A catalyst is defined as participating in a chemical reaction without being consumed itself. Oxidation of VOC is a chemical reaction between hydrocarbons and oxygen generating carbon dioxide and water. In presents of a catalyst this reaction takes place at a lower temperature compared to if no catalyst would be present.
In an industrial application a mixture of air and ethyl acetate that passes through a bed of catalyst reacts at 250-300 C (99% conversion). Without the catalyst the temperature needs to be increased to 900-1000 C in order to get the same conversion.
How does it work?
Imagine a volume of air mixed with a minor amount of hydrocarbons (VOC). On a molecular level we have molecules of nitrogen, oxygen and hydrocarbon flying around in a random pattern. When an oxygen- and a hydrocarbon-molecule collide, a reaction occur forming water- and carbon-dioxide-molecules. If the speed of the molecules increases the number of collisions per second will increase. The speed of the molecules we recognise as temperature. So the higher temperature the faster reaction.
If the molecules could be placed on a surface the probability for the molecules to collide will increase with the speed kept the same. Such surface we call catalyst. The function of the surface is to attract the oxygen and hydrocarbon. Once the reaction occurred it is also important the reactants (water and carbon dioxide) leave room for a new set of oxygen and hydrocarbon molecules.