Catalysis is a strange thing. Without being altered in any way itself, the catalytic agent has a huge effect on a chemical reaction. Just its presence is required, not any of its substance. In automotive application, the basic two-way catalytic converter promotes the oxidation of HC and CO in the exhaust, making them combine more efficiently with O2, which reaction results in the production of CO2 and water. It works well enough to allow the calibrations of other emission controls to be eased.
When you take a reading with a gas analyzer or get a smog check, here are a few
points to keep in mind.
The "noble" metals platinum and palladium are the catalytic elements of the two-way converter. Only a thin layer of these precious substances is required, which is a good thing since any appreciable weight of them would result in an astronomical price. The metals are either applied to ceramic pellets, as found in the bed of a GM unit, or to a ceramic honeycomb in the monolithic type. Since any lead in the gasoline would quickly coat the catalytic surface and slow or even stop the reaction, fuel without this octane improver must be used.
As the gases flow through the pellets or honeycomb, they start to burn rapidly, and the temperature can reach 1,600 deg. F. Extra oxygen in the exhaust stream is needed to support the blaze, and that may be provided by a very lean air/fuel ratio, pump-type air injection, or an aspirator valve setup.
Along with the main converter, which is mounted between the head-pipe and muffler, a smaller "light off" catalyst is sometimes used. Since it's located right at the outlet of the exhaust manifold, it heats up to working temperature very quickly to fight the heavy emissions an engine produces while cold.
The three-way or reduction type catalytic converter first appeared in 1976 on Saabs with the Lambda-Sond system, and has since taken over completely. It not only oxidizes HC and CO, it also breaks NOx down into ordinary nitrogen and oxygen through the action of rhodium, another precious metal.
As far as vehicular complications are concerned, it's unfortunate that reduction catalysis is such a demanding reaction. It requires the "stoichiometric" air/fuel ratio (that is, 14.7 parts air to one part fuel by weight, the ideal blend for complete combustion) or it just won't occur to any useable degree. Regular carburetors or fuel injection systems are simply too inaccurate to keep the mixture close enough to that ratio, so electronic control had to be added. This required a computer, a network of sensors (the one that reports on the oxygen content of the exhaust stream being the most important), and a carb or injection setup that can adjust the mixture according to the computer's commands, which is the reason such systems are called "feedback" or "closed-loop."
In any ordinary three-way cat, the NOx-reducing section is located upstream of the oxidation section, and air injection pump output is routed to a chamber between them. This is necessary to keep the added air from upsetting that critical mixture balance created with such care at the intake. A switching system directs air injection to the exhaust manifold during warm-up to help burn the inordinate amount of HC and CO that's produced in that mode, then shifts it to the middle of the converter once normal operating temperature is reached (that's when NOx production begins) and the system enters closed loop.