Catalysts for Car Exhaust: Oxygen Storage in Platinum/Ceria and Mass Transfer in Monoliths
Abstract: In this work, the oxygen storage/release in Pt/ceria catalysts has been investigated both experimentally and theoretically. Moreover, the flow distribution and mass transfer in monolith catalysts for car-exhaust purification have been studied.On Pt/CeO2/Al2O3 catalysts, the reduction was found to be much slower than the oxidation, and a model for the reduction of Pt/ceria by CO was developed. Water was found not to hinder the reduction of Pt/ceria by CO. Measurements and modelling of temperature-programmed isotope oxygen exchange showed that the adsorption and desorption of oxygen and the bulk diffusion were the rate-determining steps in the exchange on Pt/ceria, whereas the surface diffusion and the transfer through the Pt-ceria interface were faster processes. The low-temperature activity in CO oxidation on Pt/ceria was investigated experimentally. Reduction at 200°C was found sufficient to initiate a high catalytic activity, which started at about 45°C. We found that reduction at higher temperatures could be detrimental to the activity. Water was found to slightly increase the CO conversion. Computational fluid-dynamics simulations and Laser-Doppler anemometry measurements of the steady, isothermal flow in cylindrical monoliths showed that a conical or parabolic shape of the monolith inlet face made the flow in the monolith more uniform compared to the conventional, flat inlet face. Thus, these new designs made the monolith converter more efficient. From measurements of the CO oxidation rate in monolith channels, gas-solid mass-transfer rates were obtained. These were found to be higher, and more strongly dependent on the Reynolds numbers (Re), than in earlier studies. This is likely due to the turbulence generated at the channel inlet and its capability to survive over a longer distance at higher Re. We also investigated a new channel design of a metal monolith, with bumps on the channel walls. This monolith was found to have improved mass transfer/pressure drop performance compared with the conventional monolith.
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