How can a promoter increase the activity of a catalyst?

By increasing the active sites (surface or crystal defects) or improving reactant adsorption

By lowering the temperature to zero

By changing the products formed

How does a poison make a catalyst inactive?

It is strongly adsorbed on the active sites (or forms a stable compound), so reactants cannot be adsorbed

It lowers the activation energy too much

It increases the surface area of the catalyst

Why are reactants purified before industrial catalytic processes?

To remove poisons that would inactivate the catalyst

2068 Magh · 3 marks

Action of promoters and poisons

Short answer

Promoters enhance catalyst activity by increasing active sites or improving reactant adsorption, such as Mo boosting iron in ammonia synthesis. Poisons deactivate catalysts by strongly adsorbing on active sites, like As₂O₃ on platinum, blocking reactant access permanently or temporarily.

Explain the action of promoters and poisons.

Action of promoters

Promoters (activators) are substances that, though not themselves catalysts, increase the activity of a catalyst when added in a small quantity. They may increase the active surface or change the crystal structure (creating more defects/active sites), or they may improve the adsorption of the reactants.

In Haber's process, Mo (or Al₂O₃ + K₂O) is a promoter for the iron catalyst.
In the water-gas shift reaction, Cr₂O₃ promotes Fe₂O₃.
Cu promotes Ni in some hydrogenation reactions.

A promoter works by increasing the number of active sites (for instance by giving a rougher surface or changing the lattice spacing of the catalyst) or by improving the adsorption of the reactants. The catalyst becomes more active and lasts longer.

Action of poisons

Catalytic poisoning: a small amount of some substance (the poison) greatly reduces or destroys the activity of a catalyst. Poisons do not take part in the main reaction.

As₂O₃ poisons platinum in the contact process (2SO₂ + O₂ → 2SO₃).
H₂S, CO and compounds of S or P poison iron in the Haber process.
CO poisons platinum in the reaction 2H₂ + O₂ → 2H₂O and in hydrogenation reactions; lead compounds poison the Pt catalyst of a car's catalytic converter.

How a poison acts: the poison is strongly adsorbed on the active sites of the catalyst surface (or forms a stable compound with the catalyst). The sites are then unavailable for the reactant molecules, so the surface area available for reaction decreases and the catalyst becomes inactive. Poisoning may be temporary (poison removed by heating/washing) or permanent. For this reason the reactants are purified before use in industry.

✓ Answer checked by Claude Opus 5.5 (high effort)

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