A catalyst is a substance that can be added to a reaction to increase the reaction rate without getting consumed in the process. Catalysts typically speed up a reaction by reducing the activation energy or changing the reaction mechanism. Catalysts and their associated catalytic reactions come in three main types: homogeneous catalysts, heterogeneous catalysts and biocatalysts.
A reaction is considered homogeneously catalyzed when the catalyst and the reactant(s) are in the same physical state, most often happens with gaseous catalyst-reactant pairs. Types of homogeneous catalysts include organic acids in which the donated hydrogen atom is replaced by a metal, where a number of compounds blending carbon and metal elements in some form, and carbonyl compounds joined to cobalt or iron.
A reaction is considered heterogeneously catalyzed when the catalyst and the reactant(s) are in different phases, with the reaction occurring at the interface between them (most commonly, the gas-solid "border"). Common heterogeneous catalysts include inorganic –non-carbon-containing – solids such as elemental metals, sulfides and metallic salts, as well as a smattering of organic substances, among them hydro peroxides and ion exchangers.