comprehensive inorganic chemistry ii || fischer–tropsch synthesis: catalysts and chemistry

Co

7.20 Fischer–Tropsch Synthesis: Catalysts and ChemistryJ van de Loosdrecht, Sasol Technology Pty (Ltd), Sasolburg, South Africa; Eindhoven University of Technology, Eindhoven,The NetherlandsFG Botes, Sasol Technology Pty (Ltd), Sasolburg, South AfricaIM Ciobica, Eindhoven University of Technology, Eindhoven, The NetherlandsA Ferreira, P Gibson, DJ Moodley, AM Saib, and JL Visagie, Sasol Technology Pty (Ltd), Sasolburg, South AfricaCJ Weststrate and JW (Hans) Niemantsverdriet, Eindhoven University of Technology, Eindhoven, The Netherlands

ã 2013 Elsevier Ltd. All rights reserved.

7.20.1 Introduction: Processes, Catalysts, and Recent History 5267.20.1.1 ‘Anything’-to-liquids Technology: Syngas Production, FTS, and Product Workup 5267.20.1.2 FTS, the Product Distribution 5267.20.1.3 Fischer–Tropsch Catalysts and Modes of Operation 5277.20.1.4 Historical Development of the FTS 5297.20.2 Iron-Based FTS Catalysts 5317.20.2.1 Introduction 5317.20.2.2 Commercial Applications 5317.20.2.3 Iron Fischer–Tropsch Catalyst Preparation 5317.20.2.3.1 Fusion 5327.20.2.3.2 Precipitation 5337.20.2.3.3 Improving iron Fischer–Tropsch catalyst precursors by promoters 5347.20.2.3.4 Activation and reduction procedures 5347.20.2.4 Selectivity Manipulation of Iron Catalysts 5357.20.2.5 Catalyst Stability During FTS 5357.20.2.6 Spent Catalyst Management 5367.20.3 Cobalt-Based FTS Catalysts 5377.20.3.1 Introduction 5377.20.3.2 Composition of Cobalt Catalysts 5377.20.3.3 Preparation of Cobalt Fischer–Tropsch Catalysts 5397.20.3.3.1 Precipitation 5397.20.3.3.2 Preparation methods involving pre-shaped supports 5407.20.3.3.3 Calcination 5417.20.3.3.4 Reduction 5417.20.3.4 Cobalt Catalyst Fischer–Tropsch Performance 5417.20.3.5 Deactivation and Regeneration of Cobalt Fischer–Tropsch Catalysts 5447.20.4 Mechanisms and Kinetics of FTS Over Iron and Cobalt Catalysts 5467.20.4.1 Introduction 5467.20.4.2 Surface Science Studies and Model Reactions 5477.20.4.2.1 Adsorption of CO and hydrogen on model surfaces 5477.20.4.2.2 C–O bond scission 5487.20.4.2.3 Hydrogenation and the stability of C1Hx species 5497.20.4.3 DFT Modeling 5497.20.4.3.1 CO Dissociation 5497.20.4.3.2 CþH reactions 5507.20.4.3.3 Chain growth 5517.20.4.4 Macrokinetic Observations and Models 5517.20.4.4.1 General observations regarding kinetics 5517.20.4.4.2 Simple macrokinetic models 5527.20.4.4.3 Selectivity modeling 5527.20.4.5 Mechanistic and Kinetic Implications 5537.20.5 Conclusion 554References 554

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-097774-4.00729-4 525

http://dx.doi.org/10.1016/B978-0-08-097774-4.00729-4

526 Fischer–Tropsch Synthesis: Catalysts and Chemistry

7.20.1 Introduction: Processes, Catalysts,and Recent History

The Fischer–Tropsch synthesis (FTS) represents technology

from the 1920s1,2 that has continuously been revived to pro-

vide synthetic hydrocarbon fuels and chemicals from initially

coal, later natural gas, and nowadays also biomass. Virtually

any source of (hydro)carbon feedstock can be converted to a

mixture of synthesis gas, or syngas (CO and H2), which is in

fact a key intermediate on which theoretically the entire chem-

ical industry could be based. FTS stands for the reaction(s) of

synthesis gas to predominantly straight-chain hydrocarbons,

which can be paraffins from CH4 to waxes (CnH2nþ2 with n

from 1 to over 100), olefins from ethylene to much longer

molecules (CnH2n, with n�2), and to a lesser extent oxygen-

ated products such as alcohols. It produces as main by-

products water and/or carbon dioxide, that is, due to the

water-gas shift (WGS) reaction. Being a highly exothermic

reaction, it generates large amounts of heat. The process is

represented by the simplified reaction equations

FTS : COþ 2H2 ! �CH2 �þH2O � 165kJmol�1 [1]

WGS : COþH2O⇄H2 þ CO2 � 42kJmol�1 [2]

Reaction [1] represents in essence a polymerization, imply-

ing that the product will be a mixture of hydrocarbons with

a distribution in molecular weights. Selectivity and control

thereof are therefore of key importance in FTS technology.

Fischer–Tropsch technology represents a subject of inten-

sive research both in industry and in academia. Many excellent

reviews are available.3–9

In this chapter, we first describe the general aspects of the

technology in which the FTS features, then the more chemical

aspects of the process in relation to the iron and cobalt catalysts

that are used in practical applications, and finally mechanistic

insight, on the basis of kinetics, surface science, and computa-

tional modeling.

7.20.1.1 ‘Anything’-to-liquids Technology: SyngasProduction, FTS, and Product Workup

The overall process from original carbon source for the syngas

to the FTS product is named after the feedstock employed,

hence the terminology ‘coal-to-liquids’ (CTL), ‘gas-to-liquids’

(GTL) and ‘biomass-to-liquids’ (BTL), collectively known as

XTL (‘anything’-to-liquids).

In all instances, the carbon source is first converted to

synthesis gas (or ‘syngas’ for short), which is a mixture of CO

and H2. Solid feedstocks such as coal or biomass are gasified,

usually noncatalytically, by partial oxidation with oxygen (sup-

plying the heat for the endothermic gasification reactions) and

reaction with steam (which acts as a gasification agent, hydro-

gen source, and coolant).

When the starting material is natural gas, it can also be

adiabatically reformed in the presence of oxygen and steam.

There are different embodiments of this approach, such as

autothermal reforming (ATR), noncatalytic partial oxidation

(POX), and catalytic partial oxidation (CPOX), but in essence

the chemistry of all is the same and very similar to that of coal

gasification. Alternatively, heat can be supplied externally, in

which case the gas is reformed only with steam and/or CO2,

but no oxygen is added. Examples of this approach include

steam reforming (where mainly steam is added), dry reforming

(where mainly CO2 is added), and heat exchange reforming

(where process heat is supplied to the reformer tubes). Typical

reforming catalysts are based on nickel as the active metal.10

The second step in the XTL process is to catalytically convert

the syngas to a range of hydrocarbons via the FT synthesis,

which mainly yields linear alkanes and 1-alkenes, and which

will be the main subject of this chapter hereafter.

The third and last step is usually the workup of the

hydrocarbons to final products, which are typically fuels, but

optionally also chemicals. A popular application at present is

to target the production of long chain waxes in the FT synthe-

sis, followed by hydrocracking to middle distillate range

components, such as diesel (C9–C22) and jet fuel (C9–C15).

Hydrocracking catalysts are bifunctional in nature, with either

a noble metal (e.g., Pt) or sulfided base metals (e.g., Ni/W or

Co/Mo) as the hydrogenation function on a catalytically active

acidic support, such as a silica–alumina. We refer to the liter-

ature for further information on this subject.11,12

7.20.1.2 FTS, the Product Distribution

At the chemistry level, the FT synthesis is both a CO hydroge-

nation reaction and a polymerization reaction. The former is

reflected by the fact that the C–O bond must be broken and

new C–H bonds formed. Additionally, C–C bonds must be

formed in order to effect hydrocarbon chain growth. Since the

product carbon number distribution approximately follows a

statistical function called the Anderson–Schulz–Flory relation-

ship, it is widely accepted that chain growth occurs one carbon

atom at a time via a polymerization mechanism. Proposals for

the monomer of chain growth, which is produced in situ, have

included adsorbed CO, an enol species and a CHx species,7,13–15

and will be discussed further in the section on mechanism and

kinetics.

The competition between chain growth (yielding a surface

intermediate with one higher carbon number) and chain ter-

mination (yielding a desorbed final product) is determined by

the probability for growth, called the a-value. A higher a-valuewill result in longer hydrocarbons and thus a heavier product

spectrum (Figure 1). If a is independent of carbon number, the

scheme presented in Figure 2 applies and the total amount of

carbon contained in products with n carbon atoms (namely

Cn) can be formulated on a relative basis:

C1 ¼ 1 1� að ÞC2 ¼ 2 1� að ÞaC3 ¼ 3 1� að Þa2

Cn ¼ n 1� að Þan�1

The total amount of carbon in the product spectrum then

forms a convergent infinite sum with an analytical solution:

X11

Cn ¼X11

n 1� að Þan�1 ¼ 1

1� a

C* → C1* → C2* → C3* → …..Intermediates

Products

C1 C2 C3

a a a

1-a 1-a 1-a

Figure 2 Carbon chain growth and termination scheme for thederivation of the Anderson–Schulz–Flory equation, with a the chaingrowth probability factor, Cn (n¼1, 2, 3, . . .) the final products with ncarbon atoms, and Cn* the intermediates with n carbon atoms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Chain-growth probability, a

Car

bon

ato

m s

elec

tivity

(%)

100

90

80

70

60

50

40

30

20

10

0

CH4 C5+

C2–C4 C5–C11gasoline

C20+waxes

C9–C22diesel-

distillates

HTFT LTFT

Figure 1 Hydrocarbon product spectrum that is produced during Fischer–Tropsch synthesis for varying chain growth probability a. High-temperatureFischer–Tropsch technology (HTFT) corresponds approximately to 0.70<a<0.75, and low-temperature Fischer–Tropsch (LTFT) to about0.85<a<0.95.

Fischer–Tropsch Synthesis: Catalysts and Chemistry 527

This means that the selectivity toward products with n

carbon atoms on a carbon atom basis, namely Sn, can be

expressed as follows:

Sn ¼ CnX11Cn

¼ n 1� að Þ2an�1

After converting this equation to the logarithmic domain

and rearranging, it is found that

lnSnn

� �¼ n ln aþ ln

1� að Þ2a

[3]

As a result, a plot of ln(Sn/n) versus carbon number (n) gives

rise to a straight line with a slope equal to ln(a). However,

deviations in the actual FT product spectrum from the ideal

Anderson–Schulz–Florydistributionareusually observed.4,14,16,17

These include a higher methane and a lower C2 selectivity than

predicted by the equation. There is also an increase in the chain

growth probability factor and concomitant decrease in the

olefin/paraffin ratio with hydrocarbon chain length. In addi-

tion to linear alkanes and 1-alkenes, a variety of other products

are also formed, including branched aliphatic compounds,

alcohols, aldehydes, ketones, acids, and (at sufficiently high

operating temperatures) even aromatics. This alludes to the

complexity of the reaction and many unresolved issues remain

regarding the reaction mechanism. A further complicating

factor is that it is not always clear which of these compounds

are primary FT products and which are formed subsequently by

secondary reactions. For example, olefins and alcohols can

undergo a range of secondary reactions, such as hydrogenation,

double bond isomerization, skeletal isomerization, and con-

version to heavier compounds.4,16

7.20.1.3 Fischer–Tropsch Catalysts and Modes of Operation

Metals known to catalyze the FT reaction mainly include iron,

cobalt, ruthenium, and nickel.6 Ruthenium is a scarce and

expensive metal, whereas nickel only forms methane at reac-

tion temperatures sufficiently high to suppress nickel carbonyl

formation (note that methanation is the reverse reaction of

methane reforming, for which nickel-based catalysts are com-

monly used10). As a result, only iron- and cobalt-based FT

catalysts have found commercial application.18–20 Iron gener-

ally produces more olefins and oxygenates than cobalt (i.e., a

less hydrogenated product spectrum), which may be related to

the lower hydrogenating ability of iron. While cobalt is active

in the metallic state,19 iron catalysts change under Fischer–

Tropsch conditions to a complex mixture of iron carbides

and oxides.20,21

Byproducts of the FTS originate from the way oxygen from

CO is removed. With cobalt catalysts, essentially all oxygen

from CO dissociation (typically around 99%) is discarded as

water. Iron catalysts differ in this respect, as a significant por-

tion of the oxygen is also discarded as CO2. The latter is often

visualized as a separate, consecutive reaction, namely theWGS.

Stoichiometrically, the overall process can be represented by

reactions [1] and [2], which we repeat here:

FTS : COþ 2H2 ! �CH2 �þH2O � 165kJmol�1 [1]

WGS : COþH2O⇄H2 þ CO2 � 42kJmol�1 [2]

The net rate of hydrogen conversion divided by CO conver-

sion (sometimes referred to as the ‘usage ratio’) is extremely

important for the gas loop design around an FT reactor. In the

one extreme, where virtually no WGS takes place, the usage


ratio is only determined by the FT reaction (with a high selec-

tivity to long hydrocarbons and a low selectivity to methane)

and assumes a value of around 2. In the other extreme, where

almost all water is shifted to CO2, the usage ratio can approach

a value of 0.5. The low propensity of cobalt catalysts for the

WGS makes them the preferred catalysts for GTL application,

since the H2/CO ratio of syngas derived from natural gas is

already close to or above the usage ratio. Any additional WGS

will result in an excess of hydrogen that would not be fully

consumed by the FT reaction, even if CO is converted to

extinction. Conversely, the WGS is more facile over iron

Table 1 Fischer–Tropsch synthesis, current commercial plants and plant

Company Location Carbon feedstock Catalyst

Sasol Sasolburg, SouthAfrica

Initially coal, currentlynatural gas

Fused Fe

Precipita

Precipita(spray

Sasol Secunda, SouthAfrica

Mostly coal, nowsupplemented bynatural gas

Fused Fe

Shell Bintulu, Malaysia Natural gas Co/SiO2

Co/TiOPetroSA Mosselbay, South

AfricaNatural gas Fused Fe

Sasol-QP (Oryx) Ras Laffan, Qatar Natural gas Co/Al2O3

Shell (Pearl) Ras Laffan, Qatar Natural gas Co/TiO2

Chevron-Sasol Escravos, Nigeria Natural gas Co/Al2O3

aSAS: Sasol Advanced Synthol, fixed fluidized bed.

Moving bed1–200 μm particles

LTFT, 200–250 ∞C• 3-phase system: gas–liquid–solid• a = 0.85–0.95• Products: wax, diesel, naphta• Catalysts: supported cobalt or precipitated iron

HTFT, 320–350 ∞C• 2-phase:gas–solid• a = 0.70–0.75• Products: petrol and chemicals• Catalysts: fused iron, K-promoted

Circulatingfluid bed

7000 barrels per day 20 0

Slurry bubbcolumn,

filled with w24 000 barr

per day

Figure 3 Overview of Fischer–Tropsch technology with reactors (figure mic

catalysts, yielding amuch lower overall usage ratio that is better

suited to syngas feeds with a low H2/CO ratio, such as that

generally obtained from coal gasifiers.

Table 1 presents the current commercial application of the

FTS. There are two important aspects to note from this table.

