1. IC T IC-1/42 Lecture-8 18-11-2004 The surface science approach Simpler system - Detailed studies Well-defined system Well-defined processes Fundamental insight Input to catalyst design Single crystal surfaces as model catalysts. The structure gap The pressure gap The materials gap The price is:

2. IC T IC-2/42 Lecture-8 18-11-2004 Determination of important parameters. We want to have setailed information on important parameters like: Adsorption rates meaning: S(T)=S 0 e -Ea/RT Reaction rates: k=k 0 e -Er/RT Desorption rates: k des =k des0 e -Edes/RT Construct a microkinetic model based on reliable fundamental data resulting in deep insigth.

3. IC T IC-3/42 Lecture-8 18-11-2004 1. Order Adsorption By equalizing the chemical potentials of the gas and the surface and introducing the appropriate partition function we found We must now estimate S 0 (T) from experiments or calculations

4. IC T IC-4/42 Lecture-8 18-11-2004 1. Order Adsorption We remember that for desorption: Langmuir isotherm for adsorption on a single site Find K A from isostere: HOW?

5. IC T IC-5/42 Lecture-8 18-11-2004 2. Order Adsorption Desorption:

6. IC T IC-6/42 Lecture-8 18-11-2004 2. Order Adsorption However if B=A so it is A 2 :

7. IC T IC-7/42 Lecture-8 18-11-2004 Determination of sticking coefficients Thermalized Experiments The real data, but not always possible and they do not reveal details Beam experiments Are giving detalied information on the dependence of energy in different degrees of freedom, but may not probe the correct reaction pathway

8. IC T IC-8/42 Lecture-8 18-11-2004 1. order adsorption:

9. IC T IC-9/42 Lecture-8 18-11-2004 1. order adsorption: D0D0 S 0 =1/D 0

10. IC T IC-10/42 Lecture-8 18-11-2004 2. Order adsorption

11. IC T IC-11/42 Lecture-8 18-11-2004 Uptake on Clean Ru(0001) Agreement with S 0 =(1±0.8)·10 -12 at 300K in earlier work Dietrich, Geng, Jacobi, and Ertl, J. Chem. Phys. 104 (1) (1996) 375

12. IC T IC-12/42 Lecture-8 18-11-2004 Minority sites may rule the game

13. IC T IC-13/42 Lecture-8 18-11-2004 Some typical S 0 (T) Molecul e SurfaceTemperat ure (K) S0S0 E act (kJ/mol) H2H2 Cu(100)2505x10 -13 48 D2D2 Cu(100)2502x10 -13 56 CH 4 Ni(111)5002x10 -8 74 CH 4 Ni(100)5007x10 -8 59 CH 4 Cu(100)10008.6x10 - 9 201 N2N2 Ru(001)4001x10 -10 38 N2N2 Au/Ru(0 01) 6705x10 -15 >130

14. IC T IC-14/42 Lecture-8 18-11-2004 Determination of desorption rates The bonding energy of simple adsorbates to a specific surface On-set temperatures of reaction or decomposition Reaction pathways on the surface

15. IC T IC-15/42 Lecture-8 18-11-2004 1. Order desorption

16. IC T IC-16/42 Lecture-8 18-11-2004 The Complete Method Now collect a lot of TPD curves and read of as a function of T where  left is the same. Then for each  left we can plot Getting: As the intercept and the slope. This can be done for all  left Giving  and E des as function of coverage.

17. IC T IC-17/42 Lecture-8 18-11-2004 2. Order desorption

18. IC T IC-18/42 Lecture-8 18-11-2004 The simple method 2. order Notice symmetric and shifts down Thus by assuming  can we again estimate E des  left =1/2

19. IC T IC-19/42 Lecture-8 18-11-2004 N 2 desorption from Ru(0001) All thermal experiments with N 2 /Ru(0001) systems are dominated by steps. Mass 14 QMS signal Clean Ru(0001) 0.05 ML Au on Ru(0001)

20. IC T IC-20/42 Lecture-8 18-11-2004 More complex behaviour CO TPD from Pt(112) A typical behaviour for a two state situation But a dublet can also be due to strong lateral interaction i.e. E repulsion = E o for 

21. IC T IC-21/42 Lecture-8 18-11-2004 TPD for identification of reaction pathways CO 2 *+H* HCOO**HCOO** ½ H 2 +CO 2 +2* Tells a lot about surface reactions Can be used for analysis

22. IC T IC-22/42 Lecture-8 18-11-2004 Micro-kinetic Modeling The idea is to collect all the fundamental information I.e. any adsorption rates, desorption rates incl. prefactors and activation energies, sticking coefficients etc. and put them into a detailed model. If correct the model should then be capable of describing the process and identify what is important in the process, i.e. what is the rate limiting step, how can it be changed and what is the coverage of various species. Example: The Ammonia synthesis N 2 + 3H 2 2 NH 3

23. IC T IC-23/42 Lecture-8 18-11-2004 Micro-kinetic Modeling Ammonia Synthesis

24. IC T IC-24/42 Lecture-8 18-11-2004 The Ammonia Synthesis We can now express each coverage in terms of K, k, pressure, and  

