Catalysis Definition in Chemistry

A catalyst permits a different energy pathway for a chemical reaction.
A catalyst permits a different energy pathway for a chemical reaction which has a lower activation energy. The catalyst is not consumed in the chemical reaction. Smokefoot, Wikipedia Commons

Catalysis is defined as increasing the rate of a chemical reaction by introducing a catalyst. A catalyst, in turn, is a substance that is not consumed by the chemical reaction, but acts to lower its activation energy. In other words, a catalyst is both a reactant and product of a chemical reaction. Typically, only a very small quantity of catalyst is required in order to catalyze a reaction.

The SI unit for catalysis is the katal. This is a derived unit which is moles per second. When enzymes catalyze a reaction, the preferred unit is the enzyme unit. The effectiveness of a catalyst may be expressed using the turnover number (TON) or turnover frequency (TOF), which is TON per unit time.

Catalysis is a vital process in the chemical industry. It is estimated that 90% of commercially-produced chemicals are synthesized via catalytic process.

Sometimes the term "catalysis" is used to refer to a reaction in which a substance is consumed (e.g., base-catalyzed ester hydrolysis). According to the IUPAC, this is an incorrect usage of the term. In this situation, the substance added to the reaction should be called an activator rather than a catalyst.

Key Takeaways: What Is Catalysis?

  • Catalysis is the process of increasing the rate of a chemical reaction by adding a catalyst to it.
  • The catalyst is both a reactant and product in the reaction, so it is not consumed.
  • Catalysis works by lowing the activation energy of the reaction, making it more thermodynamically favorable.
  • Catalysis is important! About 90% of commercial chemicals are prepared using catalysts.

How Catalysis Works

A catalyst offers a different transition state for a chemical reaction, with a lower activation energy. Collisions between reactant molecules are more likely to achieve the energy required to form products than without the presence of the catalyst. In some cases, one effect of catalysis is to lower the temperature at which a reaction will process.

Catalysis does not change chemical equilibrium because it affects both the forward and reverse rate of reaction. It does not change the equilibrium constant. Similarly, the theoretical yield of a reaction is not affected.

Examples of Catalysts

A wide variety of chemicals may be used as catalysts. For chemical reactions that involve water, such as hydrolysis and dehydration, the proton acids are commonly used. Solids used as catalysts include zeolites, alumina, graphitic carbon, and nanoparticles. Transition metals (e.g., nickel) are most often used to catalyze redox reactions. Organic synthesis reactions may be catalyzed using noble metals or "late transition metals," such as platinum, gold, palladium, iridium, ruthenium, or rhodium.

Types of Catalysts

The two main categories of catalysts are heterogeneous catalysts and homogeneous catalysts. Enzymes or biocatalysts may be viewed as a separate group or as belonging to one of the two main groups.

Heterogeneous catalysts are those which exist in a different phase from the reaction being catalyzed. For example, solid catalysts the catalyze a reaction in a mixture of liquids and/or gases are heterogeneous catalysts. Surface area is critical to the functioning of this type of catalyst.

Homogeneous catalysts exist in the same phase as the reactants in the chemical reaction. Organometallic catalysts are one type of homogeneous catalyst.

Enzymes are protein-based catalysts. They are one type of biocatalyst. Soluble enzymes are homogeneous catalysts, while membrane-bound enzymes are heterogeneous catalysts. Biocatalysis is used for commercial synthesis of acrylamide and high-fructose corn syrup.

Related Terms

Precatalysts are substances that convert to become catalysts during a chemical reaction. There may be an induction period while the precatalysts are activated to become catalysts.

Co-catalysts and promoters are names given to chemical species that aid catalytic activity. When these substances are used, the process is termed cooperative catalysis.

Sources

  • IUPAC (1997). Compendium of Chemical Terminology (2nd ed.) (the "Gold Book"). doi:10.1351/goldbook.C00876
  • Knözinger, Helmut and Kochloefl, Karl (2002). "Heterogeneous Catalysis and Solid Catalysts" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a05_313
  • Laidler, K.J. and Meiser, J.H. (1982). Physical Chemistry. Benjamin/Cummings. ISBN 0-618-12341-5.
  • Masel, Richard I. (2001). Chemical Kinetics and Catalysis. Wiley-Interscience, New York. ISBN 0-471-24197-0.
  • Matthiesen J, Wendt S, Hansen JØ, Madsen GK, Lira E, Galliker P, Vestergaard EK, Schaub R, Laegsgaard E, Hammer B, Besenbacher F (2009). "Observation of All the Intermediate Steps of a Chemical Reaction on an Oxide Surface by Scanning Tunneling Microscopy.". ACS Nano. 3 (3): 517–26. doi:10.1021/nn8008245