---
title: "Action of promoters and poisons"
subject: "Engineering Chemistry"
chapter: 2
key: "semester_2/engineering_chemistry:promoters-poisons"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/promoters-poisons
site: BCT Engineering
---

# Action of promoters and poisons

Engineering Chemistry · Chapter 2: Catalyst

- Asked in: 2068 Magh, 3 marks
- Video: [Catalytic Promoters & Catalytic Poisons| Explanation & Examples| Accelerators & Inhibitors](https://www.youtube.com/watch?v=PrhNI5ngfww) — Chemistry Planet
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/promoters-poisons
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2

## Question

Explain the 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.

## Answer

#### 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How can a promoter increase the activity of a catalyst?
   A. By blocking the active sites
   B. By lowering the temperature to zero
   C. By changing the products formed
   D. By increasing the active sites (surface or crystal defects) or improving reactant adsorption
   Answer: D. Promoters create more active sites or improve adsorption, making the catalyst more active. (Key point: How promoters act)

2. In the water-gas shift reaction, which pair is correct?
   A. Fe2O3 promotes Cr2O3
   B. Cr2O3 poisons Fe2O3
   C. Cr2O3 promotes Fe2O3
   D. Cu promotes Fe2O3
   Answer: C. Cr2O3 is the promoter and Fe2O3 the catalyst in this reaction. (Key point: Promoter pairs)

3. How does a poison make a catalyst inactive?
   A. It is strongly adsorbed on the active sites (or forms a stable compound), so reactants cannot be adsorbed
   B. It lowers the activation energy too much
   C. It dissolves the reactants
   D. It increases the surface area of the catalyst
   Answer: A. Blocked active sites mean no reactant adsorption and no catalysis. (Key point: Action of a poison)

4. Why are reactants purified before industrial catalytic processes?
   A. To raise the activation energy
   B. To make the catalyst dissolve
   C. To remove poisons that would inactivate the catalyst
   D. To act as promoters
   Answer: C. Traces of poisons such as S, P compounds or CO would ruin the catalyst. (Key point: Purifying reactants)