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CHBE 551 Lecture 24

CHBE 551 Lecture 24. Catalysis I. Importance Of Catalysis. 90% of all chemical processes use catalysts Changes in catalysts have a giant influence on rates and selectivity’s of reactions. More than anything else Most real reactor design associated with optimizing performance of catalyst.

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CHBE 551 Lecture 24

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  1. CHBE 551 Lecture 24 Catalysis I

  2. Importance Of Catalysis • 90% of all chemical processes use catalysts • Changes in catalysts have a giant influence on rates and selectivity’s of reactions. More than anything else • Most real reactor design associated with optimizing performance of catalyst

  3. Catalysis Definition Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process Not being consumed  catalyst does change

  4. Catalytic Reaction Occurs Via A Catalytic Cycle: reactants + catalyst  complex complex  products + catalyst

  5. Example: Rhodium Catalyzed CH3OH+COCH3COOH Printing press analogy

  6. The Rate Enhancement Of A Number Of Reactions In The Presence Of A Catalyst 1040 1020 1042

  7. Examples Of Effects Of Solvent Table 13.2 The rate of the SN2 reaction NaCl + CH3I  NaI + CH3Cl at 350 K ______________________________________________________________ Rate Constant, Rate Constant, Solvent liter/(mol-second) Solvent liter/(mol-second) ______________________________________________________________ Gas phase ~10-45 ______ _______ Water 3.5 x 10-6 Methylcyanide 0.13 Methanol 3.1 x 10-6 Dimethylformamide 2.5 ______________________________________________________________

  8. Types Of Catalysts • Homogeneous catalysts: Soluble compounds that go in solutions • Heterogeneous catalysts: Solids that sit in reactors

  9. Common Examples Of Catalysts Homogeneous Enzymes (detergents, digestion, tears) Heterogeneous Catalytic converter

  10. Rate Laws Different Than With Gas Phase Reactions Rate proportional to the surface area not the volume.

  11. Effects Of Surface Area Consider a platinum catalyzed reaction. You can run the reaction • Run the reaction on the wire • Take the wire and smash it with a hammer and then run the reaction. The rate will be higher on the wire you smashed with a hammer!

  12. Why Does Smashing A Wire Change The Rate? • When you squashed the platinum you created more surface area. • You also changed the shape of the surface which can affect the rate.

  13. Turnover Numbers Rates of catalytic reactions often expressed as turnover number RA = Rate per unit area (molecules/cm2-sec) NS = Number of exposed metal atoms / unit area (Atoms/cm2)

  14. Turnover Numbers For Some Typical Reactions

  15. Typical Catalytic Kinetics Figure 2.15 The influence of the CO pressure on the rate of CO oxidation on Rh(111). Data of Schwartz, Schmidt, and Fisher

  16. Typical Catalyst Kinetics Called a Langmuir-Hinshelwood rate law. Also called Monod rate law.

  17. Temperature Dependence Figure 2.18 The rate of the reaction CO + 2 O2 CO2 on Rh(111). Data of Schwartz, Schmidt and Fisher[1986].

  18. Catalysts Do Not Work Over A Broad Temp Range

  19. Types Of Catalysts: Homogeneous Catalysts Heterogeneous Catalysts

  20. Homogeneous Catalysts: • Acids or Bases • Metal salts • Enzymes • Radical initiators

  21. Table 12.2-Some Reactions Commonly Catalyzed By Acids And Bases

  22. Acids And Bases As Catalysts

  23. Acids As Catalysts Continued

  24. Solid Acids And Bases As Catalysts

  25. Very Complex Pore Structure Figure 12.4 A diagram of the pore structure in Faugasite.

  26. Enzymes As Catalysts

  27. Solvents: As Catalysts

  28. Next: Heterogeneous Catalysis Examples of heterogeneous catalysts include: • Supported Metals • Transition Metal Oxides and Sulfides • Solid Acids and Bases • Immobilized Enzymes and Other Polymer Bound Species

  29. Supported Metal Catalysts Use support because platinum very expensive and only the surface is active. Spread platinum out on cheap support. Support also provides strength Figure.12.3 A picture of a supported metal catalyst.

  30. Advantage Of Heterogeneous Catalysts Compared To Homogeneous: • Cheaper separation • More selective • Generally cheaper Disadvantage • Not quite as active or a per metal atom basis

  31. Typical Mechanism Of Heterogeneous Catalysis (H2+C2H4C2H6)

  32. Summary • Catalysts make a tremendous difference to rates • 1013 enhancement average 1040 possible • Kinetics change – rarely linear • Optimum conditions often at max rate • Zero order kinetics • Two types of catalysts • Homogeneous • Heterogeneous • Homogeneous more active • Heterogeneous less expensive to use/control

  33. Query • What did you learn new in this lecture?

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