Effect of sodium on the catalytic properties of iridium black in the selective reduction of NOx by propene under lean-burn conditions

The effect of sodium promotion on the performance of an iridium black catal yst for the selective catalytic reduction of NO by propene has been investi gated. Sodium loadings above 1 at% were found to increase the nitrogen sele ctivity close to 100% in the whole temperature range investigated (150-450 degreesC). However, the presence of Na shifted the on-set of all reactions to higher temperatures (ca. 20-50 degreesC for the highest promoter loading s, i.e., 10 at% Na), leading to a narrowing of the operation temperature wi ndow. Consequently, moderate sodium loadings in the range of 1-3 at% repres ent the best compromise for a beneficial application of iridium in DeNO(x) catalysis. Combined thermogravimetry-mass spectrometry investigations revea led that not only NO dissociation, but also, e.g., the decomposition of pro pene and the adsorption of oxygen as well as the oxidation of iridium are i nfluenced by the presence of sodium. Mechanistically, our observations are consistent with a model, where the removal of adsorbed oxygen could represe nt the rate-limiting step under the applied conditions.