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

摘要

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% CO_2 and 12% water, with a volumetric space velocity of 60000 h~(-1) and at temperatures in the range 150-550 deg 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 NO_x conversion, propene and propane conversion, and NO into NO_2 oxidation increases with increasing Pt particle size. The maximum NO_x conversion reached at the optimum reaction temperature is 20-23% and does not depent on the Pt particle size. These catalysts convert NO_x selectively into N_2O. The absence of N_2 formation can be explained by a lack of sufficiently strong Bronsted acid sites on the silica support.