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9476 / 2027
Transition Elements

Lesson 6 of 6

How transition-metal catalysts work

Follow a surface cycle and an electron-transfer cycle.

A-Level 9476 (2026-2027)

  • I can explain catalytic action of transition elements and compounds.Syllabus

    Syllabus 9476, 13(m). Explain catalytic action of transition elements and compounds.

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
  1. Adsorb

    N2 and H2 attach to active sites on solid iron. Interactions with the surface weaken their bonds.

  2. React

    Surface species are brought together and react through steps with lower activation barriers than the uncatalysed route.

  3. 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 exampleAn iron-ion cycle transfers electrons in solution

Explain how Fe2+ can catalyse S2O82- + 2I- → 2SO42- + I2. All reactants and the catalyst are aqueous.

Try it first. Then show the worked stepsHide the worked steps
  1. First: S2O82- + 2Fe2+ → 2SO42- + 2Fe3+. Iron(II) donates electrons.
  2. Next: 2Fe3+ + 2I- → 2Fe2+ + I2. Iron(III) accepts electrons.
  3. Adding the steps cancels both iron species and gives the required overall reaction. Fe2+ is regenerated.
  4. 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.

Check your understanding

Why are transition metals and their ions often good catalysts?

  1. They have variable oxidation states and can adsorb reactants onto their surfaces.
  2. They shift the equilibrium to the right, so more product forms from the reactants.
  3. They are very reactive metals, so they react with and use up one of the reactants.
Show the answer

They have variable oxidation states and can adsorb reactants onto their surfaces.

Changing oxidation state lets an ion pass electrons in a cycle and be regenerated. A metal surface can hold reactants close and weaken their bonds.