Abstract
The kinetics of NOx oxidation and storage were studied over Pt/Al2O3, CexZr1−xO2 and Pt/CeO2. A detailed kinetic mechanism involving lumped nitrate and nitrite species was proposed for the interaction of NOx and O2 with ceria(–zirconia) and its parameters estimated using oxidation and adsorption/desorption experiments. Important trends in the behaviour of ceria–zirconia catalysts (x=0–0.84), platinum and Pt/CeO2 were reproduced over a wide range of temperatures. Thus zirconium affects redox behaviour of the oxide, ultimately decreasing its ability to oxidise NO to NO2. NOx storage is described via the accumulation of nitrates on the oxide surface. Oxidation and storage over Pt/CeO2 can be reproduced by fitting ceria and platinum-related parameters separately. While oxidation behaviour is mainly dictated by Pt, nitrate storage occurs on CeO2 and is shown to be more efficient under a NO+O2 flow than NO2. However when considering an equal number of active sites, Pt/CeO2 stores NOx less efficiently than Pt/BaO/Al2O3.
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