Search for new innovating catalyst for the Fischer-Tropsch … · Search for new innovating catalyst for the Fischer-Tropsch synthesis Leonor Duarte Mendes Catita Thesis to obtain

Search for new innovating catalyst for the Fischer-Tropsch

synthesis

Leonor Duarte Mendes Catita

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors: Dra. Dominique Decottignies (IFPEN)

Prof. Carlos Henriques (IST)

Examination Committee

Chairperson: Prof. Maria Filipa Gomes Ribeiro (IST)

Supervisor: Prof. Carlos Manuel Faria de Barros Henriques (IST)

Member of the Committee: Prof. José Madeira Lopes (IST)

September 2014

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“A ciência é o querer adaptar o menor sonho ao maior.”

Fernando Pessoa


v

Acknowledgments

Firstly, I would like to thank Professora Filipa Ribeiro to continuing the partnership between IST

and IFPEN, allowing us, students, to have the opportunity to work in this prestigious institution given us

a great head start into our future careers. I would also like to thank Tiago Sozinho for his support during

the beginning of this internship as well as to Joana Fernandes for her support and concerns with all

Portuguese students.

I thank Mister Tivadar Cseri (director of the Catalysis by Metals and Acid-Base Solids Department)

for the kindly welcome, which I was received with in this department.

I am very grateful to my supervisors at IFPEN, Dominique Decottignies and Antoine Fecant for their

kindness, friendship, support and availability since my arrival until my departure from IFPEN, it exceed

all my expectations. I have to thank you for all your patient in explaining me all my doubts and definitely

it was your contribution that made this a successful work. To Professor Carlos Henriques, thanks for

your availability and interest in my work.

I would also like to thank to everyone in my department for the warm and rapid integration. But is

especially to Marie Velly and Eugènie Rabeyrin that I would like to thank you the most. The availability

that you have both showed, your friendship, your professionally and personally support made my

integration and learning more easier. And thank you for all your patient when I was not able to express

myself in French. I would also like to thank Adrien Berliet for helping me when I needed, to Romain

Chenevier and also to Charles Leroux, although you were not directly related with my project you helped

me every time I needed.

To everyone I have worked in Physics and Analysis Division, Nathalie Crozet, Isabelle Clemencon

and Frederic Filali, I would like to thank your availability to teach me and to listen my doubts. Also, Denis

Roux and Nicolas Girod, I would like to thank you for attending my requests and to explain me my

questions.

To my IFPEN collegues, Svetan, Mathhieu, Remi and Tarek thank you for your warm integration,

you definitely made the difference for being here. To my Portuguese friends, Bernardo, Carolina, Marisa,

Ana Rita, Sónia and Miguel, your support during this internship, your happiness and your friendship was

a greater contribution during this time. To Pedro, for all you advices about IFPEN and for all your

availability in helping us before we arrived here. And to Filipa, although we were far, your support and

your visit here in Lyon was very important for me.

I would also like to thank my family, especially my mother and father: your efforts, your

encouragements, your leanings over all these years made me get up here and to become you I am

today. You have no idea how it was important to see you every day, even if it was just for a computer

screen. And Ruben, thank you for everything, for all your support, it would have not be the same without

you.


vii

Resumo

Este trabalho teve como objetivo estudar a influência do tamanho da partícula de cobalto no

desempenho de um catalisador à base de cobalto (8 %(m/m)), suportado em sílica-alumina comercial

(Siralox5, contento 5 %(m/m) SiO2), na síntese de Fischer-Tropsch. Assim, foram realizados diferentes

métodos de preparação de catalisadores, nomeadamente impregnação a seco e deposição-

precipitação para adição de cobalto, usando diferentes precursores do respetivo metal e diferentes

tratamentos térmicos (calcinação e steaming). Todos os catalisadores foram caracterizados por

Fluorescência Raios-X, Difração Raios-X, Microscopia Eletrónica de Transmissão e Redução a

Temperatura Programada, de forma a determinar o teor em cobalto, o tamanho da partícula de cobalto

e a redutibilidade do catalisador.

Obtiveram-se partículas de cobalto entre 2 a 32 nm. Constatou-se que a redutibilidade dos

catalisadores depende, tal como era esperado, do tamanho da partícula de cobalto. Catalisadores com

partículas menores que 5 nm resultaram em baixas taxas de redução, mesmo nos casos em que foi

adicionada platina. Por outro lado, catalisadores com partículas maiores mostraram elevadas taxas de

redução.

Os testes catalíticos foram realizados em reatores tipo slurry, a 220°C, 20 bar e razão H2/CO igual

a 2, com os catalisadores mais promissores, tendo em conta o tamanho da partícula e a taxa de

redução.

Concluiu-se que para o intervalo de partículas testado (8-32 nm), a atividade do catalisador,

medida em termos TOF, tende a estabilizar com o aumento do tamanho da partícula de cobalto. A

mesma tendência foi observada em relação à seletividade em C5+ e CH4.

Palavras-chave: Fischer-Tropsch; Tamanho da partícula de cobalto; Atividade; Seletividade


ix

Abstract

The aim of this work was to study the impact of cobalt particle size on the performance of cobalt

Fischer-Tropsch catalysts (8 wt% Co) supported on a commercial silica-alumina support (Siralox5,

containing 5 wt% of SiO2). For that purpose, different preparation methods were selected, by either

wetness impregnation or homogenous deposition precipitation for cobalt addition, using different metal

precursors and different thermal treatments (calcination or steaming). All prepared catalysts were

characterized by X-Ray Fluorescence, X-Ray Diffraction, Transmission Electron Microscopy and

Temperature-Programmed Reduction in order to determine cobalt content, cobalt particle size and

catalyst’s reducibility.

A range of Co particle size was obtained between 2 and 32 nm. Catalyst’s reducibility, as expected,

depends strongly on cobalt particle size. Actually, catalysts having less than 5 nm of cobalt particle size

were not reducible at all, not even when promotion with platinum was done. On the other hand, catalysts

with larger particles have shown reduction rates much higher.

Catalytic tests were performed in slurry reactors at 220°C, 20 bar and H2/CO equal to 2 with the

most promising catalysts, concerning their particle size and reduction rate.

For the range of particle size tested (8-32 nm), catalyst activity, measured by meanings of TOF,

shows a tendency to stabilize with increasing cobalt particle size. The same behavior was found for C5+

and CH4 selectivity.

Key words: Fischer-Tropsch; cobalt particle size; activity; selectivity


xi

Table of contents

Acknowledgments ....................................................................................................................................v

Resumo .................................................................................................................................................. vii

Abstract .................................................................................................................................................... ix

Table List ............................................................................................................................................... xiii

Figure List ............................................................................................................................................... xv

Abbreviations List .................................................................................................................................. xix

1 Introduction ...................................................................................................................................... 1

2 Bibliographic Study .......................................................................................................................... 3

2.1 Fischer-Tropsch Synthesis ...................................................................................................... 3

2.2 Fischer-Tropsch Catalysts ....................................................................................................... 9

2.3 Characteristics of Fischer-Tropsch Catalysts ........................................................................ 14

2.4 Preparation methods of Fischer-Tropsch Catalysts .............................................................. 19

2.4.1 Active phase deposition on support ............................................................................... 19

2.4.2 Post Treatment and Activation ...................................................................................... 22

3 Objective of the study .................................................................................................................... 25

4 Experimental Work ........................................................................................................................ 27

4.1 Support Characteristics ......................................................................................................... 27

4.2 Preparation Methods ............................................................................................................. 28

4.2.1 Wetness Impregnation ................................................................................................... 28

4.2.2 Steaming ........................................................................................................................ 32

4.2.3 Deposition-Precipitation ................................................................................................. 34

4.3 Characterization Methods ...................................................................................................... 36

4.3.1 X-Ray Fluorescence ...................................................................................................... 36

4.3.2 N2 adsorption-desorption ............................................................................................... 36

4.3.3 Mercury porosimetry ...................................................................................................... 37

4.3.4 Temperature Programmed Reduction ........................................................................... 37

4.3.5 X-Ray Diffraction ............................................................................................................ 38

4.3.6 Transmission Electronic Microscopic ............................................................................ 39

4.4 Catalytic Tests ....................................................................................................................... 39

xii

5 Results and Discussion ................................................................................................................. 43

5.1 Physic-Chemical properties of synthesized cobalt catalysts ................................................. 43

5.1.1 Determination of cobalt content by XRF and oxide phases by XRD ............................. 43

5.1.2 Determination of cobalt particle size.............................................................................. 47

5.1.3 Textural properties ......................................................................................................... 55

5.1.4 Catalyst reducibility from TPR on oxide catalysts ......................................................... 57

5.1.5 Catalyst Reduction Rates from TPR on reduced catalysts ........................................... 64

5.1.6 Global summary of physic-chemical properties ............................................................. 68

5.2 Catalytic performances of chosen catalysts .......................................................................... 69

5.2.1 Evaluation of activity ............................................................................................................. 69

5.2.1 Evaluation of selectivity ................................................................................................. 72

5.2.2 Subjected tendencies and summary ............................................................................. 74

6 Conclusion and Future Work ......................................................................................................... 75

7 References .................................................................................................................................... 77

8 Appendix ........................................................................................................................................ 81

8.1 TEM Characterization .................................................................................................................. 81

8.2 Textural properties ....................................................................................................................... 83

xiii

Table List

Table 1 – Principal characteristics of the support.................................................................................. 27

Table 2 – List of catalysts prepared by wetness impregnation ............................................................. 29

Table 3 – List of catalysts prepared by steaming .................................................................................. 32

Table 4 – Solutions prepared for deposition-precipitation method ........................................................ 35

Table 5 – Determination of Co content (wt%) by XRF for catalysts prepared by wetness impregnation

............................................................................................................................................................... 43

Table 6 - Determination of Co content (wt%) by XRF for catalysts prepared by steaming ................... 45

Table 7 - Determination of Co content (wt%) by XRF for catalysts prepared by deposition-precipitation

............................................................................................................................................................... 46

Table 8 – Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD for

catalysts prepared by wetness impregnation ........................................................................................ 48

Table 9 - Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD of

catalysts prepared by steaming ............................................................................................................. 49

Table 10 - Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD for

catalysts prepared by deposition-precipitation ...................................................................................... 50

Table 11 – Principal results for cobalt particle size and dispersion determined by TEM for cobalt iodide

and cobalt acetate catalyst .................................................................................................................... 51

Table 12 - Principal results of cobalt particle size determined by TEM for cobalt iodide and cobalt acetate

catalyst ................................................................................................................................................... 53

Table 13 – Principal results for the average particle size obtained from TEM and XRD ...................... 54

Table 14 – Porous volume and BET surface for catalysts (T1;D1;R1) and (T2 ;D2 ;R1) prepared by

steaming ................................................................................................................................................ 57

Table 15 – Rate of reduction for reduced catalysts prepared by wetness impregnation ...................... 66

Table 16 - Rate of reduction for reduced catalysts prepared by steaming ........................................... 67

Table 17 – List of catalysts to perform in catalytic test .......................................................................... 69

Table 18 – Results of dispersion and TOF ............................................................................................ 70

Table 19 – Principal results obtained for catalytic tests ........................................................................ 74


xv

Figure List

Figure 1- Growth in total primary energy demand [1] .............................................................................. 3

Figure 2 – FT process: production and conversion of syngas and FT products upgrading .................... 3

Figure 3 - FT stepwise growth process, where α is the probability of chain growth and d corresponds to

desorbed products [8] .............................................................................................................................. 6

Figure 4 – Slurry bubble column reactor ................................................................................................. 8

Figure 5 – Molecular average weight of hydrocarbons produced in FT synthesis [12] ........................... 9

Figure 6 – Compositional overview of modern Co-based FT catalysts [14] .......................................... 11

Figure 7 – Scheme of chain polymerization for FT synthesis (adapted from [15]) ................................ 11

Figure 8 – Hydrocarbon selectivity for various hydrocarbons fractions as a function of α [16] ............. 12

Figure 9 – Product distribution as a function of probability of chain growth α [16] ................................ 12

Figure 10 – Influence of Co particle size on the (a) TOF and (b) C5+ selectivity at 35 bar, 210°C, H2/CO

= 2 in black [23], data obtained from [20] at 250oC are plotted in grey (reproduced from [23]) ............ 15

Figure 11 – Clusters of particles of active precursor within the support [30] ........................................ 21

Figure 12 – Scheme of the catalysts preparation by wetness impregnation ......................................... 28

Figure 13 - Wetness impregnation apparatus ....................................................................................... 30

Figure 14 – Calcination profile for catalysts prepared by wetness impregnation .................................. 30

Figure 15 – Calcination profile for cobalt acetate catalyst (LD-E2-1) .................................................... 31

Figure 16 – Reduction profile of catalysts prepared by wetness impregnation ..................................... 31

Figure 17 - Scheme of catalysts preparation by steaming .................................................................... 32

Figure 18 – Steaming reactor ................................................................................................................ 33

Figure 19 – Generic steaming profile .................................................................................................... 33

Figure 20 – Preparation of catalysts by deposition-precipitation ........................................................... 34

Figure 21 – Tornado apparatus ............................................................................................................. 35

Figure 22 – Simple scheme of the catalytic tests unity 5 (TCD - thermal conductivity detector; FID - flame

ionization detector) ................................................................................................................................ 40

Figure 23 – XDR diagram of the reference catalyst Co(NO3)2.6H2O .................................................... 44

Figure 24 - XDR diagram of the cobalt nitrate with sodium nitrite having NO2-/Co2+ equal to 0,5 (LD-E2-

3) and equal to 1 (LD-E2-4) and reference catalyst .............................................................................. 44

Figure 25 - XDR diagram of the cobalt acetate (LD-E2-1), cobalt iodide (LD-E2-2), cobalt nitrate with

ethylene glycol (LD-E1-11) and reference catalyst ............................................................................... 45

Figure 26 - XDR diagram of (T1; D1; R1) catalyst prepared by steaming (LD-S-9) and reference catalyst

............................................................................................................................................................... 46

Figure 27 - XDR diagram of the cobalt nitrate prepared by deposition-precipitation under air atmosphere

(LD-DP-2) and reference catalyst .......................................................................................................... 47

Figure 28 - TEM micrograph of the (a) CoI2 and (b) Co(CH3COO)2.4H2O catalysts ............................. 50

Figure 29 - TEM micrograph for Co(NO3)2/NaNO2 (1): (a) low magnification image and (b) higher

magnification image ............................................................................................................................... 51

xvi

Figure 30 - Cobalt metallic particle size distribution for (a) Co(CH3COO)2.4H2O and (b) CoI2 catalysts

............................................................................................................................................................... 52

Figure 31 - Cobalt metallic particle size distribution for the Co(NO3)2/NaNO2 (1) catalyst ................... 52

Figure 32 - TEM micrograph for (T1; D1; R1) catalyst (a) low magnification image and (b) higher


Figure 33 - TEM micrograph for (T2; D2; R1) catalyst: low magnification image and (b) higher


Figure 34 - Cobalt metallic particle size distribution for (a) (T1;D1;R1) and (b) (T2;D2;R1) catalysts .. 54

Figure 35 - Nitrogen adsorption as a function of relative pressure over (T1;D1;R1) catalyst and support

............................................................................................................................................................... 55

Figure 36 – Pore size distribution of the support and of catalysts (T1;D1;R1) and (T2;D2;R1) determined

by volume of nitrogen absorbed ............................................................................................................ 56

Figure 37 – Volume of Hg absorbed as a function of pore diameter of catalysts prepared at (T1;D1;R1)

and (T2;D2;R1) conditions ..................................................................................................................... 56

Figure 38 – Porous distribution obtained from Hg porosimetry of (T1;D1;R1) and (T2;D2;R1) catalysts

prepared by steaming ............................................................................................................................ 57

Figure 39 - TPR profile for the reference catalyst ................................................................................. 58

Figure 40 - TPR profiles for catalysts prepared by wetness impregnation: reference catalyst,

Co(NO3)2/NaNO2 (0.5) and Co(NO3)2/NaNO2 (1) .................................................................................. 59


Co(CH3COO)2, CoI2 and Co(NO3)2/EG ................................................................................................. 60

Figure 42 - Comparison of TPR profiles between (a) CoI2 and CoI2/Pt and (b) Co(CH3COO2).4H2O and

Co(CH3COO2).4H2O/Pt.......................................................................................................................... 61

Figure 43 – TPR profile for catalyst prepared by steaming (T1;D1;R1) ................................................ 61

Figure 44 - TPR profiles for catalysts prepared by Steaming at T2 with different durations (D1<D2<D3)

and ratio H2O/Air (R1<R2) ..................................................................................................................... 62

Figure 45 - TPR profiles for catalysts prepared by Steaming at T3 with different ratios H2O/Air (R1<R2)

............................................................................................................................................................... 63

Figure 46 - Comparison between the oxide and reduced TPR profile for the reference catalyst ......... 64

Figure 47 - Comparison between the oxide and reduced TPR profile for the Co(NO3)2.6H2O/NaNO2

catalyst ................................................................................................................................................... 65

Figure 48 - Comparison between the oxide and reduced TPR profile for (a) CoI2 and (b)

Co(CH3COO)2.4H2O catalysts ............................................................................................................... 65

Figure 49 - Comparison between the oxide and reduced TPR profile of catalyst prepared by steaming

at (T1; D1; R1) conditions ...................................................................................................................... 66

Figure 50 - Comparison between the oxidized and reduced TPR profile for (a) (T2;D3;R1), (b)

(T2;D2;R1) and (c) (T2; D2; R2) catalyst .............................................................................................. 67

Figure 51 – CO consumption during catalytic test................................................................................. 70

Figure 52 – Relation between cobalt particle size and activity, recorder at the same time on stream (70h)

............................................................................................................................................................... 71

xvii

Figure 53 – TOF as a function of cobalt particle size comparing with the literature ............................. 71

Figure 54 – Average CH4 and C2-C4 selectivity as a function of Co particle size .................................. 72

Figure 55 – Average C5+ selectivity as a function of Co particle size .................................................... 72

Figure 56 - CH4 selectivity as a function of cobalt particle size – comparison with the literature ......... 73

Figure 57 – C5+ selectivity as a function of cobalt particle size – comparison with the literature .......... 73


xix

Abbreviations List

α – probability of chain growth IWI – Incipient Wetness Impregnation

Δ – Difference operator LTFT – Low-Temperature Fischer Tropsch

ASF – Anderson-Schulz-Flory MM – Molecular Mass (g/mol)

BET – Brunauer-Emment-Teller n – stoichiometric number

bcc – body-centered cubic P – Pressure (bar)

BJH – Barret-Joyner-Halenda PZC – Point of Zero Charge

BTL – Biomass to Liquid Q – Volumetric flow (NL/h)

Cx - Hydrocarbons with x or more carbons T – Temperature (oC)

CTL – Coal to Liquid TCD – Thermal Conductivity Detector

D – Duration TEM – Transmission Electron Microscopy

d – diameter (nm) TOF – Turnover Frequency (s-1)

EG – Ethylene Glycol TON – Turnover Number

fcc – face-centered cubic TPR – Temperature-Programmed Reduction

FID – Flame Ionization Detector R – Ratio; Rate

FT – Fischer-Tropsch RR – Reduction Rate (%)

FWHM – Full-Width Half-Maximum V – Volume (mL)

GTL – Gas to Liquid Vm – Molar volume (L/mol)

H – Enthalpy (kJ/mol) wt% - Mass fraction (%)

hcp – hexagonal-closed packed XRD – X-Ray Diffraction

HDP – Homogeneous Deposition Precipitation XRF – X-Ray Fluorescence

HTFT – High-Temperature Fischer Tropsch XTL – Gas, Coal or Biomass to Liquid


1

1 Introduction

Energy is fundamental for all sectors of modern economies and is directly related to well-being and

prosperity across the world. Nowadays, the world fuel consumption is highly depended on fossil fuels.

However, due to the changing environment of demand and supply of fossil energies and to the more

stringently upcoming aspects of pollution control, of cleanliness of the automotive fuels and of energy

saving, it is important to promote alternative energy sources. Besides that, it is expected that energy

demand will increase significantly in the future and so that, meeting this growing demand in a safe and

environmentally responsible way is a challenge.

All these factors have contributed in renewed interest in XTL (gas, coal or biomass to liquid)

processes via Fischer-Tropsch synthesis. The FT process involves conversion of syngas (mixture of CO

and H2), which is obtained from gas, coal or biomass, to produce heavy hydrocarbons that could be

converted to fuels such as diesel, gasoline and jet fuels by hydrocracking or hydroisomerization, for

instance. The obtained products have low aromatic and sulfur contents, which is important from an

environmental point of view.

