AUTOMOTIVE EXHAUST EMISSION CONTROL: OXIDATION OF CO ON PEROVSKITE CATALYSTS

Abstract

Air pollution generated from mobile sources is a problem of general interest. In the last 60 years the world vehicle fleet has increased from about 40 million vehicles to over 700million, this figure is projected to increase to 920 million by the year 2010. Due to incomplete combustion of petroleum derived fuels in automobile engines, the exhaust contains various pollutants like CO, HC and NOx. The reduction of these pollutants before entering into the environment is of greatest concerned. The reduction can be done either by engine modification, fuel modification or by exhaust modification. The current practice to eliminate these harmful gases is by using catalytic converters at the tail pipe of the exhaust using noble metals as catalysts. Since the availability and cost of these noble metals do not permit and the costs of these noble metals do not permit them to be used for a long time to come, it is desirable to look into equivalent catalysts which are cheaper and easily available. Perovskite is a combination of transition and rare earth metals, having the general formula ABO3, where A is a large cation and B is a small cation. A-cations are believed to be inactive. The partial substitution of A and B-site ions by other metal ions can modify the nature and concentration of defects in perovskite lattice. An A-site substitution by ions in lower valence favors the total oxidation of CO, whereas partial substitution at B-site reinforces the redox property of the perovskite. In the present study, several catalysts were prepared by the partial substitution of La3+ with Ag+, Ce4+, Sr2+ and Co3+ with Bi5+ in LaCoO3 using co precipitation and dry mixing methods and calcined at 800°C. The catalysts were characterized by using XRD, TGA and SEM. The catalytic activity tests were performed in a fixed bed micro size tubular reactor. These tests were conducted in the temperature range of 50-300°C at a constant space velocity of 198501f1 during all the tests. The exhaust gases from the reactor were analyzed by using a gas chromatograph integrated with the computer. From the results, the partial substitution of B-site ion with Bi5+ in LaCoO3 causes a slight increase in the catalytic activity of the catalyst, but the simultaneous substitutions of both cationic sites enhances the catalytic activity of perovskites for the CO oxidation. A