35nd acs national meeting april 6-10, 2006 new orleans, louisiana organizers : presiding: tuesday...

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


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


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


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


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


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


9


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

10


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 !


11


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


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


13


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


14


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


15


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

C CH CHCH2

CH2CH3 CH3 CH4

CH CHCH2

16


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

17


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)

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


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


19


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


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

C-C bond coupling reactions are usually kinetically demanding processes


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


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


22


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


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


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


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


Kinetic potential energy surface for propane formation

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

27


Kinetic potential energy surface for propane formation


28


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


29


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


30


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


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


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


Acknowledgments

Dr. John Lo

Department of Chemistry, University of Calgary

The Western Canada Research Grid (Westgrid)

Alberta Ingenuity Fund

34


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


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


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 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

35nd acs national meeting april 6-10, 2006 new orleans, louisiana organizers : presiding: tuesday...

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