35nd ACS National Meeting April 6-10, 2006 New Orleans, Louisiana Organizers : Presiding: Tuesday April 8, 2008, 1:30 -2:00 PM. Morial Convention Center, Room: Rm. 337 John Lo T.Ziegler Department of Chemistry University of Calgary,Alberta, Canada T2N 1N4 New Orleans National Meeting Modeling the Fischer-Tropsch Synthesis Catalyzed on a Fe(1,0,0) Surface Symposium on Computational Catalysis Division of Computers in Chemistry

2 The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study Outline § Introduction to F-T synthesis  Historical and economical perspectives  Mechanistic investigations and findings § Methods of computations § Methanation on iron surface  Thermodynamics & kinetics of CH 4 formation § C-C bond coupling reactions  Selectivity of ethane over ethylene § Chain propagation in F-T synthesis  Elucidating a consistent F-T mechanism according to the present work § Effects of defects and alloying  CO activation on steps  Synergetic effects between Fe and Co

3 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) H 2 + n CO  C n H 2n+2 + n H 2 O 2n H 2 + n CO  C n H 2n + n H 2 O CO + H 2 O  CO 2 + H 2 2 CO  C + CO 2 Commercialized by Shell (Malaysia), Sasol (S. Africa) and Syntroleum (USA) Øyvind Vessia, Project Report, NTNU, 2005.

4 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Mechanisms of F-T synthesis CO insertion mechanism (Pichler and Schultz (1970s)) Enol mechanism (Emmett et al. (1950s)) condensation insertion A B C A B C

5 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? How do the C 1 units couple? How does the chain grow? Maitlis et al. JACS 124, (2002) A B C D E F

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)  PW91 exchange-correlation functional at GGA level  Vanderbilt’s ultra-soft pseudopotentials  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 ModelExperiment Lattice constant Å Å Bulk modulus156 GPa170 GPa Magnetic moment 2.30   0

7 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Methanation on Fe(100) Surface  General reaction network for CH 4 formation (including all byproducts such as CO 2, H 2 O, H 2 CO and CH 3 OH) A B C DE F G H I J K L Lo and Ziegler, J. Phys. Chem. C 111, (2007)

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

9 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic PES of CH 4 Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007) Gokhale and Mavrikakis, Prep. Pap. - Am. Chem. Soc. Div. Fuel Chem. 50, U861 (2005) Gong, Raval and Hu, J. Chem. Phys. 122, (2005) Ciobica et al., J. Phys. Chem. B 104, 3364 (2000)  Stability of CH n 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 C 1 species on Fe(100) while CH, CH 2 and CH 3 have significant coverage on Co under the F-T conditions  A possible F-T mechanism: proceeding via CH coupling reaction

10 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 Activation barrier increases with  Desorption barrier decreases with  CO is less strongly bound at higher  Calculations predict full coverage by CO?Something is missing …ENTROPY ! Lo and Ziegler, J. Phys. Chem. C 111, (2007)

11 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)

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

13 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; ~ and 473 K 150 K: 50% *CO; 50% vacancy; no *C and 473 K: 27% *CO; 27% vacancy; 23% *C; 23% *O Lo and Ziegler, J. Phys. Chem. C 111, (2007)

14 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 H 2 Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007)

15 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Formation of CH x species on iron surface Fe is active catalyst for the CH x formation Reaction of C and H on Fe(100) in the absence of Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007) CCH CH 2 CH 3 CH 4 CH CH 2

16 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Temperature effects on the rate of CH 4 formation Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007) Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993) Simulations including both CO and H 2 at industrial reaction conditions P(CO)/P(H 2 )=1/3

17 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Pressure effects on the rate of CH 4 formation  The rate of CH 4 formation exhibits a strong dependence on the partial pressures of CO and H 2 Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007) Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993)  The computed rates are much higher than the experimentally observed values  Reason: the coupling of C 1 fragments is ignored in all simulations  Fixed pressures of CO and H 2 : p(CO) = 0.2 MPa, p(H 2 ) = 0.9 MPa. p(CO) = 0.2 Mpa T=525 K p(H 2 ) = 0.9 Mpa T=525 (a) (b)

18 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions on Fe(100) surface To figure out the origins of the product selectivity of ethane/ethylene mixture (a) (b) (c) (d) (e) (f) Lo and Ziegler, J. Phys. Chem. C 111, (2007)

19 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic stability of C 2 species Direct formation of C 2 from *C is not favorable Lateral interaction is an important factor determining the relative stability 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 Lo and Ziegler, J. Phys. Chem. C 111, (2007)

