35nd acs national meeting april 6-10, 2006 new orleans, louisiana organizers : presiding: tuesday...
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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 CatalysisDivision 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 CH4 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:
CO +H2 CH4Ni,Fe,Co
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
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 synthesisCO insertion mechanism
(Pichler and Schultz (1970s))
Enol mechanism(Emmett et al. (1950s))
condensation
insertionA
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 C1 units couple?
How does the chain grow?
Maitlis et al. JACS 124, 10456 (2002)
AB
C
DE
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
Model Experiment
Lattice constant
2.8553 Å 2.8665 Å
Bulk modulus 156 GPa 170 GPa
Magnetic moment
2.30 0 2.22 0
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)
A
B
CDE
F
GH
I
J
K L
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (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, 11012 (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, 11012 (2007)
9
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Thermodynamic PES of CH4
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)
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
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, 11012 (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
Eact generally increases
C + O becomes less stable w.r.t. CO
+0.06 kcal/mol
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (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; ~28 hours; @ 150 and 473 K
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)
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 H2
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
15
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
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
€
Cn + H ⇔ Cn+1
€
Cn + H ⇔ Cn+1
C CH CHCH2
CH2CH3 CH3 CH4
CH CHCH2
16
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Temperature effects on the rate of CH4 formation
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993)
Simulations including both CO and H2 at industrial reaction conditions
P(CO)/P(H2)=1/3
17
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Pressure effects on the rate of CH4 formation
The rate of CH4 formation exhibits a strong dependence on the partial pressures of CO and H2
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (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 C1 fragments is ignored in all simulations
Fixed pressures of CO and H2: p(CO) = 0.2 MPa, p(H2) = 0.9 MPa.
p(CO) = 0.2 MpaT=525 K
p(H2) = 0.9 MpaT=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)
(c)
(d)
(d)
(e)
(e)
(f)
12
3
4
5
6
7
8
9
10
11
12
13
14
15
Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)
19
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Thermodynamic stability of C2 species
Direct formation of C2 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, 13149 (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, 13149 (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 CH3CH3 is kinetically feasibleThe rate-determining step is the C + CH2 coupling reactionThe 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)
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, *CCH2 and *CCH3 are stable C2 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 C3 fragments
Reference: Lo and Ziegler, J. Phys. Chem. C (to be submitted)
Kca
l/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-CHn fragments are generally endothermic important only at high reaction temperatures
Reactions between *C and CHCH2/CH-CH3 and CH2CH3 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 CHCH2/CH-CH3 or CH2CH3 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, 11568 (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
The formation of propane and propylene can be traced back to CCHCHCCHCH33
The production of propylene is more kinetically controlled in the first step, while the path leading to CHCH2CH3 intermediate is endothermic
Turnover may occur at CHCH2CH3: either proceeding further to form propyl and propane, or undergoing dehydrogenation to yield CCH2CH3 that is then transformed into propylene (thermodynamic selectivity)
Selectivity toward propylene is thus attributed to the thermodynamically driven thermodynamically driven turnoverturnover of CHCH2CH3
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
Nextcoupling
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 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
31
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Use of AlloysUse of Alloys
Lo and ZieglerJ. Phys. Chem. C 2008, 112, 3667-3678Lo and ZieglerJ. Phys. Chem. C 2008, 112, 3667-3678
1. H2 activation1. H2 activation
2. CO activation2. CO activation
J. Phys. Chem. C.; (Article); 2008; 112(10); 3679-3691. J. Phys. Chem. C.; (Article); 2008; 112(10); 3679-3691.
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 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.
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 stepTwo 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, 14062 (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 stepTwo 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, 11568 (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 CalgaryChemistry 601 Seminar
December 6, 2007
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