--- title: "Adsorption theory; poisoning; industrial applications" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:adsorption-poison-applications" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/adsorption-poison-applications site: BCT Engineering --- # Adsorption theory; poisoning; industrial applications Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2067 Mangsir, 3+1+1 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/adsorption-poison-applications - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Describe the adsorption theory of catalysis with an example. How does a poison paralyze the activity of a catalyst? Give any two industrial applications of catalysts. ## Short answer The adsorption theory describes catalysts adsorbing reactants, activating them, and releasing products, as in ethene hydrogenation on nickel. Poisons paralyze catalysts by covering active sites, e.g., As₂O₃ on Pt. Industrial applications include ammonia synthesis (Fe catalyst) and sulfuric acid production (V₂O₅ catalyst). ## Answer #### Adsorption theory 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). #### How a poison paralyses a catalyst A poison is adsorbed strongly on the active centres of the catalyst, or forms a stable compound with it. The active sites are covered, so the reactant molecules cannot be adsorbed and the surface can no longer carry out the reaction. Examples: As₂O₃ on Pt, H₂S on Fe. #### Two industrial applications 1. Manufacture of ammonia by Haber's process: N₂ + 3H₂ ⇌ 2NH₃ with Fe catalyst (Mo promoter). 2. Manufacture of sulphuric acid by contact process: 2SO₂ + O₂ ⇌ 2SO₃ with V₂O₅ (or Pt). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. 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) 2. 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) 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. In Haber's process N2 + 3H2 <=> 2NH3, which catalyst and promoter are used? A. V2O5 catalyst with Mo promoter B. Pt catalyst with As2O3 promoter C. Ni catalyst with Cu promoter D. Fe catalyst with Mo promoter Answer: D. Fe is the catalyst, with Mo (or Al2O3 + K2O) as promoter. (Key point: Haber's process) ## Related in Engineering Chemistry - [Catalyst action; heterogeneous catalysis; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Acatalyst-heterogeneous-adsorption) — 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 - [Promoters, adsorption theory, catalyst criteria](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apromoters-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 - [Catalytic poisoning; adsorption theory](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apoisoning-adsorption) — Engineering Chemistry, chapter 2 - [Autocatalysis; positive/negative catalysis; poison](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aautocatalysis-poison) — Engineering Chemistry, chapter 2