 In the video effects on health of air pollutants, we learned about the negative effects on our health from fuels combusted in cars. One such pollutant is nitrogen dioxide, another is carbon monoxide. Instead of releasing these products, the catalytic converter release harmless gases into the atmosphere. Can you recall what a catalyst does in a chemical reaction? A catalyst is a substance that causes a chemical reaction to proceed more quickly without being used up. In a catalytic converter, there are two types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and or palladium. These metals are usually formed in a honeycomb arrangement. The honeycomb structure exposes the maximum surface of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as these metal catalysts are extremely expensive. The reduction catalyst is the first stage of the catalytic converter. When a nitrogen oxide or nitrogen dioxide molecule contacts the catalyst, it causes a nitrogen oxygen bond to break, so that an O2 molecule, a harmless product is formed along with a nitrogen molecule. All of this happens on the surface of the catalyst. Could you write an equation for this process? Pause, try it now and check your equation when you're ready. The two balanced equations are as follows. The oxidation catalyst is the second stage of the catalytic converter. It oxidizes the unburned hydrocarbons and carbon monoxide by burning them over a platinum and palladium catalyst. The equation for this is shown here. Finally, the catalytic converter is able to monitor the level of oxygen in the exhaust stream with an oxygen sensor. This sensor is linked to an automatic control system which adjusts the carburetor in order to change the air to fuel ratio. This allows for more or less air to mix with the car's fuel before it enters the cylinders. This ensures that there is just enough oxygen in the exhaust to allow for the oxidation of unburned hydrocarbons and the carbon monoxide.