Fachhochschule Köln Cologne University of Applied Sciences

SHEONGWEI NG

REVIEW OF CARBON NANOTUBE APPLICATIONS, SYNTHESIS

METHODS AND PROCESSES FOR MASS PRODUCTION

Fachhochschule Köln Cologne University of Applied Sciences

Referent: Thomas Rieckmann, Prof. Dr.-Ing.

Korreferent:

Fakultät für Anlagen-, Energie- und Maschinensysteme

Institut für Anlagen und Verfahrenstechnik

MASTERPROJEKT 2

Review of Carbon Nanotube Applications,

Synthesis Methods and Processes for Mass

Production

von SheongWei NG

Köln, 30.09.2015

Mat.-Nr.: 11107729

I

Summary

Numerous of works were carried out to have better understanding of the new carbon allotrope,

carbon nanotubes (CNTs) since their discovery in 1991. As the result of the works, CNTs have been

proven to possess high thermal and electrical conductivity, high mechanical strength, low density

and good chemical and environmental stabilities. These remarkable properties make them a

cutting edge material, which play an importance role in the society.

CNTs can already be found in our daily life products such as sport equipment, conductive

production, automotive parts and plastic reinforcement despite their production as bulk

unorganized architecture nanomaterials. So it can be said that CNTs have not yet fulfilled their

potential for the commercial products, which was tested and proven in the lab. Efforts are still

being committed to produce CNTs with desired properties and organized architecture at industrial

scale. Apart from that, research is still ongoing to explode the application of CNTs in other

technological areas such as catalyst and catalyst support, energy production and storage, and

medicine. And those works have showed promising results for the replacement of conventional

materials with CNTs.

Different methods of CNT synthesis was introduced for the past two decades, but only three are

being widely used, specifically arc discharge, laser ablation and chemical vapor deposition (CVD).

Modification was done to further improve their yield, quality, energy efficiency as well as

construction and operation cost. Both the arc discharge and laser ablation generally require high

energy input for the production, but they produce high quality of CNTs, which are crucial for the

research. The major drawbacks for their application in industrial are the low energy efficiency and

batch type nature. On the other hand, CVD methods can be operated continuously at relative low

temperature and moderate pressure. Beside that the diameter, length and alignment of the CNTs

produced by CVD methods can be easily controlled through variation of synthesis conditions.

Those advantages make CVD methods best-suited method for large-scale production of CNTs.

Scaling up the lab scale equipment to industrial scale apparatus for the production of CNTs had

been a problem. But the problem was addressed by many researchers and several processes were

purposed and developed for large-scale production. Notably, high-pressure carbon monoxide

(HiPco) process, cobalt-molybdenum catalytic (CoMoCAT) process, Endo’s process, multiwall

nanotube process and Baytube process are those CVD processes currently used in industrial for

commercial purpose. Reaction temperature, pressure, catalyst size, morphology and composition,

reactant concentration and flow rate, type of carbon feedstock along with type of reactor used

are the essential considerations, which have to be made for the implication of those processes for

sustainable manner of high yield, selective and low cost production of CNTs in industrial.

II

Content

Content 1 Introduction ..................................................................................................................................... 1

2 Type and growth mechanism of carbon nanotubes ....................................................................... 3

2.1 Single-walled carbon nanotubes (SWNTs) ............................................................................... 3

2.2 Multi-walled carbon nanotubes (MWNTs) ............................................................................... 4

2.3 Growth mechanism of carbon nanotubes................................................................................ 5

3 Applications of carbon nanotubes .................................................................................................. 6

3.1 Composite materials ................................................................................................................ 6

3.2 Catalyst and catalyst support ................................................................................................... 7

3.3 Energy storage .......................................................................................................................... 8

3.4 Environment ............................................................................................................................. 9

4 Synthesis methods ........................................................................................................................ 10

4.1 Physical process ...................................................................................................................... 10

4.1.1 Arc discharge ................................................................................................................... 10

4.1.2 Laser ablation .................................................................................................................. 11

4.2 Chemical process .................................................................................................................... 12

4.2.1 Chemical vapor deposition (CVD) .................................................................................... 12

4.2.2 Modification of CVD ........................................................................................................ 13

4.3 Comparison between the physical and chemical process ..................................................... 14

5 Process for mass production ......................................................................................................... 15

5.1 High-pressure carbon monoxide (HiPco) process .................................................................. 15

5.2 Cobalt-molybdenum catalytic (CoMoCAT) process................................................................ 18

5.3 Endo’s catalytic chemical vapor decomposition (CCVD) ........................................................ 20

6 Conclusion ..................................................................................................................................... 22

Literature and Reference ................................................................................................................. 23

III

Symbols and indices

Al2O3 Aluminum oxide

Ar Argon

C2H2 acetylene

C2H4 Ethylene

C2H6 Ethane

C4H10 Butane

C4H6 Butadiene

CH4 Methane

CNTs Carbon nanotubes

CO Carbo monoxide

Co Cobalt

CO2 Carbon dioxide

CoMoCAT Cobalt-molybdenum catalyst

Cu Copper

Fe Iron

Fe(CO)5 Iron pentacarbonyl

H2 Hydrogen

H2S Hydrogen sulfide

HiPco High-pressure carbon monoxide

Li Lithium

MgO Magnesium oxide

Mn Manganese

Mo Molybdenum

MWNTs Multi-walled carbon nanotubes

N2 Nitrogen

NaOH Sodium Hydroxide

NH3 Ammonia

Ni Nickel

NiS2 Nickel disulfide

NO Nitric oxide

O2 Oxygen

ODH Oxidative dehydrogenation

ORR Oxygen reduction reaction

Pd Palladium

PECVD Plasma enhanced chemical vapor deposition

Pt Platinum

S Sulfur

SiC Silicon carbide

SiO2 Silicon dioxide

SWNTs Single-walled carbon nanotubes

Ti Titanium

WACVD Water assisted chemical vapor deposition

wt% Weight percent

Zn Zinc

1

1 Introduction

It was well known that charcoal, graphite and diamond are the 3 main allotropes of carbon. But

not until the discovery of Buckminsterfullerene in 1985 (Kroto, et al., 1985), carbon nanotubes

(CNTs) in 1991 (Iijima, 1991) and graphene in 2004 (Novoselow, et al., 2004), which brought

carbon allotropes to nanoscale. They all have the same structural unit: a single-layer graphene

sheet, but exist in different forms. This unique individual morphology provides them special

characteristics and properties, which are distinct from each other. These discoveries and their

possible applications have great impacts on material science and our daily life. Figure 1-1 shows

the 3 newly found carbon allotropes and how they are interconnected.

Figure 1-1: Illustration of Buckminsterfullerene, CNTs and graphene. The white arrows show how the carbon allotropes are linked (Tessonnier, et al., 2011).

Particularly, CNTs have been receiving a lot of attentions due to their extraordinary properties

such as high thermal and electrical conductivity, high mechanical strength, low density and good

chemical and environmental stability. For example, CNTs are capable to carry an electrical current

density of 4 x 10-9 A cm-2 (Hong, et al., 2007) and their thermal conductivity is 3500 W m-1 K-1 at

room temperature (Pop, et al., 2006). These magnitudes are 1000 and 10 times higher than

copper electrical and thermal conductivity respectively. These interesting properties of CNTs

caused the journal publications and issued patents related to CNTs rise annually and their

production capacity increase exponentially since 2004, as shown in Figure 1-2.

