Innovative preparation of catalysts by aerosol route for ... · Usually Fischer – Tropsch (FT)...
Transcript of Innovative preparation of catalysts by aerosol route for ... · Usually Fischer – Tropsch (FT)...
Innovative preparation of catalysts by aerosol route for the
Fischer – Tropsch synthesis
Maria Joana Figueiredo Rodrigues
Thesis to obtain the Master of Science Degree in
Chemical Engineering
Supervisors: Dr. Alexandra Chaumonnot (IFPEN)
Dr. Antoine Fécant (IFPEN)
Prof. Carlos Henriques (IST)
Examination Committee
Chairperson: Prof. José Madeira Lopes (IST)
Supervisor: Prof. Carlos Manuel Faria de Barros Henriques (IST)
Members of the committee: Prof. Maria Filipa Gomes Ribeiro (IST)
October 2015
This page was intentionally left blank
“If we knew what we were doing it would not be called research, would it?”
Albert Einstein
This page was intentionally left blank
v
Resumo
Em termos gerais, os catalisadores usados no processo de Fischer – Tropsch (FT) são obtidos
através da deposição do cobalto (fase ativa) no suporte. Este óxido é habitualmente obtido pelo spray
– drying de uma solução que contém precursores inorgânicos. O principal objetivo deste trabalho é não
só a síntese de catalisadores para o processo de FT através da técnica de spray – drying, mas também
a sua caracterização.
Foram sintetizados sólidos suportados quer em sílica quer em alumina, onde se verificou uma
grande dispersão do cobalto, o que originou uma forte interação molecular entre o precursor de cobalto
e o precursor inorgânico molecular, confirmando-se assim a existência quer de silicatos de cobalto ou
aluminatos de cobalto. De modo a diminuir esta interação, foram modificados os vários parâmetros da
preparação, como por exemplo, a alteração do pH das soluções de atomização, de modo a induzir
atração ou repulsão electroestática entre o precursor de cobalto e o precursor de sílica ou alumina na
solução inicial. Foram obtidos melhores resultados quanto à acessibilidade e redutibilidade do cobalto
no caso da sílica para um pH mais ácido, apresentando, ainda assim, a formação de silicatos de cobalto.
Deverá ser realizado um estudo mais detalhado quer à sílica quer à alumina relativamente aos
parâmetros de síntese. Deverá ser igualmente testada a incorporação de um metal promotor de modo
a aumentar a redutibilidade do cobalto.
Palavras – chave: Catalisadores de fischer – Tropsch, spray – drying, aerossol, cobalto, sílica,
alumina.
This page was intentionally left blank
vii
Abstract
Usually Fischer – Tropsch (FT) catalysts are obtained through the deposition of cobalt active phase
onto the support. This oxide support is normally obtained by a spray – drying synthetic pathway. The
present work aims the synthesis and characterization of FT catalyst through spray-drying pathway.
Silica and alumina based solids were investigated. It was found a very high dispersion of cobalt
leading to a high molecular interaction between the cobalt precursor and inorganic molecular precursor.
Therefore, it was confirmed the existence of cobalt silicates or aluminates. In order to weaken Co and
carrier atoms interactions, several preparation parameters were modified, like modifying pH of the
atomizing solution to induce electrostatic attraction or repulsion of Co and Si or Al precursors in the initial
solution. Better results regarding the cobalt accessibility and reducibility were achieved for a more acidic
media. Nevertheless, it was still noticed the presence of cobalt silicates.
A more exhaustive study should be done concerning synthesis parameters for both alumina and
silica carriers. Furthermore, the loading of a promoter metal should be tried in order to improve the cobalt
reducibility.
Keywords: Fischer – Tropsch catalysts, spray – drying, aerosol, cobalt, silica, alumina.
This page was intentionally left blank
ix
Acknowledgements
First of all I would like to express my special thanks of gratitude to Prof. Filipa Ribeiro for making
possible, every year, the internships between IST and IFPEN. This opportunity allow us, students, the
chance of working in this well - known institution. Also, Victor Costa and Joana Fernandes thank you for
all your help and advices during this six months.
Secondly, I would like to thank my supervisors at IFPEN, Alexandra Chaumonnot, Antoine Fécant
and Souad Rafik – Clément for all the support since my arrival. Also, I am very grateful for your endless
patience answering to my questions, as well as, for all your efforts that helped me improving myself.
Surely, this was a successful work due to your contribution. Furthermore, I would like to thank to Prof.
Carlos Henriques for all you interest and availability as well as your suggestions to this work.
I would like to thank everyone in my department that made possible my integration, and how patient
they were when I could not speak a word of French. Also I would like to thank to Eugènie Rabeyrin for
your kindness and patience and effort to teach me French and to Adrien Berliet for helping me everytime
that I needed.
To all the phD students in Lyon: Leonor, Rúben, Sónia, Svetan, Fabien, Matthieu , Pedro, Leonel
and Dina thank you for receiving us so kindly.
To my family in Lyon: Catarina, Solange, David, Ana, Diogo, Tiago, João and Pedro thank you for
making me feel not so far from home. Also, Frederico thank you for visit, it was really important for me.
To Sofia, Gonçalo, Constança, Tomás and Tiago thank you for your support and comprehension
during all my course. Without your true friendship would not have been the same.
Last but not least, I would like to thank my mother, father, sister and brother for all you support
during my course. For certain I would not have done it without your presence in my life. Thank you for
all your encouragements and advices that made who I am today. Also, João and Carolina, thank you for
all your support and interest in my progress, is was truly meaningful for me.
This page was intentionally left blank
xi
Table of Contents
Resumo ....................................................................................................................................................v
Abstract ................................................................................................................................................... vii
Acknowledgements ................................................................................................................................. ix
Table of Contents .................................................................................................................................... xi
Table List ............................................................................................................................................... xiii
Figure List ............................................................................................................................................... xv
Glossary ................................................................................................................................................ xvii
1. Introduction ...................................................................................................................................... 1
2. Bibliographic Study .......................................................................................................................... 3
2.1. Fischer – Tropsch Process ...................................................................................................... 3
2.1.1. Reactors .......................................................................................................................... 4
2.1.2. Reactions ......................................................................................................................... 5
2.1.3. Reaction mechanism ....................................................................................................... 5
2.1.4. Product distribution .......................................................................................................... 6
2.2. Fischer – Tropsch catalysts ..................................................................................................... 7
2.2.1. Active phase .................................................................................................................... 7
2.2.2. Supports .......................................................................................................................... 8
2.2.3. Metals and oxide promoters ............................................................................................ 8
2.2.4. Usual catalyst preparation methods ................................................................................ 9
2.2.5. Activity and selectivity .................................................................................................... 11
2.2.6. Catalyst deactivation...................................................................................................... 12
2.3. One – Pot synthesis: Incorporation of the metallic phase on a mesoporous oxide matrix.... 13
2.3.1. Sol-gel chemistry ........................................................................................................... 13
2.3.2. Mesostructured materials: definition .............................................................................. 14
2.3.3. Mesostructured materials: mechanisms ........................................................................ 15
2.3.4. Mesostructured materials: synthesis techniques ........................................................... 15
3. Experimental Work ........................................................................................................................ 21
3.1. Preparation Methods ............................................................................................................. 21
3.1.1. Supports ........................................................................................................................ 21
3.1.2. Catalysts ........................................................................................................................ 23
3.2. Spray – Drying ....................................................................................................................... 26
3.2.1. Spray Drying working principle ...................................................................................... 26
3.2.2. Spray – Dryer Parameters ............................................................................................. 28
3.3. Characterization Methods ...................................................................................................... 29
3.3.1. N2 adsorption-desorption ............................................................................................... 30
3.3.2. Temperature Programmed Reduction (TPR) ................................................................ 31
xii
3.3.3. X-Ray Diffraction (XRD)................................................................................................. 31
3.3.4. Low angles X-Ray Diffraction ........................................................................................ 32
3.3.5. X-ray Photoelectron Spectroscopy (XPS) ..................................................................... 32
3.3.6. Transmission Electronic Microscopy (TEM) .................................................................. 32
3.3.7. Scanning Electronic Microscopy (SEM) ........................................................................ 32
4. Results and Discussion ................................................................................................................. 35
4.1. Silica ...................................................................................................................................... 36
4.1.1. Supports ........................................................................................................................ 36
4.1.2. Cobalt catalysts ............................................................................................................. 40
4.2. Alumina .................................................................................................................................. 55
4.2.1. Supports ........................................................................................................................ 55
4.2.2. Cobalt catalysts ............................................................................................................. 57
5. Conclusions and future perspectives............................................................................................. 63
6. References .................................................................................................................................... 65
Appendix A: t- plot curves ........................................................................................................................ A
xiii
Table List
Table 1 – Reference molar composition for all the supports. ................................................................ 22
Table 3 – Synthesized supports. ........................................................................................................... 22
Table 4 – Reference molar composition for the catalysts on a silica matrix. ........................................ 23
Table 5 – Reference molar composition for the catalysts on an alumina matrix. .................................. 23
Table 6 – Synthesized catalysts. ........................................................................................................... 24
Table 7 – Spray – drier parameters for all the samples. ....................................................................... 29
Table 8 – Relation between analysis and the expected characterized properties. ............................... 29
Table 9 – BET surface, porous volume and pore diameter for JFR003, JFR005, JFR006 and JFR007.
............................................................................................................................................................... 37
Table 10 – Results of the elementary particle size determinate by SEM. ............................................. 39
Table 11 – BET surface, porous volume and pore diameter for JFR005, JFR014 and JFR016. ......... 41
Table 12 – BET surface, porous volume and pore diameter for JFR005, JFR028, JFR016 and JFR021.
............................................................................................................................................................... 45
Table 13 – BET surface, porous volume and pore diameter for JFR016, JFR019, JFR021 and JFR022.
............................................................................................................................................................... 48
Table 14 – Results from XPS analysis for JFR019 and JFR021. ......................................................... 52
Table 15 – BET surface, porous volume and pore diameter for JFR009 and JFR010. ........................ 56
Table 16 – BET surface, porous volume and pore diameter for JFR010, JFR023, JFR025 and JFR024.
............................................................................................................................................................... 58
Table 17 – BET surface, porous volume and pore diameter for JFR024 (reference) and JFR037. ..... 60
This page was intentionally left blank
xv
Figure List
Figure 1 - Fuel consumption since 1965 up to 2035. [1] ......................................................................... 1
Figure 2 –FT process-based production scheme. Adapted from [4] ....................................................... 3
Figure 3 - FT stepwise growth process, where α stands for probability of chain growth and d stands for
desorbed species. [9] .............................................................................................................................. 6
Figure 4 - Product selectivity as function of ASF chain growth probability α. [10] .................................. 7
Figure 5 - Hydrolysis (H), Condensation (C) and Dissolution (D) kinetics for a system of TEOS. [20] . 14
Figure 6 – Micelle structure A: Sphere, B: Cylindric, C: Lamellar, D: Inverse micelle, E: Bicontinuous
phase, F: Liposomes. [20] ..................................................................................................................... 15
Figure 7- Main synthesis pathways to mesostructured materials. [21] ................................................. 15
Figure 8 - Spray - Dryer (Büchi B 290) working principle and material's structuration mechanism.
Adapted from [20] .................................................................................................................................. 18
Figure 9 – Calcination profile for the synthesized supports. ................................................................. 23
Figure 10 - Calcination profile for JFR014 and JFR016. ....................................................................... 25
Figure 11 - Calcination profile for JFR019, JFR021, JFR022, JFR023, JFR024 and JFR037. ............ 25
Figure 12 - Spray drying apparatus. [27] ............................................................................................... 26
Figure 13 – Overview over the spray-dryer parameters and their influence in the final product. [28] .. 28
Figure 14 – Typical isotherm for a mesostructured solid. [29] .............................................................. 30
Figure 15 – Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR003, JFR005, JFR006 and JFR007. ...................... 36
Figure 16 – SEM micrographs for (A) JFR005, (B) JFR006 and (C) JFR007. ...................................... 38
Figure 17 – Particles size distribution for JFR005, JFR006 and JFR007. ............................................ 38
Figure 18 – Low angles XRD patterns for JFR005, JFR006 and JFR007. ........................................... 40
Figure 19 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR005 (support reference), JFR014 and JFR016. .... 41
Figure 20 - Scheme representing the solid surfaces calcinated at 350°C and 550°C. Adapted from [20]
............................................................................................................................................................... 42
Figure 21 – Low angles XRD patterns for JFR014 and JFR016. .......................................................... 43
Figure 22 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR005 (reference), JFR016 (reference), JFR028 and
JFR021. ................................................................................................................................................. 44
Figure 23 – TPR profile for JFR016 and JFR021. ................................................................................. 46
Figure 24 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR016 (reference), JFR019, JFR021 and JFR022. ... 48
Figure 25 - Low angles XRD patterns for JFR019, JFR021 and JFR022. ............................................ 49
Figure 26 - TPR profile for JFR019, JFR021 and JFR022. ................................................................... 50
Figure 27 - XRD diagrams: A) JFR019 and B) JFR021 and JFR022. .................................................. 51
Figure 28 - TEM micrographs for the support of JFR019. ..................................................................... 52
xvi
Figure 29 - TEM micrographs for metallic phase of JFR019. ................................................................ 52
Figure 30 – Cobalt oxide particles size distribution in volume for JFR019. .......................................... 53
Figure 31 - TEM micrographs for the supports of: (A) JFR021 and (B) JFR022. ................................. 53
Figure 32 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR009 and JFR010. ................................................... 55
Figure 33 - SEM micrographs for JFR009. ............................................................................................ 56
Figure 34 - Low angles XRD patterns for JFR009. ............................................................................... 57
Figure 35 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption for JFR010 (reference), JFR023, JFR025 and JFR024. ... 58
Figure 36 - TPR profile for JFR023 and JFR024................................................................................... 59
Figure 37 - Nitrogen adsorption – desorption as a function of relative pressure and pore size
distribution determined by BJH adsorption: JFR024 and JFR037. ....................................................... 60
Figure 38 - TPR profile for JFR024 and JFR037................................................................................... 61
xvii
Glossary
α– Probability of chain growth
β – Peak Breadth
λ – X – Ray wavelength (m)
Θ – Bragg angle (°)
BET – Brunauer – Emment – Teller
BJH – Barret – Joyner - Halenda
BTL – Biomass – to – liquid
C – Bragg constant
CMC – Critical Micelle Concentration
CTL – Coal – to – liquid
EDS – Energy Dispersive Spectroscopy
EISA – Evaporation Induced Self - Assembly
FT – Fischer – Tropsch
FWHM – Full – width half – maximum
GTL – Gas – to - liquid
HPA – Phospomolybdic heteropolyacid
HTFT – High Temperature Fischer –
Tropsch
L – Volume – averaged size of crystallites
LTFT – Low Temperature Fischer - Tropsch
𝑛 – Carbon number
PO – Saturation pressure
PPO – Polypropylene oxide
PS – Vapor pressure at the droplet surface
PZC – Point of zero charge
ROR – Reduction – Oxidation - Reduction
SEM – Scanning Electron Microscopy
TCD – Thermal Conductivity Detector
TEM – Transmission Electron Microscopy
TEOS – Tetraethyl Ortosilicate
TOF – Turnover frequency (s-1)
TON – Turnover number
TPAOH – Tretaprolylammonium hydroxide
TPR – Temperature Programmed Reduction
XPS – X – ray Photoelectron Spectroscopy
XRD – X – Ray Diffraction
WGS – Water Gas Shift reaction
𝑊𝑛 – Mass fraction of species with carbon
number, 𝑛
This page was intentionally left blank
1
1. Introduction
According to the BP energy outlook for 2035, the primary energy demand will increase by 41%
between 2012 and 2035. [1] As one can see in Figure 1 more than 50% of the global energy consumption
is provided by oil, gas and coal. Among fossil fuels, gas presents the fastest growth, accounting to 1.9%
per year. Nowadays, coal represents the largest source of volume growth. However between 2025 and
2035, it is expected that coal adds less volume than oil, which is justified by China’s shift away from
coal-intensive industrialization.
Figure 1 - Fuel consumption since 1965 up to 2035. [1]
Therefore, with the development of the emerging economies (China, India, Brazil, etc.), it is
expected an increase in the number of vehicles. Thus, to keep up with the transports energy
requirements, it will be necessary to increase the diesel fuel production. In addition, it is crucial to have
an alternative fuel synthesis. Indeed, crude oil is becoming more and more heavier, shale gas only
provides very light cuts and on the other hand it is expected that environmental requirements become
more exigent.
The Fischer - Tropsch process (FT) transforms a mixture of H2 and CO, often called syngas, into
liquid fuels. FT plays an important role in the search for an alternative way to produce liquid fuels with
several plants all over the world. For instance, Sasol operates a coal-based plant in South-Africa which
has a capacity of 160 000 bpd (barrels per day), where the main products are waxes, naphtha, high
quality diesel and kerosene. [2] Furthermore, in order to produce middle distillates (diesel and kerosene)
and waxes, low temperature FT process (LTFT) is used. In this process, cobalt catalysts supported on
alumina matrix are preferred. These catalysts are normally obtained through the incipient wetness
impregnation method. During this synthesis, the metallic phase solution is deposited onto the oxide
2
support and this process could take numerous impregnations. After impregnating the desired amount of
cobalt onto the support, the solid should be dried, calcinated and reduced to form the catalyst active
phase.
