Nitrogen dioxide reduction with methane over palladium-based sulfated zirconia catalysts. A component of a lean exhaust aftertreatment system

Nitrogen oxides emitted from the combustion of fossil fuels represent a significant environmental risk. These compounds directly contribute to global warming, acid rain deposition, and the formation of both smog and ground level ozone. Catalytic aftertreatment is a proven method for removal of these pollutants from certain exhaust streams, but current technologies are unsuitable for use with next-generation lean-burn engines. This work examines the development of catalysts active for the reduction of NO2 with CH4, and evaluates their performance as a component of a two-catalyst aftertreatment system. In this proposed system NO is first oxidized to NO2 on an oxidation catalyst. NO2 is a more easily reduced species. The reduction reaction is then carried out over a reduction catalyst.; Highly selective NOx reduction catalysts must be used for hydrocarbon-based reduction under lean conditions, as in excess oxygen the hydrocarbon tends to unselectively combust. Two novel Pd-sulfated zirconia catalysts were prepared and demonstrated to be active for the reaction. Pd-sulfated zirconia was prepared through incipient wetness techniques on a monoclinic zirconia support. Characterization showed the presence of surface sulfate groups stable under reaction conditions. Activity testing demonstrated N2 yields of 60% during the reduction of NO2. These results were significantly better than observed during the reduction of NO, validating the two-stage concept. In-situ DRIFTS investigations comparing the reduction of NO and NO2 identified potential reaction intermediates. These intermediate species formed to a greater extent during the reduction of NO2, possibly explaining the improved results.; Pd/SZ was also prepared through a single-step sol-gel procedure. Addition of Pd to the sol-gel preparation was shown to directly affect the formation of crystallinity in the zirconia support. Catalyst calcination temperature directly affected the activity of the catalysts through the formation of different surface sulfate species associated with catalyst acidity. Catalysts calcined at 700°C reached N2 yields of 70%.; Testing of a mixed catalyst bed, containing both an active oxidation catalyst and Pd/SZ prepared by incipient wetness, showed very good activity for the reduction of NOx with CH4 under lean conditions. N 2 yields of 80% were achieved under simulated exhaust conditions. Additionally the two-catalyst system was shown to be active for the removal of unburned hydrocarbons and carbon monoxide. Testing of the system in the presence of water vapor showed a loss of NOx reduction activity through a competitive adsorption phenomenon on the Pd/SZ catalyst. Modification of the catalyst through an increase in sulfate content was shown to improve resistance to this deactivation effect.