Recent Trends in Catalyst Development for Diesel Engine Exhaust Emission Control

Abstract

The increased industrialization and particularly increased traffic in developing countries has been accompanied by a negative impact on air quality, the environment and human health. Diesel engines have both carbon monoxide and unburned hydrocarbon outlet concentrations much lower than those produced by spark-ignition engines (gasoline engines). But diesel engines emit large quantities of particulate matter (called PM) and nitrogen oxides (NOx), both precursors of photochemical smog. Growing alarms on health effect and pressure to fulfill the governmental directives for the reduction of pollutants emissions from diesel engines are paving the way for the development of tailored treatment systems. These cannot be accomplished by engine modifications, fuel pretreatments or simply better tuning of the combustion process; thus a convenient way of treating diesel off-gases is needed. Perovskite type oxides of general formula ABO3 where A and B are usually rare earth, alkali earth, alkali or other large ions and transition metal cation respectively are less expensive, thermally more stable, diverse physicochemical properties, increase thermal stability of the support, store and release oxygen under conditions fluctuating between oxidizing and reducing than noble metal used in catalytic converter to reduce emission from diesel engines. This paper discusses use and potential of perovskites based catalysts for simultaneous removal of soot and nitrogen oxides in a single, suitably catalyzed trap from diesel engine. The various synthesis methods like citric acid, co-precipitation, sol–gel, freeze drying, combustion synthesis, reactive grinding of catalysts are incorporated. Perovskite compounds can also tolerate significant partial substitution and non-stoichiometric while still maintaining the perovskites structure. Metal ions having different valence can replace both A and B ions. This may generate a non-integral number of oxygen atoms. It is explained the mechanism of generation of a non-integral number of oxygen atoms and doping of PGM in perovskites which work as highly active and clean catalyst and can also reduced 70 – 90 % amount of precious metals needed to meet new emission vehicle standards.