A catalyst is a substance that alters the rate of a chemical reaction, but is itself unchanged at the end of the reaction. Catalysts are specific for particular reactions. In a reversible reaction, the forward and back reactions are catalyzed equally, and the equilibrium position is not altered.
Catalysis is either homogenous (the catalysts and reactants being in the same phase, usually gas or liquid), in which case the catalyst usually forms a reactive intermediate which then breaks down; or heterogeneous, in which adsorption of the reactants occurs on the catalytic surface. heterogeneous catalysis is often blocked by impurities called poisons.
Catalysts are widely used in industry, as in the contact process, the hydrogenation of oils, and the cracking of large hydrocarbons. Metals or their compounds catalyze by adsorbing gases to their surface, forming intermediates that then readily react to form the desired product while regenerating the original catalytic surface.