Catalytic NOx reduction in lean burn exhaust over Pt silica catalysts with controlled Pt particle size

Platinum-on-silica catalysts with 1.5 wt.% platinum and average metal particle diameters of <3, 3.5, 8.7 and 22.5 nm prepared by sol-gel processing of platinum organo(alkoxy)silane complexes were evaluated as lean DeNO(x) catalysts. The catalytic experiments were run with a synthetic exhaust gas containing 500 ppm NO, 175 ppm propane, 175 ppm propene, 350 ppm CO, 6% O-2, 10% CO2 and 12% water, with a volumetric space velocity of 60 000 h(-1) and at temperatures in the range 150-550 degrees C. Catalysts with platinum particle sizes of 3.5 nm and larger showed only little platinum metal sintering during catalytic testing, whereas significant sintering was observed in the catalyst with the smallest platinum particle size of <3 nm. The lightoff temperature of NOx conversion, propene and propane conversion, and NO into NO2 oxidation increases with increasing Pt particle size. The maximum NOx conversion reached at the optimum reaction temperature is 20-23% and does not depend on the Pt particle size. These catalysts convert NOx selectively into N2O. The absence of N-2 formation can be explained by a lack of sufficiently strong Bronsted acid sites on the silica support. (C) 1999 Elsevier Science B.V. All rights reserved.