Molecule sieving catalysts for NO reduction with hydrocarbons in exhaust of lean burn gasoline and diesel engines

NO oxidation catalysts with small pores and low hydrocarbon oxidation activ ity owing to molecular sieving were prepared by dispersing platinum in the pores of ferrierite and chabazite zeolites. These small pore platinum zeoli te crystals were physically mixed with large pore silver mordenite zeolite crystals and evaluated as HC-SCR catalysts for selective catalytic reductio n of NO in a synthetic gas mixture doped with octane or isooctane, mimickin g exhaust from lean burning engines with unburned hydrocarbons. A synergeti c effect on Nz formation and hydrocarbon efficiency was obtained in these p hysical zeolite mixtures. In the pores of the small pore zeolite where the diffusion of NO, O-2 and NO2 molecules with small kinetic diameters was rap id, NO was effectively oxidized by the platinum into NO2, while the small w indow apertures suppressed the diffusion and reaction of the hydrocarbon. T he silver plated large pore zeolite catalyzed the reaction between the NO2 formed in the small pore zeolite and the hydrocarbon. The importance of mol ecular sieving was demonstrated in experiments with permutation of pore siz es and catalytic functions. (C) 2001 Elsevier Science B.V. All rights reser ved.