katalysis

Catalysis is the acceleration of a chemical reaction by a substance called a catalyst, which is not consumed in the overall reaction. The catalyst provides an alternative reaction pathway with a lower activation energy, enabling more reactant molecules to reach the transition state and form products.

Catalysis is classified by the phase relationship between catalyst and reactants. In homogeneous catalysis, the catalyst and reactants are in the same phase, typically both in solution. In heterogeneous catalysis, the catalyst is in a different phase, usually a solid at solid–gas or solid–liquid interfaces. Biocatalysis, or enzymatic catalysis, employs biological molecules such as enzymes to achieve highly specific transformations under mild conditions.

Mechanistically, catalysts work by offering active sites where reactants adsorb, react, and then desorb as products. In heterogeneous catalysis, surface processes and the properties of the solid support (such as metals or zeolites) are crucial. Enzymes achieve selectivity and efficiency through well-defined active sites and sophisticated active-site chemistry.

Catalysts are not consumed in the reaction, although they can be inhibited or poisoned by impurities. The efficiency of a catalyst is often described by metrics such as turnover number and turnover frequency, which relate the amount of product formed to the amount of catalyst over time.

Industrial and practical relevance is extensive. The Haber-Bosch process uses iron catalysts for ammonia synthesis; catalytic cracking and hydrocracking employ solid acid and metal catalysts; automotive catalytic converters use platinum, palladium, and rhodium to reduce pollutants. The concept has wide application across chemistry, energy, and biology, reflecting catalysis as a foundational principle in modern science and industry.