THE UNIVERSITY OF NEW SOUTH WALES
School of Chemical Engineering
Nano sized Cobalt for Catalysis of Hydrogen Storage
CEIC 8321 EXT RESEARCH PROJECT
Thesis B
Supervisor: Dr. Kondo – Francois Aguey Zinsou
By: Bertha Elizabeth Ibarra Lopez
Z3392989
Semester 2, October 2013
I
Declaration
"I, Elizabeth Ibarra, hereby declare that this submission is my own work and that, to the
best of my knowledge and belief, it contains no material previously published or written
by another person nor material which to a substantial extent has been accepted for the
award of any other degree or diploma of the University or other institute of higher
learning, except where due acknowledgment is made in the text."
Name: Elizabeth Ibarra Lopez
Signed: ______________________
Date: ________________________
II
Acknowledgments
Firstly, I want to thank God, for the strength, support, and life that he has given me to
complete this work.
I am grateful to my supervisor Dr. Kondo Francois Aguey Zinsou, and his energy
research group for their support, guidance, patience, and valuable time to complete the
work on this year, and for providing me the working place and materials.
I want to thank the University of New South Wales for providing me the space and the
opportunity to study in a ranked university.
I want to thank to my sponsor Senescyt and the Government of Ecuador for the
opportunity given me to study overseas. To learn, improve and get involved in a higher
system of education. I have great memories of this year of learning in the research area.
I want to thank my family, and my spiritual family for their support, the church for
helping me, getting involved and study in Australia. Also, to my flatmates for their
support, time and shared their experience at UNSW.
III
Table of Content
DECLARATION ........................................................................................................................................ II ACKNOWLEDGMENTS ......................................................................................................................... III ABSTRACT .............................................................................................................................................. XI INTRODUCTION ................................................................................................................................... 12 CHAPTER 1: LITERATURE REVIEW ............................................................................................... 14 1.1 ENERGY DEMAND AND SUPPLY .................................................................................................................... 14 1.2 ENERGY STORAGE ........................................................................................................................................... 16 1.3 HYDROGEN ........................................................................................................................................................ 18 1.4 HYDROGEN PRODUCTION .............................................................................................................................. 19 1.5 HYDROGEN STORAGE TECHNOLOGIES ......................................................................................................... 20 1.5.1 Compressed hydrogen gas .............................................................................................................. 21 1.5.2 Liquid hydrogen .................................................................................................................................. 22 1.5.3 Solid Hydrogen ..................................................................................................................................... 22 1.6 NANO CATALYST FOR HYDROGEN STORAGE ............................................................................................... 23 1.6.1 Heterogeneous Catalysis .................................................................................................................. 24 1.6.1.1 Catalysis .............................................................................................................................................. 24 1.6.1.2 Kinetics of catalytic conversion ................................................................................................ 26 1.6.1.3 Hydriding Mechanism ................................................................................................................... 27 1.6.1.4 Thermodynamic Conditions ....................................................................................................... 29 1.6.1.5 Transitional Metals ........................................................................................................................ 29 1.7 METHODS FOR SYNTHESIS NANO CATALYST ............................................................................................... 33 1.7.1 Impregnation and thermal decomposition method ............................................................ 34 1.7.2 Reduction with Sodium Boro hydride ........................................................................................ 36 1.8 STABILIZATION OF METAL NANOPARTICLE .......................................................................................... 37 1.8.1 CONTROL SIZE OF THE NANOPARTICLE BY NUCLEATION AND GROWTH ............................................ 37 IV
1.8.2 PROTECTION STABILIZATION AND SUPPORT .......................................................................................... 41 1.9 HYDROGEN STORAGE IN MAGNESIUM DOPED WITH COBALT AND GRAPHITE .................................. 42 1.9.1 HYDROGEN STORAGE IN MAGNESIUM ..................................................................................................... 43 1.9.2 HYDROGEN STORAGE UPTAKE IN CARBON NANOTUBES ...................................................................... 45 1.9.3 MECHANISM OF HYDROGEN STORAGE USING NANO CARBONS ........................................................ 45 1.9.4 HYDROGEN STORAGE IN MAGNESIUM DOPED WITH TRANSITION METAL. ........................................ 46 CHAPTER 2: EXPERIMENTAL METHODOLOGY .......................................................................... 51 2.1 MATERIALS AND APPARATUS ....................................................................................................................... 51 2.2 SAMPLE PREPARATION ................................................................................................................................... 51 2.3 RESULTS ............................................................................................................................................................ 54 2.4 CONCLUSION ..................................................................................................................................................... 82 2.5 DISCUSSION ...................................................................................................................................................... 83 REFERENCES ......................................................................................................................................... 88 V
List of Figures
FIGURE 1. RESIDENTIAL/ COMMERCIAL FUEL DEMAND BY SECTOR (2) ........................... 14 FIGURE 2. GLOBAL FUEL MIX BY DECADE (2) .............................................................................. 15 FIGURE 3. VOLUMETRIC VERSUS GRAVIMETRIC ENERGY DENSITY OF THE MOST IMPORTANT ENERGY CARRIERS (3). .............................................................................................. 17 FIGURE 4. HYDROGEN CYCLE, SOLAR ENERGY AND PROCESS OF WATER ELECTROLYSIS USED TO GENERATE, STORE AND COMBUST HYDROGEN (3) .................. 18 FIGURE 5. PRIMITIVE PHASE DIAGRAM FOR HYDROGEN (6) ................................................. 19 FIGURE 6. RATE OF HYDROGEN ABSORPTION BY LANI5 (A) POLYCRYSTALLINE (B) NANO CRYSTALLINE; (C) NANO CRYSTALLINE WITH CATALYST (17) ................................. 24 FIGURE 7. THE DENSITY OF NUCLEATION SITES IS RELATED TO THE FREE ENERGY NUCLEATION DUE TO THE INTERFACE ENERGY BETWEEN THE ∝ AND Β PHASES. (13) 25 FIGURE 8. VOLCANO TYPE CORRELATION OF CATALYTIC ACTIVITY OF DIFFERENT METALS AND THE ME-­‐H BINDING ENERGY. (15) ......................................................................... 27 FIGURE 9. LENNARD-­‐JONES POTENTIAL DIAGRAM CORRESPONDING TO THE SUCCESSIVE ENERGY BARRIERS ENCOUNTERED BY HYDROGEN DURING ABSORPTION/DESORPTION IN A METAL. (3). ............................................................................. 28 FIGURE 10. T – P PHASE DIAGRAM OF THE FE – H SYSTEM. (14) ........................................... 31 FIGURE 11. ISOTHERM OF NICKEL (15) .......................................................................................... 32 FIGURE 12. FORMATION OF NANOPARTICLES VIA REDUCTION OF METAL SALT PRECURSORS (19) .................................................................................................................................. 36 FIGURE 13. TGA/DSC CURVES OF DI-­‐N-­‐BUTYL MAGNESIUM ................................................ 44 FIGURE 13. SHOWS A CONCEPT MODEL OF MGH2 (A) PLAIN (B) NANO CRYSTALLINE (C) NANO CRYSTALLINE DOPED MATERIAL (22) ........................................................................ 46 FIGURE 14. EFFECT OF GRAIN SIZE ON HYDROGEN ABSORPTION OF BALL MILLED MAGNESIUM POWDER(17) ................................................................................................................. 48 VI
FIGURE 15. HYDROGEN ADSORPTION OF MAGNESIUM – BASED COMPOSITES FOR DIFFERENT SYSTEMS AT 300OC (27) ............................................................................................... 49 FIGURE 16. HYDROGEN ADSORPTION WITH MAGNESIUM – FETI-­‐CNTS (27) .................... 49 FIGURE 17. SYNTHESIS OF COBALT NANOPARTICLES ............................................................ 52 FIGURE 18. TPR PROFILES OF THE CO/ACTIVATED CARBON .............................................. 54 FIGURE 19. XRD PATTERNS OF CO/ACTIVATED CARBON ..................................................... 55 FIGURE 20. A, B, C AND D) TEM IMAGES OF 20% CO/ACTIVATED CARBON E) PARTICLE SIZE DISTRIBUTION 20%CO/ACTIVATED CARBON. ............................................................... 55 FIGURE 21. TEM IMAGINES OF HSAG ............................................................................................ 56 FIGURE 22. XRD PATTERNS OF HSAG 500 .................................................................................. 56 FIGURE 23. A,B,C,D AND E) TEM IMAGES OF 20% CO/HSAG F) PARTICLE SIZE DISTRIBUTION OF 20%CO/ HSAG. ............................................................................................... 58 FIGURE 24. EDX 20% CO/HSAG ...................................................................................................... 59 FIGURE 25. TPR OF 20% CO/HSAG ................................................................................................ 60 FIGURE 26. XRD 20%CO/HSAG ....................................................................................................... 60 FIGURE 27. TEM IMAGES OF 8% CO/HSAG PARTICLE SIZE DISTRIBUTION. ................... 61 FIGURE 28. PARTICLE SIZE DISTRIBUTION OF 8% CO/HSAG 20 H OF IMPREGNATION
................................................................................................................................................................... 62 FIGURE 29. XRD OF 8 % CO/HSAG ................................................................................................. 62 FIGURE 30. TEM IMAGES OF 5% CO/HSAG ................................................................................. 63 FIGURE 31. PARTICLE SIZE DISTRIBUTION OF 5% CO/HSAG 20 H OF IMPREGNATION
................................................................................................................................................................... 64 FIGURE 32. TEM IMAGES OF 3% CO/HSAG ................................................................................. 64 FIGURE 33. PARTICLE SIZE DISTRIBUTION OF 3% CO/HSAG 20 H OF IMPREGNATION
................................................................................................................................................................... 65 VII
FIGURE 34. TEM IMAGINES FOR 5% COBALT IN CARBON HSGA 500 (A, B, C) REPRESENTS 600OC; (D,E,F ) REPRESENTS 500 OC 6H OF IMPREGNATION AND 2 H REDUCTION ........................................................................................................................................... 66 FIGURE 35. PARTICLE SIZE DISTRIBUTION A) 600 OC B) 500OC, WITH 6H OF IMPREGNATION AND 2 H REDUCTION AND C) EDX OF COBALT FOR 5% COBALT IN CARBON HSGA 500 .............................................................................................................................. 68 FIGURE 36. TEM IMAGINES OF NANOPARTICLE A) 600 OC B) 500 OC WITH 3% COBALT, 48H IMPREGNATION AND 2H REDUCTION ................................................................................. 69 FIGURE 37. PARTICLE SIZE DISTRIBUTION A) 600 OC B) 500 OC WITH 3% COBALT, 48H IMPREGNATION AND 2H REDUCTION ................................................................................. 70 FIGURE 38. TEM IMAGINES OF 5%CO/HSAG, 500 C, 6H IMPREGNATION AND 5H REDUCTION ........................................................................................................................................... 71 FIGURE 38. SIZE NANOPARTICLES 5%CO/HSAG ...................................................................... 72 FIGURE 39. PARTICLE SIZE DISTRIBUTION OF 5% COBALT/ HSAG, 500 C, 6H IMPREGNATION, 5H REDUCTION ................................................................................................... 72 FIGURE 40. TEM IMAGINES FOR 3% COBALT/HSAG A, B, C) 500 OC, 6H IMPREGNATION, D, E,F) 600OC 6H IMPREGNATION, G, H, I) 500 OC, 48H IMPREGNATION ................................................................................................................................... 73 FIGURE 41. PARTICLE SIZE DISTRIBUTION OF 3% COBALT/HSAG A) 500 C, 6H IMPREGNATION, B) 600C 6H IMPREGNATION, C) 500 C, 48H IMPREGNATION ............. 74 FIGURE 42. PARTICLE SIZE DISTRIBUTION COMPARE WITH THE TYPE OF CARBON .. 76 FIGURE 43. PARTICLE SIZE DISTRIBUTION WITH DIFFERENT UPLOADS OF COBALT 76 FIGURE 44. NUMBER OF PARTICLES A) 5%CO/HSAG AND 6H IMPREGNATION B) 3% CO/HSAG AND 48H IMPREGNATION ............................................................................................. 77 FIGURE 45. PARTICLE SIZE DISTRIBUTION VARYING IMPREGNATION, REDUCTION TIME AND CONCENTRATION A) 3%CO/HSAG, 5 H REDUCTION 600 OC B) 5%CO/HSAG, 6H IMPREGNATION 500 OC ............................................................................................................... 78 FIGURE 46. TEM IMAGINES OF MAGNESIUM DOPED WITH 5%CO/HSAG ........................ 79 FIGURE 47. XRD OF MG DOPED WITH CO/HSAG ....................................................................... 80 VIII
FIGURE 48. EDX OF THE SAMPLE CO/HSAG WITH MAGNESIUM ......................................... 80 FIGURE 49. TGA .................................................................................................................................... 81 FIGURE 50. MS ...................................................................................................................................... 82 FIGURE 51. XRD PATTERNS OF NATURAL GRAPHITE AND DETONATION SOOT (29) .... 99 IX
List of Tables
TABLE 1: PROPERTIES OF SOME REPRESENTATIVE HYDROGEN STORAGE SYSTEMS. (7)
................................................................................................................................................................... 21 TABLE 2: SUMMARY OF THE TECHNOLOGIES FOR HYDROGEN STORAGE (10) ................ 23 TABLE 3. HYDROGEN STORAGE CAPACITY WITH TRANSITIONAL METALS ................... 42 TABLE 4. HYDROGEN STORAGE PROPERTIES OF SOME COMPLEX TRANSITIONAL METAL HYDRIDES (28) ......................................................................................................................... 47 TABLE 5. SUMMARY OF THE DIFFERENT UPLOADS OF COBALT ......................................... 65 TABLE 6. SUMMARY SIZE OF THE NANOPARTICLES WITH THE VARIATION OF IMPREGNATION, REDUCTION TIME AND REDUCTION TEMPERATURE. ........................... 75 X
Abstract
The advantages of the nano catalyst are their size, activity, selectivity, thermodynamics
and kinetic of diffusion of hydrogen. There are several ways to storage hydrogen and the
most useful is the solid state, since it has high volumetric and gravimetric hydrogen
densities. The aim of the project is to have small nanoparticles in the order of two
nanometers, and improve the kinetics of absorption of hydrogen in magnesium hydrides
using nano catalyst. The synthesis of cobalt nanoparticles impregnated in high surface
area graphite by the wetness impregnation method is studied.
