Download
slide1 n.
Skip this Video
Loading SlideShow in 5 Seconds..
The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study PowerPoint Presentation
Download Presentation
The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study

The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study

307 Vues Download Presentation
Télécharger la présentation

The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study Fischer-Tropsch synthesis: An Introduction (2n+1) H2 + n CO  CnH2n+2 + n H2O 2n H2 + n CO  CnH2n + n H2O CO + H2O  CO2 + H2 2 CO  C + CO2 1

  2. 2. Fischer-Tropsch Synthesis Absorption of CO + H2 Syngas molecules Dissociation of CO + H2 Hydrogenation of C and O

  3. Hydrogenation of C and O Methanation Propagation chain Monomer Initiation 2. Fischer-Tropsch Synthesis

  4. Monomer Propagation chain Initiation Propagation chain Propagation + Termination Monomer Propagation + Termination 2. Fischer-Tropsch Synthesis

  5. Franz Joseph Emil Fischer Hans Tropsch

  6. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Methods of Computations •  Fe system (less extensively studied than Co and Ru) •  Surface energy: Fe(100) ≈ Fe(110) < Fe(111) • Spin-polarized periodic DFT with plane-wave basis sets (VASP) + Band with STO basis set •  PW91 exchange-correlation functional at GGA level •  PAW • Energy cutoff: 360 eV • k-point sampling of Brillouin zone • 5-layer p(2  2) slabs mimicking Fe(100) surface separated by 10 Å vacuum layer

  7. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Methanation on Fe(100) Surface  General reaction network for CH4 formation (including all byproducts such as CO2, H2O, H2CO and CH3OH) Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) A H G F B I C E D J L K

  8. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Reactive intermediates on Fe(100) surface  Three adsorption sites available: on-top, bridge and hollow sites  Determine the most preferred adsorption sites  Calculate the binding energies at various surface coverage 4-fold 2-fold 1-fold 8 Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  9. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Chemisorption of CO: Kinetics Lateral interaction: crucial factor affecting the adsorption kinetics of CO Desorption barrier decreases with  45 35 25 Activation barrier increases with  15 CO is less strongly bound at higher  5 -5 -15 Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  10. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Dissociation of CO: Coverage dependence Lateral interaction: affects the CO dissociation Eact generally increases +0.06 kcal/mol C + O becomes less stable w.r.t. CO CO dissociation is suppressed at  = 0.75 ML Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  11. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Phenomenological kinetic simulation of CO addition and dissociation Langmuir-Hinshelwood approach: all sites in (2x2) units are energetically homogeneous Simulation parameters: CO:Ar (1:19) gas at 1 atm; ~28 hours; @ 150 and 473 K 150K 473K Results: @ 150 K: 50% *CO; 50% vacancy; no *C and *O @ 473 K: 27% *CO; 27% vacancy; 23% *C; 23% *O Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  12. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Formation of carbon filaments on iron surface Fe is active catalyst for the Boudouard reaction Boudouard reaction assists the formation of coke on Fe(100) in the absence of H2 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  13. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Reactive intermediates on Fe(100) surface  Three adsorption sites available: on-top, bridge and hollow sites  Determine the most preferred adsorption sites  Calculate the binding energies at various surface coverage 4-fold 2-fold 1-fold Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  14. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Formation of CHx species on iron surface Fe is active catalyst for the CHx formation Reaction of C and H on Fe(100) in the absence of Methanation and Hydrogenation CH2 C CH CH CH3 CH3 CH2 CH4 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  15. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic PES of CH4  Stability of CHn assuming the infinite separation approximation  For Fe(100), Co(0001) and Ru(0001), CH is the most thermodynamically stable intermediate  For Fe(110), surface carbide is the most preferred species CH is likely the most abundant active C1 species on Fe(100) while CH, CH2 and CH3 have significant coverage on Co under the F-T conditions  A possible F-T mechanism: proceeding via CH coupling reaction Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Gokhale and Mavrikakis, Prep. Pap. - Am. Chem. Soc. Div. Fuel Chem. 50, U861 (2005) Gong, Raval and Hu, J. Chem. Phys. 122, 024711 (2005) Ciobica et al., J. Phys. Chem. B 104, 3364 (2000)

  16. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Temperature effects on the rate of CH4 formation Simulations including both CO and H2 at industrial reaction conditions P(CO)/P(H2)=1/3 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993)