First, the worldwide FTS capacity is expected to reach a total of

just over 400000 barrels per day by 2013 (1 barrel¼159 l),

which is very small compared to the total crude oil production

of around 80–85 million barrels per day. Second, the FTS has

been applied in a variety of forms, which determines the type

of reactor employed. As indicated in Figure 3, these reactors

s under construction

type Reactor type Start-up date Approximate plantcapacity (barrelsper day)

/K HTFT circulatingfluidized bed

1955 to�1985

ted Fe/K LTFT multitubularfixed bed

19555000

ted Fe/Kdried)

LTFT slurry phase 1993

/K HTFT circulatingfluidized bed

HTFT SAS reactora

1980–1999

1995

160000

2

LTFT multitubularfixed bed

1992 14500

/K HTFT circulatingfluidized bed(Sasol technology)

1993 22000

LTFT slurry phase 2007 34000LTFT multitubularfixed bed

2011 140000

LTFT slurry phase 2013 34000

Stationary bed reactors200–5000 μm particles

High heat flux10 times higher heat fluxthan conventional reactors

Fixed fluid bed00 barrels per day

le

axels

Multitubularfixed bed

6000 barrelsper day

Microchannel reactor~ 200–1000 b/d (for assembly)

Microchannel processtechnology module

Boiling heat transfer

FT

rochannel reactor: courtesy of the Oxford Catalysts Group).


can broadly be classified in two classes, namely two-phase or

three-phase reactors, and moving or stationary catalyst bed

reactors.22–24

The high-temperature Fischer–Tropsch (HTFT) synthesis

process is characterized by operating temperatures of about

320–350 �C and the products are essentially only in the gas

phase under reaction conditions, giving rise to a gas–solid

system without any bulk liquid phase. Originally, this process

was operated in circulating fluidized bed reactors and more

recently in fixed fluidized bed reactors. Cobalt catalysts would

essentially only produce methane at these temperatures, mak-

ing alkali-promoted iron catalysts the only option for this

application. Due to the mechanical demand that these moving

bed reactors place on the catalyst, particle strength is an impor-

tant consideration; consequently, only fused bulk iron catalysts

have been employed commercially. The light product spectrum

is best suited to the production of gasoline, but the high

selectivity toward linear 1-olefins and (to a lesser extent) oxy-

genates allows for the extraction of chemicals from the product

slate. These include monomers such as ethylene and propyl-

ene, co-monomers such as 1-hexene and 1-octene, and sol-

vents (e.g., propanol, butanol, methyl ethyl ketone (MEK),

and acetaldehyde).25

The low-temperature Fischer–Tropsch (LTFT) synthesis is

operated between 200 and 250 �C.26,27 Both cobalt and iron

catalysts are suitable for this application, although cobalt cat-

alysts would typically be used toward the lower half of the

quoted temperature range. The heavy product spectrum

extends well into the domain of waxes, which are liquid

under reaction conditions. The presence of a bulk liquid phase

gives rise to a three-phase gas–liquid–solid system. Originally,

only fixed-bed reactors operating in a trickle bed mode were

employed for this synthesis. In order to limit the pressure

drop over the stationary catalyst bed, catalyst particle sizes

must be in the millimeter range, which brings about signifi-

cant intra-particle diffusion limitations. This not only limits

catalyst utilization, but also adversely affects product selectiv-

ities due to the differences in diffusion rates between hydro-

gen and CO that causes higher H2/CO ratios toward the

center of the particles. The highly exothermic nature of

the FT reaction causes axial and radial temperature profiles

in the catalyst bed.

More recently, slurry bubble-column reactors have been

developed to overcome some of these drawbacks. Syngas is

bubbled through a suspension of fine catalyst particles in the

liquid product phase. The catalyst particle sizes are usually less

than about 100 mm, which is sufficiently small to prevent intra-

particle diffusion limitations, while the well-mixed liquid phase

ensures virtual isothermal operation of the reactor. There are,

however, certain technical challenges associated with FTS slurry

reactors. A prerequisite of a slurry process is the development of

an efficient solid–liquid separation step to remove product wax

from the reactor. It is extremely important to ensure the

mechanical integrity of the catalyst to limit the extent of break-

up and attrition in the moving bed environment.

Of late there have been some new reactor developments for

FTS application, but none of these have been commercially

applied yet. Microchannel reactors can support very high heat

and mass transfer rates and thereby address the problems of

traditional fixed-bed reactors, while the stationary bed circum-

vents the challenges of slurry reactors. This approach shows

promise, especially with respect to the small-scale application

of a few hundred or a few thousand barrels per day production

capacity, and some relatively new commercial companies are

actively pursuing this technology.28

Structured reactors (monolith type reactors) for FT applica-

tion have also attracted the attention of mainly academia,

although there has been some limited interest from commer-

cial companies as well.29 In this approach, the active FT metal

(e.g., cobalt) is coated onto a large structure with a specific

geometry, which is then inserted into a reactor tube.

The LTFT synthesis is ideally suited for the production of

high-quality middle distillates (diesel and jet fuel) after hydro-

cracking of the long chain waxes. In addition, the heavy prod-

uct spectrum provides chemical opportunities in the form of

speciality waxes and base oils. The naphtha from the process is

also a high-quality feedstock for naphtha steam crackers that

produce mainly ethylene, but also some propylene.

7.20.1.4 Historical Development of the FTS

Historically, the first syngas conversion results were pub-

lished by Sabatier30 in 1902 where it was shown that a mix-

ture of carbon monoxide and hydrogen could be converted

into methane over nickel and cobalt catalysts. In the 1920s,

Franz Fischer and Hans Tropsch took this process a step

further and showed that syngas could be converted into a

mixture of higher hydrocarbons that could be used as petrol

or diesel (i.e., FTS).1,2 In their first patent,31 they described

the production of higher hydrocarbons using iron- and

cobalt-based catalysts operated at atmospheric pressure and

at temperatures below 300 �C. Further research in Germany

led to improved versions of this process. The first commercial

plant started in 1936. Several others followed and provided

Germany and Japan with synthetic fuel during the Second

World War. These plants used mainly cobalt catalysts

supported on kieselguhr (i.e., silica-based supports) and pro-

moted by magnesia and thoria, in fixed-bed reactors. Further,

China had FTS plants in the 1940 through 1960s, all based on

cobalt catalysts.32

After the war, the German FT technology came in the hands

of the Allied Forces. Many scientists and engineers who con-

tributed to the German developments were interrogated and

the entire Fischer–Tropsch technology was extensively investi-

gated at the US Bureau of Mines, which resulted in new two-

phase HTFT technology. The classical textbook by Storch,

Golumbic, and Anderson originates from this period.3 Small

plants were built in the US and operated in the 1950s.

Large-scale FTS developments mainly occurred in South

Africa.33 Sasol started an FTS plant in 1955 based on HTFT to

make petrol and on LTFT to produce wax. Both HTFT and LTFT

used iron-based catalysts. The HTFT technology formed the

basis for the large expansion of Sasol in the late 1970s/early

1980s when Sasol 2 and 3 were built in Secunda (see Table 1).

The main reasons for this expansion were the oil crises in the

1970s, which led to a significant increase in the crude oil price

(see Figure 4).

These oil crises also initiated renewed interest in FTS from

other companies like BP, ExxonMobil, Gulf, Shell, and Statoil,

which was mainly based on cobalt FTS catalysts.34,35 In the last

20 years, this has led to new commercial GTL plants by PetroSA

(South Africa; 1993), Shell (Malaysia; 1992), Sasol-Qatar

19700

50

100

150

200

250

Num

ber

of a

rtic

les/

pat

ents

US

cru

de

oil p

rice

($)

300

350

400 Patents

Patents

Oil price

Articles

US crude oil price

450

1975 1980 1985 1990

Publication year

1995 2000

Articles

2005 20100

10

20

30

40

50

60

70

80

90

100

Figure 4 Patents and articles per year compared with the crude oil price (figure inspired by de Smit and Weckhuysen21).

FTS

ATRASU

Figure 5 Photo of the Sasol-QP Oryx GTL plant in Qatar, showing the air separation units (ASUs), the auto-thermal reformers (ATRs), and theFischer–Tropsch synthesis (FTS) slurry reactors. The product work-up section located behind the FTS reactors is not visible (photo courtesy of Sasol).


Petroleum (Qatar; 2007; see Figure 5), Shell (Qatar; 2011),

and Sasol Chevron (Nigeria; under construction – start-up

2013). An overview of the current commercial operations

using FTS technology is shown in Table 1.

The investment decision to build the Sasol-Qatar Petroleum

Oryx-GTL plant was taken in 2003 when the oil price was $25/

barrel. The facility was built at a cost of $1 billion. Currently, a

yearly profit is generated of about $500 million.36 Sasol’s

much larger Secunda CTL facility is generating currently

about $2 billion profit annually.36 Shell’s Pearl plant (both

the FTS production – 140000 barrels per day – and the

upstream natural gas condensates – 120000 barrels per day –

together) was built at a cost of $20 billion,37 and Shell

announced to make annually $4 billion cash when Pearl is at

full production with the oil price at $70/barrel. It is clear that

new GTL/CTL facilities require large capital investments, and

are heavily dependent on the prevailing crude oil price. How-

ever, over the long term these large-scale GTL/CTL facilities do

make economic sense.

A new challenge to the GTL/CTL technology is global warm-

ing and the emission of CO2. Fuel products from GTL facilities

have a similar environmental footprint compared to crude

oil-derived fuels. However, products from CTL facilities have

a much larger CO2 impact, which is immediately clear from the

overall stoichiometric equations [1] and [2]. In the hypothet-

ical limit of using a carbon feedstock which does not contain

any hydrogen, one CO2 molecule is formed for every carbon

atom that ends up in a hydrocarbon. A large portion of the

CO2 produced in CTL plants is removed and concentrated, and

is therefore ideally suited for capturing, that is, ‘capture ready’.

At the same time new focus on products from biomass can

stimulate interest in FTS further, as biomass can be used as a

carbon source for syngas generation. Recently, Oxford Cata-

lysts has demonstrated their FTS technology using syngas made

from wood chips.28

Other opportunities for GTL applications in the future

might be the use of associated natural gas in small-scale plants

(<1000 barrels per day, as Oxford Catalysts and CompactGTL

are pursuing), as well as the use of shale gas in large-scale

facilities (as pursued by Sasol).

From an academic point of view, renewed interest in FTS

was clearly observed in twomain waves (see Figure 4). The first

wave occurred in the late 1970s, while the second one started

around 1995 and is still gaining momentum. The latter


observation is also matched by an increase in patenting activ-

ity. Of course, this revived activity is related to the increase in

the crude oil price, although the present academic interest is

certainly also inspired by the notion that crude oil resources are

limited. Typical topics in Fischer–Tropsch research with a high

academic interest are catalyst preparation methods, deactiva-

tion studies, and mechanistic and kinetic studies, in which

sophisticated tools such as in situ catalyst characterization,

surface science, molecular modeling, and transient kinetic

studies are the common ingredients. The increased interest

from both the academic as well as the commercial world has

created excellent scientific interactions and discussions, which

enabled further progress on this exciting topic of FTS. Many

questions are still outstanding, such as the state of the catalyt-

ically active surface under reaction conditions, and the reaction

mechanism in terms of elementary steps.

7.20.2 Iron-Based FTS Catalysts

7.20.2.1 Introduction

The iron-catalyzed FTS process is, along with ammonia synthe-

sis, one of the most studied systems in the field of heteroge-

neous catalysis. The reason for this is possibly the fact that the

application of the process is so versatile. Not only can iron FTS

produce a light hydrocarbon product stream ideal for the fuel

and chemical industry, it can also produce heavier hydrocar-

bons (C35þ) suited for the waxes market. Iron is also a cheap

raw material when compared to its cobalt counterpart (cobalt

is on average 250 times more expensive than iron raw mate-

rials) and it has been commercially applied since the late 1950s

by Sasol38 (Table 1). Iron is believed to be more tolerant of

poisons, for example, sulfur in synthesis gas than cobalt. It is

also known to be responsive to selectivity manipulation by the

addition of promoters and a variation of typical process param-

eters, for example, temperature, pressure, and H2/CO ratio. The

disadvantage, however, is the fact that iron FTS catalysts deac-

tivate rather quickly (activity or selectivity loss) and this will be

discussed in more detail later in this section. As already men-

tioned the iron FTS process can be manipulated to produce a

range of carbon number distributions with the final product

stream depending mainly on the temperature applied during

FTS. At lower temperatures, for example, 220–250 �C the chain

growth probability (a) of the catalyst is approximately 0.94

indicating that the bulk of the products will consist of hydro-

carbons longer than C21. In the case of higher temperatures, for

example, 320–350 �C, the chain growth probability decreases

to 0.7 and even lower with the main products being light

hydrocarbons utilized for the production of transportation

fuel and chemical feedstocks. Figure 1 shows the carbon

chain length as a function of chain growth probability. The

influence of promoters on selectivity will be discussed later in

this section.

Although there are many advantages with regard to iron-

catalyzed FTS, the transformations of the iron catalyst during

activation and FTS are rather complex and still not fully under-

stood. During catalyst preparation, iron oxides (e.g., hematite

(Fe2O3) and magnetite (Fe3O4)) are produced and these are

transformed to either a-Fe or iron carbides during activation

depending on the conditions.

7.20.2.2 Commercial Applications

Sasol is a leader in the field when it comes to commercializing

iron-catalyzed FTS processes. In the early 1950s, Sasol com-

mercialized Fe-catalyzed FTS based on the Ruhrchemie process

to produce a variety of synthetic petroleum products using the

Arge Tubular Fixed bed reactors (see Table 1 and Figure 3).

Later on, they developed the slurry-bed reactor and this reactor

together with the Arge reactors are used to produce high-

molecular-weight hydrocarbons for the wax industry.38 In the

late 1950s, Sasol also commercialized a circulating fluidized

bed reactor at their Sasolburg facilities in which fused iron

catalyst is fluidized at high temperatures to produce lighter

hydrocarbons ideally suited for producing fuel and chemical

feedstock. In the late 1970s/early 1980s Sasol’s Secunda plant

was built using this circulating fluidized bed technology, which

was replaced in the late 1990s by the improved fixed fluidized

bed technology. Subsequent to Sasol’s successful commercial-

ization of iron-catalyzed FTS, South Africa’s national oil

company (PetroSA) commercialized a GTL facility using

previous-generation high-temperature FTS technology (Sasol

licensed technology). This technology is based on a fused

iron catalyst operated in a fluidized bed reactor at high tem-

perature (330–350 �C). In 2010, it was still recognized as one

of the world’s largest GTL refineries, producing about 22000

barrels per day of high-quality FTS-derived fuels.

Rentech, based in Colorado, USA has long been investing in

iron-based FTS research. Rentech demonstrated their iron-

based FTS technology in their Product Demonstration Unit

(PDU) in the middle of 2008. The PDU produces approxi-

mately ten barrels per day of ultra-clean diesel, aviation fuels,

and naphtha.39

Synfuels China has recently emerged as an important player

in the Fischer–Tropsch industry.32 Their development of a so-

called high-temperature slurry-phase technology (HTSFTP™)

and associated iron-based catalyst is novel to the industry. The

integrated technology promises improvements in process ther-

mal efficiency and a highly active catalyst. This technology has

been demonstrated in a 4000 barrels per day semi-commercial

CTL facility owned by the YiTai Coal Liquefaction Company in

Xue JiaWan, Erdos, Inner Mongolia.

7.20.2.3 Iron Fischer–Tropsch Catalyst Preparation

There are several preparation methods available in the litera-

ture for the synthesis of Fe FTS catalysts, like precipitation and

fusion. Iron catalysts prepared commercially are actually iron

oxides, hydroxides, or oxy-hydroxides, which undergo an acti-

vation step such as reduction or pre-treatment in syngas prior

to FTS. General requirements for the catalysts are, among

others, selectivity (low for methane; high for the targeted

hydrocarbon fraction), activity and stability, and mechanical

robustness. The operation conditions, for example, high or low

temperature (HTFT and LTFT), and the type of reactor

employed put specific demands on the catalyst synthesis

procedure. For a tubular fixed-bed-type reactor, minimization

of mass transfer limitations is an important consideration, and

here catalyst strength is less important than catalyst shape

and form. For a fluidized bed reactor, however, catalyst

strength as well as particle size and density are very important.40


Table 2 summarizes the typical preparation methods used for

the various applications.

Key factors when choosing an iron source are cost and

availability. Although most of the iron oxides, hydroxides,

and oxy-hydroxides are readily available in nature, precursors

for iron catalysts are rather chemical grade raw materials.41

This is done to ensure that impurities that can influence

the catalyst are either removed or carefully controlled. It

is typical for commercial manufacturers of iron catalysts to

produce chemical grade iron(III)nitrate from sufficiently

pure scrap iron on site as part of the preparation. Large-scale

fusion preparation methods use iron ores or mill scale from

steel mills. Complex preparation methodologies that involve

novel chemicals and/or intricate transformations are rarely

commercially viable when compared to the tried and trusted

methods of precipitation and fusion. The gains from such

novel preparations must be truly unique to justify the addi-

tional expense.