Thereby, it is crucial to develop an alternative synthesis pathway that reduces the number of
preparation steps.
As one can find in the bibliographic study, a review [3] reported the synthesis in one step of
mesostructured materials (well-organized structure with an uniform and periodic porosity (2-50 nm)
leading to a narrow pore size distribution) containing heteroelements (Ca, Fe, Zr and Al, Pd) on a silica
matrix. This type of synthesis was accomplished due to the combination of sol – gel chemistry and the
aerosol process.
The main objective of the present work is the synthesis of active FT catalysts, in one step, by spray
– drying. More precisely, this technique accomplishes the direct incorporation of the metallic phase onto
the inorganic matrix synthesis.
Concerning the report outline, in the bibliographic study (chapter 2), it is described the FT process,
the usual FT catalysts and its normal synthesis procedure. After, it is given a brief explanation in the sol
– gel chemistry and synthesis techniques to obtain mesostructured solids, where it is included the spray
– drying technique.
In chapter 3, it is described the different preparations methods as well as a description on the spray
– dryer work and the characterization methods (experimental work).
In chapter 4, it is possible to find the results and discussion in the physic – chemical properties of
the synthetized materials.
Finally, in the last chapter, chapter 5, are presented conclusions and future work on this theme.
Study objectives
The main goal of this work is the synthesis of FT catalyst by the spray – drying process in one step.
This synthesis pathway accomplishes a reduction in the number of steps required to synthetize a usual
FT catalyst.
To do that, two solutions should be prepared: the first one contains an inorganic molecular
precursor, acidified water and a cobalt precursor; the second one contains a surfactant, ethanol and
acidified water. These solutions should be mixed and the resulting solution is spray – dried. The obtained
powder should be dried, calcinated and reduced in order to form the active phase.
Moreover, as referred in the bibliographic study, the aerosol process can form mesostructured
materials, however it is not the aim of this work, as the FT process does not requires mesostructured
catalysts. The priority is to have a mesoporous catalyst.
3
2. Bibliographic Study
2.1. Fischer – Tropsch Process
The FT process was proposed by Hans Fischer and Franz Tropsch in 1925. This process converts
a mixture of hydrogen and carbon monoxide, usually called synthesis gas (or syngas), into a
hydrocarbon mixture. As one can note in the following equations, this mixture is mainly composed by
paraffins, olefins and oxygenated compounds (mainly alcohols) with water, respectively.
(2𝑛 + 1)𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+2 + 𝑛𝐻2𝑂 (Eq. 1)
2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛 + 𝑛𝐻2𝑂 𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2 (Eq. 2)
2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+2𝑂 + (𝑛 − 1)𝐻2𝑂 (Eq. 3)
Depending on the syngas source (non-petroleum feedstocks), the technology is often referred as
coal-to-liquids (CTL), when using coal to obtain syngas, or gas-to-liquid (GTL) when is used gas for
syngas production, and finally it is also possible to produce syngas from biomass, which is often referred
as biomass-to-liquid (BTL). The most used feedstock is methane due to its availability, and because it
is cheaper to build a methane-based plant than a coal-based plant (coal-based plants can cost up to
50% more than a methane-based plant). [3]
This process allows the production of methane (C1), petroleum gas (C2-C4), gasoline (C5-C11),
diesel and jet fuel (C12-C20) and wax (C21+), the latter being subsequently valorized into smaller
molecules (diesel and gasoline) through hydrocracking.
However, methane or petroleum gas production is undesirable due to less carbon conversion and
also since they are possible feedstocks for syngas synthesis.
The following figure exemplifies a FT synthesis based production scheme (Figure 2).
Figure 2 –FT process-based production scheme. Adapted from [4]
4
2.1.1. Reactors
The FT process can occur at high temperature (HTFT) (300-350°C) or at low temperature (LTFT)
(150-240°C). The first one is preferentially applied in circulating bed and fixed fluidized bed reactors with
iron catalysts, where it is only present the catalyst and gases. Regarding the second process (LTFT), it
occurs in a multibular fixed bed reactor where several tubes contain the catalyst and water is surrounding
them. Or it can happen in a slurry reactor where the catalyst is suspended in a liquid wax with syngas
bubbling through, with either an iron or cobalt catalyst.
Concerning the circulating bed and fixed fluidized bed reactors, they are normally used to generate
hydrocarbons between C1-C15, olefins. Concerning the generated oxygenated compounds, they are
separated and purified to produce alcohols, acetic acid and ketones. [5] Waxes cannot be produced in
these reactors because these compounds are liquid under normal FT conditions, thus the catalyst could
agglomerate and de-fluidize the reactor. Therefore, for wax production, it is preferred the LTFT process,
hence a multibular fixed bed or a slurry reactor.
Comparing all the above described reactors, for short-life catalysts, it is ideal to use a multibular or
a slurry reactor, in order to have longer runs, hence not having to stop the reactor to feed more active
catalyst (as required in fixed bed reactors). However, a drawback in using a slurry reactor is the need of
an additional device in order to remove the generated wax. This device is not needed in fixed bed
reactors because, as the wax runs through the bed, it is relatively easy to separate the catalyst from the
wax.
Khodakov et al. [6] stated several drawbacks from each reactor type. On one hand, in fixed bed
reactors there are diffusion limitations, a considerable pressure drop and an insufficient heat removal
that leads to substantial temperature gradients. Hence high levels of methane are expected due to
excessive cracking. On the other hand, in slurry reactors the major disadvantages are the catalyst
deactivation and attrition which leads to formation of fine powders.
An important benefit from the use of slurry reactors is the easier temperature control due to a well-
mixed slurry, which is crucial since FT reactions are exothermic. Hence, the operation in this reactor is
practically isothermal. Also, with slurry reactors, higher temperatures can be used without coke
formation, which prevents catalyst deactivation, and the pressure drop across the bed is much smaller
than in fixed bed reactors. [7]
The last FT developed technologies were about LTFT process and involved syngas with high H2/CO
ratio, which is originated through steam reforming (Eq. 4) of methane, where the latter reacts with steam
under pressure, and the syngas obtained has a H2/CO of 3.
Autothermal reforming (Eq. 5), presents the required energy for the endothermic steam reforming
which is provided by the exothermic oxidation reaction. Moreover, the obtained syngas will have a H2/CO
ratio of 1 or 2.5, whether the reagent used is CO2 or O2, respectively.
Syngas can also be obtained through partial oxidation of methane (Eq. 6), where the natural gas
reacts with less than the stoichiometric quantity of oxygen, generating a mixture with a H2/CO ratio of 2.
𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2 (Eq. 4)
𝐶𝐻4 + 𝐶𝑂2 → 2𝐶𝑂 + 2𝐻2 (Eq. 5)
5
𝐶𝐻4 +1
2𝑂2 ↔ 𝐶𝑂 + 2𝐻2
(Eq. 6)
2.1.2. Reactions
The main FT process reactions were already present in (Eq. 1), (Eq. 2) and (Eq. 3). However in
each process there are side reactions, and the FT process is not an exception. As one can see in the
previous equations, water is a FT product, and it slows the reaction rate. The generated water can react
with carbon monoxide (CO) to form carbon dioxide (CO2) and hydrogen (H2), which is called the water-
gas shift reaction (WGS) (Eq. 7) and is one of the most important side reactions.
𝐶𝑂 + 𝐻2𝑂 ↔ 𝐶𝑂2 + 𝐻2
(Eq. 7)
Also, α-olefins can participate in side reactions (α-olefins hydrogenation), as well as paraffins
(paraffins isomerization). [8] It can also occur the transformation of CO into solid carbon and carbon
dioxide, which is the so-called Boudouard equilibrium (Eq. 8). Nevertheless, CO can react with H2 to
produce methane which is the inverse reaction of (Eq. 6).
2𝐶𝑂 ↔ 𝐶 + 𝐶𝑂2
(Eq. 8)
All FT products are free of sulfur and nitrogen compounds, which make FT products very attractive.
With the LTFT process, better cetane numbers are achieved: diesel after hydrotreatment presents a
cetane number of 75, and the market requires cetane numbers of 45 up to 50.
Also, in the LTFT technology, waxes (mainly linear) represent 50% of the total products. One can
note that the FT straight-run naphtha, as well as the one produced in waxes hydrotreatment, contains
no aromatics, consisting in mainly linear hydrocarbons, hence an extensive isomerization and Pt-
reforming is needed to generate a high octane gasoline.
2.1.3. Reaction mechanism
One of the proposed FT mechanisms is the hydrogenation of the adsorbed CO generating CHx
monomers which are polymerized in order to produce hydrocarbons with a wide range of chain length.
Chain growth occurs by addition of surface methylene (CH2) groups either by β-hydrogen abstraction to
produce linear α-olefins or by hydrogen addition to form n-paraffins. The first one is a reversible
termination at FT normal conditions. Nevertheless, chain termination can also occur by CO insertion into
the surface which will lead to alcohols formation. [8]
The initial step involves the dissociation of CO in order to form chemisorbed carbon, which is
hydrogenated to surface methyl and methylene groups, as it is a catalytic reaction. [8]
Therefore, at the catalyst surface, each specie has the option to desorb to generate an olefin, or to
be hydrogenated and desorb afterwards to produce a paraffin, or it can simply continue the chain growth
by adding another methylene group (Figure 3).
6
.
Figure 3 - FT stepwise growth process, where α stands for probability of chain growth and d stands for desorbed species. [9]
2.1.4. Product distribution
The product distribution of FT synthesis can be described by the equation of Anderson-Schulz-
Flory (ASF). The ASF equation (Eq. 9) refers to the chain length distribution, which is related with the
probability of chain growth, given by α.
𝑙𝑜𝑔 (𝑊𝑛
𝑛) = 𝑛 𝑙𝑜𝑔𝛼 + 𝑙𝑜𝑔
(1 − 𝛼2)
𝛼
(Eq. 9)
In the above equation, 𝑊𝑛 is the mass fraction of species with carbon number 𝑛. This equation
assumes that the chain is formed by adding C1 monomers, and the probability of chain growth is given
by α, which is independent from the chain length.
There are several factors that influence the α-value, for instance, the operating temperature, the
catalysts promoters, as well as the partial pressures in contact with the catalysts. When the temperature
increases, the α-value decreases. Thus it produces a higher amount of light hydrocarbons, which is
undesirable. Besides, increasing the pressure will lead to an increase in the α-value.
Therefore, representing 𝑙𝑜𝑔 (𝑊𝑛
𝑛) as function of 𝑛, it is possible to obtain the value of α. Figure 4
shows the product selectivity towards α, and, as one can see, the selectivity of gasoline and diesel is
about 40%, for a α-value of 0.73 and 0.84, respectively. As these selectivities are too low for industrial
applications, FT synthesis has to be operated with a α-value higher than 0.9, which will lead to a
production of a reasonable amount of liquid fuels and large amounts of waxes that will be cracked
afterwards. Cobalt catalysts have a high α-value, more precisely in the range of 0.90. [5]
7
Figure 4 - Product selectivity as function of ASF chain growth probability α. [10]
Some deviations from the ASF distribution were reported concerning higher methane selectivities,
than expected, and lower C2 (mainly ethylene) yields. Besides these deviations, products with higher
carbon numbers do not always follow the ASF distribution. There are several explanations for these
phenomena, for instance, different termination mechanisms or side reactions are causing the deviation
for products with longer chains. The latter topic is due to olefins readsorption on the catalyst that can
lead to secondary reactions, such as hydrogenation and isomerization, and even olefins condensation
leading to longer chains.
2.2. Fischer – Tropsch catalysts
The FT catalysts are composed by an oxide support, a metallic active phase and sometimes metal
and/or oxide promoters. The following paragraphs will described these topics.
2.2.1. Active phase
For FT applications only Fe-, Co-, Ni- and Ru- based catalysts have a satisfactory activity. Due to
the low availability of Ru, and consequently its high price, Ru-based catalysts are not used.
Ni catalysts have a high hydrogenolysis capability and hence, produce large amounts of methane.
Moreover, at the temperature and pressure at which FT plants operate, they present a low selectivity for
long chain hydrocarbons, which is not the FT purpose. Also, Ni based catalysts generate volatile
carbonyls, which result in continuous loss of metal.
Concerning Fe- and Co-based catalysts, despite the higher cost of cobalt catalysts, the latter
presents a higher activity and longer life than Fe-based catalysts. Indeed, cobalt is the most active metal
for long chain hydrocarbons. However, cobalt based catalysts are very sensitive to temperature
changes, thus a small increase in temperature could lead to a high production of methane. [9] [11]
More in detail, cobalt catalysts have a higher resistance to deactivation than iron based catalysts.
In addition, the WGS reaction has higher activity on Fe-based than on Co-based catalysts, thus it slows
the reaction rate on Fe-catalysts. Besides, for Fe-based catalysts, the syngas should not contain more
than 0.2 ppm of sulfur, and for Co-based catalysts, the sulfur content must be less than 0.1 ppm. [12]
8
Finally, cobalt catalysts are more suitable for higher H2/CO ratios, approximately 2 and on the other
hand, iron catalysts are preferred for lower H2/CO ratios.
This study will be focused on Co-based catalysts, due to their stability, higher activity and higher
hydrocarbon productivity. These catalysts are considered an optimal choice for long-chain hydrocarbons
production in the LTFT process. [5]
2.2.2. Supports
Concerning the support material, it is crucial to assure the stabilization of the Co particles in order
to provide a good catalyst activity. Changing the support’s surface, structure and pore size could lead
to an improved metal dispersion, reducibility and the diffusion coefficients of reactants and products. As
previously mentioned, Al2O3 and SiO2, are the most used catalysts supports due to their high surface,
and strong mechanical strength. [13] The following paragraphs will describe the most often supports
used in the FT process.
Silica – Supported catalysts
A better cobalt reducibility is achieved in this type of support due to a relatively weak interaction
between the support and cobalt. However, the cobalt dispersion is much lower in silica-supported
catalysts than the one achieved with alumina-supported catalysts. Khodakov et al. [5] reported that
catalysts with a pore size of 6 – 10 nm displayed higher FT activity and higher C5+ activity.
It was found that periodic mesoporous silicas including MCM-41, SBA-15 and SHS have expanded
their utilization in cobalt based catalysts, due to their desirable properties such as large surface area,
controllable pore size and narrow pore size distributions. [13]
Moreover, Jung et al. [13] reported successful catalytic tests with cobalt dispersed into periodic
mesoporous silicas by incipient wetness impregnation method (method described in the following
paragraphs). Also, the support’s periodic mesoporosity enhances the reactants access to active sites
as well as the transportation of higher hydrocarbon products. Industrially, silica is not the most used
support.
Alumina – Supported catalysts
Alumina has been one of the mostly used supports for cobalt FT catalysts. There are formed
small cobalt crystallites due to strong interactions between cobalt oxide with this support. Cobalt
reducibility is one of the most important problems of alumina-supported cobalt FT catalysts. However,
promotion with noble metals can improve cobalt reducibility.
2.2.3. Metals and oxide promoters
According to the literature [11], cobalt-based catalysts are composed by an oxide support, metal
and oxide promoters. Khodakov et al. [5] reported that the metal promoters are normally Pt, Ru, Ir and
Re, which are described as “reduction promoters” that will result in an ease of cobalt reduction and an
enhancement in the cobalt dispersion. Iglesia et al. [14] revealed that the promotion with ruthenium
delayed irreversible catalyst deactivation.
9
Khodakov and co-workers [5] described that the most often oxide promoters are ZrO2, La2O3, MnO
and CeO2 and the implementation of these oxides could lead to the catalyst texture and porosity
modification, as well as reduction of the formation of cobalt mixed oxides, increase in the cobalt
dispersion, and enhance mechanical and attrition resistance of the catalysts. In this study it was found
that promoting the catalyst with zirconia would lead to higher FT reaction rates, along with an increase
in C5+ selectivity.
Finally, the shape of the catalyst depends on the type of the reactor.
2.2.4. Usual catalyst preparation methods
The choice of the deposition method of the active phase will strongly influence the catalytic activity
of the final catalyst. The following paragraphs will address the most usual preparation methods to
prepare cobalt-based catalysts for FT synthesis: impregnation and deposition-precipitation methods. [5]
The preparation of cobalt-based FT catalysts includes the following steps [5]:
1) Synthesis of the catalyst support
Normally the supports used in the FT process are obtained through the spray-drying of a solution
which contains inorganic precursors of the oxide support.
2) Preparation of cobalt precursors, and possibly promoters
3) Cobalt precursors dispersion onto the catalyst support
The purpose of dispersing the active phase, cobalt in this study, is to spread it along the porous
support and to create metal clusters. [5] Moreover, it is necessary to generate a significant concentration
of stable cobalt metal surface sites, and this depends on the size of cobalt particles as well as their
reducibility.
Incipient wetness impregnation is the most common method for preparing cobalt-based catalysts
for the FT synthesis. This method consists on preparing a solution of a cobalt salt which is contacted to
a dry porous support.
After being contacted, the solution is aspired by capillary forces within the support pores. The
incipient occurs when all pores of the support are filled with the liquid and there is no excess of moisture.
The fundamental phenomena underlying impregnation and drying are extremely complex. Thus, it is
needed a careful control of temperature, time of support drying, rate of addition of impregnating solution,
etc.
