- 2072 Magh · 3+1+1 marks
Positive/negative catalysis; promoter; catalyst criteria
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.
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.
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 |
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Catalyst; positive and negative catalysis with examples · Engineering Chemistry, chapter 2
- Autocatalysis; positive/negative catalysis; poison · Engineering Chemistry, chapter 2
- Positive/negative catalyst; intermediate compound theory · Engineering Chemistry, chapter 2
- Heterogeneous catalysis; adsorption theory; criteria · Engineering Chemistry, chapter 2
- Catalysis, equilibrium effect, promoter · Engineering Chemistry, chapter 2
- Promoters, adsorption theory, catalyst criteria · Engineering Chemistry, chapter 2
- Criteria for choosing HVDC or HVAC · High Voltage Engineering, chapter 1
- Effective mass can be positive or negative · Electrical Engineering Material, chapter 1 · asked 4×
- Positive XOR equals negative XNOR · Digital Logic, chapter 2 · asked 3×
- Prove positive XOR equals negative XNOR · Digital Logic, chapter 2 · asked 3×