There are millions of vehicles on roads today. Majority of
them are run by internal combustion engines which become a major contributor to
the air pollution. Especially in cities, where huge traffic is unavoidable can
result in poor air quality that can affect not just the environment, but even
our health.
To solve this problem, the governments of different
countries around the globe have taken necessary steps to implement stricter
emission norms for vehicles. Catalytic converter is one of the devices that
could lessen the intensity of harmful emissions coming out of a vehicle.
What is a Catalytic
Converter?
Catalytic converter is a device that converts harmful toxic
emission gases from a vehicle into less harmful emission gases. The process is
achieved either through oxidation or reduction. Catalytic converters can be
used to treat exhaust gases of both gasoline and diesel engines.
History of Catalytic
Converters:
Eugene Houdry, a French Mechanical Engineer invented
catalytic converter. He was awarded a patent by the U.S. Government for his
research on developing catalytic converters for a gasoline engine.
Catalytic converters were a failure then due to the presence
of tetraethyl lead in gasoline. Presence of lead in gasoline poisoned the
catalytic converters and disabled it from treating the exhaust gases. U.S. Government
started phasing out the use of leaded gasoline in the mid-1970s because of its
neurotoxicity and its degrading effect on catalytic converters. In 1996, U.S.A
completely banned the use of leaded gasoline.
The first catalytic converter for production was developed by
engineers such a John J. Mooney and Carl D. Keith at the Engelhard Corporation.
3-way Catalytic
Converters:
A 3-way catalytic converter is used to treat the 3 main
harmful gases coming out of exhaust such as the nitrogen oxides (NOX),
hydrocarbons (HC) and carbon monoxide (CO) emissions.
There are 2 different catalysts at work: one is the
reduction catalyst and the other is the oxidation catalyst. The reduction
catalyst is made of platinum and rhodium. The oxidation catalyst is made of
platinum and palladium.
Alumina Coating:
The catalysts are in the form of thousands of micro ducts
that resemble the form of a honeycomb. The gases pass through these catalyst
ducts. The honeycomb structure is applied with a coat of alumina (aluminum
oxide). Alumina is highly porous which greatly increases the surface area of
the structure. The alumina coat carries the precious catalyst materials which
help in oxidation and reduction. This design maximizes the surface area of the
catalyst, thus giving it more gases to react on.
Catalytic converter is installed between exhaust pipe and muffler.
It works best when it is heated and can give efficiency up to 90%. As a result,
we get less harmful toxic gases from the tail pipe.
Working of a
catalytic converter:
The exhaust gases from the engine are first sent through the
reduction catalyst. The reduction catalyst (platinum + rhodium) will try to
eliminate NOX as much as possible.
Reaction at reduction
catalyst:
When NOX (NO or NO2) molecules pass
through the contact area, the catalyst breaks the bond between nitrogen and
oxygen atoms. Since, it is a reduction reaction; the catalyst will try to get
rid of oxygen atoms and would attract the nitrogen atoms. The individual oxygen
atoms (O) pair up with other oxygen atoms (O) to form oxygen molecule (O2).
The nitrogen atoms (N) stick to the catalyst surface unless they find other
nitrogen atoms (N) to bond with to form nitrogen molecules (N2).
2 NO → N2 + O2
(or)
2
NO2 → N2 + 2O2
The reduction catalyst won’t eliminate the CO and HC
molecules.
Reaction at oxidation
catalyst:
The oxidation catalyst is another honeycomb shaped structure
made of platinum and palladium. Since, it is an oxidation reaction; the
catalyst will try to attract the oxygen atoms. The CO bond is strong enough to be
split by the catalyst. Thus CO molecule as a whole will be attracted towards
the catalyst surface. The catalyst surface will also attract the oxygen
molecules (O2). Now, since O2 molecules’ bonds are weaker
than the CO bond, the O2 is split into individual oxygen atoms (O).
The oxygen atoms will bond with the CO molecules to form CO2.
2CO + O2 → 2CO2
The hydrocarbon (HC) molecules also get treated by the
oxidation catalyst. When HC molecules come in contact with the catalyst
surface, they are split into hydrogen (H) and carbon (C). Both hydrogen and
carbon bond with oxygen to form water vapor (H2O) and carbon dioxide
(CO2).
C8H18 + 17O2 →
8CO2 + 18H2O
(or)
CH4 + 2O2 →
CO2 + 2H2O
Etc….
Related Topics:
Precious metal catalysts are also termed “noble metal catalysts”, showing unique characteristics in our lives. They possess an incompletely filled d-orbital to lend electrons or withdraw electrons from the reagent, precious metal catalyst
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