  17. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Pressure effects on the rate of CH4 formation  Fixed pressures of CO and H2: p(CO) = 0.2 MPa,  The rate of CH4 formation exhibits a strong dependence on the partial pressures of CO and H2 p(CO) = 0.2 Mpa T=525 K (b) Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993)

  18. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Initiation C-C bond coupling reactions on Fe(100) surface (c) (e) (d) 15 13 5 9 2 1 4 10 12 6 8 14 3 11 7 (f) (a) (b) (e) (c) (d) Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  19. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Mechanisms of F-T synthesis Most widely accepted carbene mechanism (Fischer & Tropsch (1926)) How is methane formed? A B How do the C1 units couple? Maitlis et al. JACS 124, 10456 (2002) F C E How does the chain grow? D

  20. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetics of the C-C coupling reactions on Fe(100) C-C bond coupling reactions are usually kinetically demanding processes 20 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  21. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetics of the C-C coupling reactions on Fe(100) C-C bond coupling reactions are usually kinetically demanding processes C with CH/CH2 bond coupling reactions kinetically and thermodynamically favorable 21 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  22. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetics of the C-C coupling reactions on Fe(100) C-C bond coupling reactions are usually kinetically demanding processes CH to CH/CH2 bond coupling reactions kinetically favorable but thermodynamically unfavorable 22 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  23. Hydrogenation reactions occur rather rapidly at room temperatures Many hydrogenation reactions are indeed endothermic and require energy The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetics of the C-C coupling reactions on Fe(100) 23 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  24. Isomerization processes are not kinetically favorable The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetics of the C-C coupling reactions on Fe(100) With this information we may construct the kinetic profile for the formation of ethane ethylene 24 Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  25. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetic profile of ethane formation Monomer Propagating chain The formation of CH3CH3 is kinetically feasible The rate-determining step is the C + CH2 coupling reaction The C + CH step has to overcome a much higher barrier (> 29 kcal/mol), and is thus less likely Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  26. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study General chain propagation reactions on Fe(100) surface Very complicated processes because of a large number of active surface species For Co and Ru, the following mechanisms have been proposed: Information obtained from previous sections: *C and *CH are the most abundant surface species (monomers) *CCH, *CCH2 and *CCH3 are stable C2 fragments on Fe(100)(growing chains) Unsaturated carbon) one hydrgen two hydrgens

  27. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study General chain propagation reactions on Fe(100) surface Very complicated processes because of a large number of active surface species For Co and Ru, the following mechanisms have been proposed: Information obtained from previous sections: *C and *CH are the most abundant surface species (monomers) *CCH, *CCH2 and *CCH3 are stable C2 fragments on Fe(100)(growing chains) Unsaturated carbon) one hydrgen two hydrgens 27

  28. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions Coupling reactions with C-CHn fragments are generally endothermic important only at high reaction temperatures

  29. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions Reactions between *C and CHCH2/CH-CH3 and CH2CH3 possess lower activation barriers on Fe 29

  30. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions Reactions between *CH/*CH2 and CHCH2/CH-CH3 or CH2CH3 possess higher activation barriers on Fe Therefore, the carbide route should be the dominant mechanism in the Fe-catalyzed F-T synthesis (thermodynamically favorable but kinetically demanding) 30

  31. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions Therefore, the carbide route should be the dominant mechanism in the Fe-catalyzed F-T synthesis (thermodynamically favorable but kinetically demanding) 31

  32. Lateral interaction is an important factor determining the relative stability Direct formation of C2 from *C is not favorable Ethane is more preferred to ethylene thermodynamically in the F-T synthesis Highly unsaturated -C species are more stable because of their high coordination to Fe surface The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic stability of C2 species 32 Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)

  33. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Plausible reaction scheme of chain propagation According to the computed C-C bond coupling reaction barriers, the following possible reaction scheme leading to the formation of propane and propylene can be deduced: The kinetic profiles for the production of propane and propylene can be obtained if the activation energies for all these hydrogenation reactions are known Reference: Liu and Hu, J. Am. Chem. Soc. 124, 11568 (2002).

