1 The Fe-catalyzed F-T synthesis of Hydrocarbons: A DFT study Fischer-Tropsch synthesis: An...

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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

2. Fischer-Tropsch Synthesis2. Fischer-Tropsch Synthesis

Absorption of CO + H2

Syngas moleculesCH HO

H2 +CO

CH H ODissociation of CO + H2

CH OHHCH2CH3 Hydrogenation of C and O


H OHHCHnCH4

CHm

H2OCH4

Initiation

CHp

C2Hp

HMonomer

H OHHCHnCH4

CHm

H2OCH4

Methanation

CH OHHCH2CH3 Hydrogenation of

C and O

Propagationchain


CHp

C3Hp+q

H

C3Hp+q+1

CHp

C2+nHnp+q

H

C2+nHnp+q+1

Initiation

CHp

C2Hp

HMonomer Propagation

chain

Propagation+ Termination

CHpC2+nHnp+q

C2+nHnp+q+1

Propagation+ Termination

Monomer

CHp

C3Hp+q

H

Propagationchain

Franz Joseph Emil Fischer Hans Tropsch

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) + 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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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)

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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


4-fold4-fold

2-fold2-fold

1-fold1-fold

C

CO

O

9


Chemisorption of CO: KineticsLateral interaction: crucial factor affecting the adsorption kinetics

of CO

Activation barrier increases with

Desorption barrier decreases with

CO is less strongly bound at higher


45

35

25

15

5

-5

-15

C

CO

O

10


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


C O C O

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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


150K150K

473K473K

C O C O

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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


13








4-fold4-fold

2-fold2-fold

1-fold1-fold

H HH-H

H2

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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


€

Cn + H⇔ Cn+1

€

Cn + H⇔ Cn+1

CCH

CH2

CH3 CH3

CH4

CHCH2

H OHHCHnCH4

CHm

H2OCH4

Methanationand Hydrogenation

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Thermodynamic PES of CH4


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

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Temperature effects on the rate of CH4 formation


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

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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


Lox and Froment, Ind. Eng. Chem. Res. 32, 61 (1993); 32, 71 (1993)

Fixed pressures of CO and H2: p(CO) = 0.2 MPa,

p(CO) = 0.2 MpaT=525 K

(b)

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Initiation C-C bond coupling reactions on Fe(100) surface

(a) (b)

(c)

(c)

(d)

(d)

(e)

(e)

(f)

12

3

4

5

6

7

8

9

10

11

12

13

14

15


CHp

C2Hp

H

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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

20


Kinetics of the C-C coupling reactions on Fe(100)

C-C bond coupling reactions are usually kinetically demanding processes


21





C with CH/CH2 bond coupling reactions kinetically and thermodynamically favorable

22





CH to CH/CH2 bond coupling reactions kinetically favorable but thermodynamically

unfavorable

23




Hydrogenation reactions occur rather rapidly at room temperatures

Many hydrogenation reactions are indeed endothermic and require energy

24




With this information we may construct the kinetic profile for the formation of ethane ethylene

Isomerization processes are not kinetically favorable

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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


Propagating chainMonomer

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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 (monomers)

*CCH, *CCH2 and *CCH3 are stable C2 fragments on Fe(100)(growing chains)

For Co and Ru, the following mechanisms have been proposed:

Unsaturated carbon)Unsaturated carbon)

one hydrgenone hydrgen

two hydrgenstwo hydrgens

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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 (monomers)

*CCH, *CCH2 and *CCH3 are stable C2 fragments on Fe(100)(growing chains)

For Co and Ru, the following mechanisms have been proposed:

Unsaturated carbon)Unsaturated carbon)

one hydrgenone hydrgen

two hydrgenstwo hydrgens

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C-C bond coupling reactions

Coupling reactions with C-CHn fragments are generally endothermic important only at high reaction temperatures

29



Reactions between *C and CHCH2/CH-CH3 and CH2CH3 possess lower activation barriers on Fe

30



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

31



Therefore, the carbide route should be the dominant mechanism in the Fe-catalyzed F-T synthesis (thermodynamically favorable but kinetically demanding)

32


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)

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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


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

35


Kinetic potential energy surface for propane formation

Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

36


Kinetic potential energy surface for propane formation

Lo and Ziegler, J. Phys. Chem. C 111, 2008,submitted

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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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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.

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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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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.

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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


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.

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Mechanisms of F-T synthesis

CO insertion mechanism(Pichler and Schultz (1970s))

insertion

A

B

C

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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 !


IncreaseIn free energyWith 4 kcalFor each 100K

45


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)

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4-fold4-fold

2-fold2-fold

1-fold1-fold

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4-fold4-fold

2-fold2-fold

1-fold1-fold

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