Selective reduction of nitric oxide over noble metals

Abstract

Laboratory studies of the partitioning of CO between NO and O 2 under net lean conditions over the noble metals Ir, Rh, Pt, and Pd showed that over Ir alone is the CO + NO reaction favored over the CO + O 2 reaction. Furthermore, Ir exhibited a particle size dependence such that small Ir crystallites were more effective for reducing NO than large particles. Space velocity experiments showed that NO and O 2 react simultaneously with CO over Ir. NO reduction is positive order in NO concentration under the conditions studied. The NO reaction is enhanced by CO (CO is not inhibiting) but inhibited by competitive adsorption of oxygen. The conversion of oxygen was determined by CO availability and was slightly retarded by NO. A reaction mechanism is proposed for the reduction of NO in the presence of excess O 2 at 550 °C. We hypothesize that a different site requirement for NO and O 2 adsorption may be important in determining the selectivity of the catalyst when the number of vacant adsorption sites is limited. Ir differs from Rh in that Ir is able to maintain high NO conversion at elevated temperatures in the presence of excess oxygen. Rh, on the other hand, promotes NO reduction at lower temperatures than does Ir. Rh is expected to make a greater contribution to both CO oxidation and NO reduction during catalyst warmup than Ir. Iridium can be lost from the catalyst under our reaction conditions, presumably via volatilization.