CNTs are currently mass produced as unorganized bulk nanotubes, which have limited properties

compare to those lab-scale synthesized CNTs. Despite that CNTs have been commercialized and

applied in diverse commercial products such as sport equipment, conductive production,

automotive parts, plastic reinforcement, scaffolding for bone growth and so forth. But there is still

ongoing research to discover and develop new application area of CNTs, for instance catalyst and

catalyst support, medicine, energy production and storage, and so on. Mass production of CNTs

with desired and organized structure is one of the focuses of the research.

2

Figure 1-2: Annual number of journal publications, issued patents and production capacity of CNTs (De Volder , et al., 2013)

Their outstanding properties and wide range of applications on commercialized products draw the

interests of the scientists and researchers to develop various synthesis methods and investigate

the effects of the synthesis parameters for better production or modification of CNTs with special

properties. Arc discharge, laser ablation and chemical vapor deposition (CVD) are the three main

developed CNT syntheses. CNTs synthesized with arc discharge and laser ablation require high

energy input, which limit them from being used for continuous industrial production. On the

other hand, CVD-based methods can be operated under mild conditions and the ease of

controlling the CNT properties through variation of parameters, make them the focus of research

for large-scale production. The biggest challenge for the mass production of CNTs is to synthesize

them at industrial scale with high yield, low cost and in sustainable manner.

Among the 3 newly discovered carbon allotropes, only CNTs have reached large-scale industrial

production in order to meet their high market demand (Thayer, 2007). Numerous of scalable CVD-

based processes have been developed, modified and adopted into industrial production, including

HiPco process from Rice University, CoMoCAT process from University of Oklahoma, Endo process

from Shinshu University, Nano agglomerate fluidized process from Tsinghua University, Multiwall

nanotube process from Hyperion Company and Baytube process from Bayer Material Science

(Zhang, et al., 2011). The market value for CNTs in 2010 was estimated to be 90.5 million dollar

and global revenues are projected to exceed 1 billion dollars by 2015 (Apul, et al., 2015).

This review will highlight three different aspects of CNTs, namely applications, synthesis methods

and mass production processes of CNTs. First, examples for promising present and future

applications of CNTs, which are related to chemical and process engineering, will be extracted

from literatures and elaborated. In the next section, the three main synthesis methods of CNTs

are illustrated and explained. And their modification progresses will be addressed as well. Beside

that advantages and disadvantages for large-scale production of each method will also be

reviewed. Finally, the three mass production processes, which are currently used in industrial for

large-scale production, will be explained and the considerations in the design of industrial scale

production for each process will be introduced. And further discussion of the processes will be

supported by the parametric study results, which are extracted from the literatures.

3

2 Type and growth mechanism of carbon nanotubes

Theoretically, CNTs are cylindrical structure made by rolling one-atom-thick graphite, called

graphene, into a seamless cylinder. If the tube is made out of one cylindrical graphene, it is called

single-walled carbon nanotubes (SWNTs). But if the tube system is made out of more than one

layer of cylindrical graphene, it will be named as multi-walled carbon nanotubes (MWNTs). The

synthesis of SWNTs and MWNTs can be differentiated through the reactor used, reaction

parameters, types of catalyst and also the size and morphology of the catalyst. For instant, small

catalyst particles (0.5-5 nm) are mostly used to synthesize SWNTs with CVD method, whereas big

catalyst particles (8-100 nm) are favored for the synthesis of MWNTs (Zhang, et al., 2011). The

different between the 2 types of CNTs are illustrated in Figure 2-1.

Figure 2-1: Schematic representation of SWNT and MWNT (Martins, et al., 2013).

Apart from being a straight SWNTs or MWNTs, many attempts have been conducted to vary the

tubule morphology of CNTs since its discovery in 1991, for example: waved, coiled, regularly bent,

branched CNTs and CNTs with nanobud. The reason behind the attempts is the special properties

and potential applications that come along with the special designed tubule morphologies.

2.1 Single-walled carbon nanotubes (SWNTs)

As was mentioned in the previous section, SWNTs are cylindrical tube made by wrapping a one-

atom-thick layer of graphite. It was first discovered by Iijima and Ichihashi in 1993, 2 years after

the discovery of CNTs, by arc discharge method with iron (Fe) as catalyst (Iijima, et al., 1993).

SWNTs can be categorized into 3 different forms such as zig-zag, armchair and chiral, depending

on the way the graphene is wrapped (see Figure 2.1-1). And the properties of the SWNTs,

especially their electrical properties, are strongly depending on the existed form. The armchair

form of SWNTs is considered as metallic or highly conducting nanotubes, whereas other forms can

make the SWNTs as semiconductor (Eatemadi, et al., 2014). Owning to these properties, SWNTs

are mostly used in electronic devices and sensors, which require highly structured CNTs. But the

price of SWNTs remains higher than MWNTs due to their complex mass production process,

which limits them from widespread applications.

4

Figure 2.1-1: The three different forms of SWNTs (Eatemadi, et al., 2014)

2.2 Multi-walled carbon nanotubes (MWNTs)

MWNTs consist of more than one layer of cylindrical shape of graphene and were first obtained

by Iijimal in 1991 with arc discharged method (Iijima, 1991). They have diameter ranging from few

nanometers up to several hundred nanometers depending on the number of layers the system

possesses. There are 2 commonly used models for the description of MWNTs, namely Russian Doll

model and Parchment model. In the Russian Doll model, MWNTs are a set of SWNTs with the

largest diameter at the outer most layers and the diameter of the SWNT decreases when moving

to next inner layer until the inner most layers, which has the inner diameter of the MWNT. In the

Parchment model, a single graphene sheet is rolled in around itself for manifold times, resembling

a scroll of parchment or a rolled paper. The Russian Doll model is more commonly observed.

Double-walled carbon nanotubes (DWNTs) are a special form of MWNTs, which contain only 2

layers of SWNTs. They were first synthesized in 2003 using catalytic chemical vapor deposition

(CCVD) method (Flahaut, et al., 2003). DWNTs have identical morphology and properties like

SWNTs. But their chemical and environmental stabilities are significantly better than SWNTs due

to the protection from the outer layer. The outer layer of DWNTs provides extra CNT layer for

functionalization, which add new properties to the CNTs without destroying the tubule

morphology of the inner SWNT and their properties.

Following table shows some differences between SWNTs and MWNTs.

SWNTs MWNTs

Single layer of graphene Multiple layers of graphene

Catalyst is requires for synthesis Can be produced without catalyst

Bulk synthesis is difficult as it requires proper control over growth and atmospheric condition

Bulk synthesis is easy

Purity is poor Purity is high

A chance of defect is more during functionalization

A chance of defect is less but once occurred it is difficult to improve

Characterization and evaluation is easy It was very complex structure

It can be easily twisted and is more pliable It cannot be easily twisted

Table 2.2-1: Comparison between SWNTs and MWNTs (Eatemadi, et al., 2014).