At the moment right after impregnation, the interactions between the metal precursor and the
support are relatively weak, which allows the redistribution of the active phase over the support during
drying and calcination steps.
The distribution of Co2+ cations depends on the support charge, whether it is silica, alumina or
titania. For each material, there are different points of zero charge (PZC), which correspond to the pH
at which the positive and negative charges on the surface are equal and cancel. Therefore, at a pH
below the PZC, the oxides surface is positively charged, and at a pH higher than PZC the oxides surface
is negatively charged. Hence, if the impregnation solution has a pH below PZC, repulsion between the
support surface and Co2+ ions will lead to a nonhomogeneous distribution of Co2+ ions. On the other
10
hand, if the impregnation solution has pH above the PZC, Co2+ ions will be homogeneously distributed.
[5] In a first approximation this last statement is correct. However, at a pH of 12-13, Co in water is not
in the usual form (Co2+), it is in the form of Co(OH)42-, thus repulsion would occur.
4) Post-treatment
After impregnation, the catalyst does not present the active phase in the final form, hence it is
required additional treatments. In order to eliminate the solvent, it is needed a drying step followed by
thermal treatments, as calcination and activation. The following paragraphs will be focused on the post-
treatments in cobalt-based catalysts.
Drying
As previously mentioned, the drying step wishes the removal of the solvent present in the catalyst
pores. Coulter and Sault [15] have stated that the surface properties of Co/SiO2 catalyst, prepared by
incipient wetness impregnation with a cobalt nitrate precursor, are extremely influenced by the drying
and calcination conditions. They showed that, after drying the samples at 110°C and calcinating it in air
at 400°C, a surface phase of Co3O4 is created and it is easily reduced. However, drying under vacuum
conditions formed an irreducible Co3O4 phase due to the migration of Co2+ cations into the silica matrice.
Therefore, vacuum dried samples present a higher nitrate concentration on the surface, which leads to
the formation of a silicate surface. On the other hand, the presence of a gas phase NOx formed during
the decomposition of the cobalt nitrate precursor promotes the oxidation of the intermediate Co2+ to form
large Co3O4 particles.
Calcination
During calcination, several chemical reactions can take place. These reactions are responsible for
the precursor’s thermal decomposition, as well as the relief of volatile compounds.
According to Krylova et al. [16], for Co/SiO2 and Co/Al2O3 catalysts, the calcination temperature
influences the metal reducibility and the active phase dispersion. On one hand, the calcination at higher
temperatures of Co/Al2O3 catalysts causes the formation of new surface compounds which increases
the hydrocarbon yield (however, the selectivity on C5+ decreases leading to a higher methane
production). On the other hand, the calcination of Co/SiO2 catalysts at higher temperatures leads to a
higher selectivity on C11-C18, thus an increase in the average carbon number.
5) Reducing treatments
The activation is the last step on the catalyst preparation. This operation involves a thermal
treatment at high temperatures and it occurs usually under a hydrogen flow.
The reduction mechanism is generally assumed to occur in two steps, which are showed in the
following equations. The first step, (Eq. 10), usually takes place at low temperatures, 100-350°C, and
(Eq. 11) normally takes place at 400-600°C. [5]
11
According to Petru and co-worker [17], the metal particle size is a determinant factor in the reduction
process of oxidized cobalt catalysts. In this study, it was showed that the degree of reduction of Co3O4
strongly depends on the conditions of its preparation. For instance, the particles prepared at 600°C were
easily reduced to Co at 300°C, while those prepared at 900°C were not reduced even at 500°C. Two
effects were proposed to explain this: diffusion limitations occurring with increasing particle size, and
differences in the microstructure of the particles. Moreover, high reduction temperatures (>400°C) can
lead to sintering cobalt particles. [5]
Therefore, as above mentioned, cobalt-supported FT catalysts are normally loaded with Pt, Ru, Ir
and Re which will result in an ease of cobalt reduction and an enhancement in the cobalt dispersion.
However, this can increase the catalyst cost and affect the economic efficiency of the overall FT
technology. [5]
2.2.5. Activity and selectivity
The activity of the catalyst will dictate the process economics, thus, regarding the FT process the
size of the particles that create the active phase will interfere in the catalyst activity. Hence, there is a
necessity to find out the influence of the cobalt particles size and the most effective method to disperse
the active phase.
One of the most important catalyst characteristics is the quantity of reactant transformed per time
unit, per unit of mass, or area unit of active phase. The catalyst activity can be quantified by the turnover
number or frequency number, TON or TOF, respectively. The turnover frequency describes the number
of moles transformed per active site per time unit.
Bezemer et al. [18] studied the influence of cobalt particle size on the catalyst activity, turnover
frequency (TOF), methane selectivity and C5+ selectivity, at 1 and 35 bar, and at 210 and 250°C
regarding C5+ selectivity. Referring to the catalytic performance at 1 bar, it was found that in particles
ranging from 27 to 6 nm the activity increases from 0,64 × 10-5 to 3,51× 10-5 molCOgCO-1s-1, respectively.
This increase in activity is a result of a higher specific cobalt surface area. However, for particles smaller
than 6 nm the activity quickly decreases. [18]
Concerning the TOF, it has been concluded that, for particles ranging from 6 to 27 nm, TOF is
relatively constant, but for smaller particles it rapidly decreases. Therefore, for small cobalt particles
(smaller than 5 nm) there is a higher selectivity for methane than in particles larger than 5 nm.
Furthermore, the methane selectivity remains constant for particles ranging from 5 to 27 nm. This high
selectivity in small cobalt particles might point out a lower quantity of active sites for chain growth,
leading to a higher methane production. [18]
The C5+ selectivity depends on the cobalt particle size, and varies from 76 to 84 wt.% at 210°C and
from 51 to 74 wt.% at 250°C. [18]
Regarding the catalytic performance at 35 bar, Bezemer and co-workers [18] showed that TOF is
constant for samples with cobalt particles larger than 8 nm, while for smaller particles it starts to
𝐶𝑜3𝑂4 + 𝐻2 → 3𝐶𝑜𝑂 + 𝐻2𝑂 (Eq. 10)
3𝐶𝑜𝑂 + 3𝐻2 → 3𝐶𝑜 + 3𝐻2𝑂 (Eq. 11)
12
decrease. Thus, it can be concluded that the catalytic performance at 1 and 35 bar shows the same
trend, with constant TOF-values for larger Co particles and size dependency for small Co particles.
According to Khodakov et al. [5], catalysts containing small cobalt particles do not exhibit adequate
catalytic activity, as they are very hard to reduce and as these particles are not stable at normal FT
conditions. As they are not stable, they could lead to catalyst deactivation (sinter, carburize, reactions
with the support) or the use of small particles can radically change the adsorption properties.
Therefore, Bezemer and co-workers [18] concluded that one should aim for catalysts with cobalt
particles size close to 6 – 8 nm. As larger particles show a lower activity and smaller particles show a
lower selectivity and activity.
2.2.6. Catalyst deactivation
About the catalyst deactivation, there are several mechanisms that are responsible, such as,
poisoning by sulfur, chlorine and nitrogen compounds, sintering of cobalt crystallites, carbon effects, re-
oxidation, attrition, etc. The following paragraphs will briefly describe each one of these topics.
Sulfur is potentially present in the feed when the feedstock is coal or biomass, and Tsakoumis et
al. [19] stated that one sulfur atom adsorbed on a Co/Al2O3 catalyst poisons more than two cobalt atoms.
Also, it was found that small amounts of N-compounds have an immediate effect on the catalyst activity.
Nevertheless, the deactivation seems to be reversible with an in situ hydrogen treatment, which can
recover 100% of the catalyst activity.
Sintering of cobalt crystallites leads to a reduction of the active surface area, and it is
thermodynamically driven by the surface energy minimization of the crystallites. Tsakoumis and co-
workers [19] reported that high temperatures and water accelerate the sintering process, and this is
recognized as an irreversible process. However, by a reduction-oxidation-reduction (ROR) sequence it
is possible to have a re-dispersed cobalt phase. As FT synthesis accomplish an exothermic reaction,
the potential for sintering is relatively high, thus, as above mentioned, special attention should be given
to the reactor choice, since it is extremely important to have isothermal conditions.
Side reactions, as the Boudouard reaction, generate carbon that could interact with the metal and
form either inactive species or species that act like reaction inhibitors (amorphous, graphitic or carbon
species (coke)). As water is an oxidizing agent, and as it is the most abundant byproduct of FT synthesis,
it could lead to surface oxidation of the cobalt particles. Thus, as inactive cobalt oxides are generated,
the catalyst activity decreases.
Attrition is the breakdown of solid particles, which could be due to abrasion/erosion or fracture. This
fracture can result in the production of fine powders which means a low catalytic performance. Some
studies [19] reported that the addition of cobalt to alumina or silica supports increase the resistance
towards attrition. Catalyst deactivation due to attrition effects is particularly valid for moving bed reactors.
Lastly, Sadeqzadeh et al. [12] reported that higher H2/CO ratios promote higher initial catalytic
performance, as well as higher initial deactivation rate. This could be related to a higher water
production, which is directly associated with catalyst deactivation (sintering).
13
2.3. One – Pot synthesis: Incorporation of the metallic phase on a mesoporous oxide
matrix
The supports used in the FT process, as previously mentioned, are normally silica or alumina,
which are obtained through the spray – drying of an inorganic solution. In this procedure, sol – gel
reactions take place.
The following paragraphs will briefly describe the sol-gel chemistry and a particular case of the
application of sol – gel chemistry: mesoporous materials.
2.3.1. Sol-gel chemistry
The chemistry involved in the sol-gel process is based on inorganic polymerization reactions. Thus,
it allows the formation of inorganic polymers through polycondensation of precursors, such as, metallic
salts or alkoxides. This method is easy to apply, therefore it is widely applied in the manufacture of
catalyst supports. [3] The polymerization reactions involve two steps: hydrolysis and condensation of
the precursors.
In order to produce silica or alumina it is needed a precursor, such as Tetraethyl Orthosilicate
(TEOS) (Si(OCH2CH3)4), or aluminum chloride hexahydrate AlCl3∙6H2O, respectively. One can note that
metal alkoxides are popular precursors as they will lead to more pure solids than if a salt was used, and
also because it is easier to control the kinetics of hydrolysis and condensation reactions. In the following
equations is described the mechanism to synthesize silica. The reaction proceeds first through
hydrolysis (Eq. 12) which is the hydrolysis of alkoxy groups. Once reactive hydroxy groups are formed,
the generation of oligomers and polymers occurs via polycondensation reactions that can occur via (Eq.
13) or (Eq. 14).
One can note that the structure and morphology of the resulting network is strongly dependent on
the nature of the precursors, as well as the water content, the pH, temperature, and the relative
contribution of each one of these reactions. As mentioned before, the surface of an oxide is differently
charged depending on the pH. In the case of silica synthesized using TEOS as precursor, the
condensation reactions are catalyzed through acid-basic reactions, where the groups Si-OH2+ and Si-
O- interfere, respectively, in an acid or basic media. As one can note in Figure 5, the hydrolysis kinetics
presents a minimum at a pH of 7, and the condensation kinetics presents a minimum at a pH of 2 which
corresponds to the PZC of silica. [20]
𝑆𝑖(𝑂𝑅)4 + 𝑛 𝐻2𝑂 → 𝑆𝑖(𝑂𝑅)4−𝑛(𝑂𝐻) + 𝑛 𝑅𝑂𝐻 (Eq. 12)
𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 + 𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 → (𝑂𝑅)3𝑆𝑖 − 𝑂 − 𝑆𝑖(𝑂𝑅)3 + 𝐻2𝑂 (Eq. 13)
𝑆𝑖(𝑂𝑅)3 − 𝑂𝑅 + 𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 → (𝑂𝑅)3𝑆𝑖 − 𝑂 − 𝑆𝑖(𝑂𝑅)3 + 𝑅𝑂𝐻 (Eq. 14)
14
Figure 5 - Hydrolysis (H), Condensation (C) and Dissolution (D) kinetics for a system of TEOS. [20]
The sol-gel chemistry allows the synthesis of a large variety of amorphous or crystalline solids, with
very different porosities, and in particular it can form mesoporous materials. As the condensation
happens at ambient temperature, it is possible to add organic molecules in the reaction media, which
will create a mesostructure. [20]
The following paragraphs describe this type of materials.
2.3.2. Mesostructured materials: definition
A mesostructured material presents pores that according to IUPAC classification, consist in a well-
organized structure with an uniform and periodic porosity at the mesoporous scale and the pore diameter
is between 2 and 50 nm. The principle of synthesis is derived from the sol-gel synthesis, previously
described, except that a (supra)molecular (surfactant) is used to generate periodic mesoporosity. [21]
The surfactants can be ionic or non-ionic. In the first type, one should take in account the charge
of the oxide. On one hand, taking the example of silica, above the PZC (pH>2) the surface is negatively
charged, thus a cationic surfactant would be the better choice. On the other hand, bellow the PZC
(pH<2), the surface is positively charged, thus an anionic surfactant would be more suitable.
Therefore, using non-ionic surfactants allows the synthesis of mesostructured materials in a large
range of pH. These type of surfactants include the Brij family (CH3(CH2)x-[EO]y-OH) and the Pluronic®
family ([PEO]x-[PPO]y-[PEO]x), with PEO: polyethylene oxide and PPO: polypropylene oxide.
It is well known that amphiphilic molecules can arrange themselves in micelles (aggregate of
surfactant molecules). A typical micelle in an aqueous solution forms an aggregate with the hydrophilic
part that is in contact with the surrounding solvent, whereas the hydrophobic part forms an aggregate in
the micelle center. Figure 6 describes several forms of micelles arrangement.
One can note that the solid final geometry depends on the geometry and arrangement of the
micelles.
In order to have a mesostructured solid, it is necessary the hydrolysis and condensation reactions
of the inorganic phase (sol-gel chemistry), the micellization of organic phase and finally a good
interaction between these two phases, as it is explained in 2.3.3.
15
Figure 6 – Micelle structure A: Sphere, B: Cylindric, C: Lamellar, D: Inverse micelle, E: Bicontinuous phase, F:
Liposomes. [20]
2.3.3. Mesostructured materials: mechanisms
To explain what occurs during the synthesis of mesostructured materials, two mechanisms were
proposed, depending on the surfactant concentration (c): True Liquid Crystal templating (TLC) and
cooperative self-assembly mechanism. The first one refers to the situation where c>>CMC (Critical
Micelle Concentration (CMC): above this surfactant concentration micelles are spontaneously formed),
hence micelles soon begin to form. After, hydrolysis and condensation reactions of inorganic precursors
take place around this template. This pathway can be observed in Figure 7a).
If c≈CMC, the cooperative self-assembly mechanism takes place, where several processes occur
simultaneously, such as hydrolysis and condensation reactions of the inorganic species, the self-
assembly of the surfactant and the interaction between these two phase. In Figure 7b) it is illustrated
the pathway of this mechanism.
Figure 7- Main synthesis pathways to mesostructured materials. [21]
2.3.4. Mesostructured materials: synthesis techniques
The following paragraphs describe usual synthesis techniques for the production of mesostructured
materials.
1) Precipitation
In mildly acidic to basic conditions, it is known that silica polymerization starts with the condensation
of monosilicic acid into cyclic oligomers, which grow to three-dimensional polymer particles. These
16
particles may grow through monomer addition and if they grow large enough, they will eventually
precipitate. [22]
The formation of mesoporous solids through precipitation is rapid. In only 3 – 5 minutes in a cationic
surfactant solution, it is possible to detect well-ordered mesostructures. However, if a nonionic surfactant
is used, the formation of mesostructures is slower (normally 30 minutes). One can note that the pH value
affects the mesostructure formation rate. [22]
This technique accomplish the following steps to produce mesostructured materials: dissolution of
the reactants (inorganic precursor and surfactant) followed by the formation of a suspension. Afterwards,
this suspension is put inside an autoclave, and the autoclave goes to a pre - heated oven at 110°C and
it remains there for 24 – 48 hours. Later, the precipitate requires a wash/filtration step. Finally, a post-
treatment (drying and calcination) step is required.
2) Evaporation
The so called Evaporation Induced Self-Assembly (EISA) presents a mechanism that can be
explained by the cooperative self-assembly mechanism.
This synthesis method, EISA, consists in the progressive evaporation of a solution containing
inorganic precursors and organic surfactants. Initially, this solution is diluted, thus, the surfactant
concentration is lower than the CMC, which inhibits micelles formation, and also the lower concentration
in inorganic precursor allows a low hydrolysis and condensation kinetics. During the evaporation, the
solution will become more concentrated, which induces the mesostructuration process. Hence, this
method depends on the solvent evaporation kinetics, template structuration and inorganic species
condensation.
In order to achieve a better control on the mesostructuration process, it is necessary to understand
the influence of certain parameters, for instance pH, temperature and surfactant/silica ratio. The
parameter surfactant/silica volume ratio, studied by Sanchez et al. [23], can have a significant impact
on the level of organization. The type of obtained micelles will vary on this ratio, and more curved
micelles will form worm – like phases, that will lead to a low level of organization in the final solid.
Moreover, the surfactant will dictate the pore diameter.
Moreover, the EISA has great advantages over the precipitation method. Firstly, the
surfactant/silica stoichiometry is retained after evaporation; secondly the shaping into spheres takes
place with control of the diameter, and finally, large objects with controlled dimensions (slow
evaporation) are possible.
