Catalysts and Adsorbents

For other uses, see Catalyst (disambiguation).

A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate (speed) of a chemical reaction without itself being transformed or consumed by the reaction (see also catalysis). Chemical catalysts, the focus of this article, participate in reactions but are neither chemical reactants nor chemical products. More generally, one may sometimes call anything which accelerates a reaction without itself being consumed or transformed a catalyst (for example, a "catalyst for political change").

Catalysts and reaction energetics

Catalysts enable reactions to occur much faster or at lower temperatures because of changes that they induce in the reactants. Catalysts provide an alternative pathway, with a lower activation energy, for a reaction to proceed. This means that catalysts reduce the amount of energy needed to start a chemical reaction. Molecules that would not have had the energy to react or that have such low energies that they probably would have taken a long time to react are able to react in the presence of a catalyst. Thus, more molecules that need to gain less energy to react will go through the chemical reaction.

Catalysts cannot make energetically unfavorable reactions possible — they have no effect on the chemical equilibrium of a reaction because the rate of both the forward and the reverse reaction are equally affected.

Types of catalysts

Catalysts can be either heterogeneous or homogeneous. Heterogeneous catalysts are present in different phases from the reactants (e.g. a solid catalyst in a liquid reaction mixture), whereas homogenous catalysts are in the same phase (e.g. a dissolved catalyst in a liquid reaction mixture). A simple model for heterogeneous catalysis involves the catalyst providing a surface on which the reactants (or substrates) temporarily become adsorbed. Bonds in the substrate become weakened sufficiently for new to be created. The bonds between the products and the catalyst are weaker, so the products are released.

Homogenous catalysts generally react with one or more reactants to form a chemical intermediate that subsequently reacts to form the final reaction product, in the process regenerating the catalyst. The following is a typical reaction scheme, where C represents the catalyst:

A + C → AC (1)

B + AC → AB + C (2)

Although the catalyst (C) is consumed by reaction 1, it is subsequently produced by reaction 2, so for the overall reaction:

A + B + C → AB + C

the catalyst is neither consumed nor produced. Enzymes are biocatalysts. Use of "catalyst" in a broader cultural sense is in rough analogy to the sense described here. Other biocatalysts are ribozymes and deoxyribozymes.

Poisoning a Catalyst

A catalyst can be poisoned if another compound reacts with it and bonds chemically, but does not release. This effectively destroys the usefulness of the catalyst, as it cannot participate in the reaction with which it was supposed to catalyse.just like Rainey Nickel catalyst has reduced activity when it is in combination with mild steel. The loss in activity of catalyst can be overcomed by having a lining of epoxy or other substances .

Commonly used catalysts

Estimates are that 60% of all commerically produced chemical products involve catalysts at some stage in the process of their manufacture.^[1] Some of the most famous catalysts ever developed are the Ziegler-Natta catalysts used to mass produce polyethylene and polypropylene. Probably the best-known catalytic reaction is the Haber process for ammonia synthesis, where ordinary iron is used as a catalyst. Catalytic converters made from platinum and rhodium break down some of the more harmful byproducts of automobile exhaust. The most effective catalysts are usually transition elements.

See also

• Catalytic converter
• Coordination catalysts
• Enzymes and Ribozymes - biocatalysts
• Nanomaterial based catalysts

References

1. "Recognizing the Best in Innovation: Breakthrough Catalyst". R&D Magazine, September 2005, pg 20.