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Platinum is indeed a very good catalyst for many reactions, although of the ‘platinum group metals’ some of the others, especially palladium, are equally useful. Base metals including nickel also find many uses, for example in fat hardening, but even here attempts are being made to replace it with palladium.

A number of criteria are required to be met before a metal can be selected as a ‘good’ catalyst for any reaction. The three most important are:

(i) its activity, that is the speed with which it makes the reaction go,

(ii) its selectivity, that is, the extent to which it produces the desired product rather than any others, and

(iii) its active life, that is, how long it can be used before it becomes deactivated by poisons, or loses its mechanical stability.

Platinum scores particularly well on items (i) and (iii).

The secret of what makes a good catalyst can be summarised as follows:

• (a) It must be able to chemisorb the reactants sufficiently strongly to cover the entire surface, but not so strongly that it is difficult to persuade them to react.

• (b) They must also be chemisorbed in the right form, that is, in the way that encourages them to make only the desired product.

Platinum is especially good for reactions where selectivity is not so important, e.g. in the catalytic combustion of hydrocarbons and the reactions that occur in the treatment of vehicle exhaust.

Answer posted 25 February 2005