High-temperature catalytic reduction of nitrogen monoxide by carbon monoxide and hydrogen over La 1− xSr xMO 3 perovskites (M = Fe, Co) during reducing and oxidising conditions

Abstract

The catalytic activity of La 1− x Sr xMO 3 perovskites (M = Fe or Co) was investigated for the reduction of nitrogen monoxide. The catalytic activity for the NO+CO and the NO+H 2 reaction was investigated in a fixed bed reactor in the temperature interval 873–1223 K. The perovskites showed high activity for the reduction of nitrogen monoxide in the absence of oxygen even at 873 K. Hydrogen was less active than carbon monoxide as a reducing agent and the cobalt perovskites showed slightly higher activity than the iron perovskites in the lower temperature range. The reduction of nitrogen monoxide was concluded to proceed through the reduction of the surface by the reducing agent followed by adsorption and decomposition of nitrogen monoxide on the surface. The activated (reducing) perovskites were investigated by X-ray diffractometry. The cobalt perovskites were reduced to a compound of the K 2NiF 4 type, La 2CoO 4, while the iron perovskites retained their perovskite structure upon reduction. The perovskite structure of the cobalt perovskite was restored upon reoxidation of the La 2CoO 4 compound. The introduction of excess oxygen into the gas mixture decreased the nitrogen monoxide reduction level to zero. Nitrogen monoxide (1000 ppm) was completely reduced in a stoichiometric CO+O 2 mixture (2% CO and 1% O 2) over La 0.8Sr 0.2CoO 3 at 1173 K. LaCoO 3 showed high thermal stability in a helium atmosphere at 300–1223 K, while the perovskite was reduced to La 2O 3 and cobalt metal in a 10% CO+ He atmosphere.