rh/SiO{dollar}sb2{dollar} are shown to depend strongly on pretreatment methods and the existing gaseous environment and temperature. A quantitative estimate of the relative amounts of CO chemisorbed in different forms is made, and shown to change with the operating conditions. Transient experiments during CO oxidation on Rh/SiO{dollar}sb2{dollar} indicate that this system exhibits similar steady-state and dynamic features to those of the same reaction on Pt/SiO{dollar}sb2{dollar}. The similarities include steady-state multiplicity, oscillatory behavior prior to extinction under oxygen-rich conditions, and the occurrence of spatially non-uniform temperatures and coverages during reaction-rate oscillations. Simulations from an elementary-step model, utilizing kinetic parameters from UHV studies, help clarify many of the basic features of CO oxidation of Rh. The basis for linking the experiments and the model was the construction of the experimental and simulated bifurcation cross-sections. These depict the dependence of the ignition and extinction temperatures on the inlet CO concentration.; A similar approach was utilized in developing an elementary-step kinetic model for the complete oxidation of ethylene on Pt/SiO{dollar}sb2{dollar}. Two alternate reaction pathways were considered in describing the overall reaction. The simulations indicate that CO{dollar}sb2{dollar} formation from a surface reaction between adsorbed ethylene and adsorbed oxygen could be an important pathway under oxygen-rich conditions. Under ethylene-rich conditions, however, the intermediate formation of adsorbed CO from adsorbed ethylene, and its subsequent oxidation to CO{dollar}sb2{dollar}, appears to become more important.