A STUDY OF MOLYBDENUM CARBIDE CATALYST FOR FISCHER-TROPSCH SYNTHESIS-MEngSc Process Engineering Thesis (CEIC U

A STUDY OF
MOLYBDENUM CARBIDE CATALYST
FOR
FISCHER-TROPSCH SYNTHESIS
by
Farhan Munir
A thesis submitted for the degree of Master of Engineering Science in Process
Engineering
School of Chemical Engineering and Industrial Chemistry
The University of New South Wales
November, 2001

ACKNOWLEDGMENTS
I would like to express my gratitude to those people who contributed a lot in many ways
during the course of this project.
Firstly, I am particularly indebted to my supervisor, Dr. A. A. Adesina, for his valuable
guidance and encouragement during the course of this project.
I wish to thank Professional officer, Mr. J. Starling for his technical assistance and
helpful advices during experimental work.
To all postgraduate students, I am very grateful for their friendly behaviour during this
study.
To all my family members, in Australia and Pakistan, I am extremely grateful for their
continued support in every sense. Their moral support, encouragement and their most
valuable guidance helped me to complete this report.

TABLE OF CONTENTS
CHAPTER #
1. INTRODUCTION 1
2. FISCHER-TROPSCH SYNTHESIS 3
Background 3
Reactions 4
Thermodynamics 7
Kinetics and Mechanisms 11
3. FISCHER-TROPSCH CATALYSTS 22
Active metals for FTS 22
Catalyst supports 23
Catalyst preparation and characterisation 25
4. Mo2C CATALYST 33
High surface transition metal carbides 33
Thermodynamic consideration in preparation of carbides 34
Preparative methods for carbides 34
Molybdenum carbide catalyst 36
5. OBJECTIVES 38
6. EXPERIMENTAL 39
Materials 39
6.1.1Chemicals 39
6.1.2Gases. 40
6.2Catalyst preparation 40
6.2.1Preparation of MoS2 catalyst by PFHS method 41
6.2.2Carburisation of MoS2 to Mo2C 43
6.3Experimental apparatus 46
6.3.1For MoS2 preparation 46
6.3.2For Mo2C preparation 47
6.4Catalyst characterisation 48
6.4.1Total surface area 48
7. RESULTS AND CONCLUSIONS 51

1
Chapter 1
INTRODUCTION
The realisation of the fact that current petroleum and natural gas reserves are limited
and will not be available to supply all the energy requirements are the vital motivation
behind the researches on the hydrogenation of carbon monoxide to produce
hydrocarbons. Coal still comprises the majority of world fossil fuels resources compare
to petroleum. Coal gasification to CO and H2 followed by a CO hydrogenation step is
the one possible route to produce synthetic natural gas (SNG) via methanation reaction
or to manufacture longer chain hydrocarbons via Fischer-Tropsch synthesis (FTS)
reaction. It can be considered as an alternative to crude oil for the production of both
liquid fuels (gasoline and diesel) and chemicals like alkenes.
Fischer-Tropsch synthesis is a catalytic process and can occur on almost any group VIII
transition metals [20]. However, the product distribution differs greatly from one to
another. It is generally conceded that Fe, Co and Ru yield high molecular weight
compounds while Ni favours an almost exclusive production of CH4 [22]. Some of the
metals other than group VIII can also be used as FT catalysts, for instance Mo and W.
[5]. It is also believed that Mo is sulphur resistant and Molybdenum carbide (Mo2C) is
active for FTS [3] and has high olefin selectivity with the promotion of potassium [23].
According to Anderson [3], metallic Mo, Mo2C and MoN have the largest tendency to
produce higher hydrocarbons As mentioned earlier that Mo is sulphur resistant and that

2
is reason Mo is very useful component for catalysts, which needed to operate with
sulphur-containing or CO- rich feed.
Literature evidence shows that the Mo2C active for FTS and has high olefin selectivity.
In this project, an attempt was made to study the preparation method of Mo2C. Initially,
silica supported MoS2 was prepared from Precipitation from homogenous solution.
After getting silica supported MoS2 was than carburise to get he Mo2C with different
carburising conditions and surface area measurements were taken.

3
Chapter 2
FISCHER TROPSCH SYNTHESIS
2.1 Background
Fischer-Tropsch synthesis (FTS) look back to a history of about seventy years.
From early experiments of inventions in 1925 up to a 600,000 t per annum industrial
capacity in 1945[1], the main development took place in Germany, particularly in Franz
Fischer’s laboratories at Kaiser Wilhelm Institute for Coal Research (presently Max
Plank Institute) at Mulhein (Ruhr) in collaboration with Ruhrchemie Company for
commercialisation of FT processes [2].
The process is first commercialised in 1938 in which hydrogenation of carbon
monoxide was carried out at 400 – 450 o
C and about 7 – 30 bar pressure over alkalised
iron turning catalyst. Earlier catalysts had consisted of cobalt and of nickel or
manganese oxides. Large-scale plants in Germany produced about 800 000 t per anum
of liquid fuels during World War II, using fixed bed reactors. After World War II, large
scale use of the Fischer-Tropsch synthesis was cantered in South Africa where, since
1955, Sasol has operated first fixed bed reactor plant with a capacity of 250 000 t per
annum and later, two fluid bed reactors plants with over 2 500 000 t per annum
capacity. Since 1993, shell Oil Company has operated a modified Fischer-Tropsch plant

4
in Malaysia, which uses syngas from natural gas to produce high-molecular weight
alkanes, which are then hydro cracked to diesel fuel.
The actual interest in FTS has grown up in consequence of environmental demands,
technological developments and changes in fossil energy reserves. Present areas with
high potential for early implementation of FT synthesis are the European North Sea, the
US State Alaska and countries around Arabian Gulf, particularly with its large natural
gas and its shrinking petroleum resources. The commercial FT synthesis on the basis of
low price coal in South Africa has now been directed towards more valuable olefins
instead of gasoline since the country has meanwhile access to the world oil market. [2]
2.2 Reactions involved in FTS
Fischer-Tropsch synthesis may be defined as the hydrogenation of carbon oxides to
produce longer-chain hydrocarbon fuels and chemicals. Generally it is regard as a
hydrogenation of CO and CO2 to produce a variety of hydrocarbons and / or alcohols
[3] and in most cases FTS refers to the hydrogenation of CO resulting in straight
paraffins, isomers, olefins, and certain oxygenated product like CO2 and water. This
reaction is normally carried out at about 453 K to 673 K and pressure is about 1 to 40
atmospheres over group metals.

