There are two kinds of catalyst homogeneous and heterogenous - if it's in the same phase (as is bleach) it is a homogeneous catalyst. These work by providing an alternative mechanism with a lower activation energy when compared to the catalyst free reaction.
The catalyst usually chemically combines with one of the intermediate species and then is regenerated in a later step of the reaction mechanism.
Classic example is the catalytic oxidation of the tartrate ion using hydrogen peroxide with a cobalt II catalyst. The pink cobalt II turns green during the course of the reaction showing the formation of Co III ions - as the reaction finishes the CoII is regenerated and the pink colour reappears.
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