1 / 33

ChE 553 Lecture 15

ChE 553 Lecture 15 . Catalytic Kinetics Continued. Object. Examine the effects of pairwise interactions on rates of surface reactions Interactions change apparent order Can fit to Langmuir, but with the wrong mechanism. Started Catalytic Kinetics Last Time.

jens
Download Presentation

ChE 553 Lecture 15

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ChE 553 Lecture 15 Catalytic Kinetics Continued

  2. Object • Examine the effects of pairwise interactions on rates of surface reactions • Interactions change apparent order • Can fit to Langmuir, but with the wrong mechanism

  3. Started Catalytic Kinetics Last Time • Catalytic reactions follow a catalytic cycle reactants + S adsorbed reactants Adsorbed reactants products + S • Different types of reactions Langmuir Hinshelwood Rideal-Eley

  4. Key Predictions Unimolecular reactions • Rate increases with pressure, levels off • 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

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

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

  7. 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.

  8. Methods Do Not Always Work In Detail • Pairwise interactions between adsorbed species • Leads to ordering, coverage dependent kinetics • Can produce oscillations, steady states that depend on how steady state is reached

  9. Key Qualitative Effects • Ordered Overlayers • Island formation • Fluctuations

  10. The Effect Of An Ordered C(2x2) Overlayer • Notice that the environment of B is independent of the coverage of A provided θA > 0.5 • The rate is almost independent of the A concentration • Not exactly independent because repulsions speed rate

  11. Monte Carlo Calculation To Estimate Rate Montecarlo to estimate coverage: • Randomly choose one of three steps • Adsorption/desorption step • Reaction • Diffusion • Use Metropolis algorithm to see whether step should be choosen • Calculate rate via an ensemble average

  12. Adsorption/desorption Similar To Previous Work • Pick a random site • If empty adsorb A or B • If filled desorb molecule • If energy goes down accept the step • If energy goes up accept the step with probability exp(-βΔE) • Repeat

  13. Diffusion Changes Algorithm Slightly • Pick a random site • Pick an adjacent site • If adjacent site empty move molecule • If adjacent site filled do nothing • If energy goes down accept the step • If energy goes up accept the step with probability exp(-βΔE) • Repeat

  14. Reaction Requires Additional Changes • Pick a random site • Pick an adjacent site • If A adsorbed on one of the sites and B adsorbed on a different site Assume A and B react with a probability of p= koexp(-EA/kT) • Repeat Note only 1 in 108 attempts leads to reaction

  15. Next: Estimate The Rate Rate = koexp(-EA/kT) * (number of adjacent pairs of molecules)

  16. Result Of Simulation Using Montecarlo Fit Langmuir βhAA = -3

  17. Implications • Can fit rate data to Langmuir kinetics even where coverage does not follow Langmuir isotherm • Langmuir kinetics calculated for the wrong mechanism (aqua line) fit the data • However, Langmuir kinetics calculated for the correct mechanism (orange line) do not fit the data • Cannot use kinetics to infer mechanism

  18. Dynamic Islanding If diffusion is slow see dynamic islanding • A molecules next to B molecules react • A molecules next to A unreactive • B molecules next to B unreactive Leads to islands of A and B

  19. Rate Oscillations Observed Experimentally Under Such Conditions

  20. Interactions Between Molecules Seen In Transient Measurements Temperature programmed desorption (TPD) • Adsorb gas on cold surface • Heat at a constant 1-100K/sec • Measure gas evolution as a function of time

  21. Typical TPD Spectrum TPD of ethylene

  22. Why Peaks In TPD?

  23. Qualitative Effects In TPD

  24. Qualitative Effects On TPD Ea =10 kcal/mole 20 30 40 50

  25. Qualitative Effects On TPD

  26. TPD To Estimate Ea Ea = (0.06 kcal/mole-K) Tp

  27. Can Use Methods To Get Approximate Activation Energies TPD of ethylene

  28. Method Assumes No Interactions Between Molecules Attractive Interactions Repulsive Interactions

  29. Repulsive Interactions

  30. Attractive Interactions

  31. Ea Varies Non-linearly With Coverage

  32. Summary • Pairwise interactions change kinetics in unexpected ways • Data fits Langmuir-Hinshellwood rate expression – but for the wrong mechanism • Ea varies non-linearly with coverage even though interactions linear with number of nearest neighbors • Multiple peaks in TPD

  33. Key Implication • Extreme care needed in using kinetics to infer mechanisms etc • Can easily get the wrong mechanisms with the wrong analysis to fit data.

More Related