--- title: "Promoters, adsorption theory, catalyst criteria" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:promoters-adsorption" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/promoters-adsorption site: BCT Engineering --- # Promoters, adsorption theory, catalyst criteria Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2075 Baisakh, 1+3+1 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/promoters-adsorption - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question What are promoters? Describe the adsorption theory of catalysis with a suitable example. Point out criteria of catalysts used for industrial purpose. ## Short answer Promoters are substances that increase catalyst activity without being catalysts themselves, e.g., Mo in Haber process. The adsorption theory: reactants adsorb on solid catalyst, bonds weaken, react on surface, and product desorbs; e.g., C₂H₄ hydrogenation on Ni. Industrial catalysts need high activity, selectivity, large surface area, stability, and poison resistance. ## Answer #### 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. #### 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). #### Criteria of an industrial catalyst - **High activity:** gives a high rate at moderate temperature and pressure. - **High selectivity:** favours the desired product and avoids side reactions. - **Stability / long life:** keeps its activity for long periods at working temperature and pressure; good mechanical strength. - **Resistance to poisons**, and easy regeneration if poisoned. - **Large surface area** (finely divided or porous, or on a support). - **Low cost and ready availability**; easily separated from the products. ## 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. 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) 3. How does adsorption help activate reactant molecules? A. It strengthens their bonds so they stay stable B. It lowers the concentration of reactants at the surface C. It weakens their bonds (e.g. H-H) and the heat released helps supply activation energy D. It raises the temperature of the whole reactor Answer: C. Adsorbed molecules have weakened bonds and the exothermic adsorption heat helps provide activation energy. (Key point: Activation by adsorption) 4. Which pair is a criterion for a good industrial catalyst? A. Low activity and high cost B. Easily poisoned and short life C. Small surface area and low strength D. High activity and high selectivity Answer: D. An industrial catalyst should give a high rate and favour the desired product. 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