The Iron-Catalyzed Fischer-Tropsch Synthesis of Hydrocarbons: A DFT Study

1
New Orleans National Meeting
35nd ACS National Meeting April 6-10, 2006 New
Orleans, Louisiana

Symposium on Computational Catalysis Division of
Computers in Chemistry
Organizers Presiding Tuesday April 8, 2008,
130 -200 PM. Morial Convention Center, Room
Rm. 337
John Lo T.Ziegler Department of Chemistry
University of Calgary,Alberta, Canada T2N 1N4
Modeling the Fischer-Tropsch Synthesis Catalyzed
on a Fe(1,0,0) Surface
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
Fischer and Tropsch reported in 1923 the
synthesis of liquid hydrocarbons with high oxygen
contents from syngas on alkalized Fe catalyst
(Synthol synthesis)
(2n1) H2 n CO ? CnH2n2 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)
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))
insertion
A
A
B
condensation
C
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?
A
B
How do the C1 units couple?
Maitlis et al. JACS 124, 10456 (2002)
C
F
E
How does the chain grow?
D
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) lt Fe(111)
• ? Spin-polarized periodic DFT with plane-wave
  basis sets (VASP)
• ? PW91 exchange-correlation functional at GGA
  level
• ? Vanderbilts 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)
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
A
H
G
B
F
I
C
D
E
J
L
K
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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
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
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)
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
Desorption barrier decreases with ?
Activation barrier increases 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
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)
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 COAr (119) gas at 1 atm
28 hours _at_ 150 and 473 K
Results
_at_ 150 K 50 CO 50 vacancy no C and O
_at_ 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
CH2
CH2
C
CH
CH
CH
CH
CH3
CH2
CH3
CH4
Reference Lo and Ziegler, J. Phys. Chem. C 111,
11012 (2007)
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
(a)
? The rate of CH4 formation exhibits a strong
dependence on the partial pressures of CO and H2
p(CO) 0.2 Mpa T525 K
? Fixed pressures of CO and H2 p(CO) 0.2 MPa,
p(H2) 0.9 MPa.
? The computed rates are much higher than the
experimentally observed values
(b)
? Reason the coupling of C1 fragments is ignored
in all simulations
p(H2) 0.9 Mpa T525
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
C-C bond coupling reactions on Fe(100) surface
To figure out the origins of the product
selectivity of ethane/ethylene mixture
(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
Thermodynamic stability of C2 species
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)
With this information we may construct the
kinetic profile for the formation of ethane
ethylene
C-C bond coupling reactions are usually
kinetically demanding processes
Reference Lo and Ziegler, J. Phys. Chem. C 111,
13149 (2007)
21
The Fe-catalyzed F-T synthesis of hydrocarbons A
DFT study
Kinetic profile of ethane formation
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 (gt 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
For Co and Ru, the following mechanisms have been
proposed
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)
23
The Fe-catalyzed F-T synthesis of hydrocarbons A
DFT study
Thermodynamic stability of reactive C3 fragments
Kcal/mol
Reference Lo and Ziegler, J. Phys. Chem. C (to
be submitted)
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
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)
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 CCHCH3
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 turnover 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
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 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
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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. CO activation
J. Phys. Chem. C. (Article) 2008 112(10)
3679-3691.
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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.
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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, 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 step
Two possible solutions widely used in industry
(ii) Surface defects
Example C-C coupling reactions on flat Ru(0001)
and Ru monolayer steps
Reference Liu and Hu, J. Am. Chem. Soc. 124,
11568 (2002)
Faster rate of coupling
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 (11)
is the most favorable configuration
Structural parameters
Surface energy stability
References J. Appl. Phys. 85, 4839 (1999) Act.
Mater. 50, 379 (2002).
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