1

Mechanistic issues in Fischer-Tropsch Catalysis

R.A. van Santen1, I.M. Ciobîcă2, E. van Steen3

1 Eindhoven University of Technology, Schuit Institute of Catalysis, P.O. Box 513, 5600 MB

Eindhoven, The Netherlands 2 Sasol Technology Netherlands B.V., Eindhoven University of Technology, P.O. Box 513,

5600 MB Eindhoven, The Netherlands 3 University of Cape Town, Department of Chemical Engineering, Private Bag X3,

Rondebosch 7701, South Africa

Corresponding Author: R.A.v.Santen@tue.nl

Abstract

Computational studies have recently generated important data on reaction intermediates and

activation barriers of elementary reactions steps that are part of the Fischer-Tropsch synthesis.

We use these data to analyse different mechanistic options that have been proposed for the

Fischer-Tropsch synthesis.

It is concluded that in contrast to the Pichler-Schultz CO insertion mediated chain growth

mechanism, the Sachtler-Biloen “C1” intermediated mechanism is consistent with

computational results and that the Gaube chain growth mechanism, that closely resembles the

Maitlis mechanistic proposals, is preferred. On Co olefin is a dominant primary product of the

chain growth reaction.

The structure sensitivity of the different elementary reaction steps, initiation, chain growth

and termination, will be analyzed. Within the Biloen-Sachtler kinetic scheme, for a high chain

growth probability chain termination has to be rate limiting. Hence CO has to dissociate with

low barrier. This is a structure demanding reaction. “CH2” appears the essential “C1” species

incorporated in the growing chain. On chain growth sites these “CH2” species have to be

present at a high concentration.

Key words: Fischer-Tropsch mechanism; structures sensitivity; computational catalysis;

surface reactivity

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1. Introduction

Whereas the Fischer-Tropsch reaction is one of the earliest heterogeneous reactions

discovered (1) and this reaction is becoming of increasing practical interest (2), there are still

major questions regarding the factors that control its activity and selectivity. This is especially

true regarding its molecular basis and the relation of catalyst activity and selectivity with

catalyst structure and composition.

Computational catalysis has made large progress especially because it allows for a

comparison of the rates of elementary reaction steps proposed for different mechanistic

reaction path options. It also enables to relate surface structure with the relative stability of

reaction intermediates and transition states.

This approach is now also applied by several groups to analyse elementary reaction

steps that are part of the Fischer-Tropsch reaction. .A detailed understanding of the relation

between activation energies and site structure is becoming possible and hence on the factors

that control activity as well as selectivity.

Using the recent new information mainly from these computational studies, we will

focus on two conflicting proposals on the key reaction steps that determine the chain growth

reaction. In this review paper we limit ourselves to the mechanisms of primary product

formation of the Fischer-Tropsch reaction. Secondary reactions as olefin insertion and

hydrogenation reactions, that also affect the production distribution. The presence of a wax

environment may have additional important consequences.

A fundamental question to the Fischer-Tropsch reaction is the nature of initiation step

and chain growth intermediates. One can distinguish two basically conflicting proposals.

According to one school of catalysis, CO inserts into hydrogen and CHxOH species

are incorporated into the growing chain (3,4 ). According to the alternative point of view, the

CO or C-OH bond has to cleave first and this generates the “CHx” species that is incorporated

into the growing chain (4).

A classical experiment in Fischer-Tropsch catalysis that supports this view is due to

Sachtler and Biloen (5,6). In a mechanistic study using isotopes they demonstrated that the

carbon chain growth reaction can occur from C1 species generated by dissociating CO. As we

will see for the overall Fischer-Tropsch reaction this proposal implies that the rate of CO

dissociation should be fast and not a rate controlling step of the overall reaction.

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The Fischer-Tropsch reaction mechanism according to Sachtler and Biloen is

illustrated by Scheme 1.

CO

CHx + CnHyCH4

C1H2

alkene alkane

-H +H

chain growth

termination

methanation

aldehyde

CO

Scheme 1

It is now well recognized that activation barriers of CO activation are strongly

structure dependent. (7,8,9) For chain growth the relative rate of CO dissociation should be

fast compared to that of the termination reaction. As we will discuss the chain growth reaction

may also be structure sensitive in contrast to the termination reaction that is not expected to be

structure sensitive. Because the three reactions mentioned all contribute to the overall

catalytic cycle significant structure sensitivity may be expected for Fischer-Tropsch

selectivity as well activity.

Elegant experiments by Wilson et al. (10) on single crystal Co surfaces exposed to

synthesis gas at Fischer-Tropsch conditions show large restructurings of initial surface

terraces to a corrugated surface in the islands of a few nm. Schultz et al. (11) have extensively

reported on the self organisation of Fischer-Tropsch catalysts that initially show increasing

activity with time on stream.

Experiments by de Jong et al. (12) have demonstrated a strong decrease in activity of

Co catalysts when the particles became reduced to a size less than 4 nm. By using an inert

support they excluded support effects. They also concluded that the Co particles remained

metallic. These results agree with earlier reports of such structure dependence of the Fischer-

Tropsch reaction (13,14) These results all indicate a large effect of particle structure on

catalyst reactivity.

In contrast to these results there are also reports of an independence of Fischer-

Tropsch activity and selectivity on particle size (15). However the particle size regime studied

was substantially larger than a few nm, indicative that structure sensitivity only appears for

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particle sizes of the order of a few nm (16). Van Hardevelt et al. (17) already proposed over

thirty years ago that so-called B-5 sites necessary for CO dissociation cannot be supported on

particles less than a few nm.

The other classical proposal is the Pichler-Schulz CO insertion chain growth

mechanism (3) (see Scheme 2), that requires the insertion step and subsequent

dehydroxylation to be fast compared to the growing chain termination reaction. Very

importantly now, CO dissociation is not required to be fast.

CO

C1

CnHy + CO

CnHyCO + H

Cn + 1 H

alcohol, aldehyde

alkane

alkene

-H+H

+ CO, H2

dehydroxylation

termination

CH4

H2

chain growth

insertion

direct dissociation orhydrogen activated initiation

y

Scheme 2

Experiments in favour of this mechanism are the disappearance of CO adsorption

features during the Fischer-Tropsch reaction (18) and the existence of the hydroformylation

reaction in which CO is inserted into alky chains (19). It is important to realise that CO

insertion requires attachment to cationic-ions with empty d-orbitals to occur (20).

To discriminate the two fundamental mechanistic Fischer-Tropsch proposals is very

important since it will determine our understanding of the structure sensitivity of the Fischer-

Tropsch reaction in relation to selectivity as well as stability.

In addition to significant new information on the reactivity of surface elementary

reaction steps computational catalysis is also having a large impact on conceptual theoretical

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understanding of heterogeneous catalytic reactions. There is a growing body of knowledge

based on Brønsted-Evans-Polanyi linear activation energy-reaction energy relationships of

surface chemical reactions (16,21,22).

These relations when incorporated in microkinetic type models of the catalytic

reaction cycles enable remarkable new and predictive insights in the factors that control

heterogeneous catalytic reactions. Predictive models of volcano curves (23,24) have been

constructed that enable study of catalytic reactivity varying the composition of catalyst as well

as reaction conditions. This can be considered application of the Sabatier principle (25,22).

We have summarized these developments in two recent papers. Structure sensitivity in

relation with BEP relations in ref. 16 and the Sabatier principle in a ref. 26. We refer to these

texts as background material to the material to be presented here. Sabatier type volcano

relations cannot only be deduced for activity as a function of adsorption energy, but also can

be used to make predictions on trends on deactivation of the F.T. catalyst by C—C bond

formation (26).

We begin our analysis with a presentation of analytic microkinetic expressions of

simplified mechanistic models of the Fischer-Tropsch reaction. We intend to show explicitly

how the relative rate of CO dissociation affects the selectivity of the chain growth reaction.

We are interested to deduce how the selectivity of this reaction depends on reactivity

parameters as the adsorption energy of C or O. This provides an approach to predict the

dependence of reactivity not only on composition but also as a function of surface stability.

Using the BEP relations trends as a function of Cads and Oads energies will be analysed.

A good starting point is an analytical expression of the kinetic expression that gives

Sabatier curve for CO methanation (26). This gives the Sabatier volcano courve maximum as

a function of BEP parameters for CO activation versus Cads hydrogenation. It also illustrates

the issue of CO pressure dependence as well as particle size dependence. It has been recently

demonstrated that the methanation is highly structure sebsitive (27). The activation of CO has

a substantially lowered barrier at step edges, hence will be highly particle size dependent. The

methanation reaction is the classical example of a catalytic reaction that has maximum rate for

a particle of a few nanometers (27).

We can also study the rate of deactivating C—C bond formation versus the rate of

initial CH formation as a function adsorbate metal carbon surface interaction (26). We then

generalize the methanation reaction expression to the chain growth reaction expression. We

discover the complex CO pressure dependence that affects the relative rate of the dissociation

reaction versus chain growth rate.