5
The reaction involved in the FT synthesis can be schematically represented in the form
of general equations as [2.4]:
(2n+1) H2 + nCO = CnH2n+2 + nH2O (1)
2nH2 + nCO = CnH2n + nH2O (2.2)
2nH2 + nCO = CnH2n+1OH + (n-1)H2O (2.3)
Or
(n+1) H2 + 2nCO = CnH2n+2 + CO2 (2.4)
nH2 + 2nCO = CnH2n + nCO2 (2.5)
(n+1) H2 + (2n-1) CO = CnH2n+1OH + (n-1) CO2 (2.6)
The first three equation represents the case where water is produce as a predominant
oxygenated product while in the last three equations showing the case where CO2 .
These general equations can be used to represent various hydrocarbon reactions. In case
where n = 1, equation (1) will represents methanation reactions and if n > 1 results in
the production of Fischer-Tropsch hydrocarbons.

6
In addition to the above-mentioned reactions, there are also possibilities of some side
reactions depending on the reaction condition employed. These reactions includes:
Water gas shift reaction
CO + H2O = CO2 + H2 (2.7)
Boudouard reaction
2CO = C + CO2 (2.8)
Coke deposition
H2 + CO = C + H2O (2.9)
Carbide formation
XM + C = MxC (2.10)
Depending on the process conditions and the catalysts used, a different product
distribution can be obtained, such as low or high boiling compounds, and saturated or
unsaturated hydrocarbons. Thus, the kinetics of FT synthesis is complicated and
depends on the catalyst and operating conditions.

7
2.3 Thermodynamics of FTS
The FTS is an exothermic reaction system. Information on the heats of reactions is of
great practical importance as removal of the heat is one of the most difficult engineering
aspects in FTS. Excessive catalyst temperature usually leads to less desirable products,
carbon deposition, and catalyst disintegration. Figure 2.1 shows the behaviour of the
heats of reactions per carbon atom, ∆H/n, for reactions (2.1) and
(2.4) With temperatures. It is seen from the figure that:
1. For n-paraffins and 1- olefins, ∆H/n vary only slightly with temperature,
especially at very high temperature,
2. For reaction (2.1), which produces paraffins, ∆H/n increases (become less
negative) with increase in carbon number, that is, reaction heat decreases with
increase in product chain length,
3. For reaction (2.4), which produces olefins, ∆H/n decreases with increase in
carbon number, an opposite trend to that seen for paraffins
4. Reactions producing CO 2 give off heat than that producing H2O by the amount
of ∆H of water gas shift reaction (9 Kcal mol-1
).

8
The relationship between the standard free Gibbs energy, ∆Fo
of a chemical reactions
and its equilibrium constant is defined by the following equations:
- ∆Fo
= RT ln Keq (2.11)
According to Anderson [4], the free energy characteristics of the FTS can be described
as follows:

9
1. Reactions producing CO2 [(2.4) and (2.5)] have more negative Gibbs free
energy change values ∆Fo
(larger equilibrium constant) the corresponding
reactions producing water [(2.1) and (2.2)] as reactions (2.4) and (2.5) are
combination of reactions (2.1) and (2.2) with water gas shift reaction (2.7)
respectively, and the Gibbs free energy change of water gas shift (WGS)
reaction is negative.
2. The equilibrium constant of WGS reaction is large (>20) at FTS temperature
(453-623 K). Hence, if equilibrium were sustained for the WGS reaction, almost
all of the water produced by primary synthesis reaction should be converted to
CO2. But, in fact, nearly all of the oxygen appears as water in the synthesis on
Co and Ni and more than 25% in the synthesis on Fe. Thus, the kinetics rather
thermodynamics probably controls product distribution, since WGS reaction has
a relatively low rate of reaction within this temperature range.
3. Under usual FTS temperature, the standard free energy changes per carbon
atom, ∆Fo
/n, for reaction producing CH4 and reactions producing carbon are
more negative the for corresponding reactions producing higher hydrocarbons.
Therefore, the production of higher hydrocarbons (rather than CH4 and carbon)
must depend on the nature of the catalyst.
4. The equilibrium conversion of synthesis increases with increasing pressure. For
a given conversion, the higher the operating pressure the higher the synthesis
temperature needed.
5. With increase in reaction temperature, ∆Fo
for CH4 and other paraffins increases
(less negative) quicker than that for carbon formation, i.e., carbon formation will
be favoured under high temperature.

10
6. In this range of temperature and pressure, sizable yields of all hydrocarbons with
the exception of acetylene are thermodynamically possible. The equilibrium
constants of cyclic and aromatic hydrocarbons, which are formed for mono-
olefins of the same carbon number.
7. The ∆Fo
of reactions resulting in hydrocarbons containing the same number of
the carbon atoms becomes more negative in the order of di-olefins, mono-
olefins, and paraffins.
8. Hydrogenation of carbon monoxide to hydrocarbons or alcohols, except for
acetylene and methanol is thermodynamically possible under most synthesis
conditions
9. Hydrogenation of olefins and dehydration of alcohols are thermodynamically
possible under usual synthesis conditions.
10. Reaction of any amount of ethylene or ethanol with H2 - CO mixtures is
thermodynamically possible at all FTS temperatures. Ethylene has a greater
thermodynamic tendency to incorporate than higher olefins.
11. Reactions of water plus graphite to give hydrocarbons have positive values of
∆Fo
and heat change. These reactions, which approximate the desired overall
equation for the production of synthetic fuels from coal, are thermodynamically
impossible under all practical conditions.

