Substance that speeds up a chemical process without actually changing the products of reaction. It lowers the activation energies of both, the forward and the reverse reaction. A catalyst provides a different pathway for a reaction. It may appear in a rate law. A catalyst does not change thermodynamic data, such as the equilibrium constant or the free energy of a reaction and is not consumed in the reaction.
Enzymes are catalysts for many biochemical reactions.
Emission control catalysts are used to promote reactions that change exhaust pollutants from internal combustion engines into harmless substances.
Catalysts are used for the electrodes to break down hydrogen into electrons and protons.