catalyst

A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing permanent chemical change. It works by providing an alternative reaction pathway with lower activation energy, so more reactant molecules can react per unit time. In most cases, the catalyst is regenerated at the end of the reaction and can participate in multiple cycles.

Catalysts can be classified by phase and type. Homogeneous catalysts operate in the same phase as the reactants, typically in solution. Heterogeneous catalysts are in a different phase, usually a solid surface that interacts with reactants on its surface. Other categories include biocatalysts, such as enzymes, which catalyze biological reactions with high specificity, as well as electro-, photo-, and organocatalysts designed for particular transformations.

Mechanistically, catalysts may work by adsorbing reactants on a surface, orienting them properly, forming reactive intermediates, or stabilizing transition states. They can alter selectivity, steering products toward desired outcomes, and enable milder conditions such as lower temperatures and pressures.

Applications of catalysis span industrial chemistry, energy, and biology. Industrial examples include ammonia synthesis with iron catalysts and hydrocarbon processing with precious-metal catalysts, as well as catalytic converters that reduce emissions. Enzymes power biological processes such as digestion and metabolism. In energy technologies, catalysts are important for fuel cells, electrolysis, and photocatalytic routes to convert light into chemical energy.

Practical considerations include catalyst lifetime, susceptibility to poisoning by impurities, and deactivation. Design goals emphasize high turnover frequency, selectivity, and ease of separation and recovery. The term catalysis was coined by J. A. G. Berzelius in 1835.