| abstract |
Multi-stage catalytic systems and methods are provided for converting NO x , CO, and hydrocarbons in a mixture, to innocuous products. The systems include first and second stages, each with substrate and catalyst. The mixture is contacted with the first stage for conversion of the major portion of the NO x and a portion of the CO and hydrocarbons. The output is contacted with the second stage to for conversion of CO and hydrocarbons. In one system, the first stage catalyst consists essentially of rhodium and a zeolite; the second stage catalyst comprises a noble metal including platinum and/or palladium, and a support. The mixture has a mole ratio of reducing to oxidizing agents of at least 0.8. Low ammonia is generated. In another system, the first stage substrate is made of material capable of being heated to at least 150 o C in no more than about 20 seconds, has heating means for doing same, and is at this temperature at least part of the time of contact with the mixture. The first stage catalyst comprises a support and rhodium with optionally platinum and/or palladium. The second stage catalyst comprises a support, and platinum and/or palladium, and optionally rhodium. Most of the rhodium is in the first stage; most of the platinum and/or palladium is in the second stage. At least 70% by volume of the NO x , CO, and hydrocarbons is converted. The weight ratio of Pt and/or Pd:Rh in each system is at least about 15:1. |