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Essential to our approach is to prevent the assumption of one rate limiting step. We

intend to deduce which step will have to be fast or rate limiting in order for the reaction to

have high selectivity. Our kinetic analysis complements that of others assuming indirectly,

that chain termination should be limiting (28).

Once we have deduced adequate kinetic expressions we will use computed quantum-

chemical data to estimate the values of the relevant parameters. We especially will make

extensive use of computational results of Hu and his colleagues (29,30), who also published a

series of important computational papers on the Fischer-Tropsch reaction. It will enable us to

understand structure sensitivity and most importantly helps us to define the conditions that

particular mechanisms can operate. We will not explicitly discuss the different methods that

have been used to neither deduce the computational data nor embark on a critical evaluation

of the reliability of these numbers,

For this we refer to available literature (22,26). Most of the data that we will consider

have been obtained using state of the art DFT computational techniques applied to models of

surfaces, based on the choice of metal-slabs of particular thickness and direction.

Whereas the absolute accuracy of these methods is not better than 10 kJ/mol for

adsorbed reaction intermediates and 20-30 kJ/mol for transition state energies, the difference

between computed energies is usually large enough to draw chemically relevant conclusions.

The microkinetic discussions lead to detailed proposals of relevant intermediates and

their respective reactions. We will relate this to a comparison of two competitive chain growth

mechanisms both proposing “CH2” intermediates as the key chain building units: the Brady-

Pettit chain growth model (21) versus that of Gaube-Maitlis (31,32). In one case the growing

chain is adsorbed alkyl, in the other case it is a growing alkelydene or alkelidyne chain. We

will find that available computational data mainly favour the Gaube proposal. Interestingly

and consistent with this finding Dry (33) suggested a mechanism with a metallocycle as

intermediate, involving an allyl intermediate as in the Maitlis mechanism as well as reaction

with olefin intermediate as proposed by Gaube to explain the selectivity for branched alkenes

as a function of chain length.

We then will focus on the Pichler Schulz CO insertion mechanism (34). This reaction

has been much less studied then the previous mechanism. Of course, in homogeneous

catalysis hydroformylation has been extensively studied. It appears that this reaction is much

more difficult on metallic surfaces.

The CO insertion mechanism (35,36) is of interest since with this mechanistic scheme

now CO dissociation and initiation of the reaction can be accepted as rate limiting. The chain

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growth intermediate that is incorporated in the growing chain is CO itself and not a “C1”

species that first has to be generated.

We will conclude from available computational data that the CO insertion mechanism

is unlikely for chain growth, but can be operational to terminate growing alkyl chains. In the

later case it will give alcohols, aldehydes, or even carboxylic acids as initial products.

In a final chapter we will summarize the chemical insights on the Fischer-Tropsch

reaction that are discussed in this paper.

2. The microkinetic expressions.

2.1 The Sachtler- Biloen mechanism

According to this mechanism reaction is initiated through CO adsorption followed by

CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate

has been obtained by incorporation of pre-deposited 13C during the Fischer-Tropsch synthesis

utilizing 12CO (6). An important issue is whether during the Fischer-Tropsch synthesis CO

dissociation is strictly monomolecular or CO dissociation is promoted by initial reaction with

H. Another important question is how these rates depend on surface structure.

It is of critical importance to understand the relation between the relative values of the

elementary rate constants for CO dissociation, chain growth and chain growth termination.

We will initially present microkinetic expressions assuming “C1” formation through

direct CO dissociation and Oads removal being fast. Oads is removed by reactions with H2 or

possibly CO that we will not explicitly consider. The system that is only weakly suppressed

by water is Co (2).

The computational data that are available indicate that the activation energy for the

recombination of adsorbed hydrogen with adsorbed oxygen on Co is typically , this is to be

compared with value found for Rh of 90 kJ/mol (37). Subsequent water formation occurs by

recombination of OHads with a barrier between 5-20 kJ/mol (38). Alternativelly it has been

proposed by Gong et. al (39) that oxygen removal is water assisted. Hydroxyle formation

occurs through reactions at steps with barriers less than 40 kJ/mol. The subsequent OH

hydrogenation step has barriers less than 70 kJ/mol. With increasing metal-oxygen interaction

the reaction of OHx with Co becomes preferred over removal of oxygen by hydrogen. As has

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been shown by the same authors oxygen removal by CO is also assisted by water. Reaction of

OH with CO occurs at low temperature.

The issue of deactivation of the reaction by H2O has been extensively studied. We will

not discuss this issue here, but refer to the relevant papers (see ref. 2, chapter 7). The

reduction reaction predominantly occurs with H2 on Co to produce H2O. On Fe CO2 is an

important secondary product (40,41), which can be deduced from classical space velocity

studies (Figure 1). The oxygen removal step with CO is an essential elementary step in the

steam reforming reaction (42). This reaction most likely occurs with low barrier be reaction of

OH with CO. Coadsorbed oxygen can lower the barrier further by accepting the hydrogen

atom. Surface “C1” hydrogenation proceeds via consecutive surface reaction steps in which

different “CHx” species are subsequently formed. This we will discuss in detail later.

0

10

20

30

40

50

0 0.5 1 1.5 2

Space velocity, W/F, gcat. hr/ml

SC

O2,

C-%

100 Fe/30Al2O3/5 K2O

100 Fe/30Al2O3

Figure 1. Influence of space velocity on the selectivity of CO conversion for formation of CO2

over iron-based Fischer-Tropsch catalysts (T = 250oC, p = 20 bar, (H2/CO)inlet = 2)

showing secondary formation of CO2 with possibly a small contribution of

primarily formed CO2 (43)

The equations that we will discuss in this section we have deduced based on the additional

simplifying assumption that only one type of “C1” is formed. We will later extensively

discuss the consequences of the presence of different “CHx” species on the surface for the

9

kinetics of the Fischer-Tropsch reaction. This discussion will appear to be very relevant to

understand and calculate the chain growth parameter, α.

In the microkinetic model the only surface species accounted for are the adsorbed CO, C(1)

and the growing chain C(n) species. For dissociation of CO an empty site is necessary in

order to accommodate the two atoms generated by dissociation. Chain growth is assumed to

be independent of chain length. There is one type of growing chain and one type of

termination.

The equations describe the primary intrinsic Fischer-Tropsch reaction kinetics, secondary

reactions are not included.

These simplifying assumptions enable the deduction of closed kinetic expressions without the

necessity to assume a rate limiting step. Hence, we have a tool to evaluate the condition for

chain growth to occur. This will be formulated as relations between the rate of CO

dissociation, the rate of chain growth and the rate of chain termination.

Since the expressions for methanation are easier to interpret we will first present the

corresponding equations for this reaction. They are presented in equations 1 (26, see appendix

A):

21 (1)

(1)c

c

θλ

θ

(1a)

1

1)1(c (1b)

1Hcodiss

rλ A

k

(1c)

)1(C is the coverage with “C1”species, Hr the rate of hydrogenation. It depends implicitly

on the hydrogen surface coverage, COdissk is the elementary rate constant for CO dissociation. λ

can be considered as a control parameter. It depends on the ratio of the relevant rates, the rate

of “C1” hydrogenation versus CO dissociation as well as the CO adsorption constant and

pressure.

2

1 1

[ ]

[ ]eq eq

eq eq

K CO K COA

K CO K CO

(2)

Expression (1b) is an important result. It gives the surface coverage C as a function

of the rate constants of “C1” hydrogenation and CO dissociation.

The rate of methanation is given by:

10

)1(4 CHCH rr (3)

Theoretical catalysis analyzes the rate of a catalytic reaction as a function of a

reactivity index, that varies with surface, site or material. This is the basis of the volcano

curve constructions discussed before. We will deduce a volcano type relation from equation

(2) and (3). As reactive index we take the interaction energy of a M-C, that varies dependent

on the metal. The dependence of a rate constant on changes in heats of adsorption of surface

fragment is to be deduced from Brønsted-Evans-Polanyi relations (22).

As has been elegantly demonstrated by Hammer and Nørskov (21a) changes in energy

of adsorption of molecules are an order of magnitude less than those of atoms. To evaluate

Eq. (3) we therefore ignore the small variotiona in heat of adsorption of CO. This implies that

A in Eq. (1c) is constant. According to (BEP) activation energies depend experimentally on

adsorption energies. If the BEP expression for Hr is:

1aα E

Hr C l (4a)

then:

2 1 0 1 0;a

a

α E

β E

eλ C α β

e

(4b)

Expression 4b implies that shen the rate of hydrogenation of “C1” species decreases with

increasing M-C energy, the activationn energy for CO dissociation increases. For these

reactions typical values of and α and β are:

1α β (5)

The Sabatier maximum in expression (3) is now to be at:

2

2

1aEe

c (6)

and hence 1maxλ (7)

For this value of λ the rate of methanation and apparent CO dissociation are in balance and

surface is half covered.