The results of the synthesis of cobalt nano particles show a size distribution of 0.5 to 2
nanometers. Cobalt nitrate was used as a precursor, and impregnated in high surface area
graphite 500. The HSAG 500 was pre treated with nitric acid, at 80 oC for 4 hours. The
carbon was filtered and washed until reached the pH of the water. Then, the sample was
dried at 120 oC for 10 hours. Cobalt nitrate was used as a precursor and the sample was
prepared for 5 % of Cobalt impregnated in high surface area graphite 500, the placement
was done for 6 hours at room temperature. Then, the sample was dried at 120 oC for 10
hours. Finally, an argon treatment and reduction with hydrogen was done to the sample.
The analysis to determine the size of the nano particle was done by transmission electron
microscopy. To determine the presence of cobalt and graphite in the sample XRD and
EDX analysis were performed.
XI
Introduction
The energy demand had increased lately, and the energy supply has been from fossil
fuels. Therefore, there is a need to seek for new sources of energy that are clean and
sustainable for the environment. The relevant aspect of the energy sector is to store
energy efficiently, and deliver it on demand. As an example, energy storage can be
maximized on mobile applications, transportation systems and portable electronic
devices. Under this energy demand, is important to highlight that hydrogen is a good
energy vector because it can be produced from different sources and its combustion
produces water. Different technologies to storage hydrogen exist and the most important
is to storage hydrogen in the solid state.
The aim of the project is to synthetize cobalt nanoparticles by the wetness impregnation
method, and produce particles of less than 2 nm. The present report is divided in two
chapters, chapter one explains the literature review of transitional metals used in catalysis
for hydrogen storage. The second part describes the experimental part of the project. Is
relevant to mention the importance of the mechanism of catalysis for hydrogen storage
using nano transitional metals as nano catalysts. And consider the kinetic,
thermodynamic of the diffusion of hydrogen and the application of nano transitional
metal as nano catalyst, so it can improve the rate of absorption and desorption of
hydrogen. Important properties of transition metals to use as nano catalysts are activity
and selectivity.
12
Nanoparticles of a size distribution of 0.5 nm to 2 nm were synthesized by wetness
impregnation method, 5 % of Cobalt was impregnated in high surface area graphite. First,
HSAG 500 was pre treated with nitric acid for four hours at 80oC. Then the carbon was
dried for 10 hours at a temperature of 120 oC. The variables that changed to obtain a
better size distribution of nano particles were, time of impregnation the sample was
impregnated at 48 hours, 20 hours and 6 hours using as a precursor cobalt nitrate. Then
the sample was dried for 10 hours at a temperature of 120 oC. And a thermal treatment of
Argon was performance for 4 hours at 200oC. Other variables that changed where time
and temperature of reduction; 2 hours and 5 hours were the variables and the temperature
of 500 oC and 600 oC.
13
Chapter 1: Literature Review
1.1 Energy Demand and Supply
Energy Demand increases as the population increases
(2).
People need energy to power
homes, business, industry, transportation, electricity generation and other vital services
(1).
Figure 1 shows the energy demand until 2040 in the commercial and residential
sector, for example in the residential sector the energy demand increases up to 75 %, (see
figure 1). Also, the demand of energy can be supplied by different sources of energy,
such as: biomass, coal, oil, gas, hydro, nuclear are some of the renewable sources to
produce energy. Figure 2 shows the different sources to produce fuel until the 2040, and
the source that is increasing in the last years is the renewable energy although coal, oil
and gas are the predominant sources of energy.
Figure 1. Residential/ Commercial fuel demand by sector (2)
14
Figure 2. Global fuel mix by decade (2)
From the different kinds of energy, electrical energy is the most used because it is
available at the lowest possible cost, invisible and an omnipresent commodity
(1).
It
makes the 12% of the total energy processed by humanity, and will increase over 34% for
2025 in a context of not using fossil fuels, growing use of renewable energy, and
considering environmental impacts. (9)
The load of energy is initially based on the prediction of daily and seasonal needs, it is
highly centralized and often a long distance away from its end users (1).
The energy is first, generated, transmitted, converted and then stored. Therefore, the
storage of electrical energy has become a necessity. However, electricity is difficult to
store, as this requires bulky, costly equipment. The storage capacity worldwide is the
equivalent of about 90 GW
(11)
of a total production of 3400 GW, or roughly 2.6%
(11).
The capacity demand that is facing the on board hydrogen energy system is in the range
of 5 -13 kg hydrogen for light duty vehicle (57).
There are different ways to storage energy, mechanical energy for example as potential
energy or rotation energy of a flywheel, in an electric or magnetic field (capacitors),
chemical energy of reactants and fuels or as a nuclear fuel (1).
15
Chemical and electric energy can be transmitted easily because it involves electronic
coulomb interaction. Section 1.2 discusses the different ways to storage energy.
1.2 Energy Storage
Electricity storage can be achieved effectively; the storage techniques can be divided into
four categories. Low power application in isolated areas, essentially to feed transducers
and emergency terminals; medium power application in isolated areas (individual
electrical systems, town supply) which are for small scale system where the energy could
be stored as kinetic energy, chemical energy, compressed air, hydrogen (fuel cells), or in
super capacitor or super conductors (1).
The network connection application with peak leveling and power – quality control
applications are for large scale systems where the energy can be stored as a gravitational
energy (hydraulic system, thermal), thermal energy (sensible, latent), chemical energy
(accumulators, flow batteries) or compressed air (or coupled with liquid or natural gas
storage) (1).
The importance of hydrogen to use as an energy carrier of the future, is because of its
advantages that are high gravimetric and volumetric density (see figure 3). Hydrogen
avoids the carbon dioxide emissions that causes the green house effect which means that
is an alternative to current carbon dependence for energy production, it minimize issues
related to increasing energy consumption, energy security and related geopolitical
tensions
(4).
Figure 4 shows the hydrogen cycle, in which solar energy process of water
electrolysis is used to generate, store and combust hydrogen with oxygen and the energy
is used.
16
Figure 3. Volumetric versus gravimetric energy density of the most important energy
carriers (3).
Figure 4 shows the process, in which the cycle of hydrogen is developed, 25% of the
energy produced form the sun is used to electrolyze the water. From this electrolysis it is
generated two products: oxygen that recirculates in the earth, hydrogen which is stored in
the metal hydride form 39 kWhkg-1, from this 85% of the hydrogen is and forms metal
hydride. Then 85 % of this hydrogen is combusted and the rest reacts with oxygen and
produces water.
17
Figure 4. Hydrogen Cycle, solar energy and process of water electrolysis used to
generate, store and combust hydrogen (3)
There are different ways to produce and storage hydrogen which will be discussed in
section 1.4 and 1.5, respectively.
1.3 Hydrogen
The properties of hydrogen are important to understand when there is a need of searching
and developing technologies to produce and storage hydrogen. The phase diagram shows
the properties of hydrogen, (see Fig. 5), for example hydrogen can be found in the solid
state at 62°C with a density of 70.6 kg·m−3 and gas at 0oC . At higher temperatures
hydrogen has a density of 0.089886 kg·m−3 with a pressure of 1 bar. Hydrogen behaves
as a liquid in a small zone between the triple and critical points with a density of 70.8
kg·m−3 at -253°C (5).
18
Figure 5. Primitive phase diagram for hydrogen (6)
Hydrogen can be produced from different sources such as fossil fuel, water splitting,
biomass and its combustion produces only water (7). That is a reason why, hydrogen is an
ideal energy carrier, which is considered for transportation, such as automotive
applications. In this context storage of hydrogen is one of the key challenges in
developing hydrogen economy.
1.4 Hydrogen Production
Hydrogen can be produced from different raw materials, and hydrocarbons are the main
source; other raw materials are coal, heavy oil, light oil, methane or biomass. In an
industrial scale, natural gas and oil are the main sources (25).
For example hydrogen is produced from natural gas via steam methane reforming; the
process can be done at high temperatures of 700 oC – 1100 oC, the steam reacts with
methane to yield syngas. The heat required to drive the process is generally supplied by
burning some natural gas
(25).
Coal produces considerable amounts of hydrogen and
electricity because of the large size of available deposits (4).
19
Biomass is a sustainable source of production of hydrogen, but cannot supply hydrogen
in the amounts required, so its uses are more for food, chemical feedstock or as an energy
source (43).
A mature technique applied for hydrogen production is electrolysis, and its uses have
been for many years; most commercial electrolysers are based on the principle of alkaline
electrolysis and solid polymers can be used. The efficiency of an electrolysers range
between 65 and 75 %
(25),
and only 3% of the world’s hydrogen is produced by
electrolysis (42).
1.5 Hydrogen storage technologies
Hydrogen is an alternative carrier of energy, because in an environmental point of view,
it does not generate pollutants such as particles, nitrogen oxides, sulfur oxides,
hydrocarbons, and carbon monoxide
(4).
Using this energy carbon emissions and global
warming issues can be dismissed. In the economic point of view, hydrogen can be used in
the transportation sector as a fuel cell. Because of this argument and benefits of
hydrogen, the actual challenge faced is in developing and improving technologies for
hydrogen storage that operate at moderate temperature, pressure, minimum weight,
volume, and maintaining a reasonable cost
(8).
This section will discuss the different
technologies for hydrogen storage. It is important to mention that the solid-state
technology has a high volumetric density. The use of carbon nano tubes has a high
volumetric density compared with liquid hydrogen storage and this area is in
investigation and development. Table 1, shows different hydrogen storage technologies
and the one with the highest volumetric density is the metal hydride.
For example, the use of compressed technology for hydrogen storage is not so useful
because of the weight of the tanks and fuel cells. Therefore, using metal hydride will
improve the capacity of hydrogen storage. In the case of liquid hydrogen the
inconvenience is the liquefaction energy, dormant boil off, and safety. In the case of solid
state technology, the improvements are to decrease the temperature of desorption of
20
hydrogen, and the time of absorption and desorption of hydrogen. Table 2, summarizes
the different technologies used for hydrogen storage.