Catalyst is a vital part for any industrial FT process. Catalysts containing iron are employed in the

production of light hydrocarbons and olefins (high temperature FT – HTFT) and they are essential for

the conversion of syngas with low H2/CO ratio (<<2). Although, a more recent generation of FT

processes involves syngas with higher H2/CO ratio, which is produced from natural gas and focus on

the production of middle distillates and waxes (low temperature FT – LTFT); these new processes mostly

use cobalt supported catalysts.

Cobalt catalysts represent the optimal choice for low temperature FT processes due to their higher

stability, higher per single pass conversion, higher productivity and relatively smaller negative effect of

water on conversion. Nowadays, cobalt catalysts contain small amounts of a second metal promoter

(typically noble metals) and oxide promoter. The active phase is usually supported by an oxide with high

surface area such as silica, alumina or titanium.

The catalytic performance of cobalt supported FT catalysts depends on catalyst support and

support texture, cobalt precursor, cobalt phase composition, preparation method as well as the

conditions of thermal treatments and promotion with noble metals. However, in case of cobalt FT

catalysts, the effects of cobalt particle size on catalyst activity and selectivity are of prime scientific and

industrial importance.

Activity and selectivity of cobalt FT catalysts have been shown to be size dependent. In general for

cobalt catalysts, the decrease of the particle size below 10-11 nm is accompanied by a drastic decrease

of the reaction rate. Also, with decreasing metal size, the methane selectivity increases, with a

concurrent decrease in selectivity to carbon chains containing 5 or more carbons.

Thus, the present work pretends to report on the size sensitivity of cobalt supported FT catalysts

and to understand its impact on the catalyst performance.

For that, cobalt catalysts supported on silica-alumina, having 8 wt% of cobalt were prepared by

different preparation methods (wetness impregnation and homogenous deposition-precipitation), using

different thermal treatments (calcination and steaming) in order to achieve a wide range of particles size.

2

These catalysts were then characterized to determine their particle size and reducibility, being the

most promising ones tested in a slurry reactor at real FT operating conditions (220°C, 20 barg, H2/CO =

2).

Firstly, this report reviews the state of art of FT synthesis, concerning FT catalysts, its important

characteristics and its preparation methods, concerning active phase deposition, post-treatments and

activation. Then, in chapter 3, the objectives of this work are presented.

In chapter 4 is described the different preparation methods performed as well as the

characterization methods. A brief description of the catalytic tests unity is also presented.

In chapter 5 are presented the results of physic-chemical properties and of catalytic performance.

The last chapter (chapter 6) will be for conclusions and future trends.

3

2 Bibliographic Study

2.1 Fischer-Tropsch Synthesis

Nowadays, the world fuel and chemical production is based predominantly on fossil fuels: more

than 80% of the world fuel consumption is provided by natural gas, coal/peat and crude oil [1], Figure 1.

As reserves of crude oil are depleted, the price of crude rises [1] and the environmental concerns are

more and more important, it is essential to have an alternative production of fuel, for which Fischer-

Tropsch synthesis could play an important role.

Figure 1- Growth in total primary energy demand [1]

The FT synthesis is the key step of the XTL processes for converting synthesis gas from methane,

coal (non-petroleum feedstocks) or even biomass to liquid fuels. When using natural gas for synthesis

gas production, the process is called GTL (gas to liquid), whereas CTL and BTL processes used,

respectively, coal and biomass as feedstock.

The main hydrocarbon products of FT synthesis are alkanes (or paraffins), alkenes (or olefins) and

oxygenated products (namely alcohols). In more detail, this process allows to obtain methane (C1),

petroleum gas (C2-C4), gasoline (C5-C11), diesel (C12-C20) and wax (C21+), being the latter valorized into

gasoline and diesel via hydrocracking. However, the formation of large amounts of methane as well as

C2-C4 fraction is undesirable, since it represents a loss of carbon that could be converted directly in liquid

products [2] [3]. The following figure depicts all the steps that occurs in XTL process: production of

syngas from different feedstocks (which will be described later on), conversion of syngas into FT

products and final products separation and upgrading in order to obtain petroleum products (diesel and

kerosene, for instance).

Figure 2 – FT process: production and conversion of syngas and FT products upgrading

4

Historical Perspective

The first half of the 20th century was a period of great interest in FT synthesis due to efficient use

of coal, economical security and military constrains [4]. The first experiments on synthesis of

hydrocarbons from carbon monoxide and carbon dioxide were carried out in 1902 by Sabatier and

Senderens; the hydrogenation of these two compounds over nickel catalyst allowed the production of

methane [5].

In 1925, Hans Fischer and Franz Tropsch developed a process which could converted a mixture

of hydrogen and carbon monoxide over a metallic catalyst (as iron or cobalt) in hydrocarbons; this

process was then known as the Fischer-Tropsch synthesis [6].

After the Second World War, the importance of FT synthesis was mostly determined by energy

independence concerns, while the world economy was mostly oriented on oil consumption. At the same

period, numerous FT commercial units had been built in South Africa [4] [5].

In the seventies, there was a return of the interest in this process due to oil crisis and gasoline

shortage. Then, in the eighties, the problems of utilization of stranded gas, changes in fossil energy

reserves, environmental demands and the possible utilization of biomass has led to an increase of the

importance of FT synthesis [3].

In recent years, Fischer-Tropsch synthesis has experienced a resurgence of interest as a result of

resource utilization considerations and environmental concerns. Moreover, the depleting reserves and

increasing prices of crude oil combined with technological developments have been a mainspring for a

growing number of large FT applications [7].

Syngas production

Synthesis gas (or syngas) is a mixture of carbon monoxide and hydrogen produced from non-

petroleum feedstocks such as natural gas, coal and biomass.

Due to its availability, methane is preferred to coal for syngas production [8]. The capital cost of the

methane conversion plant is lower and the process is more efficient than with coal, for instance, since

this compound has a much lower hydrogen content when compared with methane.

There are three ways to convert natural gas into syngas: steam reforming, partial oxidation or

autothermal reforming [4] [9]. Steam reforming consists in converting light hydrocarbons into syngas, by

reaction with steam.

𝐶𝐻4 + 𝐻2𝑂 ↔ 𝐶𝑂 + 3𝐻2 ∆𝑟𝐻0(𝑇=298𝐾) = 206,1 𝑘𝐽/𝑚𝑜𝑙 (Eq. 1)

This reaction occurs at high temperatures (840-950oC). Besides hydrogen and carbon monoxide,

water, methane and carbon dioxide can also be reaction products. This reaction is endothermic and

syngas produced by steam reforming of methane is characterized by having a ratio H2/CO equal to 3.

Partial oxidation (Eq. 2) and (Eq. 3) is also directed for light hydrocarbons, but it can be applied to

heavier hydrocarbons. This reaction, which is exothermic, takes place at elevated temperatures (1200-

1500oC).

5

In the presence of oxygen and water vapor, which acts like a temperature regulator, partial oxidation

of hydrocarbons leads to syngas production. This syngas has a lower H2/CO ratio, ranging from 1 to

1,5.

𝐶𝐻4 +1

2𝑂2 ↔ 𝐶𝑂 + 2𝐻2 (Eq. 2)

𝐶𝑛𝐻2𝑛+2 + 𝑂2 ↔ 𝑛𝐶𝑂 + (𝑛 + 1)𝐻2 (Eq. 3)

Finally, autothermal reforming is a process which combines the advantages of both steam

reforming and partial oxidation: the required energy of the endothermic steam reforming is provided by

the exothermic oxidation reaction. The ratio H2/CO of the syngas produced depends on the reagent

used: if it is CO2, then H2/CO will be equal to 1 and if it is steam it will be 2,5.

In case of coal and biomass, syngas can be obtained by pyrolysis and direct gasification [4]. In

case of biomass, pyrolysis can be used to obtain carbon either in vegetable phase, liquid or gas phase,

depending on the operating conditions. In order to obtain the gas phase, pyrolysis is carried out at

elevated temperatures 800oC, while to obtain liquid phase the temperature is generally 500oC.

Concerning direct gasification, it consists on thermal cracking. Heavy hydrocarbons such as

mineral carbon, oils and vegetable carbon by reaction with steam (at temperatures higher than 1000oC)

can lead to syngas production (Eq. 4). Side reactions such as water gas shift reaction, CO methanation

and Boudouard equilibrium can also occur, leading to CO2, C, CH4, H2 and H2O production.

𝐶 + 𝐻2𝑂 ↔ 𝐶𝑂 + 𝐻2 (Eq. 4)

Reactions

The main products obtained by FT synthesis are alkanes (or paraffins), alkenes (or olefins) and

alcohols, which are given by the following reactions, respectively [5].

(2𝑛 + 1)𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+2 + 𝑛𝐻2𝑂 (Eq. 5)

2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛 + 𝑛𝐻2𝑂 (Eq. 6)

2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+1𝑂𝐻 + (𝑛 − 1)𝐻2𝑂 (Eq. 7)

Besides these main reactions, a lot of secondary reactions can also take place, such as

hydrogenation of olefins, isomerization of alkanes, dehydration of alcohols, disproportionation of CO

(Boudouard equilibrium) and methanation (Eq. 8). Moreover, part of the water formed during FT

reactions might be converted with carbon monoxide into hydrogen and carbon dioxide by the water gas

shift reaction, which is another important side reaction (Eq. 9) [5]. CO2 formed in this reaction is very

hard to hydrogenate during FT synthesis.

𝐶𝑂 + 3𝐻2 ↔ 𝐶𝐻4 + 𝐻2𝑂 ∆𝑟𝐻0(𝑇=298𝐾) = −206,2 𝑘𝐽/𝑚𝑜𝑙 (Eq. 8)

𝐶𝑂 + 𝐻2𝑂 ↔ 𝐶𝑂2 + 𝐻2 ∆𝑟𝐻0(𝑇=298𝐾) = −41,1 𝑘𝐽/𝑚𝑜𝑙 (Eq. 9)

Fischer-Tropsch reactions can be described as polymerization reactions that occur at the surface

of the catalyst. These reaction are not a classic polymerization since the monomer is produced in situ

from carbon monoxide and hydrogen.

6

As it is a catalytic reaction, the initial steps are activation of carbon monoxide and hydrogen in the

surface of the metal by an adsorption step [10]. In more detail, the hydrogenation of CO leads to CHx

surface species which act as monomers and chain initiators in the chain growth processes.

As it was mentioned before, the initial step involves the dissociation of CO to form chemisorbed

carbon, which is hydrogenated to surface methyl and methylene groups in subsequent steps. Chain

growth occurs by stepwise addition of C1 monomers to surface alky group, Figure 3 [8].

At each stage of growth the adsorbed hydrocarbon species has the option of desorbing as paraffins

and olefins or being hydrogenated to form primary FT products or of adding another monomer to

continue the chain growth. Chain termination can occur either by β-hydrogen removal, forming α-olefins

or by α-hydrogen addition to form n-paraffins. CO insertion into surface alkyls can also occurred and

leads to the formation of predominantly primary alcohols.

Thermodynamics

The reactions that occurs in FT synthesis are strongly exothermic. In the temperature range of 190

and 400°C, alkanes (or paraffins) are the most thermodynamically stable products. Above 400°C,

alkenes (or olefins) become more stable than alkanes, while alcohols remaining the less stable products

[11]. Nevertheless, within the usual range of temperature for FT synthesis (200 – 400°C), the formation

of methane and the disproportionation of CO are more thermodynamically favorable than the formation

of long alkanes chain.

Concerning pressure conditions, these synthesis reactions lead to a decrease in the number of

molecules. So that, these reactions are favorable by an increase in pressure [11], according to the Le

Chatelier’s principle.

Figure 3 - FT stepwise growth process, where α is the probability of chain growth and d corresponds to desorbed

products [8]

7

This means that if the pressure is high, the selectivity in methane decreases, while the selectivity

in alkanes with long chain increases. Although, when the pressure is too high, the formation of alkenes

predominates over the formation of alkanes.

Products

As it was mentioned before, FT reaction produces a wide range of products. However, the most

desired products are gasoline and diesel. Depending on the reactor and technology used as well as on

the catalyst, which will be described after, it is possible to maximize either the production of gasoline or

the production of diesel.

FT process can produce almost a half straight run gasoline, but also propene and butene using the

high capacity of fixed fluidized bed reactors at about 340oC, via iron catalysts. These two last products

can be oligomerised to gasoline and because the oligomers are highly branched it has a high octane

value. However, the straight run gasoline has a low octane number due to its high linearity and low

aromatic content [8]. It would need sever reforming to be converted into high octane gasoline, preferably

it could be steam cracked as it would produce a high yield of ethylene.

On the other hand, the high linearity and low aromatic content are very positive for production high

cetane diesel fuel. So, using a high capacity slurry bed reactors with cobalt catalysts and operated to

maximize wax production is possible to obtain 20% of selectivity in straight run diesel. After

hydrotreatment, its cetane number is about 75. The heaviest cut accounts for about 45%-50% of the

total amount of FT products and mild hydrocracking produces a large proportion of high quality diesel,

virtually free of aromatics. The final diesel pool has a cetane number of about 70. Since the market

usually requires a cetane number of 45, FT diesel can either be used in areas where there are very tight

constrains on diesel quality or as blending stock to upgrade lower quality diesel fuel [8].

FT synthesis can also produce a large amount of linear α-olefins and as petrochemicals they can

be sold at much higher prices than fuels. Typically, olefin contents of C3, C5-C6 and C13-C18 fractions are

85, 70 and 60%, respectively. Ethylene goes to production of polyethylene, polyvinylchloride and

propylene to polypropylene and acrylonitrile. The extracted and purified C5-C6 linear α-olefins are used

as comonomers in polyethylene production. The longer chain olefins can be converted into linear

alcohols by hydroformylation, which are used in the production of biodegradable detergents [8].

FT reactor and technology

The choice of the reactor and the type of technology of FT synthesis depends on the desired

products. In order to choose the right reactor and the right technology for FT synthesis, it is necessary

to take into account several factors: intrinsic chemical kinetics, interphase mass transfer, heat transfer

and flows’ hydrodynamics [5].

Presently, there are two FT operating modes: HTFT for high-temperature FT (300-350oC) process

and LTFT for low-temperature (200-240oC) FT process. The first one, with iron-based catalysts, is used

for the production of gasoline and linear low molecular mass olefins.

8

In more detail, this high-temperature process leads to yield hydrocarbons in the C1-C15 hydrocarbon

range and is primarily used to produce liquid fuels, despite the fact that a number of valuable chemicals

(α-olefins) can be extracted from the crude synthetic oil.

Oxygenates in the aqueous stream are separated and purified to produce alcohols, acetic acid,

and ketones including acetone, methyl ethyl ketone and methyl isobutyl ketone [5]. For HTFT, the

reactors mostly used are circulating bed and fluidized bed, which lead to gaseous products.

The low-temperature process with either iron or cobalt catalysts is used for the production of linear

long-chain hydrocarbon waxes and paraffins; the diesel fuels that are produced in this process are

sulfur-free. Most of FT technologies developed in the last two decades are based on LTFT process and

these new LTFT processes have involved syngas with a high H2/CO ratio. For LTFT, normally

multitubular fixed bed or slurry reactors are commonly used, in order to maximize liquid middle distillate

fractions and hydrocarbon waxes production [5].

Since nowadays the objective of FT reaction is mainly to achieve hydrocarbons with long chain, it

is important to focus on LTFT as well as on slurry reactors. Slurry reactors are triphasic reactors, called

slurry bubble column, coming from the fact that catalyst should be suspended in the liquid product,

forming the slurry phase in which bubbles of syngas have to pass through. In this type of reactors (Figure

4), the wax produced accumulates inside the reactor and so the net wax produced needs to be

continuously removed from the reactor.

In more detail, the reacting feed gas is introduced at the bottom of the reactor through spar gears,

forming bubbles, which help keeping the catalyst in suspension, aerating the liquid and supplying the

necessary mixing for mass transfer as it reacts. Since the reaction is highly exothermic, cooling coils

are provided in the reactor zone, containing the liquid phase with cooling medium normally in the form

of steam generation.

The advantages of slurry reactors over fluidized or fixed bed reactors are as follows: better

temperature control which is essential since FT reactions are highly exothermic, better heat recovery

and the differential pressure over the reactor is about four times lower which results in lower gas

compression costs. Moreover, a constant overall catalytic activity can be maintained easily by addition

of small amount of catalyst, in order to compensate catalyst deactivation [5].

Figure 4 – Slurry bubble column reactor

9

It allows also to work with a lower ratio H2/CO without the risk of coke formation and it offers higher

conversion rates, since it is more isothermal and so can operate at a higher average temperature.

However, in slurry reactors the attrition resistance of the catalyst is a very important parameter,

since catalyst must have good mechanical resistance as well as good chemical resistance. The second

drawback of slurry reactor comes from catalyst separation: removal of catalyst from the liquid produced

is much more difficult to handle for slurry reactors when compared to fixed bed reactors.

2.2 Fischer-Tropsch Catalysts

FT Catalyst: metal and support

The activity of the catalyst is a crucial parameter that influences the performance of the process

and it can be defined as the amount of reactant transformed per unit time per unit mass or unit area of

the active phase. This parameter can be described by the turnover number (or frequency), TON (or

TOF), which is expressed as the number of reactant moles transformed per active site (per unit time).

Another significant parameter of the process performance is the selectivity and as it was mentioned

before, the aim of the FT reaction is to achieve hydrocarbons with a long chain.

All the metals of group VIII have shown a manifest activity in the hydrogenation of carbon monoxide

to hydrocarbons [12], being ruthenium the most active metal followed by iron, nickel and cobalt.

According to the literature [12], the molecular average weight of hydrocarbon produced by FT synthesis

increased in the following sequence:

Figure 5 – Molecular average weight of hydrocarbons produced in FT synthesis [12]

Consequently, only Ni, Co, Fe and Ru have the required FT activity [8], which allow considering

them for commercial production.

Concerning the selectivity in methane, nickel is the one that shows the highest methane production

and that is why it is not likely to be used in the FT synthesis, since formation of methane is a side

reaction. On the other hand, ruthenium produces the highest molecular weight hydrocarbons [6].

However, like nickel, the selectivity changes mainly to methane at higher temperatures [6] and

furthermore, it is too expensive and its worldwide reserves are insufficient for large-scale industry [4].

Cobalt and iron are the metals which were proposed by Fischer and Tropsch as the first catalysts

for syngas conversion. Cobalt catalysts are more expensive, but they are more resistant to deactivation

by water. The productivity at higher conversions is higher with cobalt, since there is a less significant

effect of water on the rate of carbon monoxide conversion. In contrast, water has a strong negative effect

on the rate of CO conversion on iron catalyst and this catalyst also produces more olefins. The activity

at low conversion is comparable for both iron and cobalt as well as the sensitivity to sulfur [5].

Besides that, a new generation of FT processes involves syngas derived from natural gas, which

has a higher H2/CO ratio.

10

In this situation, FT synthesis proceeds at lower temperatures and is focused on the production of

middle distillate fractions and waxes. These new processes use cobalt supported catalysts due to the

following reasons [4] [5] [13]:

higher stability

high per pass conversion (up to 60-70%)

high selectivity to linear paraffins

high resistance toward deactivation

low-water gas shift activity

Taking these factors into account, cobalt catalysts should represent the optimal choice for the

synthesis of long-chain hydrocarbons in the low temperature process. This study will be then focused

on this type of catalysts.

As it was mentioned before, FT catalysts are supported catalysts. In general, the role of the support

is to achieve high surface area, to stabilize the dispersed active phase and to confer chemical and

mechanical resistance, thermal stability and sintering resistance.

In this specific case, the principal function of the support is to disperse cobalt and produce stable

cobalt metal particles in the catalyst after reduction and activation [4] [5]. For slurry reactors, the

chemical and mechanical resistance given by the support are the most important properties to be

assessed, in order to avoid fines formation. Sintering of metal particles should be avoided because it

causes a reduction of catalysts activity due to the loss of active sites. Since FT synthesis is an

exothermic reaction, the support also aims at heat removal, thus reducing temperature gradients in fixed

bed reactors [4]. Moreover, the catalytic support could modify diffusion of reagents and products inside

the catalyst grains and capillary condensation of the reaction products could also occurred in the catalyst

pores [4].

There are several combinations of support materials, such as titanium (TiO2), silica (SiO2), alumina

(Al2O3), carbon (C) and magnesium (MgO). Among these supports, SiO2 and Al2O3 are by far the most

widely used support materials for FT catalysts [7], since these supports feature large specific surface

areas and appreciable pore volumes. Thus, it will be discuss in detail their effects in the catalyst activity

and selectivity in 2.3.