2.1) Spray - drying
The spray-drying method is a sub-process of the evaporation technique, where a limpid solution (in
fact it is a colloidal solution, as hydrolysis and condensation reactions have already begun some
oligomers will be in solution) is transformed into a dry powder.
The aerosol technique presents many benefits when compared to the precipitation methods. It is
a continuous process (the time between a droplet and a dry solid is between 1 to 4 seconds), it can be
17
easily scaled-up to industrial scale, and, lastly, it allows a perfect control of the chemical composition of
the final solid due to the existence of non volatile components that once were present in the atomization
solution.
Therefore, this process permits the formation of materials that were not possible to be generated
through the usual preparation methods, due to the fast evaporation that the precursors are obliged to
co-arrange inside the matrix under a metastable state. Another benefit in using this method is the small
amount of waste that is generated, in terms of energy, it is only needed to treat the gas that is released.
There are several spray-drying techniques, all related with the mechanical destabilization of the
solution/atmosphere interface. The liquid feed can be atomized by several nozzles types depending on
the required droplet size and this colloidal dispersion of liquid droplets in a gas is usually called “aerosol”.
[24]
As one can see in Figure 8, firstly a very diluted solution is pumped to the nozzle. Furthermore, the
aerosol is generated where liquid droplets are carried by the vector gas. Concerning the drying step, the
solvent evaporation allows the material’s structuration through a mechanism of auto assembly which is
induced by the solvent evaporation. The evaporation driving force is the difference between (P0 – PS),
where PS is the vapor pressure at the droplet surface and P0 is the saturation pressure. Therefore, when
P0 = PS the evaporation process stops. After solids that are smaller than what is required are sent to an
air filter, and the solids that meet the specifications are recovered in a collector.
Moreover, the non volatile species are responsible for the polarity fluctuation and the viscosity
change in the droplet depth profile. Hence, the diffusion of volatile species inside the particles through
the solid/gas interface, has an important role in the particles’ structure. Therefore, parameters related
with this diffusion process, such as droplet size, residence time in the chamber, concentration, the carrier
gas relative pressures in volatile species, temperature and flux, must be controlled.
18
Figure 8 - Spray - Dryer (Büchi B 290) working principle and material's structuration mechanism. Adapted from [20]
Sanchez et al. [25] reported a direct aerosol synthesis of aluminosilicates in a basic medium. The
microstructure-directing agent used was the tretaprolylammonium hydroxide (TPAOH) in a Pluronic®
F127 template. The porosity showed to be homogeneous throughout the sample, and the pores size
was between 6 to 17 nm. Therefore, it is possible to conclude that the micillization and self-assembly
can occur between the aluminosilicate and the F127 copolymer until a pH of 11. Also, it has been
reported in this study that the pore size varies with the amount of TPAOH added to the solution, due to
the fact that this last compound slows the inorganic condensation reactions.
Heteroelements can be loaded onto the oxide matrix synthesis. It is important to understand and
control this incorporation.
If mesostructured solids are manufactured by spray-drying, the localization, structure and
properties of the final particles will depend on the precursor solution chemistry and on the fast drying
conditions. The uniform dispersion of the heteroelements onto the mesostructured matrice depends
mainly on two parameters: the homogeneity of the nanoparticles dispersion onto the precursor
silica/surfactant solution, and the nanoparticles surface/silica/surfactant interactions.
It was reported in a review [3] that spray-drying is a very convenient way to synthesize powders
with a high loading of heteroelements. It was also reported the incorporation of the following
heteroelements in acidic conditions: aluminum, zirconium, calcium, iron, phosphate ions and palladium.
At low concentrations, heteroelements are found homogeneous distributed within the final particles.
Normally the mesostructures obtained with pure silica are not affected by the incorporation of small
19
amounts of heteroelements. One should note, that the trapping limit of heteroelements is conditioned
by its attractive interactions with silica oligomers or with the structuring agent.
Furthermore, Sanchez and co-workers [26] reported a synthesis procedure that allows the
formation of mesoporous silica with cobalt (Co) and molybdenum (Mo) already loaded. In this study,
the co-location of molybdenum HPA precursor close to the micelles upon the evaporation of aerosol
droplets, allowed the complete accessibility of the active phase. It is interesting to draw attention to the
fact that the simultaneous existence of Co and Mo did not affect the mesostructuration. However, this
study reveals that the presence or absence of Co may interfere with the localization.
This page was intentionally left blank
21
3. Experimental Work
The catalysts prepared in this study are cobalt based catalysts either on silica or alumina matrix
and they were obtained through spray-drying performed with an ultrasonic nozzle.
Several supports and cobalt catalysts were produced, and concerning the latter, all of them have
15 wt.% of cobalt regarding the total mass of catalyst.
All the prepared supports and catalysts were characterized through N2 adsorption – desorption and
the latter was also characterized by TPR. Besides this, in the most promising catalysts or supports other
analysis were performed, such as XRD, low angles XRD, SEM and TEM.
This chapter will focus on describing the preparation methods (typical solution molar composition
and the differences between each sample), the spray-drying working principle as well as its parameters.
Finally, it will describe the characterization techniques.
3.1. Preparation Methods
The materials were obtained through a solution referred as “atomization solution”. The preparation
of this solution requires several steps, the first one being the hydrolysis of the matrix precursors. After
that, a surfactant solution (which will induce the mesostructuration), and a solution of the metallic
precursors are prepared. Finally, these three solutions should be mixed and afterwards atomized in a
Büchi B-290 spray – dryer. One should note that the atomization solution should be limpid to the eye
during all the atomization period (without any precipitation or gelification phenomenon).
Solutions nature
The atomization solution results from the mixture of two solutions: an inorganic solution and an
organic solution.
Inorganic solution: is a mixture of a silica or alumina precursor and acidified water, and if
a catalyst is synthesized, it will also contain a solution of the metallic precursor.
Organic solution: is a mixture of surfactant (Pluronic®P123 (PEO20-PPO70-PEO20), where
PEO stands for polyethylene oxide and PPO stands for polypropylene oxide), ethanol and
acidified water.
One should note that the water in both solutions can be acidified with a solution of HCl or HNO3.
3.1.1. Supports
Silica and alumina matrices were produced. To obtain a silica matrix, tetraethyl orthosilicate (TEOS
(Si(OEt)4), 98% Sigma – Aldrich) was used as precursor, and, regarding alumina, aluminum chloride
hexahydrate (AlCl3·6H2O, 99% Sigma – Aldrich) was used as precursor. The precursor’s solution
contains the inorganic precursor and acidified water at a pH of 2. The organic solution contains Pluronic®
P123, ethanol and acidified water at a pH of 2. In Table 1 it is described the molar composition for all
the synthesized supports.
22
Table 1 – Reference molar composition for all the supports.
For preparing a typical silica support, a solution with 15.97 g of TEOS (98% Sigma – Aldrich) and
24.15 g of acidified water at a pH = 2, was left stirring during one night. The organic solution contained
4.45 g of Pluronic® P123, 10.69 g of ethanol and finally 44.84 g of water at a pH = 2. In this procedure
the water in both solutions was acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.% or with a
HNO3 solution (HNO3, Sigma – Aldrich 68 wt.%).
Regarding the alumina supports, 18.05 g of AlCl3·6H2O were mixed with 23.55 g of water at a pH
of 2. This inorganic solution was left hydrolyzing during 30 minutes. Finally, the organic solution,
contained 4.34 g of Pluronic® P123, 10.33 g of ethanol and finally 43.73 g of water at a pH = 2. In this
procedure, the water in both solutions was acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.%)
or with a HNO3 solution (HNO3, Sigma – Aldrich 68 wt.%)..
In Table 2 is a full description of all synthesized supports, and for all the samples the molar ratio
between (EtOH:H2O) was 0.06, and the surfactant was always Pluronic®P123.
Table 2 - Synthesized supports.
After spray-drying, all the solids were dried at 100°C during one night, in a ventilated oven, and
calcinated at 550°C. The calcination profile for all the supports can be seen in Figure 9.
TEOS or AlCl3·6H2O H2O HCl or HNO3 EtOH Surfactant (P123)
1 50 0.009 (pH = 2) 3 0.01
Reference Matrix/Inorganic precursor nature pH;
(HCl/HNO3) Post - Treatment
JFR003
Silica/TEOS 2;HCl
Calcination at
550°C
JFR005
JFR006
JFR007
JFR028 2;HNO3
JFR009
Alumina/AlCl3·6H2O 2;HCl
JFR010
JFR025 2;HNO3
23
Figure 9 – Calcination profile for the synthesized supports.
3.1.2. Catalysts
In order to synthesize a catalyst, a metallic precursor solution has been added to the atomization
solution. The metallic precursor solution used in this study was cobalt nitrate solution in water (Co(NO3)2
with 13.4% of Co, with a solution density of 1.48). Usually, the cobalt nitrate solution is added to the
inorganic solution just before atomization starts. Furthermore, the organic solution remains equal as
described in the supports. The only difference is in the inorganic solution that accomplishes the addition
of the cobalt nitrate solution. Catalysts with 15 wt.% of cobalt, concerning the final catalyst mass, were
produced.
In Table 3 and Table 4 it is described the reference molar composition for catalysts on a silica
matrix and alumina matrix, respectively.
Table 3 – Reference molar composition for the catalysts on a silica matrix.
Table 4 – Reference molar composition for the catalysts on an alumina matrix.
For preparing a typical cobalt catalyst on a silica matrix, a solution with 13.53 g of TEOS (98%
Sigma – Aldrich) and 24.22 g of acidified water at a pH =2, was left stirring during one night. Just before
atomization, 5.03 g of cobalt nitrate solution in water (13.4% of Co with a solution density of 1.48) were
added to the latter inorganic solution. The organic solution contained 4.46 g of Pluronic®P123, 10.63 g
of ethanol and finally 44.98 g of water at a pH = 2. In this procedure, the water in both solutions was
0
100
200
300
400
500
600
0 2 4 6 8 10 12 14 16 18 20 22 24
Te
mp
era
ture
(°C
)
Time (h)
JFR003; JFR005; JFR006;JFR007; JFR028; JFR009;JFR010; JFR025
TEOS Co H2O HCl or HNO3 EtOH Surfactant (P123)
0.85 0.15 50
0.09 (pH = 1)
0.009 (pH = 2)
0.0009 (pH = 3)
3 0.01
AlCl3·6H2O Co H2O HCl or HNO3 EtOH Surfactant (P123)
0.87 0.13 50 0.009 (pH=2) 3 0.01
12h
2°C/min
24
acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.%) or with a HNO3 solution (HNO3, Sigma –
Aldrich 68 wt.%).
Regarding the preparation of a cobalt catalyst on an alumina matrix, 15.77 g of AlCl3·6H2O were
mixed with 23.69 g of water at a pH of 2. This inorganic solution was left hydrolyzing during 30 minutes.
Just before atomization, 4.27 g of cobalt nitrate solution in water (13.4% of Co with a solution density of
1.48) were added to the latter inorganic solution. Finally, the organic solution, contained 4.36 g of
Pluronic®P123, 10.40 g of ethanol and finally 43.99 g of water at a pH = 2. In this procedure, the water
in both solutions, was acidified with (HCl, Sigma – Aldrich 37 wt.%) or with a HNO3 solution (HNO3,
Sigma – Aldrich 68 wt.%).
Moreover, besides aluminum chloride other alumina precursor was tested: aluminum nitrate
(Al(NO3)3, Sigma – Aldrich 99.997%).
In Table 5 it is a full description of all synthesized catalysts. All of them were produced through an
ultrasonic nozzle and for all the samples the molar ratio between (EtOH:H2O) was 0.06.
After spray-drying, all the solids were dried at 100°C during one night, in a ventilated oven. Three
calcination temperatures were performed in the muffle at 350, 400 and 550°C.
The calcination profile for all samples can be seen in Figure 10 and Figure 11.
Table 5 - Synthesized catalysts.
Reference Matrix/Inorganic precursor nature
pH; (HCl/HNO3)
Surfactant Post - Treatment
JFR014
Silica/TEOS
2; HCl
Pluronic®
P123
Calcination at 550°C
JFR016 Calcination at 350°C
JFR019 1; HNO3
Calcination at 400°C
JFR021 2; HNO3
JFR022 3; HNO3
JFR023 Alumina/AlCl3·6H2O
2; HCl
JFR024 2; HNO3
JFR037 Alumina/Al(NO3)3
25
Figure 10 - Calcination profile for JFR014 and JFR016.
Figure 11 - Calcination profile for JFR019, JFR021, JFR022, JFR023, JFR024 and JFR037.
0
100
200
300
400
500
600
0 2 4 6 8 10 12 14 16 18 20 22 24
Te
mp
era
ture
(°C
)
Time (h)
JFR014
JFR016
0
50
100
150
200
250
300
350
400
450
0 2 4 6 8 10 12 14 16 18 20 22 24
Te
mp
era
ture
(°C
)
Time (h)
JFR019
JFR021; JFR022; JFR023;JFR024; JFR037
2°C/min
12h
1°C/min
12h
2°C/min
26
3.2. Spray – Drying
3.2.1. Spray Drying working principle
The main objective is the transformation of a solution, which contains reagents of the desired
product, into solid spherical elementary particles that were dried through an atomization process
(evaporation). The following paragraphs describe the main steps of the spray drying process.
1) Solution pumping: The equipment responsible for
the solution pumping is a peristaltic pump. This pump
is responsible for feeding the solution to the nozzle.
2) Aerosol generation: The aerosol is generated
through a nozzle where the solution is mixed with a
vector gas. The diameter reduction in the outlet of the
nozzle combined with the vector gas flux allows the
formation of small droplets that are carried by the
gas. This vector gas is chosen according the desired
atmosphere, for instance, it can be an inert gas if an
inert atmosphere is required. There are different
kinds of energy used to disperse the liquid feed into
fine droplets.
Two fluid nozzles: The energy required for
atomization is provided by a rapid ejection of
the spray gas, which was previously mixed
with the liquid feed within the nozzle. It is
more suitable for a laboratory scale, due to
its low pressure consumption, low particle
velocity and thus shorter required length in the spray chamber. The droplets produced
by this kind of nozzle range from 5 - 30 µm.
Ultrasonic nozzles: The droplet size is controlled by the frequency at which the nozzle
vibrates, and by the surface tension and density of the liquid being atomized. One can
note, that the higher the frequency, the smaller the median droplet size. Moreover, there
is no need to use air pressure, the liquid is pumped to the nozzle vibrating surface. The
produced droplets range from 2 - 100 µm.
There are other type of nozzles, such as rotary disks and pressure nozzles, however they are more
suitable for an industrial scale, and because of that they are not mentioned in this report. In this study,
an ultrasonic nozzle was chosen instead of a two fluid nozzle (the nozzle often used in the laboratory
scale), because the size of the produced elementary particles is more appropriate for the final application
of these catalysts (FT synthesis).
Figure 12 - Spray drying apparatus. [27]
27
3) Aerosol evaporation: The liquid droplets have to be in contact with a hot gas, which allows the
evaporation of the solvents and the droplets transportation. Therefore, the inlet temperature
corresponds to the temperature of this heated vector gas. This temperature needs to be
controlled which is detailed described in the spray – drier parameters.
The droplets can shrink, agglomerate or lose sphericity as moisture and the solvent are
evaporated. At the end, the dried particle surface temperature approximates the temperature of
the surrounding gas, as it is described later (spray – drier parameters).
The manner in which the sprayed liquid droplets contact with the drying gas has a major impact
on the droplet’s behavior and in the dried product properties. The usual flow configurations are:
Co-current flow: The solution is sprayed in the same downwards direction as the flow of
the drying gas. The gas temperature in the outlet is the lowest because the gas comes
in contact with the droplets at the top. Therefore, this configuration is appropriated for
heat sensitive products.
Counter-current flow: The solution is sprayed in the opposite direction of the drying gas.
The drying gas enters at the bottom and flows upwards. The product becomes hotter at
the end in comparison with the co-current mode. This approach is suitable for thermally
stable products.
In this study it is used the co-current flow, because it is required that the liquid droplets
experiment a higher temperature at the chamber inlet, in order to start the solvent evaporation.
4) Powder collection: is done through a cyclone that confers the particles separation by size.
Based on inertial forces, the particles flow to the cyclone wall and are separated from the gas
as a downwards strain. The smaller particles are sent to an air filter while the particles with a
larger diameter are collected in the vessel under the cyclone.
28
3.2.2. Spray – Dryer Parameters
The final product characteristics (particle size, final humidity, yield and outlet temperature of the
drying gas) depend on several parameters that are set in the beginning of the atomization. There are
several parameters that influence the final product characteristics, however only the following
parameters will be described, as they are the only parameters that were modified.
1) Inlet temperature: The heated vector gas is sucked by the aspirator and it is heated up before
it enters in the spray chamber. This measurement takes place at the inlet of the drying chamber.
The heating is done though a resistance, and it can heat up until 220°C.
2) Aspirator rate: the aspiration motor sucks the drying gas creating vacuum. A higher or lower
drying gas flow rate will have a great impact on the drying performance. Also, a higher
aspiration rate provides a better separation in the cyclone. However, it will lead to a larger
moisture amount, due to a shorter residence time in the drying chamber.