Table 1: Properties of Some Representative Hydrogen Storage Systems. (7)
Gravimetric
Conditions for reversible
density
Volumetric
Medium
(mass%)
density /kgL-1
H2 gas 700 bar
100
0.03
Near room temperature with
H2 liquid
100
0.07
moderate pressures
NaAlH4
5.5
1.24
MgH2
7.6
1.40
Mg2NiH4
3.8
2.60
LiAlH4
10.5
0.92
LiBH4
18.4
0.12
Need high temperatures and/or
NH3 liquid
17.7
0.60
pressures
H2O
11.1
1.00
Practically irreversible
1.5.1
hydrogen storage
Compressed hydrogen gas
Hydrogen can be found in the gas phase when it is stored in tanks with a pressure of 700
bar with a volumetric density of 0.03 kg/L near room temperature as Table 1 shows. The
review of Andreas Zuttel, points out that the gas can be mostly stored in high-pressure
gas cylinders, which operates at a maximum pressure of 20 MPa. The safety of
pressurized cylinders is a concern, especially in highly populated regions and the vessels
will consist of three layers, as an example, the industry has set itself a target of 110 kg 70
MPa cylinder with a gravimetric storage density of 6 mass % and a volumetric storage
density of 39 kgm-3 (5).
21
1.5.2
Liquid hydrogen
Another technology to storage hydrogen is the liquid state with volumetric densities of
0.07 kg/L. The process to have liquid hydrogen is to understand the simplest liquefaction
cycle of Joule-Thompson. First the gas is compressed and then cooled in a heat
exchanger, before it passes through a throttle valve where it undergoes an isenthalpic
Joule- Thomson expansion, producing some liquid; the cooled gas is separated from the
liquid and returned to the compressor via the heat exchanger (11).
1.5.3
Solid Hydrogen
Storage of hydrogen in solid material has a potential to become safe and efficient way to
store energy
(10).
There are four main groups of suitable materials such as: carbon and
other HAS materials like activated charcoals, nanotubes, graphite nano-fibers, MOFs,
zeolites, clathrate hydrates; chemical hydrides (H2O-reactive) examples are encapsulated
NaH, LiH and MgH2 slurries, CaH2, LiAlH4; Rechargeable hydrides like alloys and inter
metallic, nano crystalline, complex; Chemical hydrides (thermal) like ammonia borozane,
aluminum hydride
(10).
The ideal storage material should have a plateau pressure of few
3–8 bar and near 80 oC.(7)
Table 2, summarizes the different technologies that are applied for hydrogen storage, it
mentions the status on the market, how far it has been developed and the best option to
consider for this technology gives an overview of the research and development issues.
Solid hydrogen storage investigation and developed issues are: weight, lower desorption
temperatures, higher desorption kinetics, recharge time and pressure, heat management,
cost, pyrophoricity, cyclic life, container compatibility and optimization. In the case of
the kinetics studies, the use of nano catalyst developed by transition metals is the best
option. Specific matching of metals with different affinities to hydrogen allows the
properties of ternary hydrides to be altered, and in fact provides an important alloying
guideline for metal hydrides.
22
Table 2: Summary of the Technologies for Hydrogen Storage (10)
Technology
Gaseous H2
storage
Liquid H2
Storage
Solid H2
Storage
Status
Commercially
available but
costly
Commercially
available but
costly
Best Option
C - fiber composite
vessels ( 6 - 10 wt % H2
at 350 - 700 bar)
Cryogenic insulated
dewars ( ca. 20 wt% H2
at 1 bar and -253 C)
R & D issues
Fracture mechanics, safety,
compression energy and
reduction of volume.
High liquefaction energy,
dormant boil off, and
safety.
Very
development
There are many options:
Rechargeable hydrides,
chemical hydrides (H2O
& thermally reactive),
carbon, and other high
surface area materials.
Most developed option:
Metal hydrides (potential
for >8 wt.% H2 and >90
kg/m3 H2-storage
capacities at 10-60 bars)
Weight, lower desorption
temperatures, higher
desorption kinetics,
recharge time and pressure,
heat management, cost,
pyrophoricity, cyclic life,
container compatibility and
optimization
1.6 Nano catalyst for hydrogen storage
Reactions involving hydrogen are very sensitive to catalysis; if not using the correct
catalyst, the reaction may not proceed at all. (17) Nano catalyst are very important since its
performance helps to increase the reaction to accelerate.
According to the traditional definition, a catalyst affects the reaction kinetics (and
possibly the reaction route), but does not alter the reaction substrates or products (22). The
catalyst does not play a role in the final hydrogen storage; and the amount present in the
system should be kept as low as possible to avoid reducing the overall storage capacity
and the advantage of the solid-state catalyst is that it can be combined with the nano
structured of the hydride. (17)
The sorption kinetics can be improved by the combination of nano structured and nano
catalysis. This can be seen in the case of magnesium-based hydrides, where the case of
23
kinetics allows the operational temperature to be lowered (18). In catalyzed Mg based nano
crystalline hydrides, hydrogen absorption occurs at room temperature. The effect of
combining nano catalyst with LaNi5 is presented in Figure 6 (17), in which curve a, shows
the rate of hydrogen absorption by LaNi5, curve b shows when it is nano crystalline and
curve c shows that the time for hydrogen absorption decreases but still remains the same
quantity when a nano catalyst is used.
Figure 6. Rate of hydrogen absorption by LaNi5 (a) polycrystalline (b) nano crystalline;
(c) nano crystalline with catalyst (17)
1.6.1
Heterogeneous Catalysis
1.6.1.1 Catalysis
The mechanism of catalysis in hydrogen dissociation/ adsorption has been identified
widely and is relevant in both low temperature fuel cells and the formation of metal
hydrides
(16).
Schlapbach describes the mechanism of how hydrogen atoms dissolve into
the lattice, forming solid solution or the ∝-phase. As the concentration of Hydrogen
atoms in the lattice increases, the Hydrogen atoms occupy interstitial sites in ordered
24
manner forming the hydride phase or the 𝛽-phase. The lattice expands to accommodate
hydrogen atoms.
The physical arrangement of metal atoms may also undergo changes during hydride
formation of the catalytic cycle its diffusion of the molecules through the gas phase to the
metal surface where the molecules may bond in a molecule form, adsorption of hydrogen
molecules on the surface, dissociation of molecular hydrogen into atomic Hydrogen,
followed by diffusion of Hydrogen atoms into interstitial sites to form metal hydrides.
The overall kinetics of hydrogen absorption is determined by the slowest step.
The increased volume in the amorphous grain boundary weakens the binding between
metal and hydrogen atoms, the hydrogen molecules dissociate on the catalyst, some
hydrogen atoms remain attached to the catalyst, while others diffuse to the catalyst
support and subsequently penetrate into the metal, where the hydrogen is spillover and
interacts directly with the metal (7).
Figure 7. The density of nucleation sites is related to the free energy nucleation due to the
interface energy between the ∝ and 𝛽 phases. (7)
25
1.6.1.2 Kinetics of catalytic conversion
A catalyst can be evaluated by how well it converts reactants into products, and this is a
measure in the activity and selectivity of the catalyst. The activity of a catalyst is defined
as the rate of the consumption of the reactant, and the selectivity is the fraction of the
total products, which a particular substance represents (13).
The mechanism of catalysis can be describe in two steps as follow, in which metal
hydride can be cleaved in three different ways – by loss of a proton, hydrogen atom or a
hydride anion (13):
𝐻! + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 ↔ 𝐻! Eq. 1
𝐻! + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 ↔ 𝐻∗ 𝐻! + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 ↔ 𝐻! 𝐿! 𝑀! + 𝐻! ↔ 𝐿! 𝑀𝐻
Eq. 2
𝐿! 𝑀! + 𝐻! ↔ 𝐿! 𝑀𝐻
𝐿! 𝑀∗ + 𝐻∗ ↔ 𝐿! 𝑀𝐻
The Mechanism in which consider the process to be rate limited is Equation 2.
The activity is an important reason to choose and use the transitional metals. Figure 8,
shows the current densities measured for cathodic release of hydrogen at a given over
potential are plotted vs. the strength of the Me-Hydrogen bond (calculated in kJ/ mol).
26
Figure 8. Volcano Type correlation of catalytic activity of different metals and the Me-H
binding energy. (15)
1.6.1.3 Hydriding Mechanism
The Hydriding mechanism consist of several steps which may be described using the
long-range attractive/short-range repulsive Lennard–Jones potential described in Figure
9, the molecular hydrogen approaches the metallic surface, it encounters successive
minima in the potential curve corresponding to molecular adsorption, atomic adsorption
and bulk absorption (12).
27
The first step of the mechanism is physic sorption and it describes when the molecule
adheres to a surface of the catalyst without the formation of a chemical bond, usually by
van der Waals forces or electrostatic attraction with energy levels approximating those of
condensation (1–5 kJ mol-1) (26).
The next step is chemisorption, and it is the chemical combination of hydrogen with the
metal to form a new compound, which characteristically has a binding energy well above
50 kJmol-1 (26). Chemisorption is a strong adsorption and involves real bond breaking or
weakening in the reactant and making of the bonds to the surface.
Analyzing the graph the reaction cannot take place at low pressure and temperature
because of the energy barrier for adsorption ∆E that must be overcome
(26).
The energy
barrier ∆E bar is larger for the desorption process, and the heat released is directly related
to the difference in internal energy ∆E between the initial and the hydride states (26).
Destabilizing the hydride will speed up the desorption kinetics and reduce the heat
released, while the use of a catalyst can help both the hydriding and dehydriding kinetics
(26).
Figure 9. Lennard-Jones potential diagram corresponding to the successive energy
barriers encountered by hydrogen during absorption/desorption in a metal. (4).
28
1.6.1.4 Thermodynamic Conditions
A metal can store hydrogen depending on the plateau pressure or equilibrium pressure
(Peq) (35). In the studies of Aguey – Zinsou and Ares Fernandez (see Figure 4) shows
that under a certain hydrogen pressure, the metal dissolves some hydrogen as a solid
solution (∝ phase) and while the concentration of hydrogen increases and the pressure
rises the metal hydride phase (∝phase) nucleates and grows. At a given temperature,
when the pressure is applied above the plateau pressure hydrogen is stored. The
thermodynamic properties of metal-hydrogen systems are usually characterized by the
strength of the metal-hydrogen bond and thus the enthalpy of the (de) hydriding reaction.
For the practical utilization of hydrogen storage materials in vehicles, the metal-hydrogen
bond 10 strength should correspond to an enthalpy of around 30 kJ mol-1H2 (26).
Figure 5. Pressure composition isotherm (PCI) (4)
1.6.1.5 Transitional Metals
Transition metals are the prime candidates for catalysis, because the d orbital gives
transitional metals their distinguished properties
(44).
The outermost d orbitals are
incompletely filled with electrons so they can easily give and take electrons
(44).
Transitional metals can contribute to the essential ingredient in catalyst systems and can
be summarized because of the bonding ability, cathodic choice of ligands, ligand effects,
variability of oxidation state and variability of coordination number (77).
29
A d block metal ion has nine valence shell orbitals to accommodate its valence electrons
and to form hybrid molecular orbits in bonding with other groups. A special
configuration enables the d metal to form both 𝜎 and n bonds which is one of the key
factors in imparting catalytic properties to the transition metals and their complexes (77).
The importance of transitional metal for hydrogen storage and their alloys is because it
reacts with gaseous hydrogen or H atoms from electrolytes to form metal hydrides, such
reversible Hydriding reactions render these metals and their alloys potential hydrogen
storage materials (77).
Transitional metals nano particles in catalysis are important since it mimic metal surface
activation, brings selectivity and efficiency to heterogeneous catalysis. Nano particles
have a few tens to several thousand metal atoms, stabilized by ligands, surfactants,
polymers or den drimers protecting their surface (5).