The shape of the support depends on the type of technology and reactor that is used. Thus, for

slurry reactor a powder support is used, while extrudates are used for fixed bed reactor.

According to the literature [14], cobalt-based FT catalysts composition is illustrated in Figure 6.

Besides cobalt and oxide support, both metal and oxide promoter could be added in order to promote

either metal reduction or support characteristics. The role of the reduction promoter will be also

explained in 2.3.

11

Figure 6 – Compositional overview of modern Co-based FT catalysts [14]

Selectivity: ASF distribution

FT product distribution depends to some extent on the catalyst used, on the type of reactor and on

the operating conditions. It could be determined from the chain growth probability commonly known as

α, from the so called Anderson-Schulz-Flory (ASF) distribution.

This product distribution of FT reaction was firstly proposed by Schulz and Flory and quantified for

the first time by Herington and after by Anderson. This model is based in the following principles [11]:

1) The polymerization occurs by successive additions of C1 monomers to surface alky group;

2) The probability of chain growth (α) is independent of the chain length.

So, the polymerization rates and therefore the kinetics are independent of the products formed,

which are determined by the ability of the catalyst to promote chain propagation versus chain termination

reactions. Since the probability of chain growth is independent of chain length, the efficiency of

producing various hydrocarbons can be predicted based on simple statistical distributions based on the

chain growth probability and carbon number. The reaction scheme of chain polymerization is illustrated

in the following figure.

Figure 7 – Scheme of chain polymerization for FT synthesis (adapted from [15])

In the above figure, Ro is the rate of the formation of the first adsorbed specie, SCn is the surface

specie with n carbons, k1 and k2 are, respectively, the rate constants of the propagation and termination

reactions related to the terminal carbon and Cn is the product with n carbons. The chain growth

probability, α, is given by (Eq. 10).

𝛼 = 𝑘1

𝑘1 + 𝑘2

(Eq. 10)

12

So, the ASF model, which expresses the relation between the mass fraction (Wn) of the product Cn

and the chain growth α is given by:

𝑊𝑛 = 𝑛 × 𝛼𝑛 ×(1 − 𝛼)2

𝛼 (Eq. 11)

Analyzing the previous equation, it can be observed that the mass fraction of the hydrocarbon Cn

decreases according to a geometric progression. Linearizing this equation and representing log (Wn/n)

as a function of n it is possible to obtain the value of α, (Eq. 12).

𝑙𝑜𝑔𝑊𝑛

𝑛= 𝑛𝑙𝑜𝑔𝛼 + 𝑙𝑜𝑔

(1 − 𝛼)2

𝛼 (Eq. 12)

The representation of Wn (multiplied by 100) as a function of α shows the selectivity for the principal

hydrocarbons fractions, Figure 8.

Figure 8 – Hydrocarbon selectivity for various hydrocarbons fractions as a function of α [16]

Analyzing the figure above, it can be seen that only methane can be produced with 100% of

selectivity, while for the mass fraction for light hydrocarbons the maximum selectivity (C2-C4) that could

be reached is 56%, for gasoline fraction (C5-C11) it is 47% and for diesel (C12-C18) it is 30%. For a

probability of chain growth in the range between 0,75 and 0,95, it can be seen in Figure 9 that the

production of middle distillates (diesel, kerosene) with high selectivity is not possible by direct synthesis,

unlike waxes, which is obtained more than 40% for a probability of chain growth between 0.91 and 0.95.

Figure 9 – Product distribution as a function of probability of chain growth α [16]

13

Although, there are some deviations from the classical ASF distribution concerning methane, C2

(especially ethylene) and heavy hydrocarbons [17].

On the one hand, high methane and lower C2 selectivity observed in FT synthesis were attributed

to either elevated termination probability of the methane precursor or differences in the nature and type

of active sites or higher surface mobility or reactivity of C2 precursors.

Moreover, some studies interpreted the deviations to be a result of two competitive and

incompatible chain growth mechanisms, attributed to differences in the nature and type of sites [17].

On the other hand, primarily formed 1-olefins can be re-adsorbed on the catalyst surface and

undergo further competitive reactions such as hydrogenation to produce n-paraffins, dehydrogenation

to produce olefins or reinsertion to initiate new growing chains to produce larger hydrocarbons. Among

these possible reactions, re-adsorption and reinsertion of 1-olefins in chain growth will modify the chain

growth probability, resulting in higher hydrocarbons and a deviation from a typical ASF distribution [17].

In the case of cobalt catalysts, the chain growth probability is high (above 0,85) and has to be

maximized, since LTFT process is dedicated to diesel and wax production.

Besides these former aspects, the product distribution and selectivity is also affected by operating

conditions: temperature, pressure and ratio H2/CO.

FT operating conditions: temperature, pressure and H2/CO ratio

Performance of FT synthesis depends strongly on reaction temperature. Increasing temperature

favors selective methane formation, deposition of carbon and thereby deactivation of the catalyst and

reduces the average chain length of the product molecules, a feature being undesirable when aiming at

maximum ultimate diesel yields in a process combination with hydrocracking. The degree of branching

increases and the amount of secondary products formed such as ketones and aromatics also increases

as the temperature is raised [8]. However, FT synthesis products distribution is determined by the

kinetics and not by the thermodynamics data. So, even that rising the temperature increases the amount

of olefins, the kinetics will favor the formation of paraffins, thus reducing the olefins fraction.

As FT reactions are highly exothermic it is important to rapidly remove the heat of reaction from the

catalyst particles in order to avoid overheating of the catalyst, which otherwise result in an increased

rate of deactivation due to sintering and also in undesirable high production of methane [8].

This effect of increasing the temperature is quite pronounced in Co catalyst since it is a more active

hydrogenating catalyst, the products in general are more hydrogenated and also CH4 selectivity rises

more rapidly with increasing temperature than in Fe catalysts [8].

Concerning pressure conditions, FT synthesis is favorable by an increase in pressure, as it was

already explained in 2.1.

Another parameter that is essential on the performance of FT synthesis is the ratio H2/CO. So, if

this ratio is high then the formation of alkanes prevails over the formation of alkenes, since the

termination reaction by hydrogenation is more favored than the termination by β-elimination. However,

the hydrogenation also favors the desorption of hydrocarbons on the active sites, leading to lighter and

more saturated hydrocarbons [8].

14

Nevertheless, it might be an oversimplification to assume that if H2/CO ratio increases, the

selectivity in heavier hydrocarbons decreases, since the presence of CO2 and H2O might also play an

important role. Actually, the composition and partial pressure change along the length of the reactor and

so possibility FT selectivity, in case of fixed bed reactors [8].

For LTFT technology using a slurry reactor and a cobalt supported catalyst, H2/CO ratio is normally

between 1,7 and 2,15, while for HTFT is much less than 2 [5].

2.3 Characteristics of Fischer-Tropsch Catalysts

The economy of Fischer-Tropsch process is critically dependent on the effectiveness of cobalt

catalyst used. Therefore, the design of FT catalyst tends to focus on methods to improve cobalt

utilization by optimizing metal dispersion, improving reducibility and/or increasing stability.

In the following paragraphs, it will be discussed, the influence of cobalt particle size, cobalt phase

composition, catalyst support and promotion with noble metals on FT reaction rates, hydrocarbon

selectivity and stability.

Effect of particle size: activity and C5+ selectivity

One of the most important propriety of a catalyst is its activity and in case of FT synthesis catalyst,

the effects of metal particle size in the activity and selectivity of the catalyst are of prime scientific and

industrial importance.

This effect has been studied by various authors with original influences from Bartholomew [18] and

Yermakov [19]. The first author reported an increase in the turnover frequency by a factor of 3 on

decreasing dispersion from 20 to 10%, using cobalt catalyst at atmospheric pressure. On the other hand,

this study also demonstrated that the product selectivity is correlated with dispersion: the molecular

weight of hydrocarbon products is lower for catalysts having higher dispersions. This effect may be due

to stable oxides in the well-dispersed catalyst that catalyze the water-gas-shift reaction.

Studies of Iglesia and co-workers [20] showed that for relatively larger cobalt particles supported

on alumina, silica and titanium at 2 MPa, FT reaction rate is usually proportional to the number of

available cobalt surface atoms. This suggests that the cobalt size time yield (moles CO converted per

gram atom of metal surface per second) is independent on the sizes of cobalt particles, for diameters in

excess of 10 nm. Thus, this proposes that FT synthesis can be considered as a structure insensitive

catalytic reaction, for diameters larger than 10 nm.

This situation is however different with smaller cobalt particles (< 10 nm) since a decrease in FT

performance has been reported. Studies from Barbier and co-workers [21] found a decrease in TOF for

Co particles smaller than 6 nm in a catalyst supported on silica. For this support, it has been suggested

that this decrease in the activity is due to an enhancement in the proportion of Co-SiO2 interfacial sites,

likely caused by nanoparticle flattening [22]. For the variations in chain growth probability, Barbier also

showed that the selectivity toward C2+ hydrocarbons increases with cobalt particle size, then stabilizes

for diameters larger than 6 nm.

15

Various theories have been proposed to account for the parallel variations in intrinsic activity and

chain growth probability with cobalt particle size [18]. It has been speculated that the CO coordination

could be favored on larger particles, thus increasing CO dissociation and the intermediate concentration.

Bezemer and co-workers [23] reported a strong influence of the cobalt particle size on intrinsic

activity and C5+ selectivity: intrinsic activity of the catalyst have been shown to increase linearly as

particle size increases, in a range of 4-10 nm. This study was done for cobalt carbon nanofibers and

compared with the results obtained by Iglesia, Figure 10.

It can be seen that the catalytic data show the same general trend, with constant TOF for larger

Co particles, as it was expected and size dependency for smaller crystallites. The C5+ selectivity was

clearly dependent on cobalt particle size and varied from 76 to 84 wt% at 210°C and from 51 to 74 wt%

at 250°C (data in grey) with higher selectivity for larger particles. Furthermore, larger particles also

showed a larger chain growth probability, that is, a product distribution more shifted toward heavy

hydrocarbons. It is important to notice that the differences in selectivity are apparent for cobalt sizes

larger than approximately 8 nm, where the activity was not influenced by size. One of the possible

explanations of part of these differences is the 4-fold variation of the cobalt site density in the catalysts,

which has been reported to affect the C5+ selectivity [22].

Figure 10 – Influence of Co particle size on the (a) TOF and (b) C5+ selectivity at 35 bar, 210°C, H2/CO = 2 in

black [23], data obtained from [20] at 250oC are plotted in grey (reproduced from [23])

On the other hand, some studies [3] have reported that with increasing particle size of Co, the C5+

selectivity for Co supported on alumina passes through a maximum at a particle size of about 8-9 nm.

For larger particles the C5+ selectivity decreases before approaching a constant value. However, this

decline in selectivity is not easily understood, but it might be attributed to the rate of side ractions such

as olefin readsorption [4].

The analysis of these facts suggests that the catalytic properties of small cobalt particles in FT

synthesis could be different from larger ones. There might be two possible reasons responsible for this

phenomenon: the catalyst deactivation and the kinetics of elementary steps [4].

Concerning the first reason, at the conditions of FT synthesis, the catalyst productivity can decrease

due to several phenomena such as oxidation, formation of cobalt support mixed compounds, coke

deposition, cobalt particle sintering and catalyst attrition [24].

(a) (b)

16

As it was already seen, one of the probable reasons of catalyst deactivation is the cobalt oxidation

by water, since it is a major product of FT synthesis. According to the literature [24], larger cobalt metal

particles are most stable at FT reaction conditions due to their thermodynamic properties.

Concerning the second reason, the elementary steps of FT synthesis were studied on cobalt

catalyst supported by nanofibres and it was shown that the surface of coverage of cobalt catalyst by

reaction intermediates varied as a function of cobalt particle size.

It was reported that C5+ selectivity depends on the surface coverage of the CHx, OHx and CO

intermediates, which decreased for small particles and appeared to be constant for large particles.

On the contrary, an increase in H coverage was observed for small Co particles (< 6 nm), which

might explains the high methane selectivity of them [23]. Thus, different reactivity of small and large

cobalt particles could be probably related to this effect.

Another reason that might explains the lower activities of catalysts with small particles is the lower

reducibility of small cobalt oxide cluster. These cobalt oxide such as cobalt aluminate or cobalt silicate

can be formed during preparation and testing, since the mostly supports used are oxide [23]. Thus, a

complication with these very small cobalt crystallites might be their increased tendency to reoxidise

under certain FT conditions, after which the resulting cobalt oxide may react with active alumina or silica

sites resulting in inactive “cobalt aluminate” or “cobalt silicate” species [8]. Actually, according to the

literature [25], Co0 reoxidation is significant below 4 nm.

The relationship between the Co0 particle size and Fischer-Tropsch stability has also been studied.

For Co/SiO2, it was found that catalyst stability increased as particle size increased over the range 3.8-

9.2 nm, which was suggested to be associated with a deactivation mechanism of cobalt-support

compound formation [21]. It was also studied by Tsakoumis [26] that small cobalt crystallites are more

susceptible to deactivation by carbon fouling and also more vulnerable to sintering.

The variation in stability with particle size distribution follows two separate linear trends: first, as the

size distribution broadens, the catalysts stability increases. However, apex point is reached, after which

further broadening of the size distribution destabilizes the catalysts [25].

Taking together these facts, it appears that there is an optimal size range for a catalyst with both

high FT activity and good stability. Despite these studies, there still is some limitations in understanding

the behavior of TOF and selectivity as a function of cobalt particle size. The uncertainty in the size

sensitivity may be related to the limits or to the ambiguity of the method used for determining particle

size [21]. Another important point is that it is strongly depend on FT operating conditions and on support

nature, for instance [25].

Cobalt phase composition

Despite different specific activity of cobalt surface sites in smaller and larger particles, cobalt phase

composition could also affect FT catalytic performance. In general, metals tend to adopt a crystalline

structure that maximizes the space occupation, such as face-centered cubic (fcc), hexagonal close-

packed (hcp) and body-centered cubic (bcc) [27].

Cobalt cubic phase is usually the dominant phase in alumina, silica and titanium supported cobalt

catalyst reduced at relatively high temperatures (> 450oC).

17

According to the literature [28], catalysts with similar dispersion and similar number of cobalt

surface sites, but with higher fractions of hcp phase exhibited higher conversion in CO hydrogenation

rather than those which contain cobalt fcc metallic phase. This evokes that cobalt sites situated on cobalt

hcp phase might have higher specific activity in FT synthesis [4].

Cobalt reducibility and promotion with noble metals

The process of reduction of supported cobalt catalysts in H2 flow has been widely investigated. It

is assumed that the reduction of unsupported cobalt oxide, Co3O4, is a two-stage process which can be

attributed to successive reduction of Co3O4 to CoO and then to Co.

𝐶𝑜3𝑂4 + 𝐻2 → 3𝐶𝑜𝑂 + 𝐻2𝑂 (Eq. 13)

3𝐶𝑜𝑂 + 3𝐻2 → 3𝐶𝑜 + 3𝐻2𝑂 (Eq. 14)

Although it is usually difficult to attribute the corresponding peaks in TPR profiles for each reaction

clearly, the low-temperature peaks at 100-350°C are commonly assigned to either partial reduction of

Co3O4 to CoO or reduction or decomposition in hydrogen of residual cobalt precursors. The medium-

temperature peaks at 400-600°C are attributed to the emergence of cobalt metallic phases (CoO → Co),

while peaks at temperatures higher than 600°C are usually related to reduction of barely reducible mixed

cobalt oxides (cobalt silicate, cobalt aluminate) [29].

During the reduction of CoO to Co, the production of water is three times higher, per mol of Co.

This step, kinetically limited, essentially depends on the particle size since it influences the interaction

between the metal and the support. As it was already mentioned, the particle size is a determining factor

in the reduction process of oxidized cobalt catalysts: an increase of the particle-support interaction with

decreasing particle size is to be expected and resulting species can be reduced only at elevated

temperatures. Which means that, for supported catalysts, an elevated dispersion of cobalt at the surface

of the support will make the reduction process harder [29]. As a consequence, this might leave a fraction

of inactive cobalt after reduction, if temperature is not high enough.

Another important factor that influences cobalt reducibility is the nature and the texture of the

support that is used, which will be discussed in next topic.

So, in order to ease cobalt reduction frequently small amounts of noble metals such as Ru, Rh, Pt

and Pd are added via co-impregnation or subsequent impregnation. These promoting metals have a

strong impact on dispersion of cobalt species, FT reaction rates and selectivity [5] [4]. The transition

metals promote the reduction of the Co2+ species to cobalt metal keeping Co particles highly dispersed.

The divalent cobalt may be reduced by split-over hydrogen activated on noble metals because they are

known to have high capacity for hydrogen activation [30].

According to the literature [5], introduction of noble metals might result in the following phenomena:

easing of cobalt reduction, improvement of cobalt dispersion, inhibition of catalyst deactivation,

formation of bimetallic particles and alloys, high concentration of hydrogen activation site, increase in

intrinsic reactivity of surface sites, enhancement in cobalt dispersion and decreasing fraction of barely

reducible mixed oxides.

As well as noble metals, metal oxides such as ZrO2, La2O3, MnO and CeO2 could reduce formation

of hardly reducible cobalt mixed oxides, increase cobalt dispersion and reducibility.

18

On the other hand, the addition of this type of promoters may also modify the catalyst texture and

porosity, increase fraction of different cobalt metal crystalline phases, heighten mechanical and attrition

resistance of the catalyst and improve the chemical stability of the support [5].

Influence of support characteristics

Support texture influences the activity of the catalyst and in case of FT catalysts, the porous

structure of the support could control the sizes of supported cobalt particles.

Depending on support material, the activity and the reaction mechanism of catalysts might be

modify due to, for instance, metal-support interactions. Thus, cobalt catalysts supported on silica and

alumina have shown different specific activities (number of CO molecules converted per total cobalt

atom per second), being most active in silica support [10].

The support pores size is an important parameter that influences catalyst activity, affects product

distribution, controls the sizes of supported cobalt particles and cobalt reducibility [5] [13].

It was found that small particles are formed in narrow pores, while larger particles are formed in

wide pores. Concerning product distribution, selectivity in methane decreases with an increase in the

pore size [13]. This is not surprising, since Co3O4 particle size clearly increased with increasing support

pore size and, as it was mentioned, larger particles have a higher C5+ selectivity.

About cobalt reducibility, as it was demonstrated by (Eq. 13) and (Eq. 14), when Co3O4 is reduced,

one molecule of water is produced, for the first reaction and for the second reaction there is a production

of three molecules of water for three moles of CoO. For supported catalysts, water penetrates in porous

support, acting like a diffusional barrier for the reduction process. As it was mentioned before, operating

conditions play an important role in this process since, for instance, a decrease in hydrogen

concentration in gas flow as well as a decrease in gas flow rate will increase the amount of water in

pores with respect to the amount of hydrogen present, thus an inhibition of reduction will occur [15].

Cobalt species are much difficult to reduce in alumina than in silica, titanium and zirconia supports.

Even so, a higher reducibility for wide-pore silica-supported catalysts than for cobalt deposited on

narrow-pore supports was reported, which was predictable since larger particles are expected to behave

more like bulk Co3O4 with respect to their reducibility than smaller particles [13].

Another consideration about the support is its acidity, since it influences the catalytic behavior. The

acidity of the support may promotes side reactions such as cracking and isomerization, which lead to

lower chain growth probability and higher selectivity to lighter hydrocarbons [5]. An example of a support

which has a high acidity is zeolite.

Hence, SiO2 and Al2O3 are the most widely used support materials for FT synthesis. One factor

that these supports have in common is their reactivity toward cobalt, which may lead to formation of

cobalt support mixed compounds (aluminate and silicate). These mixed oxides should be avoid as they

do not produce active sites for FT synthesis [4] and they are reducible only at high temperatures [5].

However, the interaction between silica support and cobalt is weaker than in alumina supported

catalysts, which leads to better cobalt reducibility. In the latter, cobalt oxide strongly interacts with

alumina, forming relatively small cobalt crystallites, which may result in diffusion of cobalt active phase

into alumina and formation of cobalt aluminate.

19

On the other hand, cobalt dispersion is much lower in silica supported catalyst than in alumina [5].

Despite that fact, catalysts supported by periodic mesoporous silica have demonstrated good

performance in FT synthesis: carbon monoxide conversion was higher as also catalytic activity and C5+

selectivity [4].