3) Feed pump rate: a higher feed flow rate means a higher energy to evaporate the droplets into
solid particles, resulting in a lower outlet temperature. Thus, the feed pump rate will influence
the inlet and outlet temperature difference.
Moreover, the parameters optimization is done by using the trial and error procedure. In Figure
13 is given a summary over the spray dryer parameters and their influence in the final product
characteristics.
Figure 13 – Overview over the spray-dryer parameters and their influence in the final product. [28]
29
In Table 6 is possible to find the used parameters in the synthesis of all samples.
Table 6 – Spray – drier parameters for all the samples.
3.3. Characterization Methods
The characterization methods that were performed and will be described in the following
paragraphs are: N2 adsorption – desorption, Temperature Programed Reduction (TPR), X-Ray
Diffraction (XRD), low angles XRD, X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron
Microscopy (TEM) and Scanning Electron Microscopy (SEM). In Table 7 it is described the properties
that are expected to get from the analysis and characterize each samples.
Table 7 – Relation between analysis and the expected characterized properties.
Reference Nozzle Type
Inlet temperature
(°C)
Aspiration rate (m3/h)
Feed pump rate (mL/min)
Ultrasonic nozzle power
(kW)
JFR003 Two – fluid
nozzle
220
35 9.0 -
JFR005 –
JFR009 Ultrasonic
nozzle 17,5
1.8
2.0 JFR010
2.1 JFR014 –
JFR037
Analysis Characterized properties
N2 adsorption -
desorption Textural properties (BET area, porous volume, pore diameter)
TPR Cobalt reducibility and accessibility
XRD Cobalt crystalline phases and crystal’s size
Low angles XRD Organization of the porosity of the support (mesostructuration)
XPS Degree of oxidation of various elements
TEM
Information of the support: organization of the porosity of the support
(mesostructuration).
Information on the metallic phase: form in which the cobalt is, size of the
cobalt particles
SEM Information of the support: average size and morphology of the aerosol
elementary particles
30
3.3.1. N2 adsorption-desorption
This method is widely used for studying textural properties of catalysts. Adsorption equilibrium is
represented by isothermal plots, which describe the adsorbed quantity as function of the equilibrium
pressure P of the gas in contact with the solid. Actually, relative pressure P/P0 is used instead of P,
where P0 is the saturated vapor pressure of the adsorbate at the measurement temperature (≈ 77 K in
case of nitrogen). Therefore, an adsorption – desorption isotherm consists in measuring the quantity of
gas that is adsorbed on (or desorbed from) the surface of the solid, at a given temperature. [29]
Nitrogen is the most common molecule used in this technique. However, other molecules can be
used for specific purposes: for instance, argon, a very small monoatomic gas, is more appropriated for
the study of microporous samples, or krypton, where the low saturated vapor pressure (≈ 2 torr) can be
used to measure small adsorbed quantities precisely (for specific surface areas below 1 m2/g). [29]
Before measuring the adsorbed quantity, a degassing (or pre-treatment) stage is carried out in order
to eliminate the compounds adsorbed on the surface of the sample (H2O, CO2, etc.). The isotherms are
obtained by gradually increasing the pressure, where the small pores are filled first. The gas condenses
in successively larger pores until a saturated vapor pressure level is reached at which the entire porous
volume is saturated with liquid. Furthermore, the adsorption – desorption phenomena is highly suitable
for the study of samples where the pore size is in the mesoporous domain. [29]
The desorption isotherm is not often superimposed over the adsorption isotherm, which shows up
as a hysteresis phenomenon. This fact happens due to capillary condensation, which corresponds to a
phase transition effect caused by the interactions with the surface of the solid where the gas phase
abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid – gas
interface.
In Figure 14 it is described a typical isotherm of a mesostructured solid with a very good level of
organization. Moreover, the pore size distribution of a mesostructured solid is very narrow, as the pore
diameter is very uniform.
Figure 14 – Typical isotherm for a mesostructured solid. [29]
The main parameters obtained through this technique are the specific surface area, pore size
distribution, specific porous volume and information on the structure (pore shape, interconnection, etc.).
These information can be given by several models, where the BET model is used to determine the
specific surface area, the BJH method is used in the pore size distribution, while the t-plot method
consists in comparing the adsorption isotherm of a given solid in terms of the adsorbed thickness, what
makes possible to characterize the micro- and mesoporosity.
31
In this study, these analyses were performed in a Micromeretics ASAP 2420 equipment.
3.3.2. Temperature Programmed Reduction (TPR)
This method is used to evaluate the catalyst reducibility and the oxidation degree of the active
phase. The hydrogen consumption is followed as a function of temperature. TPR is extremely attractive
due to its high sensitivity to chemical changes induced by a catalyst promoter or by the support. [29]
This technique consists in measuring the consumption of hydrogen while heating a catalyst with a
linear temperature rate under continuous gas flow. TPR profiles do not provide direct information about
the modification of the catalyst structure, because hydrogen consumption could be attributed to different
reduction processes. In this study, the TPR analysis were performed in a Mircrometrics Auto Chem II
2920. The catalyst, during this analysis, was under a mixture of 5% H2 in air, at 58 cm3/min, and the
temperature was raised up to 1000°C, at a rate of 5°C/min. The hydrogen consumption is followed
through a thermal conductivity detector (TCD), and, it can be seen that, during the reduction process,
several products such as H2O, CO2 or CO are formed.
3.3.3. X-Ray Diffraction (XRD)
XRD is often used for identification of cobalt crystalline phases and evaluation of the crystal size
using the Debye - Scherrer equation (Eq. 15), where, β is the angular breadth of a diffraction line, C is
a constant, λ is the X-ray wavelength, L is volume – averaged size of crystallites, and finally, θ is the
Bragg angle.
The peak breadth, β, can be either calculated through the full-width half-maximum (FWHM) or by
the “integral width”, which corresponds to the area under diffraction peak divided by peak maximum.
Therefore, is often some uncertainty in measuring the size of cobalt crystallites with the Debye - Scherrer
equation, because the β definition will determine the value of the Bragg constant (C).
For very small and very large crystallites, it has been noticed a low accuracy in measuring the
crystallite sizes. The XRD technique is not very sensitive to the presence of small crystallites of cobalt
oxides (< 2 – 3 nm), since the peaks get too broad to be identified and measured. Broadening of the
XRD lines is caused by structural imperfections of the sample. [5]
The values measured during an analysis are the relative intensity levels of the diffraction lines,
corresponding to beam intensities. Moreover, the measurement of the scattering angle is done in lattice
planes which are identified using indices in three dimensions: h, k and l. These last three are related to
the directions of axes defining the crystalline system. [29]
Therefore, in order to follow the Bragg condition (Eq. 16), there is no possibility of scattering planes
with a spacing less than half the wavelength. Where n is an integer, λ is the wavelength, d is the plane
spacing, and finally, θ is the Bragg angle.
𝛽 =𝐶𝜆
𝐿 cos 𝜃
(Eq. 15)
𝑛𝜆 = 2𝑑 sin 𝜃 (Eq. 16)
32
These analyses, ranging from 5 to 72° (2 𝜃), were performed in a PANalytical X’Pert Pro equipment
with the reflection configuration.
3.3.4. Low angles X-Ray Diffraction
Low angles X-Ray diffraction analysis is often used to obtain parameters such as size, morphology
and distribution of particles. Normally, X-ray diffraction (XRD) covers angles in the range of 10° to 100°,
corresponding to a typical wavelength of 0.1 nm, to interatomic distances. Low angles XRD concerns
angles under 5°, thus, distances are greater than a nanometer. [29]
Therefore, low angles XRD processes at a higher resolution and scattering strength while
producing the scattering peaks, making it a suitable method for characterize mesostructured materials.
These analysis were performed in a PANalytical X’Pert Pro equipment in the transmission configuration,
and the analysis range was from 0.2 to 10° (2 𝜃). Moreover, the analysis were done without beam-stop,
in order to prevent the detector saturation. Also, it was used a Cu0.2 attenuator.
3.3.5. X-ray Photoelectron Spectroscopy (XPS)
XPS allows the characterization of the external surface layers of the catalyst (5 to 10 nm) [35] and
the degree of oxidation or electronic state of various elements (chemical neighborhood). The sample
that will be characterized is bombarded by an X-ray photon beam, thus, electrons of different elements
are emitted in terms of number and energy by an appropriate detector. The measured kinetic energy is
directly connected to the electron biding energy by the photoelectric effect. [29]
The characterization in this study was carried out in an ESCA KRATOS Axis Ultra spectrometer
with a monochromatic source of Al. The obtained spectrum are compared with references in order to
identify the signals.
3.3.6. Transmission Electronic Microscopy (TEM)
TEM provides detailed information concerning the composition and structure of heterogeneous
catalysts with real-space resolution down to the atomic level.
Once a sample is crossed by an electron beam, this may be partially adsorbed or deflected. A
certain fraction of these electrons, and those that have not been deflected, are combined to form an
image. Therefore, the use of transmission electron microscopy is based in controlling the electrons
involved in image formation. Thus, the image of the sample depends on the electron – matter
interactions. One can note that atoms with high atomic number scatter more electrons and at larger
angles than those with lower atomic number. Therefore, a particle of heavy metal on a light oxide
support, for instance silica or alumina, will appear dark. [29]
This characterization was carried out in a JEM 2100F equipment. To improve the contrast between
the support and the metallic phase, it is easiest to analyze it in a dark – field image, by selecting the
plan (220) for metallic cubic cobalt and the plan (440) for cobalt oxide (Co3O4).
3.3.7. Scanning Electronic Microscopy (SEM)
This characterization technique allows, along with TEM, to a local chemical and textural
characterization of catalysts. The principle of operation is very close to the one used in TEM. Therefore,
the image is obtained through the interaction of material’s and the electron beam, also known as electron
33
probe, with energies between 0.5 and 35 kV. However, when using SEM microscopes, the image
resolution is limited by the size of the electron probes. [29]
SEM provides information about the size, shape and 3D arrangement of the catalyst particles.
Nevertheless, SEM is not able to fully identify the intrinsic structure of the catalyst, thus, TEM should be
performed to fully characterize the pore structure.
In the present study, this type of analyses was performed with a Supra 40 equipment, with no pre-
treatment of the sample.
This page was intentionally left blank
35
4. Results and Discussion
Generic remarks
As the final application of these catalysts is the FT synthesis, it is more suitable the use of an
ultrasonic nozzle than a two fluid nozzle (normally used at the laboratory scale). Indeed, it produces
larger elementary particles. Therefore, the produced elementary particles would be closer to the ones
of the industrial catalysts that have elementary particles ranging from 80 to 100 µm.
In the following paragraphs, first are described the silica results and after alumina results. One
should take into account, that the main priority of this study is not the synthesis of a mesostructured
solid, but the synthesis of a mesoporous solid, since the FT process does not require mesostructured
catalysts.
On one hand, alumina matrix is the most used support in the FT synthesis. On the other hand,
there is more information in the literature concerning silica sol – gel chemistry and also IFPEN has a
vast experience in synthetizing silica based materials through the aerosol process. Therefore, a more
detailed study was performed on silica matrixes.
Supports were firstly tried to produce instead of trying to produce directly a catalyst, in order to
know if it was possible to synthesize it and also to characterize the elementary particles obtained through
the ultrasonic nozzle, which had never been used at IFPEN.
All the synthesized supports and catalysts were characterized through N2 adsorption – desorption
isotherm and the latter was also characterize through TPR. Besides these analysis, other
characterization techniques were applied when needed.
36
4.1. Silica
The following paragraphs tackle the synthesis of supports and the introduction of cobalt onto a
silica matrix, using as inorganic precursor TEOS.
4.1.1. Supports
Comparison with the two-fluid nozzle, generic characteristics and trials reproducibility
First of all, a trial was done with the two-fluid nozzle (JFR003) as it was the usual nozzle used in
the laboratory at IFPEN, as previously mentioned. This trial allowed a comparison between the two –
fluid and the ultrasonic nozzle, making it possible to evaluate the major differences between these two
equipments.
Concerning trials with the ultrasonic nozzle, two trials (JFR005 and JFR006) with the same spray
– drier parameters were performed in order to evaluate the trials reproducibility. Moreover, two samples
(JFR006 and JFR007) were collected along one trial, in order to evaluate if there were any differences
in the samples collected in the begging and the end of one trial.
Textural properties
Concerning the supports textural properties (BET surface and pore size distribution) nitrogen
adsorption – desorption isotherms were performed, which are described in Figure 15.
Figure 15 – Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR003, JFR005, JFR006 and JFR007.
In Table 8 it is described the surface area, the porous volume and the pore diameter for all the
previous samples.
0
50
100
150
200
250
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d
Vo
lum
e (
mL
/g)
Relative Pressure
JFR003
JFR005
JFR006
JFR007
0
0,02
0,04
0,06
0,08
0,1
0,12
0 10 20
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
37
Table 8 - BET surface, porous volume and pore diameter for JFR003, JFR005, JFR006 and JFR007.
Comparing the isotherms of JFR003 and JFR005, one can see that the solid obtained with the two
– fluid nozzle presents a hysteresis loop with more vertical lines than the solids obtained with the
ultrasonic nozzle. Thus, JFR003 seems more organized than JFR005. However, low angles XRD and
TEM analysis should be performed to fully verify this statement.
Furthermore, the isotherms shape of JFR005 and JFR006 are very similar, hence, it is possible to
conclude that the trials are reproducible. Likely, the isotherms shape of JFR006 and JFR007, are very
similar too, so it is possible to conclude that there is no change along time in solids belonging to the
same trial.
Higher surface areas are achieved with the two – fluid nozzle which is due to an increase in the
mesoporous volume.
Silica particles size distribution
In order to characterize the elementary particles morphology produced through the ultrasonic
nozzle, SEM analysis were performed in JFR005, JFR006 and JFR007.
As is possible to conclude through Figure 16 the synthesized silica particles are spherical, and as
one can see, the particles present the same shape throughout the trials, showing once more the trials
reproducibility (Figure 16 (A) and (B)). Furthermore, looking at the SEM micrographs (B) and (C) one
can conclude once more that there are no significant changes along the same trial.
Reference Porous type BET surface (m2) Porous volume (mL/g) Pore diameter (nm)
JFR003
Mesoporous
183 0.37 7.8
JFR005 114 0.20 7.8
JFR006 101 0.18 7.9
JFR007 107 0.18 7.9
(A) (B)
(C)
38
Figure 16 – SEM micrographs for (A) JFR005, (B) JFR006 and (C) JFR007.
Moreover, it was not possible to precise if the elementary particles are hollow as there are no
damage elementary particles.
For each sample it was done a particle’s size distribution, which are shown in Figure 17.
Figure 17 – Particles size distribution for JFR005, JFR006 and JFR007.
0
0,05
0,1
0,15
0,2
0 10 20 30 40 50
Fre
qu
en
cy (
vo
lum
e)
Elementary particles diameter (µm)
JFR005
0
0,05
0,1
0,15
0,2
0,25
0 10 20 30 40 50 60
Fre
qu
en
cy (
vo
lum
e)
Elementary particles diameter (µm)
JFR006
0
0,05
0,1
0,15
0,2
0,25
0 10 20 30 40 50 60
Fre
qu
en
cy (
vo
lum
e)
Elementary particles diameter (µm)
JFR007
(A) (B)
(C)
39
Regarding these histograms, it is possible to find the same results as before concerning the trials
reproducibility.
Table 9 - Results of the elementary particle size determinate by SEM.
The particle size distribution presents for the three supports a wide range of particles size, as it can
be seen in the histograms, since the particles size range from 2 to 60 µm. However, the maximum of
population is found for 30 and 35 µm.
As previously mentioned, the ultrasonic nozzle was used instead of the two – fluid nozzle due to
the final application of these catalysts in the FT process. It was expected to have the majority of
elementary particles ranging from 80 – 100 µm, and as one can see the medium elementary particle
diameter is around 30 µm, which value is acceptable for testing.
Organization of the porosity of the support
In order to confirm the existence of a mesostructured solid, low angles XRD was performed which
is shown in Figure 18. The presence of a peak at 0.75° (2θ) for the three samples confirms the existence
of an organized structure.
One can see that there are not three peaks as it is characteristic in mesostructured solids with a
very good level of organization. There are two hypotheses that can justify this absence: it can be
characteristic of samples where parts of them are very well organized and other parts do not present
organization at all. It can also be characteristic of samples where the entire sample is organized but not
with a good level of organization. Moreover, one can suppose that the three samples have a worm-like
structure (second option) and it will be confirmed thanks to a TEM analysis.
Moreover, the above assumption seems to be true, as the TEM analysis of a catalyst (later
described) reveals an organization throughout the sample, however with a low level of organization,
which is characteristic of worm – like structures.
Reference
Elementary particles medium
diameter (µm)
Elementary particles minimum
diameter (µm)
Elementary particles maximum
diameter (µm)
Standard deviation (µm)
JFR005 34.18 2.15 52.68 14.28
JFR006 34.51 2.34 59.38 17.40
JFR007 29.99 2.83 56.55 16.51
40
Figure 18 – Low angles XRD patterns for JFR005, JFR006 and JFR007.