Table 2 Catalyst preparation methods used for high and low temperature

Reactor Important catalyst properties

HTFTCirculating or fixed fluidized bed reactors,320–350 �C

Low surface area (<10 g m�2), hhigh strength

LTFTTubular fixed bed reactor, 220–250 �C High surface area, sufficient stre

Slurry bed reactors, 220–250 �C High surface area, small particle(50–250 mm)

Promoters

Mill scale Oxidation Arc furnace

Figure 6 Catalyst preparation diagram for the HTFT fused iron catalyst. Mil

80 mm

Inclusion

(a)

Figure 7 (a) SEM image of a HTFT Fe catalyst cast ingot, showing inclusions

7.20.2.3.1 FusionFusion produces oxidic iron particles of low surface area, high

density, and high strength, which are ideally suited for appli-

cation in circulating fluidized bed reactors (Figure 3). During

the fusion process, the iron oxide raw material together with

the promoters are fed into an arc furnace where it is subjected

to temperatures above 1000 �C. After fusion, the molten mate-

rial is cast into flat bars (ingots) and cooled. These ingots are

milled to a specified particle-size range to ensure optimum

fluidization (Figure 6).

The disadvantage of fusion is the fact that the inorganic

impurities, for example, silica and alumina oxides present in

the raw mill scale starting material, form inclusions during

cooling of the ingot. The alkali promoters migrate and bind

during cooling to these inclusions, which negates the promo-

tion effect. Figure 7(a) is a scanning electron microscopy

(SEM) image of a fused ingot showing clearly the inclusions

and with scanning electron microscopy energy-dispersive X-ray

iron-based Fischer–Tropsch processes

Raw material Synthesis method

igh density, Mill scale Fusion followed by crushing andmilling

ngth Fe(NO3)3 and silicasource

Precipitation followed byextrusion/shaping

s Fe(NO3)3 and silicasource

Precipitation followed by spraydrying and calcination

Coolingstep

Sizereduction(milling)

Reduction

l scale means iron metal pieces from the steel industry.

FeSiAlkali 1Alkali 2

(b)

of silica and alkali and (b) SEM EDX mapping on one of these inclusions.

Goethite: 8 – 200 m2g-1

Ferrihydrite: 100 – 700 m2g-1

Magnetite: 4 – 100 m2g-1Degree

ofcrystallinity

Hematite: 12 – 27 m2g-1

Figure 8 Degree of crystallinity is decreasing with an increased surfacearea for various iron oxides.


spectroscopy (SEM EDX)(Figure 7(b)) one can see the pres-

ence of the alkali in these inclusions.

The development of fixed fluidized bed reactors diminished

the need for particles of high mechanical strength, and permit-

ted the use of catalysts with lower density and higher surface

area. One of the prospective routes to alternative HT FTS

catalyst precursors is the precipitation of high-density, low-

surface-area iron oxides, hydroxides, and (oxy)hydroxides

from solutions of iron(III) salts followed by calcination.

7.20.2.3.2 PrecipitationPrecipitation of iron(III)oxides from iron(III)nitrate solutions

was one of the first methods reported in the literature for the

preparation of iron FTS catalysts.42 In the late 1930s, Ruhrch-

emie developed a large-scale preparation based on pre-

cipitation. In this procedure, the iron(III) salt is reacted with

a base to form an iron(III) oxide–(oxy)hydroxide precipitate.

By variation in process conditions, for example, pH, precipita-

tion rate, and temperature, catalyst properties such as surface

area and crystallite size can be controlled. Figure 8 illustrates

the decrease of crystallinity versus surface area for the various

iron oxides known as precursors for Fe FTS catalysts.

After precipitation, the slurry is filtered and washed to

remove all the salts (e.g., NH4NO3) from the filter cake (see

Figure 9). The latter is then reslurried and impregnated with

structural promoters like Si, Al, etc. The application of chemical

promoters in the iron catalyst is discussed inmore detail later in

this section. Next, the slurry is spray-dried to yield spherical

particles that are suited to slurry-bed and fixed fluidized-bed

reactors. The final step in the catalyst preparation is calcination.

Dissolution PrecipitationFw

HNO ,(aq)

Base(aq)

Iron

Figure 9 Catalyst preparation diagram for the slurry-bed and fixed-bed prec

This high-temperature treatment removes volatile impurities

such as water and NOx and increases the strength of the catalyst

particles. Figure 10 shows spherical particles obtained from

spray drying. For fixed-bed reactor applications, the impreg-

nated filter cake is extruded and dried at about 150 �C.The methodology described above is commercially applied

by Sasol for the synthesis of their slurry-bed reactor (SBR) and

tubular fixed-bed reactor catalyst precursors. Rentech uses a

similar method to synthesize their Fe FTS catalyst precursor.39

As mentioned above, Sasol developed ‘in-house’ precipitation

methodology to synthesize a suitable Fe-HTFTS catalyst that

offers many advantages over the fused FeHTFT catalyst.43 Some

of these include improved promoter distribution, increased

strength, and spherical particles which improve fluidization

of the catalyst. By replacing fusion with precipitation, the

negating effect of the alkali promoters could be eliminated

owing to the purity of the starting iron(III)salt.

iltration/ashing

Salt(aq)

Re-slurry andimpregnation

OR

Spray drying

Calcination

Slurry bedcatalyst

Extrusion

Drying

Fixed bedcatalyst

Water

ipitated iron LTFT catalyst.

Figure 10 Spray-dried iron LTFT slurry-bed catalyst particles.


7.20.2.3.3 Improving iron Fischer–Tropsch catalystprecursors by promotersFTS processes catalyzed by unmodified and unpromoted

iron catalysts suffer from poor selectivity, low activity, and

sintering, but the addition of structural and chemical pro-

moters addresses most of these issues.

Structural promoters, for example, Si, Al, and Mg, may sup-

press sintering, stabilize the active phase, and improve mechan-

ical strength. The addition of alumina and silica typically

increases the stability of hematite under FTS conditions.44 In

general, it is observed that in the presence of a structural pro-

moter such as silica the surface area of the iron oxide remains

high even after calcination at relatively high temperatures.

A potential disadvantage, however, of adding structural

promoters is that the activation, for example, reduction of the

iron oxide, becomes more difficult due to the formation of iron

silicates or aluminates. For this reason, chemical promoters

such as Cu or Ag are added during catalyst synthesis to increase

the rate of reduction, most likely due to hydrogen spillover

from the Cu surface to the iron oxide surface. Apart from

increasing the rate of reduction, chemical promoters are

known to (i) enhance nucleation of iron intermediates which

leads to higher surface areas, (ii) increase the number/type of

CO adsorption sites, (iii) stabilize selected phases, and (iv)

influence the rate of secondary reactions.45

Addition of alkali metals (e.g., potassium) to LTFT iron oxide

catalyst precursors is known to enhance the chain growth prob-

ability (increased C5þ selectivity), to diminish methane forma-

tion, and inhibit secondary hydrogenation reactions, leading to

higher olefin to paraffin ratios.46 In a similar way, but perhaps

not as effective, alkali earth metals have been shown to increase

alpha values and to suppress methane formation.

7.20.2.3.4 Activation and reduction proceduresIron oxides and (oxy)hydroxides are inactive for FTS and must

be activated to render an active catalyst. Depending on the

application – HTFT or LTFT – and iron oxide precursor, activa-

tion is performed in hydrogen, carbon monoxide, or synthesis

gas. The optimum activation conditions are influenced by the

type and quantity of chemical and structural promoters.

The stability and composition of the final activated iron phase

determine the performance of the catalyst under Fischer–

Tropsch conditions. Because of this, the activation procedure

has a strong effect on selectivity and activity of the catalyst.

For activation in hydrogen, the extent of reduction of iron is

governed by the role of water removed during the activation.

The degree of reduction is governed by the equation47:

DG ¼ DG� þ RT lnpH2O

pH2[4]

where ΔG is the free energy change for the reduction under the

conditions employed, ΔG� is the standard free energy change

for the reduction reaction, R is the gas constant, T is the

temperature, and p is the partial pressure of the gases indicated.

The equation implies that the rate at which water is

removed from the reactor plays a critical role: the faster the

water is removed, the faster the reduction process proceeds,

and the higher the degree of reduction is. Rewriting exp-

ression [4] to include the equilibrium constant in the form of

the ratio (pH2O/pH2)eq at equilibrium, enables one to estimate

the degree of reduction that can be obtained:

DG ¼ nRT lnpH2O

pH2

� ��pH2O

pH2

� �eq

" #[5]

As the equilibrium ratio for reduction from Fe2O3 to FeO is

0.7, and for FeO to a-Fe is 0.1, the theoretical degree of reduc-tion would be 50% at 10% water in the gas phase. Hence, for

high degrees of reduction the water content should be well

below 1%.47,48 Structural promoters such as silica and alumina

increase the resistance against reduction.

To fully understand activation, it is necessary to understand

the possible phase transformations that can occur. x-Ray dif-

fraction (XRD) and Mossbauer spectroscopy are ideal tech-

niques to distinguish between the different iron phases that

can arise, while small particle effects, which so often limit the

information content of XRD, are generally absent in the unsup-

ported iron FT catalysts.49–51 Activation with CO present in the

gas typically leads to a mixture of metallic iron (a-Fe), ironcarbides (general formula FexCy), and magnetite (Fe3O4). The

relative quantities are a function of the reducing gas, the gas

hourly space velocity, and the temperature.

It is generally believed that carbides such as Hagg-carbide

(w-Fe5C2) are the active phase for FTS.52 The exact nature of the

surface carbidic species is still a subject of debate. The stabilities of

different bulk iron carbides have been reported to be in decreas-

ing order: e0-Fe2.2C>e-Fe2C>w-Fe5C2>y-Fe3C.53 Depending on

the type of catalyst (e.g., fused or precipitated), different iron

carbides were found to be characteristic for each type after acti-

vation. However, the possibility that a-Fe plays a role during FTScannot be ignored.21 Typically during activation, precipitated

catalyst precursors, for example, hematite (Fe2O3), are converted

to magnetite (Fe3O4) irrespective of the activation gas used.

However, after this transformation the final iron phase will

depend on the activation gas used, for example, a-Fe in the case

of hydrogen or Hagg-carbide (w-Fe5C2) in the case of CO or

synthesis gas. In a study by Herranz et al., it was found that the

activation of hematite using CO resulted in mainly cementite

(y-Fe3C) while activation in synthesis gas yielded Hagg carbide

(w-Fe5C2) (Table 3).54


The fused magnetite HTFT catalyst is reduced with hydrogen

at temperatures between 350 and 450 �C and high linear flow

rates to avoid re-oxidation by water, as explained above. During

reduction, oxygen atoms are removed from the lattice leading to

an increase in surface area from <1 g m�2 to �5–8 g m�2 55.

The extent of reduction under these conditions was measured at

about 80% (a-Fe). Under CO or synthesis gas, virtually no

reduction of the nonporous magnetite was observed.

In the case of precipitated iron oxide catalyst precursors for

LTFT catalysts, the activation is usually done under muchmilder

conditions than for fused catalysts. These catalysts are more

amorphous with a high pore volume and surface area and the

oxide crystallites can sinter under too harsh activation condi-

tions. It is important to note that the success of activation of

precipitated iron catalyst precursors is coupled to FTS activity,

stability, and selectivity. This is dependent not only on the type

of reduction gas but also on process conditions, for example,

temperature and pressure. From the literature, it seems as

though activation under CO yields the optimally activated pre-

cipitated iron catalyst for FTS synthesis, as these gave the best

syngas conversion and lowest methane selectivity when com-

pared to catalysts activated with H2 or synthesis gas.56 However,

the final catalyst also had a relatively high WGS activity.

7.20.2.4 Selectivity Manipulation of Iron Catalysts

A key advantage of iron-catalyzed FTS is the fact that the

selectivity of the process can be manipulated, either by process

conditions (less responsive) or by catalyst composition (more

responsive). Lowering the temperature shifts the selectivity

Table 3 Names of the various iron phases

a-Fe2O3 Hematitea-FeOOH GoethiteFe3O4 MagnetiteFeO Wustitew-Fe5C2 Hagg carbidey-Fe3C Cementite

0 5

Ln (X

)

Alpha 1(LT FT) = 0.80

LT

HT

Alpha (H

10 15 20 2

Carbon

Figure 11 Anderson–Schulz–Flory distribution of hydrocarbons formed over

from lighter to heavier hydrocarbons. Although the Anderson–

Schultz–Flory (ASF) distribution curve (see Section 7.20.1 and

Figure 2) gives a good indication of expected selectivities, alkali

promotion of iron catalysts leads to selectivities that tend to

deviate from ASF and are characterized by two alpha values

(see Figure 11).

In attempts to increase the selectivity toward valuable base

chemicals, mixed-metal oxides and/or multicomponent metals

are typically incorporated in precipitated iron catalysts. Addi-

tion of manganese to a typical Ruhrchemie catalyst increases

the selectivity toward alpha-olefins at low-temperature FTS

conditions (�230 �C and 20 bar total pressure).57 Another

example is the Fe/Zn/Mn/Cu/K/SiO2 catalyst for direct conver-

sion of synthesis gas to chemicals.58 The development of such a

technology is known as ChemFT, and has as primary focus to

shift the selectivity toward alcohols. Table 4 compares the

selectivities of the various iron FTS technologies (HTFT, LTFT,

and ChemFT) and illustrates that the alcohol selectivity in

ChemFT is much higher than that of a typical Ruhrchemie

catalyst under similar LTFT conditions.58

7.20.2.5 Catalyst Stability During FTS

Stability is a key characteristic of a successful catalyst. The ideal

FTS catalyst shouldmaintain constant activity and a correspond-

ing stable selectivity during time on stream. Commercial reac-

tors and product workup sections are designed for a very narrow

set of optimum process conditions. The catalyst must perform

within these design constraints for as long as possible. This

determines the useful catalyst life. Unfortunately, iron catalysts

show considerable loss of performance over time. During recent

years, the focus of research has shifted from improving catalyst

activity to increasing the lifetime of the catalyst.

Deactivation of iron FTS catalysts is usually attributed to the

following factors:

(i) ‘free’ carbon formation, leading to catalyst fouling,

(ii) activity loss due to transformation of the phase, for exam-

ple, oxidation,

Alpha 2 (LT FT) = 0.94

FT

FT

T FT) = 0.75

5

number

30 35 40 45 50

an LTFT catalyst (typical Ruhrchemie catalyst) and a fused HTFT catalyst.


(iii) mechanical break-up of the catalyst,

(iv) deposition of poisons in the synthesis gas on the catalyst’s

surface, and

(v) sintering.

Buildup of ‘free’ carbon is one of the major causes of

deactivation in HTFT. It leads to a decrease in density and

strength of catalyst particles and results in catalyst bed expan-

sion, particle break-up, and carry-over of fine catalyst material

into downstream processes.25 Figure 12 shows an SEM image,

along with the distribution of elements of a ‘spent’ catalyst

retrieved from a commercial fixed fluidized bed reactor. Cata-

lyst break-up and fines formation are easily recognized. As

carbon is dispersed through the bulk of the particle, break-up

may lead to exposure of new active surfaces, thus helping to

maintain activity. At the same time, the carbon that is lost in

the form of fines is rich in alkali and thus removes some of the

chemical promoter, which degrades the selectivity.

Much has been discussed regarding the origin of the free

carbon in the catalyst. A plausible explanation is given by the

Table 4 Selectivity comparison between LTFT, HTFT, andChemFT25,58

Product Fe HTFT Fe LTFT Fe ChemFT

CH4 (%) 8.0 3.0 18.0C2–C4 olefins (%) 24.0 4.0 21.0C2–C4 paraffin (%) 6.0 4.5 17.7C5–C6 (%) 16.0 7.0 13.3C7–350

�C (middle distillateproduct)

36.0 26.5 20.5

350 �C (wax products) 5.0 51.0 0.0Oxygenates as alcohols (%) 2.8 3.8 8.3Oxygenates as acidsþketones 2.2 0.2 0.9% breakdown (C5–C12 cut)% total paraffins 13.0 29.0 49.6% total olefins 70.0 64.0 37.8% aromatics 5.0 0.0 0.0% oxygenates 12.0 7.0 12.5

-Fe -C -Si

Figure 12 Scanning electron microscopy (SEM) image of a spent,fused Fe HTFT catalyst; color coding: red, iron; yellow, carbon; and green,silicon.

so-called competition model.59 After the adsorption and dis-

sociation of CO and H2, three reactions are possible:

(i) C*þ iron!carbides

(ii) C*þxH*!CHx*

(iii) C*þ yC*! inactive carbon

The first reaction describes the formation of iron carbides

from the reduced a-Fe under FTS conditions. The dissociated

carbon (C*) can either react with dissociated hydrogen atoms

(H*) to yield hydrocarbons or react with another carbon atom

(C*) to yield inactive/ so-called ‘free’ carbon.59 This type of

deactivation can be suppressed by chemical promoters. In

recent years, Sasol developed another propriety catalyst involv-

ing the addition of chromium to reduce the amount of ‘free’

carbon formed during FTS.60

The formation of ‘free’ carbon is less pronounced in the case

of the precipitated LTFT iron catalysts. The main deactivation

mechanisms in this case are sintering and oxidation of the

active phase. Interconversion of different carbides may lead

to a stoichiometric excess of carbon which in turn leads to

weakening of catalyst particles. Figure 13 shows a deactivation

curve for a typical Ruhrchemie catalyst under low-temperature

FTS conditions. Samples of this catalyst taken from the reactor

usually contain mixtures of highly dispersed magnetite and

iron carbide (both containing around 2 nm particles).61 The

highly dispersed magnetite particles can either react in synthe-

sis gas to the required iron carbide, or they can agglomerate or

sinter into larger inactive particles (about 40 nm). The larger

magnetite particles can agglomerate further to yield large glob-

ules (around 400 nm). Surprisingly, agglomeration or sinter-

ing of highly dispersed iron carbide into less active or inactive

iron carbide particles of about 20 nm has also been observed

(Figure 14).

