catalystsubstances

Catalyst substances are chemical species that increase the rate of a chemical reaction without undergoing permanent chemical change themselves. A catalyst works by providing an alternative reaction pathway with lower activation energy, thereby increasing the fraction of molecules that react per unit time. Although catalysts participate in reaction steps, they are regenerated by the end of the cycle and remain present after the reaction, in principle unchanged. In practice some catalyst material may be consumed or lost by side processes or deactivation, but the net transfer of mass is typically zero.

Catalysts are broadly classified by phase: homogeneous catalysts are in the same phase as the reactants (for example, a soluble metal complex in a solution); heterogeneous catalysts are in a different phase (such as a solid catalyst with dissolved reactants). Biocatalysts or enzymes are biological catalysts, functioning under mild conditions with high specificity. Photocatalysts use light to drive reactions, while electrocatalysts facilitate redox processes in electrochemical systems.

Key properties include activity (rate enhancement), selectivity (chemoselectivity, regioselectivity, stereoselectivity), and stability under operating conditions. Metrics such as turnover frequency (TOF) and turnover number (TON) quantify how rapidly and how long a catalyst remains active. Catalysts are often sensitive to poisons, heat, impurities, or sintering and may require recovery and recycling.

Applications span industrial synthesis, energy conversion, and environmental protection. Iron catalysts enable ammonia synthesis; platinum-group catalysts facilitate hydrogenation and automotive exhaust treatment; zeolites and solid acids enable petrochemical processing. In biochemistry, enzymes act as highly selective catalysts under near-physiological conditions. The design of catalysts aims to maximize activity and selectivity while minimizing cost and deactivation.