4.1.2. Cobalt catalysts
It was synthetized a silica matrix containing 15 wt.% of cobalt concerning the total mass of solid
(JFR016). This solid was obtained by atomizing a solution which contained HCl and a pH, approximately,
of 2. This synthesis consisted mainly in introducing the cobalt precursor solution into the solution that
was used to produce the support. This solid was tested in a FT catalytic unit, as it will be later described.
The following paragraphs describe a preliminary study on the calcination temperature in order to
determine the best conditions to favor the formation of active catalytic species.
One should note, that at this point it is not possible to know if the solids with cobalt loaded are
catalysts or not, as they were not catalytic tested. However, as a simplification they will be referred as
“catalysts”.
Calcination temperature influence
JFR014 and JFR016 were calcinated at different temperatures, 550°C and 350°C, respectively.
The first one experimented the same calcination temperature as the support, and JFR016 was
calcinated with a lower temperature in order to prevent the formation of cobalt silicates, which are hardly
reducible species. [5]
Textural properties
Hereby are presented several analysis results to evaluate the influence of the calcination
temperature on the textural properties of the support.
JFR005 (silica matrix) is only represented in Figure 19 so it can be seen the differences in
introducing cobalt on the textural properties.
Blanc - File: C15N2584.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 - Di
140187*0.33 - File: C15K2589.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3:
140090 - File: C15K2588.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
140089 - File: C15K2587.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
139943 - File: C15K2586.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
Lin
(C
ou
nts
)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
14000
15000
16000
17000
18000
19000
20000
21000
22000
23000
24000
25000
2-Theta - Scale
0.22 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
0.75° (118Å)
JFR005
JFR007
JFR006
41
Figure 19 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR005 (support reference), JFR014 and JFR016.
In Table 10 is summarized the surface area (BET surface), the porous volume and the pore
diameter.
Table 10 - BET surface, porous volume and pore diameter for JFR005, JFR014 and JFR016.
The support isotherm shape is very close to the one of the catalysts. Comparing the t-plot of the
catalyst (JFR014) and the support (JFR005), which have the same calcination temperature, a higher
microporous volume is achieved for the catalyst (0.054 cm3/g), while the support (JFR005) presents a
microporous volume of 0.015 cm3/g. In terms of microporous surface area, JFR014 has 131 m2/g and
JFR005 has 42 m2/g. Therefore, the introduction of cobalt nanoparticles induces a higher microporosity,
and also a higher mesoporosity, as there was an increase in the porous volume along with a decrease
in the pore diameter. (Find the t-plot in appendix A)
Moreover, the isotherms shape of both catalysts is very similar. It was confirmed by the t-plot that
a higher microporosity is found in JFR014, which was calcinated at 550 °C. This is expected because
0
20
40
60
80
100
120
140
160
180
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d
Vo
lum
e (
mL
/g)
Relative Pressure
JFR005 (Support)
JFR014 (Catalyst, calcination at 550°C)
JFR016 (Catalyst, calcination at 350°C)
Reference Porous type BET surface (m2) Porous volume (mL/g) Pore diameter (nm)
JFR005
Mesoporous
114 0.20 7.8
JFR014 226 0.26 6.6
JFR016 164 0.26 6.5
0
0,01
0,02
0,03
0,04
0 5 10 15
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
42
with a higher calcination temperature small voids in the microporosity domain are created in the
surfactant hydrophilic part. Figure 20 illustrates this phenomenon.
Figure 20 - Scheme representing the solid surfaces calcinated at 350°C and 550°C. Adapted from [20]
Both catalysts present a very similar porous size distribution. For JFR014 the maximum population
is found at 6.6 nm while for JFR016 is found at 6.5 nm, therefore both solid have mesopores.
The support (JFR005) presents higher pore diameters than the catalysts (JFR014 and JFR016).
This smaller pore diameter could be due to a different micelle – inorganic precursor interaction.
Organization of the porosity of the silica matrix
To attest the existence of a mesostructured solid, low angles XRD was performed which is shown
in Figure 21. For JFR014 and JFR016 it was not found a long distance organization because no peak
at small angles was observed.
It is also possible to conclude that the introduction of cobalt onto the support has made solids with
no long distance organization, due to the impact of cobalt species on the chemical reactions inducing
the mesostructuration process.
Besides that, the calcination temperature does not have influence on the mesostructuration.
Vµ pore: 0.054 cm3/g
Mesoporous volume: 0.11 cm3/g
Vµ pore: 0.015 cm3/g
Mesoporous volume: 0.12 cm3/g
43
Figure 21 – Low angles XRD patterns for JFR014 and JFR016.
Finally, the calcination temperature was set up as 400°C, an intermediate value that accomplishes
a higher surfactant removal and trying to avoid the formation of cobalt silicates. Also, there is no interest
in creating micropores for the final application of these catalysts, because the long produced
hydrocarbons chains would have difficult leaving the catalyst.
Catalytic test
Moreover, before having all the analysis results, a part of JFR016 (before being dried or calcinated)
was reduced under hydrogen flow, and tested in a FT catalytic test unit (slurry reactor). After 16 hours,
there was no activity, in another words, there were no products formation. As there were no other
catalytic tests in this work, no setup or reaction condition are presented here.
Therefore, two main hypotheses were proposed for the catalyst lack of activity: first, the presence
of chlorine could affect the catalyst activity, and second, it could exist a strong interaction between the
silica precursor and the cobalt precursor, consequently affecting the cobalt dispersion. In the fowling
paragraphs is described in detail each hypothesis.
1) Presence of chlorine in the catalyst
The presence of chlorine in FT catalysts has showed a significant decrease in activity. This lack of
activity could be due to the poising of several surface sites by Cl atoms. [30] This could be related with
the chlorine strong electronegativity, which prevents the CO dissociation on the catalyst. Moreover, it
was found that the effect of chlorine atoms on the catalyst was slowly reversible.
1 4 2 59 4 i*0.7 - F ile : C 1 5 K 2 58 3 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 20 0 . s - T em p. : 2 5 °C (R o om ) - T im e S ta rte d : 0 s - 2 -Th e ta : 0.21 2 ° - Th e ta : 0.10 5 ° - A u x1: 0 .0 - A ux2 : 0.0 - A u x3 :
1 4 2 38 0 - F i le : C 15 K 2 5 8 0. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1 4 0 97 1 - F i le : C 15 K 2 5 8 1. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1)
1 4 2 13 9 - F i le : C 15 K 2 5 8 5. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1 4 1 41 6 - F i le : C 15 K 2 5 8 2. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
B la n c - F i le : C1 5 N 25 8 4 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 2 0 0 . s - T em p . : 2 5 °C (R o om ) - T im e S ta rte d: 0 s - 2 -Th e ta : 0 .21 2 ° - Th e ta : 0 .10 5 ° - A u x1 : 0 .0 - A ux2 : 0.0 - A u x3 : 0 .0 - D i
1 4 2 1 39 - L ef t A n g le: 0 .5 8 0 - R ig h t A n gle : 1 .1 9 8 - L e ft In t. : 5 5 9 8.1 00 C o u nts - Rig h t I nt .: 12 1 1 .4 2 9 Co u n ts - P e aks : 0 - P a ram s : 0 - W e ig ht : -1 .0 0 0 - kA 2 Ra tio : 0.5 - R e l. : 1 .8 0 8 % - T h .R .: 1 .5 46 % - R .In t. : 0 . 00 0 %
Lin
(C
ou
nts
)
0
1 000
2 000
3 000
4 000
5 000
6 000
7 000
8 000
9 000
1 000 0
1 100 0
1 200 0
1 300 0
1 400 0
1 500 0
1 600 0
1 700 0
1 800 0
1 900 0
2 000 0
2 100 0
2 200 0
2-Theta - Scale
0.24 0.3 0.4 0.5 0.6 0.7 0.8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
0.90° (d=98Å)
0.80° (d=110Å)
JFR014 JFR016
44
In order to prevent this effect HCl was replaced for HNO3 in the initial solution. The following
paragraphs describe the effect of HNO3, either on the supports and catalysts.
Textural properties
In Figure 22, one can see the nitrogen adsorption – desorption isotherms for the supports (JFR005
(HCl) and JFR028 (HNO3)) and for the catalysts (JFR016 (HCl) and JFR021 (HNO3)). One can note that
JFR005 and JFR016 are just represented as references, since they were synthetized with HCl.
Moreover, JFR016 was calcinated at 350°C and JFR021 at 400°C. A different calcination
temperature induces changes in the textural properties, but it was made the choice to compare them.
Figure 22 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR005 (reference), JFR016 (reference), JFR028 and JFR021.
Concerning the isotherms shape (supports and catalysts isotherms), one can conclude that, there
are no significant changes, except that JFR021 presents a hysteresis loop with more straight lines,
leading to a more organized structure, which is further confirmed.
In Table 11 it is summarized the surface area (BET surface), the porous volume and the pore
diameter for all the previous samples.
0
50
100
150
200
250
300
350
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d
Vo
lum
e (
mL
/g)
Relative Pressure
JFR005 (Support, HCl)
JFR016 (Catalyst, HCl)
JFR028 (Support, HNO3)
JFR021 (Catalyst, HNO3)
0
0,02
0,04
0,06
0,08
0,1
0 5 10
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
45
Table 11 - BET surface, porous volume and pore diameter for JFR005, JFR028, JFR016 and JFR021.
Both supports either with HCl (JFR005) or HNO3 (JFR028) present a different porous size
distribution.
Concerning the supports, one can conclude that the introduction of HNO3, increases (variation of
13%) the surface area along with an increase in the porous volume and in the pore diameter, possibly
due to a different micelle – inorganic precursor interaction.
Regarding the catalysts, using HNO3 instead of HCl increases significantly (variation of 103%) the
surface area as well as the porous volume, and reduces the pore diameter. This change in the pore
diameter could be due to the presence of cobalt species on the chemical reactions inducing the
mesostructuration process.
Organization of the porosity of the silica matrix
Concerning the support (JFR005) and catalyst (JFR016) synthetized with HCl, the organization of
the porosity of the support has changed with the introduction of cobalt, since JFR005 was an organized
structure and JFR016 did not present any organization.
Regarding the samples that were synthetized with HNO3, the organization of the porosity of JFR028
(support) should be confirmed thanks to low angles XRD analysis, and the organization of JFR021
(catalyst) was confirmed thanks to the same analysis (Page 49, Figure 25).
Therefore, one can conclude that the presence of HCl or HNO3 has influence in the
mesostructuration of the catalysts, probably due to a different interaction of the cobalt species on the
chemical reactions inducing the mesostructuration process.
Reference
Porous type
BET surface (m2)
Porous volume (mL/g)
Pore diameter
(nm)
Supports JFR005 (HCl)
Mesoporous
114 0.20 7.8
JFR028 (HNO3) 129 0.24 8.4
Catalysts JFR016 (HCl) 164 0.26 6.5
JFR021 (HNO3) 345 0.46 7.1
46
Cobalt reducibility
The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 23, it is the
TPR profile for both samples (JFR016 and JFR021) after calcination. After calcination both samples
were violet (which normally corresponds to the Co nitrate precursor coloration).
As previously mentioned, in chapter 2, the cobalt oxide reduction to metallic cobalt happens in two
reactions. The first one (Eq.10) corresponds to the reduction of Co3O4→CoO, which normally happens
between 100 - 350°C and the second reaction (Eq.11) corresponds to the reduction of CoO→Co°, which
normally happens between 400 - 600°C.
As one can see in Figure 23, for JFR016 there is a major peak at 796°C and for JFR021 there are
two main peaks, the first at 842°C and the second at 899°C.
Both samples present peaks at high temperatures which does not correspond to the reduction of
CoO→Co°, since there is no peak corresponding to the reduction of Co3O4→CoO (between 100-350°C)
and as it is at a much higher temperature than usual (400-600°C). Actually, these peaks may suggest
the existence of cobalt silicates which are due to a strong interaction between the support and cobalt
oxide. These species are hardly reducible, thus, it can explain the existence of peaks at elevated
temperatures.
At this point, we can conclude that the hypothesis aforementioned of strong Co-O-Si interactions
may be responsible for the lack of catalytic activity due to difficulties in the Co reduction.
Furthermore, using HCl (JFR016) or HNO3 (JFR021) does not have influence in the cobalt
reducibility. However, as previously stated, the presence of Cl atoms can affect negatively the activity
of the catalysts in the FT synthesis. Thus, from now on, all the catalysts were synthesized with HNO3 in
the initial solution.
Figure 23 – TPR profile for JFR016 and JFR021.
0
0,2
0,4
0,6
0,8
1
1,2
0 200 400 600 800 1000
Hyd
rog
en
co
ns
um
pti
on
(mL
/g c
ata
lys
t)
Temperature (°C)
JFR016 (HCl)
JFR021 (HNO3)
47
2) Strong interaction between the silica precursor and the cobalt precursor
As it can be seen in Figure 23, hardly reducible cobalt species were being formed, which supports
the hypothesis of a strong interaction between silica precursor and the cobalt precursor. These oxides
(cobalt silicates) are often amorphous, which makes it harder to characterize those using conventional
techniques, such as XRD. A low cobalt content and a high surface area favor the formation of hardly
reducible oxides. Mixed cobalt – silicium or aluminum oxides can be formed during the catalysts
preparation, oxidative and reductive pretreatments, and in the course of FT reaction. [5]
Therefore, in this case if there is a strong interaction between cobalt and TEOS in the solution,
cobalt in the final powder could be ultra – dispersed which leads to cobalt silicate and/or very small
cobalt particles that are very hard to reduce, as seen in chapter 2.
In order to change the interaction between silica precursor (TEOS) and the cobalt precursor (cobalt
nitrate), were made three solutions at different pH. The formed oligomers are mainly linear and easily
condensable in an acid media (0<pH<2) and ramified in a less acid media (pH>2). The silica PZC is
approximately at pH=2, hence a solution was made approximately at a pH=PZC (as in the usual
procedure), and the other two at a pH above and below the PZC.
Consequently, changing the pH of the solution would affect the size and the dispersion of the cobalt
particles. Atomizing a solution with a pH below the PZC provides a silica surface positively charged,
hence repulsion will occur between Co2+ ions and the surface leading to a “non – homogenous
distribution” (creating perhaps cobalt domains and hence larger particles of cobalt species). On the other
hand, atomizing a solution with a pH above the PZC provides a silica surface negatively charged, and
the Co2+ ions will be “homogenous distributed”.
Consequently, the following samples were prepared: pH=1 (JFR019), pH=2 (JFR021) and pH=3
(JFR022).
Textural properties
Hereby is described the pH influence on the textural properties. In Figure 24 it is described the
nitrogen adsorption – desorption isotherms for the three samples (JFR019, JFR021 and JFR022), and
JFR016 is only represented as a reference (this solid was synthesized through an atomization solution
which contained HCl and present a pH approximately of 2).
48
Figure 24 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR016 (reference), JFR019, JFR021 and JFR022.
In Table 12 it is summarized the surface area (BET surface), porous volume and pore diameter for
the previous samples.
Table 12 - BET surface, porous volume and pore diameter for JFR016, JFR019, JFR021 and JFR022.
Concerning the isotherms shape, one can see that JFR019 (pH = 1) is slightly different from the
others. JFR019 presents a hysteresis loop with more vertical lines, indicating the existence of a more
organized solid, which is confirmed by TEM analysis, further presented. The shape of JFR021 (pH = 2)
presents a hysteresis loop with more straight lines than JFR022 (pH = 3), but less pronounced than
JFR019.
All the three samples (JFR019, JFR021 and JFR022) present different porous size distributions.
This is due to different interactions between the surfactant and the inorganic species induced by the
different pH.
0
50
100
150
200
250
300
350
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d V
olu
me
(m
L/g
)
Relative Pressure
JFR016
JFR019 (pH = 1)
JFR021 (pH = 2)
JFR022 (pH = 3)
Reference Porous type BET surface
(m2) Porous volume
(mL/g) Pore diameter
(nm)
JFR016
Mesoporous
164 0.26 6.5
JFR019 (pH = 1) 338 0.38 5.2
JFR021 (pH = 2) 345 0.46 7.1
JFR022 (pH = 3) 258 0.35 6.3
0
0,03
0,06
0,09
0,12
0 5 10 15
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
49
JFR019 and JFR021 present higher surfaces areas than JFR022 due to a higher mesoporosity, as
JFR021 presents a higher porous volume, and, JFR022 presents a slightly higher porous volume, but a
much smaller pore diameter.
Therefore, is possible to conclude that regarding the textural properties, atomizing solutions with
different pH leads to solids with different properties.
Organization of the porosity of the silica matrix
In order to study the influence of the pH on the mesoporosity, low angles XRD analysis was
performed.
As one can see in Figure 25, for the sample JFR022 (pH = 3) there is no long distance organization.
While for JFR019 (pH = 1) and for JFR021 (pH = 2), it was confirmed the existence of a mesostructured
organization.
This can be explained, since a higher condensation rate is expected at pH = 1 and pH = 3. However,
JFR019 (pH = 1) is more organized than JFR022 (pH = 3) due to a better interaction between the
surfactant and the inorganic molecular precursor, as for each pH, the silica surface is differently charged.
The mesoporosity was also confirmed by TEM analyses, which are described later on.
Furthermore, one can conclude that the pH of the atomized solution has influenced in the
mesostructuration process, as it was expected.
Figure 25 - Low angles XRD patterns for JFR019, JFR021 and JFR022.
