Introduction to Surface Chemistry

Transcription

1 Introduction to Surface Chemistry Reinhard Schomäcker Institut für Chemie TU Berlin Strasse des 17. Juni Berlin

2 Surface Chemistry - Modification of surface - Grafting - Coating - Chemical Vapor Deposition - Atomic Layer Deposition - Impregnation Techniques - Etching - Corrosion - Electrochemical Resolution or Deposition - Surface Analytics - Microscopy, Scattering and Diffraction Methods - Spectroscopy - Catalysis - heterogeneous oxidation catalysis Wetting Diffusion Electrostatics Adsorption Structure Geometry = unknown local concentration and reactivity Oxygen as a Probe molecule

3 Outline - Industrial examples -Fundamentals - Oxidations reactions with lattice oxygen -the active component -the support effects -Oxidation reactions with adsorbed oxygen

4 Selective Oxidation of Hydrocarbons 20% of all industrial organic chemicals O 2 molecules: triplet state with 2 unpaired elctrons

5

6

7 Required material properties of oxidation catalysts

8 Nucleophilic and electrophilic oxidation Oxides are nonstochiometric

9 Dynamic surface equilibrium Nucleophilic lattice oxygen Electrophilic surface oxygen species C-H actication Oxidative dehydrogenation Insertion into activated C-H bonds Vacancy formation Radical reactions, Total oxidation Bi 2 O 3 -MoO 3 Co 3 O 4 18 O isotope exchange exp.

10 Oxidation reactions with nucleophilic lattice oxygen Lattice transport of O 2- Redox mechanism

11 The Deacon process 2 HCl +1/2 O 2 H 2 O + Cl 2-57,5 kj/mol Cu, Mn, Ni, Co, V, Mo, Hg, Ag, Nb, Ti, Zn, W, Ru, Ce

12 Fundamentals of the Deacon process Micro kinetic model Energy profile for RuO 2 /SnO 2 1. O 2 + 2* O 2 ** 2. O 2 ** 2 O* 3. O* + * + HCl OH* + Cl* 4. 2 Cl* 2 * + Cl OH* O* + * + H 2 O r k f p 1 O2 p 0.5 HCL K Cl2 k f 0.5 K p p O2 p K H 1 Cl2 1.5 HCl 2O p p 1 Cl2 H 2O p 1 H 2O Nuria Lopez, ICIQ Teschner et al, J. Catal. 2012, 285, DOI: /j.jcat

13 p Cl2 / bar Institut für Chemie Experimental Investigation of the Deacon process 0, C 350 C 325 C 300 C pcl2, exp 0,225 0, t / s r k f p 1 O2 p 0.5 HCL K Cl2 k f 0.5 K p p O2 p K H 1 Cl2 1.5 HCl 2O p p 1 Cl2 H 2O p 1 H 2O 0, ,075 0,15 0,225 0,3 Kinetic parameter for RuO 2 /SnO 2 : E A,0 = 59.4 kj/mol DH Ads,Cl2 = kj/mol DH Ads,H2O = kj/mol 400 C 350 C 325 C 300 C p Cl2, sim

14 Comparison of Deacon catalysts MO HCl MOCl 2 + H 2 O D R H ad MOCl 2 + 1/2 O 2 MO 2 + Cl 2 D R H ox D. Teschner et al, J. Catal. 2012, 285, J. Perez-Ramirez et al., J. Catal. 2012, 286, J. Allen et al, J. Appl. Chem. 1962, 12, 406 M.W.M. Hisham, S.W. Benson, J. Phys. Chem. 1995, 99, 6194

15

16 Simplified energy profile of Deacon reaction

17 Typical Feature of Surface Chemistry: Vulcano Behaviour

18

19 Oxidations with lattice oxygen: Mars-van Krevelen - Mechanism Lattice oxygen

20 Selectivity in hydrocarbon oxidation Thermodynamics favours the ultimate formation of CO 2 and H 2 O Desired products of partial oxidation reactions are intermediates derived by kinetic control Competing parallel and consecutive reactions C-H bonds of reactants usually stronger than those in intermediate products All oxidation processes are highly exothermic Expolsive regime in gas mixture

21 Nucleophilic and electrophilic oxidation Oxides are nonstochiometric

22

23 Ethanol oxidation reaction network X 1 n n C 2 0, C H 2 5 H OH 5 OH S i ni i n j j j ODH of propane: O O +5 V O O O O +5 V O O H CH O +4 V O O O O O +5 V O O H H O V O O O H O V O O O O V O O O H O V O O O O 2 -H 2 O O O O +5 V +5 O V O O O O X. Rozanska, R. Fortrie, J. Sauer, J. Phys. Chem. C, 111 (2007)

