--- title: "Positive/negative catalysis; promoter; catalyst criteria" subject: "Engineering Chemistry" chapter: 2 key: "semester_2/engineering_chemistry:pos-neg-promoter-criteria" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/pos-neg-promoter-criteria site: BCT Engineering --- # Positive/negative catalysis; promoter; catalyst criteria Engineering Chemistry · Chapter 2: Catalyst - Asked in: 2072 Magh, 3+1+1 marks - Video: [Catalyst ll Type of Catalyst ll Catalyst Promoter ll Catalyst Poison ll](https://www.youtube.com/watch?v=Z_5udrbNtHI) — My Academy Online - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/pos-neg-promoter-criteria - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-2 ## Question Distinguish between positive and negative catalysis with graphically and give one example of each. Define catalytic promoter. Write the criteria for choosing a catalysis for industrial purpose. ## Short answer Positive catalyst increases rate by lowering activation energy (e.g., MnO2 in KClO3 decomposition); negative catalyst decreases rate by raising it (e.g., glycerol in H2O2 decomposition). A promoter enhances catalyst activity, like Mo in Haber process. Industrial catalysts need high activity, selectivity, stability, poison resistance, large surface area, and low cost. ## Answer #### Positive and negative catalysis | Point | Positive catalyst | Negative catalyst (inhibitor) | |---|---|---| | Effect on rate | Increases the rate | Decreases the rate | | Activation energy | Lowered | Raised | | Mechanism | Gives an easier path | Blocks the path / destroys a chain carrier or a positive catalyst | | Example | MnO₂ in 2KClO₃ → 2KCl + 3O₂; Fe in Haber's process | Glycerol or acetanilide slows the decomposition of H₂O₂; alcohol slows the oxidation of Na₂SO₃ | ```text Energy barrier (Ea) on the path R → P negative catalyst : ████████████ highest uncatalysed : ████████ medium positive catalyst : ████ lowest (reactant and product energies stay the same) ``` Graphically, the positive catalyst gives a lower peak of activation energy and the negative catalyst a higher one than the uncatalysed path, as shown above. Examples: positive: MnO₂ in decomposition of KClO₃; negative: glycerol in decomposition of H₂O₂. #### Catalytic promoter **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. #### Criteria for 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. Which statement distinguishes a negative catalyst from a positive one? A. A negative catalyst lowers the activation energy B. A positive catalyst decreases the rate C. A negative catalyst decreases the rate by raising the activation energy D. Both lower the activation energy Answer: C. Positive catalysts lower Ea and increase rate; negative catalysts (inhibitors) raise Ea and decrease rate. (Key point: Positive vs negative catalyst) 2. Which is the correct order of activation energy barrier from highest to lowest? A. Negative catalyst > uncatalysed > positive catalyst B. Positive catalyst > uncatalysed > negative catalyst C. Uncatalysed > negative > positive D. Positive > negative > uncatalysed Answer: A. A negative catalyst raises the barrier and a positive catalyst lowers it relative to the uncatalysed path. (Key point: Activation energy order) 3. 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) 4. Why should an industrial catalyst be stable and resistant to poisons? A. To make it react with the products B. To keep its activity for a long time at working conditions C. To make separation impossible D. To lower its surface area Answer: B. Long life, mechanical strength and poison resistance keep the process economical. 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