1 4 2 59 4 i*0.7 - F ile : C 1 5 K 2 58 3 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 20 0 . s - T em p. : 2 5 °C (R o om ) - T im e S ta rte d : 0 s - 2 -Th e ta : 0.21 2 ° - Th e ta : 0.10 5 ° - A u x1: 0 .0 - A ux2 : 0.0 - A u x3 :
1 4 2 38 0 - F i le : C 15 K 2 5 8 0. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1 4 0 97 1 - F i le : C 15 K 2 5 8 1. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1)
1 4 2 13 9 - F i le : C 15 K 2 5 8 5. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
1 4 1 41 6 - F i le : C 15 K 2 5 8 2. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -
B la n c - F i le : C1 5 N 25 8 4 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 2 0 0 . s - T em p . : 2 5 °C (R o om ) - T im e S ta rte d: 0 s - 2 -Th e ta : 0 .21 2 ° - Th e ta : 0 .10 5 ° - A u x1 : 0 .0 - A ux2 : 0.0 - A u x3 : 0 .0 - D i
1 4 2 1 39 - L ef t A n g le: 0 .5 8 0 - R ig h t A n gle : 1 .1 9 8 - L e ft In t. : 5 5 9 8.1 00 C o u nts - Rig h t I nt .: 12 1 1 .4 2 9 Co u n ts - P e aks : 0 - P a ram s : 0 - W e ig ht : -1 .0 0 0 - kA 2 Ra tio : 0.5 - R e l. : 1 .8 0 8 % - T h .R .: 1 .5 46 % - R .In t. : 0 . 00 0 %
Lin
(C
ou
nts
)
0
1 000
2 000
3 000
4 000
5 000
6 000
7 000
8 000
9 000
1 000 0
1 100 0
1 200 0
1 300 0
1 400 0
1 500 0
1 600 0
1 700 0
1 800 0
1 900 0
2 000 0
2 100 0
2 200 0
2-Theta - Scale
0.24 0.3 0.4 0.5 0.6 0.7 0.8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
0.90° (d=98Å)
0.80° (d=110Å)
JFR019
JFR021
JFR022
50
Cobalt reducibility
The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 26, it is the
TPR profile for JFR019, JFR021 and JFR022 after calcination.
It is important to refer that JFR019 after calcination was black, which is the characteristic color of
cobalt oxide (Co3O4), whereas JFR021 and JFR022 were violet after the same post – treatment.
Figure 26 - TPR profile for JFR019, JFR021 and JFR022.
As one can see in Figure 26, for JFR019 there is a first peak at 375°C and a second peak at 816°C.
As the first peak is at a higher temperature than usual, it could correspond to the reduction of
Co3O4→CoO and/or to the decomposition of residual NOx groups (exothermic reaction). These NOX
groups are due to the cobalt precursor solution (cobalt nitrate) and the HNO3 used to acidify the solution,
because JFR019 is the more acidic solution so the more NO3- concentrated. [5] If one decomposes the
second peak, a part of it might correspond to the CoO→Co° reduction. And the majority of this peak, as
it happens at an elevated temperature may suggest, once more, the existence of cobalt silicates which
are due to a strong interaction between the support and cobalt oxide.
Regarding JFR021 it shows two peaks at 842°C and 899°C, and, JFR022 presents one peak at
825°C. These peaks may only suggest again the existence of cobalt silicates due to a strong interaction
between silica and cobalt oxide.
A XRD analysis was performed in order to characterize the crystalline cobalt oxide phases present
in the three catalysts.
Oxide phases
In order to know in which phase is the cobalt XRD analysis were performed. The results are
described in the following XRD diagrams. In Figure 27 A) the green line is refered to another sample
not mentioned in this part.
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0 200 400 600 800 1000
Hyd
rog
en
co
ns
um
pti
on
(mL
/g c
ata
lys
t)
Temperature (°C)
JFR019 (pH = 1)
JFR021 (pH = 2)
JFR022 (pH = 3)
51
Figure 27 - XRD diagrams: A) JFR019 and B) JFR021 and JFR022.
As one can see in Figure 27, for JFR019 it was found a small peak of cobalt oxide (Co3O4) which
is in agreement with the TPR analysis. These particles were measured at 36.8° (2θ) and they have a
size of 22 nm.
Moreover, for all the three samples it was found a peak of metallic cobalt and this was not expected
since the samples were only calcinated, and not reduced.
It was expected after the conclusions taken by the TPR, which revealed the possible existence of
cobalt silicates, to found a peak of cobalt silicates in the XRD diagrams. The absence of this peak may
be justified by the cobalt silicates being amorphous, thus does not appear on the XRD diagrams.
Therefore, a XPS analysis was performed in JFR019 (which presented a first peak in TPR and a peak
of Co3O4 in XRD), and in JFR021. This analysis was not done in JFR022 since it present very similar
results to the ones of JFR021. The results from XPS are summarized in Table 13, which describe in
which form is the cobalt.
140971 - 142139
0 4-00 5 -96 5 6 (A) - C ob alt - C o - Y : 8 .4 7 % - d x b y: 1 . - W L: 1 .5 4 06 - C u bic - a 3.54 4 30 - b 3 .5 44 3 0 - c 3.54 4 30 - a lp ha 9 0 .0 00 - b eta 90 .0 0 0 - g a m m a 9 0.00 0 - Fa ce -ce ntered - Fm -3m (2 25 ) - 4 - 44 .5 2 37 - I/ Ic PD F 7 .3 - F7 =10 0 0(0 .0
0 0-00 9 -04 1 8 (A) - C ob alt O x id e - C o 3 O 4 - Y : 9.05 % - d x b y : 1. - W L : 1 .54 06 - C u b ic - a 8 .0 84 0 0 - b 8 .0 8 40 0 - c 8 .0 84 0 0 - a lp h a 90 .0 0 0 - b e ta 9 0.00 0 - ga m m a 9 0.00 0 - Fa ce-ce n te re d - F d-3 m (22 7 ) - 8 - 5 2 8.29 8 - F1 9= 3 4(0 .0 2 30 ,
O p e rat io n s: S li ts F ixed | Im po rt
1 42 1 39 - Fi le : F1 5K 15 6 85 .raw - Typ e: PS D fas t -scan - S tar t: 5 .0 17 ° - End : 7 1.99 5 ° - S te p: 0 .0 3 3 ° - S tep t im e : 49 9 .7 s - Te m p .: 2 5 °C (R oo m ) - T im e S ta rte d : 0 s - 2-T he ta : 5 .0 17 ° - Th eta: 2 .5 08 ° - Au x1 : 0.0 - Au x2 : 0.0 - Aux3 : 0.
O p e rat io n s: S li ts F ixed | Im po rt
1 40 9 71 - Fi le : F1 5K 15 6 82 .raw - Typ e: PS D fas t -scan - S tar t: 5 .0 17 ° - End : 7 1.99 5 ° - S te p: 0 .0 3 3 ° - S tep t im e : 49 9 .7 s - Te m p .: 2 5 °C (R oo m ) - T im e S ta rte d : 0 s - 2-T he ta : 5 .0 17 ° - Th eta: 2 .5 08 ° - Au x1 : 0.0 - Au x2 : 0.0 - Aux3 : 0.
Lin
(C
ps)
0
1
2
3
4
5
6
7
8
2-Theta - Scale
6 10 20 30 40 50 60 70
142380 -142594
0 4-00 5 -96 5 6 (A) - C ob alt - C o - Y : 8 .4 7 % - d x b y: 1 . - W L: 1 .5 4 06 - C u bic - a 3.54 4 30 - b 3 .5 44 3 0 - c 3.54 4 30 - a lp ha 9 0 .0 00 - b eta 90 .0 0 0 - g a m m a 9 0.00 0 - Fa ce -ce ntered - Fm -3m (2 25 ) - 4 - 44 .5 2 37 - I/ Ic PD F 7 .3 - F7 =10 0 0(0 .0
0 0-00 9 -04 1 8 (A) - C ob alt O x id e - C o 3 O 4 - Y : 9.05 % - d x b y : 1. - W L : 1 .54 06 - C u b ic - a 8 .0 84 0 0 - b 8 .0 8 40 0 - c 8 .0 84 0 0 - a lp h a 90 .0 0 0 - b e ta 9 0.00 0 - ga m m a 9 0.00 0 - Fa ce-ce n te re d - F d-3 m (22 7 ) - 8 - 5 2 8.29 8 - F1 9= 3 4(0 .0 2 30 ,
O p e rat io n s: S li ts F ixed | Im po rt
1 42 5 94 - Fi le : F1 5K 15 7 39 .raw - Typ e: PS D fas t -scan - S tar t: 5 .0 17 ° - End : 7 1.99 5 ° - S te p: 0 .0 3 3 ° - S tep t im e : 49 9 .7 s - Te m p .: 2 5 °C (R oo m ) - T im e S ta rte d : 0 s - 2-T he ta : 5 .0 17 ° - Th eta: 2 .5 08 ° - Au x1 : 0.0 - Au x2 : 0.0 - Aux3 : 0.
O p e rat io n s: S li ts F ixed | Im po rt
1 42 3 80 - Fi le : F1 5K 15 6 86 .raw - Typ e: PS D fas t -scan - S tar t: 5 .0 17 ° - End : 7 1.99 5 ° - S te p: 0 .0 3 3 ° - S tep t im e : 49 9 .7 s - Te m p .: 2 5 °C (R oo m ) - T im e S ta rte d : 0 s - 2-T he ta : 5 .0 17 ° - Th eta: 2 .5 08 ° - Au x1 : 0.0 - Au x2 : 0.0 - Aux3 : 0.
Lin
(C
ps)
0
1
2
3
4
5
6
7
8
2-Theta - Scale
6 10 20 30 40 50 60 70
JFR019
(A)
(B)
Co°
Co3O4
JFR021
JFR022
Co°
52
Table 13 - Results from XPS analysis for JFR019 and JFR021.
The results from XPS analysis, confirmed the existence of cobalt oxide in JFR019, and it reveals
the existence of Co2+ in both samples. XPS analysis is only able to specify that there is a specie in the
catalyst that presents an oxidation state of Co2+, but it could most likely be cobalt silicates.
Furthermore, it is interesting to note that no Co° was observed by XPS.
Organization of the porosity of the silica matrix and cobalt dispersion
In order to study with more detail the silica mesoporosity and the cobalt phase, TEM analysis were
performed. The following pictures show TEM micrographs for the previous three samples.
Figure 28 - TEM micrographs for the support of JFR019.
Regarding the silica matrix, as one can see in Figure 28, the porous have a worm-like shape. Also,
TEM showed that the pore size was approximately 5 nm, what is in agreement with the value taken from
N2 adsorption-desorption isotherm.
Figure 29 - TEM micrographs for metallic phase of JFR019.
Reference Co2+ Co3O4
JFR019 55,3% 44,7%
JFR021 100% -
Co3O4
Co°
53
The metallic phase (Figure 29) is present in the form of particles which are heterogeneously
distributed and ultra – dispersed onto the matrix.
Moreover, there are zones where cobalt particles were not visible. However EDS analysis showed
that Co is also present. Images in a dark – field mode did not show any diffraction, what makes possible
to conclude that cobalt, in these zones, is in an amorphous form. Besides that, cobalt was found in two
crystalline forms: cobalt oxide (Co3O4) and metallic cobalt (Co°).
For JFR019 was done a particle size distribution. The cobalt oxide particle size distribution in
volume is represented in Figure 30, and the size range is very broad going from about 10 to 50 nm.
Figure 30 – Cobalt oxide particles size distribution in volume for JFR019.
Concerning the samples JFR021 and JFR022 (Figure 31), globally they present the same
morphology. The oxide matrix presents a mesoporous domain as expected, and the structuration was
still happening, leading to a worm – like structure. The metallic phase was present in an ultra-dispersed
form, and, as seen in JFR019, it was found an amorphous phase rich in cobalt.
Figure 31 - TEM micrographs for the supports of: (A) JFR021 and (B) JFR022.
Conclusions
To conclude, it was found that JFR019 presented the most organized structure since it was
synthesized under the most acidic conditions, as previously stated.
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54
Fre
qu
en
cy (
Vo
lum
e)
Co3O4 particles diameter (nm)
JFR019
(B) (A)
54
Furthermore, these results are in agreement with the initial hypothesis: there is a peak in TPR at
elevated temperatures which is due to existence of cobalt silicates, however this species are not
detectable on XRD due to their non – crystallinity, and finally TEM analysis show that the cobalt in its
majority is in an amorphous form.
To sum up, there is a strong interaction between silica and cobalt, and changing the pH was not
enough to change the Si-O-Co interaction. However, with the solution at pH = 1 (JFR019), better results
were achieved, therefore more acidic conditions can be tested, to improve the solids obtained with TEOS
as inorganic precursor.
Silica conclusions
It was possible to load cobalt on a silica oxide matrix by spray – drying. Therefore, a solid with
cobalt already loaded (15 wt.% concerning the final catalyst mass) was synthetized by atomizing a
solution that had TEOS as inorganic precursor and acidified water with HCl at pH = 2. The resulting solid
was tested under a catalytic unit and there was no product formation. In order to modify the lack of
catalytic activity, the pH of the initial solution was modified.
First of all, the water was acidified with HNO3 instead of HCl to avoid chlorine atoms that can poison
active sites of the catalyst. It was supposed that the lack of activity was not mainly due to the presence
of Cl but to cobalt silicate. Also, the presence of HCl or HNO3 influences the mesostructuration process
of the solids. However, as chlorine atoms can affect the activity of the FT catalysts, all the solids started
to be synthetized with acidified water with HNO3.
Moreover, it was notice by TPR analysis the existence of peaks at high temperatures. This could
indicate the presence of cobalt silicates, which are hardly reducible species. [5]
In order to change the Si-O-Co interaction solutions with different pH were atomized. Three
solutions were made at pH = 1, pH = 2 and pH = 3. For all the samples different textural properties were
achieved (surface area, porous volume and pore diameter). Also, it was concluded that the pH has
influence on the mesostructuration process, as only the samples at pH = 1 and pH = 2 were
mesostructured. Moreover, it was concluded that the Si-O-Co interaction was only modified at pH =1,
however not enough to prevent the formation of cobalt silicates.
55
4.2. Alumina
Study of Aluminum chloride as inorganic precursor
The following paragraphs tackle the synthesis of alumina and cobalt alumina solids, using as
inorganic precursor AlCl3·6H2O. As previously mentioned, this study is not so detailed as the one of
silica, due to a larger knowledge and experience at IFPEN in synthetizing silica based materials by spray
- drying.
4.2.1. Supports
Generic characteristics and spray – drier parameters influence
JFR009 was synthesized in order to characterize the alumina supports produced by the ultrasonic
nozzle. Moreover, JFR010 was synthesized with a faster feed pump rate (2.1 mL/min) in order to know
the influence of this parameter in the support textural properties and to know if further atomizations could
proceed with this feed pump rate. Finally, the feed pump rate was set up at 2.1 mL/min.
Textural properties
Concerning the supports textural properties nitrogen adsorption – desorption isotherms were
performed, which are described Figure 32.
Figure 32 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR009 and JFR010.
In Table 14 it is summarized the surface area (BET surface), the porous volume and the pore
diameter for all the previous samples.
0
50
100
150
200
250
300
350
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d V
olu
me
(m
L/g
)
Relative Pressure
JFR009
JFR010
0
0,02
0,04
0,06
0,08
0 5 10 15 20dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
56
Table 14 - BET surface, porous volume and pore diameter for JFR009 and JFR010.
In Figure 32, one can see that the isotherm shape of JFR009 and JFR010 is far different from the
typical isotherm for a mesostructured solid, and it is possible to state that this solid is probably not
mesostructured. Furthermore, this statement is confirmed by low angles XRD analysis, which is
described in the following paragraphs.
Concerning the difference in the isotherm shape of JFR009 and JFR010 and the differences in the
pores diameter, a more detailed study on alumina oxide matrix should be performed in order to justify
these differences. But it seems that the organic molecule (surfactant) does not play the role of templating
agent as with silica matrix, because of the much lower pore size.
Only JFR009 was characterized with more detail, because it was synthesized in the same
conditions as silica supports. Thus, it is possible to make comparisons between them.
Morphology of the aerosol elementary particles
In order to characterize the aluminum elementary particles morphology produced through the
ultrasonic nozzle, SEM analysis was performed in JFR009.
Figure 33 - SEM micrographs for JFR009.
Comparing Figure 33 with Figure 16 (page 37), one can see that alumina elementary particles have
a more fragile aspect than silica elementary particles. This can be due to different hydrolysis,
condensation and micellization reactions, and also, because alumina is not mesostructured (described
in Figure 34). Also, it is possible to see that alumina elementary particles are less spherical than the
ones of silica, and, that alumina elementary particles are hollow inside, as they are crushed, which did
not happen with silica elementary particles.
Reference Porous type BET surface (m2) Porous volume (mL/g) Pore diameter (nm)
JFR009 Mesoporous
248 0.41 1.8
JFR010 258 0.47 3.2
57
Organization of the porosity of the support
In order to confirm the existence of a mesostructured solid, low angles XRD was performed which
is shown in Figure 34. It is possible to conclude that alumina is not mesostructured as there is no peak
in the low angles XRD analysis.
Figure 34 - Low angles XRD patterns for JFR009.
As previously mentioned, this lack of organization is not restricting in the final application of these
catalysts. The priority is to have a porous solid, which is the case but with a lower pore diameter.