Another factor that may influences the activity of the catalyst is the preparation method, so that, in

the next chapter, it will be discussed some preparation methods.

2.4 Preparation methods of Fischer-Tropsch Catalysts

Catalytic performance of FT catalysts toughly depends on methods of catalyst preparation, since it

influences its activity, selectivity, stability and mechanical strength. Moreover, the goal of catalyst

preparation is also to avoid loss of cobalt atoms in support matrix during catalyst preparation,

pretreatments and FT reaction [4].

Preparation methods can be divided in two main groups: deposition of the active phase on support

already formed or addition of the active phase during shaping of support. Impregnation and deposition-

precipitation methods correspond to first group, while co-precipitation and sol-gel methods correspond

to the second one. Cobalt-supported catalysts for FT synthesis are very often prepared by impregnation

and deposition-precipitation [5] and so that, they will be describe in more detail.

After chosen the appropriate catalyst support and the method of deposition of the active phase, the

preparation of cobalt-supported FT catalysts can be divided as follows:

1) Conditioning, modification and promotion of catalyst support

2) Preparation of cobalt precursors and eventually promoters

3) Deposition of cobalt and promoters on catalyst support

4) Post-treatment (decomposition of cobalt precursor)

5) Activation (reducing treatments)

2.4.1 Active phase deposition on support

The purpose of the active phase deposition, which is the most important step is to spread cobalt

onto porous support and provide the precursors of cobalt metal clusters. On the other hand, the

promotion with noble metals pretends to control the properties of the catalyst, number of metal cobalt

sites, their characteristics and localization on the support [5].

Impregnation method

In general, impregnation method can be described as a deposition of precursors (salts in aqueous

phase) of the active phase over a support. This deposition can be done with or without interaction

between support and active species. In the latter, there are many types of interactions such as Van der

Waals’ force, covalent bonds and ionic bonds.

In case of catalyst for FT synthesis, the most commonly used impregnation is incipient wetness

impregnation [5]. In this method, a solution of cobalt salt (normally cobalt nitrate) is contacted with a dry

porous support.

20

After being contacted, the solution is aspired by the capillary forces inside the pores of the support.

The incipient wetness occurs when all the pores of the support are filled with the liquid and there is no

excess moisture over and above the liquid require to fill the pores.

In this method it is very important to control some parameters such as temperature and support

drying, rate of addition of impregnation solution and temperature and time drying [5].

During impregnation step many parameters can influence metal-support interaction, such as pH,

which will depend on the different ionic exchanges between support and metallic precursor, solvent

nature and nature and concentration of the dissolved metal. These two last factors will influence the

solvation of support.

The initial dispersion of cobalt on the support depends mainly on the type, concentration and

distribution of hydroxyl groups on the surface and pH of impregnation solution. Immediately after

impregnation, the interaction between the metal precursor and the support is relatively weak, thereby

allowing redistribution of the active phase over the support during drying and calcination [5].

Concerning the pH, there is a pH known as point of zero charge (PZC), for which the number of

positive and negative charges on the surface cancel. At pH below the PZC, the surfaces of the oxides

are charged positively and at pH higher than the PZC, the surface of the support is charged negatively.

If the impregnating solution has a pH below the point of zero charge, repulsion between the surface of

the support and Co2+ atoms results in nonhomogeneous repartition of cobalt ions. At pH higher than the

PZC, Co2+ ions are distributed much more homogenously [5].

Deposition-Precipitation

Deposition-precipitation method is based on precipitation combined with deposition from a liquid

medium. The method combines all the advantages of the precipitation such as control of the size and

size distribution of precipitated particles, but reduces the risk of formation of bulk mixed compounds of

support and active phase.

This method enables one to apply an active precursor finely divided on the surface of a support,

whereas the initially dissolved precursor is present in a solution, whose volume is considerably greater

than the pore volume of the support. So that, this procedure allows loadings of the active precursor

significantly higher than would be possible by incipient wetness impregnation [31]. Thus, this technique

has been employed for preparation of highly loaded and highly dispersed oxide-supported metal catalyst

[5].

In general, with this method, a solvated metal precursor is deposited exclusively onto the surface

of a suspended support by slow and homogeneous introduction of a precipitating agent. The process

consists of two steps: the first one is the precipitation from the bulk solution both in support pores and

over support and the second one is the interaction of the precipitate with the support surface, which acts

as a nucleating agent. A fine and homogeneous phase can be obtained by involving surface OH groups

of the support in the precipitation process. In the deposition process, adsorption of the metal ions onto

the support coincides with nucleation and growth of surface compound.

However, there is a competition between the nucleation of a solid precursor compound in the

support surface and in the bulk solution.

21

So that, the most important challenge in this method is prevent the precipitation in the bulk solution;

in order to avoid that it must exists a greater interaction of the precipitated with the support surface,

diminishing the nucleation barrier. Thus, the nucleation on the support is easier, harming the nucleation

in the bulk solution.

The size of precipitated particles depends on the rate of nucleation and growth [31]. When the

nucleation and growth of the precipitate of the precursor are rapid, large crystallites of the precursor

result. With a rapid nucleation and slow growth, clusters of small particles of the active precursor outside

the pore system of the support are often obtained. At high concentrations, at a point where the

precipitant enters the suspension, the small particles rapidly and irreversibly flocculate, which leads to

the clustering of small particles. Catalysts prepared in this way are likely to rapid deactivation during

pretreatment or use at elevated temperatures, as the small elementary particles of the precursor are

intimately connected and therefore sinter readily. A cluster which is usually found with supported

catalysts prepared by precipitation is in Figure 11.

As a significant number of catalytic active precursor can be precipitated as hydroxides or basic

salts, many deposition-precipitation procedures involve an increase in pH level of a suspension of the

support in a solution of the active precursor. The interaction of the precipitating species and the

suspended support can be influenced by the pH. The injection of an alkaline or acid solution below the

level of the liquid suspension of the support can avoid concentrations so high that significant precipitation

can proceed in the bulk of the solution [31].

Thus, a new deposition-precipitation method has been proposed in the literature by Lok [32] [33]

and other authors [34]. This method is based on slow decomposition of aqueous cobalt amine

complexes formed from cobalt carbonate (or cobalt nitrate). The pH is homogeneously decreasing from

moderate basic to neutral values by controlled evaporation of ammonia from an ammonia/carbonate

suspension at elevated temperatures. Cobalt amine carbonate solution may be prepared by dissolving

basic cobalt carbonate in an aqueous solution of ammonium carbonate contains ammonium hydroxide.

This method has led to an uniform distribution of very small Co crystallites of 3-5 nm and due to the high

dispersion and Co loadings, high activity in FT synthesis has been reported.

Some studies [35] have compared the incipient wetness impregnation (IWI) and homogeneous

deposition precipitation (HDP) using ammonia, using Co/TiO2 catalyst.

Figure 11 – Clusters of particles of active precursor within the support [30]

22

Incipient wetness impregantion led to the formation of small, but clustered supported Co3O4

particles. Similarly sized, but more uniformly distributed supported particles were found for catalysts

prepared by ammonia evaporation method.

2.4.2 Post Treatment and Activation

After impregnation of the active phase, supported catalysts have already a porosity from the

support, but they do not present the active agent in the final form. So, the thermal treatments such as

calcination and steaming and the activation, which can also be consider a thermal treatment have the

objective to improve the dispersion of the active agent. Their operating conditions are also crucial for

the activity and selectivity of cobalt supported catalysts. However, before thermal treatments the catalyst

can also be submitted to a drying step in order to eliminate the solvent. These steps will be describe in

the following paragraphs.

Drying

As, it was mentioned before the aim of drying is to eliminate the solvent (usually water) present in

the solid porous. There are not many studies concerning this step, however Coutler and Sault [36] have

shown that the particle size distribution, precursor decomposition and cobalt reducibility depend strongly

on drying and calcination procedures, for Co/SiO2 catalysts prepared by wetness impregnation.

Calcining in air at 400°C after drying in air at 110°C forms larger Co3O4 particles that easily reduce under

hydrogen at 300°C. On the other hand, drying under vacuum led to the formation of an irreducible cobalt

phase attributed to the migration of Co2+ ions into silica framework. So, prolonged air drying eventually

converts the surface silicate into Co3O4, while vacuum drying disperses the nitrate precursor on the

support, forming cobalt silicates islands.

According to the same study, the final characteristics of the catalysts depends on the drying

conditions, but also the chemical environment associated during drying. Actually, the presence of gas

such as NOx favors the formation of cobalt silicate.

Calcination

The calcination influences directly the textural properties of the catalyst such as specific area,

porous volume and porous size distribution. In general, the calcinations start with a steady increase in

temperature until it reaches a plateau of variable duration. Normally it is performed under air or nitrogen

flow, at elevated temperatures in order to generate porosity and to give mechanical strength to the

catalyst [27].

During calcination it can occur various transformation such as precursor’s decomposition, with

active sites and porosity generation, changes in crystal structure, surface cleaning and textural changes

by sintering (increasing dimensions of the particles).

According to the literature [37], for Co/SiO2 and Co/Al2O3, the calcination temperature influences

the activity of cobalt catalysts at atmospheric pressure as well as the support nature.

23

Strong temperature elevations decreases not only the reducibility of the catalysts, but also the

metallic dispersion due to the formation of cobalt silicates and cobalt aluminates. Moreover, the increase

in calcination temperature had different consequences for Co/SiO2 and Co/Al2O3. For the first one, the

total hydrocarbon yield has increased as well as C5+ selectivity, unlike the second one.

The calcination of a Co/SiO2 catalyst at a moderate temperature and at a progressive temperature

augmentation avoid the formation of cobalt silicates and thus, increases the quantity of cobalt active

phase [38]. It has also been reported that for calcination temperatures between 100 and 150°C, the

formation of smaller Co3O4 particles is more favorable.

In general, for calcination temperatures higher than 500°C, the cobalt reducibility decreases.

The calcination step can also be performed under inert atmosphere (with azote or argon, for

instances), which reduces the exothermic phenomenas during the precursor decomposition and allows

to obtain a higher variety of oxide phases such as Co3O4, CoO and CoAl2O4. It also favors the cobalt

dispersion and increases the activity of the catalysts supported on silica.

Activation

Finally, during catalyst preparation the last step to perform is the activation step. This operation

consists in a thermal treatment at elevated temperatures, usually under hydrogen flow in order to obtain

the active agent and also to improve the metallic dispersion.

In this case, the activation step consists in obtain the cobalt metallic particle by reducing Co3O4. The

reduction mechanism of Co3O4 was already shown in 2.3. The reduction of Co3O4 in CoO on silica does

not need of aggressive conditions: CoO can be obtained at 350°C under nitrogen atmosphere. On the

other hand, under hydrogen flux at 400°C, CoO appears more rapidly at the surfaces of silica or alumina.

The most important parameter in this stage is the reduction temperature, since it has a significant

influence in the final dispersion and consequently in the metallic area of the active agent. In this specific

case, the reduction step also influences directly the cobalt phase composition. According to the literature

[4], for a cobalt catalyst supported on an alumina, the cobalt cubic phase (fcc) predominates over cobalt

hexagonal closed-packed phase (hcp) if the reduction temperature is higher than 450°C. It has also

been reported that catalysts with higher fractions of hcp phase exhibited higher conversion in CO

hydrogenation than those with cobalt fcc metallic phase. So, this might suggests that cobalt sites

situated on cobalt hcp phase could have higher specific activity in FT synthesis.

According to the same study, it was shown that the direct reduction of nitrate precursor leads to

weaker metal support interactions than in the case of calcinated catalysts and increases the quantity of

amorphous or poorly crystalline hexagonal metallic cobalt, which are assumed to be the active phases

in FT synthesis.

Other parameters as the degree of temperature augmentation, the hydrogen flux and the partial

pressure of hydrogen also influence the activity of the catalyst.


25

3 Objective of the study

Cobalt particle size plays an important role on the efficiency of cobalt FT catalysts, since it

influences its activity and selectivity, thus on the economy of Fischer-Tropsch synthesis. So that, the

objective of this trainee course is to determine the particle size effect on the performance and reactivity

of cobalt catalyst (8%wt Co) supported on a commercial silica-alumina (Siralox5 with 5 wt% of SiO2).

In fact, the chemical background of the possible cobalt particle size effects has remained largely

unclear. The exact position of the plateau point at which TOF and C5+ selectivity no longer varies with

particle size, varies amongst the literature, ranging from 4 to 10 nm. The uncertainty in the size sensitivity

may be related to the limits or to the ambiguity of the method used for determining particle size [21]. It

has been demonstrated that the position of this plateau may depend on the pressure used in FT

synthesis and it shifts to larger size as pressure increases. However, these remains uncertainty, since

the nature of the support used alters the intrinsic activity of the catalyst [25]. Another point that is missing

in the literature is the effect of cobalt particle size in the probability of chain growth, α.

Taking into account these facts, specifically the objectives of this work are:

Prepare a set of industrial catalysts with a broader range of particles sizes than commonly

studied for FT catalysts (especially towards larger particles)

Perform catalytic tests with industrial FT conditions (220oC, 20bar, H2/CO = 2)

Report a tendency between α selectivity as a function of cobalt particle size

Compare C5+ and CH4 selectivity obtained in the operating conditions described above with the

data obtained from the literature

So as to achieve these objectives, different preparation methods using different cobalt precursors,

with different thermal treatments were performed in order to play with the complexation role of the

counter-ion obtaining smaller particles and to increase sintering phenomena to obtaining larger particles.

One of the thermal treatments studied was steaming, which influence directly the properties of the

catalysts. The aim of this treatment is to understand how steam can affect such properties. There are

not many studies among the literature about steaming conditions, so that one of the aim of this work will

be to understand how steaming temperature, duration of the plateau and ratio between water and air

influences the catalyst characteristics. The possible transformations that can occur in steaming are

similar to the ones already presented for calcination.

In order to obtain information about catalyst activity and selectivity, catalysts were chosen,

according to the most favorable results of particle size and cobalt reducibility and tested in a slurry

reactor.


27

4 Experimental Work

Catalysts prepared in this study were supported cobalt catalysts. In order to achieve different

particles sizes, different preparation methods were chosen, via either wetness impregnation method or

homogeneous precipitation for cobalt addition, using different cobalt precursors and different thermal

treatment for catalyst calcination. The purpose was either to play with the complexation role of the

counter-ion leading to small particles or to improve the sintering phenomena in order to obtain larger

particles. For catalysts having small hardly reducible particles, noble metals promotion was tested in

order to improve cobalt reducibility and to obtain catalysts with a sufficient activity. So as to have a

comparative basis between the catalysts, a reference catalyst was defined and prepared, which will be

described later. All the prepared catalysts were characterized in order to determine particle size and

catalyst reducibility. In order to study the influence of cobalt particle size on Fischer-Tropsch catalyst

performances, catalytic tests were performed in slurry reactors with the most promising catalysts, trying

to scan a large range of cobalt particle size.

In this chapter, experimental protocols followed for catalyst preparation as well as the description

of apparatus used are firstly presented. Then, characterization methods performed are described as

well as their theoretical background and equipment. The end of this chapter is dedicated to the

description of catalytic tests performed to measure FT catalyst performances.

4.1 Support Characteristics

All the catalysts were prepared with the same support, which was a commercial silica-alumina

support (Siralox5) containing 5 wt% of SiO2. The principal characteristics of the support are gathered in

the following table.

Table 1 – Principal characteristics of the support

Support characteristics

Shape Powder

Composition Si, Al

BET surface (m2/g) 158

Average particle size (μm) 70

Porous volume (mL/g) 0,55

Average porous size BJH (nm) 11

Volume of water retention (mL/g) 0,52

28

4.2 Preparation Methods

Catalysts were prepared by three different methods: wetness impregnation, using different Co

precursors followed by calcination, wetness impregnation followed by steaming and deposition-

precipitation. In the following paragraphs the preparation conditions are described.

4.2.1 Wetness Impregnation

The goal of wetness impregnation was to achieve smaller or medium particles sizes by using

different precursors. After cobalt impregnation into the support, calcination and reduction of the solid

were done. The following figure schematizes the preparation of the supported catalysts.

Figure 12 – Scheme of the catalysts preparation by wetness impregnation

Preparation of the impregnation solutions

The choice of the precursors was done based on their solubility in water, on their decomposition

conditions and on their tendency to complex Co2+ in order to avoid the cobalt particles to agglomerate.

So that, the following precursors were chosen: cobalt nitrate hexahydrate, cobalt iodide, cobalt acetate

tetrahydrate, cobalt nitrate hexahydrate with sodium nitrite and cobalt nitrate with ethylene glycol.

Among all the catalysts prepared by wetness impregnation, the one prepared with cobalt nitrate

was chosen to be the reference catalyst. Actually, this precursor is commonly used in the preparation of

Fischer-Tropsch catalysts due to its high solubility in water and due to its easy decomposition, which is

endothermic and so that enables the development of cobalt crystallites and the formation of Co3O4

particles [5].

Cobalt acetate is the organic precursor most often used in the preparation of Fischer-Tropsch

catalysts and it is much less soluble in water than cobalt nitrate [5]. According to the literature [39], on

catalysts with cobalt acetate as precursor the cobalt dispersions are high, but the reduction degrees are

low, which indicates very strong metal-support interactions.

About the addition of ethylene glycol, according to the literature [40], this compound avoid the

formation of aggregates leading to a more uniformly distribution of Co3O4 particles in the support.

The addition of ethylene glycol was made in order to obtain a ratio R of 0,8, in which R is equal to

the following expression [40].

𝑅 =𝑚𝑤𝑎𝑡𝑒𝑟

𝑚𝑤𝑎𝑡𝑒𝑟 + 𝑚𝐸𝐺

(Eq. 15)

http://en.wikipedia.org/wiki/Water_of_hydration

29

With,

mwater: mass of water in the solution of dissolution (g)

mEG: mass of ethylene glycol in the solution of dissolution (g).

Concerning the cobalt iodide as well as cobalt nitrate with sodium nitrite, there is no evidence in

the literature of using these precursors on Fischer-Tropsch synthesis.

The impregnation solutions were prepared according to cobalt content desired and water retention

volume of the support. Specifically, 20g of catalyst were obtained by impregnation of a solution

containing cobalt precursor dissolved in a certain quantity of water. It is important to mention that for

some catalysts, the impregnation was done in multiple steps due to problems of solubility in water. So

that, the total mass of precursor needed was divided by the number of impregnations.

In order to improve catalysts reducibility, promotion with platinum was also performed by wetness

impregnation method. The addition of this metal was done to cobalt acetate and cobalt iodide. Firstly,

platinum was added in a third impregnation step for both catalysts. However, as it will be see in 5.1.4,

the reducibility of the catalysts was not enhanced. So that, addition of platinum was then performed by

co-impregnation with cobalt acetate.

To summarize, in the following table are the chosen precursors and the number of impregnations

performed as well as some important specifications.

Table 2 – List of catalysts prepared by wetness impregnation

Reference Catalyst precursor %(m/m) Co to impregnate

Number of impregnations

Specifications

LD-E1-1 Co(NO3)2.6H2O 8 1 Reference

LD-E1-11 Co(NO3)2.6H2O/EG 7 1 R = 0,8

LD-E2-1 Co(CH3COO)2.4H2O 8 2 -

LD-E2-2 CoI2 8 2 -

LD-E2-3 Co(NO3)2/NaNO2 8 2 NO-2/Co2+ = 0,51

LD-E2-4 Co(NO3)2/NaNO2 8 2 NO-2/Co2+ = 12

LD-E2-7 Co(CH3COO)2.4H2O/Pt 8 2 500 ppm Pt

Co-impregnation

LD-E3-1 CoI2/Pt 8 3 100 ppm Pt

LD-E3-2 Co(CH3COO)2.4H2O/Pt 8 3 100 ppm Pt

Experimental conditions and apparatus

Wetness impregnation preparations were performed in the equipment shown in Figure 13. It

consisted of a metal rod, which was rotating thanks to a stirring motor at a velocity of 60 rpm. The rod

had, at its end, a metallic blade for stirring the catalyst support, which was in a metal vessel. The

precursor solution was introduced dropwise by a syringe pump system at a flow of 0,5 mL/min. The

metallic blade also has ensured the continuous stirring of the impregnated solution.

1 Molar ratio

2 Molar ratio

30

In order to obtain an homogeneous distribution of the precursor solution over the support, the

impregnation step lasted almost 1 hour and 30 minutes.

Calcination was performed in air flow of 1 NL/h/gcata and in a tubular four, equipped with a glass

reactor, which had a porous plat and a thermowell. The calcination conditions are in the following figure.

