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ChE 553 Lecture 14

ChE 553 Lecture 14. Catalytic Kinetics. Objective. Provide an overview of catalytic kinetics How do rates vary with concentration Simple modles: langmuir. Key Ideas For Today. Generic mechanisms of catalytic reactions Measure rate as a turnover number Rate equations complex

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ChE 553 Lecture 14

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  1. ChE 553 Lecture 14 Catalytic Kinetics

  2. Objective • Provide an overview of catalytic kinetics • How do rates vary with concentration • Simple modles: langmuir

  3. Key Ideas For Today Generic mechanisms of catalytic reactions • Measure rate as a turnover number • Rate equations complex • Langmuir Hinshelwood kinetics

  4. Generic Mechanisms Of Catalytic Reactions Figure 5.20 Schematic of a) Langmuir-Hinshelwood, b) Rideal-Eley, c) precursor mechanism for the reaction A+BAB and ABA+B.

  5. Turnover Number: Number Of Times Goes Around The Catalytic Cycle Per Second • Printing press analogy • Reactants bind to sites on the catalyst surface • Transformation occurs • Reactants desorb Figure 12.1 A schematic of the catalytic cycle for Acetic acid production via the Monsanto process.

  6. Typical Catalytic Cycle Figure 5.10 Catalytic cycles for the production of water a) via disproportion of OH groups, b) via the reaction OH(ad)+H)ad)H2O

  7. Definition Of Turnover Number (12.119) Physically, turnover number is the rate that the catalyst prints product per unit sec.

  8. Typical Turnover Numbers

  9. Next Topic Why Catalytic Kinetics Different Than Gas Phase 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.

  10. More Typical Behavior Figure 2.18 The rate of the reaction CO + 2 O2 CO2 on Rh(111). Data of Schwartz, Schmidt and Fisher[1986]. A) = 2.510-8 torr, = 2.510-8 torr, B) = 110-7 torr, = 2.510-8 torr, C) = 810-7 torr, = 2.510-8 torr, D) = 210-7 torr, = 410-7 torr, E) = 210-7 torr, = 2.510-8 torr, F) = 2.510-8 torr, = 2.510-8 torr,

  11. Physical Interpretation Of Maximum Rate For A+BAB • Catalysts have finite number of sites. • Initially rates increase because surface concentration increases. • Eventually A takes up so many sites that no B can adsorb. • Further increases in A decrease rate.

  12. 1 S + A A ad 2 3 A C Ad ad 4 5 C C + S ad 6 (12.121) Derivation Of Rate Law For AC Mechanism   Rate Determining Step  

  13. Derivation: Next uses the steady state approximation to derive an equation for the production rate of Cad (this must be equal to the production rate of C).

  14. 1 S + A A ad 2 3 A C Ad ad 4 5 C C + S ad 6 (12.121) Derivation  SS approximation on Aad and Cad   People usually ignore reactions 3 and 4 since their rates very low rates compared to the other reactions.

  15. Dropping The k3 And k4 Terms In Equations 12.124 And 12.125 And Rearranging Yields:

  16. Rearranging Equations (12.126), (12.127) And (12.128) Yields:

  17. Derivation Continued Equations (12.129) and (12.130) imply that there is an equilibrium in the reactions: Aad A + S  Cad C + S 

  18. Site Balance To Complete The Analysis If we define S0 as the total number as sites n the catalyst, one can show: Pages of Algebra

  19. Substituting Equations (12.140) And (12.141) Into Equation (12.123) Yields: (12.142) In the catalysis literature, Equation (12.142) is called the Langmuir-Hinshelwood expression for the rate of the reaction AC, also called Michaele’s Menton Equation.

  20. Qualitative Behavior For Unimolecular Reactions (AC)

  21. Langmuir-Hinshelwood-Hougan-Watson Rate Laws: Trick To Simplify The Algebra Hougan and Watson’s Method: • Identify rate determining step (RDS). • Assume all steps before RDS in equilibrium with reactants. • All steps after RDS in equilibrium with products. • Plug into site balance to calculate rate equation.

  22. Example:

  23. Solution

  24. Solution Continued

  25. Qualitative Behavior For Bimolecular Reactions (A+Bproducts) Figure 12.32 A plot of the rate calculated from equation (12.161) with KBPB=10.

  26. Physical Interpretation Of Maximum Rate For A+BAB • Catalysts have finite number of sites. • Initially rates increase because surface concentration increases. • Eventually A takes up so many sites that no B can adsorb. • Further increases in A decrease rate.

  27. S + A A ad 2 3 A C Ad ad 4 5 C C + S ad 6 Another Example: Consider a different reaction (12.120) 1    (12.121)

  28. Derivation The Same

  29. What Does This Look Like?

  30. Summary • Catalytic reactions follow a catalytic cycle reactants + S adsorbed reactants Adsorbed reactants products + S • Different types of reactions Langmuir Hinshelwood Rideal-Eley

  31. Summary • Calculate kinetics via Hougan and Watson; • Identify rate determining step (RDS) • Assume all steps before RDS in equilibrium with reactants • All steps after RDS in equilibrium with products • Plug into site balance

  32. Key Predictions Unimolecular reactions • Rate increases with pressure, levels • Rate always increases with temperature • Very sensitive to poisons Bimolecular reactions • Rate rises reaches a maximum at finite temp and pressure, then drops • Sensitive to poisons

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