7.20.2.6 Spent Catalyst Management

Regeneration of ‘spent’ iron FTS catalysts is difficult, due to the

sintering of the particles during FTS. Successful regeneration

requires redispersion of the sintered phase, and this cannot

easily be achieved. Reactivation by re-reduction is possible, but

the activity of the reactivated catalyst is lower because the

0.0

0.5

1.0

1.5

2.0

0 50 100 150 200 250

Rel

ativ

e ac

tivity

rat

io

Time on line (h)

Figure 13 Relative activity versus time on stream for a precipitatedRuhrchemie-type iron LTFT catalyst.

(Fe3O4)HD (FexCy)HD

(Fe2C)LP(Fe3O4)G

HD – highly dispersed phase » 2 nmLP – larger particles » 20 nmG – globules » 400 nm

50 nm

200 nm

50 nm

(Fe3O4)LP

CnHm

(FeO)HD (Fe)HD

H2 + CO

H2 + CO

FTS

-H2O

H2O OxidationSintering

Sintering

Sintering

CO

-CO2

CO

-CO2

Figure 14 Deactivation mechanism for a typical Ruhrchemie iron catalyst under low-temperature Fischer–Tropsch synthesis conditions.


original surface area cannot be recovered. Multiple reactivation

steps are therefore not viable. Spent HTFT catalysts may in

principle be recycled to make new catalysts. However, using

the material in the fusion process has a high energy cost associ-

ated with it, due in part to its high carbon content. Iron is a

cheap material, and there is little economic incentive for recov-

ering it. Therefore, spent catalysts have usually been landfilled.

Currently, awareness of the environmental impact of such pro-

cedures is growing, and reclamation of metal – even iron – from

spent catalysts, for example, by acid dissolution is more and

more seen as a social responsibility of the industry to reduce the

impact of commercial processes on the environment.

7.20.3 Cobalt-Based FTS Catalysts


Cobalt as an FTS catalyst was already claimed by Fischer and

Tropsch in their original patent of 1925.31 The commercializa-

tion of the FTS by Germany and Japan in the period 1938–45

relied fully on cobalt catalysts. Only after World War II did the

focus shift to the use of iron catalysts for FTS applications.

Since the oil crises of the 1970s the interest in cobalt-based

FTS catalysts reappeared, which has resulted in numerous sci-

entific papers and patents (see Figure 4). Many companies

showed interest in cobalt FTS, for example, BP, ConocoPhilips,

Gulf, ExxonMobil, IFP, Johnson Matthey, Sasol, Shell, Statoil,

and Syntroleum. Almost all focused on wax production, fol-

lowed by hydrotreating to produce diesel. This is also the

application that will receive most attention in this section.

Cobalt FTS catalysts are exclusively utilized in low-

temperature synthesis or LTFT, and are applied in fixed-bed,

slurry-phase, and micro-channel FTS reactors. Catalyst design

needs to be adjusted to the targeted reactor as well as the applied

FTS conditions. Important for catalyst design are the composi-

tion, method of preparation, activity and selectivity behavior,

deactivation and regeneration, and mechanical integrity.

Cobalt FTS catalysts are currently commercially applied by

Sasol/QP in the Oryx GTL plant, Qatar (Co/Al2O3), in a slurry-

phase reactor, and by Shell in the SMDS plant in Bintulu,

Malaysia, as well as in the Pearl plant, Qatar (both Co/Mn/

TiO2), in a fixed-bed reactor. Figure 15 shows the catalyst that

is used in slurry-phase application.

Exciting academic and industrial research in the last 20

years has increased the fundamental knowledge of cobalt FTS

catalysts substantially on topics like the nature of the active

site, impact of crystallite size on activity and selectivity, and

deactivation mechanisms, owing to the application of surface

science techniques, model catalysts, in situ analyses at relevant

industrial conditions, and molecular modeling.8,19,27,62–67 The

literature of the last 20 years shows that quite a wide variety of

cobalt catalyst compositions prepared by numerous methods

can result in academically and industrially relevant cobalt-

based FTS catalytic systems.

7.20.3.2 Composition of Cobalt Catalysts

Modern cobalt catalysts are similar to the ones prepared by

Fischer and Tropsch in the sense that they consist of promoted

cobalt on a metal oxide support. An inspection of the literature

and patents on this topic reveals the following general charac-

teristics, with almost all companies with FTS catalysts having a

similar formulation for them18,35,68 (Table 5):

(a) Cobalt as the FTS active metal (typically 10–30 wt%)

(b) A second metal (usually noble) as a reduction promoter

(0.05–1 wt%)

(c) A structural oxidic promoter (e.g., Zr, Si, and La) (1–10 wt%)

(d) A refractory oxidic support (most likely modified)

Cobalt is expensive and to maximize its use, it needs be well

dispersed on the support. Since cobalt metal is considered the

active phase, it is imperative that there is a high density of

cobalt metal sites available. The number of cobalt surface

sites is a function of particle size and morphology, extent of

Slurry phase reactor

Cobalt catalyst

Wax

10 m

0.1 m

5 � 10-4 mSupport particles

1 � 10-7 mCobalt and support

Support

Cobalt

~ 60 m

1 � 10-8 m

A cobalt nanoparticle

Cobalt

Cobalt

Cobalt

Modified support

Structural promoter

Reduction promoter

Figure 15 Cobalt catalysts for application in a slurry-phase reactor, and schematical composition of a typical cobalt-based Fischer–Tropsch catalyst.

Table 5 Examples of catalyst formulations, as patented by several industrial Fischer–Tropsch synthesis companies

Company Composition Reference Preparation route

Co (wt%) 2nd metal Structural promoter Support

Shell 20 – MnO (Co/Mn¼12) TiO2 WO 199700231 CoprecipitationExxonMobil 12 Re (1 wt%) Al2O3 (6 wt%) TiO2 US 5268344 ImpregnationSyntroleum 20 Ru (0.1 wt%) La (1 wt%), SiO2 (0.1–10.6 Si/nm

2) Al2O3 WO 2005058493 ImpregnationBP 10 – Al2O3 (0.5 wt% Al) ZnO WO 19913400 ImpregnationSasol 20 Pt (0.05 wt%) SiO2 (0.8 Si/nm

2) Al2O3 US 7365040B2 Impregnation


reduction, and particle stability.68 It is preferred to have a fairly

high extent of reduction (>60%), but it should also be noted

that the cobalt is further reduced during the FTS reaction. An

optimum cobalt particle size of just above 8–10 nm is pre-

ferred as particles below those have shown to have a lower

turnover frequency (TOF).69 Additionally, very small particles

(4–6 nm) could bemore prone to sintering and alsomay prove

very difficult to reduce due to an increased metal-support

interaction. It is important that there is a minimum amount

of cobalt-support compounds as these are reducible at very

high temperatures and are inactive for the FTS reaction.70 The

two most common phases of metallic cobalt in supported

cobalt FTS catalysts are face-centered cubic (fcc) and hexago-

nally close-packed (hcp), which often coexist.19,71 It has been

reported that for cobalt particles less than 40 nm, the predom-

inant phase should be fcc.72 The mode of activation, addition

of promoters, and support may influence the relative amounts

of the phases.19 Some authors have reported that the hcp phase

is more active for FTS.73

Nanometer-sized cobalt particles when supported on tradi-

tional oxidic carriers like silica, alumina, and titania are diffi-

cult to reduce due to strong interactions with the support.

Therefore, catalysts are often promoted with a second metal

(e.g., Ru, Pt, or Re) which leads to improved reducibility of the

cobalt oxide particles; the increase in amount of active sites

results in higher activity compared to un-promoted catalysts.

The more facile cobalt reduction is attributed to faster hydro-

gen activation in the presence of promoter metals and


subsequent spillover of hydrogen to cobalt oxides and reduc-

tion of cobalt species.19,68 In many cases, the promotion with

noble metals leads to a smaller average size of either cobalt

oxide or cobalt metal particles. Promotion with noble metal

may also play a role during the decomposition of the cobalt

precursors and can lead to crystallization of smaller cobalt

oxide particles. This increased dispersion is most likely due to

a higher rate of nucleation, enabled by the promoter.68

Promoter metals such as Ru have also been claimed to lead

to the formation of bimetallic particles and alloys. This influ-

ences catalyst activity and selectivity, may inhibit deactivation

by keeping the surface clean, and allows easier regeneration of

the cobalt surface.74 The metal promoter is usually present at

levels of 0.1–0.5 wt%. At these low concentrations reduction is

efficiently promoted, and the hydrocarbon selectivity is hardly

negatively affected.

Structural promoters affect the formation and stability of the

active phase of a catalyst material. For Co/silica catalysts, it has

been shown that promotionwith Zr results in a decreased cobalt–

silica interaction, which in turn leads to a higher degree of cobalt

reduction and increase in the metallic atoms on the surface.19,75

Zr promotion of cobalt/alumina catalysts has been claimed to

prevent formation of cobalt aluminate.76 Incorporation of ele-

ments such as B77 and Ni78 increases the stability of cobalt

catalysts by suppressing carbon formation. Irreducible oxides

such as MnO and CeO2 may also slow down cobalt sintering.63

Awide range of promoters has been studied; the reader is referred

to a detailed review by Morales and Weckhuysen.63

The support provides mechanical strength and thermal sta-

bility to the cobalt crystallites, while facilitating high cobalt

dispersion. The properties of the support are an important

factor. For alumina, high purity, low acidity, and relatively

high surface area (150–250 m2g�1) are required, according to

patents from the 1980s.79–81 More recently, however, alumina-

based supports of relatively low surface area (50 m2 g�1), such

as Ni-promoted a-Al2O3, have been reported to have a positive

effect on both mechanical strength and C5þ selectivity.82 The

pore size of the support can also influence the size of the cobalt

crystallites, as shown by Saib et al. for SiO2-supported

catalysts.83 Van Steen and Claeys reported that the desired

pore size of the support for the optimum cobalt crystallite

size should be around 12–16 nm.61

The support needs to be robust under FTS conditions, imply-

ing that it should be able to cope with the presence of several

bars of steam that occur at high conversion levels. Van Berge

et al. found that an unprotected alumina-supported cobalt FTS

catalyst was susceptible to hydrothermal attack during realistic

FTS conditions, which resulted in contamination of product wax

with ultra-fine, cobalt-rich particulates.23,84,85 This problem was

solved by pre-coating the support with silica as structural pro-

moter. TiO2 seems to be the support of choice for both Exxon

and Shell based on the most recent patents (Table 5). An advan-

tage of TiO2 is that it has a high hydrothermal stability and can

withstand high water partial pressures. The rutile/anatase ratio

can be tailored, which influences the surface area and mechan-

ical properties.

Supported cobalt catalysts should also be resistant to attrition

especially if applied in a slurry bubble-column environment.

Wei et al.86 noted that the attrition resistance of supported cobalt

catalysts follows the sequence: Co/Al2O3>Co/SiO2>Co/TiO2.

There has also been work conducted on less conventional

supports such as MCM-41, SBA-16, and carbon nanofibers,

nanotubes, and spheres.19,69,87 These studies are mainly aca-

demic in nature but further fundamental understanding of

cobalt FTS catalysts considerably. Carbon supports interact

weakly with cobalt and allow for a high degree of cobalt reduc-

tion, thus enabling the study of cobalt particle-size effects.69,87

7.20.3.3 Preparation of Cobalt Fischer–Tropsch Catalysts

The preparation of cobalt FTS catalysts aims to achieve the

optimal crystallite size distribution in a particle that is optimal

for its application in a specific fixed-bed, bubble-column, or

microchannel reactor. As the optimum size range of catalyst

particles for the different reactor types varies (see Figure 3),

preparation methods and equipment depend on the targeted

reactor application. Important considerations for choosing a

particular method of preparation and starting components are

to minimize poisons (e.g., Na, S, Cl) in the catalyst and the

type of waste streams resulting from the chosen method.

A number of procedures for preparing cobalt FT-catalyst

precursor exist:

• coprecipitation of cobalt, promoters, and support, followed

by catalyst particle shaping. In a variation of this method,

the support is added just before particle shaping,

• precipitation or impregnation of cobalt and promoters

onto pre-shaped support particles, and

• impregnation of cobalt (oxide or metal) particles onto pre-

shaped supports.

7.20.3.3.1 PrecipitationMost of the initial FTS-catalysts (e.g., Co/ThO2/kieselguhr)

were made by coprecipitation.88 This method has been applied

for some of the modern cobalt catalysts as well, for example,

for Co/Mn catalysts,89 Co/Mg/SiO2 and Co/ZnO2.90

Catalyst preparation based on coprecipitation usually con-

sists of three steps: precipitation, washing and drying, and

shaping. Selection of chemicals is of course an important con-

sideration in view of the associated waste streams.

Chemical precipitation of the cobalt, promoter, and sup-

port by a precipitation agent can be done batchwise or contin-

uously at constant pH. The cobalt precipitates as a hydroxide,

which can exist as green a-Co(OH)2 or pink b-Co(OH)2 poly-

morphs. The former is metastable and readily transforms into

the stable b-phase. Crystallite size and composition of the

precipitate are controlled by temperature, precipitation agent,

precursor salts, structure directing or organic hydrolysis

reagents, aging time, and reaction atmosphere (air or N2).

Using Na2CO3 or KOH as precipitation agents in the prepara-

tion of Co/SiO2 catalysts would lead to cobalt silicate

formation.91 To prevent formation of the inactive cobalt sili-

cate, the silica is added after the precipitation.

Filtration and washing of the precipitate is required to

remove excess chemicals. Even low levels of alkali metals and

halogens left in the washed precipitate can severely degrade the

catalyst’s performance.

Shaping of the catalyst precursor depends on the reactor

application. For bubble beds, the precipitate is usually reslurried

and spray-dried to obtain the required particle-size distribution.


For fixed-bed reactors, the precipitate is extruded or pelletized.

Addition of acids to the washed and dried precipitate is done to

improve the final catalyst’s particle strength.92

7.20.3.3.2 Preparation methods involving pre-shapedsupportsSupport morphology and characteristics play an important role

in optimizing the preparation of cobalt catalysts on pre-shaped

support particles. As the aim is to get a desired amount of

cobalt crystallites onto the support and maintain a crystallite

size of around 8–10 nm, the following support characteristics

need consideration:

• The support pore volume dictates howmuch cobalt precursor

can be added per impregnation. As shown in Table 6, 30 g of

metallic cobalt per 100 g of support occupies only 0.03 ml g�1

of support material, but when using Co(NO3)2�6H2O as the

precursor, a pore volume of 0.79 ml g�1 is required.

• The time required to get a homogeneous cobalt distribution

throughout a support particle during impregnation depends

0

10

20

30

40

50

60

-10 0 10 20 30 40 50 60 70 80

Mas

s% C

o3O

4

Distance from edge of catalyst particle (mm)

Figure 16 Macroscopic cobalt crystallite distribution, as measured by scanimpregnation followed by immediate fast drying and (b) slurry-phase impregnaparticle, followed by fast drying.