This section will discuss and consider the thermodynamics and kinetics of hydrogen in
the nano catalysts. As a reminder that the effective dissociation of hydrogen into a metal
particle with subsequent spillover strongly depends on the particle size and the contact
between the support and the particle as reported by Yang et al, Chen et al and other
groups, the aim of the experimental section is to synthesize transitional metals
nanoparticles of the size around of two nanometers.
30
a.
Iron
The studies of Antonov V E, show that hydrides formed at high pressures in the systems
other than Fe–H show to have close-packed metal sub lattices with an fcc (γ) or hcp (ε)
structure, in which hydrogen occupies octahedral interstitial positions.
Figure 10. T – P phase diagram of the Fe – H system. (14)
The description that Antonov V E gives for T–P diagram of phase transformations of iron
placed in an atmosphere of gaseous hydrogen is shown in figure 10, in which the
coordinates of the triple point are T = 280 oC and P=5GPa and the lines of phase
transformations divide the T–P plane of the diagram into three regions: α, ε, and γ. Theα
→ε transition in the Fe–H system is very sluggish, and a noticeable amount of nonreacted α-Fe was observed even in Fe–H samples loaded with hydrogen at a pressure as
high as 9 GPa, at room temperature of 350◦C.
31
b. Nickel
Hydrogen acts as a hardening agent, preventing dislocations in the nickel atom crystal
lattice from sliding past one another. Varying the amount of alloying hydrogen and the
form of its presence in the nickel hydride (precipitated phase) controls qualities such as
hardness, ductility, and tensile strength of the resulting nickel hydride (14).
Figure 11. Isotherm of Nickel (15)
32
c.
Cobalt
Studies of Driessen A, predicted the hydride formation properties of cobalt hydride at 1
Mbar. A low hydrogen pressures absorb only small amounts of hydrogen, cobalt has an
HCP to FCC phase transition at approximately 450 oC and ambient pressure. Cobalt
hydride becomes possibly unstable at very high pressure (P>300kbar)(16). Metal Hydrides
are of extreme interest for high-pressure chemistry. As another example, the reaction for
producing cobalt hydrides proceeds in the presence of the high electric field.
In a field evaporation process of cobalt in an atmosphere of hydrogen 10- 6 Pa at a
temperature of 20 K, both the ions of cobalt and the ones of CoHx (where x= 1, 2 and 3).
Cobalt hydride is produced in the direct reaction of cobalt with hydrogen under a pressure
of 5 GPa a t 600 K (17).
The main idea of this project is to use nano catalyst transitional metals for hydrogen
storage and improve the kinetics of Hydriding of magnesium hydrides. Section 1.9
describes the mechanism and how the transitional metals improve the kinetics of
magnesium hydrides.
1.7 Methods for synthesis nano catalyst
For the methods of synthesis nano catalyst the report of Acres G, mentions that a properly
designed catalyst should have the essential attributes of activity, stability, selectivity and
regenerability
(40).
This can be related to the physical and chemical properties of the
catalyst. The first stage is known as dispersion and is achieved by impregnation, while
the second stage is called calcination or reduction. It is brought about by a thermal
treatment on an active atmosphere of hydrogen. Once the metal species have been bound
to the support, degree of dispersion and location will be retained. (40)
It is important to mention that these two stages are relevant for the preparation of
catalysts. The primary aim of applying a catalytically active component to a support is to
33
obtain the catalyst in a highly dispersed form and hence in a highly active form when
expressed as a function of the weight of the active component. (40)
The parameters that characterized the catalyst are activity, stability and selectivity, which
determined the correct dispersion and location of the active ingredients. Dispersion,
location and regenerability are each in their turn determined by the interaction of the
active components with the support surface and with each other during preparation,
activation, use and regeneration.
Reports of Acres K, mention that the activity of a catalyst can be arised from maximizing
the dispersion and the availability of the active catalytic material. The material should be
highly dispersed and concentrated on the external part of the support.
The next section will discuss the effects of these stages in the process of preparation of
catalysts.
1.7.1
Impregnation and thermal decomposition method
The impregnation method of a supported catalyst can be achieved by filling the pores of a
support with a solution of the metal salt from which the solvent is subsequently
evaporated. The precursor that is used is cobalt nitrate.
The same importance of the ionic character of the impregnation solution is the ionic
exchange type and capacity of the catalyst support surface, and it is related to the
chemical structure of the support surface.
Activated carbon and graphite are used in the experiment; these two types of carbons are
commonly used as supports in heterogeneous catalysts, because of their relatively low
cost, high surface area and how the metals can later be removed (40), although in the case
of hydrogen storage it is important because of its gravimetric capacity of storage
hydrogen.
34
The carbon material was treated with nitric acid before the adsorption of the active
component. Studies of Bouleghlimat Emir, Davis Philip et al, discuss the effect of the
acid treatment on the surface of graphite and mentions that acid is used to help to remove
unwanted contaminants such as sodium, potassium, silica, calcium and other substances
that accumulate on the carbon surface that appear during preparation, and introduce
hydrophilic functional groups that changes the carbon to the adsorption of the active
component and the reaction solvent. (19, 20, 21)
Also, studies of Wang and Lu mention that the use of an acid wash significantly
increased the dispersion of transitional metals on carbon and contribute to the formation
of acidic functional groups that could react with the transitional salt (21).
After the pre treatment of carbon with nitric acid and after washing the carbon with
deionized water. The thermal treatment is applied, and the sample is heated in an oven
until it reaches the temperature of 120oC. Is important to mention that thermal treatment
alone to form the metal is not always desirable and can lead to crystallite growth
and in extreme cases metal relocation.
(40)
(46, 47)
Thermal treatments may take the form of low
temperature drying operations (up to 150 C) used to remove water, and some
decomposition of species such as chloroplatinic acid is known to occur within this this
temperature. (40)
After drying the sample, the next step is impregnation of the sample, in which setting the
distribution of solute concentration during the impregnation there are some assumptions
that the carbon is homogenous porous sphere which is initially dry, the support surface is
complete by an aqueous solution that is considerable more than what would be needed to
saturate the surface of the support, the pores are large enough so no physical exclusion of
the solute occur (24).
Studies of Acres mention that after impregnation, a catalyst needs a thermal treatment or
a reduction stage to render the adsorbed metal species active. The first thermal treatment
is of low temperatures drying operations (up to 150 oC) the main purpose is to remove
water. Then, thermal treatment after impregnation at temperatures of 150 oC and 500 oC
35
are used to decompose the adsorbed species to the metal or metal oxide. Reduction in
hydrogen at 400 oC, highly dispersed, nanoparticles formed (48).
TPR and oxidation help to the thermal treatment and reduction stages of catalyst
preparation (46-55).
[Co(NH3)4]++ + O2
[Co(NH3)4]++
1.7.2
Co O
Co
Eq. (3)
Eq. (4)
Reduction with Sodium Boro hydride
Synthesis of nano particles using the reduction with sodium Boro hydride is another
method which uses various reductants such as sodium boro hydride, hydrogen and
alcohols. Boro hydride reduction of metal ions has been used extensively for the removal
of aqueous ions as well as the production of fine particles of metals and metal borides (19).
The reaction (reduction, nucleation, and growth) takes place inside the droplet, which
controls the final size of the particles. Once the particle inside the droplets attains its full
size, the surfactant molecules attach to the metal surface, thus stabilizing and preventing
further growth. Figure 12 shows the formation of the Nanoparticles.
Figure 12. Formation of Nanoparticles via reduction of metal salt precursors (19)
Eq. (5)
In Equation 5, the reducing agent boro hydride is mixed with the metal precursor salt in
36
the presence of stabilizing agents such as ligands polymers or surfactants). As an
example, the study of Klabunde and coworkers synthesized cobalt nanoparticles with
sizes in the range of 1.8 to 4.4 nm by the reduction of a cobalt salt with NaBH4 in reverse
micelles formed using dido decyldimethylammonium bromide.
From this two methods the method applied for the synthesis of cobalt nanoparticles is the
wetness impregnation method studied by Tingjun Fu et al, which average of the
nanoparticles are of 1.4 nm (30).
1.8 Stabilization of Metal Nanoparticle
The stabilization of nanoparticle is relevant because it can help to obtain uniform nano
particles. This section describes the process of nucleation and growth to control the size
of the nanoparticle and the support used to stabilize the nanoparticle.
1.8.1 Control size of the nanoparticle by nucleation and growth
The report presented by Clemens Burda et al. mention that the preparation of
nanoparticles of different shapes are important when the concept of nucleation and
growth is introduced
(62).
The chemical growth of bulk or nanometer sized materials
inevitably involves the process of precipitation of a solid phase from a solution (62). In the
case of nanoparticles to occur the solution must be supersaturated by directly dissolving
the solute at higher temperature and the cooling to low temperature or by adding the
necessary reactants to produce a saturated solution (90-91). The precipitation stage consists
of nucleation step followed by particles growth stages
(62).
There are three kinds of
nucleation process: homogenous, heterogeneous and secondary nucleation.
Homogenous nucleation occurs in the absence of a solid interface by combining solute
molecules to produce nuclei
(62),
it happens due to the driving force of the
thermodynamics because the supersaturated solution is not stable in energy. The overall
free energy change, ∆ 𝐺, is the sum of the free energy due to the formation of a new
37
volume and the free energy due to the new surface created.
The stabilization of metal nanoparticle can be controlled by nucleation and growth; the
preparation of a highly uniform nano crystal is necessary to induce a single nucleation
event and to prevent additional nucleation during the subsequent growth process (62).
!
∆𝐺 = − ! 𝜋𝑟 ! 𝑘! 𝑇𝑙𝑛 𝑆 + 4𝜋𝑟 ! 𝛾
Eq. (6)
Clement Burda et. al, mention that when S > 1, ∆𝑮 has a positive maximum at a critical
size (r*), this maximum free energy is the activation energy for nucleation. Nuclei larger
than the critical size will further decrease their free energy for growth and form stable
nuclei that grow to form particles. The critical nuclei size r* can be obtained by setting
d∆𝑮/dr=0.
𝑟 ∗ = !!
!!"
Eq. (7)
! !"#(!)
For a given value of S, all particles with r > r* will grow and all particles with r<r* will
dissolve. From the above equation, it follows that the higher the saturation ratio S, the
smaller the critical nuclei size r* is.
Nanoparticles are small and are not thermodynamically stable for crystal growth
kinetically. To finally produce stable nanoparticles, these nanoparticles must be arrested
during the reaction either by adding surface protecting reagents, such as organic ligands
or inorganic capping materials or by placing them in an inert environment such as an
inorganic matrix or polymers (93).
This section describes Jongnam Park studies, which performed the first theoretical studies
on the narrowing of the size distribution during the growth process. Equation 6, gives the
relationship between the monomers supplied to the particles (J), Jongnam Park studies
mention that the “growth by diffusion” model shows that the growth rate of spherical
particles depends on the flux of the monomers supplied to the particles (J) (93).
𝐽=
!!! ! !"
!,!
!"
Eq. (8)
38
“Fick's law Equation. 8 gives the flux J of monomers diffusing through the surface of a
sphere enclosing the particle (D is the diffusion coefficient, C is the concentration, and
x(≥r) is the distance from the center of the particle)”. (93)
!"
𝐽 = 4𝜋𝑟 ! 𝐷 !"
Eq. (9)
“If J is assumed to be constant for x, the integration of C(x) from r to r+δ with respect to
x gives Equation 9” (93).
𝐽 = 4𝜋𝐷 !(!!!)
!
[𝐶 𝑟 + 𝛿 − 𝐶! ] Eq. (10)
“Cs(=C(r)) is the concentration at the surface of the particle. For sufficiently large values
of δ (r≪δ), Equation 9 is reduced to Equation 10, in which Cbulk is the concentration of
the bulk solution”(93).
𝐽 = 4𝜋𝑟𝐷 (𝐶!"#$ − 𝐶! ]
Eq. (11)
Equation 11 follows from Equations (6) and (9).
!"
!"
=
!! !
!