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 Reference: Lo and Ziegler, J. Phys. Chem. C 111, (2007) With this information we may construct the kinetic profile for the formation of ethane ethylene Hydrogenation reactions occur rather rapidly at room temperatures Many hydrogenation reactions are indeed endothermic and cause energy Isomerization processes are not kinetically favorable

21 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Kinetic profile of ethane formation The formation of CH 3 CH 3 is kinetically feasible The rate-determining step is the C + CH 2 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, (2007)

22 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 Information obtained from previous sections: *C and *CH are the most abundant surface species *CCH, *CCH 2 and *CCH 3 are stable C 2 fragments on Fe(100) For Co and Ru, the following mechanisms have been proposed:

23 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Thermodynamic stability of reactive C 3 fragments Reference: Lo and Ziegler, J. Phys. Chem. C (to be submitted) Kcal/mol Propylene Lo and Ziegler, J. Phys. Chem. C 111(2008),submitted

24 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study C-C bond coupling reactions Coupling reactions with C-CH n fragments are generally endothermic  important only at high reaction temperatures Reactions between *C and CHCH 2 /CH-CH 3 and CH 2 CH 3 possess lower 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) Reactions between *CH/*CH2 and CHCH 2 /CH-CH 3 or CH 2 CH 3 possess higher activation barriers on Fe

25 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, (2002).

26 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

27 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

28 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Propane/Propylene selectivity in the F-T synthesis CCHCH 3 The formation of propane and propylene can be traced back to CCHCH 3 The production of propylene is more kinetically controlled in the first step, while the path leading to CHCH 2 CH 3 intermediate is endothermic Turnover may occur at CHCH 2 CH 3 : either proceeding further to form propyl and propane, or undergoing dehydrogenation to yield CCH 2 CH 3 that is then transformed into propylene (thermodynamic selectivity) thermodynamically driven turnover Selectivity toward propylene is thus attributed to the thermodynamically driven turnover of CHCH 2 CH 3 Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

29 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Overall reaction scheme of the F-T synthesis Combining all information collected in previous sections, a reasonable reaction mechanism for the F- T synthesis on Fe can be constructed Next coupling Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

30 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 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;

31 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Use of Alloys Lo and ZieglerJ. Phys. Chem. C 2008, 112, H 2 activation 2. CO activation J. Phys. Chem. C.; (Article); 2008; 112(10);

32 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 C 1 species on Fe(100) is *CH, but the chain initiation takes place making use of *CH 2 instead. The carbide mechanism, in which *C inserts into surface *C n H m 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.

33 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study Acknowledgments  Dr. John Lo  Department of Chemistry, University of Calgary  The Western Canada Research Grid (Westgrid)  Alberta Ingenuity Fund

34 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study How to improve the catalytic properties of Fe? The bottleneck of the F-T synthesis: CO dissociation constitutes the rate-determining step Two possible solutions widely used in industry:(i) metal promoters NO decomposition on Cu and CuSn surfaces (Reference: Gokhale, Huber, Dumesic and Mavrikakis, J. Phys. Chem. B 108, (2004)) DFT predicts that the NO decomposition is an endothermic process on pure Cu surface (red), but can be promoted when Cu is doped with Sn (green and blue) The presence of Sn alters the reaction mechanisms of adsorbed NO molecules:

35 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study How to improve the catalytic properties of Fe? The bottleneck of the F-T synthesis: CO dissociation constitutes the rate-determining step Two possible solutions widely used in industry:(ii) Surface defects Example: C-C coupling reactions on flat Ru(0001) and Ru monolayer steps Faster rate of coupling Reference: Liu and Hu, J. Am. Chem. Soc. 124, (2002)

36 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study CO activation on Fe(310) surface Fe(310) has been extensively studied for its magnetic properties and anisotropic multilayer relaxation because of its small packing efficiency It can be generated via spark-cutting strain-annealed ultra-pure Fe sample at [310]

37 The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study The F-T synthesis on Fe-Co alloy surfaces Fe-Co alloys have been known for their high saturation magnetization, high Curie temperature and low magnetocrystalline anisotropy Fe-Co alloys exist in a range of 0 to 100 at. % Co, in which the BCC CsCl-B2 phase of Fe-Co (1:1) is the most favorable configuration Structural parameters: References: J. Appl. Phys. 85, 4839 (1999); Act. Mater. 50, 379 (2002). Surface energy: stability

38 The Iron-Catalyzed Fischer-Tropsch Synthesis : A DFT Study John Lo and Tom Ziegler Department of Chemistry, University of Calgary Chemistry 601 Seminar December 6, 2007