Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative reaction pathway with a lower activation energy. Catalysts can be classified into homogeneous (same phase as reactants) and heterogeneous (different phase from reactants). For example, in the acid-catalyzed esterification, sulfuric acid acts as a homogeneous catalyst, whereas in the Haber process, iron serves as a heterogeneous catalyst.