(𝐶!"#$ − 𝐶! )
Eq. (12)
If Cs and Cbulk are constant for all particles, the growth rate of a particle is inversely
proportional to its radius. It is understood that the number of monomers diffused onto the
surface of a particle increases in proportion to the square of its radius, whereas the
volume of a particle consisting of the monomers increases in proportion to the third
power of its radius (93).
This model does not consider the reaction kinetics of crystal growth and it depends on the
particle size. During the growth process, there are two reactions acting in opposition to
each other, namely, precipitation and dissolution [Equation 12] (93).
39
𝑛𝑀
𝑘!
!↔
𝑘!
𝑀!!
Eq. (13)
Ms and Mc refer to monomers in solution and in the crystal, and kp and kd are the reaction
rate constants for precipitation and dissolution, respectively (93).
!
𝐶!,!" = !!
Eq. (14)
!
Finally the equations after doing the analysis are:
!"
𝑟 ∗ = !!! r
Eq. (15)
!
𝜏=
!
! ! ! ! !!!,!"
!! ! !!
!"
𝐾 = !!!
!
!
! !!
𝑡
Eq. (16)
Eq.(17)
Taken in consideration small values of K (≪1) and large values of S (≫1) has a
∗
∗
maximum at 𝑟 ∗ = 𝑟!"#
. For 𝑟 ∗ > 𝑟!"#
, the size dependence of the crystal chemical
potential is relatively small such that the variation of the growth rate with r* mainly
depends on mass-transport effects rather than on kinetic effects. Consequently, the slope
of the graph is negative and a narrowing of the size distribution occurs in this region (the
“focusing” region) (93).
The studies of Sugimoto, Reiss and Talapin reveal two underlying mechanisms for the
control of the size distribution: 1) The “focusing” effect is a kinetically driven process
that actively reduces the variance of the particle size distribution during the growth
process. It works when the growth process is diffusion controlled and the degree of super
saturation is high. 2) Ostwald ripening occurs when the super saturation is low.
Studies from Went Hartmut show that the reactions in the synthesis of nano crystals are
more complicated, for example the precipitation reaction and the dissolution reaction are
40
not reversible. Precursors do not seem to act directly as monomers, but instead undergo
several intermediate reactions before the crystallization (93).
1.8.2 Protection stabilization and Support
In the synthesis of metal nanoparticle stability is a crucial requirement application of the
magnetic nanoparticle, especially with pure metals such us Cobalt, Iron and Nickel
because they are sensitive with air (61). The main reason of the instability is the oxidation
in air, and the susceptibility toward oxidation becomes higher when particles are smaller
(61).
Also, the support function helps the nano catalyst for hydrogen storage.
Hydrogen storage is critical for the improvement of mobile fuel cell. Carbon nanotubes
(CNTs), metal organic frame works (MOFs), activate carbon (AC), carbon nanofibers
(CNFs), ordered mesoporous carbon (OMC), and zeolite, which are highly porous
materials, demonstrate promising hydrogen storage capacities at 77 K by Physisorption
(58-60).
Dillon et al., investigate the first report about nanotubes, which shows that the
uptake of hydrogen could achieve 5 – 10 wt% under room temperature and 300 torr
(57),
and was promising to achieve by U.S. DOE target. Later Chamber et al. reported a
hydrogen storage in graphite of 6.7wt%. This section will mention the types of supports
used for hydrogen storage.
Studies of Yongde Xia. et al. report that an activated porous carbon AX 21 had a capacity
of 0.6wt% at 298 K and 100 bar, and was increased by 1.2 wt% by doping Pt under the
same conditions
(38).
Four broad classes of carbon support must be differentiated,
activated charcoals, chemically activated charcoals, carbon blacks and graphite.
In the case of graphite, report of Strobel mention that graphite is one of the four ordered
carbon allotropes (diamond with sp3 bonding, graphite with sp2 bonding, fullerene with
perturbed sp2 bonding and carbine with sp1 bonding) (23). The characteristic for hydrogen
storage is that is an inexpensive material and is available on an industrial scale. The
hydrogen absorption of graphite is of 5% at 0 oC (23).
41
Hydrogen storage with transition metals and carbon nanotubes are explained in Table 3,
which the highest hydrogen density is with Cobalt (1.51%), Iron (0.75%) and finally
Nickel (0.4%). Also, the highest desorption activation energy after palladium has the
cobalt nano tubes, the desorption activation energy with carbon nano tubes sample has 39
kJ/mol and when it is doped with cobalt the desorption activation energy is of 65 kJ/mol
(95).
Table 3. Hydrogen storage capacity with transitional metals (95)
Samples Desorption activation Hydrogen storage energy (kJ/mol) capacity (wt %) CNT
Co-CNTs
Fe-CNTs
Ni-CNTs
Pd-CNTs
39
65
48
47
71
0.3
1.51
0.75
0.4
7
Comparing these parameters for hydrogen storage in carbon materials, cobalt is the
predominant transitional metal that is used for hydrogen storage. Another aspect to
consider is that cobalt when is doped with magnesium has higher hydrogen storage
capacities as it is shown in table 3.
1.9 Hydrogen Storage in magnesium doped with cobalt and graphite
There are different approaches for hydrogen storage, although the sorption of molecular
H2 in solid-state materials is attractive because of the fast kinetics, excellent cyclabilities
and high adsorption capacity. Research in this field is focus on using catalyst, alloys or
additives to catalyze or destabilize the magnesium hydride system
(32-35),
formation of
complex hydride systems and decreasing the particle size down to 1 nm
(35-36),
can
decrease the H2 release temperature, increase the kinetics and improve the cyclabilities of
hydrides (38). Is relevant to mention that magnesium hydride forms ternary and quaternary
42
hydride structures by reacting with transition metals (Co, Ni, Fe, etc.) and it improves the
kinetics (22), and do not intend to change its thermodynamics. Also, small nanoparticles of
transition metals offer an additional hydrogen sorption mechanism via its active surface
site. (40-41)
Metals can be effective catalyst for the hydride such as palladium
(42).
Doping transition
metal nanoparticles with a certain percentage of carbon nanotubes on the sorption
behavior of MgH2, have been investigated and the addition of carbon nano tubes
promotes hydrogen diffusion in the host metal lattice of MgH2.
(22)
The mechanism of
hydrogen storage using carbon has an interest since carbon materials have a high
volumetric density but still is under research and development. The properties that should
be evaluated are the lightweight, cost and availability, high volumetric and gravimetric
density of hydrogen, fast kinetic, ease of activation, low temperature of dissociation or
decomposition, appropriate thermodynamic properties, long term cycling stability and
high degree of reversibility at room temperature. All these characteristic and properties
are to consider for a technology to storage hydrogen but they are still in studies, and
currently do not exist.
1.9.1
Hydrogen storage in Magnesium
Magnesium Hydride can store ∼ 7.6wt% hydrogen, Table 1 (25) mention that from all the
alternatives of solid state materials magnesium, has a promising future; from that
perspective studies of Aguey Zinsou and Ares Fernandez have established that
magnesium remains a very interesting option because it has high volumetric capacity,
gravimetric capacity, is abundant and non toxic.
Although, the use of magnesium as hydrogen storage remains hindered by the high
temperature (>300 oC) that is needed to reversibly store hydrogen and to release
hydrogen from magnesium hydride (MgH2), and also the slow kinetics for hydrogen
absorption and desorption.
(79)
Studies of Eki J. Setijadi et. have structured magnesium
nano crystalline synthesized by the hydrogenolysis of Grignard reagents and found a
43
storage capacity of 6.8 mass %, hydrogen desorption without catalyst at 250 oC in less
than 2 hours (78), hydrogen desorption for 15 min. There is an improvement in the kinetics
One of the solutions of this issue is found by reducing crystalline grain particle sizes (4,19,
80).
As an example of how the size of the nano particle influenced in the hydrogen
adsorption; 5 nm of magnesium hydride were found to release hydrogen from 85 oC. (81)
For 7 nm magnesium hydride nanoparticles mechano-chemically produced, a small
reduction of 3 kJmol-1 for ∆H and 3 J mol-1 for ∆S
(82).
For controlling the properties of
magnesium when the approach is nano sizing is the synthetic method, and there are only
a few methods reported, which are based on physical vapor deposition
chemical methods
(82),
decomposition of magnesium anthracene
magnesium salts or Grignard reagents
porous support
(87, 88, 83, 89).
(81 - 86)
(85),
(83 – 84).
Mechano-
the reduction of
and melt infiltration or impregnation of a
The report of Eki et. al, studies the synthesized by the
hydrogenolysis of Grignard reagents, and the magnesium hydride used in this experiment
is prepared by Di – n – butyl magnesium (78). The temperature required to decompose the
selected Grignard reagents was investigated by TGA/DSC and evolved gases were
followed by MS
(78).
Figure 13, shows the TGA/DSC curves of Di-n-butylmagnesium
decomposed from 120 oC in a sharp step leading to a mass loss of 81.7% at 280 oC and an
endothermic peak at 260 oC. This corresponds to an almost complete decomposition of
the butyl group as evidenced by the release of 1-butene.
Figure 13. TGA/DSC curves of Di-n-butyl magnesium
44
1.9.2
Hydrogen Storage uptake in carbon nanotubes
Carbon nanotubes have plenty characteristics, because of its high surface area and
abundant pore volume; its porous carbon is considered good absorbent. For example,
Yuda Yurum studies shows that carbon materials tested are up to 5% and 7.5 wt%
hydrogen can be stored in porous carbon and MOFs. Section 1.8.2, discusses the use of
carbon nanotubes as supports of transitional metals.
1.9.3
Mechanism of hydrogen storage using nano carbons
Studies of Michael et al. mention that Van der Walls attractive forces (Physisorption) can
be based by interaction or the overlap of the highest occupied molecular orbital of carbon
with occupied electronic wave function of the hydrogen electron, overcoming the
activation energy barrier for hydrogen dissociation (chemisorption). The Physisorption of
hydrogen limits the hydrogen-to-carbon ratio to less than one hydrogen atom per two
carbon atoms (i.e., 4.2 mass %). While in chemisorption, the ratio of two hydrogen atoms
per one carbon atom is realized as in the case of polyethylene
(43–45).
Physisorbed
hydrogen has a binding energy normally of the order of 0.1 eV, while chemisorbed
hydrogen has C–H covalent bonding, with a binding energy of more than 2-3 eV.(22)
45
1.9.3 Hydrogen Storage in magnesium doped with transition metal.
Figure 13. Shows a concept model of MgH2 (a) plain (b) nano crystalline (c) nano
crystalline doped material (22)
Table 4 shows that using nano transitional metals doped with Magnesium will increase
hydrogen density, and the use of iron has the higher hydrogen density about 5.5% wt.
compare with cobalt that is 4.0% and nickel is 3.6 %. For example, in the case of Iron –
magnesium hydride, Mg2FeH6 are not clear therefore do not form stable binary alloys,
and this metal hydride is difficult to prepare. In a similar way, the synergistic approach of
doping nanoparticles of Fe and Ti with a few mol% of carbon nanotubes (CNTs) on the
sorption behavior of MgH2 has recently been investigated
(22).
The addition of CNT
significantly promotes hydrogen diffusion in the host metal lattice of MgH2 due to the
short pathway length and creation of fast diffusion channels (22).
46
The desorption, temperature for iron (320oC), Cobalt (280 oC) and finally Nickel (280
o
C).
The hydrogenation properties of magnesium hydride with carbon (graphite, CNT, carbon
black), shows temperature absorption, desorption of 300 oC, kinetics of 20 min and
maximum hydrogen storage of 6.2wt%. (39)
Table 4. Hydrogen Storage Properties of some complex Transitional Metal Hydrides
(28)
Formula
Hydrogen anions
Hydrogen Density
Desorption
wt.%
gL-1
temperature (1 bar)
Mg2FeH6
[FeH6]4-
5.5
150
320 ºC
Mg2CoH5
[CoH5]4-
4.5
126
280 ºC
Mg6Co2H11
[CoH4]5-, [CoH5]4-, H-
4.0
97
370 ºC
4.2
106
˃ 480 ºC
3.6
98
280 ºC
α-MgH2
7.7
109
280 ºC
H2(liquid)
100
71
˂ 253 ºC
Ca4Mg4Co3H19
Mg2NiH4
4-
[CoH5] , H
[NiH4]
-
4-
Figure 14, mentions the effect of the grain size on hydrogen absorption, while decreasing
the size of the particles higher absorption of hydrogen appears. The experiment is done at
300 oC, first cycle.
