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 (bala nce=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 presen ce of excess oxygen. Pt-MFI has been employed as an oxidation catalyst and various metal ion-exchanged MFI zeolites have been examined as th e 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 oxi dation 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 eff ective for the IAR method under the present conditions. For example, t he 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 muc h higher than 1.4 of conventional reduction on Cu-MFI. The reaction me chanism has been discussed on the basis of the dependence of the react ion on the partial pressure of NO2. (C) 1998 Elsevier Science B.V. All rights reserved.