The first plateau at 85°C was for eliminating all the water presented in the catalyst, while the plateau at

360°C was for eliminating all the volatile groups present, such as NOx in the case of cobalt nitrate.

Figure 14 – Calcination profile for catalysts prepared by wetness impregnation

All catalysts have shown a calcination profile identical to this one, except catalysts prepared with

cobalt acetate, whose calcination profile has revealed an exothermic peak. Indeed decomposition of

cobalt acetate is highly exothermic and it proceeds at slightly high temperature.

The highly exothermic decomposition of cobalt acetate might lead to higher fractions of hardly

reducible cobalt silicates and cobalt aluminates [5], as it will be seen in 5.1.4. The respective calcination

profile is in the following figure.

Figure 13 - Wetness impregnation apparatus

Qair=1NL/h/gcat

a

31

Figure 15 – Calcination profile for cobalt acetate catalyst (LD-E2-1)

The reduction was done in a quartz reactor, under H2 flow of 2NL/h/gcata. The particularity of this

step was that the reduced catalyst was covered by wax, paraffin nC80 named Sasol wax, in order to

avoid the reoxidation of the cobalt reduced in contact with air.

The most important parameter in this stage is the reduction temperature, since it influences directly

cobalt phase composition as well as the kinetics for reduction and the final rate of reduction that can be

reached. For instance, if the reduction temperature is less than 450°C, catalyst might have a higher

specific activity in FT synthesis, since cobalt sites situated on cobalt hcp phase predominate. For this

reason and also to avoid the sintering phenomena, it was chosen a reduction temperature of 400°C.

Besides that, this temperature allows to achieve good reduction rates, according to the kinetics of

reduction reaction.

So, the catalysts were reduced in hydrogen for 16h at 400°C and a plateau of 150°C during 1h

was done in order to dry the catalyst. The reduction conditions are in the following figure.

Figure 16 – Reduction profile of catalysts prepared by wetness impregnation

32

4.2.2 Steaming

The goal of steaming method was to achieve larger particles, by enhancing sintering phenomena.

So, different operating conditions were chosen in order to vary the severity of steaming treatment:

steaming temperature (T1<T2<T3);

duration of the plateau (D1<D2<D3);

ratio between water and air (R1<R2).

Cobalt precursor was first impregnated into the support as it was already described, followed by

drying at 85°C during one night, steaming and reduction, as it can be seen in the following scheme.

Figure 17 - Scheme of catalysts preparation by steaming

Preparation of the catalysts

Concerning the precursor, cobalt nitrate was chosen, in order to compare the catalysts prepared

by steaming with the reference catalyst. Cobalt impregnation into the support was done exactly as it was

already described in 4.2.1. For all catalysts, the goal was to impregnate 8% (m/m) of cobalt into the

support. After that, catalysts were dried. To summarize, in the following table are all the catalysts

prepared by steaming. It is important to mention that the catalysts LD-S-9 and LD-S-10 were prepared

more than one time. The latter one was well reproduced, unlike LD-S-9 whose particle size results were

very different for catalysts prepared at the same conditions.

Table 3 – List of catalysts prepared by steaming

Reference Catalyst precursor wt% Co to impregnate Steaming conditions

LD-S-2 Co(NO3)2.6H2O 8 T3; D2; R1

LD-S-3 Co(NO3)2.6H2O 8 T3; D2; R2

LD-S-4 Co(NO3)2.6H2O 8 T2; D2; R2

LD-S-6 Co(NO3)2.6H2O 8 T2; D3; R1

LD-S-7 Co(NO3)2.6H2O 8 T2; D1; R1

LD-S-9 Co(NO3)2.6H2O 8 T1; D1; R1

LD-S-10 Co(NO3)2.6H2O 8 T2; D2; R1

33


Steaming was performed in a glass reactor, which had a bed of SiC separated from the other half

of the reactor by a porous plate. So, steam was generated within the bed of SiC, passed through the

solid to be steamed, which was placed in the porous plate and left at the top of the reactor, Figure 18.

This step was carried out under air flow of 0,5L/h/gcata to avoid the condensation of water in the

reactor. Water, whose flow depended on the experiment executed, was injected at the bottom of the

reactor, thanks to a syringe pump system.

So that, its vaporization and its heating up occurred while passing through the bed of SiC. An

important point is that the injection of water was done before the temperature reached the plateau

temperature.

The following figure presents a generic temperature profile performed during steaming. Concerning

the last step, reduction, it was done as the same way as for catalysts prepared by wetness impregnation.

Figure 19 – Generic steaming profile

Figure 18 – Steaming reactor

34

4.2.3 Deposition-Precipitation

The goal of homogeneous deposition-precipitation at high pH was to prepare highly loaded and

highly dispersed oxide-supported cobalt catalyst. This method consisted on putting in contact the

support with the precursor solution, to which was added ammonia solution. After that, the mixture was

heated to boiling until 95°C while stirring and then it was cooled to room temperature, centrifuged,

washed and dried at 85°C during one night, according to [34]. The following scheme presents the steps

followed during this preparation method.

Figure 20 – Preparation of catalysts by deposition-precipitation

Preparation of solutions

There are several studies about deposition-precipitation for cobalt catalysts and, so that, according

to the literature cobalt nitrate and cobalt carbonate were chosen as cobalt precursors [21] [33] [34].

This method has already been tested with nickel nitrate and silica support and it was successful to

produce highly dispersed Ni catalysts. Thus, due to the similarities between nickel and cobalt, cobalt

nitrate was also chosen as cobalt precursor [21]. On the other hand, a cobalt ammine complex could

also be formed by using cobalt carbonate, according to [33] [34].

For preparation with cobalt nitrate precursor the preparation method described in [21] was followed.

So that, 20 g of solid having 8% by weight of cobalt was prepared, maintaining the concentration of

cobalt and ammonia used in that work. Specifically, 8,07 g of Co(NO3)2.6H2O was dissolved in 79,06

mL of water, to which was added 70,59 g of 28 wt% ammonia in order to cause Co(OH)2 to precipitate;

this solution was prepared under air atmosphere. Then, the support was added and the temperature

was hold at 95°C during the precipitation process as well as the stirring for almost four hours. After that,

the mixture was cooled to room temperature and centrifuged. The solid was washed five times with 75

mL water and dried at 85°C during one night. It is important to mention that the pH was recorded in this

process: before the addition of the support the pH was 10, after that it was 8 and it has stabilized in that

value during the rest of the process.

For preparation with cobalt carbonate precursor, the preparation method described in [34] was

chosen. Once more, 20 g of solid was prepared, with a target final cobalt content of 15 wt%, applying

the same concentrations of cobalt, carbonate and ammonia as the ones used in that publication.

35

For the preparation of cobalt amine carbonate solution, 6,09g of cobalt carbonate were dissolved

in 59,80 mL of water, to which was added 55,84 g of 28 wt% ammonia as well as 6,06 g of ammonium

carbonate. Two experiments were done with this precursor: one solution (A) was prepared under air

atmosphere and the other (B) under helium atmosphere, in order to avoid the oxidation of Co2+. Then,

the support was added and the temperature was hold at 95°C as well as stirring during almost 8 hours.

The rest of the procedures were the same as for the cobalt nitrate solution. The pH was also recorded

during the trialing: before addition of support, the pH was 10,8 and after it was 8 and it stayed there for

the rest of the process, for both solutions.

In the following table is summarized the solutions prepared for deposition-precipitation.

Table 4 – Solutions prepared for deposition-precipitation method

Reference Catalyst precursor wt% Co Atmosphere Time of stirring (h)

LD-DP-1 Co(CO3) 15 Air 8

LD-DP-2 Co(NO3)2.6H2O 8 Air 4

LD-DP-3 Co(CO3) 15 Helium 8


The preparations were made in a tornado apparatus (Figure 21) that uses a single overhead stirrer

to agitate up to six flasks simultaneously. Each flask has a volume of 250 mL and a centrifugal stirrer

shafts made of PTFE. The agitation was maintained at 360 rpm, while the heating was made directly by

the stirrers hotplate and it had a digital temperature control (±0,5°C). It has also an integral water cooled

reflux head and allowed to operate under an inert atmosphere, in this case helium.

After that, the mixture was centrifuged (4000 rpm during 20 minutes). The resultant solid residue

was washed with water (75 mL) and then went again to centrifugation; this step was done 5 times.

Finally, the solid went to drying under air at 85°C overnight. It is important to mention that for all samples

prepared, the supernatant has never become colorlessness, as it was expected [34]. Also, at the end of

every centrifugation, it still remained much supernatant which might mean that the precipitation process

did not occur, suggesting that this preparation method was not successful.

Figure 21 – Tornado apparatus

36

4.3 Characterization Methods

Catalysts characterization provides important information about the structure of cobalt FT catalysts

and their precursors. It allows identification of the active sites for FT reaction and reveals possible routes

for optimization of catalyst structure [5]. In this chapter, the characterization methods performed are

described: X-ray fluorescence, N2 adsorption-desorption, mercury porosimetry, temperature-

programmed reduction (TPR), X-ray diffraction (XRD) and transmission electron microscopy.

4.3.1 X-Ray Fluorescence

X-ray fluorescence is based on excitation using an X-ray tube and is a global elementary analysis

technique. The X-ray emitted from the excited atoms relaxation provides information on the composition

of the sample: the energy of the characteristic rays offers information on the nature of the elements

contained in the sample, while the measured intensity, for a given energy level, depend on the mass

concentration level of the element concerned [41].

In this work, these analysis were made in order to determine cobalt content of the catalysts. These

analyses were performed on a Thermo Scientific instrument, model ARL PERFORM’X. The cobalt

content will be present with an accuracy of ±0,3-0,4 wt% Co.

4.3.2 N2 adsorption-desorption

Nitrogen adsorption-desorption is a very used technique to characterize textural properties of

catalyst. So, adsorption equilibrium are represented by isothermal plots which show the adsorbed

quantity as a function of the equilibrium pressure P of the gas in contact with the solid. In practice,

relative pressure P/P0 is used where P0 is the saturated vapor pressure of the adsorbate at the

measurement temperature (approximately 77 K in the case of nitrogen). Determining an adsorption-

desorption isotherm thus consists of measuring the quantity of gas that is adsorbed on (or desorbed

from) the surface of the solid at a given temperature [41].

Prior to measuring the adsorbed quantities, a pre-treatment stage is carried out to eliminate the

compounds adsorbed on the surface of the sample (H2O, CO2, etc.). So that, the sample is placed under

vacuum and then, the temperature is increased. Then, the sample is placed in a tube submerged in

nitrogen liquid at 77 K, a feeble nitrogen pressure is created in the sample and the adsorbed volume is

measured. The adsorption isotherm is obtained by gradually increasing the pressure.

The smallest pores are filled first; the gas then condenses in successively larger pores until a

saturated vapor pressure level is reached at which the entire porous volume is statured with liquid. By

measuring, from P0, the quantities of gas that reaming adsorbed for decreasing relative pressure levels,

the desorption isotherm is obtained. These measures were made in a Micromeretics ASAP 2420

equipment.

The forms of the isotherms and hysteresis loops can be classify according to IUPAC, whose

classification shows the relationship between the form of the isotherms, the average radius of the pores

and the intensity of the adsorbate-absorbent interactions.

37

To complete the information given by these isotherms, BET model, t-plot and BJH method are also

used. BET model is used to determine the specific surface area of solids, while t-plot is a simple method

that consists on comparing the adsorption isotherm of a given solid in terms of adsorbed thickness and

BJH method consists of plotting the curve of the derivation of the absorbed volume as a function of pore

diameter in order to obtain the porous distribution.

Finally, this technique is highly suitable for study of samples where the pore size is between

approximately 2 and 50 nm, which corresponds to mesoporous domain [41].

4.3.3 Mercury porosimetry

As the last technique described, mercury porosimetry also enables the textural characterization of

catalysts, since it allows the determination of the specific surface area and pore distribution. Unlike

nitrogen adsorption-desorption, this technique is widely used for macroporous samples and for the upper

range of mesoporous (20-50 nm) [41].

In Hg porosimetry, the penetration of a non-wetting liquid (mercury) in the pores is only possible

with the action of an external force (applied pressure), this force countering the resistance created by

the surface tension of the liquid. Thus, the liquid penetrates a pore with a radius rp under a pressure P

given by Washburn’s equation.

𝑃 =2𝛾𝑐𝑜𝑠𝜃

𝑟𝑝

(Eq. 16)

Where 𝛾 is the surface tension (485 dyn/cm) at the liquid-gas interface and 𝜃 (140°) is the wetting

angle of the liquid with the material.

The experimental measurement consists on measuring the quantity of liquid consumed in

penetrating the pores as a function of the applied pressure. This gives the distribution of the porous

volume versus pore size.

Firstly, the sample is prepared by degassing in an oven under standardized conditions. The sample

is then subjected to a low pressure treatment, followed by a high pressure phase (4000 bar). The

pressure is increased in stages which are adapted to the porosity of the solid under examination. In this

case, as pressure increases, the smaller pores become filled. The examinations were done in a

Micrometrics Autopore V 1.09.

The results of this technique are given in different graphs. One corresponds to the cumulate volume

of mercury penetrated as a function of pore diameter logarithmic and the other corresponds to the

derivate of this curve as a function of pore diameter.

4.3.4 Temperature Programmed Reduction

Temperature-programmed reduction (TPR) is commonly used for characterizing heterogeneous

catalysts and allows to study the reduction behavior of the catalysts. It enables the characterization of

the oxides phases based on their resistance or their tendency to reduction, which depends on the

interaction between the metal and the support.

38

The TPR experiments were performed with 500 mg sample of catalyst under a mixture of 5% H2 in

Ar flowing at 58 cm3/min, and the temperature was raised at a rate of 5°C/min until it reached 1000°C.

The consumption of hydrogen was followed as a function of temperature under continuous gas flow and

it was determined by using a thermal conductivity detector (TCD) with a Wheatstone bridge circuit.

During the reduction process, several products such as water, CO or CO2 are formed.

Thus, it is important to remove all undesired gas molecules that can interfere in the signal output

and for that, a molecular sieve is used, in this case a zeolite. The experiments were done in a

Micrometrics Auto Chem II 2920 apparatus.

Due to its high sensitivity to chemical changes induced by the catalyst promoter or by the support,

TPR is a particularly attractive method for studying carrier-active phase or dopant-active phase

interactions. Also, it is important to notice that TPR profiles do not provide direct information about the

modification of catalyst structure, which means that the hydrogen consumption could be attributed to

different reduction processes [5]. Also, the quantity of consumed hydrogen as well as the formation of

water during TPR and the position of the peaks constituted a footmark of the catalyst.

The operating conditions of these experiments are of extremely importance since they influence

the profiles obtained. For instances, if the increasing rate of temperature is too high or if the gas flow

rate is too high, the peaks will move to higher temperatures and will be more narrow.

On the other hand, if the concentration of hydrogen in the gas flow is too high, the reduction peaks

will move for the lower temperatures. Another important parameter is the sample amount, if it is too

much the resolution of the experiments will be lower [5].

4.3.5 X-Ray Diffraction

X-ray diffraction (XRD) is commonly used for identification of crystallite phases and evaluation of

the crystallite sizes, using their degree of crystallization. It has been shown that the angular bready β of

a diffraction line is given by Debye-Scherrer equation [5]:

𝛽 =𝐶𝜆

𝐿𝑐𝑜𝑠𝜃

(Eq. 17)

Where C is a constant, 𝜆 is the X-Ray wavelength, L is the volume-average size of the crystallites

and 𝜃 is the Bragg angle. In this expression either the full-width half-maximum (FWHM) or the “integral

width” can be used for definition of the peak breadth. In this case, the FWHM was used.

The value of the Bragg constant (C) will depend on the way that the peak breadth was measured.

Moreover, the crystallite sizes are measured for a plane characterized by the Miler indices hkl.

This technique is not very sensitive to the presence of very small crystallites (<2-3 nm), since the

peaks are getting too broad to be identified and measured. The broadening XRD lines could be related

to not only a finite size of crystallite, but also the presence of strained and imperfect crystals [5].

Generally speaking, when a monochromatic X-ray beam with a wavelength λ reaches the catalyst

sample, the diffraction phenomena is only observed for a certain number of crystallites that present a

given group of planes (hkl) to the beam with an incidence angle compatible with the Bragg condition.

XRD pattern is then given by the angular positions as a function of the intensities of the resultant

diffracted peaks, which constitutes the fingerprint of the crystalline species [41].

39

A PANaytical X’Pert PRO diffractometer instrument, in a Bragg-Brentano configuration, with a

stationary X-ray source and a movable reactor was used for the XRD measurements. For that, the

samples (0,5 g) were prepared by powder compaction.

The diagrams were scanned at 0,05°/step, using 5 seconds acquisition time per step and the

analysis range was from 2 to 72° (2𝜃). The software used in order to obtain the diffractograms was

DiffracPlus commercialized by Siemens/Socabim.

In this study, Co3O4 peak located at 2𝜃 equal to 36,9°C was used, in order to calculate the average

cobalt oxide crystallite thickness for almost all the samples, by Scherrer equation (Eq. 17). The accuracy

of the measurements were ±10%.

4.3.6 Transmission Electronic Microscopic

Transmission electron microscopy (TEM) provides a detailed information about the composition

and electronic structure of heterogeneous catalysts with real-space resolution down to the atomic level

[5]. An electron beam may be partially adsorbed and partially deflected, when crossing a sample. By

using electromagnetic lenses, a certain fraction of these electrons and of those that have not been

deflected can be recombined to form an image. The use of TEM is based on controlling the electrons

involved in image formation. Thus, it offers an image of the sample that depends on the electron matter

interactions. Besides that, electron diffraction can also be used in TEM to gain information on the

crystallographic structure of the sample [41].

In this case, TEM analysis were made in passivated catalysts in order to determine the size of

cobalt particles. A representative sample (with hundreds of particles) was taken in order to determine

the frequency of particles belonging to a certain size range. So that, it was possible to do the histograms

representatives of particle sizes distribution. For reasons of contrast between the metallic phase and

the support, the metallic phase was more easily observed in dark-field image, by selecting the diffraction

spots of the (002) plane of Co metal. The TEM analysis were performed in a JEM 2100F instrument.

The software used in order to process the data obtained was LOGRAMI.

4.4 Catalytic Tests

Catalysts were tested in a slurry reactor. As it was mentioned before, this type of reactor is more

isothermal and so can operate at a higher average temperature resulting in higher conversions [8].

Description of the unity catalytic tests

The tests have been performed in a slurry reactor tank having a total volume of 50 mL and the

stirring was performed by a magnetic stirrer at a rate of 1000 rpm. The temperature regulation of reaction

medium was made by a thermocouple and the control of liquid level in the reactor was made by overflow

liquid level sensor.

40

The syngas entered in the reactor via a line, that was placed in the liquid phase until the reactor’s

bottom and a porous plate placed on the top of the reactor, which allowed the withdrawal of the product

to the separation process.

The separation process between the gas, water and wax was performed in a single three-phase

separator (850 mL), whose level was controlled by a pressure differential sensor between the top (gas

phase) and the bottom (liquid phase). This separator has a heating system, to maintain the temperature

at 80°C and at the top has a refrigerant system to sustain the temperature at 150°C.

Concerning the density of the different products, the aqueous phase is firstly extracted from the

bottom of the separator and after is the organic phase. The gas products obtained were analyzed in a

gas on line chromatography, while the liquid products were analyzed in a laboratory.

A simply scheme of this procedure is represented in the following figure.

Figure 22 – Simple scheme of the catalytic tests unity 5 (TCD - thermal conductivity detector; FID - flame

ionization detector)

Start-up and loading of the reactor

The loading of the reactor was done by the introduction of both catalyst covered by wax and solvent.

The solvent used was Squalane, which consists of a mixture of polyalphaolefins. This solvent is liquid

at ambient conditions and has good physic-chemical properties in the conditions of the FT synthesis in

a slurry reactor.

In this study, the mass of catalyst introduced in the reactor was between 4 and 8 g and the rest of

the volume of the reactor was filled with Squalane. Then, the reactor was heated until 120oC in order to

melt the wax which was covering the catalyst. The pressure was set under nitrogen flow at 40 bar, in

order to perform the leakage test. Afterward, pressure was decreased down to the target pressure of

the catalytic test (20 barg), then syngas was introduced.

The ratio between H2 and CO was maintained at 2 and in order to verify such value,

chromatography analysis were performed. Then, reactor temperature was increased up to the chosen

temperature of the test (220°C). Start of run corresponded to the moment when stirring started.

