A catalyst is a chemical compound that acts to speed up a reaction, but in the process is not itself changed. Therefore the catalyst, at the end of the reaction, is free to act again to assist another reactant through the reaction.
Catalysts work by lowering the energy barrier between the reactants and the products. In this case:
2H2O + ENERGY = 2H2 + O2
where it normally takes a tremendous amount of energy to convert reactants to products – the addition of a catalyst can decrease the amount of energy required and therefore speed the reaction up!
2H2O + CATALYST+ energy = 2H2 + O2 + CATALYST
