The purpose of using a catalyst is to either speed up or slow down the rate of a reaction.  Some chemical reactions have high energies of activation, causing the progress of the reaction to proceed at a slow rate.  A catalyst effectively lowers the energy of activation, allowing the reactants of the reaction to interact, break bonds and form new bonds at a faster rate.  Catalysts are not part of the initial reaction, they simply enhance the likelihood of the reaction taking place.  Catalysts may also be used to slow down the rates of reactions, as well.  Such substances are called inhibitors, because they "inhibit" the reactants from reacting to and with each other.  Catalysts serving as inhibitors raise the energy of activation of the reactants, causing the rate of the reaction to slow from what it was initially.  This reaction is probably slower than others because of the energy stored in the arrangement of the molecular composition atoms.  This reaction is called a "decomposition" reaction, because the singular reactant is decomposing into the individual components that make it up.