Catalytic Properties

Why transition metals make good catalysts

Transition metals and their compounds are widely used as catalysts. This catalytic activity is traced to two abilities that are characteristic of the d-block: the ability to adopt multiple oxidation states, and the ability to form complexes with the reactant species.

Some well-known industrial examples:

Vanadium(V) oxide, $\mathrm{V_2O_5}$, in the Contact Process (manufacture of sulphuric acid).

Finely divided iron in the Haber Process (manufacture of ammonia).

Nickel in catalytic hydrogenation of alkenes.

How solid-surface catalysis works

When the catalyst is a solid and the reactants are gases or are in solution, catalysis proceeds through the formation of bonds between the reactant molecules and atoms at the surface of the catalyst. First-row transition metals use their 3d and 4s electrons to form these surface bonds. This has two effects:

1. It increases the local concentration of the reactants at the catalyst surface.
2. It weakens the bonds within the reacting molecules themselves, which lowers the activation energy of the reaction.

The role of variable oxidation states

Because transition-metal ions can readily change their oxidation state, they are particularly effective catalysts — the metal ion can cycle between two oxidation states, being reduced by one reactant and then re-oxidised by the other, so that it is regenerated at the end and can catalyse the next cycle. A classic example is the $\mathrm{Fe^{3+}}$-catalysed reaction between iodide and persulphate ions:

The catalytic action can be explained as proceeding through two steps, in which the iron ion shuttles between the $+3$ and $+2$ states: …