Transcript Document

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. Fischer-Tropsch Synthesis
H2
CH3
+CO
H
H
C
H
H
C
CH2
O
CH
Absorption of CO + H2
Syngas molecules
O
Dissociation of CO + H2
H
H
O
Hydrogenation of C and O
2. Fischer-Tropsch Synthesis
CH3
CH4
CH2
CH
H
H
Hydrogenation of
C and O
O
CH4
CH4
Monomer
CHn H H H
CHCH
n 4
H
O
H
O
CHm
CHm
CHp
C2Hp
H2O
H
H
H2O
Methanation
Propagation
chain
Initiation
2. Fischer-Tropsch Synthesis
Monomer
CHp
Propagation
chain
H
C2Hp
C3Hp+q+1
CHp
Monomer
CHp
Initiation
H
C3Hp+q
Propagation
chain
Propagation
+ Termination
H
C3Hp+q
Hnp+q+1
2+n
CC
Hnp+q+1
2+n
CH
CH
pp
H
C2+n
Hnp+q
2+n
C
Hnp+q
Propagation
+ Termination
Franz Joseph Emil Fischer
Hans Tropsch
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
Model
Experiment
Lattice constant
2.8553 Å
2.8665 Å
Bulk modulus
156 GPa
170 GPa
Magnetic
moment
2.30 0
2.22 0
6
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
K
L
7
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
4-fold
2-fold
1-fold
 Calculate the binding energies at various surface coverage
CO
C
Lo
and Ziegler,
J. Phys.
Chem.
111,11012
11012
(2007)
Reference:
Lo and Ziegler,
J. Phys.
Chem. C
C 111,
(2007)
O
8
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
15
Activation barrier
increases with 
CO is less
strongly
bound at
higher 
5
-5
-15
CO
C
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
O
9
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)
C O
C
O
10
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
C O
C
O
@ 473 K: 27% *CO; 27% vacancy; 23% *C; 23% *O
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
11
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
12
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
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
4-fold
2-fold
1-fold
 Calculate the binding energies at various surface coverage
H2
H-H
H
Lo
and Ziegler,
J. Phys.
Chem.
111,11012
11012
(2007)
Reference:
Lo and Ziegler,
J. Phys.
Chem. C
C 111,
(2007)
H
13
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Formation of CHx species on iron surface
CH4
H2O
Fe is active catalyst for the CHx formation
Cn  H  Cn 1
CHn
CH4
H
H
H
O
CHm
Reaction of C and H on Fe(100) in the absence of
Methanation
and Hydrogenation

CH2
C
CH
CH2
CH
CH3
CH3
CH4
14
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
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)
15
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)
16
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)
17
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Initiation C-C bond coupling reactions on Fe(100) surface
CHp
H
C2Hp
(c)
(e)
(d)
15
5
2
1
4
13
9
10
6
12
8
3
14
11
7
(a)
(f)
(b)
(e)
(c)
(d)
Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)
18
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
19
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)
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)
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)
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Kinetics of the C-C coupling reactions on Fe(100)
Hydrogenation reactions occur rather
rapidly at room temperatures
Many hydrogenation reactions are indeed
endothermic and require energy
23
Reference: Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Kinetics of the C-C coupling reactions on Fe(100)
Isomerization processes are not
kinetically favorable
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)
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)
25
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
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
C-C bond coupling reactions
Coupling reactions with C-CHn fragments are generally endothermic  important only at high
reaction temperatures
28
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
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
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
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
32
Lo and Ziegler, J. Phys. Chem. C 111, 13149 (2007)
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).
33
The Fe-catalyzed F-T synthesis of hydrocarbons: A DFT study
Thermodynamic stability of reactive C3 fragments
Kcal/mol
Propylene
Reference:
Lo and
J. Phys. Chem. C (to be submitted)
Lo and Ziegler, J. Phys.
Chem.
C Ziegler,
111(2008),submitted
34
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
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
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
37
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.
38
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.
39
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.
40
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.
41
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
Ni,Fe,Co
CH4
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)
42
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
43
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 …
Lo and Ziegler, J. Phys. Chem. C 111, 11012 (2007)
ENTROPY !
44
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 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)
This term will be completely
lost because of the
assumption that the
adsorbed species is
immobile
45
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
4-fold
2-fold
1-fold
 Calculate the binding energies at various surface coverage
Lo
and Ziegler,
J. Phys.
Chem.
111,11012
11012
(2007)
Reference:
Lo and Ziegler,
J. Phys.
Chem. C
C 111,
(2007)
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
4-fold
2-fold
1-fold
 Calculate the binding energies at various surface coverage
Lo
and Ziegler,
J. Phys.
Chem.
111,11012
11012
(2007)
Reference:
Lo and Ziegler,
J. Phys.
Chem. C
C 111,
(2007)
47