5

2.3 Growth mechanism of carbon nanotubes

Despite many studies have been done for better understanding of the CNT growth mechanism

and several possibilities have been proposed, the actual CNT growth mechanism is yet to be

discovered and established. Generally, the widely-accepted mechanism can be considered as a

three-step process. Firstly, the carbon feedstock will decompose into elementary carbon atoms

on the surface of the catalyst particles. Secondly, the carbon atoms will either diffuse through or

diffuse on the side of bulk catalyst particles. The latter is mostly accepted because it explains the

hollow core of the CNTs (Tessonnier, et al., 2011). The main driving force of the diffusion was

suggested to be the temperature and concentration gradient (Harris, 2009). Finally, the carbon

atoms will precipitate out and form the cylindrical network of CNTs on the surface of the catalyst

particles.

During the final step of the process, there are 2 possible growth models, which can take place

depending on the strength of the interaction between the catalyst particles and the substrate,

namely tip-growth and base-growth models. If the catalyst-substrate interaction is weak, the

carbon atoms are capable to push the catalyst particle off the substrate and it will stay on top of

the CNTs while the growth process continues on the bottom part of the particle. This

phenomenon is called tip-growth model. On the other hand, if the catalyst-substrate interaction is

too strong to be detached by the carbon atoms, the catalyst particle will remain anchored on the

surface of the substrate while the CNT growth process continues on the top part of the particle.

This phenomenon is knows as base-growth models. In both models, the growth process will be

terminated once the catalyst particle is fully covered with excess carbon and subsequently their

catalytic activity will cease. The widely-accepted growth mechanisms of CNTs are illustrated in

Figure 2.3-1.

Figure 2.3-1: Illustration of the general CNT growth mechanism: (a) tip-growth model, (b) base-growth model (Kumar, 2011).

The mechanism discussed above is the basic and widely-accepted CNT growth mechanism. The

uncertainness of the physical and chemical state of the catalyst particles during the growth and

the mode of diffusion during the second step of the mechanism are still troubling the researcher

before the establishment of correct and accurate CNT growth mechanism can be conclusively

done (Kumar, 2011).

6

3 Applications of carbon nanotubes

The number of CNT applications related articles is still increasing despite the decrease of the

number of articles related to CNT synthesis since 2009 (Zhang, et al., 2011). This phenomenon

shows that the focus point of the studies about CNTs is shifting from synthesis to application,

especially for the application in catalyst, energy and environmental area. Despite the existing

wide range of applications, research about the new application and usage of this new type of high

performance carbon nanomaterial is still ongoing due to the market demand.

The applications of the nanotubes can be categorized into 2 groups, namely large and limited

volume applications. Large amounts of CNTs of good quality are needed for the large volume

applications e.g. as components in conductive, electromagnetic, high strength composites,

supercapacitors, fuel cell catalyst and transparent conducting films. While high structure and

reproducibility standards are required for limited volume applications, such as drug delivery

system, electronic devices and sensors.

Despite the annual substantial production of CNTs, the applications of CNTs in commercialized

products have mostly been limited to the use of bulk CNT powders, which is a mass of

unorganized fragment of nanotubes. Specific structures and agglomeration states are required for

many CNT applications. Bulk nanotubes might not be showing the similar properties as the

organized CNT architectures, which were tested in lab. Nonetheless, the bulk CNT powders still

yield promising performance for commercial applications compare to conventional materials

used.

In the following section, the promising present and future applications of CNTs related to

chemical and process engineering will be focused on four different application areas, namely

composite materials, catalyst and catalyst support, energy storage, and environment.

3.1 Composite materials

Since the first report regarding the preparation of CNTs and polymer composite materials in 1994

(Ajayan, et al., 1994), many efforts have been made to composite the CNTs with others polymer in

order to produce desired functional composite materials. The difficulty in structure control, poor

process ability and existence of impurities remain as the main challenges for the application of

individual bulk CNTs. Compositing CNTs with other polymers seems to be one of the solutions to

the problems by further enhancing the properties of the bulk CNT materials.

CNTs can be composited with polymer through functionalization. Figure 3.1-1 shows some

possible functionalization mechanism for SWNTs.

A) Functionalization of the defect group at the end of the tubes and side walls;

B) Functionalization of the covalent side wall through addition reactions and subsequent

nucleophilic substitution;

C) Functionalization of noncovalent exohedral with surfactants;

D) Functionalization of noncovalent exohedral with polymers;

E) Functionalization of endohedral with C60.

7

Figure 3.1-1: Some possible ways of functionalization for SWNTs (Hirsch, 2002).

CNTs are composited with others polymers to form conductive CNT polymer, for example

composition of CNTs with poly (p-phenylenevinlene-co-2,5-dicotoxy-m-phenylenevinylene)

(PmPV) showed a very high conductivity due to the conducting path provided by the CNTs to the

polymer (Coleman, et al., 1998). The conductive CNT polymers have been used in automobile

industries for electrostatic-assisted painting of mirror housing, as well as fuel lines and filters that

dissipate electrostatic charge (De Volder , et al., 2013). Furthermore, attempts are in progress to

composite CNTs with aluminum for the advanced lightweight of automobile parts (Peng, et al.,

2015) and with carbon fibers for lightweight wind turbine blades (De Volder , et al., 2013).

3.2 Catalyst and catalyst support

Currently there is no concrete industrial application of CNTs as catalyst and catalyst support, but

research has proven that CNTs have the potential to replace metals as catalyst and catalyst

support for many organic and inorganic reactions. Better environmental acceptability, favorable

management of energy with good thermal conductivity and inexhaustible resources make carbon-

based materials such as CNTs an interesting alternative to some current industrialized chemical

process (Su, et al., 2010).

Many studies have been conducted to replace conventional metal and metal oxide catalyst with

CNTs for the oxidative dehydrogenation (ODH) of unsaturated hydrogen carbons and alkane

activation, oxygen reduction reaction (ORR) and the transesterification of triglycerides. For

instance, surface functionalized CNTs showed an increase of selectivity for the ODH of ethane

(C2H6) due to the suppression of electrophilic oxygen (O2) intermediates on the carbon surface

(Frank, et al., 2010). In another study, Butadiene (C4H6) was efficiently catalyzed by surface

modified CNTs during the ODH of n-butane (C4H10) due to the capability of CNTs to keep O2/ C4H10

8

at low ratio during the reaction (Zhang, et al., 2008). Apart from ODH, studies showed that CNTs

are capable of reducing (Matsumoto, et al., 2004) and even replacing (Gong, et al., 2009) the high

price and limited supply platinum (Pt), which is conventionally used as catalyst for ORR. Alkaline

earth oxides, calcined hydrotalcites and nano-magnesium oxide (MgO) are the classical

heterogeneous basic catalyst used for biomass conversion. Small surface area and partial

dissolution into reaction media make them a drawback for the application. But CNTs seem to be

an alternative solution for the problem, as study proved that amino group grafted MWNTs show

high activity and stability in transesterification of triglycerides (Villa, et al., 2009).

