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

1
The Fe-catalyzed F-T synthesis of Hydrocarbons A
DFT study
Fischer-Tropsch synthesis An Introduction
(2n1) H2 n CO ? CnH2n2 n H2O 2n H2 n CO ?
CnH2n n H2O CO H2O ? CO2 H2 2 CO ? C CO2
1
2
2. Fischer-Tropsch Synthesis
3
2. Fischer-Tropsch Synthesis
4
2. Fischer-Tropsch Synthesis
5
Franz Joseph Emil Fischer
Hans Tropsch
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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) 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

Model Experiment
Lattice constant 2.8553 Å 2.8665 Å
Bulk modulus 156 GPa 170 GPa
Magnetic moment 2.30 ?0 2.22 ?0
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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
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 COAr (119) gas at 1 atm
28 hours _at_ 150 and 473 K
150K
473K
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)
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 T525 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)
C
F
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 unfav
orable
22
Reference Lo and Ziegler, J. Phys. Chem. C 111,
13149 (2007)
23
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
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 (gt 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
The Fe-catalyzed F-T synthesis of hydrocarbons A
DFT study
Thermodynamic stability of C2 species
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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
The Fe-catalyzed F-T synthesis of hydrocarbons A
DFT study
Thermodynamic stability of reactive C3 fragments
Kcal/mol
34
Reference Lo and Ziegler, J. Phys. Chem. C (to
be submitted)
Lo and Ziegler, J. Phys. Chem. C
111(2008),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. CO activation
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. CO activation
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)
(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)
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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)