Table 6 Pore volume requirements for different cobalt components

Cobalt compound Molar mass (g mol�1) Cobalt mass fraction

Co 59 100CoO 75 0.79Co3O4 241 0.73Co2O3 166 0.71CoOOH 92 0.64Co(OH)2 93 0.64Co(NO3)2 183 0.32Co(NO3)2�6H2O 291 0.20CoCl2�6H2O 237 0.25

on particle geometry (diffusion path length and pore size),

viscosity of the suspension, interaction between solution and

support surface (contact angle and surface tensions), and

diffusion coefficients.93–95 Figure 16 shows the effect of dif-

fusion on the different cobalt distributions observed on small

alumina particles for incipient wetness impregnation com-

pared to slurry-phase impregnation, as obtained from SEM/

EDX line scans.96,97

• For fixed-bed catalysts, eggshell-type cobalt distributions

are sometimes preferred to overcome pore diffusion limita-

tions on performance and selectivity. Concentrating the

cobalt in the outer layers of the support is, among others,

achieved by adding viscosity enhancers or using cobalt salt

melts for impregnation.98

• For deposition precipitation onto pre-shaped supports, the

same parameters that determine the time required during

impregnation to get a homogenous distribution (e.g., par-

ticle geometry, diffusion path length, pore size, viscosity of

the suspension, interaction between solution and support

surface, contact angle and surface tensions, and diffusion

coefficients) are important.99

0

10

20

30

40

50

60

-10 0 10 20 30 40 50 60 70

Mas

s% C

o3O

4

Distance from edge of the catalyst particle (mm)

ning electron microscopy (SEM) line scans for: (a) incipient wetnesstion allowing 3 h for the cobalt to slowly disperse throughout the alumina

(%) Density (g cm�3) Pore volume required for a loading of 30 gof Co per 100 g of support (ml)

8.9 3.46.4 5.96.1 6.75.2 8.25.0 9.43.6 13.22.5 37.71.9 79.01.9 62.5


• In addition, diffusion differences between the precipitation

agent and cobalt will impact the final cobalt distribution

during precipitation. When depositing bulky cobalt

hydroxide crystallites or cobalt metal particles100 onto pre-

shaped support particles, the important parameters are pore

size, particle diameter, and bulkiness of the precipitate.

• During the drying of the catalyst precursor, the same

parameters as highlighted above for impregnation and

deposition should be taken into account to prevent the

cobalt precursor (if not chemically fixed to the support)

from migrating out of the particle again. In addition, heat

transfer coefficients, evaporation enthalpies, and particle

outer surface area need consideration for optimizing the

drying phase of the cobalt catalyst preparation.

Each preparation method needs to be optimized carefully,

and one cannot assume that the optimum procedures for one

type of support and support particle shape will be the same for

all other supports and support particle shapes.

7.20.3.3.3 CalcinationUsually, drying is not fully accomplished and therefore the first

stages of calcination actually complete the drying phase. To

maintain the cobalt distribution achieved by impregnation or

precipitation during drying and calcination, the cobalt compo-

nent mobility must be hindered. One way of achieving this is

to ensure that the cobalt component stays in a viscous or solid

form. For catalysts obtained by impregnation from cobalt

nitrate solutions, this implies that during calcination the com-

bination of heating rate and air flow must be such that water

and NOx are immediately removed.101 As the mobility of the

cobalt phase can be minimized by fast calcination, heat flow

into the system is also important as both the drying and nitrate

decomposition are endothermic.

Performing calcination under different atmospheres pro-

vides a way to affect the dispersion of the cobalt phase. NO

addition during calcination leads to the formation of a less

mobile cobalt hydroxyl nitrate.102 Using H2 or CO as decom-

positionmedium at temperatures below those where reduction

takes place also gives catalysts with good cobalt dispersions.103

Adding organic additives during impregnation is another

method to influence cobalt nitrate decomposition. Oxidation

of the additive is an exothermic process, which provides heat

for the endothermic nitrate decomposition, and thus acceler-

ates its decomposition.

The transmission electron microscopy (TEM) images in

Figure 17 illustrate how cobalt distributions change when

different calcination conditions are applied.

7.20.3.3.4 ReductionCobalt catalysts are usually reduced in hydrogen or a diluted

hydrogen atmosphere. Examples of CO reductions are also

found, but carbon formation on the cobalt crystallites should

be avoided. Reduction of cobalt oxide to cobalt metal occurs in

two exothermic steps:

Co3O4 þH2 ! 3CoOþH2O [6]

CoOþH2 ! CoþH2O [7]

For optimal reduction, care must be taken to optimize heat

transfer, minimize hydrogen diffusion and mass transfer limi-

tations, and to remove water effectively. The latter benefits

from high hydrogen space velocities and application of low

heating rates.104 Figure 18 shows the impact of the water

partial pressure during reduction of a 30 g Co/0.075 g

Pt/100 g alumina catalyst on the starting FTS performance,

indicating that low water content should be targeted for max-

imum activity, in agreement with the thermodynamics of

reduction as expressed in eqn [5]. Hence, the hydrogen stream

used for reduction must be as dry as possible. For small catalyst

particles as used in slurry-bed reactors, fluidized bed reduction

reactors are preferred and the above requirements are easily

met. For reductions in fixed-bed reactors, more care must be

taken to overcome the limitations especially toward the reduc-

tion reactor outlet.

The maximum temperature required for reduction of cobalt

catalysts depends on the level of reduction promoter present

(Pt, Ru, Pd, etc.), the presence of other promoters (e.g., alkali

metals make reduction more difficult), the support, the sup-

port modifiers, and the catalyst precursor used in the prepara-

tion. Reduction temperatures that are too high can cause

sintering and loss of cobalt metal surface area. In the case of

Co/SiO2 catalysts, cobalt silicate formation has been reported

for temperatures higher than 350 �C.Catalyst performance depends critically on the reduction

procedure.105 Application of reduction–oxidation–reduction

(ROR) cycles has been reported to improve the FTS perfor-

mance of cobalt catalysts by up to 30%.106 Some of the reasons

given in the literature for this improved performance from

ROR treatment are: (1) rougher (more steps on the surface)

cobalt crystallites, (2) higher degree of reduction, and (3) re-

dispersion of the cobalt on the support surface.

7.20.3.4 Cobalt Catalyst Fischer–Tropsch Performance

Both activity and selectivity are of course important parameters

for cobalt-based FTS catalysts. High activity is important for

slurry-phase catalysts, while for fixed-bed catalyst the heat

removal capacity needs to be balanced with the activity of the

catalyst. From a selectivity point of view, a low methane selec-

tivity is normally desired, combined with a high C5þ selectivity

or a high chain growth probability (a). Determining the intrin-

sic catalytic performance of cobalt FTS catalysts is not a

straightforward exercise, as it is influenced by the choice of

reactor and conditions. Khodakov et al.19 summarize a num-

ber of issues and choices related to the testing of FTS catalysts,

for example: (i) reactor choice: fixed-bed, slurry-bed (or con-

tinuous stirred-tank reactor, CSTR), or high-throughput reac-

tors, (ii) hydrodynamics, (iii) heat transfer and hot spots, (iv)

intra particle and external mass-transfer limitations, and (v)

atmospheric or elevated pressure. As testing catalysts under

different FTS conditions (H2/CO ratio, T, and P) will result in

different catalyst performances35,107 and therefore possibly

selection of different catalysts, it is important in the early stages

of research to understand the long-term scaling-up view, with

respect to reactor choice and FTS conditions. Taking the same

cobalt catalyst and testing it in different manners can result in

very different catalytic activity behavior. Figure 19(a) clearly

shows that the FTS activity is influenced by the water partial

0.5 μm

(a) (b)

(c) (d)

0.5 μm

0.5 μm 0.5 μm

Figure 17 Cobalt crystallite distributions as measured with STEM for cobalt alumina catalysts calcined in different manners. (a) Cobalt oxidemicroglobical formation of a 30 g Co/100 g alumina catalysts using a heating rate of 1 �C min�1 and an air space velocity of 1mn

3 per kg Co(NO3)2.6H2Oper hour. (b) Cobalt oxide distribution of a 30 g Co/100 g alumina catalysts using optimized heating rate and air space velocity to ensure optimumcalcination. (c) Cobalt oxide distribution on a 30 g Co/100 g alumina catalysts using carbon coated alumina , using the same heating rate and air flow rateas in (a). (d) Cobalt oxide distribution on 30 g Co/100 g alumina catalysts using the same heating rate and flow rate as (a) but with 1% NO in He ascalcination atmosphere.


pressure applied during the test, which possibly impacts factors

such as sintering and carbon deposition, as well as surface and

active site reconstruction.65 The selectivity behavior of cobalt

catalysts is also strongly influenced by parameters such as

temperature, hydrogen and carbonmonoxide partial pressures,

and conversion. Comparing catalysts tested at different condi-

tions should thus be done with care.

Figure 19(a) clearly shows that cobalt catalysts are more

active in fixed-bed than in slurry-bed reactors. However, cobalt

catalysts in slurry-phase reactors are normally applied at tem-

peratures around 230 �C, while in fixed-bed reactors they are

normally used at temperatures around 210 �C. The productiv-ity per gram of catalyst is therefore higher in slurry-phase

reactors (Figure 19(b)).

For heterogeneous catalysts the activity often increases with

smaller particle size, as the metal surface area increases. This

was confirmed by Iglesia108 who showed that the activity of

cobalt catalysts is directly proportional to the amount of cobalt

metal surface in the catalyst. The TOF or the reaction rate per

unit of cobalt surface area was stable over the range of cobalt

particles that was investigated (9–200 nm). FTS over cobalt

catalysts was therefore regarded to be structure insensitive.

Thereafter, a number of authors have investigated the effect

of cobalt metal particle size on the intrinsic activity of sup-

ported cobalt catalysts for smaller cobalt particles, that is, well

below 10 nm.69,87,109–112 Bian et al.110 using Co/SiO2 con-

firmed Iglesia’s results for samples with cobalt particles

between 11 and 29 nm, but Barbier et al.,109 Bezemer et al.,69

Martinez and Prieto,111 Coville and coworkers87 all showed

that the TOF was stable for catalysts with cobalt particles above

8–10 nm, while it decreased sharply for catalyst with smaller

particles. Only Borg et al.113 reported no sensitivity for the


activity of cobalt particles with sizes down to 3 nm. The

relationship between TOF (or reaction rate per unit cobalt

surface area) and cobalt particle size for above-mentioned

publications is summarized in Figure 20. As the TOF numbers

were obtained under different FTS conditions (i.e., tempera-

ture and partial pressures), they were normalized to enable

comparison of trends in the different papers and therefore

expressed in arbitrary units. It is clear that for catalysts with

cobalt crystals above 10 nm the TOF is structure insensitive,

while there is a sharp decrease in activity for particles smaller

than 8–10 nm.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 200 400 600 800 1000

Act

ivity

(au)

Time (h)

Figure 19 (a) Three Fischer–Tropsch synthesis runs with same Co/Al2O3 caPH2O inlet¼0; PH2O outlet¼3.0 bar. Red open: fixed bed with water co-feedingPH2O inlet¼0; PH2O outlet¼4.5 bar; all the catalyst is exposed to outlet water pTropsch synthesis runs with same Co/Al2O3 catalyst, as tested at 20 bar, 60%slurry phase run at 230 �C (blue triangles).

100.7

0.8

0.9

Rel

ativ

e FT

act

ivity

1.0

30 50 70 90PH20

(mbar)110 130 150 170

Figure 18 Impact of water partial pressure during reduction of a 30 gCo/0.075 g Pt/100 g alumina catalyst on its initial Fischer–Tropschsynthesis performance.

As mentioned above, the C5þ selectivity depends strongly

on the FTS process conditions. For a constant set of conditions,

Iglesia114 found no particle-size effect on selectivity for parti-

cles between 9 and 100 nm. A few years later, however, Barbier

et al.109 reported a strong dependency of the chain growth

probability, a, on particle size. Increasing the cobalt diameter

from 4.5 to 9.5 nm caused the a-value to increase from 0.74 to

0.87 (at 170 �C, 1 bar). Bian et al.110 showed a similar, though

less pronounced, trend with the a-value increasing from 0.85

to 0.89 when the particle size increased from 11 to 29 nm (at

200 �C, 10 bar). Bezemer et al.69 reported a very clear particle-

size effect on selectivity at atmospheric pressure, with a meth-

ane selectivity that was stable for particles larger than 6 nm, but

increased sharply for smaller particles (220 �C, 1 bar).

However, the reported data at high pressure (35 bar and

210 �C) clearly show that the C5þ selectivity still increases with

increasing particle size up to 15 nm. Xiong et al.,87 Prieto

et al.,112 and Borg et al.113 all confirmed the general trend on

an increasing C5þ selectivity with increasing particle size

extending beyond 10 nm, and up to 20 nm.

Little fundamental understanding has been offered to

explain this particle-size effect on both activity and selectivity.

Interestingly, the effect of size on activity is very pronounced

for particles smaller than 10 nm, while the impact on selectiv-

ity seems to be more gradual and does not level off above

10 nm. The particle-size effects cannot, as previously suggested,

be explained by the oxidation of the smallest particles. Bezemer

et al.69 showed with x-ray absorption near-edge structure

(XANES) measurements that oxidation did not occur. This is

in line with extensively reported research that cobalt oxidation

during FTS does not occur for cobalt particles larger than

0 200 400 600 800 1000

Pro

duc

tivity

(mol

e C

O c

onve

rted

/g c

at/s

)

Time (h)

talyst, as tested at 20 bar, 230 �C, and H2/CO¼2. Red solid: fixed bed,; PH2O inlet¼2.6, PH2O outlet¼4.0 bar. Blue solid: slurry bed;artial pressure in a CSTR laboratory slurry reactor. (b) Two Fischer–conversion, with the fixed bed run at 210 �C (red circles) and the


2 nm.65 The particle-size effect can also not be explained by

sintering as this was not observed by Bezemer et al.69 and by

the Coville group.87 It is clear that the Co particle-size effect in

FTS extends beyond the classical impact of size, which derives

from the fraction and type of surface atoms as a function of

crystallite size and normally does not extend beyond 4 nm

particles. It was suggested69 that the optimum combination

of active sites for the different elemental reactions of FTS (i.e.,

CO dissociation, hydrogenation, and insertion) requires rela-

tively large cobalt particles, possibly combined with a CO-

induced surface reconstruction. This might be related to the

presence or absence of the so-called B5 site,65 which has been

speculated to be the most active site for CO dissociation, and

needs a certain particle size to be present in high abundance.

Another speculation is that the particle-size effect might be

related to specific bonding modes of CO, such as the bridge-

bonded CO coordination; this mode is believed to be favored

on large particles and held responsible for an increased CO

dissociation rate, which would lead to an increased reaction

rate.87,109 Based on in situ Fourier transform infrared

spectrometry results, Prieto et al.112 proposed an enhancement

0 5 10 15

TOF

(au)

Co particle

Figure 20 The TOF or Fischer–Tropsch synthesis rate per unit surface area, ato be scaled due to variations in process conditions, and is therefore reported

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20

Time on

RIA

F

Figure 21 Normalized activity stability for a Co/Pt/Al2O3 catalyst during reareactor at fixed CO conversion (230 �C, 20 bar, H2þCO conversion of 50–70%from van de Loosdrecht, J.; Bazhinimaev, B.; Dalmon, J. A.; NiemantsverdrieCatal. Today 2007, 123, 293–302.

of Codþ–SiO2 sites in small particles, caused by flattening of

the crystals during FTS, and possibly due to carbon-induced

surface reconstruction. Another study showed that carbon and

oxygen atoms, originating from dissociated CO, were very

strongly bonded on small particles, possibly blocking active

sites for further CO dissociation.117

As the impact of the particle size is different for activity from

for selectivity, the fundamental explanation of these effects

seems to involve more than one kinetically relevant mechanis-

tic step in the FTS process. Understanding this requires more

fundamental research.

7.20.3.5 Deactivation and Regeneration of CobaltFischer–Tropsch Catalysts

Catalyst stability is crucial for the economics of cobalt FTS,

in addition to other important factors such as high activity,

selectivity, and mechanical strength. Understanding catalyst

deactivation is essential for improving catalyst stability and for

developing effective regeneration procedures. Figure 21 shows a

typical deactivation profile for Co FTS catalysts under

20 25 30 size (nm)

Coville (2011)

Fischer (2010)

Pietro (2009)

Borg (2008)

Martinez (2007)

De Jong 1 bar (2006)

De Jong 35 bar (2006)

Bian (2003)

Barbier (2001)

Iglesia (1997)

s a function of the cobalt metal particle size.69,109,110,112–116 The TOF hadin arbitrary units.