24 Mechanistic aspects r 1 K p 1 1 n C k 2 2 K p 5 1 H OH 1 n C 2 H OH k 5 2 K p 1 k 5 1 n C p O H OH 2 5 k 2 with n 1 k 2 /k 5 p O2 << 1 K 1 p C2H5OH > 1 B. Kilos, A.T. Bell, E. Iglesia, J. Phys. Chem. C, 113 (2009)

25 Impact of vanadia dispersion on performance - dispersion of vanadia influenced by support material - different performance of monomers, oligomers and polymers - different contribution of uncovered support surface - no comparability of low loaded catalyst with different supports

26 Catalyst preparation - Preparation of near-monolayer catalysts by thermal spreading of vanadyl acetylacetonate - No over oxidation of the products under differential conditions (X < 10%) and stoichiometric feed support surface area [m²/g] surface density [V/nm²] wt% V TiO 2 17,1 3,5 0,6 % Al 2 O 3 200,9 3,1 4,9 % ZrO 2 52,1 3,1 1,4 % CeO 2 19,8 3,9 0,7 % No crystalline V 2 O 5 in all samples

27 E A,app [kj/mol] TOF (200 C) [mol/mols] Institut für Chemie Support effect 0,3 0,25 0,2 0,15 0,1 0,05 VO x /Support E, app lntof R T A 1 0 Al2O3 V2O5 ZrO2 CeO2 TiO2 n C 2 H4O TOF at X 10% n V VO x /Support Al2O3 V2O5 ZrO2 CeO2 TiO2

28 E A,app [kj/mol] Institut für Chemie Support effect ODH of propane and methanol vs. ethanol 120 VO x /Al 2 O VO x /ZrO VO x /CeO 2 V 2 O 5 90 VO x /TiO propane methanol* E A,app ethanol [kj/mol] *L. J. Burchman, I. E. Wachs, Catal. Today, 49 (1999) P.R. Shah, I. Baldychev, J.M. Vohs, R.J. Gorte, App. Cal. A, 361 (2009) 13-17

29 ln X(C 3 H 8 ) (%) DH f [kjmol -1 ] Energy Institut für Chemie Theorie of defect formation O x O V O 1 2e O 2 2 Defect formation ΔH f K e V O p O 2 Mass action law d ln K dt ΔH RT f 2 Van t Hoff s law ΔE m V O ΔE ΔEm 1 3 ΔH f X 10-7,0-7,5-8,0 ΔE m DH f this work DH f Allersma1967 DH f Sauer2004 DH f Todorova2007-8,5-9,0 ΔE* , , T [ C] TiO2 ZrO2 SiO2 V2O5 MgO a-al2o3 T. Allersma, R. Hakim, T. N. Kennedy, J. D. Mackenzie, J. Chem. Phys., 46 (1967) J. Sauer, J. Doebler, Dalton Transactions, 19 (2004) T. K. Todorova, M. V. Ganduglia-Pirovano, J. Sauer, J. Phys. Chem. C., 111, (2007),

30 Energy profile of ethanol partial oxidation TS 1 VO x /Al 2 O 3 E a,app E [kj/mol] ΔH R ΔH def V V V V O2 + C2H5OH(g) + ½ O2(g) V V V V (OH)2-C2H4O + ½ O2(g) V IV V IV O+ H2O(g) + C2H4O(g) + ½ O2(g) reaction cordinate coordinate V V V V O2+ H2O(g) + C2H4O(g)

31 E A,app [kj/mol] TOF [mol/mols] Institut für Chemie BEP- Correlations with defect formation enthalpy propane ethanol methanol* VO x /Al 2 O 3 0,45 0,40 ethanol (200 C) methanol ** propane (400 C) 100 VO x /ZrO 2 0,35 90 V 2 O 5 0,30 80 VO x /TiO 2 0,25 0,20 VO x /TiO , , ,05 V 2 O 5 VO x /ZrO 2 VO x /Al 2 O H f [kj/mol] 0, H f [kj/mol] *L. J. Burchman, I. E. Wachs, Catal. Today, 49 (1999) P.R. Shah, I. Baldychev, J.M. Vohs, R.J. Gorte, App. Cal. A, 361 (2009) **I. E. Wachs, Catal. Today, 100 (2005) 79-94

32 Preparation of a high performance catalyst for ODP Institut für Chemie 4V/13Ti/SBA-15 catalyts structure based on spectroscopical and reactivity results.