Besides being used as catalyst, CNTs can also be applied as catalyst support and their

performances are promising and even better than the traditional catalyst supports such as metal,

metal oxide and active carbon. The high mechanical strength of CNTs makes them suitable to be

used in mechanically taxing stirred batch reactor. Their high surface area and inherently

microporous provide a better place for dispersion and impregnation of catalyst. And they have a

longer life span than conventional catalyst supports due to their high chemical and environmental

stability. MWNTs supported catalytic nickel disulfide (NiS2) nanoparticles showed better

desulfurization activity and better resistance to the solid sulfur (S) deposition of selective

oxidation of hydrogen sulfide (H2S) into elemental S compare to silicon carbide (SiC) supported

NiS2 nanoparticles (Nhut, et al., 2004). Another study showed that catalytic palladium (Pd)

nanoparticles supported on CNTs showed higher selectivity for the oxidation of benzylic alcohol to

benzaldehyde in comparison to activated carbon (Villa, et al., 2010). Furthermore, CNTs

supported catalytic Pd also showed promising result for the hydrogenation of alkene, alkyne and

nitric oxide (NO) as well as conversion of nitro to amino group (Oosthuizen, et al., 2011).

3.3 Energy storage

Owning to their high chemical stability, electrical conductivity, surface area, electrolyte

accessibility and low resistance of charge transport, CNTs have been used as anode materials for

lithium (Li) ion batteries, which can be found in notebook computers and mobile phones. When

MWNTs are assembled in bundles, the interlayer space of MWNTs provides the room for the

storage of large amount of Li+ ion (Leroux, et al., 1999). A study reported that the defection on the

CNT wall by nitrogen atom (N) doping further increased the storage capacity of MWNTs, as larger

portion of interwall space is available for the storage of Li+ ions (Shin, et al., 2012).

Pt has been used as catalyst to improve the performance of fuel cell. The major drawback factor

for large-scale practical applications of fuel cell is the high price and limited supply of Pt. Reducing

the amount of Pt used in fuel cells is an essential step for commercialization of fuel cells as energy

source. A study showed that the usage of Pt can be reduced as much as 60 %, if CNTs are used as

catalyst support instead of carbon black. The reasons behind the improved performance are the

formation of triple phase boundaries of the electrode and the high conductivity of CNTs

(Matsumoto, et al., 2004). A recent study even reported that fuel cell can have a better

performance, when doped CNTs were used as electrode without the present of Pt as catalyst

(Gong, et al., 2009).

Apart from storing electrical energy, CNTs were reported to be capable for hydrogen (H2) storage.

The capillary effects of the small size CNTs provide space for high density of condensation of H2

gas inside SWNTs (Jones, et al., 1997). Thus, the H2 can be stored as gas phase instead of liquid

9

phase. It was believed that current storage methods, which store H2 in liquid phase, can be

possibly replaced by CNT-based method. Because the major problem with available methods is

the potential energy lost during the cooling and condensation of H2 gas. But it was reported that

CNTs have a maximum hydrogen uptake capacity of only 0.2 wt% (Barghi, et al., 2014), which is

significantly lower than commercially available hydrogen storage need to be. Arguably, it is

because of the impurities present in the CNTs. But even the very pure sample of CNTs, which

were purified with the help of Microwave digestion method, showed only a maximum capacity of

3.7 wt% (Yuca, et al., 2011). Yet the value is still lower than the targeted value of 5.5 wt% required

by U.S. Department of Energy (DOE) for automotive application (Froundakis, 2011). Researches

are still ongoing to overcome the problems caused by limited H2 uptake capacity of CNTs.

3.4 Environment

Adsorbent in water purification is an upcoming application for the CNTs. For example,

commercialized portable filters now contain CNT meshes to purify contaminated drinking water

(De Volder , et al., 2013).

Beside commercial applications, CNTs are potential adsorbent for wastewater treatment in

industrial due to their hollow and layered structures, high specific surface area, and

hydrophobicity. Many reports have showed that CNTs are capable of removing natural organic

matter and synthesis organic contaminants through adsorption, for example polycyclic aromatic

hydrocarbon, benzene derivatives, phenolic compounds, pharmaceuticals, polychlorinated

biphenyls, dialkyl phthalate esters, protein, organic dyes and dioxin (Apul, et al., 2015).

Furthermore, current adsorbents for metal ions have low adsorption capability and removal

efficiencies. With the unique properties of CNTs, they might be an alternative solution for the

problems (Rao, et al., 2007).

The drawback of the instant industrial application of CNTs in this field can be considered in two

different perspectives, namely nature system and engineering. From nature system perspective,

CNTs might enter the environment through the wastewater treatment either intentional or

unintentional and their toxicity can be enhanced by the adsorbed organic contaminant (Apul, et

al., 2015). This raises the concept about the risk of human health, as reports showed that CNTs

are capable of causing cell death due to the their accumulation after entering human cell (Porter,

et al., 2007) and causing side effects to human lungs (Lam, et al., 2006). Whereas from the

perspectives of engineering application, study showed that with the present of natural organic

matter, microporous activated carbon fiber and granular activated carbon have better adsorption

capacity of synthesis organic contaminants such as phenanthrene and 2-phenyl-phenol in

comparison to CNTs (Zhang, et al., 2011).

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4 Synthesis methods

The research on the fascinating science and technology of CNTs is mainly promoted by the

development of controllable synthesis methods, which provide more desired samples for

investigation purpose. As many applications require CNTs to have specific structures and

agglomeration states, studies are still ongoing to discover new methods and further modify the

existing methods in order to meet the needs. Numerous methods were purposed for the

production of CNTs since their discovery and the methods can be classified into following 2 major

groups:

1. Physical process

2. Chemical process

In the following section, three widely used methods, namely arc discharge, laser ablation and

chemical vapor deposition, will be presented individually and discussion about their modification

will be made.

4.1 Physical process

4.1.1 Arc discharge

The first ever used method for the synthesis of CNTs is the arc discharge method (Iijima, 1991).

Figure 4.1.1-1 shows a typical illustration of arc discharge method. Normally, graphite rods will be

used as the electrode for anode and cathode in an enclosed chamber, which will be pressurized

with inert gas like helium (He) and argon (Ar) at a given pressure. The electrodes are connected to

a voltage stabilized direct current (DC) power supply. The adjustable anode will be moved closer

to the cathode until an arc appears. The arcing gap between the electrodes should be constantly

kept at approximately 1 mm or less during the synthesis. The current will discharge the carbon

from the anode and the evaporated carbon atoms will recondensed as CNTs on the cathodic rod.

The synthesis will only last for few minutes and the anodic rod has to be replaced as the rod will

be consumed during the process. This prohibits the continuous production of CNTs. The arc

discharge method typically generates deposits on cathodic rod at the rate of 20-100 mg min-1

(Kingston, et al., 2003).

Figure 4.1.1-1: Arc discharge apparatus for the synthesis of CNTs (Saito, et al., 1996).

