A catalyst is a substance that increases the speed of a chemical reaction without being consumed by the reaction. For example, nitric oxide (NO) is a catalyst in the breakdown of ozone (O3) in the upper atmosphere. An oxygen atom (O) will react with an ozone molecule form two oxygen molecules (O2), but at a very slow rate. However, in the presence of nitric oxide, ozone is quickly broken down with the following series of steps:
Step 1 - An oxygen atom combines with a molecule of nitric oxide, forming a molecule of nitrogen dioxide (NO2):
O + NO → NO2
Step 2 - A molecule of nitrogen dioxide combines with a molecule of ozone, forming two molecules of oxygen and a molecule of nitric oxide:
NO2 + O3 → 2O2 + NO
Although the molecule of nitric oxide participates in the reaction, it is not consumed by it and is available for additional reactions.