33 Activity scaling with H 2 -TPR peak temperatures G. Deo, I.E.Wachs, J. Catal, 146 (1994)

34 Catalytic cycle of propene oxidation

35 Mars-van-Krevelen Rate Law Rate of reduction Rate of reoxidation Steady state assumption + Surface oxygen coverage r 1 1 k K p 2 1 n CH K p 1 1 n CH k 2 k K p 5 1 p 1 2 O 1 n CH 2 Overall rate

36 Oxidation reactions with adsorbed Oxygen - Total Oxydation reactions of VOC - Partial Oxidation of Methane - Epoxydation of Ethene - Oxidative Coupling of Methane

37 Oxidation reactions with electrophilic oxygen Epoxidation of Ethene Singularities: Ag as selective catalyst, oxygen as oxidant, only working with ethene Proposed Pathways Of selective and unselective reaction

38 Oxygen is more electrophilic at high coverage Kinetics: Eley-Rideal

39 Possible intermediate DFT calculations of activation barriers for model discrimination

40 Proposed structures For DFT calculations

41 Vulcano Plot for EO-TOF

42 Oxidative Coupling of Methane (OCM) to Ethylene o Worldwide research efforts to convert CH 4 into chemicals (C 2 H 4, CH 3 OH ) o Oxidative Coupling of Methane (OCM) is a promising direct conversion route of methane to ethylene but has not reached industrial practice yet Desired: H 2 CH 4 + O 2 C C H H H + 2 H 2 O DG (800 C) = kj mol -1 Undesired: 2 CH O 2 DG (800 C) = kj mol -1 2 CO + 4 H 2 O 2 CH O 2 2 CO DG (800 C) = kj mol H 2 O kinetic control by means of a catalytic process needed 43

43 papers Institut für Chemie Pioneering work G.F. Keller, M.M. Bhasin, J. Catal. 1982, 73, 9 catalyst M. Hinsen, M. Baerns, Chem. Z. 1983, 107, 223 A.C. Jones, J.J. Leonardo, A.J. Sofranko, periodic US patent , 1984 feed strategy year Reihe1 U. Zavyalova, M. Holen, R. Schlögl, M. Baerms, ChemCatChem, 2011, 3, 1935

44 Ethylene Production by Oxidative Coupling of Methane Model Studies for Understanding Complexity table adopted from E. V. Kondratenko, M. Baerns

45 Mn-doped Na 2 WO 4 /SiO g batches with spray drying technology X.Fang et al, J. Mol. Catal. (China), 6 (1992) 427 *U. Simon, O. Görke, A. Bertold, S. Arndt, R. Schomäcker, H. Schubert, Chem. Eng. J.168 (2011)

46 E A,app + DH vap [kj/mol] Institut für Chemie Common features of Catalysts for Oxidative Coupling of Methane Very low specific surface < 5 m 2 /g Similar activation energies E A,app = 150 kj/mol Inactive < 600 C, no lattice oxygen involved 280 VO x /TiO VO x /CeO VO x /ZrO E A,0 Ethanol Cyclohexane Propane Ethane D 0 (C-H) [kj/mol] Hypothesis: 180 adsorbed Mn/Na 2 WO 4 /SiO 2 atomic oxygen 160 Gasphase with O* Methane E A,app Ethanol Propane D 0 (C-H) [kj/mol] Ethane Methane

47 Experiments in a TAP reactor with J. Perez-Ramires, ICIQ, Taragona, now ETH Zurich

48 Mechanism of Oxidative Coupling of Methane /cm Li Defects? TM n+ TM (n+1)+ + e - J. Lunsford, Angew. Chemie, Int. Ed, 1995, 34, 970 Stability? Selectivity? Operation conditions? Arndt et al., Catalysis Reviews, 53, 2011,

49 O 2 Institut für Chemie Mechanism of Oxidative Coupling of Methane /cm 3 C H /cm 3 C O x + O 2 + O 2 + O 2 C H C H C H + C H C 2 H 4 * + C H 3 C 2 H 6 O O O OCM is possible in the gas phase but not at 1bar and with poor selectivities CH 3 + CH 3 + M C 2 H 6 + M* (trimolecular) Catalyst = M? R. Horn, S. Mavlyankariev

50 C 2 Selectivity [%] Institut für Chemie Catalyst Testing and kinetic studies Mn-Na 2 WO 4 /SiO 2, 100 mg, 750 C, Feed Gas: 8:1 4:1 2: CH 4 Conversion [%]

51

52 Product selectivity of Catalysts

53 Nucleophilic and electrophilic oxidation The surface is, What matters!

54 Selectivity in hydrocarbon oxidation Thermodynamics favours the ultimate formation of CO 2 and H 2 O Desired products of partial oxidation reactions are intermediates derived by kinetic control Competing parallel and consecutive reactions C-H bonds of reactants usually stronger than those in intermediate products Surface structure and reactivity is the essential control parameter - Definded sites of selective reactivity are required - Stability of sites against reaction conditions and product essential for processes