11
2.4 Kinetics and Mechanisms of Fischer-Tropsch Synthesis
2.4.1 Mechanism of the Fischer-Tropsch Synthesis
Fischer-Tropsch synthesis converts two of the simplest compounds, H2 and CO, into a
complex array of products, containing predominantly of alkenes and alkanes but also a
variety of minor compounds including a range of oxygenate compounds. The
motivation behind the study of mechanism is to understand how catalyst composition
and reaction conditions govern product distribution in CO hydrogenation. Currently, the
mechanisms available are more and due to the complexity of FTS, no single mechanism
are capable of explaining all the various observations made during the synthesis
reaction. Generally, the synthesis mechanism is divided into the following steps [5]:
1. Adsorption of reactants,
2. Chain initiation,
3. Chain growth,
4. Chain termination,
5. Desorption of products, and
6. Reabsorption and further reaction.
More specifically, the synthesis gas mechanism starts with the chemisorption of CO and
H2 on the catalyst surface with the formation of a primary complex that lead to
weakening of a C-O bond and formation of a C-H bond followed by chain growth by

12
reaction of the primary complex with synthesis gas or with the products already formed
and adsorbed with chain termination for example by hydrogenation or by reaction of the
growing chain with synthesis products followed by desorption from the surface. The
pertinent feature for FTS is given in the figure 2.2.
Most of the mechanistic studies focus on steps 2 to 4. Various mechanisms reported can
be grouped into four types. [5,13,14]
1. Surface Carbide Mechanism,
2. Enolic mechanism,
3. CO insertion Mechanism, and
4. Alkoxy Mechanism.

13
2.4.1.1 Surface carbide mechanism
This mechanism is related CO hydrogenation and the authors hypothesized that the CO
reacts with metal of the catalyst to form bulk carbide, which subsequently undergoes
hydrogenation to form methylene groups [15]. The methylene species were assumed to
polymerise to form hydrocarbon chains that then desorb from the surface as saturated
and/or unsaturated hydrocarbons. Figure 2.3 shows the schematic of carbide
mechanism.

14
Schematic diagrams of the above mentioned mechanism is given in Appendix 2.

15
2.4.1.2 Enolic Mechanism
This mechanism is based on the work of Anderson [3] and Storch [16]. In this
mechanism, it is believed that enol, formed from simultaneous chemisorption of CO and
H2 on the catalyst surface, is the intermediate in the synthesis. The reaction between
two primary enolic complexes leads to the formation of C-C bond and water with the
concurrent release of a carbon atom from the surface by hydrogenation. Through this
stepwise condensation and hydrogenation, chain growth continues by the addition of
one carbon at a time.

16
2.4.1.3 CO insertion mechanism
This mechanism involves the insertion of a carbonyl group into a meral alkyl bond.
The resulting alkyl intermediate can then undergo a variety of reactions to form
acids, aldehydes, alcohols, and hydrocarbons. In addition branched hydrocarbons
can also be formed [17,18].

17
2.4.1.4 Alkoxy mechanism
This mechanism is based on the idea that CO chemisorbs on metal surface and reacts
with hydrogen to yield oxygen rather than a carbon coordinated species. Thus the bond
strength of metal oxides should correspond directly to specific activity of machination
but should be related inversely to methane selectivity.

18
More recent studies on the mechanism are done by Davis, B.H [43]. He studies the
mechanism for generation of hydrocarbon and oxygenated products from synthesis gas
using FTS, as describe earlier. The data indicate from the study shows that there are
similarities between iron and cobalt catalytic synthesis mechanisms, the details differ. It
also appears from his study that the surface carbide mechanism is a better choice. The
detailed schematic diagrams of these mechanisms are given in Appendix2.

19
2.4.2 KINECTICS OF FTS
It is very obvious from the available literature that the kinetic expressions were
influenced by the type of catalyst and the operating conditions employed by the various
investigators [5]. Nevertheless, it is generally conceded that FT reactions are about first
order in H2 partial pressure and zero order in CO as long as the H2/CO ratio is between
1 and 3 [6,7,8].
Vannice [9] has summarized almost all known rate expressions for the FTS reported
before 1974.Recently Wojciechowski [10] and Sarup and Wojciechowski [11] presented
a rather comprehensive kinetic study of FTS over a Co catalyst. For Fe catalysts,
Anderson [3] found that the first order equation
r = k PH2 (2.12)
fit the data well up to synthesis gas conversion of 60%. Considering that inhibition by
water may occur at conversion larger than 60%, Anderson [3] proposed a rate equation
including water inhibition. Thus
r = ko PCO PH2 (2.13)
PCO + a PH2O

20
Which can be derived from the enolic mechanism by assuming that the hydrogenation
of chemisorbed CO is the rate-determining step [12]. To handle the observed H2
dependence, Huff and Satterfield [13] derived an alternate rate form of
r = ko PCO P2
H2 (2.14)
PCO + a PH2O
Using two different mechanism, the carbide mechanism taking the hydrogenation of
surface carbon as the rate determining step, and an enol / carbide mechanism with
hydrogenation of surface enol as the rate limiting step. Ledakowicz [14] develop the
following relationship in order to accommodate situations with high water gas activity
and /or low H2/CO ratios as
r = ko PCO PH2 (2.15)
PCO + PCO2
Based on the enolic mechanism with hydrogenation of surface CO as the rate
determining step assuming that CO and CO2 are only gaseous species which adsorb
significantly on the catalyst surface. They also presented the following rate equation

21
r = ko PCO PH2 (2.16)
PCO + a PH2O + cPCO2
To account for inhibition by both water and CO2. This generalised rate expression may
be used for catalysts with low WGS activity, where water concentration are high, as
well as for catalysts with high shift activity which shows inhibition by CO2.The
activation energy reported by the authors for Fe Catalysts is about 80 to 103 kJ mol-1
,
regardless of catalyst type.

