Catalyst: Encyclopedia II - Catalyst - Types of catalysts

Catalyst - 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.

For example, in the Haber process to manufacture ammonia, finely divided iron acts as a heterogenous catalyst. The metal uses active sites to allow partial weak bonding to the reactant gases, which are adsorbed onto the metal surface. As a result, the bond within the molecule of a reactant is weakened and the reactant molecules are held in close proximity to each other. In this way the particularly strong triple bond in nitrogen is weakened and the hydrogen and nitrogen molecules are brought closer together than would be the case in the gas phase, so the rate of reaction increases.

Other heterogenous catalysts include vanadium V oxide in the Contact process, nickel in the manufacture of margarine, alumina and silica in the cracking of alkanes and platinum rhodium palladium in catalytic converters.

In car engines, incomplete combustion of the fuel produces carbon monoxide, which is toxic. The electric spark and high temperatures also allow the oxygen and nitrogen to react to form nitrogen monoxide, which is acidic. Catalytic converters reduce such emissions by adsorbing CO and NO onto the catalytic surface, where the gases undergo a redox reaction. Carbon dioxide and nitrogen are desorbed from the surface and emitted as relatively harmless gases:

2CO + 2NO → 2CO(2) + N(2)

Example of homogenous catalysts are H+(aq) which acts as a catalyst in esterification and chlorine free radicals in the break down of ozone. Chlorine free radicals are formed by the action of ultraviolet radiation on chlorofluorocarbons (CFCs). They react with ozone forming oxygen molecules and regenerating chlorine free radicals:

Cl(.) + O(3) → ClO(.) + O(2)

ClO(.) + O → Cl(.) + O(2)

N.B. Full stops in brackets denote free radicals that should be superscripted. Numbers in brackets should be subscripted

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:

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

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.

Adapted from the Wikipedia article "Types of catalysts", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki