Topic 6 of 6

How transition-metal catalysts work

Follow a surface cycle and an electron-transfer cycle.

A-Level 9476 (2026-2027)

Catalysis uses accessible bonding and oxidation states

A catalyst participates in steps and is regenerated overall.

Transition metals and their compounds can offer a pathway with lower activation energy. Their ability to interact with reactant electron density helps surface catalysis; accessible oxidation states help electron-transfer cycles. These are chemical reasons for catalytic activity, not a claim that every transition metal catalyses every reaction.

Iron in the Haber process: heterogeneous catalysis

Adsorb
N₂ and H₂ attach to active sites on solid iron. Interactions with the surface weaken their bonds.
React
Surface species are brought together and react through steps with lower activation barriers than the uncatalysed route.
Desorb
NH₃ leaves, freeing sites for another cycle. Iron is regenerated; N₂ + 3H₂ ⇌ 2NH₃.

In a catalytic converter, transition-metal surfaces also help CO react with NO: 2CO(g) + 2NO(g) → 2CO₂(g) + N₂(g). Adsorption holds reactants near each other and facilitates bond changes. The catalyst does not change the reaction enthalpy or equilibrium constant.

Worked example

An iron-ion cycle transfers electrons in solution

Explain how Fe²⁺ can catalyse S₂O₈^2- + 2I^- → 2SO₄^2- + I₂. All reactants and the catalyst are aqueous.

First: S₂O₈^2- + 2Fe²⁺ → 2SO₄^2- + 2Fe³⁺. Iron(II) donates electrons.
Next: 2Fe³⁺ + 2I^- → 2Fe²⁺ + I₂. Iron(III) accepts electrons.
Adding the steps cancels both iron species and gives the required overall reaction. Fe²⁺ is regenerated.
Oppositely charged reactants can meet in each catalysed step, replacing the direct encounter between two anions with a lower-barrier route.

Answer

This is homogeneous catalysis using the Fe(II)/Fe(III) pair. A positive overall cell potential establishes feasibility, while the catalyst addresses the kinetic barrier.