22
Chapter 3
FISCHER-TROPSCH CATALYSTS
3.1 Active Metals for FTS
The discovery of the gasoline synthesis by Fischer and Tropsch is based on iron and
cobalt as catalysts, both the metals remaining until today the only ones for industrial
applications [21]. Essentially, Fischer-Tropsch synthesis can occur on almost any group
VIII transition metals [20]. However, the product distribution differs greatly from one to
another. It is generally conceded that Fe, Co and Ru yield high molecular weight
compounds while Ni favours an almost exclusive production of CH4 [22]. Some of the
metals other than group VIII can also be used as FT catalysts, for instance Mo and W.
[5]. It is also believed that Mo is sulphur resistant and Molybdenum carbide (Mo2C) is
active for FTS [3] and has high olefin selectivity with the promotion of potassium [23].
According to Anderson [3], metallic Mo, Mo2C and MoN have the largest tendency to
produce higher hydrocarbons. As mentioned earlier that Mo is sulphur resistant and that
is reason Mo is very useful component for catalysts, which needed to operate with
sulphur-containing or CO- rich feed.

23
3.2 Catalyst Support
The major drawbacks with bulk metal catalyst are catalyst efficiency and they are
costly. These types of catalysts also shows low thermal stability and surface area losses
occurs during sintering. To avoid these problems the active metals component is very
often supported on high surface area carriers. Chen [5] summarize the major purposes
for the employment of support in a catalysts as:
1. To stabilize the catalyst against agglomeration and coalescing, usually referred
to as a thermal stabiliser,
2. To introduce resistance to poisons or resistance to by-product formation,
3. To decrease the density of the catalyst and also to dilute the costly ingredient by
less costly materials,
4. To prepare the catalyst in such a form that its resistance to breakage and
minimization of pressure drop is accomplished.
Snel [33] also pointed out the influence of support on catalytic behaviour of a
supported catalyst by support basicity effect, support dispersion effect, electronic
modification effect, and strong metal-support interaction effect.
There are various materials, which can be utilized as catalyst support. Table 3.1
shows the general classification.

24
Table 3.1 General classification of Catalyst Support [Chen [5]]
Class Supports
Inert Support SiO2
Catalytically active
supports
Al2O3, SiO2-Al2O3 and
Zeolites etc
Those influence active
component by strong
interaction
TiO2, Nb2O5, V2O5 etc
Structural supports Monoliths
The most widely used supports in FT catalysts are Al2O3 and SiO2 .According to
Ishihara [34] others metal oxides are used as FT catalysts for improved activity and
selectivity.
Silica
Silica is a refractory oxide, which is widely used as a catalyst support after alumina. It
occurs in many polymorphs depending on the temperature and pressure. Silica has
characteristics that make it useful in many cases in which alumina is inapplicable. Silica
is also primarily much more resistant to acid media and as a consequence is more
satisfactory than alumina in this type of environments. However, alkali environments
adversely affect both silica and alumina and neither is suitable for use in basic system.

25
At 1 atm and below 846 K, silica exists in low-quartz form. It may also react with
catalytic compounds and cause the incipient formation of silicates when the temperature
exceeds 723-773 K. [35]. The thermal stability range and the figure of colloidal silica
are mentioned in Appendix 3.
3.3 Catalyst Preparations and Characterisation
3.3.1 Catalyst Preparation
The production of supported catalysts can be divided into two main groups [36]:
1. Application of the active precursor onto a separately produced carrier, and
2. Selective removal of one or more components from solids of an initially small
specific surface area.
The first group of preparation methods has the advantage that a number of properties of
the support can be adapted to the requirements of a catalytic process. Especially when
shaped pellets are utilised, the pore-size distribution and mechanical strength of the
pellets may be adjusted. The active precursor can be applied onto the support by the
following methods:
1. Adsorption
2. Impregnation and drying
3. Precipitation

26
Adsorption of active metals is mostly done from liquids. While impregnation and
subsequent drying is utilised to obtain higher loadings or to apply active precursors that
do not markedly adsorb onto the support. The first two methods are important but are
not relevant to the objectives of current studies.
Precipitation
Precipitation of an active precursor in the presence of suspended support is also utilised
to produce supported catalysts. After completion of the precipitation the solids are
filtered, dried, processed to pellets and thermally treated. Since precipitation can be
carried out more rapidly than drying, this procedure has some advantages. However, the
distribution of the active material throughout the support is even worse than with
impregnation and drying.
On addition of a precipitating agent to a suspension of the support in a solution of the
precursor, the precipitant initially contacts the dissolved precursor outside the pore
system of the support. To get very small precipitated particles of the active components,
nucleation of the precipitate must proceed rapidly. Consequently, small precipitated
particles of the precursor will develop outside the pores of the carrier. Provided the
precipitated particles are attracted by support, they will be attached to the carrier.
However, the diffusivity of colloidal particles rapidly drops with the particle size. As a
result, the porosity of the carrier must be kept small to limit transport problems as much
as possible. Transport problems are completely prevented by coprecipitation with the
support, if the active precursor and the support nucleate simultaneously [36].

27
Using a compound that slowly reacts to a precipitant can separate addition and reaction
of a precipitating agent. The slowly reacting compound can be added and a
homogeneous solution can be established before the precipitating agent has attained
marked concentration. Though nucleation of the active precursor can occur as rapidly as
require to generate very small particles, homogenizing the suspension can be completed
before the precursor starts to precipitate. Working at different temperatures can extend
the time available to homogenize the suspension considerably. The solution can be
mixed and homogenized at a temperature where no marked formation of the precipitant
takes place after which the temperature is raised and the precipitant develops rapidly.
Using this method the active material to be applied on to the support has to be present
within the pores of the carrier before formation of the precipitant set in. Consequently
the volume of the dissolved active precursor together with the inchoate precipitant can
be at most equal to the pore volume of the support. When a highly loaded support is to
be produced, the support must be impregnated by a concentrated solution. With
concentrated solutions the precipitated particles of the active precursor are likely to
cluster, subsequent thermal treatment causes the cluster to sinter, which leads to
relatively large active particles [36].

29
To avoid clustering of active particles especially at high loadings of the support would
be favourable. The concentration of a saturated solution is given as a function of the
temperature (solubility curve) is given in figure 3.1 and 3.2. The figure on the top is
showing the difference in free energy between a solution with a solid particle and
homogeneous solution of equal overall composition is represented as a function of
particle size. When the concentration of the solution is below that of the solubility
curve, the free energy grows on formation of a solid particle. Since the free energy of
larger particles increases linearly with the volume of the particles, the increase is
proportional to the third power of the particle size. At concentrations above that of the
solubility curve, the free energy of a solid particle and a saturated solution is lower that
that of the homogeneous solution. With large particles the decrease in free energy is
proportional to the third power of the particle size. The decrease in free energy per unit
volume grows with the difference between the concentration of the homogeneous
solution and that of the solubility curve the difference in the free energy is zero.
Considering the above figure in which case finely divided carrier suspended in a
solution of the active precursor. It is assumed that the ions of the active species
chemically interact with the surface of the carrier. We also assumed in this case that the
solubility curve has been shifted to higher concentrations. The top of the figure shows
the difference in free energy for the same concentration.
Simple addition of a precipitating agent to a suspension of the carrier in a solution of the
precursor does not lead to the homogeneous increase in concentration required to get

30
deposition precipitation. When the solution of the precipitant is poured into the
suspension of the support, the concentration can locally raise oboe the super solubility
of the bulk compound.
Local concentration differences in the suspension of the support can be minimized by
the following two methods. The first procedure separated addition and reaction of a
precipitating agent. As an example we can consider the increase in hydroxyl ion
concentration by hydrolysis of urea. Since the hydrolysis occurs at a marked rate only
about 60oC, the solution can be homogenized at a lower temperature and subsequently
brought t at a temperature where the reaction rapidly proceeds. In the second method, a
solution of the precipitant is injected into the suspension of the support below the level
of the liquid.
Precipitation according to the first method is known as precipitation from a
homogeneous solution used in gravimetric analysis to prepare well-crystallized,
relatively large crystallites that are easy to filter. Deposition precipitation on the other
hand, can provide extremely small particles. Besides the well describe utilisation of urea
a number of other methods has been developed, which are summarize in Figure 3.3.
Raising the pH- level of a solution of the active component can precipitate many active
precursors.

31
Cyanate is utilized when precipitation has to be done at lower temperatures than about
70 o C, the temperature at which urea hydrolyses rapidly. To avoid formation of soluble
amine complexes, nitrite can be favourably used.

32
3.3.2 Catalyst Characterisation
Generally, the two principal objectives in the application of physical techniques in the
study of catalysis are:
1. The characterisation of the catalysts; and
2. The acquisition of the information relevant to understand the catalytic
phenomenon.
The first objective consists of establishing the identity for the catalyst, to indicate its
structure, morphology and other physiochemical data. The second objective concerns
the catalytic process.
For physical properties of catalysts, physisorption of gases and mercury porosimetry are
the most common techniques to determine total surface areas and pore structure [45].
Selective chemisorption is a classic method for the measurement of the number of
surface metal atoms, metal surface areas and average particle size [46]. There are also
numerous methods employed for the characterisation of catalysts, from X-rays to
Infrared spectroscopy, to transmission electron microscopy. All these techniques are
employed or can be employed for catalyst characterisation, depending properties we
intend to find out. Schematic representation of physical techniques principal along with
the comparative physical characteristics for many physical techniques are given in the
Appendix 3 in the form of tables and charts.

33
Chapter 4
MOLYBDENUM CARBIDE (Mo2C) CATALYST
4.1 High Surface Transition Metal Carbides
The alloying of main group elements such as C, N or O, with early transition metals
produces a class of materials known as carbides, or oxycarbide [24-26]. The materials
have high melting points (>3300K), hardness (>2000 kg mm-2) and strength (> 3 * 105
Mpa).
The monometallic carbides often adopt simple crystal structure with the metal atoms
arrange in cubic close-packed (ccp), hexagonal close-packed (hcp) or simple hexagonal
(hex) arrays. The non-metallic elements C, N, and O, occupy interstitial spaces between
metal atoms, and for this reason the materials are also known as interstitial alloys.
The crystal structure adopted by the binary carbides is similar to those found in the
noble metals. The resemblance is not coincidental, and has been explained using Engel-
Brewer valence bond theory. The crystal structure and composition of carbides and
nitrides are given in Figure 1, Appendix 4.

34
The carbides have been found to be exceptional hydrogenation catalysts [27]. They have
activity close to or surpassing those of group VIII noble metals.
4.2 Thermodynamic considerations in the Preparation of
Carbides
Strategies for preparing are numerous and involve widely differing starting metallic
compounds, as well as different carbon sources. Carbide formation from elemental
carbon and transition metals show a number of trends (Table 1 – 4 Appendix). First, the
free energy of formation is strongly negative for the early transition metals, and
becomes less favourable in going to the group 8 metals. As temperature is raised,
carbide stability decreases slightly among the early transition metals, but increases
markedly for the late metals. In general, trends in free energy are mirrored by values of
the heats of reaction but, towards the right in the periodic table, entropic effects are
important in stabilizing the compounds.
4.3 Preparative Methods for Carbides
Oyama [29] surveyed many types of preparation methods for carbides and nitrides.
Because of the interest in transition metal carbides, this section of the thesis discusses
some important preparation methods for carbides.

35
A. Direct Reaction of Metals and Non-metals
M + C MC
This method of preparation is carried out by contacting metallic powders and solid
carbons, sometimes in the presence of gaseous hydrocarbons, at 1500-2300 K.
Thermodynamics indicate that carbide formation from the elements is favourable at
lower temperatures, but high temperatures are used to counter solid-state diffusion
limitations [28].
B.Reaction of Metal Oxides in the Presence of Solid Carbons
MO + 2C MC + CO
This transformation is carried out by intimately mixing metal oxides, powders with
carbon, again as with pure metals, at temperature 1500 – 2300K.
C. Reactions of Metals or Metals Oxides with Gas- Phase Reagents
M + 2CO MC + CO2

36
MO + HxCy MC + H2O + CO
Carburisation with gaseous carbon sources such as methane, higher hydrocarbons, and
carbon monoxide was initially carried out mainly with metal wires. For catalytic
applications metals have been carburised with methane and ethane [30], propane [31],
and carbon monoxide [32].
4.4 Molybdenum Carbide (Mo2C) Catalyst
Molybdenum carbide, Mo2C, has been shown to have excellent catalytic activity for
hydrogen transfer reactions and has been suggested as a possible substitute for noble
metals [26,37].
Saito and Anderson [38] compared the performance of unsupported molybdenum metal,
carbide, nitride, oxide, and sulphide for CO methanation, and found that the Mo2C had
the highest activity.
Considerable attention has been focused in the recent years on the chemical and
physical properties of transition metal carbides and nitrides. The utility of these
materials ranges from wear-resistant coatings, to superconductors, to heterogeneous
catalysts [27]. The carbides of transition metals are catalytically active for number of
reactions including hydrogenation [39].

