DeNOx performance of Ag/Al2O3 catalyst using simulated diesel fuel–ethanol mixture as reductant

The NOx reduction activity of Ag/Al2O3 catalyst has been investigated systematically, using a mixture of simulated diesel fuel (SD) and ethanol (E) as the reductant with special interest in the low temperature deNOx performance below 350 °C. The Ag loading and catalyst operating conditions such as C1/NOx and ethanol/simulated diesel fuel (E/SD) ratios have been optimized in order to achieve the maximum deNOx performance of the Ag/Al2O3 catalyst. Ammonia has been identified as the most abundant reaction intermediate over the Ag/Al2O3 catalyst under the optimized operating conditions. To further enhance the deNOx performance by utilizing the NH3 formed over the Ag/Al2O3 catalyst, a dual-bed reactor system has been employed with CuZSM5 catalyst in the rear bed consecutively following the front bed containing the Ag/Al2O3. The NOx-to-N2 conversion in this dual-bed system increased up to 85% from 275 to 450 °C, mainly due to the NH3 oxidation to N2 by the CuZSM5 in the rear bed. Physicochemical characterization of the Ag/Al2O3 catalysts by UV–vis, TEM and EELS has indicated that both ionic and metallic Ag formed on the catalyst surface play important roles for the high deNOx performance of the present catalytic system; the ionic Ag including Ag+ and Agδ+ is the active reaction site for the reduction of NOx to N2, while the metallic Ag is responsible for the partial oxidation of hydrocarbons (HCs) which promotes the initiation of the HC/SCR process in the low temperature range. The optimum ionic/metallic (I/M) ratio of Ag species on Al2O3 support surface appears to be in the range of 1.4–1.7, depending upon the catalyst temperature range of interest.

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► When C1/NOx and E/SD ratios were 4, the highest NOx-to-N2 conversion, 70% over Ag(3.8)/Al2O3 was achieved at 300 °C.
► A proper combination of the ionic and metallic Ag on the catalyst surface, 1.4–1.7 is critical for the high deNOx performance.
► The CuZSM5 in the dual-bed system improved the deNOx performance of Ag/Al2O3, mainly due to the NH3 oxidation to N2.