Note that because of the pressure dependence of COdissr the actual position of this

Sabatier maximum will strongly depend on CO partial pressure.

A recent computational study on CO methanation by Ni (27) has demonstrated that

hydrogen assisted CO dissociation on surfaces that contain double steps have relatively low

overall barriers for CO dissociation. At such sites activation energies depend only weakly on

CO coverage. As a result the rate of catalysis by such surfaces should be significantly

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enhanced and the rate for methanation should depend only weakly in CO pressure. We will

discuss this in detail later, but it is interesting to mention here that in Chockendorff et al.

study hydrogen activation took place not through intermediate formyl formation but by

addition to the oxygen atom of adsorbed CO.

Since the Fischer-Tropsch chain growth reaction will strongly depend on θC(1) this

gives already an indication that this reaction will occur in sites with a high rate of CO

dissociation. We now will explicitly demonstrate this.

For the Fischer-Tropsch reaction analogous expressions to Eq. 1 can be deduced (see

appendix B). The expression for the coverage with C(1) intermediates becomes:

21 11 1

1 1 1

1H

mc cα cα

codiss c

r r θ θ

k A θ

(8a)

The surface concentration θc(1) now implicitly depends on the surface concentration of the

growing chain through the chain growth parameter α:

1, 1

1, 1c c

nc c t

r n n θα

r n n θ r

(8b)

In expression 5 rc(n-1,n) is the chain growth reaction rate that converts an adsorbed

hydrocarbon chain of length n-1 to a chain of length n. rt is the rate at which the growing

chain of length n-1 is terminated and is desorbed. In expression 4 we have made the

assumption that the chain growth probability, , is independent of n. This is the Anderson-

Flory-Schulz assumption that often holds over a significant interval of n (4,44,45). It is

typically observed for high carbon number (>C15-C20) and only when the olefin content in

the product is constant.

Details of the derivations are presented in appendix B. To deduce Eq. (8a) we also

made use of the relation between α and total surface coverage:

0

lim 1 1

11

1

N Ni

c cN

i i

c

θ α θ

θα

(9a)

Since:

1

( ) 1N

ci

θ i

(9b)

It follows that:

(1) 1cθ α (9c)

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When (1) 1cθ α , one deduces that the rate constant of dissociation has to satisfy the

following relation:

1 mdiss H ck A α r r (10)

The rate of dissociation of adsorbed CO has to be fast compared to other two rates that

control the selectivity of the Fischer-Tropsch reaction. As directly follows from the

expression of α (Eq. 8) its value will only be close to one as long as the rate of hydrocarbon

chain termination is rate limiting. Since α is related to θC(1) one deduces:

; 1c

c t

rα α

r r

(11)

Then the apparent activation energy of CO has to satisfy relation:

12( )co term c c

act act actE app E E (12a)

This relation is strictly only valid as long as differences of reaction rate pre exponents

are small. In addition to Eq. (12a) we also find that high chain growth rate requires:

t c cact actE E (12b)

Eqs. (9) are of great interest since they give a quantitative condition that the activation

energies of different elementary reaction steps have to satisfy for the Fischer-Tropsch

reactions to have a high chain growth selectivity.

Interestingly recent simulation data by the Marin group (46, 47) seem to confirm Eq.

(12a). They used a single event microkinetic model (SEMK) to analyze exmperimental data

on the iron catalyzed Fischer-Tropsch reaction.

For CO dissociation they find an activation energy of only 57 kJ/mol, whereas the

activation energies for the chain growth reaction and termination reaction through alkane or

alkene formation are 45 kJ/mol, 118 kJ/mol and 97 kJ/mol respectively.

One should be aware that often the quoted values of activation energies are apparent

activation energies. For instance, the rate of CH formation depends on concentration C, as

well as hydrogen. The apparent activation energy for “CH” formation from “C1”, is the

intrisic activation energy for CH formation corrected by the adsorption energy of hydrogen,

multiplied by a factor that depends on the order of this reaction in hydrogen.

It is also interesting to study the expression of α for intermediate values. For λ instead

of Eq. 1b, we then have Eq. (13b):

11

1cθ λ

(13a)

13

COdiss

CH

r

rr

1

1 (13b)

so that becomes:

)1(

tC

C

rr

r (13c)

Again we note the strong dependence of α on the apparent rate of CO dissociation.

When the rate of CO dissociation is small λ′ is large. λ′ is also large when the rate of

methanation is fast. These high values of λ′ reduce the value of chain growth parameter α.

Figures 2a and 2b summarize the main conclusions one can draw for the analysis of

this section. Figure 2a schematically illustrates the different chain growth regimes one expects

as a function of varying apparent activation energy of CO. This variation is due to changes in

A (see Eq. 2b and appendix A) that relate the apparent activation energy of CO dissociation

with CO gas phase pressure. It implies a large variation of the CO coverage θCO.

Figure 2a Fischer-Tropsch chain growth regimes as a function of the relative value of the

apparent activation energy of CO dissociation with respect the respective activation energies

of chain growth and chain growth termination. The apparent activation energy of CO controls

θC(1) to a significant extent.

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At low CO activation energy a high selectivity for long hydrocarbon chains is

expected. When the activation energy of CO dissociation increases beyond the activation

energy of the chain growth reaction one finds an intermediate chain growth. For high apparent

activation energies of CO (CO activation energies high compared to the activation energy of

chain termination) and high CO coverage the selectivity to methanation is high.

Figure 2b Fischer-Tropsch selectivity as a function of reaction order in CO. The coverage

θCO is a direct function of reaction order.

As Figure 2b illustrates similar regimes, which can be distinguished when one

analyses the Fischer-Tropsch reaction as a function of reaction order in CO.

Interestingly chain growth can only be expected in the CO kinetic order regime where

the order is around zero. When reaction order is first order in CO, the rate of dissociation is

fast, but surface coverage of “C1”species is low. As a result the main product is methane.

When reaction order is very negative the apparent activation energy for CO

dissociation is high. As a consequence again the surface concentration of “C1” species will be

low and the main product will again be methane. Experimentally (45) the reaction order in

CO is slightly negative corresponding to the chain growth regime.

2.2 The micro kinetic expressions according to the Pichler-Schulz mechanism. (CO

insertion route)

15

The advantage of the Pichler-Schulz mechanism presented in Scheme 2 is that the

chain growth reaction does not require high C(1) coverage but is favoured by a high CO

coverage. In contrast to the Biloen-Sachtler mechanism initiation by CO dissociation is

considered rate limiting in the Pichler-Schultz mechanism. This implies “C1” formation to be

rate limiting as often assumed in engineering studies (44,45,48,49).

In this stage of our analysis it is not very useful to remark that this is in apparent

agreement with experiment. As we will see later we will conclude that unique step site sites

are required for efficient chain growth. Since they may only be present in a very low

concentration in a particular experimental system, the overall coverage measured for a

particular surface species does not have to have any relevance for the Fischer-Tropsch chain

growth reaction (28). Quite interestingly in this context is the very low coverage of an Fe

catalyst surface of less than 10-4 by Govender et. al (49).

For initiation, a C(1) surface species may still be required, but its rate of formation

may be slow similar as in conventional polymerization catalysis; initiation is usually the rate

limiting step. Now the slow reaction step of the reaction is not the termination reaction as in

the Sachtler-Biloen scheme but the initiation reaction generating initiating C(1) species.

Because of the structure sensitivity of the CO dissociation reaction, but also because of the

expected structure sensitivity of the chain growth reaction it will only proceed at very unique

sites. Now the rate of CO insertion and consecutive steps should be fast compared to the rate

of CO dissociation.

Whereas the CO insertion reaction has been extensively investigated in organometallic

chemistry and homogenous catalysis, there are only few first principle investigations available

on transition metals surfaces (30,50).

In homogeneous reactions the reaction proceeds readily on Pd2+, or Rh1+ when proper

ligand environment is used. Typical barrier energies are 50 kJ/mol (51). The reaction has also

been investigated on carbonyl complexes of Co, as Co(CO)4CH3 (51). Now a substantially

higher barrier is found mainly because the CO ligands have to be reorganized to a particular

configuration. Once this reconfiguration has happened CO insertion again occurs with a

barrier of the order of 50 kJ/mol.

This indicates that most likely a uniquely configured kink site is needed for the CO

insertion reaction into CH3 to occur easily. The study by Cheng, Hu et al (29) on surface

terraces and steps indicates that the barrier to CO insertion is quite high because of strong

repulsive interactions between CH3 and CO. They find in essence that the insertion reaction is

16

not structure sensitive. They find a barrier of 150 kJ/mol for the forward reaction and 90

kJ/mol for the reverse reaction. For the same reason recombination of CH3 species does not

happen on terraces because of the high repulsive barriers of repulsively interacting CH3

species adsorbed in parallel configuration to the surface (52).

