--- title: "Heterogeneous catalysis and its theory" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:heterogeneous-catalysis-theory" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/heterogeneous-catalysis-theory site: BCT Engineering --- # Heterogeneous catalysis and its theory Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2067 Chaitra, 1+4 marks - Video: [Principles of Heterogeneous Catalysis](https://www.youtube.com/watch?v=evBiIO6fuHI) — Professor Dave Explains - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/heterogeneous-catalysis-theory - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Define heterogeneous catalysis. Give a brief account on theory of Heterogeneous catalysis. ## Short answer Heterogeneous catalysis occurs when the catalyst is in a different phase from reactants (e.g., solid Fe with gaseous N2/H2). The adsorption theory explains it: reactants adsorb on active sites, bonds weaken, react, and products desorb, freeing sites for reuse. Fine division increases surface area, poisons block sites, promoters enhance activity. ## Answer #### Heterogeneous catalysis **Heterogeneous catalysis:** the catalyst is in a different phase from the reactants; usually a solid catalyst with gaseous or liquid reactants. - N₂(g) + 3H₂(g) → 2NH₃(g), catalyst Fe(s) (Haber's process) - 2SO₂(g) + O₂(g) → 2SO₃(g), catalyst V₂O₅(s) or Pt (contact process) - Vegetable oil + H₂ → vegetable ghee, catalyst Ni(s) (hydrogenation) - 4NH₃ + 5O₂ → 4NO + 6H₂O, catalyst Pt gauze (Ostwald's process) #### Theory: 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In heterogeneous catalysis the catalyst is: A. In the same phase as the reactants B. Always a gas C. In a different phase from the reactants, usually a solid with gaseous or liquid reactants D. Consumed during the reaction Answer: C. Heterogeneous catalysis has the catalyst in a separate phase, commonly a solid. (Key point: Heterogeneous catalysis definition) 2. Which is an example of heterogeneous catalysis? A. Haber's process: N2(g) + 3H2(g) -> 2NH3(g) over solid Fe B. Acid hydrolysis of methyl acetate with H+ C. Inversion of cane sugar with H+ D. Lead chamber process with NO(g) Answer: A. Solid Fe with gaseous reactants are different phases. The others have catalyst and reactants in the same phase. (Key point: Example of heterogeneous catalysis) 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. Which observation does the adsorption theory explain? A. A finely divided catalyst is more active because of its larger surface area B. Catalysts shift the equilibrium position C. Homogeneous catalysts work in solution D. Catalysts change the value of DeltaH Answer: A. A larger surface gives more active sites for adsorption. The theory also explains poisons (blocked sites) and promoters. 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