Understanding Catalysts in Chemical Reactions

1. Catalysts

2. Learning Objectives • Catalysts can be classified as either heterogeneous or homogeneous. • Heterogeneous catalysts work by the adsorption of reactant molecules. • The surface activity of a catalyst can be reduced by poisoning. • Impurities in the reactants result in the industrial catalysts having to be regenerated or renewed. • Catalytic converters are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen.

3. Definition of Catalysts • Speed up the rate of a chemical reaction • Are not chemically changed or used up in the reaction • May be specific for a particular reaction

4. Types of catalysts • Homogeneous catalysts –reactants and catalyst in same state • Heterogeneous catalysts – reactants and catalysts in different states

5. How do heterogeneous catalysts work? • Catalyst is usually solid • Reactants are adsorbed onto surface • Adsorb – formation of weak bonds

6. Catalyst

7. Reactant A Catalyst

8. Adsorption occurs at active site Catalyst

9. Catalyst

10. Reactant B Catalyst

11. Catalyst

12. Catalyst

13. Catalyst

14. Catalyst

15. Products Catalyst

18. H H • E.g Hydrogenating alkenes (turning oils into fats) H H C CH H H H

19. H H C CH H H H H H • Bonds within reactant molecules are weakened and break

20. H H C CH H H H • New bonds form between the reactants H H

21. H H C CH H H H H • New bonds form between the reactant molecules H

22. H H C CH H H H • The product(s) leave the catalyst surface H H

23. Catalysts NUMBER OF MOLECUES WITH A PARTICULAR ENERGY NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER Ea MOLECULAR ENERGY

24. Catalysts • Catalysts lower the activation energy for a reaction EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER NUMBER OF MOLECUES WITH A PARTICULAR ENERGY Ea MOLECULAR ENERGY

25. EXAMPLES OF CATALYSTS • Metals Ni, Pt hydrogenation reactions Fe Haber Process Rh, Pd catalytic converters • Format is FINELY DIVIDED • This increases the surface area and provides more active sites

26. Catalyst poisoning • In heterogeneous catalysis Catalyst poisoned by a chemical binding to surface • Poison bonds irreversibly with catalyst • Sulphur and lead are two examples of catalyst poisons

27. Poisoning Catalyst

28. Poisoning Catalyst

29. Poisoning • Active site taken up by poison • Effectiveness of catalyst reduced. Catalyst

30. Cleaning a catalyst • Catalysts can sometimes be regenerated by blowing hot air over catalyst – oxidises chemicals on surface • Used to remove carbon (soot) after catalytic cracking

31. Catalytic Converters • Found in car exhaust systems • Reduce levels of pollutant gases • Pollutants react in exhaust to form less harmful gases.