Catalytic properties of La0.8A0.2MnO3 (A = Sr, Ba, K, Cs) and LaMn0.8B0.2O3 (B = Ni, Zn, Cu) perovskites: 2. Reduction of nitrogen oxides in the presence of oxygen

The selective catalytic reduction of nitrogen oxides in the presence of oxygen and water vapor was investigated over perovskite-type mixed oxides La0.8A0.2MnO3 (A=Sr, Ba, K, Cs) and LaMn0.8B0.2O3 (B=Ni, Zn, Cu) with propene as the reductant, in the temperature range of 150–550°C. The catalysts were contacted with a reactant mixture containing 1000 ppm NO, 500 ppm C3H6, 5% O2 and 5% H2O in helium at a space velocity of 14.4 cm3/g s. A maximum was observed in the conversion of nitrogen oxides to nitrogen with temperature in the range of 300–400°C. In the absence of propene, the activity of these perovskites for NO removal in the 300–550°C range increased from approximately 20% (with propene) to 40% (without propene). Water inhibited the NOx removal process as well, the conversion levels in the propene and water-free gas mixtures rising to about 50–55%. The removal of nitrogen oxides did not take place in the absence of oxygen. Two different reaction mechanisms were postulated: (i) the selective reduction with propene at 150–300°C; (ii) the direct decomposition, which is inhibited by the presence of propene and water, at 300–550°C. The nitrogen dioxide formed by the perovskite-catalyzed oxidation of nitrogen monoxide appears to play a key role in both mechanisms.