41

The catalytic test was operated at a constant target CO conversion (40%). For that purpose, syngas

flow rate has to be adjusted daily, in order to compensate catalyst deactivation.

So, the preparation of the reactor usually takes three days and comprises the following steps:

loading of the reactor, leakage tests and material balance in order to verify if the ratio H2/Co is equal to

2. This last analyze is done in a chromatograph and normally the value is between 1,97 and 2.

Since the present tests were done in a slurry reactor, the presence of the initial solvent might

misrepresent the results, thus it was necessary to have a longer test (300 hours) in order to the wax

produced by the catalyst replaced the solvent introduced in the beginning.

Gas products analysis

The reactor gas effluents were analyzed with on line gas phase chromatography, which lasts

approximately 40 minutes. The apparatus used was from Agilent and was equipped with four columns

and three detectors. A concentric column connected to a thermal conductivity detector (TCD) and a

capillary column connected to a flame ionization detector (FID). The first column allowed to separate

the gas such as CO, CH4, N2 and CO2, in cases of higher conversions. The other column was used to

analyze hydrocarbons C1-C7.

The CO conversion corresponds to the CO quantity transformed in hydrocarbons. The quantity of

CO transformed is proportional to the peaks areas obtained from the TCD. The CO conversion was

obtained from the ratio between the residual quantity of CO present at the reactor outlet and the initial

quantity of CO (obtained by by-pass), by standardizing with nitrogen.

The selectivity of methane is calculated by the ratio between the outflow of methane and the volume

of CO consumed.

The C1-C7 selectivity is determined from the FID results. The area of each peak is proportional to

the number of carbons, which form the hydrocarbon specie. This selectivity is obtained in carbon atoms.

The decoupling of the peaks concerning paraffins and olefins (the latter has a lower retention time)

allows to calculate the selectivity relative to each other.

Liquid products analysis

Liquid products analysis were done by chromatography, but not on line as the gas products

analysis. The result chromatogram allowed to quantify paraffins (≤ C70) and olefins (≤ C16). The analysis

of this products allows to determine the selectivity for each hydrocarbon higher than C8 and to indirectly

determine the probability of chain growth α.


43

5 Results and Discussion

5.1 Physic-Chemical properties of synthesized cobalt catalysts

In paragraphs below, the results obtained from the characterization methods described in chapter

4.3 will be presented and discussed. Catalyst characterization has provided important information about

the structure of the prepared catalysts, since it allowed identification of the active sites for FT reaction,

information about the chemical composition, textural properties and reducibility behavior. At the same

time, it revealed routes for optimization of catalyst preparation.

5.1.1 Determination of cobalt content by XRF and oxide phases by XRD

In order to know the real content of cobalt in catalysts, XRF analysis were performed. XRD analysis

were also performed in order to identify the oxide phases in the catalyst. According to the literature [5],

oxidized cobalt catalysts supported by porous oxides typically contain Co3O4, CoO, intermediate cobalt

oxides (Co3O4-x), mixed cobalt-support oxide phases as cobalt aluminate and silicate and perhaps non-

decomposed cobalt precursors. Between these compounds, Co3O4 crystallites always exhibit well-

defined XRD patterns, as it will be discussed in the paragraphs below.

5.1.1.1 Results of Wetness Impregnation

The results obtained for cobalt content in catalysts prepared by wetness impregnation are in the

following table. In this case, the objective was to impregnate 8% (wt%) of cobalt, except for the catalyst

with ethylene glycol, whose objective was to impregnate 7% (wt%) of cobalt.

Table 5 – Determination of Co content (wt%) by XRF for catalysts prepared by wetness impregnation

Reference Catalyst wt%Co ±wt% Co

LD-E1-1 Reference (Co(NO3)2.6H2O) 8,2 0,3

LD-E1-11 Co(NO3)2.6H2O/EG 6,8 0,3

LD-E2-1 Co(CH3COO)2.4H2O 7,2 0,3

LD-E2-2 CoI2 7,4 0,3

LD-E2-3 Co(NO3)2/NaNO2 (0,5) 7,7 0,3

LD-E2-4 Co(NO3)2/NaNO2 (1) 7,4 0,4

LD-E2-7 Co(CH3COO)2.4H2O/Pt3 6,9 0,3

According to Table 5, the experimental measured values are in good agreement with the target

value. However, it is noticed that for catalysts with sodium nitrite, cobalt content is less than it was

expected. As calcination was performed with air flow, the decomposition of sodium nitrite in sodium

oxide, nitrogen oxide and nitrogen dioxide may occurred (Eq. 18).

3 Catalyst prepared by co-impregnation.

http://en.wikipedia.org/wiki/Sodium_oxide

http://en.wikipedia.org/wiki/Sodium_oxide

http://en.wikipedia.org/wiki/Nitric_oxide

http://en.wikipedia.org/wiki/Nitrogen_dioxide

44

2𝑁𝑎𝑁𝑂2 → 𝑁𝑎2𝑂 + 𝑁𝑎𝑂 + 𝑁𝑂2

(Eq. 18)

So, the residual Na2O presence could explain the lower Co content than expected as it might have

caused a dilution of cobalt present in the catalyst.

The figure below shows the XRD pattern of the reference catalyst and the support contribution. It

exhibits characteristic peaks of aluminum oxide (red peaks) and crystallite Co3O4 (green peaks).

Figure 23 – XDR diagram of the reference catalyst Co(NO3)2.6H2O

So, for the reference catalyst CoO crystallites were not found. It is possible to conclude that this

catalyst presents a well crystallize structure, since the two diffraction patterns are quite different and the

diffraction lines of the catalyst are relatively narrow.

The comparison between reference catalyst and catalysts using also cobalt nitrate, but with sodium

nitrite is in Figure 24. The behavior of its diffraction lines is very similar to the reference catalyst,

suggesting that the crystallites size might be also very similar.

Figure 24 - XDR diagram of the cobalt nitrate with sodium nitrite having NO2-/Co2+ equal to 0,5 (LD-E2-3)

and equal to 1 (LD-E2-4) and reference catalyst

Co3O4

Inte

nsity (

a.u

.)

2θ

Reference catalyst LD-E2-3 LD-E2-4

Support

Reference catalyst

Co3O4

Al2O3

45

The diffraction peaks of cobalt acetate, cobalt iodide and cobalt nitrate with ethylene glycol are

weaker and broader than those in the reference catalyst’s XRD pattern (Figure 25). This suggests that

almost no crystallite phase was detected and that cobalt crystallites size are smaller than in reference

catalyst, which will be discussed in detail in 5.1.2.1. It could also indicates the fine dispersion of cobalt

oxides in these three catalysts.

Figure 25 - XDR diagram of the cobalt acetate (LD-E2-1), cobalt iodide (LD-E2-2), cobalt nitrate with

ethylene glycol (LD-E1-11) and reference catalyst

The same behavior was found for cobalt acetate promoted with platinum (prepared by co-

impregnation).

5.1.1.2 Results of Steaming

Concerning cobalt content of catalysts prepared by steaming, the results are in the following table.

Once more, the goal was to prepare catalysts with 8 wt% of Co.

Table 6 - Determination of Co content (wt%) by XRF for catalysts prepared by steaming

Reference Catalyst wt% Co ± wt% Co

LD-S-4 T2; D2; R2 8,1 0,4

LD-S-6 T2; D3; R1 7,8 0,3

LD-S-9 T1; D1; R1 7,5 0,3

LD-S-10 T2; D2; R1 7,5 0,4

Since these catalysts have been prepared with cobalt nitrate, during steaming the decomposition

of this precursor has occurred, which was noticed by the reddish vapors (NOx).

The decomposition of cobalt nitrate is endothermic and leads to release of nitrogen dioxide (which

causes an orange color), water and oxygen, (Eq. 19).

Inte

nsity (

a.u

.)

2θ

Reference catalyst LD-E2-1

LD-E2-2

LD-E1-11

Co3O4

46

3𝐶𝑜(𝑁𝑂3)2. 6𝐻2𝑂 → 𝐶𝑜3𝑂4 + 6𝑁𝑂2 + 𝑂2 + 18𝐻2𝑂 (Eq. 19)

Taking this into account, the decomposition of cobalt nitrate might have caused a premature depart

of cobalt, which explains the cases where cobalt content is less than the target value. Other explanation

is that nitrates might have not been totally eliminated by steaming, especially for steaming at lower

temperature (T1). So that, residual nitrates might have diluted cobalt present in the catalyst.

To illustrate a XRD pattern of catalysts prepared by steaming, in the following figure is shown the

XRD diagram of catalyst prepared at (T1; D1; R1) conditions.

Figure 26 - XDR diagram of (T1; D1; R1) catalyst prepared by steaming (LD-S-9) and reference catalyst

Regarding this figure, peaks corresponding to cobalt diffraction are very evident and narrow,

suggesting that this catalyst has a structure very well crystallized. Comparing this XRD pattern with the

reference catalyst, the main difference between them is the fact that the diffraction peaks are stronger

and less broadened. That means either that the crystallites size are larger, which will be analyze in

5.1.2.1 or that cobalt structure contains fewer structural defects.

For the rest of catalyst prepared by steaming, their XRD diagrams are very similar with the one

presented in the above figure.

5.1.1.3 Results of Deposition-Precipitation

Cobalt content determined by XRF of catalysts prepared by deposition-precipitation method are

shown in the following table.

Table 7 - Determination of Co content (wt%) by XRF for catalysts prepared by deposition-precipitation

Reference Catalyst precursor Atmosphere wt% Co Target value

(wt% Co)

LD-DP-1 Co(CO3) Air 2,6 15

LD-DP-2 Co(NO3)2.6H2O Air 4 8

LD-DP-3 Co(CO3) Helium 3 15

Lin

(C

ounts

)

0

1000

2000

3000

4000

5000

6000

7000

8000

2-Theta - Scale

16 20 30 40 50 60

Inte

nsity (

a.u

.)

2θ

Reference catalyst LD-S-9

Co3O4

47

So, cobalt content obtained for all catalysts prepared by deposition-precipitation is much lower than

the ones that were expected. With these results, it could be assumed that the active precursor did not

migrate into the pores of the support and so that, the precipitation within the porous support did not

occur.

In the next figure is the XRD diagram of catalyst with cobalt nitrate comparing with the reference

catalyst. The diffraction peaks are very weaker and broader, which means that almost no crystallite

phase was detected. It also suggests that this catalyst has smaller crystallites than the reference

catalyst. A rapid nucleation and a slow growth of the precipitate of the precursor might have happened,

which caused the formation of clusters of small particles of the active precursor. Lower cobalt loading

used can also explain these behavior. The other catalysts prepared by deposition-precipitation present

the same behavior as this one.

Figure 27 - XDR diagram of the cobalt nitrate prepared by deposition-precipitation under air atmosphere

(LD-DP-2) and reference catalyst

5.1.2 Determination of cobalt particle size

Cobalt particle size was determined either by XRD analysis on oxide catalysts or on reduced

catalysts by TEM. The results are presented in the following paragraphs.

5.1.2.1 Average particle size from XRD

As it was mentioned before, XRD technique is commonly used for identification of cobalt crystallite

phases as well as for evaluation of the crystallite sizes using Scherrer equation (Eq. 17). So, the average

Co3O4 particle sizes were calculated from the most intense Co3O4 line (typically at 2θ equal to 36,9°).

As the active phase of the catalysts during FT reaction is known to be metallic cobalt, the average

Co3O4 crystallite size is not expected to be equal to the average particle size of the active phase.

Inte

nsity (

a.u

.)

2θ

Reference catalyst LD-DP-2

Co3O4

48

According to the literature [42], the diameter of a given Co3O4 particle could be converted to the

corresponding size of metallic cobalt considering the change in the relative molar volume that occurs

during the transition of Co3O4 to metallic cobalt. The resulting conversion factor for the diameter d of a

given Co3O4 particle being reduced to metallic cobalt is:

𝑑(𝐶𝑜0) = 0,75 × 𝑑(𝐶𝑜3𝑂4) (Eq. 20)

However, some limitations of this approach should be taken into consideration. First of all, this

approximation suggests that large and small particles of cobalt oxide have the same reducibility. As it

was mentioned before, smaller particles of cobalt oxide are much more difficult to reduce than larger

ones. This could lead to underestimation of the sizes of cobalt metal particles using the method based

on the average size of cobalt oxide particles.

On the other hand, the approximation assumes that decreasing the particle diameter during catalyst

reduction proceeds according to the molar volume.

However, some studies [5] have showed, for instances for silica-supported cobalt oxide particles,

that the decrease in its volume after reduction could vary between 30% and 50%.

Results of Wetness Impregnation

The results obtained for the average crystallite and metallic particle sizes of catalysts prepared by

wetness impregnation are in the following table.

Table 8 – Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD for catalysts

prepared by wetness impregnation

Reference Catalyst Co3O4

Crystallite size (nm)

Co0 Particle

size (nm)

LD-E1-1 Reference (Co(NO3)2.6H2O) 11 8

LD-E1-11 Co(NO3)2.6H2O/EG 3 2

LD-E2-1 Co(CH3COO)2.4H2O 5 4

LD-E2-2 CoI2 3 2

LD-E2-3 Co(NO3)2/NaNO2 (0.5) 13 10

LD-E2-4 Co(NO3)2/NaNO2 (1) 13 10

LD-E2-7 Co(CH3COO)2.4H2O/Pt4 3 2

By regarding the results obtained by XRD, it is concluded that the main objective of wetness

impregnation preparation was achieved: smaller particles, besides medium particles, were obtained,

which suggests a strong metal-support interaction. However, it is important to mention that contrary to

what was expected, sodium nitrite did not reduce particle size comparing to reference catalyst. Finally,

Co3O4 crystallite size does not seem to be considerably affected by promotion with platinum, as expected

[43].

4 Catalyst prepared by co-impregnation.

49

Using cobalt acetate, cobalt iodide or cobalt nitrate with ethylene glycol as cobalt precursors

allowed to obtain catalysts with the smallest particles size. However, due to the weak intensity of peaks,

there is a lack of precision in particle size determination.

Concerning these three last catalysts, Co particles are expected to be highly dispersed, but hardly

reducible as it will be seen in 5.1.3.

Results of Steaming

The average crystallite size of catalysts prepared by steaming are gathered in the following table

as well as the calculated average particle size of metallic cobalt.

Table 9 - Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD of catalysts

prepared by steaming

Reference Catalyst Co3O4


Co0 Particle

size (nm)

LD-S-2 T3; D2; R1 17 13

LD-S-3 T3; D2; R2 15 12

LD-S-4 T2; D2; R2 21 16

LD-S-6 T2; D3; R1 20 15

LD-S-7 T2; D1; R1 29 22

LD-S-9 T1; D1; R1 43 32

LD-S-10 T2; D2; R1 30 22

Comparing these results with the ones obtained by wetness impregnation, the average

crystallite size increases within steaming, which suggests that sintering phenomena has occurred. As a

result, the dispersion of cobalt particles might be reduced as well as the exposed surface area of metallic

phase. On the other hand, the reduction rate of these catalysts will be much higher than for catalysts

prepared by wetness impregnation. Indeed larger particles are likely to behave more like bulk Co3O4

with respect to their reducibility than smaller particles [43].

Another important conclusion that can be drawn from these results is that an increase in steaming

temperature results in a decrease in particle size. On the one hand, diffusion rate increases with

increasing temperature, on the other hand increasing temperature might have induced a rapidly

formation of Co3O4, and thus finishing the diffusion process leading to smaller particles.

The duration of the plateau, which was the second parameter studied, also influences particle size.

As a result of an increase in duration of steaming, metal-support interaction might be favored leading to

formation of smaller particles, which caused the average particle size to decrease.

To summarize, the main conclusion is that steaming is a good way to obtain cobalt catalyst with

large crystallites. Steaming conditions have a great impact on the final crystallite sizes, since within the

range of studied operating conditions, catalysts with average Co3O4 crystallite size from 15 to 43 nm

have been obtained.

50

Results of Deposition-Precipitation

Average crystallite sizes for catalysts prepared by deposition-precipitation are given in the following

table. It is very important to say that the peaks in the XRD diagrams for these catalysts were very weak

and very broadened, which means that almost no crystallite phase was detected and so that these

measurements are not very precise.

Table 10 - Determination of Co3O4 crystallite and assumed cobalt metallic particle sizes by XRD for

catalysts prepared by deposition-precipitation

Reference Catalyst precursor Atmosphere Co3O4


Co0 Particle

size (nm)

LD-DP-1 Co(CO3) Air 2 1

LD-DP-2 Co(NO3)2.6H2O Air 3 2

LD-DP-3 Co(CO3) Helium 4 3

5.1.2.2 Distribution and average particle size from TEM

The TEM analyses were performed in passivated catalysts in order to study the particle size

distribution of metallic cobalt as well as to determine the average particle size.

Results of Wetness Impregnation

Concerning catalysts prepared by wetness impregnation, TEM analysis were performed in the

following catalysts: cobalt iodide, cobalt acetate and cobalt nitrate with sodium nitrite (Co2+/NO-2 equal

to 1). TEM pictures of the first two catalysts are given in the following figures.

Figure 28 - TEM micrograph of the (a) CoI2 and (b) Co(CH3COO)2.4H2O catalysts

(a) (b)

51

For both catalysts, the support is in the form of platelets tangled into each other and cobalt particles

are the ones present in white. Also, metallic cobalt particles are homogeneously distributed on the

support particles, since no agglomerates are visible. Another consideration is that the shape of cobalt

particles are quite spherical and for cobalt acetate, some larger particles seems to exist.

TEM micrographs of cobalt nitrate with sodium nitrite are in the following figures.

Figure 29 - TEM micrograph for Co(NO3)2/NaNO2 (1): (a) low magnification image and (b) higher

magnification image

For this catalyst, the metal phase is quite uniformly distributed over the support, but some

agglomerates can be observed. Regarding Figure 29, a fraction of particles seems to be located not

inside the single pores, but rather on the outside or adjacent of pore cavities, which might explains the

fact that cobalt particles appears as agglomerates.

Results concerning average particle size of both catalysts as well as dispersion, which was

calculated assuming an octahedral fcc model, are in the following table. These results do not agree with

the results obtained from XRD analysis. This fact suggests that the degree of reduction of CoI2 and

Co(CH3COO)2.4H2O catalysts might be too low due to their small particles. Another factor that might

explains this difference is that, since TEM analysis were done on reduced catalysts, they might have

been readily oxidized to cobalt oxide in the presence of air, as it was demonstrated in some studies [44].

The inconsistency between these two methods will be analyzed in more detail in 5.1.2.3.

Table 11 – Principal results for cobalt particle size and dispersion determined by TEM for cobalt iodide and

cobalt acetate catalyst

Reference Catalyst

Average particle size in

number (nm)

Average particle size in surface

(nm)

Average particle size in volume

(nm)

Dispersion (%)

Standard deviation

in number

(nm)

LD-E2-1 Co(CH3COO)2.4H2O 7 9 10 14 3

LD-E2-2 CoI2 7 9 10 14 2

LD-E2-4 Co(NO3)2/NaNO2 (1) 9 14 15 9 4

(a) (b)

52

The respective histograms of Co(CH3COO)2.4H2O and CoI2 catalysts, which represent the particle

size distributions in number are in the following figures. The particle size distribution in volume is in

Appendix 8.1, page 81.

Figure 30 - Cobalt metallic particle size distribution for (a) Co(CH3COO)2.4H2O and (b) CoI2 catalysts

Regarding these histograms, the particle size distribution for both catalysts is very similar. Another

conclusion that can be drawn from these graphs is that they do not provide evidence of a bimodal

distribution.

Concerning Co(NO3)2/NaNO2 catalyst, particle size distribution in number is in the following figure.

Once more, particle size distribution in volume is in Appendix 8.1, page 81. In this case, particle size

distribution is much larger than the other catalysts, which emphasizes the dissymmetry observed. Also,

this catalyst presents a higher heterogeneity concerning the particle size since it has particles from 2 to

27 nm.

Figure 31 - Cobalt metallic particle size distribution for the Co(NO3)2/NaNO2 (1) catalyst

Results of Steaming

For the catalysts prepared by steaming, TEM analysis were performed in catalysts at (T1; D1; R1)

and (T2; D2; R1) conditions. For both of them, TEM analysis showed that the metallic phase is quite

uniformly distributed over the support and agglomerates are rarely observed, unlike for Co(NO3)2/NaNO2

catalyst, for instances. To support these facts, TEM micrographs are presented for each catalyst in the

following figures.