Inderwildi et al. (53) found for hydrogen assisted cleavage CO bond cleavage on

Co(0001) an overall barrier of 130 kJ/mol. Studying Ru earlier Ciobîcă et al. (7) concluded

also that this dissociation path of CO through adsorbed formyl intermediate has a lower

barrier for dissociation on the CO terrace than direct dissociation of adsorbed CO. On the

other hand they concluded that direct dissociation of CO on the stepped surface is more

favourable than dissociation through the formyl intermediate.

Interestingly Inderwildi et al. found a rather low activation barrier for cleavage of the

CO bond in formaldehyde. This motivated Zhuo et al. (50) to study the insertion reaction of

CO into surface carbene and subsedquent CO cleavage species. Their results are summarized

in Figure 3.

Figure 3. Reaction energy diagram of CO insertion and C-C bond formation on the hcp Co

(0001) surface according to Zhuo et al. (50)

17

The reactions were studied on the HCP-Co(0001) surface. Interestingly for all

elementary rate constants rather low barriers were found. The activation energy for CO

insertion into “CH2” is 80 kJ/mol. Cleavage of the CO bond in the adsorbed aldehyde has a

barrier of only 40 kJ/mol. However the overall barrier for the chain growth reaction: CO +

CH2 → CH3CO is 190 kJ/mol, mainly due to the instability of intermediate reaction

intermediates.

For chain growth the essential rates in this case to compare are that of CO insertion

and CO cleavage versus alkyl chain termination. The rate of C-O cleavage reaction

determines the selectivity for hydrocarbon formation and chain growth versus termination as

oxygenat. The results of Zhuo et al. indicate an unlikely role for CO insertion in the chain

growth reaction. In agreement with this P. Maitlis and V. Zanotti (54) arrive at a similar

conclusion based on comparison with experimental studies.

If no oxygenate formation occurs one deduces the following expression for the chain

growth probability:

;;ins t

c cins

ins deh decarb r rc cins t

rα k k

r r

(14)

In Eq. (14) insα is the overall rate of the CO insertion to form the growing chain intermediate.

This rate is linear in CO coverage:

c c appins ins cor r θ (15)

The rate of termination rt is the rate at which the growing alkyl chain is terminated by

hydrogen addition or hydrogen removal.

If one assumes formation of C(1) to be rate limiting the rate of production of a

hydrocarbon with chain length n is given by Eq 16:

1

(1 )n

nC ins diss CO CO sR α k θ θ N

(16)

Key for long chain growth is a barrier for CO insertion and CO cleavage that is low

compared to the overall barrier of the termination reactions. So far the overall activation

energies found for CO insertion on the transition metal terraces and stepped surfaces cannot

compete with the generally lower values for the activation energies of the termination reaction

of adsorbed hydrocarbon intermediates. This makes the reaction mechanism discussed in this

paragraph unlikely.

The results of Zhuo et al. (50) indicate that chain growth through CO insertion with

CO cleavage into a surface carbene intermediate has a barrier that is too high compared to

18

termination by hydrogen addition or (β) CH cleavage, that have typical values of 70-90

kJ/mol. The presence of oxygenates and in particular the presence of carboxylic acids and

methyl alkyl ketones in the Fischer-Tropsch product is a strong indication for this (see Figure

4). OH addition has also been proposed for the formation of oxygenates in the Fischer-

Tropsch product (55), but the variety of oxygen containing products cannot only be explained

by OH addition (e.g. the formation of methyl alkyl ketones). Furthermore, the latter reaction

is unlikely on Co because of the low OH coverage. For CO insertion to play also an important

role in the chain growth reaction the cleavage of the CO bond however has to be fast

compared to subsequent hydrogenation to the alcohol. For this we have so far no indication.

On the contrary, the often observed lower than expected selectivity for methanol in

comparison to the C2-oxygenates, ethanol, acetaldehyde and acetic acid, indicates that

desorption following CO-insertion might be the preferred reaction pathway. Furthermore, the

absence of significant amounts of ethyl alkyl ketones and e.g. diketones is a further indication

of a reaction pathway of CO insertion followed by a desorption step.

-3

-2

-1

0

1

2

0 5 10 15

Carbon number, NC

log

(100

*ni/

ni)

linear hydrocarbonslinear oxygenatesmethyl alkyl ketones

Figure 4: Anderson-Schulz-Flory distribution of the linear hydrocarbons, linear oxygenates

(n-alcohol, n-aldehyde and linear carboxylic acids), and methyl alkyl ketones formed in the

Fischer-Tropsch synthesis over an an iron-based Fischer-Tropsch catalyst operating at 225oC

(56)

19

Termination rate as oxygenate requires addition of hydrogen to the CO inserted

surface fragment. To form alcohol Cheng et al. (30) find a value of 136 kJ/mol for the overall

hydrogenation activation energy, but for aldehyde formation only a barrier of 35 kJ/mol. Very

few reaction studies have reported the relative rate of formation of alcohols and aldehydes

(see Figure 5). Furthermore, the rapid transformation of aldehydes into alcohols and indeed

the conversion of carboxylic acids to aldehydes and alcohols makes the interpretation of the

data available in literature difficult (57). Hence, a comparison of the calculated barriers with

experimentally observed ratios for verification of proposed reaction pathways is not easy.

However, a very high aldehyde content as expected based on the reported activation barriers

have to our knowledge not been reported.

0

10

20

30

40

0 3 6 9 12

Carbon number, NC

Ald

ehyd

e co

nte

nt

in t

he

frac

tio

n o

f n

-ald

ehye

+ n

-al

coh

ol-

(1),

mo

l-%

Figure 5. Aldehyde content in the fraction of n-alcohol-(1) plus n-aldehyde as a function of

carbon number over an an iron-based Fischer-Tropsch catalyst operating at 225oC (56)

At high CO pressure (58) the surface may reconstruct significantly with formation of

carbonyl type surface species. Such surface intermediates may catalyse olefin

hydroformylation.

One has to conclude that computed data seem to reject the Pichler-Schulz reaction as a

significant route to chain growth.

20

3. Physical chemistry of elementary surface reaction steps

3.1 The activation of CO. Structure dependence and trends as a function of metal

It is now well understood that the activation of CO is highly structure sensitive (16).

Activation of CO has been studied on most of the transition metals. Of especial relevance to

us are the computational data available for Co (8) and Ru (7), the metals that are active in the

Fischer-Tropsch reaction in the metallic state. These data can be compared with available data

on Rh (58), selective in alcohol production and Ni (27), that is a methanation catalyst.

Fe is also an important Fischer-Tropsch catalyst, but in the active state it is present as

a carbide (59) with unique chemistry, that we will not discuss. CO dissociation has been

studied computationally on Fe surfaces by Niemantsverdriet et al. (60). Lo et al. studied the

growth reaction on Fe surfaces (61). Activation and adsorption energies for CO dissociation

on the different metal surfaces are compared in Tables 1 for Fe and group VIII metals.

Table 1a Activation energies of CO dissociation at low coverage with respect to the adsorbed

state and the dense (111) type surfaces.

Estimated TS for CO dissociation * from the Brønsted-Polanyi**formula (in kJ/mol

Fe***

166 (27)

Co

251

Ni

355

Cu

517

Ru

227

Rh

315

Pd

424

Ag

592

Re

122

Os

227

Ir

336

Pt

419

Au

581

* based on 2x2Ru(0001)

** ΔTS=0.85x(ΔP-ΔR)

*** fcc and hcp (not bcc)

21

Table 1b. Adsorption energies of CO

DFT calculated adsorption energy of molecular CO (top and hcp) on selected metals in kJ/mol

Fe*

-169[-115]

(-179)[(-175)]

Co

-171

(-176)

Ni

-151

(-183)

Cu

-68

(-85)

Ru

-177

(-176)

Rh

-187

(-193)

Pd

-130

(-189)

Ag

-19

(-18)

Re

-187

(-154)

Os

-186

(-161)

Ir

-199

(-162)

Pt**

-154

(-168)

Au

-28

(-29)

* hcp and fcc, not bcc,

** relativistic problems

One notes the large differences in activation energies as a function of metal, compared

to the significant smaller energy changes of CO adsorption. It is also noteworthy that the

activation energies of CO activation are substantially higher than required for Fischer-Tropsch

chain growth.

22

Figure 6. A comparison of the energies of CO dissociaton on a stepped and non-stepped Ru

suface (7)

Figure 6 illustrates the dramatic decrease in the activation energies when the non

reactive surface terrace of a non corrugated surface is compared with stepped or more open

surfaces. Now on Ru or Co the activation erngy of CO dissociation becomes competitive with

the activation energy of chain growth termination. Differences in activation energies are large

compared to the changes in the adsorption energies of CO.

The most reactive centers for CO activation have the structure of a surface step. The

carbon atom prefers a site with four metal atoms in the plane and one metal atom below the

site. The O atom has to connect at least to two surface metal atoms, typically located at the

step edge. In the transition state there should be no sharing of substrate atom bonds with the

same surface metal atom (62). Finally that site is also the most reactive for which the surface

atoms have the lowest coordinative saturation (63). A surface site should not be too unstable

so that it reconstructs upon dissociation. This gives an extra cost of reaction (8). The lowest

computed activation energy for CO dissociation is found for the Ru( 1211 ) surface. For this

surface a value of 60 kJ/mol is reported (62). The activation energies for a comparable site on

Co is higher by 40 kJ/mol.