11

Efforts have been committed to produce good yield of high quality CNTs through arc discharge

method by using catalyst and changing the synthesis parameters and conditions. Studies were

conducted using pure metal as catalyst for the synthesis. Cobalt (Bethune, et al., 1993) and iron

(Iijima, et al., 1993) was impregnated on holed anodic graphite electrode and as a result SWNTs

were produced. Synthesis parameters such as pressure of the chamber can alternate the quality

of the CNTs produced. A study showed that the number of layer of CNTs can be increased as the

pressure of He in the pressurized chamber increased. But after 66.66 kPa (500 torr) there was no

change in sample quality but decrease in total yield (Ebbesen, et al., 1992). Apart from the

reaction pressure, current is another factor that might change the quality of the CNTs. The current

should be kept as low as possible and the stable plasma state should be maintained because the

low current prevents the formation of hard and sintered materials, which lead to low yield of

CNTs (Ebbesen, et al., 1993). Another possibility is to replace the inert gas pressurized chamber

with liquid. Studies showed that CNTs produced by arc discharge under liquid nitrogen and water

have higher quality than those produced under gas (Antisari, et al., 2003).

4.1.2 Laser ablation

Another physical process for the synthesis of CNTs is laser ablation process, which was introduced

in 1995 (Guo, et al., 1995). It was claimed that CNTs produced with this method have higher yield

and purity, and this method has better control over growth conditions compare to arc discharge

method. The apparatus set up is illustrated in Figure 4.1.2- 1. The graphite target, which contains

small amount of cobalt (Co) and nickel (Ni), will be struck by laser beam in a high temperature

reactor. The CNTs will form on the lower temperature region of the reactor as the vaporized

carbon atoms condense. The tubular reactor will be filled with continuously flow of inert gas such

as He and Ar to create inert atmosphere and carry the grown CNTs to the water-cooled copper

collector. This method was further refined by using double pulsed laser for even vaporization of

the graphite and minimization of formation of soot on the collector (Thess, et al., 1996).

Figure 4.1.2- 1: Schematic of laser ablation apparatus for synthesis of CNTs (Yakobson, et al., 1997).

12

The variation of average diameter, length, structure and yield of the CNTs can be done by

changing the process parameter such as temperature, laser used and catalyst composition.

SWNTs was successfully produced at room temperature by using 1 kW of carbon dioxide (CO2)

laser beam for the vaporization of graphite target, which contain small amount of Co and Ni

particles (Kokai, et al., 1999). The results showed that the yield of SWNTs was increased as the

temperature increased and the highest yield was recorded at 1200 °C. Production rates of SWNTs

as high as 1.5 g h-1 was reported by using 1 kW of free electron laser for the vaporization of metal

particles loaded graphite. Integration of free electron laser with this method is capable of

producing SWNTs at the rate of 45 g h-1 after process optimization (Eklund, et al., 2002).

4.2 Chemical process

4.2.1 Chemical vapor deposition (CVD)

Use of CVD method to synthesize MWNTs was first reported in 1993 by catalytic decomposition of

acetylene (C2H2) at temperature of 700 °C and graphite supported Fe was used as catalyst (Jose-

Yacaman, et al., 1993). Three years later, SWNTs were successfully synthesized through Mo

catalyzed disproportionation of CO at 1200 °C (Dai, et al., 1996). A typical set up for CVD method

is illustrated in Figure 4.2.1-1. The mixture gas of hydrocarbon gas, which acts as the carbon

feedstock such as carbon monoxide (CO), methane (CH4), ethane (C2H6), ethylene (C2H4) and

acetylene (C2H2), and process gas, which acts as carrier gas such as ammonia (NH3), nitrogen (N2)

and hydrogen (H2), will be fed into the reaction chamber. The decomposition of hydrocarbon gas

takes place in the reaction chamber and carbon atoms deposit and growth on the catalyst loaded

substrate at the temperature ranging from 400-1200 °C. Because of its higher yield and simpler

equipment compared to arc discharge and laser ablation, CVD is the most promising method for

large-scale production of CNTs.

Figure 4.2.1-1: Illustration for typical CVD set up (Mubarak, et al., 2014).

As production of SWNTs by CVD usually required high temperature (900-1200 °C), CO and CH4 are

used for carbon feedstock due to their high thermal stability. Apart from temperature and

feedstock, there are another two key factors that affect the nature and types of CNTs produced

by CVD method, namely the catalyst used and the preparation of the substrate. Silicon and glass

are normally used as the substrate material. And study showed that Ni, Fe and Co-based catalyst

13

are the most active catalyst for the decomposition of hydrocarbon in comparison to others

transition metals such as manganese (Mn), copper (Cu), zinc (Zn) and titanium (Ti) (Deck, et al.,

2006). Solution deposition, electron beam evaporation and physical sputtering are commonly

used to deposit the catalyst particles on the substrate material. These deposition methods have

to be chosen specifically for the production of desired CNTs, as the methods have influences on

CNT properties.

The carbon atoms are deposited on the surface on the catalyst particles and CNTs grow on them.

The particles will be encapsulated inside the CNTs after the termination of the growth and these

transition metals are proven to have significant influences on the CNT properties (Brukh, et al.,

2008). Silicon dioxide (SiO2) seems to be the alternative catalyst for the metal free CVD synthesis.

A study showed that SiO2 nanoparticles are capable of catalyzing the synthesis of SWNTs and their

size distribution is much narrower (Huang , et al., 2009).

4.2.2 Modification of CVD

Owning to the ability for continuous operation, simplicity for scaled up to large industrial process,

availability for abundant of raw materials and simplicity of reactor design, CVD is a promising

synthesis method for CNTs to meet the high market demand. And CVD is the only known method

for producing aligned CNTs (Rafique, et al., 2011). Numerous of researches have been done to

enhance the method for higher CNT production yield and better architecture CNTs.

CNTs produced by CVD are randomly entangled. And this limits their applications as electrodes or

electrodes filler in energy conversion and energy storage, as structurally aligned CNTs are critical

for the applications (Zhang, et al., 2011). Plasma enhanced chemical vapor deposition (PECVD)

was first successfully used to synthesize aligned CNTs at 666 °C (Ren, et al., 1998). In this work,

C2H2 was used as carbon feedstock and the CNTs were grown on Ni deposited glass. NH3 gas was

introduced into the reaction as dilution gas and it showed catalytic activity on the CNT growth.

The reason behind the conformal alignment is believed to be the electrical self-bias imposed on

the substrate surface from the plasma environment (Bower, et al., 2000). A further study showed

that higher plasma power will slow down the growth rate of CNTs due to the rapid decomposition

of carbon feedstock C2H2 at high plasma power (Bell, et al., 2006). The C2H2 has to be slowly

decomposed to prevent formation of amorphous carbon. Besides that the ratio between NH3 and

C2H2 is another crucial factor for high quality and quantity of CNTs. Higher ratio of NH3 to C2H2 is

favored as NH3 generates atomic hydrogen species to remove excess carbon and suppresses the

decomposition of C2H2 due to its weaker molecular chemical bonds (Hussain, et al., 2015).

