A three-way catalytic converter oxidises carbon monoxide and hydrocarbons, and reduces the levels of nitrogen oxides. It does this by creating the right conditions for reactions of these emissions to occur. The air to fuel ratio must be carefully controlled as oxygen is needed for the reaction of carbon monoxide and hydrocarbons. Oxygen sensors are fitted in engines with a three-way catalyst. Figure 3: The three-way catalyst system. xi Three-way catalysts are designed to produce more desirable reactions and less undesirable reactions.
Desirable reactions reduce the harmful emissions from the car exhaust. Carbon monoxide and hydrocarbons react with oxygen to produce carbon dioxide and water, and nitrogen oxides react with carbon monoxide to produce carbon dioxide and nitrogen. The products are already present in the Earths atmosphere. 2CO + O2 2CO2 C7H16 + 11O2 8H2O + 7CO2 2CO + 2NO 2CO2 + N2 xii Three-way catalytic converters contain the stoichiometric fuel mixture. This means that the catalysts have the right amount of oxygen to combust the fuel and react with the carbon monoxide and hydrocarbons.
Three-way catalysts also contain ceria, which stores oxygen when the mixture is lean, and releases it when the mixture is rich. This gives optimum conversion of pollutants because as the carbon monoxide and hydrocarbons are absorbed onto the catalysts surface, the oxygen stored by the ceria oxidises them even if there is not enough oxygen in the air to fuel ratio. xiii The future Scientists still have problems that need to be solved to reduce harmful vehicle emissions.
One problem that needs to be addressed is how to prevent harmful exhaust emissions when the catalyst is still warming up. There are several ways that this could be dealt with, including operating the catalysts at lower temperatures and also heating the catalyst before the engine is started. xiv Scientists also need to develop a system that is able to reduce nitrogen oxide emissions from diesel engines. There are several ideas on how to go about this, including using ammonia in the exhaust, but nothing has yet been developed. http://www.nutramed.com/environment/carsepa.htm