Catalysts, interstitial compounds and alloys

Transition metals and their compounds are good catalysts because they can take several oxidation states and form complexes. Examples: V₂O₅ in the Contact process, finely divided iron in the Haber process, nickel in catalytic hydrogenation.

On a solid catalyst, reactant molecules bond to surface metal atoms (first-row metals use their 3d and 4s electrons). This raises the reactant concentration at the surface and weakens bonds in the reacting molecules, lowering the activation energy.

A change of oxidation state lets an ion shuttle electrons. Iron(III) catalyses 2I⁻ + S₂O₈²⁻ → I₂ + 2SO₄²⁻ in two steps: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂, then 2Fe²⁺ + S₂O₈²⁻ → 2Fe³⁺ + 2SO₄²⁻. The iron ends as it began.

Interstitial compounds form when small atoms such as H, C or N are trapped in the holes of a metal lattice. They are usually non-stoichiometric and neither typically ionic nor covalent: TiC, Mn₄N, Fe₃H, VH₀.₅₆ and TiH₁.₇. Their formulas do not correspond to normal oxidation states.

Interstitial compounds melt higher than the pure metals, are very hard (some borides approach diamond), keep metallic conductivity and are chemically inert.

An alloy is a blend of metals. In a homogeneous solid-solution alloy one metal's atoms are scattered at random among the other's, which works when the metallic radii are within about 15 percent of each other. Transition metals, with similar radii, alloy readily, and their alloys are hard and often high-melting.

Cr, V, W, Mo and Mn are used to make many steels and stainless steel. Alloys with non-transition metals matter too: brass (copper-zinc) and bronze (copper-tin).