Catalytic properties of La(0.8)A(0.2)Mno(3) (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 oxy gen and water vapor was investigated over perovskite-type mixed oxides La(0 .8)A(0.2)MnO(3) (A = Sr, Ba, K, Cs) and LaMn(0.8)A(0.2)MnO(3) (B = Ni, Zn, Cu) with propene as the reductant. in the temperature range of 150-550 degr eesC. The catalysts were contacted with a reactant mixture containing 1000 ppm NO. 500 ppm C3H6, 5% O-2 and 5% H2O in helium at a space velocity of 14 .4 cm(3)/g s. A maximum was observed in the conversion of nitrogen oxides t o nitrogen with temperature in the range of 300-400 degreesC. In the absenc e of propene, the activity of these perovskites for NO removal in the 300-5 50 degreesC range increased from approximately 20% (with propene) to 40% (w ithout propene). Water inhibited the NOx removal process as well, the conve rsion 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 ox ygen. Two different reaction mechanisms were postulated: (i) the selective reduction with propene at 150-300 degreesC, (ii) the direct decomposition. which is inhibited by the presence of propene and water, at 300-550 degrees C. The nitrogen dioxide formed by the perovskite-catalyzed oxidation of nit rogen monoxide appears to play a key role in both mechanisms. (C) 2001 Else vier Science B.V. All rights reserved.