30 40 50 60 line (days)

listic Fischer–Tropsch synthesis in a laboratory scale micro-slurry, feed gas composition of 50–60 vol.% H2 and 30–40 vol.% CO). Adaptedt, J. W.; Tsybulya, S. V.; Saib, A. M.; van Berge, P. J.; Visagie, J. L.

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Hyd

roge

n re

sist

ant

carb

on (w

t%)

Time on stream (days)

Figure 22 Build up of polymeric carbon on a cobalt slurry-phaseFischer–Tropsch synthesis catalyst as a function of time on stream.121


commercially relevant FTS conditions.118 Deactivation is initially

stronger, after which it starts leveling off. Similar deactivation

profiles have been reported by other laboratories.18,62,119,120

Fundamental studies on catalyst deactivation essentially

involve understanding differences in the fresh and spent cata-

lysts with respect to the active site. The B5 sites on metallic

cobalt are currently considered as active sites for the FTS.65

Hence, changes to the number or nature of these sites will

contribute to deactivation. However, due to the complexity of

the FTS process and the lack of suitable techniques to charac-

terize any particular site on a surface, most studies on deacti-

vation are necessarily limited to ‘observables’ such as changes

in metallic cobalt surface area. This in itself is no trivial matter,

as cobalt catalysts are sensitive to their environment, and spent

FT catalysts are embedded in wax (which actually protects

them from being exposed to the air). As is so often the case

in fundamental studies of catalysts, a combined approach

using real catalyst and model systems, advanced in situ and

ex situ characterization, combined with molecular modeling

has given detailed insight into FTS catalyst deactivation.65

The main deactivation mechanisms of cobalt FTS catalysts

and its active sites, as proposed in the literature, are: (1) oxi-

dation, (2) mixed metal-support interaction, (3) carbon depo-

sition and carburization, (4) sintering, (5) poisoning, and (6)

surface reconstruction.19,62,64,65,70,71,108,114,118,121–133

Over the last 15 years, oxidation of cobalt by the product

water was seen as the major deactivation mechanism in the

open literature.65 However, many recent publications have dis-

proved this.62,64,65,71,118,134Following an in-depth study on oxi-

dation using model systems, molecular modeling, surface

thermodynamic calculations, and an industrial catalyst tested

under commercially relevant conditions, the key finding is that

oxidation is crystallite size and condition dependent, that is,

under realistic FTS conditions (H2O/H2¼0.5–3). Co crystallites

with diameters larger than about 2 nm will not undergo oxida-

tion. In fact, from XANES analyses of spent Co catalysts from an

extended FTS run, it was found that a further reduction took

place during FTS118 and has been confirmed by others.135–137

Further, the formation of metal-support compounds such

as cobalt aluminate have been considered as a deactivation

mechanism.129,138 Although thermodynamically favorable,

this reaction needs CoO formation as an intermediate, which

does not take place under realistic FTS conditions. Indeed, a

recent study showed that the minor cobalt aluminate formed

during FTS originates from unreduced CoO present in the fresh

catalyst and not from Co metal.65,122 This leaves sintering and

carbon deposition as the major contributors to Co FTS catalyst

deactivation.

Carbon deposition on an FTS catalyst that is covered by

growing hydrocarbons and is entirely embedded in product

wax represents a real challenge. Nevertheless, deleterious car-

bon arising from CO or FT products can have a wide range of

negative effects on Co FTS catalysts. We mention pore block-

age, resulting in mass transfer limitations, formation of bulk or

surface carbides, and blockage or alteration of active sites.121

Pore blockage by long hydrocarbon products resulting in dif-

fusion limitations of the reactants CO and H2 has been men-

tioned as a deactivation mechanism from the onset of the

discovery of the FTS.139 A hydrogen treatment or solvent

wash of the spent catalyst resulted in a partial recuperation of

FT activity. Although detailed studies are unknown to us, pore

blocking is generally accepted as a deactivation mechanism.

Recently, strong evidence has been found for the accumula-

tion of a rather inactive polymeric carbon on themetallic surface

of cobalt resulting in catalyst deactivation (Figure 22).121 Tech-

niques involved in this work were temperature-programmed

hydrogenation, low-energy ion scattering, and energy-filtered

TEM. Molecular modeling suggested that the polymeric carbon

might be a form of graphene. In general, there is good agree-

ment that bulk cobalt carbide is metastable and will not be

present in substantial quantities during FTS for cobalt, although

Karaca et al.71 observed small quantities of Co2C in their in situ

XRD experiments (which, interestingly, was entirely absent for

the first 9 h on stream, but appeared in the measurements after

10 h; exposure to pure CO made the signal grow further).

Although characterization of spent cobalt FT catalysts run for

several months in a slurry bubble column did not show bulk

cobalt carbide formation,118,121 a possible role of subsurface

carbon cannot be ruled out. To date, our knowledge on subsur-

face carbon comes from molecular modeling. Calculations on

carbon clusters by Zonneville et al.140 indicate that carbon in

subsurface positions affects the CO dissociation rate and may

therefore affect the FTS activity as well. More work is needed to

ascertain the impact of subsurface carbon.

Carbon deposition can be decreased by adding promoters.

Examples from the literature include ruthenium74 and

boron.77,141,142

Sintering is a thermodynamically driven process whereby

smaller, more unstable particles grow to form larger, more

stable particles that are lower in surface energy. Sintering as a

deactivation mechanism is easier to investigate than carbon

deposition, with TEM and XRD being the most common tech-

niques used. Due to this in general there is good agreement in

the literature on the importance of sintering as a deactivation

mechanism of cobalt FTS catalysts.62,65,70,71,121,134,137,143–145

Key factors that affect the rate of sintering are the reaction

temperature and the partial pressure of water: an increase in

either of these results in enhanced sintering. The choice of

support also plays a key role. Alumina is considered to provide

more stability against sintering than silica does, due to the

improved metal support interaction in the former. Both


particle migration and Ostwald ripening are expected to be

important, but further studies are desirable to gain full under-

standing on the contribution of each.

Poisoning of cobalt catalysts by S, NH3, HCN, Hg, and Cl is

well known and is an issue for cobalt-based FTS62,146–149 espe-

cially for coal-to-liquid applications.65 Sulfur is a strong, irre-

versible poison with a large adsorption energy. Due to its size

and electronic effect, sulfur will also poison adjacent cobalt

sites. Once adsorbed, sulfur is difficult to remove and will

accumulate with time.146 Sulfur poisoning can however rela-

tively easily be prevented by cleaning the synthesis gas feed

properly, for example, by using zinc-oxide or lead-oxide guard

beds. Poisoning of cobalt-based FTS catalysts by means of

nitrogen-containing compounds such as NH3 and HCN is a

known effect and postulated to arise from competitive

adsorption.150 The impact of N-compounds is less severe

than that of S-compounds and can be reversed by mild hydro-

gen treatment. Nevertheless, reducing their level to parts per

billion is recommended.150

Surface reconstruction is a thermodynamically driven pro-

cess which results in a lowering of the surface energy and

therefore can contribute to catalyst deactivation. Using molec-

ular modeling, Ciobica et al.124 showed that atomic carbon

from dissociated CO can cause a reconstruction of the Co fcc

(111) surface to a Co (100)-like structure, followed by a clock

reconstruction. This surface is less active and could therefore

contribute to deactivation. As the reconstruction is accompa-

nied by a change in surface density, it could, ironically, also

assist in the formation of more reactive sites, as proposed by

Wilson and de Groot.151 This is a complex phenomenon that

needs further investigation.

Methods to reverse deactivation and regenerate deactivated

Co FTS catalysts have been around since the early days of

Fischer and Trospch.3 The most common methods reported in

the open literature are treatment of the deactivated catalyst

in hydrogen or in steam, applying oxidation–reduction cycles,

and combinations of these.65,152 With carbon and sinte-

ring being the major deactivation mechanisms during FTS,

‘oxidation–reduction’ is considered to be the most robust and

preferred method to regenerate spent Co FT catalysts.65 By

careful control of the oxidation step, deleterious carbon is

removed at temperatures above 250 �C. The oxidation step

is also key for the redispersion of cobalt (see Figure 23). The

20 nm

Figure 23 Co particles supported on a flat SiO2 support during different stastate before oxidation (left). Hollow oxide particles, formed upon oxidation (msmaller metallic particles (right).65,153

mechanism of redispersion of cobalt has been proposed to be a

two-step process, that is, (1) oxidation to form hollow spheres

by the Kirkendall effect and (2) multinucleation of Co3O4

during reduction to produce smaller crystallites.65,153

Poisons such as sulfur are removed by oxidation with steam

and air to sulfates, followed by washing them out. However,

phases originating from strong metal-support interaction are

very difficult to reverse and their formation should be

prevented.154

Substantial progress has been made toward understanding

deactivation and regeneration of Co FTS catalysts, but more spe-

cific knowledge, for example, on deactivation by carbonaceous

species and sintering mechanisms, is definitely necessary. With

currently available in situ characterization, synchrotron-based

techniques, and molecular modeling, we expect major advance-

ments in the coming years.

7.20.4 Mechanisms and Kinetics of FTS Over Ironand Cobalt Catalysts


The complicated nature of the FTS is among others reflected by

its complex product spectrum, consisting of methane, C2þolefins and paraffins (linear and branched), oxygenates

(mainly alcohols, but also aldehydes and ketones), and even

aromatics (at sufficiently high operating temperatures). Three

classes of mechanisms have been proposed, each assuming a

different monomer for chain propagation.

The carbide mechanism (Table 7) was first formulated by

Fischer and Tropsch in 1926, which proposes that CO dissoci-

ates before the carbon atom is partially hydrogenated to a CHx

species.2 These CHx species combine by the addition of one

monomer at a time to effect hydrocarbon chain growth. The

growing intermediate can then terminate in different ways

before leaving the catalyst surface, giving rise to an alkane, an

alkene, or an oxygenate. A number of variations have been

considered within the basic carbide mechanism, arising from,

for instance, whether CO dissociation occurs unassisted or via

interaction with hydrogen, and the number of hydrogen atoms

in the monomer (CH or CH2). The enol mechanism (Table 7),

proposed by Storch in the 1950s, assumes that CO is partially

hydrogenated to a CHOH species (oxymethylene) which then

20 nm20 n

m

ges of an oxidation–reduction cycle. Cobalt particles in the metalliciddle). Reduction of the hollow particles, which break up into several

Table 7 Fischer–Tropsch Reaction Mechanisms

Carbide mechanism Enol mechanism CO insertion mechanism

Initiation CO!CþO COþ2H!CHOH CO!CþOOþ2H!H2O Oþ2H!H2OHCþxH!CHx

Propagation RþCHx!R–CHx R–C–OHþCHOH!R–C–COHþH2O R–CHþCO!R–CH–COR–CHxþ (2�x)H!R–CH2 R–C–COHþH!R–CH2–COH R–CH–COþH!R–CH2–CHþH2O

Termination R–CH2þH!R–CH3 R–CH2–COHþ4H!R–CH2–CH3þH2O R–CHþ2H!R–CH3

R–CH2–CH2–H!R–CH¼CH2

RþCO!oxygenates R–CH2–COHþnH!oxygenates R–CH–COþnH!oxygenates

NB : R¼H, alkyl(CH3, CH3CH2, CH3CH2CH2, . . .).


acts as the monomer.3 Chain growth occurs via a condensation

reaction with water elimination. Again, different termination

routes determine what final product molecule is formed. In the

1970s, Pichler and Schultz introduced the CO insertion mech-

anism (Table 7), which assumes a similar initiation step to the

carbide mechanism.155 The difference resides in the way that

chain growth is proposed to occur, namely by direct CO inser-

tion into the growing intermediate followed by hydrogenation

to remove the oxygen atom.

Even though the FTS has been known since the 1920s, it is

evident from the foregoing that its mechanism is still a matter

of debate. In order to progress the understanding of the mech-

anism, a multidisciplinary approach is required. Subsequently,

three such disciplines (model surface science experiments,

density functional theory (DFT) calculations, andmacrokinetic

studies) will be briefly discussed with particular emphasis on

their implications for mechanistic and kinetic understanding.

7.20.4.2 Surface Science Studies and Model Reactions

A surface science approach is very powerful to study elemen-

tary reaction steps in isolation. Conceptually, it is very close to

the approach taken by DFT calculations: take a well-defined

surface, that is, a single-crystal surface of the material you want

to study and use an ultrahigh vacuum so that the adsorbate of

interest can be introduced with high purity and with a high

accuracy, down to a sub-monolayer coverage. The model sys-

tem can then be studied with many different sophisticated

analysis techniques that give information on the atomic level.

The number of studies with cobalt and iron single-crystal

surfaces related to the FTS is relatively small. Iron carbide

single-crystal work is not available, as far as we know. Studies

on nickel and rhodium crystals are more numerous, as they are

easier to use and because of the fact that both metals are used

as a catalyst for a number of reactions. In this section, we

discuss the most relevant surface science findings on cobalt,

with a focus on studies where a single elementary step was

studied in isolation.

7.20.4.2.1 Adsorption of CO and hydrogen on modelsurfacesOne of the simplest experiments one can do is to study the

interaction of a surface with CO and H2, the reactants in the FT

reaction. In a typical experiment, a clean close-packed Co

surface is exposed to increasing amounts of CO at a sample

temperature of 180 K.156 Such a low-temperature experiment

in ultrahigh vacuum (UHV) conditions is equivalent to incre-

asing the CO pressure in a room temperature experiment.157

Initially CO adsorbs, with a sticking coefficient of 0.7, on top

sites up to a coverage of 0.33 ML (monolayer), with an adsorp-

tion energy of �115 kJ mol�1.156 This translates into a desorp-

tion temperature around 400 K. Upon further dosing, the CO

coverage increases, until a saturation coverage of �0.65 ML is

reached. Increasing the CO coverage beyond 0.33 ML leads to

complex overlayers where CO occupies bridge and threefold

sites as well as top sites. The downward shift of the CO desorp-

tion temperature for coverages beyond 0.33 ML is mainly

caused by repulsive interactions between CO molecules rather

than by the difference in adsorption site. This has important

implications for the interpretation of vibrational spectra on

supported catalyst particles, where occupation of both top

and bridge/threefold sites is typically detected. Occupation of

bridge and threefold sites can simply be caused by a high CO

coverage on the facets of the particle rather than by the pres-

ence of special sites.

Hydrogen/deuterium adsorption on a close-packed Co

surface was studied in a similar fashion: hydrogen adsorbs at

180 K with a low sticking coefficient, up to a coverage of �0.5

ML hydrogen atoms, with an adsorption energy of 33 kJ mol�1

(per H atom). Recombinative desorption occurs between 300

and 400 K.158 Hydrogen desorbs at lower temperature from

more open surfaces, around 300 K, indicating a weaker adsorp-

tion onto those surfaces. The sticking coefficient on the other

hand is much higher than on close-packed surfaces: 0.76

for an open surface compared to 0.05 on a close-packed

surface.159,160 This enhanced hydrogen sticking is commonly

observed on the more open crystal planes of different metal

surfaces.

When CO is dosed at 180 K on hydrogen-covered surfaces,

CO partially replaces the hydrogen, and only 50% of the initial

coverage remains on the surface. The remaining hydrogen is

less strongly bound due to the CO, and as a result the desorp-

tion peak shifts downward by �100 K. CO, on the other hand,

is only mildly influenced by the presence of hydrogen, and any

influence of hydrogen is only seen at low temperatures.158

These experiments show that repulsive interactions exist

between hydrogen and CO, which adds to the barrier to form

hydrogenated HxCO species. Other adsorbates such as sulfur,

oxygen, and carbon give rise to a similar downward shift of the

hydrogen desorption temperature, and those species also

(partly) block the surface for hydrogen adsorption.158


7.20.4.2.2 C–O bond scissionA key step in any FT mechanism is the cleavage of the C–O

bond. The close-packed surface of cobalt, Co (0001), is unable

to cleave the CO bond: CO desorbs as a molecule with the CO

bond intact.156 Some open surfaces of cobalt, such as Co

1012� �

161,162 and Co 1120� �

,163 and cobalt foils are capable

of cleaving the CO molecule directly: after a CO thermal

desorption experiment161,162 or a prolonged exposure to CO

at elevated temperature,163 carbon and oxygen are found to be

left on the surface. CO dissociation stops when enough carbon

and oxygen has built up to block all the active sites for disso-

ciation. UHV studies on 2-nm Co particles on alumina show

that CO dissociates during a CO thermal desorption experi-

ment, which can be explained by the high defect density on

such small nanoparticles.164 These studies did not report exact

temperatures or activation barriers for CO dissociation, but in

all cases a typical reaction temperature in the order of 400 K

can be deduced. In short, surface science shows that direct CO

dissociation is possible, but not on the (most abundant) close-

packed surface.

