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Catalysts and initiators are chemicals used to start, promote, or slow chemical reactions.

Basic Information

Catalysts affect chemical reactions by providing alternative pathways for the formation and breaking of bonds; for this reason they are considered a type of reagent. Unlike many reagent chemicals, however, catalysts are not consumed when a reaction occurs. (Although they are not consumed by the process itself, catalysts may be inhibited, destroyed, or similarly deactivated through secondary processes such as coking, dissolution, and evaporation.) Catalysts are useful because the alternate pathways typically use less energy than it would take to activate an uncatalyzed reaction. Catalytic action can also make it possible to perform chemical processes at lower temperature and/or pressure, thereby reducing fuel resources necessary to provide a special reaction environment.

The video below provides an excellent technical introduction to catalysis and the effect of catalysts on activation energy and reaction rate.

Video credit: Michael Evans/YouTube

In addition to the formation of new paths, catalysts may also reduce energy consumption by stabilizing the reaction through the transition state, as well as temporarily destabilizing the reactants themselves. The free energy graph below illustrates how catalyzed reactions reach an identical product by using less energy.

Image credit: Andy Schmitz

The vast majority of commercial chemical products use catalysts during their production process. They are becoming increasingly prominent — due to their ability to reduce chemical waste — with the rise of sustainable ("green") chemistry.

Mechanism

A catalyst's mechanism is achieved through the catalytic cycle, which is often visually portrayed as a loop due to the regenerative nature of the process. The image below illustrates a reaction in which the combination of reactants A and B produces compound C (A+B=C). The catalyst (X) first combines with reactant A, then adds reactant B, producing XAB. At this point the reaction occurs, producing XC, with C being the intended product. The catalyst then separates from C; if the catalyst is not destroyed through a secondary process the cycle is free to recommence.

Image credit: Smokefoot/Wikimedia

Homogeneous vs. Heterogeneous

Catalysts may be broadly classified as either homogeneous or heterogeneous.

Heterogeneous catalysis involves a catalyst in a different phase than the reactants. A typical heterogeneous reaction involves a solid catalyst with liquid or gas reactants. Heterogeneous catalysis begins when one or more reactants are adsorbed onto the catalyst surface. The catalyst molecules then interact with the attached reactant molecules; this phase is followed by the reaction itself. The product molecules are then desorbed (break away) from the catalyst.

Automobile catalytic converters are a common example of heterogeneous catalysis. In order to convert toxic gases into benign exhaust, catalytic converters rely on a solid precious metal catalyst — typically involving palladium and rhodium — to speed oxidation and reduction of the gases into carbon dioxide, water, and nitrogen.

Heterogeneous catalysis using a solid metal catalyst and gas reactants.

Image credit: Jake Smith

Homogeneous catalysis occurs when both the catalyst and the reactants are in the same phase. The mechanism of this type of catalysis largely depends on the reactants and catalysts present in the reaction. A common example of homogeneous catalysis is the destruction of the atmospheric ozone layer; in this reaction, both the reactants and the free radical catalyst are gases.

Image credit: Robert Stewart

Homogeneous catalysis is less common than the heterogeneous type because, in the former process, separation of the catalyst from the product does not occur naturally, and performing this process manually requires extra time and costs. However, there are certain chemicals — such as ethane-1,2-diol, shown below — which are manufactured using liquid acid or base catalysts. The image below shows the all-liquid production of ethane-1,2-diol by adding water to epoxyethane. In this case the catalyst is a trace of liquid acid, from which a hydrogen ion functions as the catalyst.

Image credit: Essential Chemical Industry Online

Positive and Negative

While the previous discussion involves catalysts used to speed chemical reactions, certain catalysts can be used to slow reactions as well. With this in mind, catalysts can be further classified as positive or negative:

• Positive catalysts quicken chemical reactions.
• Negative catalysts retard chemical reactions.

Applications

Catalysts may be broadly used in any industry involving chemical reactions. Their most obvious application is in the production of chemicals and other compounds, but catalysts may be used in a wider variety of industries as well. Some of these industries, as well as the processes involved, are listed below.

• Inorganic chemical processing: manufacture of inorganic chemicals (i.e. ammonia, sulphuric acid, nitric acid) through the contact process, the Haber process, and others.
• Organic chemical processing: manufacture of organic chemicals through nitration, hydration, and others.
• Petrochemical: hydrocarbon processing (improving octane ratings using isomerization and reforming); cracking using zeolite catalysts.

Types

Catalysts are typically specified by their mechanism and use, some of which are lised below.

• Crosslinkers and vulcanizers assist in forming polymer chains and vulcanization, respectively.
• Curing agents are added to resins or polymer adhesives to cause polymerization, resulting in a hardened product. Curing agents may be some type of catalyst (i.e. crosslinkers) or initiators.
• Initiators trigger chemical reactions. They are not true catalysts, as they become an integral part of the end product; because of this they are instead considered to be co-reactants.
• Reaction accelerators are catalysts which quicken the progress of a chemical reaction.
• Reaction terminators and retarders end (terminate) or slow (retard) chemical reactions.

Standards

Catalysts and initiators may be produced, used, and tested based on various specifications and standards. Some common standards are listed below.

• ASTM D3766 (Standard terminology relating to catalysts and catalysis)
• ASTM D4824 (Test method for determination of catalyst acidy by ammonia chemisorption)
• SAE SP-2253 (Advanced catalysts and substrates)

References

ChemGuide - Understanding Chemistry: Catalysis

IHS - ASTM Petrochemical Standards (contains many catalyst testing standards)

The Essential Chemical Industry Online - Catalysis in industry