Another modified CVD is called water assisted chemical vapor deposition (WACVD), which was

introduced in 2004 to synthesize SWNTs (Hata, et al., 2004). In this work, C2H2 was used along

with H2 and Ar or He, which contained small and controlled amount of water vapor. In the normal

CVD synthesis, the amorphous carbon formed will coat on the catalyst particles and cause the

reduction of their catalytic activity and lifetime. The results of the study showed that water can

promote and preserve the catalytic activity for a longer period of time. The reason behind it is the

ability of water to produce large amount of hydroxide groups on carbon, which convert the

deposited carbon to CO and H2 by gasification and subsequently inhibit the catalyst from ripening

(Xie, et al., 2013).

14

4.3 Comparison between the physical and chemical process

Physical processes like arc discharge and laser ablation generally have to be conducted in

advanced and costly apparatus at very high temperature. And due to the fast process time only

production of short and low yield of CNTs is possible as well as constantly replacement of graphite

target prohibits them from being used as a continuous process. But they produce high quality

CNTs especially for the production of SWNTs. These high quality samples of CNTs are critical for

the nanotube research to achieve important results. Physical processes are ideal for production at

laboratory scale for research purpose, but following disadvantages limit their use as large-scale

industrial process for commercialized applications:

1. Large amount of energy is needed for vaporization of carbon atoms from target material,

which makes them energy extensive methods. It is impossible and uneconomical for this

huge amount energy to be generated for industrial use.

2. Large graphite is needed to be targeted for vaporization of carbon atoms.

3. Highly tanged CNTs are produced and mixed with unwanted form of carbons. Thus

purification is needed to purify the CNTs and assemble them into desired form. The

designing of such refining process is expensive and difficult.

On the other hand, chemical processes require only cost effective and convenient equipment for

controllable growth of CNTs. The chemical reaction takes place at relative low temperature and

ambient pressure. Besides that CVD, PECVD and WACVD can be operated continuously without

the need of replacing carbon feedstock, which makes them promising methods for continuous

industrial scale production. Chemical processes offer following advantages for the use as large-

scale production:

1. Simple reaction process and reactor design, controllable and manipulatable reaction.

2. Easy availability of raw materials as carbon feedstock.

3. Cheap production as little amount of energy is needed and cheap raw materials are

abundant.

4. Unique process for the synthesis of vertically aligned CNTs.

5. Similar operation to chemical unit operations makes them to be easily scaled up to large

industrial process.

Following table (Table 4.3-1) summarizes the main differences between the three methods.

Property/Process Arc discharge Laser ablation CVD

Raw materials availability Difficult Difficult Easy, abundantly available

Energy requirement High High Moderate

Process control Difficult Difficult Easy, can be automated

Reactor design Difficult Difficult Easy

Production rate Low Low High

Purity of product High High High

Yield of process Moderate High High

Post Treatment Refining Refining No extensive refining

Process nature Batch Batch Continuous

Per unit cost High High Low

Table 4.3-1: Comparison of CNT production methods (Rafique, et al., 2011)

15

5 Process for mass production

5.1 High-pressure carbon monoxide (HiPco) process

HiPco process is a type of CVD method for large-scale production of SWNTs and was introduced in

1999 (Nikolaev, et al., 1999). There are at least two large-scale reactors, which are currently

operated for industrial purpose: one at Rice University and another at a spin-off company, Carbon

Nanotechnologies Inc. (Harris, 2009), which has the production capacity of 65 g/h (Eklund, et al.,

2007). As was mentioned in the previous section, in conventional CVD method catalysts are

deposited or embedded on the substrate before the decomposition of hydrocarbon and growth

of CNTs on substrate begin. Instead, in HiPco process volatile organometallics are introduced into

the feed flow along with carbon feedstock. The organometallics will react, decompose and

condense in situ to form sized clusters, upon which CNTs nucleate and growth. With this method

the CNTs produced are free from catalytic supports and the product yield and purification yield

are as high as 97 % and 90 % respectively (Isaacs, et al., 2010).

Figure 5.1-1 shows the fundamental reactor and Figure 5.1-2 illustrates the block flow process

diagram for the HiPco process. In the initial lab work for parametric study (Bronikowski, et al.,

2001), 8.4 L/min of pure CO gas were rapidly mixed with 1.4 L/min gas mixture of CO and iron

pentacarbonyl (Fe(CO)5), which contained about 33.33 Pa (0.25 Torr) of Fe(CO)5 vapor. The

standard running conditions were 30 atm of CO pressure, 1323 K (1050 °C) of reaction

temperature and 24-72 hours of reaction time. The production rate of SWNTs under the reaction

conditions is 450 mg/h or 10.8 g/day. The Fe(CO)5 thermally decomposed and reacted to produce

Fe particles for the production of Fe clusters, which act as nuclei for the CNT growth. The solid

CNTs are produced catalytically through exothermic CO disproportionation on the surface of Fe

particles according to the Boudouard reaction:

𝐶𝑂(𝑔) + 𝐶𝑂(𝑔) ⇌ 𝐶(𝑠) + 𝐶𝑂2(𝑔)

Figure 5.1-1: Schematic of CO flow-tube reactor for HiPco process (Nikolaev, et al., 1999).

16

Figure 5.1- 2: The block flow process diagram for the production of CNTs with HiPco process.

The forward reaction of Boudouard reaction is an exothermic reaction and has more gas molecule

on the left side of the reaction. According to the Le Chatelier’s principle, increasing the

temperature and pressure will theoretically favor the forward reaction and produce high yield of

CNTs. This assumption is proven with the result from a parametric study of the HiPco process

(Bronikowski, et al., 2001) as shown in Figure 5.1-3 and Figure 5.1-4. Carbon dioxide (CO2) is the

by product for the Boudouard reaction, therefore the production of CNTs can be monitored by

measuring the corresponded maximum amount of CO2 produced assuming that all carbon

products are nanotubes.

Temperature plays a crucial role in the HiPco process. The effects from temperature on the

process have to be addressed during the considerations for the design of commercial scale

process and reactor. The gas phase catalyst, Fe(CO)5, will decompose rapidly at 250 °C and the

Boudouard reaction takes place at a significant rate only at temperature above 500 °C (Nikolaev,

et al., 1999). Thus, the heating rate of the gas mixture between the temperature ranges of 250-

500 °C will determine the result of the process. If the heating rate is too slow, larger Fe clusters

will form, which make them too big for nucleation of nanotubes and will overcoat with

amorphous carbon (Hafner, et al., 1998). On the other hand, high heating rate causes smaller Fe

clusters to form, which evaporate quickly at the temperature where formation of SWNTs occurs.

This leads to low yield of SWNTs. From Figure 5.1-3, it can be seen that the production of CO2 is

very low when the temperature is lower than 800 °C. But the production increases after 800 °C

and shows its maximum production at 1050 °C before fall off at higher temperature. The reason

behind the fall off was believed to be the higher rate of evaporation of active catalytic Fe clusters

at high temperature compare to the growth rate of SWNTs and decomposition rate of the catalyst

(Bronikowski, et al., 2001).

