Enhanced NOx reduction and byproduct removal by (HC + OHC)/SCR over multifunctional dual-bed monolith catalyst

Abstract

Selective catalytic reduction of NOx over Ag/Al2O3 monolith catalyst was investigated using a dodecane–ethanol mixture as the reductant, with special attention to the increased formation of harmful byproducts due to ethanol in the reductant hydrocarbon mixture. To compensate the tradeoff effect of ethanol by fully utilizing and/or efficiently removing these hazardous byproducts formed over the Ag/Al2O3 catalyst, additional catalysts were employed in a double-layer or dual-bed configuration, or a combination thereof. Among the catalysts tested, a trimetallic dual-bed monolith catalyst was identified as the best combination for the maximum NOx-to-N2 conversion (∼90% above 300°C) with a minimum formation of harmful byproducts, through multifunctional cooperative catalytic processes on Ag/Al2O3, CuCoY and Pd/Al2O3. Reaction intermediates formed on catalyst surfaces were investigated using an in situ DRIFT for both single-bed and dual-bed catalysts, thereby identifying three important roles of Ag/Al2O3 in the front bed. The relative importance of various deNOx processes occurring on CuCoY in the rear bed strongly depends on the reaction temperature and the gas space velocity over the dual-bed catalyst system. Reaction pathways of (HC+OHC)/SCR in the dual-bed catalyst system were discussed in light of the maximum NOx conversion with a minimum consumption of OHC.