catalysis

Catalysis is the process by which the rate of a chemical reaction is increased by a substance called a catalyst. A catalyst participates in the reaction mechanism but is not consumed in the overall reaction, and it provides an alternative pathway with lower activation energy. Because the catalyst appears at the start and end of the catalytic cycle, only a small amount is needed to effect large changes in rate.

Catalysts are categorized by phase and role: homogeneous catalysts are in the same phase as reactants (often in solution); heterogeneous catalysts are in a different phase (commonly a solid surface); biocatalysts, usually enzymes, are highly selective biological catalysts; organocatalysts are small organic molecules that promote reactions without metals.

Catalysis can influence selectivity as well as rate, enabling chemoselectivity, regioselectivity, and stereoselectivity. In heterogeneous catalysis, mechanisms often involve adsorption of reactants on a surface, formation of surface intermediates, and desorption of products (Langmuir-Hinshelwood or Eley-Rideal mechanisms).

Industrial significance is high. Classic examples include the Haber-Bosch process using iron catalysts to synthesize ammonia, the contact process using vanadium(V) oxide to oxidize SO2 to SO3 for sulfuric acid production, and petroleum hydrocarbon processing with noble-metal catalysts. Enzymes enable selective synthesis in biology and industrial biocatalysis.

Catalysis also underpins environmental and energy technologies, such as catalytic converters in vehicles, fuel cells, electrochemical reduction of CO2, and water-splitting catalysts.

Important metrics include turnover number (TON) and turnover frequency (TOF), which measure how many substrate molecules a single catalyst molecule converts and how fast. Catalysts can be deactivated by poisoning, fouling, or sintering, and recycling or regeneration is often required.

Historically, the term catalysis was coined by Berzelius in 1835, and the field has grown with advances in kinetics, surface science, and computational chemistry.