As the reaction pressure increased, the maximum production of CO2 increased simultaneously as

shown in Figure 5.1-4, because left side of equilibrium Boudouard reaction contains less gas

molecules. Higher pressure leads to higher disproportionation rate of CO, subsequently higher

Reactor

CO2 (g)

Condesation of CNTs

P

CO (g)

CO (g) + Fe(CO)5 (g)

CO (g)CO (g)

CNTs (s)

Adsorption of CO2

CO (g) + CO2 (g)

CNTs (s) + CO (g) + CO2 (g)

Heating of CO gas

17

growth rate of SWNTs on the catalytic clusters. This allows the production of longer SWNTs, as the

carbon atoms have longer period of time to nucleate and growth on the clusters before they are

deactivated. Beside that higher growth rate of SWNTs leads to narrow diameter distribution of

SWNTs, because more small Fe clusters will be used for the nucleation and growth of SWNTs

before they grow into larger clusters through accretion.

The reactor of the process can be further modified by recycling the unconverted CO and mixing

with the CO feed. After the reaction, mixture of unconverted CO, SWNTs and CO2 will pass

through a series filters and cooled surfaces to collect the SWNTs by condensation and adsorption

beds containing sodium hydroxide (NaOH) to remove CO2 according to following chemical

equation: 2𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝐶𝑂2(𝑔) → 𝑁𝑎2𝐶𝑂3(𝑎𝑞) + 𝐻2𝑂(𝑙). The unconverted CO will be

recirculated back to the reactor, thus forming a closed loop. This recycle process is assumed to be

capable of reducing the amount of CO needed from 162.5 g/h to 0.045 g/h (Isaacs, et al., 2010).

Figure 5.1-3: The CO2 yield against the reactor temperature, while the reactor pressure was maintained at 30 atm (Bronikowski, et al., 2001).

Figure 5.1-4: The maximum CO2 yield and the concentration of Fe(CO)5 that produces maximum CO2 against the CO pressure, while temperature was maintained at 1050 °C (Bronikowski, et al., 2001).

18

5.2 Cobalt-molybdenum catalytic (CoMoCAT) process

CoMoCAT is another notable CVD method for large-scale production of SWNTs, which was

introduced in 2000 (Kitiyanan, et al., 2000). Currently, South West Nanotechnologie Inc. is using

CoMoCAT fluidized bed reactor for commercial production of CNTs (Agboola, et al., 2007).

CoMoCAT is a specified designed catalyst with synergistic effect of Co and Mo. Even though study

showed that Mo is capable to catalyze the CO disproportionation reaction for the production of

SWNTs, the reaction was carried out at very high temperature, to be specific at 1200 °C (Dai, et

al., 1996). Another report regarding the CoMoCAT showed that Mo is inactive for the production

SWNTs in the temperature range of 600-800 °C. And Co only has 7 % of selectivity toward SWNT.

But with the bimetallic CoMoCAT, the selectivity toward SWNTs was increased to more than 80 %

(Alvarez, et al., 2001). Beside that SWNTs produced through CoMoCAT process showed

significantly narrower distribution of diameters compare to SWNTs obtained from HiPco process

(Resasco, et al., 2002). And the SWNTs obtained from CoMoCAT can have purity higher than 90 %

(Isaacs, et al., 2010)

Figure 5.2-1 shows the basic illustration of a fluidized bed reactor and Figure 5.2-2 illustrates the

block flow process diagram of the CoMoCAT process used by South West Nanotechnologies Inc.

for the production of SWNTs. For the preparation of the bimetallic catalyst, aqueous solution of

cobalt nitrate (Co(NO3)2) and ammonium heptamolybdate ((NH4)6Mo7O24) were impregnated on

SiO2. After that the Co-Mo/SiO2 catalyst is dried in an oven at 80 °C and calcined in flowing air at

500 °C. The calcined catalyst will be placed inside the reactor, which was heated by H2 to 500 °C

and He to 700 °C. After that CO will be introduced into the reactor and undergo

disproportionation reaction (Boudouard reaction) to form SWNTs. The typical reaction

temperature and pressure ranges from 700 to 900 °C and from 1 to 10 atm respectively. The

production rate is about 0.25 g SWNT/g catalyst in a couple of hours (Rafique, et al., 2011).

Figure 5.2-1: A schematic of a fluidized bed reactor for the production of SWNTs using CoMoCAT process (Jansen, et al., 2009).

19

Figure 5.2-2 : The block flow process diagram for the production of CNTs with CoMoCAT process used by South West Nanotechnologies Inc. The uniqueness of this process is the special designed bimetallic catalyst, which shows the

synergistic effect between Co and Mo. The molar ratio of Co:Mo has influence on the total carbon

yield and selectivity to SWNTs, as shown in Table 5.2-1. The production of SWNTs by CO

disproportionation is strongly affected by the of Co2+ species, which are stabilized by Mo oxide

species (Alvarez, et al., 2001). High molar ratio of Co:Mo promotes the production of carbon but

depresses the selectivity to SWNTs. On the other hand, the reaction is more selective to SWNTs

with deceasing molar ratio of Co:Mo but has lower total yield of carbon. Co is selective toward

SWNTs by interacting with Mo in a superficial Co molybdate like structure and maintain as well-

dispersed Co2+ ions at low Co:Mo ratio. But at high ratio of Co:Mo, Co forms a non-interacting

phase, which will be reduced to metallic Co. The metallic Co forms large clusters through sintering

at high temperature, at which CNTs are normally synthesized. The large Co clusters favor the

production of less desirable forms of carbon such as fibers and graphite (Resasco, et al., 2002).

Similar to HiPco process, the CO feedstock undergoes equilibrium disproportionation reaction,

which forward reaction will be theoretically favored, when the temperature is increased. As a

result, high yield of carbon products should be obtained. Surprisingly, results from a parametric

study (Alvarez, et al., 2001) showed that the total carbon yield decreased as the temperature was

increased from 600 –800 °C, as shown in Table 5.2-1. But the selectivity to SWNT was dramatically

increased with increasing reaction temperature as higher temperature suppresses the production

of SWNTs and supports the formation of MWNTs and amorphous carbon on the catalyst. Identical

results and trends were obtained when the temperature was further increased to 850 °C and

950 °C (Resasco, et al., 2004). Higher rate of catalyst deactivation compare to the decomposition

rate of CO is the reason behind the low carbon yield at high temperature. Beside that high

reaction temperature supports the formation of large diameter of SWNTs. Because sintering of Co

clusters accelerates with temperature to form larger clusters, upon which the SWNTs nucleate

and grow (Resasco, et al., 2004).