As direct CO dissociation is not possible on the close-packed

surfaces of cobalt, one might consider if the CO molecule is

(partly) hydrogenated before the C–O bond breaks. When CO

and hydrogen are co-adsorbed onto a close-packed surface at

low temperature, the molecules just desorb upon heating, with-

out reaction.158,160 An alternative experimental approach is to

study the decomposition of (partly) hydrogenated CO mole-

cules such as methanol and formaldehyde. Methanol adsorbs as

methoxy (H3CO) when dosed at 165 K. During heating this

methoxy species is stable up to �300 K, after which it decom-

poses to CO and H2.165,166 Experimentally, this is seen as a

single step, indicating that the first dehydrogenation is rate-

limiting. The intermediate species, H2CO and HCO, decompose

534 532

O1shv = 650 eV

O1s, TP-XPShv = 650 eV

O–CH2–CH3

370 K

Oad

250

K30

0K

350

K

250 K

530 528Binding energy (eV)

Pho

to-e

mis

sion

inte

nsity

(a.u

.)

Binding energy532 530 528

Figure 24 Ethanol decomposition on a close-packed cobalt surface, followespectroscopy (XPS). At low temperature, the ethoxy intermediate is found. Dand C2Hx (acetylene) on the surface, showing clear evidence for C–O bond clCiobica, I. M.; Saib, A. M.; Niemantsverdriet, J. W. J. Phys. Chem. Lett. 2010

rapidly and are not observed in significant concentrations on

the surface. This is in line with the finding that formaldehyde

(H2CO) decomposes with 100% selectivity to CO and H2

between 100 and 200 K.160 The experiments show that partially

hydrogenated species such as HCO andH2CO are very unstable,

and the barrier to decompose via dehydrogenation is obviously

much smaller than that of (Hx)C–O bond cleavage. Similar

experiments have been reported on 2-nm cobalt particles sup-

ported on alumina. In those experiments, 60% of the methanol

that was present dehydrogenated, while 40% underwent C–O

bond cleavage.164 In this experiment, it was not clear whether

the C–O bond scissionwas assisted by the presence of hydrogen,

as CO (produced by methanol dehydrogenation in the metha-

nol experiment) also dissociates on those particles in the

absence of hydrogen.

In the CO insertion mechanism (Table 7), a COmolecule is

inserted into a CxHy, after which the C–O bond has to be

cleaved to generate a Cxþ1Hz intermediate that can insert

another CO molecule. In other words, the CO molecule is

chemically modified by insertion of an alkyl group on the

C-end of the molecule. Experiments using ethanol on a close-

packed Co surface gave information about the effect of alkyl

modification on the C–O bond cleavage.166 Figure 24 shows

the result of such an experiment: ethanol adsorbs as an ethoxy

species at 160 K. This ethoxy species decomposes around

350 K, via an acetaldehyde intermediate, with an activation

barrier of �70 kJ mol�1. The products of this dissociation

step are atomic O and a C2Hx species, demonstrating C–O

bond cleavage. This means that alkyl insertion in the CO

molecule facilitates C–O bond scission. For the CO insertion

mechanism, it implies that after the CO is inserted the C–O

bond can be readily broken and a Cxþ1Hy species is formed that

can undergo further chain growth.

C1s, TP-XPShv = 380 eV

C1s (high res.)hv = 321 eV

O–CH2–CH3

250 K

370 K

C2H2

Photo-em

ission intensity (a.u.)

Binding energy (eV)286 285 284 283 287 286 285 284 283 282

d with temperature programmed (TP) synchrotron x-ray photoelectronuring heating ethoxy decomposes around 350 K, yielding atomic oxygeneavage. Adapted from Weststrate, C. J.; Gericke, H. J.; Verhoeven, M.;, 1, 1767–1770.


7.20.4.2.3 Hydrogenation and the stability of C1Hx speciesC1Hx species are important ingredients in a carbide mechanism

as they are the monomeric species responsible for chain initi-

ation and growth. Surface science experiments can give infor-

mation about the stability of the different C1Hx species.

Regarding cobalt there is only one article that addresses this

question directly, using CHxCly intermediates on a cobalt

foil.167 The authors mention that the behavior on nickel foil

was essentially the same. In an experiment on a close-packed

nickel surface, a molecular beam was used to dissociate meth-

ane at low surface temperature, which generates CH3 species

(þH) on the surface.168 Heating of this CH3 layer showed

transformation of CH3 to CH around 200 K, and no sign of

CH2 was found. On close-packed Pt reported in the same

study, a very similar trend was seen: CH3 decomposes around

250 K, yielding solely CH, which decomposes around

500 K.168 These surface science results demonstrate that the

CH2 species, which is typically seen as the monomer for

chain growth, is particularly unstable, which means that its

concentration under equilibrium concentrations will be

much lower than that of CH3 and CH.

Generally speaking, surface science studies on cobalt and

iron are scarce in comparison to those on nickel and the more

noble metals. Studies on iron foils and single crystals,169–176

although very interesting from the point of view of surface

chemistry, are even further removed from the reality of

Fischer–Tropsch reactions than cobalt, as iron FTS catalysts

are essentially carbides. Surface science studies on iron carbides

are, to the best of our knowledge, not available.

7.20.4.3 DFT Modeling

Molecular modeling by DFT offers a relatively new way to

understand reaction mechanisms at the molecular level.177,178

Adsorption configurations along with their energetics as well as

transition states can be modeled, and thus adsorption energies,

activation energies, and heats of reactions can be obtained. In

addition, entropy changes over elementary reactions enable one

Adsorbed CO

Extent

1.14 eV(exp 110 kJ mol-1)

-0.34 eV

2.30 eV

-1.16 eV

Transition state

Figure 25 Energy diagram for the dissociation of carbon monoxide on the (1effect of repulsion between carbon and oxygen atoms that are adsorbed onto1 eV�96.5 kJ mol�1). Adapted from Bromfield, T. C.; Ferre, D. C.; Niemants

to estimate pre-exponential factors, although these are generally

less often calculated than enthalpies.

Validation of calculations has to be sought by comparing

calculated adsorption energies and activation barriers or vibra-

tional frequencies from stable adsorption states with experi-

mental values, obtained from surface science experiments with

single crystals. However, relatively little experimental data are

available, partly also because surface science studies are by

necessity often performed in vacuum, whereas FTS reaction

steps occur at higher pressures.

As this is an area of research that is emerging rapidly, we

intend to present some examples of how DFT modeling is used

in mechanistic studies. Although DFT results refer to tempera-

tures of zero K and pressure, the results give valuable insight

into the energetic of the underlying surface chemistry. It is not

our intention to give a full review here, as it is still too early for

conclusive statements on the FTS reaction mechanism.

7.20.4.3.1 CO DissociationConversion of CO and H2 into CxHyþH2O necessarily implies

that the C–O bond has to broken. The question is now if this

happens before or after reactionwithH-atoms. As an example, we

show a computational study of direct CO dissociation on the

square (100) surface of bcc-iron in Figure 25.179,180 On this

surface, the CO molecule is known to adsorb in a tilted

geometry.171 In the transition state for dissociation, the C–O

bond elongates, and the energy rises by 1.14 eV (�109 kJ mol�1),

which is the activation energy for dissociation. This value com-

pares well with the experimentally measured activation energy of

110 kJ mol�1 reported by Bernasek and coworkers.171 When the

bondbreaks, theC andOatoms each end up in a fourfold hollow

site of the Fe(100) surface. However, the two atoms significantly

repel each other, implying that the total energy decreases substan-

tially when the two atoms move apart, as shown in the last

structure of Figure 25. This series of calculations demonstrates

that the direct dissociation of a COmolecule on this (100) surface

is very well possible under reaction conditions (a barrier of

110 kJ mol�1 corresponds roughly to a reaction temperature of

Dissociated CO

Dissociated COrepulsion relieved

of reaction

0.82 eV

00) surface of iron, showing the exothermicity of the dissociation, and theadjacent sites. Energies are given in electron volt (eV);

verdriet, J. W. ChemPhysChem 2005, 6, 254–260.


400–450 K, whereas the FTS reaction temperature is at least

475 K), provided the space is available for the dissociation

products to move apart. Hence, this elementary step needs a so-

called ‘ensemble’ of iron atoms, which is determined by the

condition that the C and the O atom can end up on next nearest

neighbor sites, that is, at distances at least equal to √2 times the

lattice constant.

Table 8 compares the activation energy for dissociation on a

number of iron surfaces with increasing reactivity.181,182 The

trend is that dissociation becomes easier when the surface be-

comesmore reactive.Note that the (110) surface is unlikely to play

a role in CO activation, as the barrier is forbiddingly high.181–183

Several authors have proposed that COdissociation becomes

easier when the CO first reacts with hydrogen.183,185,186 The

reasoning is evident, as the C–O bond in an HC–O or C–OH

fragment is expected to be weaker than in the CO molecule.

However, forming the HCO or COH fragment also costs

energy.186 It appears that on surfaces of low reactivity, such as

Co(0001), Fe(110), or the (100) surface of the Hagg carbide, H-

assisted dissociation indeed leads to a lower activation barrier

than direct CO dissociation does.183,185 On more reactive sur-

faces, and notably on surfaces containing steps, the direct

Au

5

3H2(g

) + C

O(g

)3H

2(g

) + C

O*

3H2(g

) + C

* + O

*O

* + 6

H* +

C*

O* +

5H

* + C

H*

O* +

4H

* + C

H

0

-5

Ene

rgy

(eV

)

Ag

Cu

Ni, Rh

CoRuFe

W

Pd, Pt

Figure 26 Density functional calculation for the energy profile of the metharhodium, and cobalt fall close to the optimum profile for the reaction, while irCHx intermediates. Adapted from Jones, G.; Bligaard, T.; Abild-Pedersen, F.; N

Table 8 Activation energies for direct CO dissociation

Ironsurface

Characteristics CO dissocienergy

(110) Flat, close packed, least reactive surface 149 kJ mo(100) Flat, somewhat more open, and thus more

reactive than (110)103–110 k

(310) Stepped surface with narrow terraces 70–87 kJ m(710) Stepped surface with broader terraces 64–86 kJ m

dissociation is favored, and often with a lower barrier than for

the reaction HþCO.186 Nevertheless, it is good to realize that

on certain surfaces the H-assisted pathway may be an alternative

that gives less reactive facets the chance to play a role in FTS.

7.20.4.3.2 CþH reactionsMany studies have addressed the formation of CHx fragments

all the way to methane on several surfaces. Particularly inter-

esting is the comparison made by the Nørskov group.187 Their

calculations are based on a full set of DFT calculations on the

fcc (211) step of ruthenium surfaces, from which they esti-

mated the adsorbate energies on several other transition and

group IB surfaces. According to the Sabatier Principle,188 the

optimum pathway for a reaction is that in which the interme-

diates adsorb at the catalyst surface in a moderate way, that is,

not too strongly and not too weakly. The set of profiles in

Figure 26 illustrates that metals such as Ni, Rh, and Co are

close to ideal methanation catalysts, but that (the stepped

surfaces of) metals such as Ru, Fe, andW bind the intermediate

species too strongly. On the other extreme are Au and Ag,

where formation of intermediates is strongly endothermic

and therefore unfeasible. Metals such as Cu, Pt, and Pd are

CO + 3H2

CH4 + H2O

2*

O* +

3H

* + C

H 3*

O* +

2H

* + C

H 4(g

)O

H* +

H* +

CH 4

(g)

H 2O

(g) +

CH 4

(g)

nation reaction on the group VIII and I-B metals, showing that nickel,on and tungsten form too strong bonds with the carbon, oxygen, and theorskov, J. K. J. Phys. Condens. Matter 2008, 20.

ation activation Reference

l�1 Sorescu181

J mol�1 Bromfield et al.,179 Scheijen et al.,184 and Sorescu181

ol�1 Sorescu181

ol�1 Sorescu181

Table 9 Kinetic expressions for the rate of the Fischer–Tropschsynthesis, rFT

Equation number Rate expression

1 rFT ¼ APH2 PCO

PCOþKH2OPH2O

2 rFT ¼ AP0:5H2

PCO

PCOþKH2OPH2O

3 rFT ¼ AP0:5H2

PCO

1þKCOPCOð Þ2

4 rFT ¼ APH2 PCO

1þKCOPCOð Þ2

5 rFT ¼ AP0:75H2

P0:5CO

1þKC=OHP0:5CO

P0:25H2

þKOP0:5CO

P�0:25H2

� �2

6 rFT ¼ AP0:75H2

P0:5CO

1þKCOP0:5COð Þ2

K stands for equilibrium constant, P for partial pressure, and A is an effective rate

constant.


incapable of breaking the CO bond, but can carry out CO

hydrogenation to methanol.

Not shown in the figure are the barriers for the individual

steps. As shown by several authors, activation energies for

hydrogenation of adsorbed CHx (x¼0–3) species on uncorru-

gated (atomically flat) surfaces are in the range of 50–

100 kJ mol�1 and therefore not expected to be problematic

for the reaction mechanism.189–194

7.20.4.3.3 Chain growthNext comes the question how the hydrocarbon chains grow.

This has been and still is a matter of scientific debate. The

conventional view is that chains grow by a reaction between

an alkyl and a CH2 species, for example, CH3þCH2 to C2H5,

and C2H5þCH2 to C3H7. Termination to an alkane would

then occur by hydrogenation of the alkyl, and olefins would

form by b-H abstraction from the alkyl fragment. This view

originates from the work of Biloen et al., who, however, pro-

posed this mechanism with some reservation, and with a judi-

cious discussion of the assumptions involved.195,196 Ciobica

et al. found that reactions between CxHy fragments that contain

less hydrogen are energetically more favorable, making chain

growth via reactions of the type CH2þCH and CH¼CH2þCH

more likely.197 Nevertheless, the class of mechanisms, based

on CO dissociation, formation of CHx species, and incorpora-

tion of CHx in a growing chain, albeit with a rich variety in the

details, comes close to the original proposal of Fischer and

Tropsch, entitled the carbide mechanism (see Table 7).

Another class of mechanisms considers CO insertion as the

step leading to chain growth (see Table 7). This line of thought

also has a long history, the archetype being the Pichler–Schulz

mechanism.155 The most detailed mechanism has been pre-

sented by Saeys and coworkers,198 who proposed that CO

inserts in an adsorbed CH2, which then further hydrogenates

to a fragment in which the C–O bond breaks, leading to a C2

intermediate in which the next CO inserts. As discussed in the

section on surface science, experimental proof exists that C–O

bond breaking in adsorbed species derived from ethanol, such

as ethoxy, is a facile step, even on the least reactive surface of

cobalt, that is, the close-packed (0001) surface.166

At the time of writing, medio 2011, the authors believed

that DFT is a highly valuable tool for getting insight into

reaction mechanisms at the level of elementary steps. It is,

however, much too early to draw definitive conclusions on

which mechanism is prevalent in the FTS. Further, it should

be acknowledged that not all catalysts and conditions can be

captured under one dominating mechanism, and it is even not

at all certain that this would be the case on one catalyst. For

example, the initiation by CO dissociation might simulta-

neously occur via direct dissociation on highly reactive parts

of a cobalt particle, and by H-assistance on facets of moderate

reactivity. We also point to the differences between cobalt and

iron. Whereas the former is believed to operate as a metal, the

latter is active as a carbide, in which the intrinsically high

reactivity of the iron atoms is considerably decreased by carbon

neighbors. The DFT literature has suggested mechanisms vary-

ing fromMars-van Krevelen-type reactions199 (in which carbon

atoms from the lattice become the CHx species for initiation

and chain growth) to H-assisted CO dissociation and

CO insertion as the major ingredients for hydrocarbon

formation.200 It is obvious that mechanistic understanding

would greatly benefit from more surface science work, but

unfortunately the possibilities for this branch of physical

chemistry research are limited for reactions that by necessity

require high pressure conditions.

