Ir/H-ZSM-5 catalysts in the selective reduction of NOx with hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir-0 and IrO2. The presence of Ir-0 was essential for high DeNOx activity. The ratio of Ir-0/Ir4+ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir-0. Crystallite size and oxidation state of It have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.