Catalytic Converters
There are millions of cars on the road and each one is potentially a source of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.

To solve these problems, the respective government agencies create clean-air laws, and many laws have been enacted that restrict the amount of pollution that cars can produce. To keep up with these laws, automakers have made many refinements to car engines and fuel systems. To help reduce the emissions further, they have developed an interesting device called a Catalytic Converter, which treats the exhaust before it leaves the car and removes a lot of the pollution.

A catalytic converter is a device in the vehicle exhaust system that chemically changes pollutants like Carbon Monoxide (CO), Hydrocarbons (HC), Oxides of Nitrogen (NOx) and Particulate Matter (PM) into harmless substances like Carbon Dioxide (CO2), Nitrogen (N2), and water vapor (H2O).

The catalytic converter uses a reticulated ceramic foam and cordierite honeycomb structure which are coated with high alumina wash coat in order to increase the surface area to required level. Later the reticulated foam is coated with combustion catalyst and the cordierite substrate is coated with combustion catalyst, oxidation catalyst and reduction catalyst. These formulations (Pt: Pd: Rh) in the presence of the promoters effect the conversion of pollutants as the exhaust passes through the converter without affecting the engine performance and mileage.

Working
Most modern cars are equipped with three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce -- Carbon Monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually Platinum, Rhodium and / or Palladium. The idea is to create a structure that exposes the maximum surface area of a catalyst to the exhaust stream, while also minimizing the amount of catalyst required (they are very expensive).

Structure of Separate Catalysts.
There are two main types of structures used in catalytic converters - Honeycomb and Ceramic beads. Most cars today use a honeycomb structure.

Ceramic Honeycomb Catalyst Structure
The Reduction Catalyst
The reduction catalyst is the first stage of the catalytic converter. It uses Platinum and Rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO--->N2 + O2 or 2NO2--->N2 + 2O2

The Oxidation Catalyst
The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned Hydrocarbons and Carbon Monoxide by burning (oxidizing) them over a Platinum and Palladium catalyst. This catalyst aids the reaction of the CO and Hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2---> 2CO2

But where did this oxygen come from?

The Control System
The third stage is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

However, the modern 3-way Catalytic Converters consist of Single Catalyst catering to both the oxidation and reduction function.

Failing ways of Catalytic Converter

  •  It can become clogged.
  •  It can become poisoned.

There really is no "inspection port" for the consumer or mechanic to see an actual clog in a converter. Often, the only way to tell if a catalytic converter is malfunctioning (plugged) is to remove it and check the change in engine performance. When a clogged converter is suspected, some mechanics temporarily remove the O2 sensor from the exhaust pipe ahead of the catalytic converter and look for a change in performance.

A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Any additives or malfunctions that cause the mixture or the temperature of the exhaust gases to change reduce the effectiveness and life of the catalytic converter. Leaded gasoline and the over-use of certain fuel additives can shorten the life of a catalytic converter.

A catalytic converter can also fail because of :

  •  Bad exhaust valves on the engine.
  •  Fouled plugs causing unburned fuel to overheat the converter.

Sometimes you can tell that a converter is clogged because you don't go any faster when you push the gas pedal. Also, there usually is a noticeable drop in gas mileage associated with a clogged catalytic converter. A partially clogged converter often acts like an engine governor, limiting the actual RPMs to a fast idle. A totally clogged converter causes the engine to quit after a few minutes because of all the increased exhaust back pressure.