47
Figure 14. Effect of grain size on hydrogen absorption of ball milled magnesium
powder(17)
Figure 15, shows the absorption of hydrogen when the magnesium hydride is alone, then
is doped with 5% CNTs and the last graph when magnesium is doped with 5% of
transition metal. It shows that hydrogen absorption is higher when the magnesium
hydride is doped with CNTs, and almost reaches the theoretical capacity.
48
Figure 15. Hydrogen adsorption of magnesium – based composites for different systems
at 300oC (22)
Figure 16, shows the hydrogen adsorption of magnesium – FeTi-CNTs with the variation
of temperature, and higher adsorptions of hydrogen are at 300 oC. This figure shows the
effect of doping magnesium carbon nano tubes to transition metals.
Figure 16. Hydrogen adsorption with magnesium – FeTi-CNTs (22)
49
Magnesium hydride is doped with cobalt and it is attractive because of their hydrogen
storage characteristics, (4.5 wt% and 4 wt%) for Mg2CoH5 and Mg6Co2H11, respectively.
The study of Gonzales Fernandez, mention that the thermodynamic properties of Mg –
Co mixtures with hydrogen have been investigated, by pressure composition isotherms
measurements. Absorption and desorption PCTs measurements allowed to obtain
formation and decomposition Van’t Hoff diagrams and thermodynamics properties.
Temperature–pressure regions of 210–255 oC and 130 kPa, formed MgH2 in an only
hydride phase and values up to 330 oC and 970 kPa, a mixture of the two ternary hydrides
was found. The time performed at 425 ◦ C and 5.9MPa where absorption took only 3 min
instead of 80–500 h for the first and second route. The effect that the atomic composition
of Mg–Co mixture has on the hydriding reaction remains unknown and should be studied
in future investigations (94).
50
Chapter 2: Experimental Methodology
2.1 Materials and Apparatus
The carbon supports used for Co-based catalyst preparation were activated carbon Darco,
20 – 40 mesh granular supply by Aldrich Chemical Company Inc., high surface area
graphite (HSAG) 500 with a pore size of 3 – 4 nm supply by Tilcam, nitric acid of a
concentration of the 70% supplied by Ajax Finechom Pty. Ltd. Cobalt nitrate crystalline
> 99.0% (KT), produced by Sigma Aldrich; water de ionized is used for the impregnation
of the catalyst in carbon. The materials used are a reflux, heater; stir bars, water bath
heating apparatus, round flask, furnaces, and crucibles.
2.2 Sample preparation
Cobalt nanoparticles were prepared by the method of incipient wetness impregnation.
First, the carbon support was pre treated and its porous sides were activated with nitric
acid. The carbon was refluxed at 80 oC for 4 hours in 28% nitric acid in a stainless steel
water bath heating apparatus.
Secondly, the carbon material was filtered and washed with distilled water until neutral
pH was reached. After, the carbon was dried at 120oC for 10 hours. Nanoparticles with
Co content of 20, 8, 5 and 3 wt.% were prepared by incipient wetness impregnation
method. Aqueous Co(NO3)2 solution was introduced into the carbon support aided by
ultra sonication and stirring process.
51
Ultra sonication and stirring process were beneficial to the dispersion of cobalt particles
into carbon with capillary force to obtain the Co/carbon catalyst with most Co metallic
nanoparticles dispersed inside the carbon. Then, the sample was placed for impregnation,
and dried at 120 oC for 10 h with a heating rate of 0.2 oC/min, with a heat treatment at
200 oC in Ar for 5 h with a flow rate of 19.4 ml/min and a heating rate of 0.67 oC/min and
resulted in Co/Carbon nano catalyst.
Then, a Temperature Programmed Reduction (TPR) analysis was done to the activated
carbon and the carbon HSAG, in which the reduction temperature is found. And, the
samples were reduced with hydrogen at 600 oC for 5 hours with a flow rate of
20.5ml/min and a heating rate of 2oC/min.
The variables modified in the preparation of the catalyst were: time of impregnation from
48h, 20h and 6h at room temperature and the reduction time of 2h, 5h and the
temperature varied of 600 oC and 500 oC. . Figure 17, shows the synthesis of Cobalt
nanoparticles.
After obtaining the preferred size of the nanoparticle of 5 % Co/HSAG; was doped to
Magnesium the procedure was Di-n-butyl magnesium 1 M in heptane, 3.8 mL mixed
with 400 mg Carbon-Co in 50 mL cyclohexane. Then, it was transferred to Parr reactor,
heated to 180 oC with 30 bar H2. It was stirred and let it react for 24 hours. Then, it was
centrifuged and washed with cyclohexane twice, once with THF. The solids were
collected after dried under vacuum for solvent removal. Characterized with a tandem
TGA-DSC-MS. Figure 17 shows the synthesis of cobalt nanoparticles.
Figure 17. Synthesis of Cobalt Nanoparticles
52
2.3 Analysis
The analysis applied for cobalt/HSAG 500 were Temperature Programmed Reduction
(TPR), X- ray diffraction (XRD), Transmission electron microscope (TEM).
The method was consider and reproduced by the report of Tingjun Fu, Yunhui Jiang, Jing
Lv, Zhenhua Li was reproduced in active carbon and carbon HSAG 500.
The graphs of the size distribution were obtained, measuring the imagines and each
nanoparticle with the use of a ruler in a different range of capture like 20 nm, 10 nm and
50 nm. And a table in excel was elaborated to count the nanoparticles and write next to
them the size of the nanoparticle in cm and in nm that corresponds each imagine. To
obtain the graph statistic analysis was performance, to obtain the frequency of data, the
total distance was obtained, which is to subtract the highest value with the lowest value,
the type of range, which indicates the number of columns that should be constructed in
each graph, the number of columns in the table are already taken from a table that is
already done. Then, the election of the size of the range in which each value should be
consider is equal to divide the total distance to the number of class. After that the graph is
elaborated first establishing the range of the size, starting with the first measure add the
amplitude, count the total particles that are in that range, and the graph is draw in which
the frequency is in function in the range of the nanoparticles. Also the mean is obtained,
the variance and the standard deviation is obtained. Another way to obtain the
distribution of the nanoparticles is applying Linest analysis, but in this case is not
consider and the analysis is performance with statistic. The error that could occur because
of the measure of the size of the nanoparticle could be because of the ruler which
variance is of ± 1 cm and the operator how she considers the sphere of the nanoparticle
and the diameter of the nanoparticle, how she counts. In total around fifty nanoparticles
were counted and the distance between each were about 0.5 to 1 cm, depending of the
TEM imagines.
53
2.4 Results
Synthesis of cobalt nanoparticles, were prepared by the method of incipient wetness
impregnation, the procedure applied corresponds to the studies of Tinjun Fu et. al, the
experiment was done with 20 hours of impregnation, 5 hours of reduction at 600 oC. As
shown in figure 18, TPR experiments were performed to determine the reduction
behavior of the catalyst; 20 % of Cobalt was impregnated in activated carbon supplied by
the laboratory. TPR clearly shows the tendency of three partly overlapped peaks for a
carbon nanotube which temperature of reduction is of 600oC (30). The first peak at around
180 – 250 oC was attributed to the reduction of Co3O4 to CoO, the second reduction peak
at around of 250 – 350 oC (75) attributes to the reduction of CoO to Coo occurs (76). Figure
19 shows XRD patterns of Cobalt impregnated in Activated Carbon, which shows peaks
corresponding to the graphite 2h(JCPDS 41 – 1487) of 26,54o(29) in the case of cobalt
peaks of 44,31o, 55,61 o, 75,94 o, correspond to a cubic cobalt structure. Figure 20 shows
the results of TEM; using activated carbon the most frequently particle size distribution is
TCD signal, (a.u) of 3 to 10 nanometer, although the sample has a very dispersed sized of the nanoparticles.
0 100 200 300 400 500 600 700 800 Temperature (oC ) Figure 18. TPR profiles of the Co/Activated Carbon
54
900 Figure 19. XRD patterns of Co/Activated Carbon
`
e
Figure 20. a, b, c and d) TEM images of 20% Co/Activated Carbon e) Particle size
distribution 20%Co/Activated Carbon.
55
Figure 20.e, shows the average of the size particle distribution, with an average of
particle of 9.7 nm, 𝜎 = ±4.8 nm.
Another sample was prepared using the same method, but in this case High surface area
graphite 500 was used, figure 21 shows the TEM imagines of the graphite applied in the
experiment. The porous size of the carbon is 3 – 4 nm. Figure 22 shows the XRD pattern
corresponding to the high surface graphite HSAG 500 with a peak corresponding to the
graphite 2h(JCPDS 41 – 1487) of 26,54o(29). The study of Guilei Sun and et al describes
the XRD patterns of natural graphite and are presented in Appendice 1, and correspond to
the same structure of the HSAG 500.
Figure 21. TEM imagines of HSAG
Figure 22. XRD patterns of HSAG 500
56
The HSAG 500 described was used to impregnated cobalt, and the first sample was
elaborated with 20% of cobalt. Figure 23 shows TEM imagines of 20% of Cobalt in
HSAG 500. This sample was prepared with the same procedure from the other sample
2.16 grams of HSAG 500 were pre treated with nitric acid for 4 hours with a reflux in a
temperature of 80 oC, the sample was placed for 20 hours at room temperature, drying at
120 oC for 10 hours, and heat treatment at 200 oC in Argon for 5 hours and hydrogen
reduction at 600 oC for 5 hours. And resulted in one nano catalyst. The size of the nano
catalyst has a very distributed sized and the most frequency sized varied in the ranged
from 9 – 11 nm.
57
f
Figure 23. a,b,c,d and e) TEM images of 20% Co/HSAG f) Particle size distribution of
20%Co/ HSAG.
The sample of 20% Co/HSAG has the same method of the other sample, and the EDX
experiments were performed, in which cobalt metallic was found as figure 24 shows the
peaks of cobalt.
Figure 20.f, shows the average of the size particle distribution, with an average of particle
of 37.3 nm, 𝜎 = ±31.09 nm.
58
cps/ev
keV
Figure 24. EDX 20% Co/HSAG
As figure 25 shows, TPR experiments of three partly overlapped peaks for a carbon
nanotube which temperature of reduction is of 600oC
(30).
The first peak at around 180 –
250 oC was attributed to the reduction of Co3O4 to CoO, the second reduction peak at
around of 250 – 350 oC
(75)
attributes to the reduction of CoO to Coo occurs
(76).
The
experiment was performance at the same conditions.
As figure 26 shows the XRD patterns, of 20% Co/HSAG, which shows peaks
corresponding to the graphite 2h(JCPDS 41 – 1487) of 26,54o(29) in the case of cobalt
peaks of 44,31o, 55,61 o, 75,94 o.
59
0.45 0.4 TCD Signal (a.u) 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 0 100 200 300 400 500 o
Temperature ( C) 600 Figure 25. TPR of 20% Co/HSAG
Figure 26. XRD 20%Co/HSAG
60
700 800 Figure 27 shows the TEM imagines of the impregnation of 8%Co in high surface area
graphite 500, which synthesis was done for 20h of impregnation, 5 hours of reduction and
a size distribution of the nanoparticle that is demonstrated in Figure 28, which shows that
the predominant nanoparticles are in the range of 1 – 3 nm, the mean of the nanoparticles
is of 7.6 nm and 𝜎 = ±5.7 nm.
. And figure 29, shows the XRD patterns, of 8% Co/HSAG, which shows peaks
corresponding to the graphite 2h(JCPDS 41 – 1487) of 26,54o(29) in the case of cobalt
peaks of 44,31o, 55,61 o, 75,94 o. In this case at 8% Cobalt Imagines of TEM shows a
homogenous distribution of the nano particles impregnated in high surface area graphite.
Figure 27. TEM images of 8% Co/HSAG particle size distribution.
