IC T IC-1/42 Lecture 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:

IC T IC-2/42 Lecture 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.

IC T IC-3/42 Lecture 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

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

IC T IC-5/42 Lecture Order Adsorption Desorption:

IC T IC-6/42 Lecture Order Adsorption However if B=A so it is A 2 :

IC T IC-7/42 Lecture 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

IC T IC-8/42 Lecture order adsorption:

IC T IC-9/42 Lecture order adsorption: D0D0 S 0 =1/D 0

IC T IC-10/42 Lecture Order adsorption

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

IC T IC-12/42 Lecture Minority sites may rule the game

IC T IC-13/42 Lecture Some typical S 0 (T) Molecul e SurfaceTemperat ure (K) S0S0 E act (kJ/mol) H2H2 Cu(100)2505x D2D2 Cu(100)2502x CH 4 Ni(111)5002x CH 4 Ni(100)5007x CH 4 Cu(100) x N2N2 Ru(001)4001x N2N2 Au/Ru(0 01) 6705x >130

IC T IC-14/42 Lecture 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

IC T IC-15/42 Lecture Order desorption

IC T IC-16/42 Lecture 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.

IC T IC-17/42 Lecture Order desorption

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

IC T IC-19/42 Lecture 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)

IC T IC-20/42 Lecture 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 

IC T IC-21/42 Lecture 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

IC T IC-22/42 Lecture 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

IC T IC-23/42 Lecture Micro-kinetic Modeling Ammonia Synthesis

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

IC T IC-25/42 Lecture The Ammonia Synthesis

IC T IC-26/42 Lecture The Ammonia Synthesis

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

IC T IC-28/42 Lecture 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

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

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

IC T IC-31/42 Lecture 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.

IC T IC-32/42 Lecture 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

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

IC T IC-34/42 Lecture 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

IC T IC-35/42 Lecture 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) Electronic effects account for one third of the barrier change Geometrical effects for two thirds of the barrier change

IC T IC-36/42 Lecture Microkinetic Model for Ammonia Synthesis over Ruthenium N * 2 N* H * 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)

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

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

IC T IC-39/42 Lecture The Industrial Ammonia synthesis The equilibrium curve The optimum operating line

IC T IC-40/42 Lecture 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

IC T IC-41/42 Lecture 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

IC T IC-42/42 Lecture 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?