Catalytic reduction of NO over in situ synthesized Ir/ZSM-5 monoliths

The catalytic performance of Ir-based catalysts was investigated for the re duction of NO under lean-burn conditions over binderless Ir/ZSM-5 monoliths , which were prepared by a vapor phase transport (VPT) technique. The catal ytic activity was found to be dependent not only on the Ir content, but als o on the ZSM-5 loading of the monolith. With the decreasing of the Ir conte nt or the increasing of the ZSM-5 loading of the monolith, NO conversion in creased. When the ZSM-5 loading on the cordierite monolith was raised up to ca. 11% and the metal Ir content was about 5 g/l, the NO conversion reache d its maximum value of 73% at 533 K and SV of 20 000 h(-1). Furthermore, bo th the presence of 10% water vapor in the feed gas and the variation of spa ce velocity of the reaction gases have little effect on the NO conversion. A comparative test between Ir/ZSM-5 and Cu/ZSM-5, as well as the variation of the feed gas compositions, revealed that Ir/ZSM-5 is very active for the reduction of NO by CO under lean conditions, although it is a poor catalys t for the C3H8-SCR process. This unique property of Ir/ZSM-5 makes it super ior to the traditional three-way catalyst (TWC) for NO reduction under lean conditions. (C) 2001 Elsevier Science B.V. All rights reserved.