37
Miyao and co-workers [40] studied the preparation and characterisation of alumina
supported Mo2 C. Mo2C was prepared in this study by nitridation of 12.5 wt%
MoO3/Al2O3 in a flow of NH3 at 700Oc, followed by carburisation in a flow of 20 %
CH4/H2 also at 700Oc for 3 hours. The sample was compared to an unsupported
materials prepared from MoO3 in the same manner. The results suggested that Mo2C
was formed on the alumina supported by the carburisation treatments at 700oC, in the
same manner as the unsupported reference sample. Prenitridation before carburisation
resulted in the formation of carbide with a larger surface area and less free carbon,
compared to the carbide formed by direct carburisation.

38
Chapter 5
OBJECTIVES
The main objectives of this study were:
1. To study the preparation methods for Mo2C catalysts
2. To study the effects of carburisation temperature on the surface area of the
Mo2C Catalyst.
3. To investigate the effect of C/H ratio of carburising gas on the total surface area
of the catalyst.
4. To study the effect of flow rate of carburising gas on the surface area of the
Mo2C catalyst.

39
Chapter 6
EXPERIMENTAL
6.1 Materials
6.1.1Chemicals
All chemicals utilize for the catalyst preparation in this study are listed in Table 6.1.
Deionized water was used for all solution preparations.
Table 6.1 Chemicals Employed for the Catalyst Preparation
Chemicals Formula Grade Morphology Manufacturer Mol. Wt
Molybdenum
Trioxide
MoO3 AR AJAX 143.94
Silica SiO2 Pure Precipitated AJAX 60.09
Thioacetamide CH3CSNH2 AR --- AJAX 75.13
Urea NH2CONH2 AR --- AJAX
Nitric Acid HNO3 AR --- AJAX 63.01

40
6.1.2 Gases
All gases used in this study were supplied by ‘ Reaction Engineering and Technology
Group’, School of Chemical Engineering and Industrial Chemistry, UNSW Sydney
Australia. The source of these gases to reaction group is BOC Gases. All gases utilize
during the course of this study are listed in Table 6.2.
Table 6.2 Gases employed in this study with specification and Applications
Gas Specification Application
H2 Ultra High (99.999%) Reactant
N2 Inert
C3H8 Reactant
6.2 Catalyst Preparation
The main objective of this study was to prepare Mo2C catalyst. The preparation of
Mo2C was achieved in two steps. Firstly, the preparation of silica supported MoS2
catalyst via precipitation from homogeneous solution (PFHS). The next step is to
carburise MoS2 catalyst to get finally Silica supported Molybdenum Carbide Mo2C

41
catalysts. These two steps for catalyst preparation are discussed in detail in the coming
sections of this thesis.
6.2.1 Preparation of MoS2 catalyst by PFHS Method.
A weighed sample of silica was suspended in an aqueous solution of 100 ml in a 250 ml
conical flask containing 10 ml MoO3 (0.1 M) solution, 1 g urea, 1 ml of 0.75 M
concentrated nitric acid and 30 ml of thioacetamide (0.133 M). The flask content was
kept at 90o
C in a water-bath for 3 h with intermittent shaking. The precipitated obtained
was then filtered, washed and dried at 120o
C.In order to prepare 2 % of Mo/SiO2
catalyst, 4.7 g of silica were used in the suspension. The recipe for the preparation of the
catalyst is given in Table 6.3
Table 6.3
Chemicals Formula Concentration Amount
Molybdenum
Trioxide
MoO3 0.1 M 10 ml
Silica SiO2 -- 4.7 g
Thioacetamide CH3CSNH2 0.133 M 30 ml
Urea NH2CONH2 -- 1 g
Nitric Acid HNO3 0.75 M 1 ml

42
The above recipe is employed in order to prepare Silica supported MoS2 catalyst of 2 %
of Mo/SiO2 catalysts.
Precipitation
The suspensions containing all the above-mentioned ingredients were heated at 90oC in
a water bath. For shaking of the suspension, the shaker was equipped in a water bath.
The flask content was kept for 3 h wit intermittent shaking, which is provided by the
shaker. The next step was the filtration of precipitates.
Filtration
The precipitates were filtered using vacuum filtration unit. The contents of the flask
were poured into the funnel of the filtration unit and vacuum was applied. The
precipitates obtained from filtration were then dried. Washing is also done in this
section.
Drying
The washed precipitates obtained after the filtration and washing was then dried at
120o
C for about 14 h.