25. IC T IC-25/42 Lecture-8 18-11-2004 The Ammonia Synthesis

26. IC T IC-26/42 Lecture-8 18-11-2004 The Ammonia Synthesis

27. IC T IC-27/42 Lecture-8 18-11-2004 The details of ammonia synthesis Notice how the molecular state is not in play since it is assume in equilibrium: q N2 cancels

28. IC T IC-28/42 Lecture-8 18-11-2004 The rate limiting step may not even have an barrier If we assume 0.030eV-2*0.025*5/3 = -0.05eV The important message is that although the activation is low there is still an entropy barrier

29. IC T IC-29/42 Lecture-8 18-11-2004 Comparison Theory & Experiment Measured data N 2 Fe(111) Calculated N 2 Fe(111) and Fe(100) Within the accuracy of DFT

30. IC T IC-30/42 Lecture-8 18-11-2004 Example: Nitrogen coverage E des = 190kJ/mol and =1x10 13 s -1

31. IC T IC-31/42 Lecture-8 18-11-2004 Now back to the model Atomic nitrogen is MARI  N ~1 K promoted ammonia catalyst at 673 K, 100 bar approaching 68% of the equilibrium ammonia concentration.

32. IC T IC-32/42 Lecture-8 18-11-2004 Approach towards Equilibrium The exit concentration is 19% and corresponds to 75% of the obtainable equilibrium conversion for 100 bar and 673K. The approach towards equilibrium is slow because N* blocks the surface

33. IC T IC-33/42 Lecture-8 18-11-2004 Comparrision of model and exp. This is not a proof but an indication that we are on the right track

34. IC T IC-34/42 Lecture-8 18-11-2004 Structural gap in the Ammonia Synthesis over Ru Catalysts 3/2H 2 + 1/2N 2 NH 3  H 0 = -46 kJ/mol Why Ruthenium? NH 3 Conc. Fe is blocked by N (NH 3 ) Ru is not as easily blocked by N (NH 3 ), but by Hydrogen and it is expensive!! Fe Ru Reactor length

35. IC T IC-35/42 Lecture-8 18-11-2004 Energy Diagram for N 2 Dissociation Energy Terrace site Step site S. Dahl, A. Logadottir, R. Egebjerg, J. Larsen, I. Chorkendorff, E. Tørnqvist, and J. K. Nørskov, Phys. Rev. Lett. 83 (1999) 1814. Electronic effects account for one third of the barrier change Geometrical effects for two thirds of the barrier change

36. IC T IC-36/42 Lecture-8 18-11-2004 Microkinetic Model for Ammonia Synthesis over Ruthenium N 2 + 2 * 2 N* H 2 + 2 * 2 H* N* + H* NH* + * NH* + H* NH 2 * + * NH 2 * + H* NH 3 * + * NH 3 * NH 3 + * k+k+ k-k- S. Dahl, J. Sehested, J. H. Jacobsen, E. Tornqvist, and I. Chorkendorff, J. Catal. 192 (2000) 391. From assuming 1% of active sites on the clean Ru(0001) surface. k + 1 = 10 5 ·exp(-37 kJ/mol / RT) (bar -1 s -1 ) The rest of the parameters are in agreement with tailing edge of TPD spectra of N 2, H 2 and NH 3 desorption from Ru(0001)

37. IC T IC-37/42 Lecture-8 18-11-2004 Universality Fe(111) is good Ru steps are good You can now see why the catalyst bed should be varied down through the reactor

38. IC T IC-38/42 Lecture-8 18-11-2004 The equilibrium curve The optimum operating line inlet outlet A B D C The Real Ammonia reactor The schematic reactor The Industrial Ammonia synthesis

39. IC T IC-39/42 Lecture-8 18-11-2004 The Industrial Ammonia synthesis The equilibrium curve The optimum operating line

40. IC T IC-40/42 Lecture-8 18-11-2004 The Concept of Optimal catalyst curve 2:1 80bar 420 o C 3:1 200bar 450 o C Claus Jacobsen et al. J. Catal. 205 (2002) 382 High ammonia conc. 90% requires low bonding energy Low ammonia conc. 5% requires higher bonding energy 5% 90% 5% 90% Defines optimal Catalyst

41. IC T IC-41/42 Lecture-8 18-11-2004 Promoters K K  E TS =  =19 kJ/mol  eÅ CH 4 Structural promotors: Al 2 O 3 and CaO for the ammonia cat. helps stabilizing the structure ensuring a huge surface area Electronic promotors: K, Cs for the ammonia cat. Set up a dipole moment lowing the activation energy for either adsorption or desorption They may also act as inhibitors i.e. methane sticking on Ni

42. IC T IC-42/42 Lecture-8 18-11-2004 Inhibitors or poisons Poisons may as just seen work through an electronic effect Often poisons are species that just block sites, i.e. bond Strongly or irreversible to the active sites. Good examples are Sulfur, Chlorine, and Oxygen: 1 ppm H 2 O in the ammonia syn-gas reduces the activity by a factor of 2. Explain why one should not use leaded fuel on a modern car?