Air pollution generated from mobile sources is a problem of general interest. Vehicle population is projected to grow close to 1300 million by the year 2030. Due to incomplete combustion in the engine, there are a number of incomplete combustion products CO, HC, NOx, particulate matters etc. These pollutants have negative impact on air quality, environment and human health that leads in stringent norms of pollutant emission. Numbers of alternative technologies like improvement in engine design, fuel pretreatment, use of alternative fuels, fuel additives, exhaust treatment or better tuning of the combustion process etc. are being considered to reduce the emission levels of the engine. Among all the types of technologies developed so far, use of catalytic converters based on platinum (noble) group metal is the best way to control automotive exhaust emissions. This review paper discusses automotive exhaust emissions and its impact, automotive exhaust emission control by platinum (noble) group metal based catalyst in catalytic converter, history of catalytic convertor, types of catalytic convertor, limitation of catalytic convertor and also achievements of catalytic convertor.
Keywords- Automotive emission, Catalytic Converter, Catalyst, Perovskite, Platinum group metal
Introduction
Air pollution generated from mobile sources such as automobiles contributes major air quality problems in rural as well as urban and industrialized areas in both developed and developing countries. About 50 million cars are produced every year and over 700 million cars are used worldwide. Vehicle population is projected to grow close to 1300 million by the year 2030 . Most vehicular transportation relies on combustion of gasoline, diesel and jet fuels with large amount of emission of carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx) and particulates matter (PM) are especially concern . HC and CO occur because the combustion efficiency is less than 100%. The NOx is formed during the very high temperatures (>1500 0C) of the combustion process resulting in thermal fixation of the nitrogen in the air which forms NOx. Typical exhaust gas composition at the normal engine operating conditions are: carbon monoxide (CO, 0.5 vol.%), unburned hydrocarbons (HC, 350 vppm), nitrogen oxides (NOx, 900 vppm) hydrogen (H2, 0.17 vol.%), water (H2O, 10 vol.%), carbon dioxide (CO2, 10 vol.%), oxygen (O2, 0.5 vol.%) .Carbon monoxide is a noted poison that has an affinity for hemoglobin in the blood 210 times greater than the oxygen affinity prolonged exposure to levels above 9 ppm can lead to reduce mental acuity for some individuals. HC and NOx lead to photochemical smog in presences of sunlight give secondary pollutants like ozone, nitrogen dioxide & peroxyacyl nitrate which cause also global environmental problems . As the emission standards were tightened, more advanced control strategies were applied that included modifications in engine design and fuel system, control of engine parameters and use of sophisticated exhaust after treatment devices . Reduction of toxic substances emission from combustion engines can be achieved by primary (inside engine) measure and secondary (outside engine) measures. As primary measures many different possibilities and technical methods of reducing exhaust gas emission are used e.g. combustion of lean air fuel mixture, multistage injection fuel, exhaust gas recirculation, fuel gas after burning, loading of additional water into cylinder volume . Nowadays secondary measures, in automotive exhaust aftertreatment processes a range of advanced technology is applied based on oxidation and threeway catalyst adsorption storage and filtration process . This enables reduction of the carbon monoxide (CO), hydrocarbons (HC), nitrogen oxide (NOx) and particulate emissions from a gasoline or diesel engine to meet the demands of current and future exhaust emission regulations . This review paper discusses automotive exhaust emissions and its impact, automotive exhaust emission control by platinum (noble) group metal based catalyst in catalytic converter, history of catalytic convertor, types of catalytic convertor, limitation of catalytic convertor and also achievements of catalytic convertor.
CATALYTIC CONVERTER
The pollutants have negative impact on air quality, environment and human health that leads in stringent norms of pollutant emission. Numbers of alternative technologies like improvement in engine design, fuel pretreatment, use of alternative fuels, fuel additives, exhaust treatment or better tuning of the combustion process etc. are being considered to reduce the emission levels of the engine. Out of various technologies available for automobile exhaust emission control a catalytic converter is found to best option to control CO, HC and NOx emissions from petrol driven vehicles while diesel particulate filter and oxidation catalysts converter or diesel oxidation catalyst have so far been the most potential option to control particulates emissions from diesel driven vehicle. A catalytic converter (CC) is placed inside the tailpipe through which deadly exhaust gases containing unburnt fuel, CO, NOx are emitted . The function of the catalytic convertor is to convert these gases into CO2, water, N2 and O2 and currently, it is compulsory for all automobiles plying on roads in US and Japan to have catalytic converters as they use unleaded petrol. In India, the government has made catalytic converters mandatory for registration of new cars.
Fig. 1. Catalytic Converter Position in a Vehicle
HISTORY OF CATALYTIC CONVERTER
The catalytic converter was invented by Eugene Houdry, a French mechanical engineer who lived in the United States . The until the extremely effective anti-knock agent tetra-ethyl lead was eliminated from most gasoline over environmental concerns, it would “poison” the converter by forming a coating on the catalyst’s surface, effectively disabling it. The catalytic converter was later on further developed by John J. Mooney and Carl D. Keith at the Engelhard Corporation creating the first production catalytic converter in 1973. Beginning in 1979, a mandated reduction in NOx required the development and use of a three way catalyst for CO, HC and NOx abatement .
TYPES OF CATALYTIC CONVERTER
1) The oxidization catalytic converter
An oxidation catalyst is a device placed on the tailpipe of a car. The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas.
HC + O2 → CO2 H2O (1)
2CO + O2 → CO2 (2)
2) The reduction catalytic converter
A reduction catalyst to control NOx can be used as a separate system in addition to the oxidation catalytic converter. The reduction catalyst is fitted upstream of the oxidation system. The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to reduce the nitrogen oxide emissions. When such molecules come in contact with the
catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst forming N2.
2NO N2 + O2
3) The three-way catalytic converter (TWCs)
TWCs have the advantage of performing the oxidation of carbon monoxide (CO), hydrocarbons (HC) and the reduction of nitrogen oxides (NOx) simultaneously. Noble metals are usually used as the active phase in TWCs. Pd catalysts are especially attractive since Pd is by far the cheapest noble metal in the market and has better selectivity and activity for hydrocarbons. Rhodium the other essential constituent of three-way catalysts is widely recognized as the most efficient catalyst for promoting the reduction of NO to N2. The TWCs performance in the emission control can be affected by operating the catalyst at elevated temperatures (> 600 °C). Reactions occurring on the automotive exhaust catalysts are very complex as listed below. The major reactions are the oxidation of CO and HC and the reduction of NOx. Also, water gas shift and steam reforming reaction occur. Intermediate products such as N2O and NO2 are also found. The NOx storage concept is based on incorporation of a storage component into the three-way catalyst (TWCs) to store NOx during lean conditions for a time period of minutes.
Reactions in Catalytic Converter
Oxidation
2CO + O2 → 2CO2 HC + O2 → CO2 + H2O Reduction/three-way 2CO + 2NO →
2CO2 + N2 HC + NO → CO2 + H2O + N2 2H2 + 2NO →