Catalytic Converter
A catalytic converter is a device used to
reduce the emissions from an internal combustion engine. Most commonly used in
an automobile's exhaust system, catalytic converters are now commonly used on
generator sets, forklifts, mining equipment, trucks, buses, trains, and other
machines that have engines to provide an environment for a chemical reaction
where unburned hydrocarbons are more completely combusted.
A catalytic converter is a device used to
reduce the harmful emissions from an internal combustion
engine (used in most modern day automobiles and vehicles). There is not enough
oxygen available to oxidize the carbon fuel used in these engines completely
into carbon dioxide and water, thus toxic by-products are produced. Catalytic
converters are used in exhaust systems to provide a place for the oxidation and
reduction of
toxic by-products (like nitrogen oxides, carbon monoxides, and hydrocarbons) of fuel into less
hazardous substances for the environment such as carbon
dioxide, water vapor, and nitrogen gas.
Introduction
Catalytic
converters were first widely introduced in American production cars in 1975 due
to EPA regulations on toxic reductions. The United States Clean Air Act
required a 75% decrease in emissions in all new model vehicles after 1975. This
decrease was to be carried out with the use of catalytic converters. Without
catalytic converters vehicles would release hydrocarbons, carbon monoxide, and
nitrogen oxide. These gases are the largest source of ground level ozone, which
causes smog and is harmful
to plant life. Catalytic converters can also be found in generators, buses,
trucks, and trains— almost everything with an internal combustion engine will
have some sort of catalytic converter attached to its exhaust system.
How does it work?
A catalytic
converter is a very simple device using the basic redox
reactions in chemistry to help reduce the pollutants a car makes. It converts
around 98% of the harmful fumes produced by a car engine into less harmful
gases. It is composed of a metal housing that has a ceramic honeycomb-type
interior with insulating layers. This honeycomb interior has thin wall channels
that are coated with a washcoat of aluminum oxide. This is very porous and
increases the surface area, which allows for more reactions to take place. This
is where the precious metals are located. These metals include platinum,
rhodium, and palladium. No more than 4-9 grams of these precious metals are
used in a single converter. The converter utilizes simple oxidation and
reduction reactions to convert toxic fumes into gases that are not nearly as
harmful to the environment. Recall that oxidation is the loss of electrons and
that reduction is the gaining of electrons. These precious metals listed
earlier promote the transfer of electrons and in turn the conversion of toxic
fumes.
Functions
1.
A three-way catalytic converter has
three simultaneous functions: Reduction of nitrogen oxides into elemental
nitrogen and oxygen. (NOx → Nx + Ox)
2.
Oxidation of carbon monoxide to
carbon dioxide. (CO + O2 → CO2)
3.
Oxidation of hydrocarbons into
carbon dioxide and water. (CxH4x + 2xO2 →
xCO2 + 2xH2O)
There
are two types of "systems" that run in the catalytic converter-
"lean" and "rich". When the system is running
"lean", that means there is more oxygen than required, thus the
reactions favor the oxidation of carbon monoxide and hydrocarbons (at the
expense of the reduction of nitrogen oxides). On the contrary, when the system
is running "rich", meaning there is more fuel than needed, the
reactions favor the reduction of nitrogen oxides into elemental nitrogen and
oxygen (at the expense of the two oxidation reactions). With a constant imbalance
of the reactions, the system is never running at 100% efficiency. Note:
converters can store "extra" oxygen in the exhaust stream for later
use. This storage usually occurs when the system is running lean; the gas is
released when there is not enough oxygen in the exhaust stream. The released
oxygen compensates for the lack of oxygen derived from the reduction of NOx,
or when there is hard acceleration and the air-to-fuel ratio system runs rich
suddenly, faster than the catalytic converter can adapt to it. Also, the
release of the stored oxygen helps in the oxidation process of CO and HC.
Dangers of pollutants
Without the redox
process to filter and change the nitrogen oxides, carbon monoxides, and
hydrocarbons into less harmful chemicals, the air quality (especially in large
cities) would reach a harmful level to the human being.
Nitrogen
oxides- these compounds are in
the same family as nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and
nitric oxide. When NOx is released into the air, it reacts with
organic compounds in the air and sunlight, the result is smog. Smog is a pollu
tant and has adverse effects on children's lungs. NOx reacting with
sulfur dioxide produces acid rain, highly destructive to everything it lands
on. Acid rain deteriorates cars, plants, buildings, national monuments and
pollutes lakes and streams to such an acidity that is unsuitable for fish. NOx
can also bind with ozone to create biological mutations, and reduce the
transmission of light (like smog).
Carbon
monoxide- this form of CO2 is a
harmful variant of a naturally occurring gas. Odorless and colorless, this gas
does not have many useful functions in everyday processes.
Hydrocarbons- inhaling hydrocarbons from gasoline, household cleaners,
propellants, kerosene and other fuels can cause death in children. Further
complications can be central nervous system impairments and cardiovascular
problems.
Catalytic inhibition and destruction
The
catalytic converter is a sensitive device with precious metals coating the
inside. Without these metals, the redox reactions would not occur and the whole
module would be useless. There are several substances and chemicals that
inhibit the catalytic converter from doing its job.
