There are four possible cases of atmospheric pollution from the automobile. Without
emission control, a carburettor and fuel tank emit fuel vapour. The crankcase emits blow
by gases and fuel vapour, and the tailpipe emits exhaust gases that contain pollutants.
The pollutants are hydrocarbons (HC), carbon monoxide (CO), and nitrogen gases (NOx).
These air pollutants are harmful to plants and animals, including human being.
The need to control engine emissions was recognized as early as 1909. Due to the more
stringent rules and emission standards, automotive manufacturers begun to develop a
treatment device for exhaust gases known as catalytic convertor for their vehicle models.
The catalyst convertor converts the
harmful pollutants in the exhaust
gases into harmless gases. It is
located in the exhaust system and all
exhaust gas must flow through it. The
catalyst is a material in catalyst
converter that causes a chemical
change without being a part of the
chemical reaction. The result in
exhaust gas leaving the catalyst
converter contains less HC, CO, NOx .
TYPES OF CATALYTIC
1. The oxidization catalytic converter :-The
oxidation catalyst is the second stage of catalyst
converter. It reduces the unburned hydrocarbon
and carbon monoxide by burning (oxidizing) them
over a platinum palladium catalyst.
HC + O₂ → CO₂ + H₂O
2CO + O₂ → 2CO₂
2. The reduction catalytic converter :-
The reduction catalyst is the first stage of the catalytic
converter. It uses platinum and rhodium to reduce
nitrogen oxide emissions.
2NO → N2 +O2
3. The three-way catalytic converter (TWC):-
TWCs have the advantages of performing the oxidation of CO, HC and the reduction of
nitrogen oxides (NOx) simultaneously. Noble metals used as the active phase in TWCs. Pd
are generally used.
The major reactions are the oxidation of CO and HC and reduction of NOx. Also, water
gas shift and steam reforming reaction occur.
2CO + O2 → 2CO2
Oxidation HC + O2 → CO2 + H2O
Reduction/three-ways 2CO + 2NO → 2CO2 + N2
HC + NO → CO2 + H2O + N2
Water gas shifting CO + H2O → CO2 + H2
Steam reforming HC + H2O → CO2 + H2
Component of catalytic converter:-
These include oxides of base metals e.g. copper, chromium, nickel, cobalt etc. and the
noble metals platinum (Pt), palladium (Pd) and rhodium (Rh). Base metal oxides
although found to be effective at higher temperature but they sinter and deactivate
when subjected to high-end exhaust gas temperature of conventional SI (Spark-
Ignition) engine operation.
b) Substrate or support:-
i) Pellets :- The first catalytic converters of passenger cars in early 1970s used a bed of
spherical ceramic pellets. These are also known as packed bed catalytic converter. The spherical
pellets made of γ-alumina (γ – Al2O3). The material of pellets is selected to have a high
mechanical strength against crush and abrasion.
ii) Monolith:-In our daily practice a monolith is a ceramic block consisting of a large number of
small straight and parallel channels. A special mixture of clay binders and additives is pushed
through a sophisticated dye to create the monolith structure.
iii) Washcoat: -A thin layer of inorganic oxides known as wash coat is applied to the cells in
monolith structure to increase effective surface area for dispersion of active catalyst that
increase its contact with the reacting gases.
iv) Air to fuel ratio :- Conversion efficiency of NO, CO and HC as a function of the air - fuel
in a three way catalytic converter. There is a narrow range of air - fuel ratio near
stoichiometry in which high conversion efficiencies for all three pollutants are achieved.
EFFECT OF COLD START EMISSION ON THE
EFFICIENCY OF CATALYTIC CONVERTER :-
While a catalytic converter is running under the ideal
operating conditions, it is very successful at purifying exhaust:
collecting and eliminating 97% of toxins including
hydrocarbons, nitrogen oxides, and carbon monoxide. But the
issue of cold-start emissions is that automobiles do not always
meet these conditions. An obvious solution to prevent cold-start
emissions would be to make the engine hotter, as fast as
1) In the exhaust stream with temperatures up to 1000 °C the metal in
the catalyst is prone to deactivation by sintering, leading to a reduction
in surface area and hence catalytic activity.
2) Platinum used as catalyst have high cost and also vaporized at high
3) Palladium also used as catalyst with the platinum having less ability of
absorption and desorption of oxygen.
4) Three – way catalytic converter CO2, which is also harmful for the
MODIFICATION IN CATALYTIC CONVERTER:-
1) Replacement of Platinum and Palladium:-
Due to high cost we can use another material as catalyst in
catalytic converter We can use Perovskite Oxide(CaTiO3) due to low
costs, high thermal stability, better redox properties.
2) Development of four – way catalytic converter:-
In this converter we use high alunimium content Zeolite which
absorb all the CO2 produced in three – way catalytic converter.
APPLICATION OF CATALYTIC CONVERTER :-
1) A catalytic converter is a device used to reduce the toxicity of emissions
from an internal combustion engine.
2) Catalytic converters are most commonly used in motor vehicle exhaust
3) Catalytic converters are also used on generator sets, forklifts , mining
equipment, trucks , buses, trains, and other engine-equipped machines
4) A catalytic converter provides an environment for a chemical reaction
wherein toxic combustion by-products are converted to less-toxic
Three-way catalyst with stoichiometric engine control systems remain the state of art
method for simultaneously controlling hydrocarbon, CO and NOx emissions from
vehicle. Due to high cost of platinum and operating limitation platinum based catalytic
converter motivates the investigators of alternative catalyst material, which have low
coast and provide high efficiency. catalytic converter is best option for reducing the
pollutant. It is essential for investigation and modification in catalytic to increase the
efficiency of catalytic converter.
1) P. Pundir, Engine emissions pollutant Formation and Advances in Control
Technology, Narosa Publishing house, New Delhi, Chapter 1, pp. 1-10.