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ChE 553 Lecture 23

ChE 553 Lecture 23. Catalysis By Surfaces. Objective For Today. Ask How Surfaces Can Catalyze Reactions. Catalysis Definition. Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process

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ChE 553 Lecture 23

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  1. ChE 553 Lecture 23 Catalysis By Surfaces

  2. Objective For Today • Ask How Surfaces Can Catalyze Reactions

  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. Freshman Chemistry View Of Catalysts • “Catalysts lower barriers to chemical reactions” • Catalysts often raise barriers to elementary steps in a reaction mechanism!

  5. Key Mechanism Of Catalyst Action • Stabilizing intermediates by bonding to them • The Increase in intermediate concentration leads to a substantial increase in rate.

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

  7. Example: Rhodium Catalyzed CH3OH+COCH3COOH Methyl radicals are not stable in the gas phase or solution but they are stable when bound to rhodium. The higher concentration leads to a higher rate

  8. Example To Illustrate How Increasing Barriers Can Increase Rates consider AAadB with the formation of B rate determining The rate of reaction, r, is given by r= KadkBPA/(1+KadPA) Where Kad is the equilibrium constant for adsorption, kB is the rate constant for the production of B and PA is the pressure of A

  9. Note the correct rate equation is r= KadkBPA/(1+KadPA) I am assuming that the last term is small

  10. Model That I will derive In Lecture 22 • Kad = K0* exp((ΔGad+X)/RT) Where ΔGad is free energy of adsorption on some reference surface, and X is the change in free energy in moving to some other surface kB=ko*exp(-(Ea+0.5 X)/RT So as you increase X, you increase the activation energy for the formation of B.

  11. Now Combine The Equations r = K0* exp((ΔGad)/RT)* ko*exp(-(Ea)/RT)* exp((0.5*X)/RT) When X=0, r=r0 the rate on the reference surface, Combining r = r0*exp((0.5*X)/RT)

  12. Now Consider How Increases In X Changes The Rate From before kB=ko*exp(-(Ea+0.5 X)/RT r = r0*exp((0.5*X)/RT) Notice that when we increase X, we increase the activation barrier for B formation, yet the rate of B formation goes up!

  13. Physical Interpretation • The rate is given by • r= kBθB • When we increase X, we increase θB and decrease kB. As long as the increases in θB are larger than the decrease in kB the net rate will increase • Most catalysts work by increasing θB. The rate increases even though the activation barrier for the rate determining step goes up.

  14. Limit To The Analysis • θB cannot be bigger than 1 • r= kBθB = KadkBPA/(1+KadPA) • At low coverages increases in θB dominate so the net rate will increase • Once θB gets close to 1, θB cannot increase any more. The rate decreases with increasing X.

  15. Plot of actual case from lect 24

  16. Net Result Is Volcano Behavior (12.75)

  17. Sabatier’s Principle The best catalysts are substances which bind the reactants strongly, but not too strongly.

  18. What Types Of Reactions Will Be Increased By Metal Surfaces? • Bulk metals have many free electrons • Free electrons rapidly exchange with radicals • radical intermediates stabilized • Free electrons neutralize carbocations • Carbocation reactions slowed down

  19. A Selection Of The Reactions Catalyzed By Supported Metals Key intermediates, hydrogen atoms, oxygen atoms, methyls ethyls …

  20. Insulating Oxides Stabilize Ionic Intermediates • Insulating oxides have no free electrons • radical intermediates not stabilized • Insulating oxides are ionic • Ionic intermediates, i.e. carbocations can be stabilized

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

  22. Semiconductors Are In Between • Semiconductors have fewer free electrons, and also be ionic • Usually use semiconductors when you want to stabilize some radicals e.g. oxygen atoms or sulfur atoms but not other radicals, such as hydrogen atoms.

  23. A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides (Semiconductors)

  24. A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides

  25. Summary • Most catalysts work by stabilizing intermediates • Intermediate concentration goes up • Activation energy for product formation goes up (not down)! • Rate constant for product formation goes down • Net effect is an increase in rate • Metals, semiconductors, insulators stabilize different intermediates – gives different chemistry

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