43
6.2.2 Carburisation of MoS2 (Prepared via PFHS Method) to Mo2C
Catalyst.
This is the second and most important part of Mo2C Catalyst preparation. The catalyst
design for carburisation is obtained with the help of statistical method known as
Fractional Factorial Design (FFD). FFD is a statistical method, which enables
experimenters to get necessary information on a multi factor system with minimum
experiments [41].
For carburisation of MoS2, mixture of propane( C3H8) and H2 is utilize in the presence
of N2.
Catalyst Design
The catalyst designs for carburisation of MoS2 are based on FFD. Three factors namely,
C/H ratio, temperature and time at two levels were used for the catalyst design. The
general design of the carburisation step is given in table 6.4

44
Table 6.4 Designs of Carburisation Experiments
Factor Level 1 Level 2
C3H8/H2 Ratio 1 6
Temperature
(o
C)
400 600
Time
(Hours)
1 5
On the basis of the above design, the outline of experiments for carburisation We can
observe from the above table that each factors are at two levels and there are three
factors in total. The total number of experiments was 8 ( 23
= 3 factors and 2 levels)
The flow rates of the carburising gas mixture are calculated by using the calculations
given in Table 6.5. Total gas flow rate of carburising mixture were kept at about 100-
ml/ min. After calculation of C3H8 /H2 Ratio, the amount of inert gas, which was N2
was utilize in the carburising mixture.
Table 6.5: C3H8 /H2 Ratio Calculations
C3H8 /H2 Ratio C3H8 H2

45
1: 1 1/3 1
6: 1 2 1
The outline of the experiments along with the experimental details are given in Table
6.6
Table 6.6 Experimental Details for Carburisation
Catalyst
Sample
C3H8/H2
Ratio
Temperature
(o
C)
Time
(Hours)
C3H8
Ml/min
H2
Ml/min
N2
Ml/min
C1 1 400 1 20 60 20
C2 1 600 1 20 60 20
C3 6 400 1 60 30 10
C4 6 600 1 60 30 10
C5 1 400 5 20 60 20
C6 1 600 5 20 60 20
C7 6 400 5 60 30 10
C8 6 600 5 60 30 10

46
6.3 Experimental Apparatus
6.3.1 Apparatus Employed for MoS2 Preparation
The apparatus employed in this section of catalyst preparation consist of the following
items:
• Conical Flask. 100,250 and 500ml
• Measuring Cylinder 100 ml
• Volumetric Pipette
• Beakers
• Filter Papers ( Whatman’s 90 mm)
• Vacuum Filtration Unit

47
• Dryer
• Sample Bottles
6.3.2 Apparatus Employed for Mo2C Preparation
The schematic diagram of experimental rig employed for the carburisation of MoS2 is
illustrated in Fig. 6.1. The whole experimental rig was placed in a fume cupboard in
order to avoid any effluent gas escapes. The system consists of a reactor, furnace,
temperature controller, flow controllers, and mixing vessel.
Reactor
The reactor is 10 mm ID, 40 cm long quartz cylinder. The reactor is fabricated with
quartz so that it can with stand high temperature for the reaction of carburisation. The
nature of the material also ensures the inert behaviour, which is also feasible to the
carburisation. The bed of catalyst consisted of 1 g of MoS2 catalyst. The catalyst was
carefully supported by glass wool on both sides.
Temperature Controller

48
Temperature controllers are employed in order to ensure the correct temperature inside
the reactor.
Mass Flow Controllers
A Brook Instrument 3- channel mass flow controllers were utilizes to monitor the flow
rate of C3H8, H2, and N2 . In order to achieve accurate flow rate to maintain correct
C3H8/ H2 ratio, these mass flow controllers were calibrated. After calibration of these
mass flow controllers, correct values are calculated from the calibration curves. The
details of the calibration of these mass flow controllers are given in the appendix 6. The
flow was stable throughout the experiment.
Mixing Vessel
The mixing vessel is a steel cylinder about 5 cm ID and 7 cm height. The large volume
of this vessel ensures the good mixing of the coming feed gases
6.4 Catalyst Characterisation
Catalyst characterisation plays a vital role in providing important information related to
physical and chemical properties of catalysts. As catalysis is a surface phenomenon.

49
Catalytic rates and selectivities depend on the available active surface area and their
accessibility in a catalyst, the intrinsic activity of the active sites on the surface and the
process conditions. Hence catalyst characterisation studies provide a basis for the
understanding the interrelationship between the activity and selectivity of a catalyst.
6.4.1 Total Surface Area
The method utilizes to calculate the ‘total surface area’ of the catalyst is known as BET.
Brunauer, Emmett and Teller jointly developed this method and is the most frequently
used for the measurement of total surface area of the catalyst. A schematic of the BET
equipment is show in Fig 6.3 (Appendix 6). A 30% N2 in He was used as measuring gas
and He was employed as flushing gas. The adsorption and desorption of N2 from the
measuring gas were used to determine the total BET surface area of the catalyst [42]. A
mixture of 30% N2 in the He has been suggested to give the best agreement with multi-
point BET methods.
The sample was first dried and degassed at 393 K for 1 hour and then cooled to room
temperature. A flow of the measuring gas was switched to pass the sample at a
temperature of liquid nitrogen (77 K). After the adsorption equilibrium had been
established, the temperature of the sample was raised to the ambient level and the
amount of N2 desorbed was measured. The formula used for the calculation BET total
surface area is as follows:

50
ABET = 4.35 V (273/ T ) ( 1 – X (P/Ps ) ( 760 / Patm )
Ws
Where ABET is the BET surface area. m2
/ g
V is the volume of N 2 adsorbed/desorbed, ml
T is the room temperature, K
X is the mole fraction of N 2 in the measuring gas,
P is sample pressure, mm Hg,
Ps is saturation pressure of N 2, mm Hg,
Patm is the atmospheric pressure, mm Hg,
Ws is the mass of the sample, g.

52
Chapter 7
RESULTS AND CONCLUSIONS
7.1 Results
The results obtained from the BET total surface area measurement are shown in table
7.1.
Table 7.1 BET Total Surface Area for Catalyst Samples
Catalyst
Sample
C3H8/H2 Ratio Temperature(o
C) Time (Hours) ABET
m2
/g
C1 1 400 1 140
C2 1 600 1 134
C3 6 400 1 155.48
C4 6 600 1 288.08
C5 1 400 5 288.27
C6 1 600 5 210.48
C7 6 400 5 167.4
C8 6 600 5 173.18

53
BET Total Surface Area of Mo2C Catalysts
0
50
100
150
200
250
300
350
1 2 3 4 5 6 7 8
Catalyst Samples
BETTotalSurfaceArea,m2/g
Effect of C3H8/H2 Ratio on BET Total Surface Area w.r.t Carburisation Time
100
120
140
160
180
200
220
240
260
280
300
0 1 2 3 4 5 6 7
C3H8/H2 Ratio
BETTotalSurfaceArea,m2/g
Carburisation Time 1 Hour
"Carburisation Time 5 Hour"