7.20.4.4 Macrokinetic Observations and Models

In order to derive a macrokinetic model, a scheme of elemen-

tary reaction steps is usually assumed to represent the

mechanistic pathway of the reaction. Normally, a number of

simplifying assumptions are made regarding the catalyst sur-

face and the adsorption of species on it, for example, that only

one type of adsorption site is considered, and that these sites

are homogeneously distributed over the catalyst surface, that

species only adsorb in a monolayer, and that adsorbed species

do not interact apart from the involved chemical reactions. It is

usually further assumed that most reaction steps are suffi-

ciently fast to reach equilibrium, but that one or more rate-

determining steps exist that are relatively slow and control the

overall rate of reaction.188 These assumptions allow for the

derivation of simple, manageable kinetic expressions, such as

those presented in Table 9.

7.20.4.4.1 General observations regarding kineticsMacrokinetic models are mainly developed for use in process

modeling, yet their mechanistic importance stems from the

fact that they capture the overall behavior of the FT synthesis.

Considering eqn [3] from Table 9 as an example, it is seen that

the reaction rate is predicted to increase with the square root of

the hydrogen partial pressure (everything else being constant).

Therefore, it is said that the reaction order of hydrogen is 0.5 in

this kinetic model. The variation in reaction rate with CO

partial pressure is more complex, since the overall reaction

order is not constant. At very low CO partial pressures or at

high reaction temperature, KCOPCO is much smaller than 1 and

the denominator term can be ignored, yielding an overall CO

reaction order of 1. In the other extreme, at very high CO

partial pressures or at low temperatures where KCOPCO is much

larger than 1, the constant term in the denominator can be

ignored and the overall CO reaction order strives to �1.


Similarly, it can be said for eqn [1] that the reaction order in

hydrogen is constant at 1, while the overall reaction order in

CO varies from 0 to 1 as the conversion increases from low to

high values.

There are distinct differences between the chemical reaction

kinetics over iron and cobalt FTS catalysts. The reaction rate

increases roughly linearly with pressure over iron catalysts at

constant temperature and H2/CO ratio,201 but only to an order

of around 0.5 for cobalt catalysts.202 Furthermore, CO has a

strong inhibiting influence over cobalt catalysts to the extent

that it has a significantly negative reaction order under com-

mercially relevant reaction conditions. To the contrary, CO has

a positive effect on the rate over iron catalysts in the normal

operating regime and it has been estimated that CO will only

start to negatively influence the kinetics above a partial pres-

sure of around 11 bar.203

The influence of water on the reaction kinetics has proven

to be a highly controversial topic. Historically, it was firmly

believed that water (and possibly CO2) inhibited the reaction

rate over iron catalysts via competitive adsorption, but more

recently it has been shown that there is no convincing evidence

for this notion.203 The results of water co-feeding studies over

cobalt catalysts have been inconsistent, since some have

reported a positive and some a negative influence of water on

the reaction rate, while others have found no effect at all. At

least for alumina-supported cobalt catalysts, it appears as

though water in the range of 1–6 bar has no significant influ-

ence on the overall rate of CO conversion, but that it does

decrease the methane selectivity.204

7.20.4.4.2 Simple macrokinetic modelsFor the derivation of macrokinetic models, a key question is

whether or not there is a rate-determining step involved in the

formation of the monomer of chain growth. If monomer

formation is relatively facile, then the rate of CO conversion

and the product distribution obtained are intimately linked

and must be modeled together. This typically yields a complex,

implicit type model that requires an advanced numerical rou-

tine to solve. However, in order to keep these models manage-

able, questionable simplifying assumptions are oftenmade, for

example, the assumption by Yang et al.205 that the monomer is

in thermodynamic equilibrium with the gas phase concentra-

tions of CO, H2, and water. Furthermore, such models require

a large number of parameters that are inevitably highly cross-

correlated, implying that it is virtually impossible to accurately

estimate their values. If, however, there is a rate-determining

step in the formation of the monomer, the overall reaction rate

can be decoupled from the product distribution. In such a case,

the overall rate of CO conversion is determined by the CO

hydrogenation reaction (monomer formation), while the

product distribution is determined by the polymerization

part of the FT reaction (i.e., the competition between chain

growth and desorption). The steady state isotopic transient

kinetic analysis study of van Dijk206 has indeed provided

microkinetic support for the notion that there is a rate-

determining step in the formation of the monomer.

The approach followed during FTS kinetic studies by Botes

et al.202,207 has been to consider different reaction schemes and

rate-determining steps in the formation of the monomer to

obtain a variety of explicit rate expressions. A systematic

experimental approach was then followed to eliminate non-

applicable models until ultimately a robust kinetic expression

remained as the preferred rate equation. A further notable

feature of the FTS kinetic studies was to operate the reaction

at a baseline condition, where the catalyst is known to be quite

stable, for most of the run. Changes to other conditions were

only made for short intervals sufficient to allow for hydrody-

namic steady state inside the reactor, but insufficient to effect

changes in the intrinsic catalyst behavior. The importance of

this approach is related to the fact that FT catalysts, especially

those based on iron, readily respond to changes in operating

conditions. It is imperative to avoid reversible and irreversible

changes in the catalyst when chemical reaction kinetics is

studied.

Originally within Sasol, the rate equation [1] in Table 9, by

Anderson201 was used to describe the FTS over iron. Following

a systematic in-house study focusing on the reaction order of

hydrogen, its exponent was later reduced to a value of 0.5,

yielding eqn [2] (Table 9). The most recent study on iron-FT

kinetics has considered the implications of the historic rate

equations, specifically the single order denominator which

implies that hydrogen reacts directly from the gas phase. By

applying a second order denominator to obtain a more appro-

priate Langmuir–Hinshelwood–Hougen–Watson-type equa-

tion where both CO and H2 first absorb onto the surface

before reaction, and also including a constant term in the

denominator to provide for the possibility of vacant sites, it

could in fact be shown that there is no statistical justification

for including a water term in the kinetic model.207 Experiments

were designed to conclusively show that eqn [3] (Table 9) is

more accurate than the foregoing expressions. This study

highlighted the fact that the historic perception regarding the

effect of water on iron-FT kinetics was self-specified by the old

models, but never tested. Furthermore, the CO order of unity is

consistent with a mechanism where CO interacts with a hydro-

gen atom before being dissociated.

The most recent cobalt kinetic study involved the derivation

of several rate equations to cover various reaction schemes of

CO hydrogenation.202 Models assuming hydrogen-assisted CO

dissociation, such as eqn [4] that was originally proposed by

Yates and Satterfield,208 generally described the measured data

poorly and could be eliminated early on in the study. Figure 27

illustrates that eqn [4] underestimates the reaction rate at low

CO partial pressure, but overestimates it at higher CO partial

pressures. Ultimately, after further work to distinguish between

those models where CO first dissociates before it is hydroge-

nated, eqn [5] (Table 9) was the only rate expression that could

not be eliminated. Therefore, it was selected as the most appro-

priate kinetic model. It should be noted that this expression can

be very closely approximated by eqn [6], which contains one

model parameter less and is thus preferably used from a practi-

cal perspective. Figure 27 shows that the preferred equation [6]

does not suffer from the same systematic errors as eqn [4], since

it is reasonably accurate across a range of CO partial pressures.

7.20.4.4.3 Selectivity modelingThe vast number of components in the FT product slate does

not allow for the prediction of individual product selectivities;

consequently, the product spectrum is rather represented by a

product characterization model with a limited number of

00.5

1r F

T (m

easu

red

) / r

FT (p

red

icte

d)

1.5

Equation 4

Equation 6

PH2PCO

(1 + KCOPCO)2rFT = A

P0H

.2

75PC0.5

O

(1 + KCOPC0.5

O)2rFT = A

1 2 3 4CO partial pressure (bar)

5 6 7 8

Figure 27 Performance of rival kinetic expressions as a function of CO partial pressure for a data series where the CO flow rate into a slurry reactor wasvaried while that of H2 was maintained constant.


parameters. After correlating these parameters with process

conditions, an explicit selectivity model can be obtained. The

simplest model for describing selectivity is the Anderson–

Schulz–Flory distribution, with the chain growth probability

(a-value) as the only parameter. Approaches have been pro-

posed to account for deviations from the ideal distribution.

The double-a model by Donnelly et al.209 assumes that two

types of catalytic sites or two types of mechanisms simulta-

neously form the observed product spectrum. However, the

three model parameters have a high degree of covariance

when estimated from experimental data. Furthermore, neither

the C1 and C2 selectivities, nor the olefin content of the prod-

uct spectrum can be predicted. Some of these limitations have

been addressed by considering a chain length-dependent

desorption model, which assumes that termination by desorp-

tion becomes increasingly more difficult as the chain length

increases.17 It has also been reported that the chain length

effects in the FT product spectrum, in particular the positive

deviation of methane and the negative deviation of ethylene,

can be explained by symmetry effects as accounted for by the

single event kinetic theory.210 Some have ascribed the chain

length-dependent deviations to secondary olefin reactions, but

many concerns remain over this approach.203 These include

the observation that secondary olefin reactions are much less

facile (almost negligible) over iron catalysts compared to

cobalt catalysts, yet the bend in the Anderson–Schulz–Flory

graph is much more pronounced with iron catalysts.

7.20.4.5 Mechanistic and Kinetic Implications

Despite the large number of kinetic and mechanistic studies on

FTS, there is still substantial uncertainty regarding the most

relevant steps in the reaction pathway(s). A variety of elementary

reaction steps have been proven to be realistically possible under

typical FTS conditions, while very few steps could be eliminated

with certainty. For example, and as described before, DFT calcu-

lations have shown that the coupling reactions of CHx fractions

are reasonably facile, which lends support to the steps of chain

growth as proposed by the carbide mechanism. However, DFT

calculations have also shown that each of the steps required for

the CO insertion mechanism is energetically feasible, while the

surface science approach has demonstrated experimentally that

the scission of the C–O bond of a CO molecule into which an

alkyl group has been inserted is a facile reaction, even on a low

reactivity cobalt surface.166

The current inability to clearly discriminate between rival

mechanisms partly stems from the overlap between the pro-

posed reaction pathways. For instance, the two most popular

mechanisms are not mutually exclusive with respect to each

other. The carbide mechanism has to assume direct CO inser-

tion as a termination step at least in order to explain the

observed formation of oxygenates in the FTS. On the other

hand, the initiation step in the CO insertion mechanism is

similar as for the carbide mechanism. The most plausible con-

clusion currently is therefore that a variety of reaction pathways

simultaneously contribute to the overall synthesis. The question

still remains though whether one pathway is dominant over the

rest and individually determines the bulk of the observed behav-

ior of the system, or whether two or more parallel pathways

have similar contributions to the overall kinetics. The answer to

this question may not even be absolute, as it may depend on

what aspect of the reaction is of interest. Hypothetically speak-

ing, if the carbide mechanism predominates in the formation of

olefins and paraffins (the main products of the synthesis), it

may well be accurate to describe the overall rate of syngas

conversion by only considering this mechanism. However,

even in such an event, CO insertion cannot be ignored if the

object is to model oxygenate formation.

Despite all the foregoing uncertainty, some consistencies

have also emerged from the studies performed in the various


disciplines. For example, the results of DFT calculations suggest

that unassisted CO dissociation readily occurs on the more

open (high reactivity) metal surfaces, while hydrogen-assisted

CO dissociation would be required on the close packed (low

reactivity) surfaces.179,181,183,185,186 As described in the section

on surface science above, unassisted CO dissociation is not

easy on close packed surfaces, while it is facile over open

surfaces. It has further been found that HxCO species are

more inclined to dehydrogenate than to undergo C–O bond

cleavage on cobalt surfaces. During a macrokinetic study on an

actual cobalt-FT catalyst, models assuming hydrogen-assisted

CO dissociation failed comprehensively, while the preferred

model based on unassisted CO dissociation could describe the

experimental data over a range of commercially relevant con-

ditions. Together all these findings suggest that, in the case of

the cobalt-based FT synthesis, the main pathway in the conver-

sion of CO to a CHx species proceeds via unassisted cleavage of

the C–O bond.

To the contrary, it is known that the carbiding of iron cata-

lysts substantially decreases the reactivity of iron surfaces. There-

fore, one may well expect hydrogen-assisted CO dissociation to

predominate over iron-FT catalysts,199 which are in the carbided

state under actual synthesis conditions. The most preferred

macrokinetic model for iron is indeed consistent with a CO

dissociation step that occurs via interaction with hydrogen.

A further consistency that is steadily emerging relates to the

most likely nature of the species responsible for chain propaga-

tion in terms of the carbide mechanism. Originally, it was

believed (not necessarily based on strong evidence) that these

species are quite saturated with hydrogen, that is, that the grow-

ing intermediate is a CH3–CH2� � �CH2 species, while the mono-

mer being added is a CH2 species.195 DFT calculations have

shown that reactions between intermediates that are leaner in

hydrogen (e.g., CH¼CH2 and CH) are energetically more favor-

able than reactions between more hydrogen-saturated

species.197 In line with this, it has been found during surface

science experiments the coupling of two CH species to from

acetylene is facile over nickel catalysts.168,211Further support is

provided by the steady state isotopic transient kinetic analysis

(SSITKA) study of Govender,212 who concluded from H–D

switching experiments that the C2H species is the only abundant

C2 intermediate on the fully carbided, working iron-FT catalyst

surface. Therefore, even though the carbide mechanism as a

whole cannot be discarded as a prominent reaction pathway

for the FTS, it seems unlikely that it proceeds in the form that

was originally proposed. This has particular significance for the

termination toward olefins, since it implies that a hydrogen

abstraction is not required (possibly even a hydrogen addition).

7.20.5 Conclusion

The FTS represents proven technology, which has secured its

position in modern energy technology. Originally used to

convert coal into liquid fuels, nowadays the emphasis is on

monetizing natural gas, by converting it to diesel fuel, waxes,

and naphtha. It is expected that some 500000 barrels of fuel

per day will be produced using Fischer–Tropsch technology by

2013. Although small in comparison to the 85 million barrels

of crude oil that are produced daily, the 0.5 million daily

barrels of synfuels is undoubtedly significant, particularly

locally where the production takes place.

The technology has much potential for wider use, for exam-

ple, in emerging economies, or at a smaller scale in the utili-

zation of biomass. Interesting applications of FTS have been

proposed for conversion of remote natural gas at off-shore oil

production locations.

Both GTL and BTL can be important tools in strategies

aimed at reduction of CO2 emissions. CTL technology is clearly

disadvantaged here, and will in the future have to be combined

with CO2 sequestration technology. The rapid increase in dis-

coveries of shale gas (in, for example, USA and Canada) can

also provide the GTL industry with a significant boost.

Although proven technology, FTS continues to pose chal-

lenges from an industrial perspective. Stability improvement of

the catalysts is an important aspect, but also selectivity

improvement would be very advantageous. Economically one

would like to have the highest possible C5þ and the lowest

possible CH4 selectivity, because recycling of CH4 means that

the carbon atoms involved have to go through the expensive

syngas generation more than once, with the associated effi-

ciency losses. Syngas production is the most expensive part of

a GTL plant; it accounts for 40–60% of the capital investments.

Increased research efforts on reducing the costs of syngas pro-

duction will make XTL projects even more viable. Although

new XTL facilities require large capital investments and are

dependent on the price ratio of crude oil to natural gas, in

the long term they are expected to be economically successful.

From a more academic perspective, understanding the

mechanism of the FTS has been and will be a challenge. It is

more and more realized that mechanisms may differ with

conditions and catalysts. It is highly unlikely that one unique

mechanism can account for all different forms of FTS. Molec-

ular modeling represents a very important tool for getting

mechanistic insight, but the problem is that experimental val-

idation of its predictions at the level of elementary steps is very

difficult to achieve, as the opportunities for relevant surface

science experiments are limited. Mechanistic studies aimed at

describing FTS selectivity from first principles are in their

infancy and have a long way to go before accurate predictions

can be expected.

Describing the physical/chemical state of the catalysts

under reaction conditions is another field where significant

progress has been booked, but major advances would still be

very welcome. The advent of in situ imaging tools in combina-

tion with realistic catalysts,213 as well as the use of planar

model catalysts in simulated environments,153 has proven

promising and will almost certainly lead to improved insight

in the relation between catalyst properties on the nanoscale

and performance in the reaction.

The FTS is therefore expected to remain an inspiring source

of industrial and academic research for many years to come.

For a related chapter in this Comprehensive, we refer to

Chapter 7.01

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