Ok I will take a few moment to explain this how I understand it as I work as an Emissions Tec in the oil and gas industry. There are three types of catalyst. The first is the NSCR catalyst (three way) or Non Selective Catalyst Reduction, It is for a Rich Burn engine only (or Stoicometric). The NSCR Catalyst goes after reduction of Carbon Monoxide, Nitric Oxide, Nitrogen Dioxide, Formaldehydes and VOC emissions. A NSCR catalyst requires very little left over Oxygen in the exhaust to work, somewhere around .4% to operate correctly, that is why when there is a misfire it tends to get very hot as there is unburned fuel and AIR from the missing cylinder that creates a Exothermic reaction in the catalyst as it burned.
There is always some Carbon Monoxide left over and that is where the second type of catalyst comes in to play. The second type is called an Oxidation catalyst as it goes primarily after reducing Carbon Monoxide and Formaldehydes. It is also called a lean burn catalyst as it requires about 10 to 15% Oxygen in the exhaust stream for it to operate. That is usually the second catalyst in a housing or exhaust system and that is why the air pump on the engine blows into the middle of the catalyst housing. To get the air in there to help with the oxidation reaction (rust). It is also a exothermic reaction, but it will not run wild when there is a misfire in the engine.
The last type of catalyst is called the SCR, or Selective Catalyst Reduction. It is a type of catalyst that requires a catalyst or burning agent. This agent is known as Urea (Ammonia) and is injected into the exhaust stream before the catalyst. This catalyst goes after NOx (Nitrogen dioxide and Nitric Oxide). You see these primarily on the new Diesel trucks out there now as the combustion process has a very high temperature and high peak firing pressure. High temperature creates high levels of NOx and a diesel engine puts out a lot of it.
There are some combination units out there at the moment that use all three of these catalyst in line, but the NSCR (three way) is always in line first, then the Oxidization and often on industrial engines followed by the SCR catalyst.
22 years I have been working with this crap, assuring compliance with federal and state agencies. And it always pisses me off when people want to remove it from their vehicles. It is just plain wrong to remove it.