(100) stepped surfaces are extremely active in CO dissociation because the

coordination of C to (100) step sites is extraordinarily strong. Strong coordination of C to

such sites was shown to cause surface reconstruction of the Co(111) surface if small Co f.c.c.

particles (64).

The activation energies of elementary surface reactions that follows the same reaction

path on similar reaction sites are often found to change linearly with varying reaction energy

by changing metal. This is the Brønsted-Evans-Polanyi relation (21a, 63). It can be written as:

reactBforw

act EE (17a)

( 1)reverseact B reactE E (17b)

Because of microscopic reversibility:

forw backreact act actE E E (17c)

For the dissociation barrier of CO αB is typically 0.9. As long as there is no site change

clearly dissociation is very sensitive to change of surface reactivity. The reverse

recombination reaction then is rather independent of such changes.

23

As explained in ref (65) the best correlation with surface fragment energies is found

when the reaction energy of the products is compared with respect to gas phase and reaction

products that do not interact in the final state.

When for one particular metal this barrier is known the activation energies of the same

elementary reaction step on different metals can be deduced from the differences with the

adsorption energies of C and O on these metals.

There is limited information on the activation energy of CO dissociation as a function

of CO coverage. The apparent activation energy of CO dissociation depends strongly on CO

coverage. If sites next to the dissociating CO molecule are occupied the apparent activation

energy increases because a CO molecule has to desorb for the other molecule to dissociate.

Because the dissociating CO molecule generates a C and an O atoms s an additional site is

required.

In a remarkable study of the methanation on Ni Andersson et al. (27) found that on the

double step sites of the on Ni(311) surface there is essentially no suppression of CO

dissociation by coadsorbed CO, because the sites onto which O and C become adsorbed are

not favourable for coadsorbed CO. On a stepped (211) surface they found only an increase in

the activation energy of CO dissociation of 40 kJ/mol due to the presence of coadsorbed CO.

There are several computational studies that indicate that hydrogen addition to CO

will lower the overall activation energy of CO dissociation. On the (0001) terrace of Ru

Ciobîcă et al. (7) demonstrated that activation of CO through a formyl intermediate would

proceed with a barrier of 140 kJ/mol. This is substantially lower than the barrier 210 kJ/mol

found for direct CO dissociation on the same surface.

However on stepped surfaces this reaction was found not to compete with direct CO

dissociation. A similar result as for Ru has also been found for the Co(0001) terrace.

Inderwildi et al. (53) report an activation energy for formyl formation very similar as that

found for the Ru surface. Interestingly the Inderwildi et al. study indicates a significantly

lower barrier for CO cleavage of the CO bond in formaldehyde than that of the CO molecule.

A different suggestion for hydrogen activated CO dissociation has been made in the

earlier mentioned study by Andersson et al. (27) of CO activation by different Ni surfaces.

They propose activation of CO by hydrogen addition to the oxygen atom of adsorbed CO.

Their results are summarized in Table 2

Table 2. A comparison of the activation energies of CO dissociation on different Ni surfaces.

Hydrogen assisted and non assisted activation of CO is compared.

24

One has to distinguish CO bond activation through H addition to C from CO from

bond activation through addition of H to the O atom of CO. Activation of CO through COHads

on Ni proceeds with substantially lower barriers than CO dissociation, even on stepped and

double stepped surfaces. Dissociation of CO via COHads appears to be also rather insensitive

to CO overlayer coverage.

Whereas on Ni (a metal with low reactivity with respect to CO activation) hydrogen

assistance favours CO bon cleavage independent of surfaces site, we have discussed that on a

Ru stepped surface direct CO dissociation provides a lower reaction path. This in contrast to

the finding on the Ru terrace, where hydrogen assisted CO cleavage is again the favoured

pathway.

Insert Figure 7. Shetty Porquerolles slide. A comparison of reaction energy paths for

hydrogen assisted and direct activation on the Ru( 1211 ) surface (66)

25

As is illustrated in Figure 7 on the corrugated Ru( 1211 ) surface, with a low barrier of

CO dissociation, hydrogen activated CO bond cleavage has a substantially higher barrier than

direct CO dissociation. This illustrates that hydrogen assisted CO bond cleavage becomes

only competive when direct CO bond cleavage has a very high CO activation energy.

The need for rapid initiation of the Fischer-Tropsch reaction, implying a low barrier

for CO dissociation and reduced suppression of this barrier when at higher pressures the

surface tends to be covered with CO, can also be the reason that promoting oxidic cations are

beneficial. Such promoting effects have been extensively reviewed (67). The interesting

feature of these promoting systems is that they interact weakly with CO, but have a high

affinity for oxygen. An attractive proposal to understand the role of the promoting cations is

that the reducible cation provides a site for the O generated upon CO dissociation(68),

whereas CO interacts only weakly with the promoting cations. Hence CO dissociation will

not be suppressed at high coverage.

3.2 The CHx-CHy recombination reaction

A series of elegant papers by Cheng et al. (29,30) report on the structure dependence as well

as metal dependence for this class of reactions. Data on CHx-CHy recombination on flat and

stepped surface of Co are reproduced in Table 3.

Table 3 Structure dependence of CHx-CHy bond formation on stepped and non-stepped Co

(0001) surfaces (Eact. (eV)) (30)

C+C C+CH C+CH2 C+CH3 CH+CH CH+CH2 CH+CH3 CH2+CH2 CH2+CH3

flat 1.22 0.91 0.74 0.94 0.86 0.76 1.05 0.70 1.11

step 2.43 1.96 1.34 1.09 1.76 1.32 1.55 0.22 0.73

The structure dependence of different recombination reactions is quite different.

Reaction with a Cads atom is generally preferred on a terrace compared to step. As expected

from Bond Order Conservation (BOC) rules (69) the activation energies decrease with

increasing hydrogen attachment of connecting CHx fragment.

Only reaction between CH2 and CH3 fragments is preferred on steps, all the others are

favoured on the terraces. Noteworthy are the very low values of CH2-CH2 recombination on

the surface step sites. The barrier of recombination of CH2-CH3 on a step competes with the

recombination reactions of C with CH2 and CH with CH2 on the flat surfaces.

26

It is interesting to use BOC theory to understand the variation in activation energy for

different CHx fragments. We will illustrate this for the recombination of two equal CHx

fragments. The BEP relation for this surface reaction can be formulated:

)),(2()( nxEECHCHE CBEP

oactxx

recact (18)

αBEP is the BEP proportionality parameter that is close to one when the transition state is close

to the associated state, or nearly zero in the reverse case (63,65). ),( nxE C is the adsorption

energy of the CHx fragment. n is the number of metal atoms to which the C atom is attached.

Bond order conservation theory can be used to deduce a relationship between the bond energy

of the CHx fragment and that of an adsorbed C atom:

1 1 1 1, (0, 2 2 2

11c

cE x n E nx n nx

(19)

The resulting behaviour is sketched in Figure 8

Dependent on changes in the coordination of CHx fragments a different slope of the

M—C bond energy curve is deduced. The geometries chosen are typically as found for a

metal as Pt (68). The type of dependence as given by equation 12 has been called scaling law

by the Nørskov group (69). They have found such relationships for many systems.

27

Figure 8. The relation between CHx adsorption energy and that of C according to BOC

theory.

It follows from Eq. (18) that the activation energy of the recombination reaction

should decrease with increasing values of x and y as long as reaction paths are comparable.

As can be observed for the activation energies in Table 3 and 5 indeed this kind of

dependence is sometimes found. BEP type relationships can also be proposed when

recombination reactions are compared on the same surfaces but for different metals. An

example is the recombination of CH and CHx on Co and Ru terrace studies by Ge and et al.

(70) given in Table 4.

Table 4. CH and CH2 recombination on Co and Ru (0001) surface (70)

CH+CH2 (Energy kJ/mol)

EC ECH2 ∆ER Eact

Co (0001) 668 377 16.4 81

Ru (0001) 688 412 34 126

The difference in activation energies Eact closely follows the difference in energy of

the CH and CH2 fragments. Since these energies are much higher on Ru the barrier for

reaction on Ru is also higher. The transition state is late with respect to the dissociated state.

