In essence, a catalyst is a substance that can increase the rate of a chemical reaction without itself being consumed in the process. Catalysts are important in many industrial processes and are used extensively in the manufacture of chemicals, petroleum refining, and environmental remediation.
The word “catalyst” comes from the Greek word κατάλυσις (katalysis), meaning “dissolution”, which was first used to describe the action of a catalyst in 1835 by German chemist Berzelius. The term “catalysis” was coined later by Swedish chemist Jöns Jacob Berzelius.
Catalytic reactions are typically divided into two broad categories: heterogeneous catalysis, which occurs when the catalyst and reactants are in different phases (solid-liquid, solid-gas, or liquid-gas); and homogeneous catalysis, which occurs when all reactants and catalysts are in the same phase (usually liquid). Heterogeneous catalysis is generally more important industrially because it can be easily scaled up to large volumes. Homogeneous catalysis is often used for laboratory studies because it allows for better control over reaction conditions.
There are three main types of catalytic reactions: addition reactions, elimination reactions, and substitution reactions. Addition reactions involve two reactants that combine to form one product; elimination reactions involve the loss of a small molecule from a larger reactant molecule; substitution reactions involve the replacement of one atom or group of atoms in a molecule with another atom or group.
Most catalytic processes involve enzymes, which are proteins that act as biological catalysts. Enzymes typically have an active site where the reaction takes place; this active site has a specific three-dimensional shape that complementary to the shape of the substrate molecules. The binding of substrate molecules to an enzyme’s active site alters the conformation (shape) of the enzyme so that it can more effectively catalyze the reaction.
Other important classes of organic catalysts include organometallic compounds and Lewis acids/bases. Organometallic compounds contain at least one metal atom bonded directly to a carbon atom; these compounds often serve as both ligands (molecules that donate electrons) and substrates in transition metal-catalyzed processes. Lewis acids/bases are electron-deficient molecules that can accept electrons from other molecules; they play an important role in acid-base catalysis, where they act as proton donors or acceptors