4.2.2. Cobalt catalysts
Cobalt catalysts on an alumina matrix were synthesized containing 15 wt.% of cobalt, concerning
the final mass of catalyst. Following the same strategy as in silica, the presence of chloride in the catalyst
should be avoided. However, to understand the influence on having HNO3 instead of HCl, a trial was
done where the atomization solution contained with HCl (JFR023) and another where it contained HNO3
(JFR024). Also, some trials were made with aluminum nitrate (Al(NO3)3) as inorganic precursor, to avoid
the presence of chlorine atoms in the final solid.
One should note, that at this point it is not possible to know if the solids with cobalt loaded are
catalysts or not, as they were not catalytic tested. However, as a simplification they will be referred as
“catalysts”.
Blanc - File: C15N2584.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 - Di
140187*0.33 - File: C15K2589.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3:
140090 - File: C15K2588.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
140089 - File: C15K2587.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
139943 - File: C15K2586.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -
Lin
(C
ou
nts
)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
14000
15000
16000
17000
18000
19000
20000
21000
22000
23000
24000
25000
2-Theta - Scale
0.22 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
0.75° (118Å)
JFR009
58
Textural properties
In Figure 35, one can see the nitrogen adsorption – desorption isotherms for the supports (JFR010
and JFR025) and for the catalysts (JFR023 and JFR024). One can note that JFR010 is just represented
as an oxide matrix reference, since it only differs in the presence of HCl or HNO3.
Figure 35 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR010 (reference), JFR023, JFR025 and JFR024.
In Table 15 it is summarized the surface area (BET surface), the porous volume and the pore
diameter for all the previous samples.
Table 15 - BET surface, porous volume and pore diameter for JFR010, JFR025, JFR023 and JFR024.
As one can see in Figure 35, the isotherms shape change significantly with the presence of HCl or
HNO3. As previously mentioned in the supports sub – chapter, the supports (JFR010) synthetized
through an atomization solution which contained HCl are not mesostructured, and likelihood, the
catalysts which contained HCl (JFR023) are not also. Moreover, the isotherm shape of the support which
contained HNO3 is very different from the typical isotherm for a mesostructured solid, thus is reasonable
0
50
100
150
200
250
300
350
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d V
olu
me
(m
L/g
)
Relative Pressure
JFR010 (Support, HCl)
JFR023 (Catalyst, HCl)
JFR025 (Support, HNO3)
JFR024 (Catalyst, HNO3)
Reference
Porous type
BET surface (m2)
Porous volume (mL/g)
Pore diameter (nm)
Supports JFR010 (HCl)
Mesoporous
258 0.41 3.2
JFR025 (HNO3) 260 0.45 3.1
Catalysts JFR023 (HCl) 65 0.15 2.9
JFR024 (HNO3) 121 0.19 1.8
0
0,02
0,04
0,06
0,08
0,1
0 5 10
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
59
to affirm that both supports and catalysts containing HNO3 are not mesostructured. One should take into
account that to verify this statement a low angles XRD and TEM analysis should be performed.
Concerning the supports (JFR010 and JFR025), one can conclude that the introduction of HNO3,
almost does not modify the surface area, porous volume or pore diameter. Regarding the catalysts
(JFR023 and JFR024), using HNO3 instead of HCl increases significantly (variation of 86%) the surface
area. However there is a reduction in the pore diameter along with a reduction in the porous volume
with the introduction of HNO3 in the catalyst, instead of HCl.
The incorporation of cobalt on the alumina matrix changes significantly the textural properties,
reducing considerably the surface area, the porous volume and the pore diameter, whereas it is the
contrary with silica matrix. Therefore, the textural properties of the obtained solids are not satisfactory
for the application of these catalysts in the FT process. Hence, a further study should be performed in
order to improve these textural properties.
Cobalt reducibility
The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 36 it is the
TPR profile for JFR023 and JFR024.
Figure 36 - TPR profile for JFR023 and JFR024.
After spray drying both solutions, the collected powder was light blue, which is the typical color of
cobalt aluminates. In order to confirm this supposition, a TPR analysis was performed.
For both samples there is only one peak at high temperatures which does not correspond to the
reduction of CoO→Co°, since there is no peak corresponding to the reduction of Co3O4→CoO (between
100-350°C) and as it is at higher temperature than usual (400-600°C).
Therefore, these peaks may suggest the existence of cobalt aluminates, which is in agreement with
the powder color, due to a strong interaction between the oxide matrix and cobalt oxide.
To completely avoid the presence of chlorine atoms in the final catalyst aluminum nitrate (Al(NO3)3)
was tested as inorganic precursor.
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
0 200 400 600 800 1000
Hyd
rog
en
co
ns
um
pti
on
(mL
/g c
ata
lys
t)
Temperature (°C)
JFR023 (HCl)
JFR024 (HNO3)
60
Study of Aluminum nitrate as inorganic precursor
In the following paragraphs it is described the synthesis of cobalt catalysts on an alumina matrix,
with 15 wt%., concerning the final catalyst mass.
Textural properties
In Figure 37 is described the nitrogen adsorption – desorption isotherms for JFR037 and JFR024.
One should note that the latter is only represented as a reference, as the inorganic precursor is the only
difference between the both samples.
Figure 37 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by
BJH adsorption: JFR024 and JFR037.
In Table 16 it is summarized the surface area (BET surface), the porous volume and the pore
diameter for all the previous samples.
Table 16 - BET surface, porous volume and pore diameter for JFR024 (reference) and JFR037.
Comparing the isotherms shape it is possible to affirm that JFR037 is not mesostructured, however
a low angles XRD and TEM analysis should be performed to confirm this.
0
20
40
60
80
100
120
140
0 0,2 0,4 0,6 0,8 1
Ad
so
rbe
d V
olu
me
(m
L/g
)
Relative Pressure
JFR024 (AlCl3)
JFR037 (Al(NO3)3)
Reference Porous type BET surface (m2) Porous volume (mL/g) Pore diameter (nm)
JFR024 Mesoporous
121 0.19 1.8
JFR037 32 0.11 12
0
0,01
0,02
0,03
0 10 20 30
dV
/dD
vo
lum
e (
mL
/g)
Average pore diameter (nm)
61
Concerning the differences in the pore diameter and in the surface area, a more detailed study on
should be performed in order to justify these differences, but it seems that with Al(NO3)3 the organic
molecule (surfactant) plays a role in building the porosity as it is wider than with AlCl3 precursor.
The increase in the pore diameter is positive, as the produced hydrocarbons chains would be
desorbed from the catalyst more easily. However, the porous volume it is not satisfactory for the FT
process, hence a more detailed study should be done to improve the textural properties.
Cobalt reducibility
The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 38 it is the
TPR profile for JFR037 and JFR024. It is important to refer that after calcination, both samples were
black, which is the characteristic color of cobalt oxide (Co3O4), instead of violet for JFR024.
Figure 38 - TPR profile for JFR024 and JFR037.
As one can see, in Figure 38, the TPR profile for the samples obtained with Al(NO3)3 as inorganic
precursor have changed significantly.
If one decomposes the peaks of JFR037 it is found a first peak around 415°C, a second peak at
615°C and finally a third peak at 895°C.
For JFR037 sample the first peak likelihood corresponds to Co3O4→CoO reduction, whereas the
second peak may correspond to CoO→Co° reduction. Concerning the first peak, one can note that it is
at a more elevated temperature than usual, which might be explained through the decomposition of
residual NOX groups (exothermic reaction). This group exists due to the presence of NO3 in the inorganic
precursor, cobalt precursor and acidified water.
Finally, JFR037 presents a third peak that probably correspond to the existence of cobalt
aluminates. To confirm this, a more detailed characterization should be performed.
Alumina conclusions
The loading of cobalt onto an alumina matrix was possible. Moreover, it is believed that none of the
synthetized alumina based solids are mesostructured (due to the low angles XRD for the support and
to the isotherms shape).
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
0 200 400 600 800 1000
Hyd
rog
en
co
ns
um
pti
on
(mL
/g c
ata
lys
t)
Temperature (°C)
JFR024 (AlCl3)
JFR037 (Al(NO3)3)
62
To follow the same strategy as in silica, two solids with cobalt already loaded (15 wt.%, concerning
the final catalyst mass) were synthetized through two solutions containing HCl and HNO3 and aluminum
chloride as inorganic precursor. The loading of cobalt in the alumina matrix decreases the surface area,
porous volume and the pore diameter. One should note, that the achieved textural properties are not
satisfactory, therefore a further study should be performed to improve these. Moreover, for both cases
it was noticed a strong interaction between cobalt and alumina precursor, as in TPR analysis, there were
peaks at elevated temperatures.
In order, to avoid the presence of chlorine atoms in the final solid, aluminum nitrate was tested as
inorganic precursor.
One should note that the change in the aluminum precursor allows a modification in the interaction
between alumina and cobalt, hence, the reduction of Co3O4 → Co° was achieved.
63
5. Conclusions and future perspectives
The aim of this work is the synthesis of active FT catalysts by spray – drying. At this point, is not
possible to conclude if the synthetized solids are catalysts, as they were not catalytic tested. It is only
possible to affirm that it was possible to synthetize, by spray – drying, solids with a cobalt loading of 15
wt.% onto a silica and alumina matrix.
Concerning the solids synthetized onto a silica matrix, it was noticed that all of them present very
good textural properties for final application in the FT process. Nevertheless, the existence of cobalt
silicates was observed, due to high cobalt dispersion that lead to a strong interaction between the
inorganic precursor (TEOS) and cobalt. Therefore in order to reduce this interaction the pH of the
atomization solution was changed.
Only at pH = 1 better results were achieved, as there was a reduction of Co3O4 →Co°. However, it
was noticed the formation of cobalt silicates. Nevertheless, a more detailed study should be done, to
really evaluate which parameter could change the interaction between silica and cobalt. Also, it should
be understood the presence of metallic cobalt in samples that were not reduce. Depending on the
catalytic tests results, it should be understood where the cobalt nanoparticles are located on the support
and it could be tried the loading of a promoter metal in order to see if the cobalt reducibility was improved.
Moreover, concerning the solids synthetized onto an alumina matrix, it was noticed that the textural
properties are not satisfactory for the application in FT process. In solids produced with aluminum
chloride, as inorganic precursor, it was notice the existence of cobalt aluminates. Another aluminum
precursor, aluminum nitrate, was tested: the cobalt reducibility was improved since there was a reduction
of Co3O4 →Co° but, there were still cobalt aluminates. Though, concerning alumina matrixes a more
detailed study should be performed in order to improve the textural properties and also to completely
understand several differences that were reported in the results chapter.
This page was intentionally left blank
65
6. References
[1] BP Energy Outlook 2035, January 2014;
[2] B. Anantharaman, D. Chatterjee, S. Ariyapadi, R. Gualy, Hydrocarbon Processing (2012),
pp.47-53;
[3] C. Boissière, D. Grosso, A. Chaumonnot, L. Nicole, C. Sanchez, Advanced Materials, Vol.23
(2011), pp.599-623;
[4] http://www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/ftsynthesis,
visited on 13/03/2015;
[5] A. Khodakov, W. Chu, P. Fongarland, Chemical Reviews, Vol.107 (2007), pp. 1692-1744;
[6] S. Chambrey, P. Fongarland, H. Karaca, S. Piché, A. Griboval-Constant, D. Schweich, F. Luck,
S. Savin, A. Khodakov, Catalysis Today, Vol. 171 (2011), pp. 201-206;
[7] B. Jager, R. Kelfkens, A. Steynberg, Studies in Surface Science and Catalysis, Vol. 81 (1994),
pp. 419–425;
[8] E. Iglesia, S. Reyes, R. Madon, and S. Soled, Advances in Catalysis, Vol. 39 (1993), pp. 221-
302;
[9] M. Dry, Journal of Chemical Technology and Biotechnology, Vol.77 (2001), pp. 43-50;
[10] https://web.anl.gov/PCS/acsfuel/preprint%2520archive/Files/45_3_WASHINGTON%2520DC_
08-00_0592.pdf+&cd=1&hl=fr&ct=clnk&gl=fr, visited on 20/03/2015;
[11] J. Casci, C. Lok, M. Shannon, Catalysis Today, Vol.145 (2009), pp. 38-44;
[12] M. Sadeqzadeh, S. Chambret, S. Piché, P. Fongarland, F. Luck, D. Curulla-Ferré, D. Schweid,
J. Bousquet, A. Khodakov, Catalysis Today, Vol. 251 (2013), pp. 52-59;
[13] J. Jung, S. Kim, D. Moon, Catalysis Today, Vol. 185 (2012), pp. 168-174;
[14] E. Iglesia, S. Soled, R. Fiato, G. Via, Journal of Catalysis, Vol. 143 (1993), pp. 345;
[15] K. Coulter, A. Sault, Journal of Catalysis, Vol. 154 (1995), pp. 56-64;
[16] A. Lapidus, A. Krylova, J. Rathousky, A. Zuka, M. Jancalkova, Applied Catalysis A: General,
Vol. 80 (1992), pp. 1-11;
[17] D. Petru, L. Kepinski, Catalysis Letters, Vol. 73 (2001), pp. 41-46;
[18] G. Bezemer, J. Bitter, H. Kuipers, H. Oosterbeek, J. Holewijn, X. Xu, F. Kapreijn, A. Dillen and
K. Jong, J. Am. Chem. Soc., vol. 128 (2006), pp. 3956-3964;
[19] N. Tsakoumis, M. Rønning, Ø. Borg, E. Rytter, A. Holmen, Catalysis Today, Vol. 154 (2010),
pp. 162-182;
[20] F. Colbeau-Justin, Design de nouveaux catalyseurs par incorporation d’hétéropolyanion dans
une matrice mésostructurée, 2012 (pHD thesis);
[21] A. Chaumonnot, F. Tihay, A. Coupé, S. Pega, C. Boissière, D. Grosso, C. Sanchez, Oil and
Gas Science and Technology – Rev. IFP, Vol. 64 (2009), No.6, pp. 681-696;
[22] D. Zhao, Y. Wan, W. Zhou, Ordered Mesoporous Materials, Wiley-VCH, 2013;
[23] N. Baccile, D. Grosso, C. Sanchez, Journal of Materials Chemistry, Vol. 13 (2003), pp.3011-
3016;
[24] F. Iskandar, Advanced Podwer Technology, Vol.20 (2009), pp. 283-292;
66
[25] S. Pega, C. Boissière, D. Grosso, T. Azaïs, A. Chaumonnot, C. Sanchez, Angew. Chem. Int.
Ed., Vol.48 (2009), pp. 2784-2787;
[26] F. Colbeau-Justin, C. Boissière, A. Chaumonnot, A. Bonduelle, C. Sanchez, Advanced
Functional Materials, Vol.24 (2014), pp. 233-239;
[27] http://www.buchi.com/en/products/spray-drying-and-encapsulation/solution-spray-drying-large-
10-%E2%80%93-60-%CE%BCm, visited on 18/05/2015;
[28] S. Kleinhans, C. Arpagaus, G. Schönenberger, The Laboratory Assistant, Büchi labortechnik,
2007;
[29] J. Lynch, Physico-Chemical Analysis of Industrial Catalysts - A Pratical Guide to
Characterisation, Editions Technip, 2003;
[30] A. Paredes-Nunez, D. Lorito, N. Guilhaume, C. Mirodatos, Y. Schuurman, F. Meunier, Catalysis
Today, Vol. 246 (2014), pp.178-183;
[31] J. Figueiredo, F. Ribeiro, Catálise Heterogénea, 2 ed., Lisboa: Fundação Calouste Gulbenkian,
2007;
[32] P. Bruinsma, A. Kim, J. Liu, S. Baskaran, Chem. Mater., Vol.9 (1997), pp. 2507;
[33] C. Brinker, Y. Lu, H. Sellinger, H. Fan, Adv. Mater., Vol. 11 (1999), pp. 579;
[34] C. Sassoye, C. Laberty, H. Khanh, S. Cassaignon, C. Boissière, M. Antonietti, C. Sanchez,
Advanced Functional Materials, Vol. 19 (2009), pp. 1922-1929;
[35] Y. Lu, H. Fan, A. Stump, T. Ward, T. Rieker, C. Brinker, Nature, Vol. 398 (1999), pp. 223-226;
[36] V. Dufaud, F. Lefebvre, G. Niccolai, M. Aounine, J. Mater. Chem., Vol.19 (2009), pp. 1142;
[37] L. Catita, Search for new innovating catalyst for the Fischer-Tropsch synthesis, 2014 (master
thesis);
[38] E. Gorrepati, P. Wongthahan, S. Raha, H. Fogler, Langmuir, Vol. 13 (2010), pp.10467- 10474;
[39] E. Rytter, A. Holmen, Catalysts, Vol. 5 (2015), pp.478-499;
[40] R. Andreson, W. Hall, A. Krieg, B. Seligman, Reduced and Carbonized Cobalt Catalysts, Vol.
71 (1949), pp.183 – 187;
A
Appendix A: t- plot curves
Figure A1 - t – plot for samples JFR014 and JFR016.
0
20
40
60
80
100
120
140
160
180
0 0,5 1 1,5 2 2,5
Ad
sro
be
d V
olu
me
(m
L/g
)
Thickness statistiques (nm)
JFR014
JFR016