1.
Lead- most (if not all) vehicles run
on unleaded gasoline, meaning all the lead has been taken out of the fuel.
However, if lead is to somehow get inside the fuel and is burned, it leaves a
residue that coats the catalytic metals (Pt, Rh, Pd, and now even Au) and
prevents them from contacting the exhaust fumes and performing the necessary
redox reactions.
2.
Manganese and silicon- manganese is
primarily found in the organometallic compound MMT (methylcyclopentadienyl
manganese tricarbonyl). MMT is a compound used in in the 1990's to up the
fuel's octane rating (a higher octane rating means the gas is less likely to
combust, thus causing the engine to explode. This is important since higher
performing engines have a high compression ratio, which would need a higher
octane gas to complement the amount of compression in the engine), and has now
been banned from commercial sale due to the EPA's regulations. Silicon can be
leaked from the combustion chamber into the exhaust stream from the coolant
inside the engine.
These
contaminants prevent the catalytic converter from doing its job. However, this
process could be reversed by running the engine at a constantly high
temperature to increase the hot exhaust flow through the converter, thus
melting or liquefying some of the contaminants and out of the exhaust pipe.
This process does not work if the metal is coated with lead, because lead has a
high boiling point. If lead poisoning is severe enough, the whole converter is
rendered useless and a new one is needed.
Thermodynamics of catalytic converters
Recall
that thermodynamics tells us whether or not a
reaction or process is spontaneous under certain conditions, but NOT how fast
or at the rate of which the process is going. The redox reactions below occur
rather slowly without the catalyst, so even if the processes are
thermodynamically favorable, they cannot occur without proper energy. This
energy refers to the activation energy needed
to overcome the initial barrier preventing the process from happening. A
catalyst aids in the thermodynamic process by lowering the activation energy
required for the reaction to start. However, the catalyst itself does NOT
produce a product, but it does affect the AMOUNT and the speed at which the
products are formed.
Global Warming
Though the
catalytic converter has helped reduce toxic emissions from car engines, it has
also done its part in harming the environment. During the conversion of hydrocarbons
and carbon monoxide, carbon dioxide is produced. Carbon dioxide is one of the
most common greenhouse gases and contributes immensely to global warming. Along
with carbon dioxide, the converters sometimes rearrange the nitrogen-oxygen
compounds to form nitrous oxide. This is the same stuff used for laughing gas
and in cars as a speed boost. Nitrous oxide is much more potent than carbon
dioxide as a greenhouse gas. It is 300 times more potent and thus contributes
to global warming that much more.
Catalytic converter theft
Demand
for precious metals and parts that contain them is rising
If you run a car you need to be
aware of the risk of catalytic converter theft, particularly if your car has
large ground clearance. Thieves are targetting catalysts – located in the
exhaust system under the vehicle – because of the high value of the precious
metals they contain.
In these harder times and with
precious metal prices rising steadily, the demand for parts that contain them
is rising. As a result, the risk of theft of catalytic converters is on the
increase again.
On diesel cars, the diesel oxidation
catalyst is often integrated with the Diesel Particulate Filter (DPF) and there is anecdotal evidence
that these are being stolen for their scrap value too.
Catalytic converters (CATs) have
been fitted in the exhaust of the majority of petrol cars manufactured since
1992 and diesel cars since 2001. By chemical reaction CATs substantially
reduce harmful pollutants from the exhaust.
The metal case of the CAT contains a
ceramic honeycombed structure providing a massive surface area across which the
exhaust gases flow.
Precious metals – platinum,
palladium and rhodium – are coated onto this ceramic structure as catalysts (a
catalyst modifies and increases the rate of a chemical reaction without being
consumed itself) for the reactions that 'clean' the exhaust:
- Oxidising (adding oxygen) carbon monoxide and unburnt hydrocarbons to form carbon dioxide and water, and
- Reducing (remove oxygen) from other gases such as oxides of nitrogen.
Theft
The
start of the credit crunch in 2008 and high prices for precious metals resulted
in a noticeable increase in thefts of catalytic converters. Thieves simply cut
the catalytic converter from the exhaust pipe of a parked car and sell them on
to scrap metal dealers.
Taller vehicles (4x4s) are
particularly vulnerable as the converters are more accessible. Because they
tend to have larger engines, they contain more of the precious metals too.
The ceramic honeycomb has to undergo
a chemical process known as 'carbochlorination' to recover the precious metals
it contains – generally abroad in countries such as China, Poland, Canada and
Latvia.
Once an unmarked converter has been
removed from a vehicle it's quite difficult to match it to that vehicle as
there aren't any distinguishing marks.
Reduce
risk
To reduce the risk of theft of your
car's catalytic converter:
- Garage your car whenever possible
- Park in well-lit busy areas
- Look out for people working under cars
- If the car's high risk consider marking the metal shell of the converter with a unique mark, so that if it is removed by thieves it will be easier to trace back to your vehicle
- If you operate a small fleet, consider obstructing acess to vehicles with high ground clearance by parking lower vehicles close by
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