(a) (b)

53

Figure 32 - TEM micrograph for (T1; D1; R1) catalyst (a) low magnification image and (b) higher

magnification image

Figure 33 - TEM micrograph for (T2; D2; R1) catalyst: low magnification image and (b) higher magnification

image

Concerning the average particle size obtained by TEM analysis, once more they differ from the

results obtained by XRD as it can be seen in the following table.

Table 12 - Principal results of cobalt particle size determined by TEM for cobalt iodide and cobalt acetate

catalyst

Reference Catalyst

Average particle size in

number (nm)

Average particle size in surface

(nm)

Average particle size in volume

(nm)

Dispersion (%)

Standard deviation in number (nm)

LD-S-9 T1; D1; R1 8 14 18 10 4

LD-S-10 T2; D2; R1 9 13 16 10 4

On the other hand, histograms (Figure 34) show that both catalysts present a wide range of particle

sizes, since particles size between 3 and 28 nm for (T1;D1;R1) and between 2 and 29 nm for (T2;D2;R1)

were found. Particle size distribution in volume is in Appendix 8.1, page 81.

(a) (b)

a) (b) (a)

54

Figure 34 - Cobalt metallic particle size distribution for (a) (T1;D1;R1) and (b) (T2;D2;R1) catalysts

5.1.2.3 Comparison of size determination methods

In the following table is the comparison between the results obtained of the average particle size

from XRD, which was calculated from (Eq. 20) and TEM.

Table 13 – Principal results for the average particle size obtained from TEM and XRD

Reference Catalyst TEM (Average particle

size in volume (nm) dCo)

XRD (Average Co size in volume calculated from Eq.

20 (nm))

LD-E2-1 Co(CH3COO)2.4H2O 10 4

LD-E2-2 CoI2 10 2

LD-E2-4 Co(NO3)2/NaNO2 (1) 15 10

LD-S-9 T1; D1; R1 18 32

LD-S-10 T2; D2; R1 16 22

The particle size distribution obtained from TEM can also be presented in volume, considering that

all the particles are like semi-spheres and their volume can be calculated by the following expression

V=2/3πr3, where V is the particle volume and r is its radius.

On the other hand, XRD is consider as a bulk analysis and so that, the average crystallite size

obtained is in volume, which means that the average particle size calculated from (Eq. 20) is also in

volume.

The higher deviations were found for catalysts with smaller particle size, corresponding to CoI2 and

Co(CH3COO)2.4H2O. The deviation observed could be due to the fact that particles sizes are determined

either on the oxide catalyst in XRD or on the reduced catalyst in TEM.

For catalyst with small oxide particles, only the largest particles could be reduced, the smallest

ones being non or partially reduced. So that, histograms obtained from TEM might correspond to a small

fraction of reduced cobalt present in the catalysts, corresponding to the largest particles, while XRD

analysis corresponds to the bulk entire oxide cobalt phase Co3O4.

For catalysts prepared by steaming, the larger deviation found between XRD and TEM results could

be explain as a redispersion effect resulting from large cobalt particles breaking up into smaller ones

during re-oxidation, as reported in the literature [42].

To be consistent for all solids, results from XRD will be considered for crystallite size determination.

(b) (a)

55

5.1.3 Textural properties

The study of textural properties is of principal importance for porous materials. In fact,

characteristics as the pore size and pore size distribution is very useful in various stages of catalyst’s

existence such as its preparation, use and regeneration [41]. In order to know the pore size distribution,

N2 adsorption-desorption and mercury porosimetry analysis were performed in catalysts prepared by

steaming at (T1; D1; R1) and (T2; D2; R1) conditions. The first technique is more suitable for the study

of samples where the pore size is between approximately 2 and 50 nm, which corresponds to

mesoporous domain, while Hg porosimetry covers an extensive range of pores size from 2 nm to 150

μm (meso and macroporous) [41].

Figure 35 - Nitrogen adsorption as a function of relative pressure over (T1;D1;R1) catalyst and support

The forms of the isotherms and hysteresis loops give information about the average radius of the

pores and the intensity of the adsorbate-adosrbant interactions. According to IUPAC classification [41],

the above isotherms, which are very similar, can be classified as type IV, which is characteristic of solids

with mesoporous (between 2 and 50 nm). A hysteresis loop is also observed reflecting a capillary

condensation type phenomenon, which can be classified as type H1 [41].

This type of hysteresis is often associated with solids made up of agglomerates (strongly interlinked

particles) leading to narrow pore size distribution [41]. The t-plot (see Appendix 8.2, page 83), which

compares the adsorption isotherm of a given solid in terms of the adsorbed thickness, also shows clearly

the mesoporous nature of both catalysts.

The pore size distribution was made by BJH method for both catalysts and support. The porous

distribution for both catalysts is very similar and has the same distribution shape of the support. By

regarding the figure below, it can be seen that pore size distribution is relatively narrow. For all the

samples, the maximum population is found at approximately 11 nm, although at approximately 7 nm

there is a slight change in the adsorption curve, which might indicates that there is also a population

with that size. Thus, it could be concluded that steaming did not induced major changes of textural

properties.

0

50

100

150

200

250

300

350

0 0,2 0,4 0,6 0,8 1

Vo

lum

e a

dso

rbed

(m

L/g

)

Relative pressure (p/p0)

Support

T1; D1; R1

56

Figure 36 – Pore size distribution of the support and of catalysts (T1;D1;R1) and (T2;D2;R1) determined by

volume of nitrogen absorbed

In order to study the macroporosity and the upper range of mesoporosity (20 – 50 nm) [41], mercury

porosimetry analysis were performed. The volume of mercury absorbed as a function of pore diameter

for both catalysts as well as the porous distribution obtained from that curves are presented in the

following figure.

Figure 37 – Volume of Hg absorbed as a function of pore diameter of catalysts prepared at (T1;D1;R1) and

(T2;D2;R1) conditions

By regarding the results obtained from Hg porosimetry it is concluded that both catalysts do not

present macroporosity, but only mesoporosity. Actually, for both catalysts there is no evidence of porous

with 20 nm or more. The resulting pore size distribution of both catalysts is shown in the following figure.

0

0,01

0,02

0,03

0,04

0,05

0 5 10 15 20 25

dV

/dD

(m

L/g

/nm

)

Average pore diameter (nm)

T2;D2;R1

Support

T1;D1;R1

0

0,2

0,4

0,6

0,8

1

1,2

1 10 100 1000 10000 100000

Vo

lum

e o

f H

g (

mL

/g)


T1; D1; R1

T2;D2; R1

57

Figure 38 – Porous distribution obtained from Hg porosimetry of (T1;D1;R1) and (T2;D2;R1) catalysts

prepared by steaming

Catalyst prepared at (T1; D1; R1) is characterized by a bimodal distribution, since its distribution

has two peaks: one at approximately 7 nm and other, much more intense, at approximately 11 nm.

These results are according to the ones obtained from BJH method, however Hg porosimetry was able

to find another porous population. Concerning catalyst prepared at (T2; D2; R1), an unique peak is

presented corresponding to a population with approximately 11 nm and there is no evidence of

macroporosity, which is in good agreement with the conclusions already drawn from BJH method.

To summarized, in the following table is the porous volume and the surface area obtained for

each catalyst.

Table 14 – Porous volume and BET surface for catalysts (T1;D1;R1) and (T2 ;D2 ;R1) prepared by steaming

Reference Catalyst Porous type Porous

volume (mL/g) BET surface (m2)

LD-S-9 T1; D1; R1 Mesoporous 0,53 147

LD-S-10 T2; D2; R1 Mesoporous 0,54 140

5.1.4 Catalyst reducibility from TPR on oxide catalysts

In order to study the reducibility of the oxide catalysts, TPR analysis were performed. To interpret

the following TPR profiles, it is important to have in mind that the process of reduction of supported

cobalt catalysts is a two-stage process, which can be attributed to successive reduction of Co3O4 to

CoO and then to Co. This fact is also supported by the XRD analysis, which have shown that the

principal phase of oxide catalysts was Co3O4 structure.

0

0,01

0,02

0,03

0,04

0,05

0,06

1 10 100

dV

/dD

(m

L/g

/nm

)


T1;D1;R1

T2; D2; R1

58


The next figures shows the TPR profiles of the catalysts after calcination.

The reference catalyst Co(NO3)2.6H2O (Figure 39) exhibits one narrow peak between 250 and

350°C, which corresponds to the reduction of Co3O4 to CoO and a very broad reduction peak between

350°C and 830°C indicating the reduction of Co2+ to Co0 atoms, which are interacting with the support.

Above 830°C, consumption of hydrogen corresponds to the reduction of cobalt aluminate or cobalt

silicate. These species have been formed due to the substitution of Co3+ ions in Co3O4 spinel structure

by, for instance, Al3+ ions [13]. Also, the peak at approximately 180°C corresponds to the decomposition

of residual cobalt nitrate. In the same figure, the consumption of hydrogen in each step is also

represented. The volume of hydrogen consumed in the first peak should be 25% of the total hydrogen

consumed, while the ratio of hydrogen consumption between the first two peaks should be 1:3, as

predicted from Co3O4 reduction stoichiometry (Eq. 13) and (Eq. 14). So, the results obtained for the

reference catalyst are in good agreement with the theoretic values.

Figure 39 - TPR profile for the reference catalyst

The TPR profiles obtained for the set of catalysts prepared by wetness impregnation using also

cobalt nitrate are shown in the following figure.

20%

VH2,total

55%

VH2,total

24%

VH2,total

59


Co(NO3)2/NaNO2 (0.5) and Co(NO3)2/NaNO2 (1)

The first conclusion that can be drawn regarding the above profiles is that no catalyst appears to

be totally reduced at 1000ºC. As these catalysts have been prepared with sodium nitrite, the first peak

in their TPR profiles (at 280°C and 330°C for a ratio Co2+/NO-2 of 0.5 and 1, respectively) is attributed to

the decomposition of this compound, which was already explained in 5.1.1.1. It should be noticed that

for the ratio Co2+/NO-2 of 1, this peak is much more intense since the ratio is also higher.

Moreover, the peaks at 359°C and 396°C for Co2+/NO-2 of 0.5 and 1, respectively, are attributed to

the reduction of Co3O4 to CoO. The remaining peaks correspond not only to the reduction of divalent

cobalt to metallic cobalt, but once more to cobalt aluminates and cobalt silicates, which explains the

peaks at elevated temperatures such as 953°C and 920°C for Co2+/NO-2 of 0.5 and 1, respectively.

Besides, the shape of these profiles can be attributed to a non-homogeneity of the particle sizes

distribution, as it was seen in 5.1.2.2, since that could explain the peaks at lower temperatures

(corresponding to the largest particles and so that, more easier to reduce) and the peaks at elevated

temperatures (corresponding to smaller particles, resulting from the strong interactions between the

cobalt oxide and the support and so that, hardly to reduce) [6].

In the following figure is the comparison between the reference catalyst and Co(CH3COO)2, CoI2

and cobalt nitrate with ethylene glycol.

60

Figure 41 - TPR profiles for catalysts prepared by wetness impregnation: reference catalyst, Co(CH3COO)2, CoI2

and Co(NO3)2/EG

Regarding TPR profiles of CoI2, Co(CH3COO)2.4H2O and Co(NO3)2/EG, the consumption of

hydrogen until 600°C is very limited, which means that the reduction process has not occurred. This

behavior was already expected since these catalysts have shown smaller particles size, thus the

strongest interaction between cobalt oxide and the support matrix.

In the case of CoI2, the peak at 690oC might be attributed to the reduction of I2 in I-. During

calcination, the catalysts has iodide (I-), which might have been oxidized to iodine (I2) by oxygen, since

this step was performed under air flow. In fact, this oxidation reaction is very easy due to its standard

potential (E°= -0.54 V). During TPR, residual iodine is thought to be reduced to iodide which explains

the high consumption of hydrogen at approximately 690°C. The other oxidation states of iodine were

not considered, since they are very unstable.

As it was already explained, in order to improve the reducibility of catalysts CoI2 and

Co(CH3COO)2.4H2O, Pt was added. TPR profiles of catalysts with and without Pt are shown in Figure

42. In case of cobalt acetate, addition of platinum was also performed by co-impregnation, since

according to the literature [45], an intimate association between Pt and Co is required to promote

reduction. The comparison between TPR profiles of the addition of platinum in these two different ways

are in Figure 42.

61

Figure 42 - Comparison of TPR profiles between (a) CoI2 and CoI2/Pt and (b) Co(CH3COO2).4H2O and

Co(CH3COO2).4H2O/Pt

For both catalysts, the addition of Pt did not cause peaks to shift to lower temperatures, as it was

expected [13] and it did not eliminate the cluster-support interactions, since it is still remain the peaks at

higher temperatures, even by adding the platinum by co-impregnation. However, it is noticed that the

addition of Pt causes the highest peak of CoI2 to disappear. Bearing in mind that the addition of platinum

was done in a third impregnation step, it means that, the presence of Pt during the third calcination step

enables decomposition of residual catalyst precursor.


The next TPR profiles correspond to catalysts submitted to drying over one night at 85°C and to

steaming at different conditions of temperature, plateau duration and ratio between water and air. The

next figure corresponds to the TPR profile for catalyst prepared at T1, during D1 and with a ratio R1

between water and air.

(a) (b)

Figure 43 – TPR profile for catalyst prepared by steaming (T1;D1;R1)

62

Regarding the shape of TPR profile of Figure 43, this is not a characteristic two-step reduction

pattern. Nevertheless, the peak at 330ºC might be attributed to the reduction of Co3O4 to CoO and the

very broad peak can be attributed to cobalt species interacting with the support.

The principal difference between this catalyst and the ones prepared by calcination is the existence

of the first peak observed at 260ºC. Since this catalysts have been prepared with cobalt nitrate, this first

peak corresponds to the reduction of residual NOx groups.

According to the literature [46], cobalt nitrate undergoes melting at 75°C, between 80 and 170°C it

undergoes dehydration giving cobalt nitrate monohydrate and this specie, by increasing the

temperature, undergoes decomposition giving rise to an unstable intermediate of composite structure

containing Co(NO3)2, CoO, Co2O3 and Co3O4. From 240°C, it only exits Co3O4 as product of the

precursor decomposition.

The next figure shows the catalysts prepared by steaming at temperature T2.

Figure 44 - TPR profiles for catalysts prepared by Steaming at T2 with different durations (D1<D2<D3) and

ratio H2O/Air (R1<R2)

Analyzing the profiles above, once more the peaks at lower temperatures correspond to the

presence of residual nitrate. Furthermore, by analyzing these profiles it is possible to verify the influence

of the plateau duration as well as the ratio water/air.

According to the literature [15], the increasing in the duration of the plateau favors kinetically the

decomposition of cobalt precursor and possibly the presence of cobalt oxides in form of CoO(OH). This

fact can be confirmed regarding Figure 44, since for duration D1, which is the smallest one, the peak

attributed to the decomposition of nitrate is much more intense. On the other hand, by increasing the

duration of the plateau (for instances, D3) it is observed a slight increase in the broad peak which means

an increase in the hardest species to reduce.

Figure 43 and Figure 44 show the existence of various peaks of reduction, which are not

characteristic of two reduction steps of Co3O4. For instance, the peaks more narrow could be related to

monodispersed particles, while the peaks more broad could be related with multidispersed particles.

63

The other hypothesis is that steaming treatment could have generated different particles sizes,

which might explains the various peaks until 500°C. Thus, the largest particles correspond to the peaks

until approximately 350°C (besides the peak corresponding to nitrate decomposition) as they are more

easier to reduce, while the peaks between 350°C and 500°C would correspond to medium particles.

The other peaks at higher temperatures correspond to the smallest particles, which are the result of

strong interactions between the oxide metal and the support, as it was already mentioned. So that, for

the catalysts of Figure 43 and Figure 44 it is possible to talk about particle size multimodal distribution.

For the catalysts maintained at T3, the TPR profiles are the following.

Figure 45 - TPR profiles for catalysts prepared by Steaming at T3 with different ratios H2O/Air (R1<R2)

As it was expected, the peak corresponding to the decomposition of nitrate is much less intense

due to the increase of the temperature and it means that at this stage almost all the nitrate has been

decomposed. So that, it is realized that the temperature is an essential parameter for the decomposition

of the precursor in the oxide phase.

To summarize, the reduction of catalysts Co(NO3)2 prepared by steaming proceeds via the

reduction of residual NOx groups, followed by the reduction of Co3O4 to CoO and after CoO to Co. The

magnitude of the peak corresponding to reduction of residual NOx groups depends strongly on steaming

conditions, being less intense for more severe conditions. Another point that is common to all the profiles

is that an heterogeneity of particle size might exist, which explains the various peaks at lower

temperatures, corresponding to the particles more easier to reduce and the various peaks at higher

temperatures, corresponding to the species more harder to reduce. An additional interesting point is that

the TPR profiles for catalysts prepared at a temperature T3 are much more similar with the reference

catalyst than the other ones.

64

5.1.5 Catalyst Reduction Rates from TPR on reduced catalysts

The formation of the active sites for FT catalysts requires the reduction of Co3O4 particles into

metallic cobalt. In order to measure the reduction rate of catalysts, TPR analysis were performed on

reduced catalysts, which have been passivated under air. This TPR analysis allows to determine the

amount of non-reduced cobalt, by measuring total hydrogen consumption between 20 and 1000°C and

evaluating the amount of hydrogen by the passivated cobalt.

For each catalyst, the rate of reduction concerning the amount of cobalt present in the catalyst and

the stoichiometry of (Eq. 13) and (Eq. 14) was calculated by the following expression:

𝑅𝑅 (%) = (1 −𝑉𝑡𝑜𝑡𝑎𝑙 𝑜𝑓 𝐻2 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 − 𝑉𝐻2𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑏𝑦 𝑟𝑒𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛

34⁄ × 𝑉𝑡𝑜𝑡𝑎𝑙 𝑜𝑓 𝐻2 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑏𝑦 𝑜𝑥𝑖𝑑𝑖𝑧𝑒𝑑 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

) ×𝑤𝑡% 𝐶𝑜 𝑟𝑒𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑇𝑃𝑅

𝑤𝑡% 𝐶𝑜 𝑖𝑛 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡

× 100

(Eq. 21)


The comparison between the TPR profile of the reduced and oxidized reference catalyst is shown

in the following figure.

Figure 46 - Comparison between the oxide and reduced TPR profile for the reference catalyst

In the reduced TPR profile, the first peak of hydrogen consumption (between approximately 335°C

and 400oC) is assumed to be due to the reduction of passivated cobalt. Another important point is the

fact that the reduction process was not able to reduce the species formed from the strong interaction

metal-support, since the TPR profiles of oxidized and reduced catalyst almost overlap for temperatures

higher than 700°C.

For catalyst with sodium nitrate, the reduced TPR profile is the following.

65

Figure 47 - Comparison between the oxide and reduced TPR profile for the Co(NO3)2.6H2O/NaNO2 catalyst

Once more, on the reduced TPR profile, a peak at approximately 390°C is presented due to the

reoxidation of the cobalt. As it happened with the reference catalyst, there are peaks at temperatures

higher than 700°C due to the cobalt silicates and cobalt aluminates, as already explained. On the other

hand, it is noticed that the peak corresponding to the decomposition of sodium nitrate in the oxide

catalyst no longer exists on the reduced catalyst TPR profile, which means that the reduction step was

able to remove all this compound.

Concerning CoI2 and Co(CH3COO)2.4H2O, as it was expected from the TPR profiles of the oxidized

catalysts, these catalysts are hardly reduced since they present the smallest particle size. To illustrate,

in the following figures are the TPR profile for both catalysts.

Figure 48 - Comparison between the oxide and reduced TPR profile for (a) CoI2 and (b)

Co(CH3COO)2.4H2O catalysts

To summarize the results obtained from the TPR profiles of reduced catalysts, the reduction rates

for each catalyst are in the following table. As it was expected by regarding the corresponding TPR

profiles, cobalt iodide and cobalt acetate have reduction rates close to zero. For the catalyst with sodium

nitrite, the rate of reduction was not possible to calculate, since it was hardly to distinguish between the

hydrogen volume consumed due to decomposition of nitrates and to reduction of Co3O4 to CoO on the

oxidized catalyst TPR profile.

