Operation and Composition of the TWC

The operation and composition of modern TWCs has been comprehensively reviewed in a range of publications [4-7]. Here we will give a brief summary of their main features emphasising the combinations of materials used, the modifications to the catalyst and the modifications to the process stream required in order to allow them to effectively operate. 

The catalyst reacts species which can be oxidised within the exhaust (i.e. reductants such as CO and CxHy) with those that can be reduced (i.e. oxidants such as NOx and O ). Since the exhaust gas is a product of the combustion process in the engine, the relative levels of reductants and oxidants within the exhaust stream can be controlled by defining the reductant oxidant ratio entering the combustion chamber, i.e. the air-to-fuel (A/F) ratio. A range of pollutants (CO, CxHy and NOx) are generated during every combustion cycle and the concentrations of these within the exhaust gas vary as a function of A/F [4]. 

If an engine operates stoichiometrically (with an A/F ratio of 14.7 1 for gasoline combustion) then there is sufficient air in the fuel-air mixture to completely combust the hydrocarbons within. The exhaust gas produced will be neither oxidising nor reducing ([CO] + [CxHy] = [NOx] + [O ]). If it operates under fuel-rich conditions (A/F 14.7) there will be insufficient O present in the reaction stream to react with the fuel (and the exhaust gas will be net reducing, [CO] + [CxHy] [NOx] + [OJ). Finally, if it operates in a fuel lean mode (A/F 14.7) there will be an excess of O in the combustion chamber and the exhaust gas produced will be of a net oxidising nature ([CO] + [CxHy] [NOx] +[Oj). 

The three components that need to be removed from these streams (NOx, CO and CxHy) can only all be removed if the balance between oxidants and reductants in the exhaust mixture is maintained. 

Under fuel-rich conditions there will be insufficient O and NOx to oxidise the CO and CxHy (to CO and CO + H O) and therefore the catalyst (which will exist in a reduced state) will not remove all the CO and CxHy from the exhaust gas mixture. Under fuel lean conditions there will be insufficient CO and CxHy to reduce the NOx and O within the mixture (to N and H O, respectively), the catalyst will be fully oxidised and NOx will be emitted. 

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