Catalytic Converter

Catalytic converters are an excellent technology that helps the environment reduces the harmful chemicals, especially the ones coming out of motor vehicles. The catalytic converter is positioned halfway between the exhaust stream and the exhaust pipe. This device may not seem to be much, however, it is in fact a very technologically complex system which also happens to cost quite a lot. The cost is high due to the uses of the precious metals working inside of the catalysts such as platinum and palladium. For the cost issue, most manufacturers use some optimising techniques to bring the cost low. The catalytic converter has two main catalysts. One is the reduction catalyst while the other is the oxidation catalyst.

If we look at how an engine works to make the car move, we can say that the power for the wheels to move come straight from the explosion that occurs in the pistons. The cylinder which the piston is located in takes in oxygen and petrol and ignites them by a spark via the spark plug. For a car that is doing around 6000 rounds per minute; the time for the piston to take in the oxygen and petrol to when it pushes the burnt waste out, takes approximates 5 milliseconds. How can the car gain so much power within such a little time to make it move? Well, because an engine has 4 strokes in each round, it allows it to accelerate some reactions. For example as it takes in the oxygen and petrol in the first stroke, it compresses the gases in the second stroke. The compression increases which means the gases are moving at a higher pressure while also having a smaller volume to move around in. This makes the chances of the gases to collide perfectly higher. Also because the gases enter at such a high speed, increases the chances of a successful collision between the oxygen and the petrol. After being compressed the spark plug sparks, which makes the gases combust and create a mechanical force to push down the piston and make the car move. However, if the piston did not compress the gases, the reaction rate would have been slower and the mechanical power that the explosion does would have been weaker.

A three-way catalytic converter does three things.
1. Oxidation of unburnt hydrocarbons HC to carbon dioxide and water: C_xH_2x+2+ [(3x+1) / 2]O₂
= xCO₂ + (x+1)H₂O
2. Oxidation of carbon monoxide to carbon dioxide: 2CO + O₂ = 2CO₂
3. Reduction of nitrogen oxides to nitrogen and oxygen: 2NO_x  = xO₂ + N₂

These reactions occur most efficiently when the catalytic converter receives exhaust from an engine running with approximately 15 parts air to 1 part petrol. Cars that are fitted with a three-way catalytic converter are equipped with a computerised close loop feedback fuel injection system using one or more oxygen centres. These oxygen centres are where the catalytic converter stores oxygen from the exhaust stream when the air to fuel ratio is lean. Because of this, when the air to fuel ratio is rich, the catalytic converter releases some of the oxygen stored to make the ratio just right. How can the exhaust gases be rich or lean? It is because even though cars have a computerised fuel and air injection system, the procedure takes an extremely little time, which means there are chances of mistakes from the computer. A rich ratio is when there is an excess fuel to the available oxygen, while lean means there is excess oxygen to the available fuel. A place where the catalyst may not be very efficient is when for example the car doing certain manoeuvers such as a hard acceleration. During this manoeuver, nitrous oxide escapes the catalytic converter. Note: nitrous oxide is 300 times more dangerous than carbon dioxide.

A two way catalytic converter only does the first two things that the three way catalytic converter does. This means it does not separate nitrogen oxides. Two way catalytic converters are widely used on diesel engines, it was also used on gasoline engines too until 1981 which was then replaced by the three way converter.

A three way catalytic converter has two different types of catalysts at work. One is a reduction catalyst and the other is an oxidation catalyst. Both of these catalysts have a ceramic structure coated with metals. Usually these metals are platinum, rhodium and palladium. These metals are extremely expensive, that is why newer converters use a gold mix in the catalyst as well instead of the metals states before. Gold is actually increases oxidation, the reaction that reduces pollution, by 40%. The first phase, the reduction catalyst separates the nitrogen oxides to become nitrogen and oxygen. Once this compound is separated, it becomes non-toxic. The oxidation catalyst which is the second phase of the catalytic converter reduces the unburnt hydrocarbons and carbon monoxide by burning them over a platinum and palladium catalyst. These catalysts aid at reducing the toxic gases such as the carbon monoxide by mixing it with oxygen to create carbon dioxide. Carbon dioxide is bad for the environment however; it is not as bad and deadly as carbon monoxide.

The centre of the catalytic converter is made up of a honeycomb structure for the gases to move from the first catalyst to the second. The honeycomb structure helps by exposing as much surface area possible. As we all know, the more surface area exposed, the higher the chances of a successful collision between the gas and the rhodium coating on the honeycomb passageway. This technique allows the catalytic converter to be more compact which ultimately means the less coating there needs to be which finally means the cost to manufacture it decreases. Therefore it’s a win-win situation both for the manufacturer and the customer who purchases the motor vehicle.

References:

1. https://en.wikipedia.org/wiki/Catalytic_converter#Types
2. http://www.howstuffworks.com/catalytic-converter2.htm
3. http://www.youtube.com/watch?v=LyFKVqLsPiA
4. http://www.youtube.com/watch?v=KIVdPaTSot8
5. http://www.youtube.com/watch?v=nbCe68ck6qg
6. http://www.youtube.com/watch?v=umdoG7qdWWs
7. http://www.youtube.com/watch?v=3wjFEUD8P7I
8. http://www.chytecs.com/cata-converters.html
9. http://answers.yahoo.com/question/index?qid=20081120032312AADaMIP
10. http://www.molecularsieve3a.com/honeycomb-ceramic-catalytic-converter.html
11. http://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio
12. http://www.chemguide.co.uk/physical/basicrates/pressure.html