0

0,5

1

1,5

2

0 200 400 600 800 1000

Co

nsu

mp

tio

n o

f h

yd

rog

en

(m

L/m

in)/

g c

ata

lyst

Temperature (°C)

Co(NO3)2.6H2O/NaNO2 - Oxidized catalyst

Co(NO3)2.6H2O -Reduced catalyst

(a) (b)

66

Table 15 – Rate of reduction for reduced catalysts prepared by wetness impregnation

Reference Catalyst Rate of reduction (%) Co0 particle size

(nm)

LD-R-1 CoI2 ≈0 2

LD-R-2 Co(CH3COO)2.4H2O ≈0 4

LD-R-8 CoI2/Pt ≈0 2

LD-R-9 Co(CH3COO)2.4H2O/Pt ≈0 3

LD-R-15 Reference (Co(NO3)2.6H2O) 36 8


The next figure shows the comparison between the TPR profiles of the oxidized and the reduced

catalyst prepared by steaming at (T1; D1; R1) conditions.

Figure 49 - Comparison between the oxide and reduced TPR profile of catalyst prepared by steaming at

(T1; D1; R1) conditions

Regarding the above TPR profile, the first conclusion is that this catalyst was well reduced. As the

catalysts prepared by wetness impregnation, the first peak that is presented at approximately 285°C is

due to the partial reoxidation of cobalt.

The rest of comparisons for the catalysts (T2; D3; R1), (T2; D2; R2) and (T2; D2; R1) are in the

following figures. By regarding the following profiles, it is possible to see that the peak corresponding to

the reoxidation of cobalt is almost at the same temperatures (approximately 280°C). Another point that

is common for all steaming catalysts is that the oxidized and reduced TPR profiles overlap for

temperatures higher than 700oC, which correspond to cobalt aluminates and cobalt silicates that are not

reduced.

67

Figure 50 - Comparison between the oxidized and reduced TPR profile for (a) (T2;D3;R1), (b) (T2;D2;R1)

and (c) (T2; D2; R2) catalyst

The reduction rate of the previous catalysts was calculated by (Eq. 21) and the results are in the

following table. The results were as expected, since the catalysts with the highest average particle size

is the one that shows the highest rate of reduction.

Table 16 - Rate of reduction for reduced catalysts prepared by steaming

Reference Catalyst Rate of reduction (%) Co0 particle size (nm)

LD-R-5 T2; D2; R2 48 16

LD-R-13 T2; D3; R1 38 15

LD-R-16 T2; D2; R1 62 22

LD-R-20 T1; D1; R1 87 32

a) b)

c)

68

5.1.6 Global summary of physic-chemical properties

According to X-Ray Fluorescence, all catalysts have shown the desired cobalt content, except the

ones prepared by homogeneous deposition-precipitation method. So, while wetness impregnation was

able to deposit the active phase precursors over the support, the results obtained for deposition

precipitation suggests that the precipitation within the porous support did not occur. For catalysts

prepared by at lower steaming, nitrates have not been completely decomposed, causing a dilution of

cobalt present in the catalyst, which might explain a slight less cobalt content than it was expected.

Concerning textural properties, catalysts prepared by steaming show presence of mesoporosity

and there are not major changes of textural properties.

XRD patterns for all catalysts exhibit peaks of aluminum oxide and crystallite Co3O4. For catalysts

prepared by wetness impregnation using cobalt nitrate, the diffraction lines are relatively narrow,

suggesting that these catalysts are well crystallized. In contrast, catalysts prepared with cobalt acetate,

cobalt iodide and ethylene glycol show weaker and broader peaks corresponding to Co3O4, suggesting

that either almost no crystallite phase was detected or Co3O4 crystallites were too small. The same

conclusion can be drawn for promoted catalysts and catalysts prepared by deposition-precipitation.

Furthermore, catalysts prepared by steaming show cobalt diffraction lines stronger and less broadened,

which means that the cobalt structure might contain fewer structural defects.

Catalysts prepared with cobalt acetate, cobalt iodide and ethylene glycol have the smaller Co3O4

crystallite sizes, according to XRD analysis, suggesting a strong metal-support interaction. Catalysts

prepared by deposition-precipitation also show small crystallite size. These results were expected due

to the weak intensity of Co3O4 peaks, but because of that, there is a lack of precision for particle size

determination. Then again, steaming catalysts have higher crystallite size, which proposes that sintering

phenomena has occurred, as it was expected.

Besides XRD characterization, TEM analysis were also performed in order to determine particle

size. The results obtained for both analysts were different, since particle sizes are determined on the

oxide catalyst for XRD and on the reduced catalyst for TEM. The higher deviations were found for

catalysts having smaller particle sizes, since for these ones, only largest particles could be reduced,

while the smallest ones are partially or not reduced. So that, the results obtained from TEM correspond

to a small fraction of reduced cobalt present in catalysts, corresponding to the largest particles, while

XRD analysis to the entire oxide cobalt phase.

So, Co3O4 average particle size measured by XRD ranges from 2 to 43 nm. For catalysts prepared

by wetness impregnation it ranges from 3 to 13 nm, for steaming catalysts is between 15 and 43 nm

and for deposition-precipitation is between 2 and 4 nm.

TPR profiles for catalysts prepared by wetness impregnation using cobalt nitrate are a characteristic

two-step reduction pattern, corresponding to the reduction of Co3O4 to CoO and after CoO to Co.

Unlike, catalysts prepared by steaming, whose TPR profiles show different reduction peaks,

suggesting a particle size multimodal distribution, wherein the first peak corresponds to the reduction of

residual NOx groups.

69

This peak hinges on the severity of steaming conditions, being more intense for smaller steaming

temperatures. As it was expected, catalysts prepared by wetness impregnation show less reduction

rates, since they have smaller Co3O4 particles, while the ones prepared by steaming, having larger

particles, show higher reduction rates. Also, promotion with platinum did not have the desired effect,

since promoted catalysts remained hard to reduce.

5.2 Catalytic performances of chosen catalysts

The choice of catalysts to perform in catalytic test was made according to their rate of reduction

and in order to have a wide range of cobalt particle size. The chosen catalysts are in the following table.

Table 17 – List of catalysts to perform in catalytic test

Reference Catalyst precursor Preparation method wt% Co

Co particle size (nm)5

Reduction rate (%)

LD-R-15.2 Co(NO3)2.6H2O

(Reference catalyst) Impregnation 8,2 8 36

LD-R-13.2 Co(NO3)2.6H2O Steaming (T2; D3; R1) 7,8 15 38

LD-R-16.2 Co(NO3)2.6H2O Steaming (T2; D2; R1) 7,5 22 62

LD-R-20 Co(NO3)2.6H2O Steaming (T1; D1; R1) 7,5 32 87

5.2.1 Evaluation of activity

CO consumption during FT synthesis at 220°C and 20 barg was obtained from CO conversion and

CO flow rate (NL/h). CO consumptions measured for the four chosen catalysts are compared in Figure

51. The catalytic tests were run at a constant target CO conversion (40%), by adjusting syngas flow rate

daily. According to the following figure, after approximately 70h of test, CO consumption has stabilized

and for that reason activity will be compared for that time. However, the activity LD-R-13.2 is not possible

to be compared with the other catalysts, since this catalyst has never reached the target CO conversion,

thus it has a much lower CO consumption. Actually, this catalyst has not shown a sufficient activity and

so that, it reached a limiting point below which it was not possible to adjust CO and H2 flow in order to

maintain it at a constant conversion.

5 Determined by XRD (eq. 20).

70

Figure 51 – CO consumption during catalytic test

Another conclusion that can be drawn from Figure 51 is that CO consumption gradually decreased

with time on stream. This is due to catalyst deactivation coming from structural changes of cobalt such

as cobalt oxidation, formation of cobalt mixed compounds, coke deposition, cobalt particle sintering and

catalyst attrition, which could have occurred [4]. These effects seem more pronounced with smaller

particles [4], which might explain the highest decrease in CO consumption for reference catalyst,

although it still has the highest CO consumption.

The activity of each catalyst was compared by meanings of TOF (turnover frequency), for the same

time on stream (70 h). This value was calculated by (Eq. 22), taking into account the rate of reduction

(Table 17) and dispersion for each catalyst. This last parameter was obtained by R. Van Hardeveld and

F. Hartog method [47], using Co particle size calculated from XRD (Eq. 20) and considering a cub-

octahedral shape for Co particles.

𝑇𝑂𝐹 (𝑠−1) =%𝐶𝑂 𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛 × 𝑄𝐶𝑂 × 𝑀𝑀𝐶𝑜

𝑉𝑚 × 𝑚𝑐𝑎𝑡𝑎 × %𝐶𝑜 × %𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑟𝑎𝑡𝑒 × %𝐷𝑖𝑠𝑝𝑒𝑟𝑠𝑖𝑜𝑛 (Eq. 22)

The results are gathered in the following table. Dispersion is higher for smaller particles, as it was

expected. The LD-R-13.2 catalyst presents the lowest TOF due to its lower CO conversion, so that its

activity will not be compared with the other catalysts.

Table 18 – Results of dispersion and TOF

Reference Catalyst precursor Co particle size (nm)

Dispersion (%) TOF (×10-3) (s-1)

LD-R-15.2 Co(NO3)2.6H2O

(Reference catalyst) 8 15,5 84,1

LD-R-13.2 Co(NO3)2.6H2O 15 8,5 19,9

LD-R-16.2 Co(NO3)2.6H2O 22 5,8 58,1

LD-R-20 Co(NO3)2.6H2O 32 4,0 66,4

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0 50 100 150 200 250

(%C

o c

on

vers

ion

×C

O

flo

w)/

gcata

lyst

(NL

/h/g

cata

lyst)

Time on stream (h)

LD-R-15.2 - 8 nm

LD-R-13.2 - 15 nm

LD-R-16.2 - 22 nm

LD-R-20 - 32 nm

71

In the following figure is the relation between TOF and cobalt particle size, for all the catalysts

studied except the one with 15 nm (LD-R-13.2). So, for these FT operating conditions, TOF does not

seem to be dependent on particle size, within the studied range.

Figure 52 – Relation between cobalt particle size and activity, recorder at the same time on stream (70h)

The trend found for TOF and cobalt particle size is in good agreement with the literature, as it can

be seen in the following figure [22] [48]. Caution should be taken when comparing absolute TOF values

with different operating conditions and with different supports, since they might strongly influence the

activity of the catalyst. Also, the time on stream for which the TOF calculations are done also influences

the obtained results. For a higher time on stream, catalyst deactivation will be higher leading to lower

activities.

Figure 53 – TOF as a function of cobalt particle size comparing with the literature

1

10

100

0 10 20 30

TO

F (×

10

-3)

(s-1

)

Co particle size (nm)

1

10

100

0 20 40 60 80 100

TO

F (×

10

-3)

(s-1

)


Fixed bed; 220°C;H2/CO=2; 20 bar;%CO=10%;Co/SiO2 [22]

Fixed bed; 190°C;10 bar;H2/CO=2;%CO=10%;Co/Al2O3 [48]

Studied Catalysts

72

5.2.1 Evaluation of selectivity

CH4 and C2-C4 selectivity for each catalyst (except LD-R-13.2 for the reasons already explained) is

presented in the following figure. An average of selectivity in the different hydrocarbons fractions

measured during time on stream was considered, except for the point of the beginning of the test which

corresponds to adjustment of CO and H2 flow in order to achieve iso-conversion. CH4 and C2-C4

selectivity appear to stabilize with increasing cobalt particle size, even so it is slightly lower for larger

particles.

Figure 54 – Average CH4 and C2-C4 selectivity as a function of Co particle size

C5+ selectivity shows the same behavior as CH4 and C2-C4 selectivity, as it can be shown in Figure

55. However, C5+ selectivity is slightly higher for larger cobalt particle size.

Figure 55 – Average C5+ selectivity as a function of Co particle size

CH4 and C5+ selectivity follow the same trend concerning cobalt particle size, as the ones found in

the literature [22] [23]. Once more, operating conditions play an important role in the results obtained.

0

5

10

15

8 22 32

Sele

cti

vit

y(%

)

Co Particle size (nm)

CH4 selectivity

C2-C4 selectivity

70

75

80

85

90

95

0 10 20 30 40

C5+

Sele

cti

vit

y(%

)

Co Particle size (nm)

73

Comparing the results obtained with the ones at 1 bar [23], CH4 selectivity is much lower in the

present case (20 bar), since a decrease in pressure changes FT product distribution towards more

methane formation.

Figure 56 - CH4 selectivity as a function of cobalt particle size – comparison with the literature

Figure 57 – C5+ selectivity as a function of cobalt particle size – comparison with the literature

74

5.2.2 Subjected tendencies and summary

To summarize, the main results of the catalytic tests are gathered in the following table. CO

conversion was between 41 and 43%, which is in good agreement with the target value (40%), except

for LD-R-13.2. For that reason, this catalyst has shown a TOF value, which was not taken into account

for performances tendencies comparison. On the other hand, all catalysts have shown a significant

selectivity towards C5+.

Table 19 – Principal results obtained for catalytic tests

Reference Co particle size

(nm) CO conversion

(%) TOF (×10-3) (s-1)

Selectivity (%)

CH4 C5+ C2-C4

LD-R-15.2 8 41,2 84,1 8,18 81,26 9,84

LD-R-16.2 22 42,9 58,1 9,34 80,14 9,88

LD-R-20 32 43,1 66,4 7,86 82,02 9,49

For the range of particle size studied, the general trend is a stability in both TOF, CH4 and C5+

selectivity with increasing cobalt particle size.

One last annotation is that it was not possible to determine probability of chain growth, α. In order

to get this parameter, a sufficient amount of waxes has to be produced during the catalytic test. Thus,

catalyst must have a sufficient activity in order to perform the test at the target CO conversion and to

achieve a sufficient wax productivity. However, for the present catalysts, the wax productivity was not

high enough to obtain a representative wax sample within reasonable test durations. Therefore, it was

not possible to determine the α selectivity. Unless it is observed that no pronounced variation of CH4

and C5+ occurs with Co particles size modification in the range of 8-32 nm, it is not possible to assume

that no change will be expected for catalyst behavior concerning α parameter. As a matter of fact, CH4

selectivity is not correlated with chain growth probability, and even if C5+ selectivity is correlated only a

slight variation in this value (which in this work was not measure because of test precision) may induce

significant modification in α value.

75

6 Conclusion and Future Work

The aim of this work was to study the impact of cobalt particle size on the performance of cobalt

supported FT catalysts (8%wt Co), especially concerning α selectivity. In order to do that, different

preparation methods were performed, with different cobalt precursors and different thermal treatments

so as to reach a wide range of cobalt particle size.

Catalysts prepared by wetness impregnation followed by either calcination or steaming were well

succeeding, unlike the ones prepared by deposition-precipitation using ammonia evaporation. For the

latter, the precipitation process has not occurred, possibly because ammonia evaporation has not taken

place, which explains the lower cobalt content found for these catalysts. Moreover, the protocol followed

in this method was adapted from the literature; however in the present study a commercial silica-alumina

support was used, different from the one in the literature, which might also explain the ineffectiveness

of this method.

A wide range of Co0 particle size between 2 and 32 nm was achieved, concerning catalysts

prepared by wetness impregnation and steaming. For the first method, Co0 particle sizes between 2 and

10 nm were obtained as it was desired (for which may be suspected strong metal-support interaction).

For the second method, particle sizes between 12 and 32 nm were found, demonstrating sintering

phenomena for the larger ones.

Steaming method is an interesting method to synthesize catalysts with larger particles. Analyzing

the results obtained for cobalt particle size as a function of steaming conditions, a pattern was found:

higher severity of steaming conditions lead to small particles. Actually, diffusion rate increases with

increasing temperature, but increasing temperature might have induced a rapidly formation of Co3O4,

thus finishing the diffusion process leading to smaller particles.

Besides cobalt particle size, cobalt reducibility was another parameter characterized in this study.

For smaller particles, lower reduction rates were obtained even with platinum promoted catalysts,

meaning that they might not have a sufficient activity if performed, contrasting with catalysts having

larger particles.

Taking into account the results obtained of cobalt particle size and reduction rates, catalysts with

Co0 size between 8 and 32 nm were chosen to perform catalytic tests at industrial conditions (slurry

reactor, 220°C, 20 bar and H2/Co equal to 2). The aim of these tests was to measure and compare

catalytic activity, selectivity and the probability of chain growth, α.

Catalysts activity was measured by meanings of TOF and for the range of particle size studied,

TOF seems to somewhat stabilize with increasing particle size. Larger particles were slightly more

selective towards hydrocarbons with a long chain, while smaller particles led to more methane formation.

Nevertheless, the general trend is a stabilization of CH4 and C5+ selectivity with increasing particle size.

Measuring probability of chain growth was not possible as catalysts activity was not sufficient.

In these conditions, in order to measure a value of α, catalysts would have to be tested during a

too long time, which is not possible due to catalyst initial activity and deactivation. Thus, reporting α

parameter as a function of cobalt particle size was not possible.

76

Another conclusion that can be drawn is that the trend observed for TOF and selectivity is in good

agreement with the literature, although it is not possible to compare absolute values since they strongly

depends on operating conditions and on time on stream for which the calculation are done.

As perspective for this study, it would have also been interesting to perform a severe steaming of

the support, in order to create higher support porosity, thus obtaining larger particles. Another interesting

point to study will be the reproducibility of steaming method, since for some catalysts prepared at the

same steaming conditions, the results obtained concerning cobalt particle size and cobalt reducibility

were quite different. The difference in these results might be related to the faulty of water vaporization

through the bed of SiC used in the steaming reactor. Concerning cobalt reducibility, oxide promotion

could be also tested in order to promote reduction rate and consequently increase the amount of

available Co metal surface area.

Increasing cobalt loading, and thus the average cobalt particle size and cobalt reducibility is another

suggestion. In that way, catalytic activity would also be improved. However, due to the added expense

of cobalt, it would be important to determine the appropriate loading of cobalt to maximize activity as

well as selectivity, taking into account economic reasons.

Finally, the major challenge is still to set α values and investigate the real effect of particle size.

Although in this work stabilization in C5+ selectivity is reported with increasing particle size, it does not

mean that α will stabilize too. Actually, there are some errors in measuring C5+ selectivity which lead to

a lack of precision in these values. A weaker deviation in C5+ selectivity measurements, could lead to a

higher variation in α. On the other hand, there is still some deviations in the ASF distribution concerning

CH4 selectivity. Even C5+ selectivity and α are correlated, the same is not true for CH4 selectivity,

meaning that a higher methane production does not mean that a lower α will be obtained.

77

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81

8 Appendix

Figure List

Figure A 1 – Co particle size distribution in volume for cobalt iodide catalyst ...................................... 81

Figure A 2 - Co particle size distribution in volume for cobalt acetate catalyst ..................................... 81

Figure A 3 - Co particle size distribution in volume for cobalt nitrate with sodium nitrite catalyst ......... 82

Figure A 4 - Co particle size distribution in volume for catalyst prepared by steaming at (T2;D2;R1).. 82

Figure A 5 - Co particle size distribution in volume for catalyst prepared by steaming at (T1; D1; R1) 82

Figure A 6 – t-plot for catalyst prepared by steaming at (T2; D2; R1) .................................................. 83

Figure A 7 - t-plot for catalyst prepared by steaming at (T1;D1;R1) ..................................................... 83

8.1 TEM Characterization

The particle size distribution obtained in volume by TEM for catalysts prepared by wetness

impregnation is in the following figure.

Figure A 1 – Co particle size distribution in volume for cobalt iodide catalyst

Figure A 2 - Co particle size distribution in volume for cobalt acetate catalyst

0

0,02

0,04

0,06

0,08

0,1

0,12

0 2 3 5 6 8 9 11 12 14 15 17 18

Fre

qu

en

cy (

in v

olu

me)


82

Figure A 3 - Co particle size distribution in volume for cobalt nitrate with sodium nitrite catalyst

For catalysts prepared by steaming the results are the following.

Figure A 4 - Co particle size distribution in volume for catalyst prepared by steaming at (T2;D2;R1)

Figure A 5 - Co particle size distribution in volume for catalyst prepared by steaming at (T1; D1; R1)

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0 2 3 5 6 8 9 111214151718202123242627

Fre

qu

en

cy (

in v

olu

me)


0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

0,09

0,1

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Fre

qu

en

cy (

in v

olu

me)


0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Fre

qu

en

cy (

in v

olu

me)


83

8.2 Textural properties

The t-plot graphs for catalysts prepared by steaming are in the following figures.

Figure A 6 – t-plot for catalyst prepared by steaming at (T2; D2; R1)

Figure A 7 - t-plot for catalyst prepared by steaming at (T1;D1;R1)

0

50

100

150

200

250

300

0 0,5 1 1,5 2 2,5

Ad

so

rbed

vo

lum

e (

ml/g

)

Thickness statistiques (nm)

0

50

100

150

200

250

300

0 0,5 1 1,5 2 2,5

Vo

lum

e A

dso

rbé (

ml/g

)

Thickness statistiques (nm)

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