54
7.2 CONCLUSIONS
The BET Total surface area was measured for all samples. The Mo2C catalyst was
prepared at different conditions. Three parameters were used for carburisation namely
C3H8/H2 ratio, time and time . The following conclusions were made from the result
obtained. Table 7.1 showing the details of the experiments conducted for carburisation
of MoS2 catalysts to obtained Mo2S. The surface area for Mo2S is found to be 166.15
m2
/g.
Effect of C3H8 /H2 Ratio on BET Total Surface Area:
The C3H8/H2 ratio utilizes for carburisation of the Mo2S catalysts were 1:1 and 6:1.
Catalyst samples C-4 and C-5 have found to be highest BET surface area. We can easily
see that these two catalysts attained the highest surface area regardless of different
conditions for carburisation. The same surface area is achieved when we carburise the
catalyst at 6:1 and 1:1 C3H8/H2 ratio, at 600 and 400 oC but for 1 and 5 hours time were
used respectively.
We can also see in the case of sample C-1, when 1:1 C3H8/H2 ratio were used at 400oC
for 1 hour, the surface area was substantially low.

55
Effect of Temperature on BET Total Surface Area:
There have been no major changes in surface area of the catalyst at two different
temperature levels. Considering the catalyst sample C-1 and C-2 (Table 7.1), we can see
there is s decrease in surface area of the two samples dur to the increase in temperature
by 200oC. In this comparison, all other parameters are the same for the two catalysts in
question. The similar situations appear to be with catalyst sample C-7 and C-8.
Effect of Time on BET Total Surface Area:
On examination of the results obtained from BET measurements for the catalyst
samples, it is quite clear that the time has also an important role on the out comes of the
BET surface area. Considering the catalyst samples, C-7 and C-8 for instance, the
values obtained for the above samples implies that the increasing carburising time with
higher C3H8/H2 ratio has an adverse affect on the surface area of these catalysts samples.

56
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15. Fischer, F., and Tropsch, H., Brennstoffchem, 7, 97 (1926).
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23. Park, K. Y., Seo, W. K., Lee, J. S., “ Selective Synthesis of Light Olefins from
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Press, New York, 1971.
29. Oyama, S. T., “ Handbook of Heterogeneous Catalysis”, Vol.1 Academic Press,
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267-281 (1990).
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59
36. Poncelet, G., Grange, P., and Jacobs, P. A., “ Preparation of Catalysts III,” Elservier
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A Gen 165 (1997) 419-428.
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Science and Technology Press, Chendu, China, 1983.
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44. Davis, B.H., “ Fuel Processing Technology,” 71 (2001) 157-166.
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64
Fischer-Tropsch Synthesis: Current
Mechanisms
[Davis, B.H., 2001]

88
CALIBRATION CALCULATIONS AND GRAPH OF MASS FLOW METERS.
CALIBRATION OF MASS FLOW
METERS
FOR H2 CHANNEL # 1
MAIN READING FLOWRATES (ml/min)
3 26.96
5 43.44
8 68.8

89
FOR N2 CHANNEL # 3
MAIN FLOWMETER
0.26 19.1
0.3 23.1
0.4 32.1
FOR C3H8 CHANNEL # 2
MAIN FLOWMETER
1 34
0.7 21.81
2 74.07
CALIBRATION GRAPH FOR MASS FLOW METERS FOR FOR H2
y = 8.3747x + 1.7347
R
2
= 1
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5 6 7 8 9
MAIN READING
FLOWRATES

90
CALIBRATION GRAPH FOR MASS FLOW METERS FOR FOR N2
y = 92.308x - 4.7718
R2
= 0.9994
0
5
10
15
20
25
30
35
0.2 0.25 0.3 0.35 0.4 0.45
MAIN READING
FLOWRATE
Series1
Linear (Series1)
CALIBRATION GRAPH FOR MASS FLOW METERS FOR C3H8
y = 40.167x - 6.2463
R2
= 1
0
10
20
30
40
50
60
70
80
0 0.5 1 1.5 2 2.5
MAIN READING
FLOWRATE(ml/min)
Series1
Linear (Series1)

91
BET Surface Area Measurements Calculations

92
Sample: C-1
Parameters:
Area Volume (cc)
Calibration Small Loop 335016 0.50 Atm. Press 760mm Hg
Calibration Large Loop 3031314 5.00 LN2 Sat P 775mm Hg
Sample Desorption 1816535 2.97 Sample P 0mm H2O
Sample Duplicate 1855143 3.04 Room Temp 25'C
N2 Conc. 0.302
Sample Wt 0.0602g
BETSurface area 8.34m2 138.5m2/g
BETduplicate 8.52m2 141.5m2/g
Average 140
Sample: C-2

93
Parameters:
Area Volume (cc)
Calibration Small Loop 345936 0.50 Atm. Press 760 mm Hg
Calibration Large Loop 3110150 5.00 LN2 Sat P 775 mm Hg
Sample Desorption 1953330 3.12 Sample P 0 mm H2O
Sample Duplicate 1916430 3.06 Room Temp 25 'C
N2 Conc. 0.302
Sample Wt 0.0644g
BET Surface area 8.74m2 135.75m2/g
BET duplicate 8.57m2 133.13m2/g
Average 134.44
Sample: C-3
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0506g
Average 155.48

94
Sample: C-4
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0469g
Average 228.08
Sample: C-5
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0506g
Average 288.27
Sample: C-6
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0552g
Average 210.48

95
Sample: C-7
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0537g
Average 167.40
Sample: C-8
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0524g
Average 173.18
Sample: C-MoS2
Parameters:
Area Volume (cc)
N2 Conc. 0.302
Sample Wt 0.0515g
Average 166.15

A STUDY OF MOLYBDENUM CARBIDE CATALYST FOR FISCHER-TROPSCH SYNTHESIS-MEngSc Process Engineering Thesis (CEIC U

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