---
title: "Promoters in catalysis"
subject: "Engineering Chemistry"
chapter: 2
key: "semester_2/engineering_chemistry:catalyst-promoters"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-promoters
site: BCT Engineering
---

# Promoters in catalysis

Engineering Chemistry · Chapter 2: Catalyst

- Asked in: 2069 Bhadra, 1.5 marks
- Video: [Catalyst Promotor and Poison](https://www.youtube.com/watch?v=xwHqglRZ_y4) — E learning Sikkim
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/catalyst-promoters
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2

## Question

Explain the term: Promoters.

## Short answer

Promoters are substances that increase a catalysts activity when added in small amounts, such as by increasing active surface or creating defects. Examples include Mo for iron in Habers process and Cr₂O₃ for Fe₂O₃ in the water-gas shift reaction.

## Answer

**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.

## Check yourself

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

1. What is a promoter?
   A. A substance that destroys catalyst activity
   B. A substance, not itself a catalyst, that increases a catalyst's activity when added in small amount
   C. A substance that decreases the reaction rate
   D. A reactant that is consumed in the reaction
   Answer: B. Promoters (activators) boost catalyst activity, e.g. by more active sites or better adsorption. (Key point: Promoter definition)

2. 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)

3. In Haber's process, which is a promoter for the iron catalyst?
   A. Mo (or Al2O3 + K2O)
   B. As2O3
   C. H2S
   D. CO
   Answer: A. Mo or Al2O3 + K2O promotes Fe, whereas As2O3, H2S and CO are poisons. (Key point: Promoter example)

## Related in Engineering Chemistry

- [Promoters, adsorption theory, catalyst criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apromoters-adsorption) — Engineering Chemistry, chapter 2
- [Homogeneous catalysis](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahomogeneous-catalysis) — Engineering Chemistry, chapter 2, asked 2×