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The cat provides an environment for reactions to take place. Gases that come out of the engine include carbon monoxide CO, oxides of nitrogen NOx, and unburned fuel/hydrocarbons CxHx. These gases react with some left over oxygen to make less harmful compounds.
CO reacts with oxygen to form carbon dioxide.
nitrogen oxides decay into oxygen and nitrogen.
hydrocarbon combust with oxygen to form carbon dioxide and water vapor.
I'm gussing the popping is these small combustion reactions continuing after the car is turned off, until the cat cools below reaction temperature.
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