Table 5. Comparison of activation energies of CHx-CHy recombination on stepped metal

surfaces. (30)

C+C C+CH C+CH2 C+CH3 CH+CH CH+CH2 CH+CH3 CH2+CH2 CH2+CH3

Co 2.46 1.96 1.36 1.12 1.74 1.34 1.57 0.27 0.76

Rh 2.26 1.66 1.58 1.50 1.44 1.56 1.60 0.86 0.89

Ru 1.80 1.29 1.13 1.28 1.26 1.25 1.62 0.92 1.17

Eact(eV); stepped Co surfaces

In Table 5 also computed data of the Hu group are presented (30). They apply to

recombination on stepped surfaces. The trends as a function of CHx and CHy are as expected

from Eq. (18). A decrease in barrier height is generally found with increasing values of x and

y. As a consequence the difference in barrier energy between the metals also tends to be

smaller for larger values of x and y. But exceptions to these rules are present. For a particular

CHx—CHy recombination sometimes the direction of change of barrier may be completely

28

opposite. It implies that in one case the transition state is late with respect to recombination, in

the other late with respect to dissociation. Also in some case the reaction paths may be

different for the different metals.

Note also that the trend as a function of M—C interaction for CH—CH2

recombination is different for the step (Table 5) compared to the trend found for the terrace

(Table 6). In agreement with the result reported in Table 3, on Co CH—CH2 formation is

found to be preferred on the terrace, for Ru there is no difference in reported values for step

and terrace.

The barrier for C—C bond formation is always found be the highest. C-C bond

formation preferentially occurs on a terrace. This is the elementary reaction step that initiates

deactivating carbonaceous overlayer formation.

Interestingly in the Pettit experiment in which “CH2” was generated from CH2N2

chain growth was only reported when hydrogen is present. This hydrogen is necessary to

convert the adsorbed olefin to alkylidene (Gaube mechanism, see section 4). Similarly, van

Barneveld and Ponec (71) observed that chain growth may occur using CH2Cl2 or CHCl3 in

hydrogen as a feed. Interestingly, feeding CH3Cl in hydrogen did not lead to chain growth

reaction, but to methane formation.

Methanation involves the cleavage of only one M—C bond. Therefore the BEP

relation for C1 to (CH)1 hydrogenation will have a proportionality constant half of that of the

corresponding C—C bond formation reaction.

The selectivity of deactivating C-C bond formation versus chain growth or

methanation will depend on the relative rates of C-C bond formation versus that of CH

formation. Since in the methanation reaction the M—C bond is converted into that in CH4,

but the M—C bond remains partially in tact upon C—C bond formation the relative rate of

methanation should decrease with increasing M-C bond energy (72).

Assuming BEP type relations to be valid, we can make a prediction of the selectivity

of the Fischer-Tropsch reaction as function of M—C bond interaction energy. This behaviour

is sketched in Figure 8. In this figure a schematic representation is given of the relative rate of

production of a particular set of Fischer-Tropsch products as a function of M-C interaction

energies. Four types of reaction are compared: coke formation or carbide formation,

hydrocarbon chain growth, CH4 formation and CO dissociation.

When the rate of CO dissociation is low, little Cads is formed on the surface. Since the

“C1” surface concentration will be small the probability of C—C recombination is low. This

is the situation when the M—C interaction is weak as in Ni.

29

When the M—C bond strength is very weak , initiatiating graphene formation may

become preferred over competing “CH” formation from adsorbed H and C. Therefore the

“C1” coverage is relatively high and hydrogen coverage is low, graphene formation will

dominate over methanation. In the reverse case methanation will be the dominant reaction

When the interaction between adsorbate and surface increases the CO dissociation rate

will increase and hence the “C1” concentration will be increased. Because of the larger

decrease in activation energy of C-C bond formation versus that of CH formation with

increasing M-C interaction energy the relative rate of “CH” formation tends to be enhanced

over that of C—C bond formation.

With increasing M—C bond interaction energy the rate constants for “CHx-CHx”(x

larger than zero) formation may even overcome that of C—C bond formation. Also at

intermediate C—M bond energy CHx—CHy recombination may become favoured over

methanation. This is for instance illustrated by the low values of CHx—CHy recombination as

shown in Table 3, to be compared with the substantially higher values reported for methyl

hydrogenation of Figures 10 (to be discussed in the next sub section 3.3) when referenced to

the most stable CHx intermediate.

When the M—C bond interaction increases further C—C bond formation will become

suppressed and methanation as well as carbide formation is the dominant reaction.

So, the chain growth Fischer-Tropsch reaction is expected to proceed only with a high

value of chain growth parameter α in a M—C bond energy window (Figure 9). The window

boundary parameter values are defined by on the one hand the strong interaction needed for a

low barrier CO dissociation on the other hand and the higher M—C bond energy beyond

which methanation wins form CHx recombination.

30

Figure 9. The Fischer-Tropsch window. A schematic representation of the selectivity of

chaingrowth as a function of the metal-carbon atom adsorption energy

Interestingly when the M—C bond becames as strong as is found for iron, at the

Fischer-Tropsch condition reduced Fe is converted to FexCy. As a consequence the surface

M—C bond is to be decreased. The high chain growth probability of Fe is due to the

weakened M—C bond on the iron carbide surface. In Figure 8 it results in a higher selectivity

of the Fischer-Tropsch reaction in the lower M—C interaction part compared to that of

reduced iron that is in the left part of Figure 8.

The other important conclusion from this section is that at least on Co the chain

growth reaction proceeds at the edge sites of stepped surfaces by recombination of carbene

type intermediates. There is consistent with the Gaube chain growth reaction mechanistic

proposal (72), that we will discuss in section 4.

31

3.3 “C1” hydrogenation

The great difference between methane activation or the reverse reaction of CH3

hydrogenation, in which a σ bond is broken or formed and reactions as CO dissociation in

which π bond formation or cleavage occurs is the different surface atom ensemble

requirement (16). For CH4 activation this is illustrated in Figures 10.

(a) (b)

Figure 10 . The transition states of CH4 activation on Ru(0001) (a) surface (Fig. 10a) and

( 0211 ) surface (Fig 10b), respectively (73,74)

In these figures the computed transition states structures for methane activation are

shown on two surfaces of Ru (73). In the transition state the molecule only contacts with a

single surface atom. Since the reactivity of surface atoms strongly depends on its coordinative

unsaturation, the activation energy strongly decreases when we compare activation on terrace,

versus edge or kink atoms (see Table 6).

Table 6a. Comparison of CH4 activation energies (kJ/mol).

Ru (0001) * 76

Ru (1120) ** 56

Rh (111) *** 67

Rh step *** 32

Rh kink *** 20

Pd (111) *** 66

Pd step *** 38

32

Pd kink *** 41

Pd atom **** 5

* Ciobica et al.(73b);** Ciobica et al (74); *** Liu, Hu (75); ****Diefenbach et al. (76)

Table 6b. Methane activation by a metal atom and metal surface.

strE intE

Pd atom * 216 -221

Rh (111) ** 200 -130

*Diefenbach et al. (76); ** Bunnik, Kramer (77).

In Table 6b one notes the large decreases in the activation energies of CH4 activation

with decreasing coordination number of the surface atoms. For Pd comparison is made to

activation by a single metal atom that is found to have an extremely low activation energy

compared to that of surface atoms.

Table 6b provides a comparison of the strain energies of methane in the transition

state, ΔEstr for the Pd atom and Rh(111) surface. ΔEstr is defined as the deformation energy of

the CH4 molecule in the transition state configuration in the gas phase. The configuration of

CH4 in the transition states of a Pd atom and Rh in the Rh(111) surface are rather similar. This

is reflected by the similar value of ΔEstr. Eint is the interaction energy of methane in the

transition state configuration and the metal. This value is very different for the two systems.

Whereas a Rh atom should more strongly interact than Pd, the order of Eint is reverse. The

large difference in Eint is due to the energy cost of electron localisation of electrons on the Rh

atom embedded in the (111) surface.

As has been elegantly demonstrated by Liu and Hu (75) in contrast to the large

decrease in the activation energy in the forward dissociative direction with increasing

coordinative unsaturation of a surface atom, microscopic reversibility dictates that the reverse

reaction of hydrogenation of CH3 intermediates will be independent of coordinative

unsaturation of the surface atoms. This will also hold for termination of the growing

hydrocarbon chains by hydrogenation. The activation energy of this reaction will also be

33

independent of the reactivity of the surface atom. Hence alkyl chain hydrogenation will be

independent of surface structure.

It is clearly much higher than the activation energy of carbene recombination as

calculated for the step sites of Co, but comparable with the activation energy for this reaction

computed for the stepped Ru surface. Since for chain growth the activation energy of CO

should be smaller than the average value of the termination and chain growth activation

energies it appears that the activation energy for methyl or alkyl hydrogenation are consistent

with the occurrence of the chain growth reaction on both systems.

However because of the high activation energy of CO on terraces sites on a terrace

site chaingrowth will not occur, but this site is candidate for the preference of the methanation

reaction.

Figures 10 show computed activation energies and relative energies of reaction

intermediates for C(1) hydrogenation. Results are shown for two different Ru surfaces.

