Intermediate addition of reductant between an oxidation and a reduction catalyst for highly selective reduction of NO in excess oxygen

Intermediate addition of reductant (ethene) into a NO+O-2 stream (balance=helium) between an oxidation catalyst of NO to NO2 and a reduction catalyst of NOx to N-2, named an IAR method, has been confirmed to be a very effective method to selectively reduce NO to N-2 in the presence of excess oxygen. Pt-MFI has been employed as an oxidation catalyst and various metal ion-exchanged MFI zeolites have been examined as the reduction catalysts. When H, Zn, Ag, In, Ga, Mg, Ba, Ca, and Na-MFI were used as the latter catalysts, the conversion levels of NO to N-2 greatly increased from those on the respective systems without the oxidation catalyst. On the other hand the conversions on Co, Cu, Mn, Ni, Fe, and Pt-MFI little changed with or without the preoxidation of NO. The combination of Pt-MFI and Zn-MFI has been found to be the most effective for the IAR method under the present conditions. For example, the degree of conversion of NO to N-2 at 573 K was increased to 54% on the Pt- and Zn-MFI combination from 5% on Zn-MFI alone or 6% on Pt-MFI alone. The efficiency of ethene, namely the molar ratio of the amount of NO reduced to that of ethene consumed, reached about 2 and was much higher than 1.4 of conventional reduction on Cu-MFI. The reaction mechanism has been discussed on the basis of the dependence of the reaction on the partial pressure of NO2. (C) 1998 Elsevier Science B.V. All rights reserved.