Transition metals are exceptional catalysts because of their unique electronic structure — partially filled d-orbitals. This gives them two key catalytic abilities: variable oxidation states (they can gain and lose electrons reversibly, enabling redox catalysis) and the ability to form coordinate bonds with reactant molecules (enabling surface adsorption and intermediate formation).

In heterogeneous catalysis, the metal surface provides active sites where reactant molecules adsorb, have their bonds weakened, react with each other, and then desorb as products. The catalyst is regenerated in the process. The key is that the metal-reactant bond must be strong enough to hold reactants but weak enough to release products — this is the Sabatier principle, and transition metals near the middle of the d-block (Fe, Ni, Pt, Pd) satisfy it best.

Sulphuric acid (H₂SO₄) is the most produced industrial chemical in the world, with about 200 million tonnes made annually. It is used to make fertilisers, detergents, dyes, drugs, and hundreds of other chemicals. The Contact process involves three key steps:

The Haber process for ammonia synthesis is one of the most important chemical reactions in history — it enabled nitrogen fixation at industrial scale, making artificial fertilisers possible and supporting the food needs of billions of people. Fritz Haber won the Nobel Prize in 1918 for developing it.

The Wacker process was developed in the 1950s and revolutionised production of acetaldehyde. The full catalytic cycle involves three coupled redox reactions:

The hydrogenation mechanism on nickel surface follows the Langmuir-Hinshelwood model: (1) H₂ adsorbs on Ni surface and dissociates into two H atoms strongly bound to surface Ni atoms. (2) The alkene approaches and adsorbs on the surface. (3) One H atom is transferred from the surface to one carbon of the double bond (half-hydrogenated intermediate). (4) The second H atom transfers to the adjacent carbon. (5) The fully saturated alkane desorbs from the surface. This is why partial hydrogenation is possible — the half-hydrogenated intermediate can sometimes desorb before the second H is added, creating a mix of saturated and unsaturated products.

Promoters enhance catalyst activity without themselves being catalysts. Mo in the Haber process prevents Fe particles from sintering at high temperatures, maintaining high surface area. K₂O increases electron density on Fe surface, improving N₂ adsorption. Al₂O₃ acts as a structural promoter, keeping Fe particles separated.

Catalyst poisons reduce or destroy activity. Arsenic, phosphorus, and sulphur compounds poison the Fe catalyst in Haber process. Carbon monoxide poisons Pt in fuel cells and automotive catalysts. Lead compounds in petrol poisoned Pt/Pd catalytic converters (reason for switch to unleaded petrol). Enzyme inhibitors in biology work similarly — nerve agents (sarin, VX) irreversibly inhibit acetylcholinesterase.

In heterogeneous catalysis (Fe, V₂O₅, Ni), the solid catalyst and gaseous/liquid reactants are in different phases. The reaction occurs at the catalyst surface. Advantages: easy separation, continuous operation, high temperature use. Disadvantage: mass transfer limitations, surface poisoning.

In homogeneous catalysis (PdCl₂ in aqueous Wacker process), catalyst and reactants are in the same phase (solution). Advantages: every catalyst molecule is accessible, mechanistic study possible. Disadvantages: separation of catalyst from product is difficult and costly. Most modern research focuses on making homogeneous catalysts recyclable — by immobilising them on solid supports (heterogenising homogeneous catalysts).