61
30 Number of nano particles 25 20 15 10 5 0 1 -­‐3 4 -­‐ 6 10 -­‐ 12 16-­‐18 Size Nano particle (nm) Figure 28. Particle size distribution of 8% Co/HSAG 20 h of Impregnation
Figure 29. XRD of 8 % Co/HSAG
62
Figure 30, shows the TEM imagines of 5% cobalt impregnated in HSAG, the same
method is used as the other ones, 20 hours of impregnation and 5 hours of reduction is
needed. Figure 31, shows the size distribution of the nano particles for 5% Co/HSAG,
average of the size of nano particle distribution is of 10.25 nm, 𝜎 = ±5.1 nm, with a
predominant range of 1 – 3 nm.
Figure 30. TEM images of 5% Co/HSAG
63
70 Number of Nanoparticle 60 50 40 30 20 10 0 1-­‐ 6 7-­‐12 13-­‐18 19 -­‐ 24 31 -­‐ 36 Size of the nanoparticle Figure 31. Particle size distribution of 5% Co/HSAG 20 h of Impregnation
Figure 32, shows the TEM imagines of an upload of 3% Co/HSAG figure 32.a, b, c
shows the imagines of 20h impregnation, 5h reduction at 600 oC. As shows in figure 33
the size distribution of 3% Co/HSAG, with a predominant range of nano particles of 3 – 5
nm. Figure 33, shows the size of the nanoparticle with a predominant range of f the mean
of the nanoparticles is of 8.15 nm and 𝜎 = ±3.38 nm.
Figure 32. TEM images of 3% Co/HSAG
64
50 45 Number of Particles 40 35 30 25 20 15 10 5 0 3 -­‐ 5 6 -­‐ 8 9 -­‐ 11 12 -­‐ 14 15 -­‐ 17 Size of the particle Figure 33. Particle size distribution of 3% Co/HSAG 20 h of Impregnation
Table 5. Summary of the different uploads of Cobalt
% Co Support 20 Activated Carbon 20 5 600 TEM size variation, nm 3 – 10 20 HGSA 500 20 5 600 9 – 11 8 5 3 HGSA 500 HGSA 500 HGSA 500 20 20 20 5 5 5 600 600 600 1 – 3 1 – 6 3 – 5 Impregnation Reduction Reduction Time Time Temperature Table 6, summarize the results taken from the first part in which the upload of cobalt
was the variable; impregnation, reduction time and reduction temperature remain
constant. At high concentration of cobalt the size of the particles increases and when
the concentration of cobalt decreases the size is smaller.
65
The next Synthesis of nano particles were for 3 and 5 % of Cobalt impregnated in
High Surface Area Carbon. The variables to change in the section was impregnation
time, reduction time and impregnation temperature. And these variables were
evaluated and the size of the nano particle was determined. For example, time of
impregnation 6h and 48h were changed; temperature and time of reduction were
changed from 500 to 600 oC and 5h to 2h respectively.
Figure 34. TEM imagines for 5% Cobalt in carbon HSGA 500 (a, b, c) represents 600oC;
(d,e,f ) represents 500 oC 6h of impregnation and 2 h reduction
66
Figure 34, shows the TEM imagines of 5%Co/HSAG, with different captions of 100nm,
50nm and 20 nm.
Figure 35, shows the graphs of the number of particles in function with the size of the
nanoparticles of a sample of 5% Cobalt/HSAG with 6h impregnation, 2h of reduction.
Figure 34.a. represents the size of nanoparticle at 600 oC, in which the most numerous
particles are of the size of 0.5 to 1 nm.
Even though in the graph figure 35 a. shows that the range of the nanoparticle starts with
cero, actually the measurement was done to the particles to start in 0.5 nm.
The statistic analysis that were consider in this case is for 5 % Co, figure 35 a. the mean
of the nanoparticle is of 3.63nm, variance of 6.79 nm and the standard deviation of
2.61nm.
Figure 35 b. the statistic analysis applied were mean 2.83 nm, variance 4.78 nm and the
standard deviation is 2.19 nm. When the synthesis was at 3 % Co, the mean of the
nanoparticle is 3.25 nm, standard deviation of 2.28 nm and when the sample had 48hours
of impregnation the mean is 2.7 nm, variance 1.79 nm and the standard deviation of 2.19
nm.
67
Figure 35. Particle size distribution a) 600 oC b) 500oC, with 6h of impregnation and 2 h
reduction and c) EDX of Cobalt for 5% Cobalt in carbon HSGA 500
68
Figure 36, shows the TEM imagines for 3% Cobalt/HSAG 500, and the temperature
of reduction was changed, figure 35.a represents 48h of impregnation and 2 hours of
reduction at 600 oC, and figure 35.b represents 48h of impregnation and 2 hours of
reduction at 500oC.
Figure 36. TEM imagines of Nanoparticle a) 600 oC b) 500 oC with 3% Cobalt, 48h
impregnation and 2h reduction
69
Figure 37. Particle size distribution a) 600 oC b) 500 oC with 3% Cobalt, 48h
impregnation and 2h reduction
70
Figure 38 shows different TEM imagines of 5% Co/HSAG, 6 hour of impregnation,
2h of reduction at 600 oC and Figure 38, shows the size of the nanoparticles which,
shows that the relevant size of the nanoparticle is of 1 – 12 nm.
Figure 38. TEM imagines of 5%Co/HSAG, 500 C, 6h impregnation and 5h reduction
Figure 39, shows the size of the nanoparticles that corresponds to 5%Co/HSAG, and the
particles for this concentration is of 1 – 12 nm.
71
Figure 38. Size Nanoparticles 5%Co/HSAG
Figure 39. Particle size distribution of 5% Cobalt/ HSAG, 500 C, 6h impregnation, 5h
reduction
72
Figure 40. TEM imagines for 3% Cobalt/HSAG a, b, c) 500 oC, 6h impregnation, d, e,f)
600oC 6h impregnation, g, h, i) 500 oC, 48h impregnation
73
Figure 41. Particle size distribution of 3% Cobalt/HSAG a) 500 C, 6h impregnation, b)
600C 6h impregnation, c) 500 C, 48h impregnation
74
Table 6. Summary size of the nanoparticles with the variation of impregnation,
reduction time and reduction temperature.
% Co Support 5 HGSA 500 6 2 600 TEM size variation, nm 0.5 – 1 5 HGSA 500 6 2 500 2 – 3 3 3 5 3 3 3 HGSA 500 HGSA 500 HGSA 500 HGSA 500 HGSA 500 HGSA 500 48 48 48 6 6 48 2 2 5 5 5 5 600 500 500 500 600 500 1 – 2 1 – 2 1 -­‐ 12 2 – 3 5 – 7 3 -­‐ 15 Impregnation Reduction Reduction Time Time Temperature Impregnation, reduction time and temperature of reduction had been changed and the
next graphs will show how the size of the nanoparticle varied.
Figure 42 shows how the particle size distribution changes using two different types of
carbon, High Surface Area Graphite and activated Carbon. Most of the nanoparticles are
in the range of 3 – 10 nm and the HSAG is more predominant in this range than the
activated carbon, is relevant to mention that the porous of the particle is of 3 – 4 nm and
in the case of the activated carbon there is no porous size. The concentration of Cobalt is
of 20% impregnated in the carbon material, the impregnation time is of 20 hours and the
reduction time is 5 hours.
75
Figure 42. Particle size distribution compare with the type of carbon
Figures 43, shows a particle size distribution of different uploads of cobalt such us 20%,
8%, 5%, and 3% of cobalt. The size distribution of the nano particles varied from 1 to 36
nm. The predominant size of nanoparticles are from 1 to 6 nm, and at 5 % Cobalt most of
the nanoparticles are more homogenous distributed.
Figure 43. Particle size distribution with different uploads of Cobalt
76
Figure 44. Number of particles a) 5%Co/HSAG and 6h impregnation b) 3% Co/HSAG
and 48h impregnation
Figure 44 a, shows the size distribution of nanoparticles at 5 % Cobalt impregnated in
HSAG, at 6 hours of impregnation and the temperature of reduction was the variable of
500 oC and 600oC, the sized of the nanoparticles varied from 0.5 nm to 11 nm. As it
shows in figure 44b the range of size distribution of the nanoparticles are predominant
size of the nanoparticle is from 2 to 3 nm at 500 oC, in the case of 3% Cobalt the
predominant size of the nanoparticles were of 1 – 2 nm, and the most predominant
nanoparticle were at 500 oC.
77
Figure 45. Particle size distribution varying impregnation, reduction time and
concentration a) 3%Co/HSAG, 5 h reduction 600 oC b) 5%Co/HSAG, 6h impregnation
500 oC
78
Figure 46. TEM imagines of Magnesium doped with 5%Co/HSAG
Figure 46, shows TEM imagines of magnesium doped in 5% cobalt nanoparticles
impregnated in HSAG. Figure 48 shows EDX of cobalt/HSAG with magnesium. Figure
47 shows the XRD of magnesium doped with cobalt impregnated in high surface area
graphite.
79
Cobalt""
Carbon"
Magnesium"
Intensity((a.u)(
Mg.Co.HSAG"
0"
10"
20"
30"
40"
50"
60"
70"
80"
Figure 47. XRD of Mg doped with Co/HSAG
Figure 48. EDX of the sample Co/HSAG with Magnesium
80
90"
100"
Figure 49. TGA
Figure 49, shows the TGA for doping magnesium in cobalt/HSAG although the reducing
of the mass even though the graph does not shows a scale from 0 – 100 %, but while
reducing the mass, the temperature reaches the 500 oC, which almost is the temperature
of reduction of the nano catalyst.
On the other hand, figure 50, shows the MS of the sample and there is not increase on the
desorption of hydrogen because the peaks shows a temperature of 400 oC and if it is
compare with the studies of Eki et al, the sample of d – n – butyl Mg the hydrogen
desorption is over the 400 oC, and the use of Co/HSAG as a nano catalyst decrease the
temperature of 50 oC, it should be good to compare the curve of desorption of hydrogen
with the other a higher concentration of cobalt.
81
Figure 50. MS
2.6 Conclusion
In conclusion, the synthesis of cobalt nano particles for catalysis in hydrogen storage
presents, produces nano particles in the range of 0.5 nm to 1 nm, the upload of cobalt to
synthesis the nanoparticles is of 5%Co. First, the HSAG 500 is pre treated with nitric acid
in a ratio of 1 gram of HSAG with 7.5 ml of nitric acid, for four hours at a temperature of
80 oC, dried at 120 oC for 10 hours, the sample placed at room temperature for 6 hours,
dried at 120 oC for 10 hours. After, Argon treatment of 19.4 ml/min for five hours and
finally hydrogen reduction of the nano catalyst at 600 oC for two hours at a 20.5ml/min.
The analysis of XRD shows the presence of graphite in a peak of 26,54o(29) in the case of
cobalt peaks of 44,31o, 55,61 o, 75,94 o, correspond to a cubic cobalt structure.
82
The results of doped cobalt/HSAG 500 in magnesium did not improve the kinetics of the
nano catalyst, because the results of XRD did not show any cobalt – magnesium phase in
the graphic, most of the peaks represent cobalt, magnesium and High Surface area
graphite. This can be produced because in the process of doping magnesium into the
cobalt/HSAG, there was not a good growth. In the case of the Thermo gravimetric
analysis while reducing the mass consumed the temperature reaches the 500oC in the
other hand the temperature reaches 250 oC (78).
2.7 Discussion
The synthesis of cobalt nanoparticles, are described in this project. Is important to
mention that all the steps are important in the preparation of nano catalyst and have an
effect in the synthesis of cobalt nanoparticles. Starting from the initial step which is the
pre treatment of the carbon with nitric acid, this step adequate the carbon or is called also
as the purification process
(64)
studies of Nadia F. Andrade mentions that the majority of
methods used for producing carbon nanotubes have impurities, these contaminants are the
residual catalytic metals and byproducts in the form of poly aromatic compounds,
amorphous carbon and carbon nanoparticles converted into graphite, therefore a
purification process is important which, helps to eliminate this undesired species
(64),
which damage the carbon nano tubes.