(a) (b)

Figures 10. Relative energies of CHx species on Ru(0001) and Ru(1120 ) surfaces (73,74)

Most important is to realize the very different relative stabilities of the CHx species on

the two surfaces. This is very relevant since the relative concentration of a particular CHx

species controls to a significant extent the relative rates the CHx—CHy recombination reaction

The energy scheme as shown in Figures 10 assumes the hydrogen atoms to be

adsorbed to the surface. There is no equilibrium with gas phase. If equilibrium with the gas

phase is taken into account, we have to add to each reaction that involves hydrogen the

adsorption energy of a hydrogen atom. This is of the order of 40 kJ/atom.

34

(a) (b)

Figures 11. Relative equilibrium energies of CHx species in equilibrium with gas phase

hydrogen and surface hydrogen on an open and dense surface respectively (schematic).

As is illustrated in Figures 11, whereas CH3,ads is unstable on a surface when gas phase

hydrogen is absent it becomes the most stable intermediate when in equilibrium with gas

phase hydrogen. Equations (20) give the expressions for the equilibrium distribution of the

CHx,ads species.

)31(1

3

1

iK

Kt

i

nH

ieq

nH

ieq

CHi

i

i

(20a)

4

0iCHt i

(20b)

On the surface the CHx species (3≥x≥1) may be expected to rapidly equilibrate.

We have concluded in the previous subsection 3.1, that “CH2” is the preferred

intermediate for chain growth. One notes that only the open surface of Ru, when equilibrated

with gas phase shows a high probability for “CH2” and CH3”, whereas the dense surface then

only gives a high probability for “CH3” intermediate.

“CH” and “CH2” are only found to be stable intermediates only when equilibration

with gas phase hydrogen is ignored. This was the assumption on which the mechanistic work

based “CH” intermediated chain growth (78) was based.

An alternative termination reaction of the growing alkyl chain instead of by

hydrogenation addition is β CH cleavage. Barriers computed by Hu et. al (78) show low

barriers of reaction for the equilibration of adsorbed alkyl and alkene. Barriers are around 50

kJ/mol. On the Co(0001) surface the olefin is slightly more stable than adsorbed alkyl (-10

kJ/mol). For the overall barrier for termination including desorption to give gas phase alkene

35

this gives a value of 70 kJ/mol. No number is available for termination at the surface step, but

alkene may be expected to adsorb more strongly.

It should be noted that the formation of methane by hydrogenation of a surface C1-

species is even in the Fischer-Tropsch synthesis an important, albeit undesired, reaction.

Typical methane selectivity between 30-60 mol-% are obtained in the process. This implies

that the rate of hydrogenation of the C1-species is of the same order as the rate of chain

growth. The Fischer Tropsch process is only economical since the majority of carbon in the

synthesis gas ends up in long chain hydrocarbons, whereas only up to 10% will end up as

methane.

4. The chemistry of the chain growth reaction via “C1” species.

The two most relevant reaction mechanisms that we will compare are the Brady-Pettit

alkyl mechanism (31) and the Gaube-Maitlis alkelydene/alkenyl (32,33) chain growth

mechanism.

According to the Brady-Pettit mechanism (79) shown in Scheme 3 chain growth

occurs through insertion of CH2 in the growing alkyl chain. Termination occurs through H

addition or abstraction. This mechanisms predicts the primary formation of paraffins and

olefins as observed experimentally (80), and may even predict the primary formation of

oxygen containing product compounds via OH termination (55)

Scheme 3

In the Maitlis scheme (see Scheme 4) initial C—C bond formation occurs through CH

and CH2 recombination. This was also proposed by Ciobîcă et al. (78) based on calculations

Brady-Pettit alkyl mechanism (CH3+CH2)

initiation: CH3+CH2 CH3CH2

propagation: R-CH2+CH2 R-CH2-CH2

termination: R-CH2+H R-CH3 (alkane)

R-CH2-CH2-H R-CH=CH2 (alkene)

36

on the Ru(0001) surface. Subsequent steps in the Maitlis scheme (31) occur by insertion of

“alkenyl” into “CH2” species followed by an allyl-vinyl isomerisation step. A hydrogen

transfer reaction is proposed for chain growth. No calculations are available for this reaction.

Maitlis provided several elegant experimental proofs supporting this scheme. However the

intermediate allyl species may be expected to strongly interact with the surface, which would

disfavour hydrogen transfer. Consecutive initiation reactions with growing allylic

intermediates would result in the formation of surface species with more than one double

bonds and in the end to the formation of dienes, a product hardly observed in the Fischer-

Tropsch process. Furthermore, the mechanism does not predict the primary formation of n-

paraffins, which would require an additional route.

Scheme 4.

An alternative to the Maitlis scheme is given by Gaube (32) (Scheme 5). In this

scheme again “CH2” is the key inserting intermediate. The growing chain is an alkelydene

species. Reaction is terminated by desorption of olefin, the competing reaction is hydrogen

transfer to generate the alkenyl intermediate to continue chaingrowth. Also this reaction has

so far not been studied computationally.

Maitlis alkenyl mechanism (CH=CH2+CH2)

initiation: CH+CH2 CH=CH2

CH=CH2+CH2 CH2-CH=CH2

CH2-CH=CH2 CH3-CH=CH

propagation: R-CH=CH+CH2 R-CH-CH=CH2 (C-C coupling)

R-CH-CH=CH2 R-CH2-CH=CH (H transfer)

termination: R-CH-CH=CH2+H R-CH2-CH=CH2 (1-alkene)

R-CH-CH=CH2+H R-CH=CH-CH3 (2-alkene)

R-CH2-CH=CH+H R-CH2-CH=CH2 (1-alkene)

37

Scheme 5

All schemes depend on a relative high CH2 concentration on the metal surface. Indeed

an acceleration in the rate of the Fischer-Tropsch synthesis is observed if CH2Cl2 is co-fed

with synthesis gas over Fischer-Tropsch catalysts (71). We will return later to this issue, but

will first present the corresponding expressions for the chain growth parameter α. Equation

(21a) gives the expression of α according to the Maitlis mechanism:

2

2

( )

( ) ( )CC CH

mCC CH eq des

k RCH

k RCH K H shift k

(21a)

and equation 14b the corresponding expression for the Gaube mechanism:

2

2

( )

( ) ( )CC CH

gCC CH eq des

k RCH

k RCH K H shift k

(21b)

On cobalt in the Maitlis mechanism the termination reaction produces dominantly the

olefin. In the Gaube reaction termination also generates olefin. In both cases these termination

steps compete with the hydrogen bond shift reaction. It implies that the apparent activation

energy for termination equals the activation energy of olefin desorption added with the

hydrogen bond shift equilibrium energy.

For Eq. (21) to be valid hyddrogen atom transfer within the surface intermediates in

the chain growth reaction. Calculations by Ciobia et al. (78) indicate that the maximum

activation energies for these elementary reaction steps are less than 50 kJ/mol

Not only do we need sites that have a low barrier for CO dissociation and sites that

have a low barrier for recombination, we also need sites that contain the inserting “C1”

species as a major reaction intermediate (MARI). Otherwise additional penalties for the

Gaube alkelydene mechanism (CH2+CH2)

initiation: CH2+CH2 CH2=CH2

CH2=CH2 CH-CH3

propagation: R-CH+CH2 R-CH=CH2 (C-C coupling)

R-CH=CH2 R-CH2-CH (H transfer)

termination: R-CH=CH2 R-CH=CH2 (gas) (desorption)

38

apparent activation energy of the chain growth reaction are generated that increase its

activation energy. On Ru the “CH2” intermediate this requires surfaces as the Ru( 1211 )

surface. The stepped Co surface as studied by Cheng et al. (29,30) disfavours CH2 as an

intermediate, hence this surface site is most likely not the most selective surface site.

A consequence of this analysis is that even when CO dissociation is favoured still low

values of α can occur if not the right site is available that stabilizes the “CH2” intermediate.

In practice some of the reaction steps identified as part of one of the reaction

mechanisms in which CH2 is the key growth intermediate C(1) species may occur in parallel

affecting for instance alkane versus alkene selectivity. For instance hydrogen atom addition to

the olefinic intermediates may lead to alkyl species that can react in consecutive steps with

CH2 or by addition of a second hydrogen atom become converted to alkane.

The Dry mechanistic proposal (33) that we mentioned earlier to explain branching can be

considered a modification of Maitlis as well as Gaube proposal.

39

Chain growth reaction steps

C=C

HH

R HM=CH2

C=C

HH

H RM=CH2

I. RCH=CH2+"CH2"

metallocycle formation

C=C

HH

R HM=CH2

C=C

HH

H RM=CH2

R-CH-CH=CH2

R-H2C=C-R

CH3

II.

(a)

(b)

(a)

(b)

olefin formation

branched olefin formation

Scheme 6. Dry branching mechanism via metallocycle

In the Maitlis intermediate the hydrogen atom has to move to create a reactive secondary

atom, in the Gaube mechanism CH2 is also proposed to insert to the secondary carbon atom.

Dry proposed metal cycle type intermediates as chain propagating species.

