catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the overall process. They act by providing an alternative reaction pathway with a lower activation energy, allowing more reactant molecules to reach the transition state and form products at a given temperature. After the reaction, the catalyst is regenerated and can participate in additional cycles.

Catalysts are broadly classified as homogeneous or heterogeneous. Homogeneous catalysts are in the same phase as the reactants, while heterogeneous catalysts are typically solids that provide active sites on their surfaces. Enzymes are biological catalysts that operate under mild conditions with high specificity. Catalysis can be influenced by promoters, inhibitors, surface area, porosity, and electronic structure.

Industrial use relies on catalytic processes that improve efficiency and selectivity. The Haber-Bosch process uses iron catalysts for ammonia synthesis; the Contact process uses vanadium pentoxide for sulfuric acid production; refining and petrochemistry commonly employ solid catalysts such as zeolites and metal oxides. Automotive catalytic converters use platinum group metals to convert harmful gases into less polluting species.

Performance is described by activity, selectivity, stability, and lifetime. Turnover number and turnover frequency quantify how many reaction cycles a catalyst performs. Deactivation can occur through sintering, fouling, coking, or poisoning by impurities. Catalysts enable milder operation, reduced energy demand, and lower waste, but may involve material costs, finite lifetimes, and environmental considerations related to metal catalysts.