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Title of thesis

Optimization of Surface Treatment and Functionalization of Multiwalled

Carbon Nanotubes (MWCNT) for Carbon Dioxide (CO2) Adsorption

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Date : 13th October 2014 Date : 13

th October 2014

SYUHAIDAH BINTI RAHMAM

Prof. Dr. Norani Muti Mohamed

No 25, Jalan

Teruna 8, Taman Bukit Rambai,

75250 Melaka.

UNIVERSITI TEKNOLOGI PETRONAS

OPTIMIZATION OF SURFACE TREATMENT AND FUNCTIONALIZATION OF

MULTIWALLED CARBON NANOTUBES (MWCNT) FOR CARBON DIOXIDE

(CO2) ADSORPTION

by


The undersigned certify that they have read, and recommend to the Postgraduate Studies

Programme for acceptance this thesis for the fulfillment of the requirements for the degree

stated.

Signature: ______________________________________

Main Supervisor: ______________________________________

Signature: ______________________________________

Co-Supervisor: ______________________________________

Signature: ______________________________________

Head of Department: ______________________________________

Date: 13th

October 2014


Assoc. Prof. Dr. Suriati Sufian

Assoc. Prof. Dr. Suriati Sufian

OPTIMIZATION OF SURFACE TREATMENT AND FUNCTIONALIZATION OF

MULTIWALLED CARBON NANOTUBES (MWCNT) FOR CARBON DIOXIDE

(CO2) ADSORPTION

by


A Thesis

Submitted to the Postgraduate Studies Programme

as a Requirement for the Degree of

MASTER OF SCIENCE

CHEMICAL ENGINEERING

UNIVERSITI TEKNOLOGI PETRONAS

BANDAR SERI ISKANDAR,

PERAK

OCTOBER 2014

DECLARATION OF THESIS

Title of thesis Optimization of Surface Treatment and Functionalization of Multiwalled

Carbon Nanotubes (MWCNT) for Carbon Dioxide (CO2) Adsorption

I _________________________________________________________________________

hereby declare that the thesis is based on my original work except for quotations and citations

which have been duly acknowledged. I also declare that it has not been previously or

concurrently submitted for any other degree at UTP or other institutions.

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Signature of Author Signature of Supervisor

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________________________________

Date : _____________________ Date : __________________


No 25, Jalan

Teruna 8, Taman Bukit Rambai,

75250 Melaka


13th October 2014 13

th October 2014

DEDICATION

With loads of love, I dedicate this successful work to:

My sweetheart, Mohd Ikhwan b Ridzuan

My beloved son, Muhammad Mukhlis b Mohd Ikhwan

My father, mother, brother and sisters

My father, mother, and brothers in law

My supervisor, Prof. Dr. Norani Muti Mohamed

My co supervisor, Assoc. Prof. Dr. Suriati Sufian

Thank you very much for supporting me all the time.

ACKNOWLEDGEMENTS

Alhamdulillah, grateful to Allah because of his willingness, I managed to

complete the research project in two years period time. Million thanks to my

supervisor Prof. Dr. Norani Muti Mohamed, and co-supervisor, Assoc. Prof. Dr.

Suriati Sufian for their guidance, assistance, and support from the beginning, until the

end of project. Also, thank you to Universiti Teknologi PETRONAS (UTP),

Characterization and Analysis Laboratory (CAL) and the fund from Ministry of

Science, Technology, and Innovation. The research cannot be done successfully

without the contribution and support from all parties.

I would also like to express my gratitude and heartily grateful to my husband, son,

mom, dad, brother, sisters, mother in law, father in law and brothers in law. The

encouragement, advice, and support from them really make me strong and work hard

to complete this research work. Especially to my husband, my ‘third supervisor’,

thank you very much for your patience and sacrifice for the whole period of my study

time. Special thanks as well to my mother and father that helped me to look after of

my son when I was focusing in thesis writing.

