--- title: "Catalytic poisoning; adsorption theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:poisoning-adsorption" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/poisoning-adsorption site: BCT Engineering --- # Catalytic poisoning; adsorption theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2070 Bhadra, 2+3 marks - Video: [Catalyst ,types, poisoning of catalyst](https://www.youtube.com/watch?v=tyS8dz9pfuk) — Prof. Manohar Ahire - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/poisoning-adsorption - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question What do you mean by catalytic poisoning? Explain the adsorption theory of catalysis with an example. ## Short answer Catalytic poisoning is deactivation of a catalyst by strong adsorption of a poison on its active sites, blocking reactant access. Adsorption theory states that reactants adsorb on the catalyst surface, bonds weaken, they react, and the product desorbs. Example: C2H4 and H2 on Ni form C2H6 via surface reaction. ## Answer #### Catalytic poisoning **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. #### Adsorption theory of catalysis According to the **adsorption theory** (for heterogeneous catalysis), the catalyst works by adsorbing the reactant molecules on its surface. The steps are: 1. **Diffusion and adsorption:** reactant molecules diffuse to the catalyst surface and are adsorbed on the active sites, so their concentration at the surface becomes high. 2. **Activation:** adsorption weakens the bonds of the reactants (e.g. H–H breaks), forming activated, reactive species. Adsorption is exothermic, and the heat released helps to supply activation energy. 3. **Surface reaction:** the activated adsorbed molecules, now close together, react to form the product. 4. **Desorption:** the product molecules have little affinity for the surface; they leave the surface, which frees the active sites. 5. The freed sites adsorb new reactant molecules and the cycle repeats. ```text H₂ C₂H₄ H–H C=C H H ↓ ↓ ↓ ↓ | | ▒▒▒▒▒▒▒▒▒▒▒ → ▒▒▒▒▒▒▒▒▒▒▒ → H–C–C–H (leaves) Ni surface adsorbed, H–H | | (adsorption) bond broken H H ``` **Example:** hydrogenation of ethene on nickel: C₂H₄ + H₂ → C₂H₆ (Ni). H₂ is adsorbed on Ni and splits into H atoms, ethene is also adsorbed, the H atoms add to the double bond, and ethane is desorbed. The theory explains why a finely divided catalyst is more active (larger surface), why poisons stop catalysis (they occupy the active sites) and why promoters help (they increase the active surface). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Catalytic poisoning is: A. Increase of catalyst activity by a promoter B. Loss of catalyst activity caused by a small amount of a foreign substance C. Complete consumption of the catalyst in the reaction D. Loss of catalyst by melting only Answer: B. A small amount of poison greatly reduces or destroys activity. (Key point: Poisoning definition) 2. 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) 3. What is the correct order of steps in the adsorption theory of catalysis? A. Activation, desorption, adsorption, surface reaction B. Adsorption, activation, surface reaction, desorption C. Surface reaction, adsorption, desorption, activation D. Adsorption, desorption, activation, surface reaction Answer: B. Reactants are adsorbed, activated, react on the surface, and the products desorb to free the sites. (Key point: Steps of adsorption theory) 4. In hydrogenation of ethene on nickel, what happens on the surface? A. Ni dissolves in ethene and forms an ionic compound B. H2 is adsorbed and splits into H atoms, which add to adsorbed ethene to give ethane that desorbs C. Ethene splits into carbon atoms on Ni D. Ni is converted to nickel hydride permanently Answer: B. H-H bond breaks on Ni, H atoms add across C=C, and ethane leaves the surface. (Key point: Ethene hydrogenation on Ni) ## Related in Engineering Chemistry - [Catalytic poisoning; intermediate compound theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalytic-poisoning) — Engineering Chemistry, chapter 2 - [Adsorption theory; poisoning; industrial applications](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aadsorption-poison-applications) — Engineering Chemistry, chapter 2 - [Catalyst action; heterogeneous catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-heterogeneous-adsorption) — Engineering Chemistry, chapter 2 - [Heterogeneous catalysis; adsorption theory; criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ahetero-adsorption-criteria) — Engineering Chemistry, chapter 2 - [Adsorption theory of catalysis](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aadsorption-theory) — Engineering Chemistry, chapter 2 - [Catalyst, catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-definition-adsorption) — Engineering Chemistry, chapter 2