5. Deactivation

Whereas it is not our aim in this paper to analyze deactivation mechanisms of the

Fischer-Tropsch reaction in depth, we can comment on two ways of catalyst deactivation:

a. Sites can become blocked due to coke formation

b. Reactive sites can reconstruct to a non-reactive phase

40

The computational results summarized here indicate strongly that initial C—C bond

formation occurs preferably on surface terraces. Additional “C” attachment reactions to form

graphene are thermodynamically favored (81). Whereas the edge atoms of graphene may

become most stable at surface edges (82) the carbon growth mechanism on terraces is

consistent with continued reactivity of surface step edges under conditions where coke

formation occurs.

The step edges are also regenerated during reaction. The hydrogenation of the C atoms

generated upon CO dissociation increases their mobility and conversion into CH2 species that

incorporate into the growing chains at the step edges.

The graphene overlayer growth coke formation route is quite different from graphite

formation by Ni. Helveg et al. (83) argue that graphite formation on Ni occurs through carbon

detachment reaction at step edges.

Surface reconstruction is driven by stabilisation of adsorbed carbon when attached to

more reactive surface atoms. Ciobîcă et al. (64) have demonstrated that this leads to the the

Co(111) to Co(100) reconstruction on f.c.c. Co (the stable phase of small Co particles) when

an overlayer of Cads is present. Interestingly because of the change in metal atom density in

the surface layer this may cause faceting and hence create step-edge sites, which are sites that

have high reactivity in the Fischer-Tropsch reaction (10). Hence surface reconstruction by

stable carbide overlayer formation may actually result in initial activation of the catalyst, a

phenomena that has been extensively described as self organization by Schulz (84).

6. A summary of the Fischer-Tropsch mechanism discussion

In this paper we have discussed important aspects of the molecular basis of the Fischer-

Tropsch reaction. The discussion limits itself to the initial elementary reaction steps of the

reaction. Many important additional aspects have been ignored. Kinetically the incorporation

of olefins by readsorption on the catalyst is important (85,86). Also the formation of waxes

changes the environment and surface physics in an essential way (86,87) and introduces

important mass transfer effects. Additionally the reactor type used and size and shape of

catalyst particles (86,88) will influence the overall catalytic behaviour in aan essential way.

Implicit in these studies are assumptions on the molecular details of the individual

elementary reaction steps that we address here. The kinetic analysis presented employed a

41

simplified molecular kinetic scheme with the aim to make explicit the relation between

different elementary reaction rate constants that determines selectivity differences.

Figures 2 and 3 summarise the main molecular catalytic features that determine the

chain growth selectivity of the Fischer-Tropsch reaction. One of the important results of the

computational catalysis studies is that we can exclude all mechanistic proposals that involve

chaingrowth through successive insertions of CO and subsequent C-O bondcleavage. These

mechanistic steps that also recently have been reviewed (89).

The now generally accepted mechanism proceeds through the incorporation of C1

species generated at the catalytic center after initial CO bond cleavage of adsorbed CO.

Hydrogen atom assistance of this reaction at Fischer-Tropsch conditions is probably not

required.

In case methanation is the desired reaction, higher hydrocarbon forlmation as well as

alcohol formation has to be suppressed. Then chaingrowth has to be slow and the best catalyst

would have CO dissociation as rate limiting step. It implies a high CO coverage, C1 formation

as rate limiting and rapid hydrogenation of surface carbon. The reaction order in CO will be

close to −1. The preferred metal appears to be Ni, with its weak M-C bond and high

dissociation barrier for CO activation.

It is often proposed that C1 formation is also the rate limiting step of the Fischer-

Tropsch reaction. This is only consistent with chain growth through CO insertion, which we

believe to be unlikely.

Within the Biloen-Sachler scheme termination should be rate limiting step and hence

CO dissociation fast. A relative fast CO dissociation relative to chain termination will lead to

the formation of long chains, whereas a slow rate of CO dissociation relative to chain

termination will lead to the formation of short chain products, e.g. methane. In the Fischer-

Tropsch reaction the order in CO should be near to zero.

The inhomogeneous distribution of surface sites of the Fischer-Tropsch catalyst

implies that overall kinetics maybe very different. The rates of the different elementary

reaction steps will differ.

Since CO activation is only fast at B5 sites as proposed by van Hardeveld, it implies

that the chain growth reaction will only occur when step-edge type sites are present that cause

CO to dissociate with a low barrier. Terrace surfaces with a high barrier of CO dissociation

will predominantly give methane.

The Gaube chain growth mechanism is in best agreement with results of calculations

that suggest carbene type intermediate recombination as lowest activation energy chaingrowth

42

steps. In accordance with experiment on Co that alkene will be the dominant product and

“CH2” intermediates to be the dominant C1 species that is incorporated in the growing chain.

Sites that stabilise “CH2” hence have to be joined with sites that activate CO with a low

barrier.

Most likely some of the mechanisms we discussed may be operational in parallel. It is

experimentally observed that the primary olefin/paraffin rate seems to be in the range between

2 and 4. For instance, the primary olefin/paraffin ratio seems to be ca. 4 on an iron-manganese

catalysts operating at 550K (90), on a precipitated cobalt catalyst at 463K (91) and on a

Ru/SiO2 catalyst operating at 483K (92) seems to be ca. 4 (Schulz), for a ruthenium-based

catalyst iron-based catalyst. Whereas the Gaube mechanism would predict alkene as the

primary product , alkane production would occur through parallel intermediate steps as in the

Pettit mechanism.

Once it becomes understood that CO dissiociation plays a critical role in the Fischer-

Tropsch reaction and one accepts the Biloen-Sachtler mechanistic proposal, one can

appreciate why particular metals are the preferred catalysts for this reaction.

As can be deduced from Table 1a Fe, Co and Ru are unique because they have the

lower CO activation enrgies of the group VIII metals. We already mentioned that the high

reactivity of Fe leads to the formation of bulk carbides during the actual Fischer-Tropsch

synthesis. The selectivity of Rh towards lower oxygenates is due to the relatively slow rate of

dissociation of CO, that results in a high probability for CO to terminate in the growing alkyl

or alkenyl chain. An interesting recent Dynamic Monte Carlo study (93) demonstrates this

explicitly by a comparison of Ru and Rh as catalysts. The selectivity to for higher oxygenates

on Ru is low because of the significantly faster rate of CO dissociation.

Since CO activation is a highly structure sensitive proces, experiments as reported by

Bezemer et al. (12) that show a high structure sensitivity agree with this proposition. This

structure becomes explicitly apparent for very small particles, of the order of a few nm, that

cannot support the relevant B-5 type sites. Then the rate of chain growth is observed to

decrease significantly. In this context the particle size independence reported for larger

particles (85) has to imply that on this partcles the B5 site concentration is constant and CO

dissociation is fast.

Whereas C-C bond formation to form graphene has the lower barrier on the terraces,

CHx-CzHy bond formation tends to favour the rim of the step edges. The reduction in CO

coverage observed for the working Co catalyst (94) is in line with this. The increasing

coverage with different kinds of oligomerized CnHy species (graphene,growing chains, C1

43

species. will decrease the coverage with CO. CO dissociation is fast at the steps and chain

termination is the slow step in the process.

Interestingly the relative rate of chain growth and chain termination has a very

different dependence on the C-atom interaction energy with the surface than CO dissociation.

When the metal-carbon interaction is weak C-C bond formation and graphene formation will

be the dominant reaction. With increasing metal-carbon interaction partially hydrogenated

CHx tends to become stabilized with respect to the methanation reaction and chain growth

becomes more favoured.

These trends are consistent with observations made on chain growth of surface carbon

depositied by methane decomposition. In a row of the periodic system the selectivity of

hydrocarbon formation increases from right to left e.g. Pd shows lower selectivity and Ru the

higher (95). Metals as Pt and Ir have the higher selectivity for chain growth initiated from

“C1” species because of their relatively high M—C bond energies . These metals are

unsuitable Fischer-Tropsch catalysts because the dissociation of CO is too slow.

The optimum site of the Fischer-Tropsch catalyst has a relatively high surface

concentration of “CH2” species. The rate of “CH2” hydrogenation has to be slower than the

rate of “CH2” insertion in the growing chain and it has to be rapidly formed from adsorbed

CO.

Since CO dissociation happens at step edges and the site of C generation (at the

bottom of the step) and chain growth(at the rim of the step) is different, diffusion of partially

hydrogenated C1 species over the step edge has to be fast. Also the rate of Oads removal has to

be a relatively fast process. Otherwise it will block sites of CO dissociation. In case Oads

removal by hydrogen is more slow the CO shift reaction will start to compete. This is known

to occur for the Fe catalyst.

Also non-selctive graphene formation has to be suppressed. For these one needs access

to sites that activate hydrogen. Step-edges suppress graphene formation because of their high

interaction anergies with adsorbed C. Graphene formation as well as methanation are

preferred at surface terraces.

Termination as oxygenate is a slow surface reaction by recombination of CO with

growing alkyl chains, but the insertion into surface carbene type chains may be expected to

have a relatively low barrier. It tends to compete with olefin and alkane formation.

44

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