Influence of the pre-treatment on the structure and reactivity of Ir/gamma-Al2O3 catalysts in the selective reduction of nitric oxide by propene

The selective catalytic reduction (SCR) of nitric oxide by propene over Ir/ Al2O3 under lean-bum conditions (1000 vpm NO, 2000 vpm C3H6, 500 vpm CO, 10 vol.% O-2) was studied. The activity was shown to be strongly enhanced aft er exposure of the catalyst at 600 degreesC under the reaction mixture, irr espective of the oxidising or reducing pre-treatment. Simultaneously, the I r dispersion decreased from 78 to 10%. The influence of each component of t he reaction mixture on the activation process was examined. The presence of both CO and O-2 was found to be necessary to activate Ir/Al2O3 while NO wo uld not be. In situ FT-IR results revealed that initially fully oxidised Ir particles partially reduced in the feed to form Ir-0 reduced surface sites (vCO at 2060 cm(-1)) which adsorbed CO up to 350-400 degreesC. The activat ion under reactants was related to the formation of these sites. The presen ce of reduced (or partially reduced) Ir sites, possibly siting at the surfa ce of IrO2 particles and stabilised by CO adsorption, was proposed to be re sponsible for the SCR activity. (C) 2001 Elsevier Science B.V. All rights r eserved.