The Effect of an Oxidation Precatalyst on the NOx Reduction by Ammonia SCR

The performances of oxidation and SCR catalysts were investigated individually and as a combined system for the removal of NO x by ammonia. Experiments were carried out with synthetic gas mixtures matching the composition of diesel exhaust. Introducing an oxidation catalyst upstream of the SCR catalyst enhances the removal of NO x at low temperatures. The beneficial effects of combining the two catalysts are the oxidation of unburned hydrocarbons that inhibit the SCR reaction and the conversion of NO to NO 2 on the oxidation catalyst. NO 2 fractions up to 50% of total NO x enhance the NO x conversion at temperatures below 300 °C. However, excess NO 2 should be avoided because the reaction of NO 2 with ammonia is slow, resulting in a decreased NO x conversion. Above 350 °C, the NO x conversion essentially does not depend on the ratio NO 2 /NO x or on the presence of hydrocarbons in the feed. A simple mathematical prediction of the NO x conversion for the combined system is also possible. The parameters used in this calculation are the ratio NO 2 /NO x behind the oxidation catalyst and the experimentally determined conversions of pure NO or NO 2 over the SCR catalyst.