Homogeneous and heterogeneous catalysis

7 minAS Level8.3.1

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can classify a catalyst as homogeneous or heterogeneous, describe how a solid catalyst works in terms of adsorption, and explain why surface area and poisoning matter.

Two ways for a catalyst to meet its reactants

A catalyst can only work where it meets the reactants. That gives two possibilities.

Homogeneous catalysis: the catalyst is in the same phase as the reactants. Everything is dissolved in one solution, or everything is gas, so the molecules mix at the molecular level.
Heterogeneous catalysis: the catalyst is in a different phase, almost always a solid catalyst with gaseous or liquid reactants. The reaction happens on the surface of the solid.

The words are built from Greek: homo = same, hetero = different.

Definition

Homogeneous and heterogeneous catalysis

Homogeneous catalysis: the catalyst is in the same physical state as the reactants, for example $Fe^{2+}(aq)$ catalysing a reaction between two aqueous ions.

Heterogeneous catalysis: the catalyst is in a different physical state from the reactants, for example iron solid catalysing the reaction of nitrogen and hydrogen gases.

Heterogeneous catalysis in three stages: adsorb, react on the surface, desorb. The surface is left unchanged.

What happens on a solid surface

A heterogeneous catalyst works in three stages.

Adsorption. Reactant molecules attach to the surface at active sites.
Reaction. Held in place and with their bonds weakened by the surface, the molecules react through a lower-energy route.
Desorption. The product molecules leave the surface, freeing the active sites.

Because the reaction happens at the surface, only surface atoms count. A finely divided catalyst, or one spread over a support, has far more active sites per gram than a solid lump — which is why catalysts are used as powders or on honeycomb supports.

Worked example: Three industrial catalysts

Classify each catalyst and say which phase each substance is in.

(a) Iron in the Haber process: $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ .

(b) Vanadium(V) oxide in the Contact process: $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$ .

Have a go first?

Go deeper: Why catalysts are poisoned, and why that matters

An active site is a particular patch of surface where adsorption is just right. If a strongly adsorbing impurity sticks there and does not leave, that site is lost. This is catalyst poisoning.

Sulfur compounds poison the iron catalyst in the Haber process, which is why the gas mixture is purified first. Lead used to poison the platinum in car catalytic converters, which is one reason leaded petrol was phased out.

Poisoning is not the same as the catalyst being "used up" in the reaction: the catalyst is still there, but part of its surface is blocked.

Common mistakes

Homogeneous means the catalyst is the same substance as the reactants.

It means the same phase — same physical state. The catalyst is still a different substance.

In heterogeneous catalysis the catalyst dissolves in the reactants.

It stays as a separate solid phase; the reactants adsorb onto its surface.

A catalyst works equally well as a large lump or a fine powder.

Only surface atoms are active sites, so a powder with a much larger surface area is far more effective per gram.

A poisoned catalyst has been used up in the reaction.

In the exam

To classify, look at the state symbols: same state as the reactants means homogeneous; a solid with gases or solutions means heterogeneous.

For heterogeneous catalysis, use the sequence adsorption → reaction → desorption, and refer to active sites and surface area.

Common examples worth knowing: iron (Haber), vanadium(V) oxide (Contact), nickel (hydrogenation of alkenes), platinum/rhodium/palladium (catalytic converters).

Check yourself

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Recall

Answer the 2 questions above to continue.