Improved NOx reduction over the staged Ag/Al2
The reaction mechanism of reducing NOx with hydrocarbons over Ag/Al2O3 has been examined to improve its NOx reduction performance at lower temperatures, using ethanol and n-octane as representative hydrocarbon reductants. Based on the results obtained at the early stages of the hydrocarbon oxidation and NO reduction, it is proposed that the partial oxidation of hydrocarbons and the oxidation of NO are the first reaction steps over Ag/Al2O3. n-Octane is broken up into smaller hydrocarbon molecules, which are then subsequently oxidized to form various aldehydes, while ethanol is also rapidly converted to acetaldehyde. At the same time, NO is oxidized effectively to NO2 in the presence of reductants. These observations and additional experiments with variable amounts of Al2O3 placed downstream of the Ag/Al2O3 catalyst suggest that the NO reduction by hydrocarbons over Ag/Al2O3 may occur via a bifunctional reaction mechanism; NO and hydrocarbons are converted into NO2 and more reactive hydrocarbon species (i.e., smaller alkenes, oxygenated hydrocarbons), respectively, over the Ag sites, while N2 is produced from the subsequent reactions between these intermediate species over different sites including Al2O3. The proposed bifunctional reaction mechanism offers an opportunity to improve the overall NOx reduction performance of Ag/Al2O3 by optimizing individual reaction steps separately. Thus, the concept of a staged catalyst system has been examined using Ag/Al2O3 for the formation of reaction intermediates, and a secondary catalyst (e.g., Al2O3 or BaY) for the subsequent N2 production reaction. Significant improvement in NOx reduction to N2 was obtained at lower temperatures, when BaY was used as the second catalyst and ethanol was used as reductant. © 2008 Elsevier B.V. All rights reserved.
Lee, JH; Schmieg, SJ; Oh, SH
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