An oxidative treatment is often used to remove these impurities
(64),
studies of
Bouleghlimat Emir, Davis Philip et al, discuss the effect of nitric acid which mention that
acid helps to remove unwanted contaminants such as sodium, potassium, silica, calcium
and other substances that were accumulate on the carbon surface that appear during
preparation and introduce hydrophilic functional groups that changes the carbon to the
adsorption of the active component and the reaction solvent. (19, 20, 21)
Studies of Wang and Lu mention that the use of an acid wash significantly increased the
dispersion of transitional metals on carbon and contribute to the formation of acidic
functional groups that could react with the transitional salt (21).
83
The most common method used to remove impurities is submitting the carbon nano tubes
sample to a reflux process using nitric acid
(65 – 67).
This step creates functional groups
containing oxygen, especially hydroxyl, carbonyl, and others on the ends and different
positions along the carbon
(69 – 70).
The density and location of these groups are strongly
dependent on the oxidation conditions and carbon are used as starting anchoring points
for further functionalization (70)
Results of EDX and XRD, show the presence of cobalt in the sample, and figure 36
shows the presence of sodium, and the cause will be that carbon was not pre treated
correctly.
The pre treatment of carbon is affected also by temperature and time of reflux. Studies of
Nadia F et. al mention that many variables influence on the final product of the reaction,
like the type of reagents, concentration, exposure time, methods of treatment, type of
carbon and the heating temperature. This experiment performance the report of Tingjun
Fu et. al which treats the carbon at 80 oC for 4 hours with 28 % Nitric Acid.
In the case of Filtration, the solution is washed until it reaches the pH of the deionized
water, the thermal treatment occurs and the sample is dried at a temperature of 120 oC.
The method used to deposit cobalt in an active phase is impregnation, the study of Andrei
Y. Khodakov et. al mentions that impregnation is a method of cobalt deposition on
porous support, the dry support is in contact with a solution that contains the precursor
(71),
cobalt nitrate. Section 2.4 shows the results of the experiment, the concentration of
cobalt has an influence in the size distribution of the nanoparticles, is important to
mention that the maximum loading is limited by the solubility of the precursor solution
(72).
The experiment was reproduced in the same conditions as the report done by Tingjun Fu
which conditions were 20%Co, 20 h of impregnation and the results were un
homogenous particles, the most predominant particles were in a range of 9 – 11 nm and
the solution was concentrated. Although, TPR results shows that the curve clearly follows
84
the tendency of the three partly overlapped peaks for a carbon nanotube which
temperature of reduction is of 600oC.
The first peak, shows that in a temperature of 180 – 250 oC the conversion of Co3O4 to
CoO occurs, the second reduction peak at around of 250 – 350 oC(75) the conversion of
CoO to Coo occurs (76). The Results of TPR follows the tendency of the curve reported in
the studies of Tingjun Fu et al.
XRD patterns of carbon material and cobalt, shows that, high surface graphite HSAG 500
with a peak corresponding of 26,54o(29) corresponds to the graphite 2h(JCPDS 41 – 1487).
The peaks of cobalt correspond to 44,31o, 55,61 o, 75,94 o, of a cubic cobalt. For 20% of
cobalt, size of nanoparticles varied from 3 nm to 20 nm with a predominant size range of
9 – 11 nm, the HSAG 500 has a pore size of 3 – 4 nm, BET surface area 506 (m2/g), a
pore volume of 0.82 (cm3/g), although in the study of Tingjun Fu, with the conditions
detailed before the nanoparticles have a size distribution of 1.4nm with a carbon that has
BET surface area 854 (m2/g), a pore volume of 0.44 (cm3/g). The concentration of cobalt
nitrated was the variable, because the sample didn’t have a homogenous particle size
distribution and studies mentioned that in wetness impregnation method, the volume of
the solution should be just sufficient to fill the pores and wet the outside of the particles,
and this volume had been determined with preliminary test on aliquot samples (73).
Samples were prepared at different concentration of cobalt such us 8%, 5% and 3% of
Cobalt. An appropriate concentration is equal or slightly less than the pore volume of the
support, when the upload decreases for example to 5 %Co the size of the nanoparticles
varied from 1 nm to 36 nm, with a predominant size range of 1 to 6 nm, with five hours
of reduction, 20 hours of impregnation time.
Impregnation time is another variable to consider and the time varied is 48 hours, 20
hours and 6 hours of impregnation for the concentration of 3%, and 5% of Cobalt. The
size of the nanoparticles for a 3% of Cobalt shows a size range of 1nm to 8 nm for 48
hours of impregnation, in the case of 6 hours of impregnation the size of the nanoparticles
varied from 1 nm to 5 nm.
85
After impregnation the next step is drying, and it is an important step in catalyst since it
can affect the distribution of the active species, during drying the solution in the pores
will become oversaturated and precipitation takes place
(74).
Irregular and uneven
concentration distribution can be performed if drying is not done exactly
(73).
Heating
rate, final temperature, time of treatment and type of atmosphere can influence the drying
process
(74).
Thermal treatments may take the form of low temperature drying operations
(up to 150 C) used to remove water, and some decomposition of species such as
chloroplatinic acid is known to occur within this temperature. (40)
Also, studies of Acres mention that after impregnation catalyst needs a thermal treatment
or a reduction stage to render the adsorbed metal species active. The first thermal
treatment is of low temperatures and is drying operations (up to 150 oC) the main purpose
is to remove water. The time of drying is variable like 10 hours of drying is done at 120
o
C.
After drying the next step is hydrogen reduction, in this case time and temperature varied
for the reduction 2 hours, 5 hours are applied and temperature of 500 oC and 600 oC
varied. Studies mention that reduction in hydrogen at 400 oC a highly dispersed,
nanoparticles are formed
(48).
And TEM imagines show that 2 hours of reduction
homogenous nano particles are formed and the size of the nano particle varied from 1 nm
to 7 nm, with a reduction temperature of 600 oC.
From this section samples with 5% of Cobalt impregnated in HSAG 500, were prepared,
at 2 hours of reduction time with a temperature of 600oC, and doped with magnesium.
Results of these test shows, no improvements in the kinetic of magnesium doped with
5%Co/HSAG. XRD patterns, of the sample shows, before doping the sample have cobalt
and graphite; peaks of 26,54o(29) correspond to the graphite 2h(JCPDS 41 – 1487). Peaks
of 44,31o, 55,61 o, 75,94 o, corresponds of a cubic cobalt.
Analysis of XRD shows that before doping the sample has cobalt and graphite, and after
doping the sample XRD results shows the presence of magnesium, cobalt, graphite, but
does not shows the phase of magnesium – cobalt. This gives a result of the TGA because
86
while decreasing the mass of consumption the temperature decreases until it reaches the
500 oC. Studies of Eki et al. mention that the percentage of mass loss decrease until it
reaches the temperature of 260 oC
(78).
In this experiment the temperature increases until
the temperature in which cobalt is reduced.
This could be because there were not enough Co/HSAG in the sample or instead doping
Co/HSAG in magnesium needs a higher temperature, studies of Michael et al mention
that for example doping the transitional metals Fe-CNTs with magnesium was produced
at 300 oC, for one cycle, and the use of nano catalyst improve the kinetics of absorption
of hydrogen.
As a future work I will suggest the synthesis of homogenous cobalt nanoparticles. And
consider the mechanical milling of MgH2, and the size of the nanoparticle of the
magnesium. Therefore, the nucleation and growth of the nano catalyst with magnesium
should be consider since the preparation of nano structures of different shapes requires a
stable nucleation and particle growth. There are three kinds of nucleation processes and
the supersaturated solution is not stable in energy, and the phase of Co/HSAGMagnesium can be form. Also, studies of Gonzalez Fernandez et al, mention that Mg2Co
and Mg3Co have not seen as a stable phase since they present high reversibility. The
results of TGA show a temperature in which the cobalt impregnated in HSAG is reduced,
and the catalyst is formed. The process of doping magnesium in Co/HSAG 500 should be
considered. And the temperature of reduction of the cobalt impregnated in HSAG 500
should be considered.
87
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94
Appendix 1 – Abbreviation
H2
Hydrogen
H*
Hydrogen atom
H+
Proton
H-
Hydrogen Anion
Peq
Equilibrium Pressure
PCI
Pressure Composition Isotherm
Tc
Critical Temperature
J
Flux of the nanoparticle
D
Diffusion coefficient
C
Concentration
X
Distance from the center of the particle
Cbulk
Concentration of the bulk solution
MPa
Mega Pascale
Kg/L Kilogram/ liters
V
Molecular volume of the precipitate
r
Radius of nuclei
kB
Boltzmann Constant
S
Saturation Ratio
𝛿
Surface free energy per unit surface area
95
Appendix 2 – Equipment used for Preparation of the Sample
FURNACE: Nabertherm Range: (30 – 30000C)
1. Set the program of the furnace Nabertherm.
2. Click the narrow to set the temperature.
3. Click the narrow to set the time. (This procedure, repeat until I have the curve of
my temperature and time done).
4. Click the bottom save.
5. Click the bottom that has the P option and choose a program to start it can be
selected P 1, P2, P3, etc.
6. Click the bottom with the saved icon.
7. Now it is programmed
FURNACE: Shinko (Argon flow)
1. Open the furnace put the sample in the middle of the furnace
2. Close the furnace
3. Set the program, set the temperature around 120 oC for around 4 hours, set the
flow of Argon of 12.4
4. Open the valves of Argon turn on all the electrical equipment.
FURNACE: Shinko (Hydrogen Reduction)
1.
2.
3.
4.
Turn on the equipment, consider the electrical part.
Open the valves of hydrogen.
Set the flow of hydrogen around 20.
Program the temperatures, 600oC around 5 hours.
96
TPR
Setting the instrument:
a.
b.
c.
d.
e.
Take off U – tube
Put quartz – wool in the big barrel
Pour sample over the quartz wool (around 0.1g) and remember the exact amount.
Put the ferrules and o - rings.
Screw the U – tube into the equipment finger tight (remember the ferrules need to
match)
f. Close the furnace part
Setting the program
a.
b.
c.
d.
e.
f.
g.
File, open sample info.
Filename: keep the name and yes to create.
Enter the mass and add comments if any
ANALYSIS conditions: load LANI5TPR, find.
REPORT options click the TCD vs. TEMPERATURE option
Analysis Conditions: La Ni5TPR
CHECK GASES: unit 1, clicks gas selection. Remember: Argon for carrier and
preparation, hydrogen for the loop – when changing, just open and plug the ones I
need + keep the other there so people keeps track.
h. Open the gas valves.
i. Unit 1, start analysis, select file and next.
j. 2 hours afterwards put iso – propanol cold trap, ¾ of the bottle for the
isopropanol, add hydrogen, get slurry, put in pedestal and put glove to isolate.
Unloading and cleaning
a. Put gloves on and get the U – tube down
b. Take the wire and put wool out. Put the waste in the little plastic thing near the
unit.
c. Rinse the tube and submerge it into water in ultra sonic bath for 10 minutes
d. Rinse with ultra pure water
e. Keep in mind always to manipulate the U tube with gloves on
97
XRD
1. Setting the Analysis
a. Check the light in the shutter. Off, okay.
b. Open and place the sample in magazine A, place 1.
c. Check the mask and aperture slot – in my program, I use the 10” (black stripe)
and 1” mask – and change if needed. Close door.
d. Select my program – keep in mind that it is saved in the ABSOLUTE or
something folder.
e. KEEP log of the conditions (especially when using the information to compare
support, pre reduction, after, spent, etc).
f. After finishing, copy the recently acquired data into the network and check it in
the other computers.
g. Set the power back to 30 – 10 and log off.
98
Appendix 3 – Parameters of natural Graphite
Table 7. Parameters of natural graphite XRD (29)
Sample Type Natural graphite Detonation soot 2𝜽(o) 26.54 26.48 D 002 (nm) 0.34 0.34 B 002 (x10-­‐3 nm) 3.57 9.67 D (nm) 39.95 14.73 Figure 17. XRD Patterns of natural graphite and detonation soot (29)
99
100