(NH4)6Mo7O24 (aq)

Impregnation on SiO2 (s)

Drying in an oven at 80 °C

Reactor

Calcination in flowing air at

500 °C

CO2 (g)

Separation of CO2 throught

Adsorption

P

CO (g) + CO2 (g)

CO (g)

Co-Mo/SiO2 (s)

CO (g)

Co(NO3)2 (aq)

CO (g)

CNTs (s)

20

Catalyst Operating temperature

Reaction conditions

Total carbon yield (%)

Selectivity to SWNT (%)

Co:Mo (1:2) 700 °C 1 h, 50 % CO 1.5 88

Co:Mo (1:4) 700 °C 1 h, 50 % CO 1.6 96

Co:Mo (2:1) 700 °C 1 h, 50 % CO 2.2 57

Co:Mo (1:1) 600 °C 1 h, 50 % CO 2.7 25.8

Co:Mo (1:1) 700 °C 1 h, 50 % CO 1.7 62.5

Co:Mo (1:1) 800 °C 1 h, 50 % CO 1.0 86.6

Table 5.2-1: Total carbon yield and selectivity to SWNT obtained by CO disproportionation on CoMoCAT with different Co:Mo ratio and reaction temperatur (Alvarez, et al., 2001)

Apart from temperature, CO concentration in the gas phase and reaction time are other

considerations in the design of commercial scale process for the high yield and selective

production of SWNTs. Study found out that the dominant product at low CO concentration was

amorphous carbon. But the yield of SWNTs grew with increasing concentration. As for the

reaction time, when the reaction was conducted in a short period of time, the amount of SWNTs

produced was very small and the product was mainly amorphous carbon. However, the growth of

SWNTs became dominant as the reaction time gets longer (Alvarez, et al., 2001).

5.3 Endo’s catalytic chemical vapor decomposition (CCVD)

The Endo’s CCVD was introduced in 1988 for the production of carbon fibers (Endo, 1988) and

was adopted for the production of MWNTs in 1993 (Endo, et al., 1993), which is a continuous

process known as floating reactant method. The process was scaled-up by Showa Denko KK Japan

for industrial production and the capacity was reported to be 16 kg/h (Eklund, et al., 2007). Figure

5.3-1 illustrates the setup for the floating reactant method and its corresponding block flow

process diagram is showed in Figure 5.3-2. In the process, hydrocarbon vapor, metal catalyst and

carrier gas such as Ar and H2 are fed into the reactor from the top of the reactor. The metal

catalyst particles are floating in the furnace zone of the reactor and gradually falling to the

bottom of the reactor due to gravitational force. The hydrocarbon vapor reacts and followed by

deposition of carbon atoms and growth of CNTs on the catalyst particles. Thus, CNTs can be

collected at the bottom of the reactor. Mixture of benzene vapor and H2 gas was used to produce

CNTs at the temperature of 1000 °C, as reported in the study (Endo, et al., 1993).

Figure 5.3-1: Schematic setup of floating reactant method for MWNT production (Endo, et al., 2006)

21

Figure 5.3-2: The block flow process diagram for the production of CNTs with Endo’s process.

The size of the catalyst particle is curial for the controlling of the diameter and number of layers

of the CNTs. Small size catalyst favors the formation of SWNTs and small diameter of CNTs,

whereas large size catalyst promotes the production of MWNTs and large diameter of CNTs.

There are two way of controlling the catalyst particle size. The widely used method is deposition

of catalyst on the quartz substrate and subsequently limits the aggregation of the catalyst

particles. Another way is mixing with ceramic particles such as aluminum oxide (Al2O3), MgO and

zeolites to form ceramic-supported catalysts. The major advantage of using ceramic particles is

large surface area provided for the supporting of catalyst. The metal concentration and

temperature for the preparation of catalyst can be varied to obtain desired catalyst size (Endo, et

al., 2006).

Like other CVD methods, the carbon source and its flow rate is important for controlling number

of layers of CNTs. Carbon feedstock with low carbon content such as CH4 and C2H6 favors the

production of SWNTs due to their high thermal stability. With high carbon flow rate, it is difficult

for the efficient production of SWNTs and the products obtained are mainly MWNTs and

amorphous carbon (Endo, et al., 2006).

Reactor

Carrier gasAr and H2

CNTs (s) + Ar (g) + H2 (g)

Metal catalyst Hydrocarbon vapor

22

6 Conclusion

Being a cutting edge material, CNTs successfully showed the world their unique properties, which

are essential for the improvement of society daily life. Despite their production as bulk material,

with the ongoing research focusing on the mass production, the problem can possibly be solved in

the near future. Until then CNTs will be capable to show their truth potential to the world, which

were only seen by the researcher in the laboratory.

As was mentioned before, the focus of the up-to-date research is on CNT applications and mass

production instead of their synthesis methods in laboratory. This scenario indicates that the three

widely used synthesis methods are generally accepted as the most efficient and useful methods

to produce CNTs for various purposes. Arc discharge and laser ablation method provide high

quality CNTs to sustain the research of CNTs. Without them the exploration and development of

knowledge about CNTs might not be so highly promoted. Owning to the advantages such as low

production cost, controllable synthesis and so on, scalable CVD-based synthesis methods have

been developed and adapted into industrial for large scale production of CNTs.

Scaling up the laboratory apparatus to industrial scale plant used to be the major problem for CNT

mass production since their discovery, but the problem had been addressed. And currently, CNTs

have been successfully mass produced on ton scale by several companies such as, Hyperion

Company, Carbon Nanotechnologies Inc., South West Nanotechnologie Inc., Showa Denko KK

Japan, Arkema and others. As the demand of CNTs is growing fast in global market, it can be

expected that the companies will come out with improved technique and process for the mass

production of high quality CNTs in order to maintain their position in CNT industry. Process

intensification for example catalyst route innovation, feedstock saving and coupled process

(Zhang, et al., 2011) might be a new direction for the industry to further enhance their process for

the production of high quality CNTs with low cost.

There is no doubt that CNTs offer the society a lot of possibility to improve and catch up the fast

growing population and life. But the other side of the coin has to been taken into the account in

order to maintain the sustainable development of the CNTs. There are many studies focus on the

mass production and applications of CNTs, but their toxicity and negative effects on human and

environment cannot be ignored and should be addressed in order to prevent long-term harm to

human being and the nature.

As a newly discovered material, CNTs still have a long way to go and much work have to be done

in order to catch up the footstep of those traditional bulk chemicals. Following problems are some

of the current stumbling block for the mass production of CNTs, but they can be the stepping

stone for the sustainable mass production of CNTs, if the solutions to the problems are found:

1. Lack of an understanding of the CNT growth mechanism.

2. Difficulty to couple between CNT structure, mass production process and properties, as

well as between the main synthesis process and the post treatment such as dispersion,

forming of composites and others (Zhang, et al., 2011).

3. Lack of integrated overview on all the steps throughout the CNT mass production process.

4. Lack of understanding of the CNT negative effects on human being and environment.

23

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27

Erkla rung

SheongWei NG

Matr.-Nr.: 11107729

Münchener Straße 21

51103, Köln

Ich, SheongWei NG, erkläre hiermit, dass ich die vorliegende Masterprojektarbeit ohne fremde

Hilfe und ausschließlich unter Angabe der verwendeten Literatur und Software angefertigt habe.

_________________________________________________

Köln, 30.09.2015 – SheongWei NG

Fachhochschule Köln Cologne University of Applied Sciences

KURZBESCHREIBUNG

As the cutting edge material, CNTs offer spectacular

properties for the improvement of growing society.

Even though they found their place in many practical

applications, ongoing research is trying to show their

applicable potential in other technological areas. The

high market demand makes them the focus of the

research for mass production. And CVD methods

seem to be the most promising process for the

production at industrial scale. Many CVD-based

processes have been successfully adapted into

industrial for annually ton-scale production.

STICHWORTE: Carbon nanotubes, Application, Synthesis Method, Mass Production, Review