  34. Propylene The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic stability of reactive C3 fragments Kcal/mol 34 Lo and Ziegler, J. Phys. Chem. C 111(2008),submitted Reference: Lo and Ziegler, J. Phys. Chem. C (to be submitted)

  35. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetic potential energy surface for propane formation Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

  36. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetic potential energy surface for propane formation Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

  37. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study CO dissociation channel: Fe(100) v.s. Fe(310) Two stable configurations are located on Fe(310): 4f and 4f2 Barrier for CO activation on Fe(310) edge is lowered compared to that on flat Fe(100) at 0.250 ML surface coverage At higher coverage, the Fe(310) 4f2 becomes the most feasible path, having the barrier of only 22.7 kcal/mol, and a large exothermicity of 12.1 kcal/mol It is estimated that for an Fe catalyst with 10% Fe(310) steps by surface area, the resulting percentage of adsorbed CO undergoing decomposition becomes: (compared to 50% for Fe(100) surface) Lo and Ziegler J. Phys. Chem. C. 2008; 112; 3692-3700

  38. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Use of Alloys 1. H2 activation Lo and ZieglerJ. Phys. Chem. C 2008, 112, 3667-3678 2. COactivation J. Phys. Chem. C.; (Article); 2008; 112(10); 3679-3691.

  39. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Conclusions The process of Co hydrogenation on Fe catalyst has been investigated computationally, and the associated kinetics has been explored. CO addition on Fe(100) has been controlled by the entropy lost during the process, and in maximum 50% of the surface active sites can be occupied. The most abundant C1 species on Fe(100) is *CH, but the chain initiation takes place making use of *CH2 instead. The carbide mechanism, in which *C inserts into surface *CnHm units, is found to be more thermodynamically feasible than the well-known alkenyl or alkylidene mechanisms. The activity of Fe catalyst in the F-T synthesis can be improved by introducing surface defects, such as steps, or doping of other metals.

  40. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Use of Alloys 1. H2 activation Lo and ZieglerJ. Phys. Chem. C 2008, 112, 3667-3678 2. COactivation J. Phys. Chem. C.; (Article); 2008; 112(10); 3679-3691.

  41. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Conclusions The process of Co hydrogenation on Fe catalyst has been investigated computationally, and the associated kinetics has been explored. CO addition on Fe(100) has been controlled by the entropy lost during the process, and in maximum 50% of the surface active sites can be occupied. The most abundant C1 species on Fe(100) is *CH, but the chain initiation takes place making use of *CH2 instead. The carbide mechanism, in which *C inserts into surface *CnHm units, is found to be more thermodynamically feasible than the well-known alkenyl or alkylidene mechanisms. The activity of Fe catalyst in the F-T synthesis can be improved by introducing surface defects, such as steps, or doping of other metals.

  42. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Fischer-Tropsch synthesis: An Introduction First discovered by Sabatier and Sanderens in 1902: Fischer and Tropsch reported in 1923 the synthesis of liquid hydrocarbons with high oxygen contents from syngas on alkalized Fe catalyst (Synthol synthesis) (2n+1) H2 + n CO  CnH2n+2 + n H2O 2n H2 + n CO  CnH2n + n H2O CO + H2O  CO2 + H2 2 CO  C + CO2 Øyvind Vessia, Project Report, NTNU, 2005. Commercialized by Shell (Malaysia), Sasol (S. Africa) and Syntroleum (USA)

  43. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Mechanisms of F-T synthesis CO insertion mechanism (Pichler and Schultz (1970s)) insertion A B C

  44. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Chemisorption of CO: Kinetics Lateral interaction: crucial factor affecting the adsorption kinetics of CO Desorption barrier decreases with  Activation barrier increases with  CO is less strongly bound at higher  Increase In free energy With 4 kcal For each 100K Calculations predict full coverage by CO? Something is missing … ENTROPY ! Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  45. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Chemisorption of CO: Entropic contribution Different components of entropy for a gaseous molecule can be computed using statistical thermodynamics Generally speaking, one can write the total entropy as a sum (reference: Surf. Sci. 600, 2051 (2006)) This term will be completely lost because of the assumption that the adsorbed species is immobile This term is small compared to the rotational entropy, and is thus neglected This term mostly vanishes during adsorption for immobile species; but it is not possible to compute such quantity for adsorbed molecules, and is thus assumed zero after adsorption (crude approximation)

  46. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Reactive intermediates on Fe(100) surface  Three adsorption sites available: on-top, bridge and hollow sites  Determine the most preferred adsorption sites  Calculate the binding energies at various surface coverage 4-fold 2-fold 1-fold 46 Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)

  47. The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Reactive intermediates on Fe(100) surface  Three adsorption sites available: on-top, bridge and hollow sites  Determine the most preferred adsorption sites  Calculate the binding energies at various surface coverage 4-fold 2-fold 1-fold 47 Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007) Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)