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.
The last part of the converter is a control system that controls the fuel-injection system. This system is aided by an oxygen sensor that monitors how much oxygen is in the exhaust stream. This sensor in turn tells the engine computer to adjust the air-to-fuel ratio, thus keeping the catalytic converter running at the stoichiometric point and near 100% efficiency.
Functions
- A three-way catalytic converter has three simultaneous functions: Reduction of nitrogen oxides into elemental nitrogen and oxygen. (NOx → Nx + Ox)
- Oxidation of carbon monoxide to carbon dioxide. (CO + O2 → CO2)
- 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.
- 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.
- 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.
- Reduction of nitrogen oxides into elemental nitrogen and oxygen. (NOx → Nx + Ox)
- Oxidation of carbon monoxide to carbon dioxide. (CO + O2 → CO2)
- Oxidation of hydrocarbons into carbon dioxide and water. (CxH4x + 2xO2 → xCO2 + 2xH2O)
Catalytic converter theft
Due to the use of precious metals in the coating of the inner ceramic structure, many catalytic converters have been targeted for theft. The converter is for the most part easy to get to, because it lies on the outside and under the car. A thief could easily slide under the car, saw the connecting tubes on each end, and make off with the catalytic converter. Depending on the type and amount of precious metals inside, a catalytic converter can be easily sold for $200 dollars a piece.
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.
References
- Timberlake, Karen C. Chemistry : An Introduction to General, Organic, and Biological Chemistry. 10th ed. Upper Saddle River: Prentice Hall Higher Education, 2008.
- Petrucci, Ralph H., William S. Harwood, and Geoff E. Herring. General Chemistry : Principles and Modern Applications. 9th ed. Upper Saddle River: Prentice Hall, 2006.
