Oscillation of NO_x Concentration in the Selective Catalytic Reduction of Nitrogen Oxides on Platinum-Containing Zeolite Catalysts

摘要

The emission of nitrogen oxides, carbon monoxide, and hydrocarbons from spark-ignition engines can nowadays be effectively reduced with the three-way catalyst. Since diesel and lean-burn engines operate with an excess of oxygen, conventional three-waycatalysts cannot be used for purifying exhaust gases from these engines. While oxidation catalysts, which are already being used in diesel engines, do reduce the amount of carbon monoxide and hydrocarbons in the exhaust gas, they do not bring about a reduction in the amount of nitrogen oxides. Since the discovery by Iwamoto et al. and Held et al. that the selective catalytic reduction of nitrogen oxides with hydrocarbons can be achieved in an oxidizing atmosphere (hydrocarbon selective catalytic reduction, HC-SCR), the potential of HC-SCR in the purification of exhaust gases from diesel and lean-burn engines has been intensively scrutinized. In addition to copper-containing zeolite catalysts favored earlier, platinum-containing catalysts are employedin modern research, because they are insensitive to water in the exhaust gas. However, the reaction also takes place on transition metal free catalysts (for example, on γ-Al_2O_3--see below).