One of the requirements of the catalytic converter is it needs to be at high temperatures before it works properly, so when a car first starts the converter is not working at its most effiecent. Ideas to combat this is that it is heated by the car battery to allow shorter periods before it is at optimum temperature. This leads to other problems in the system.
The efficency of the catalyst is affected by the size of the metal particles.But over time due to the high temperature of the operation it causes the metal particles aging, meaning the particle size decreases further from its optimal size and so decreasing the effiency of the catalyst. Also due to the constant high temperature of the catalytic converter, the Al2O3 that the honeycomb structure is made of converters into a lower phase with a lower surface area meaning less effiency.
One of the main problems in maintaining the regulations in reducing emissions is the flow rate of the emissions. So controlling the flow rate at which it enters the catalytic converter is very important, so the emissions stay in contact with the catalyst for the optimum time. As if the ratio is greater then the optimum for air then the extra O2 present will compete with the NO for H2 and so the effiency of turning NO back to N2 is lowered greatly. However if less air in ratio, then the oxidising reagents are short and so CO, H2 and hydrocarbons all compete for NO and O2. This problem has been reduced by a sensor being located in the exhaust pipe to measure the levels of O2 that sends signals back to the carburettor or fuel injection depending on engine to adjust the air:fuel ration as needed. Also a recent idea in catalytic converters means they can contain CeO2/Ce2O3 to aid in storing oxygen, So when the vehicle is running lower then the optimum ratio of Air:Fuel then the O2 will be stored for if the ratio goes so if greater will release O2.
Author: Matthew Lilley (document modification date: 25th May 2004)