In chemistry and biology, a catalyst is a substance the increases the rate of a chemical reaction without being consumed by it. Catalysis is the process of speeding up a reaction using a catalyst. The word “catalyst” comes from the Greek word kataluein, which means to loosen or untie. British chemistry Elizabeth Fulhame first described the concept of catalysis in her 1794 book describing her work on oxidation-reductions reactions.
- A catalyst lowers the activation energy of a reaction, making it more thermodynamically favorable, and thus faster.
- Catalysts are not consumed by a reaction. They are both reactants and products.
- Around 90% of commercial chemical manufacturing relies on catalysts.
How Catalysis Works
Catalysis is a different pathway for a chemical reaction, which has a lower activation energy. When a reaction has a lower activation energy, it occurs more readily and thus more quickly. A catalyst binds to a reactant and it increases the number of collision between the reactant molecules, making the reaction more favorable thermodynamically. When the catalyst is an enzyme, the enzyme binds to a substrate, leading to catalysis. Sometimes binding a catalyst and a reactant changes the temperature of the reaction, improving its ability to proceed. Sometimes the intermediate steps of catalysis do consume the catalyst, but later steps release it before reaction completion.
Note that a catalyst does not change the equilibrium of a chemical reaction because it affects both the forward and reverse reaction rates. So, a catalyst has no effect on the equilibrium constant the or theoretical yield. Also, the Gibbs free energy of the reaction is unchanged.
Examples of Catalysts
- Enzymes are biological catalysts (proteins) that react with a substrate and form an unstable intermediate compound. Because the intermediate is unstable, the reaction proceeds toward equilibrium more quickly than it would without the enzyme. For example, carbonic anhydrase is an enzyme that catalyzes the reaction that turns carbonic acid into water and carbon dioxide:
H2CO3(aq) ⇆ H2O(l) + CO2(aq)
This enzyme helps carbon dioxide diffuse out of blood and into the lungs so the body exhales and removes it.
- Many catalysts are transition metals. For example, platinum is the catalyst in an automobile catalytic converter that turns carbon monoxide into carbon dioxide. Other metals that are good catalysts are gold, palladium, ruthenium, rhodium, and iridium (the noble metals).
- Potassium permanganate acts as a catalyst for the decomposition of hydrogen peroxide into water and oxygen. In this case, the catalyst changes the temperature of the reaction (increases it), increasing the reaction rate.
- Other common catalysts are zeolites, graphitic carbon, and alumina.
Positive and Negative Catalysts (Inhibitors)
A positive catalyst lowers the activation energy of a reaction and speeds up its rate. In contrast, a negative catalyst makes a reaction less favorable and slows its rate. Note, the IUPAC prefers avoiding this terminology and recommends using the terms “catalyst” and “inhibitor”. An example of an inhibitor is sulfuric acid, which slows the decomposition of hydrogen peroxide.
Other Terms Relating to Catalysts
- A precatalyst is a substance that converts into a catalyst during a chemical reaction.
- A promoter is a substance that increases the activity of a catalyst, but is not itself a catalyst. Another word for a promoter is a co-catalyst. Some promoters actively remove material that would interfere with the reaction. Others aid in dispersing the catalyst or binding the catalyst to a reagent.
- A catalytic poison inactivates a catalyst. Note that some inhibitors reversibly inactivate catalysts. The action of a catalytic poison is irreversible.
There are three common units for catalysis. The SI unit is the katal, which is a derived unit that expresses the rate of the reaction in moles per second. When comparing the effectiveness of a catalyst, useful units are turnover number (TON) and turnover frequency (TOF), which is TON per unit of time. TON and TOF describe the rate of catalyst recycling in the reaction.
Types of Catalysts and Catalysis
The two broad categories of catalysis are homogeneous catalysis and heterogeneous catalysis:
- Heterogeneous catalysts are in a different phase from the catalyzed reaction. An example of heterogeneous catalysis is using a solid catalyst like a zeolite or alumina to catalyze a reaction in a mixture of liquids and/or gases. Membrane-bound enzymes are another example of heterogeneous catalysts.
- Homogeneous catalysts are the same phase as the chemical reactants. Soluble enzymes are examples of homogeneous catalysts.
Demonstration: See Catalysis in Action
An excellent demonstration of catalysis is the “elephant toothpaste” reaction. In the classic reaction, potassium iodide is the catalyst for the decomposition of hydrogen peroxide into water and oxygen. The kid-friendly version uses yeast as a catalyst and a lower concentration of peroxide, but the basic principle is still the same. Normally, hydrogen peroxide decomposes slowly, giving it a shelf life of around 3 years unopened and up to six months once you break the seal on the bottle. But, in the presence of a catalyst, the reaction only takes seconds.
The “genie in a bottle” is another example of a demonstration that relies on a catalyst. This reaction produces a cloud of vapor, resembling a genie emerging from its bottle.
- IUPAC (1997). “Catalyst”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.C00876
- Laidler, Keith J.; Cornish-Bowden, Athel (1997). “Elizabeth Fulhame and the discovery of catalysis: 100 years before Buchner“. In Cornish-Bowden, Athel (ed.). New beer in an old bottle: Eduard Buchner and the growth of biochemical knowledge. Valencia: Universitat de Valencia. ISBN 9788437033280.
- Laidler, K.J.; Meiser, J.H. (1982). Physical Chemistry. Benjamin/Cummings. ISBN 0-618-12341-5.
- Masel, Richard I. (2001). Chemical Kinetics and Catalysis. New York: Wiley-Interscience. ISBN 0-471-24197-0.
- Nelson, D.L.; Cox, M.M. (2000) Lehninger Principles of Biochemistry (3rd ed.). New York: Worth Publishing. ISBN 1-57259-153-6.