Last but not least, thank you to everyone that involve directly or indirectly in

completing this project. From deepest of my heart, I appreciate every single thing that

you have done to ensure this project will end successfully. May Allah bless you all

and grant with Jannah (heaven).

ABSTRACT

The unique characteristics of multiwalled carbon nanotubes (MWCNT) such as

high porosity, high surface area and the existence of surface functional groups

through chemical modification makes it a suitable material for adsorption of carbon

dioxide (CO2). However, due to its hydrophobicity and bundling issues, MWCNT

could not function as an effective adsorbent. Thus, treatment on MWCNT is required

to overcome these problems before it could be functionalized with the amine group to

assist in adsorbing CO2. The objective of the work is to optimize the surface treatment

followed by functionalization of MWCNT with 3-Aminopropyl triethoxysilane

(APTS) to enhance the CO2 adsorption. Three types of acids used for surface

treatment were sulfuric acid (H2SO4), nitric acid (HNO3), and the mixture of sulfuric

and nitric acid (HNO3/H2SO4). Functionalization with 30% and 60% of APTS was

carried out after the surface treatment in order to observe the effect of concentration

on the attachment of amine group, hence the ability of modified MWCNT to adsorb

CO2. MWCNT treated with the mixture of HNO3/H2SO4 was found to have the most

attachment of carboxyl and hydrophilic group that can provide more active sites for

attachment of amine group in the functionalization with APTS. The efficiency of CO2

adsorption for MWCNT treated with H2SO4, HNO3, and HNO3/H2SO4 and

functionalized with 60% APTS are recorded to be higher than the pristine MWCNT

with the value of 64.99 cm3/g, 93.02 cm

3/g, and 127.61 cm

3/g, respectively,

comparing to the value of pristine MWCNT which is 2.90 cm3/g. The optimum

modification’s condition is represented by the sample treated with HNO3/H2SO4 and

functionalized with 60% APTS because its ability to adsorb the highest amount of

CO2 (127.61 cm3/g) due to the most attachment of amine group.

ABSTRAK

Ciri-ciri unik tiubnano karbon berbilang dinding (MWCNT) seperti keliangan dan

luas permukaan yang tinggi, dan kewujudan kumpulan berfungsi permukaan melalui

pengubahsuaian kimia menjadikannya bahan yang sesuai untuk penjerapan karbon

dioksida (CO2). Walaubagaimanapun disebabkan sifat hidrofobik dan keadaan

tiubnano yang bergumpal, menyebabkannnya tidak dapat berfungsi sebagai bahan

penjerap yang berkesan. Oleh itu, rawatan diperlukan oleh MWCNT untuk mengatasi

masalah-masalah ini sebelum MWCNT boleh difungsikan dengan kumpulan amina

yang berkeupayaan untuk menjerap CO2. Objektif penyelidikan ini ialah untuk

mengoptimumkan rawatan permukaan dan pengfungsian MWCNT dengan 3-

aminopropyl triethoxysilane (APTS) untuk meningkatkan penjerapan CO2. Tiga jenis

asid telah digunakan untuk rawatan permukaan iaitu asid sulfurik (H2SO4), asid nitrik

(HNO3) dan campuran antara HNO3 dan H2SO4 (HNO3/H2SO4). Proses pengfungsian

dengan 30% dan 60% APTS dilakukan selepas rawatan permukaan untuk menyiasat

kesan kepekatan APTS kepada pelekatan kumpulan berfungsi amina, seterusnya

keupayaan MWCNT yang diubahsuai untuk menjerap CO2. Sampel MWCNT yang

dirawat dengan HNO3/H2SO4 didapati mempunyai paling banyak lekatan kumpulan

karboksil and hidrofilik yang menyediakan lebih ruang aktif kepada tiubnano untuk

lekatan kumpulan amina dalam pengfungsian dengan APTS. Kecekapan penjerapan

CO2 untuk MWCNT yang dirawat dengan H2SO4, HNO3, dan HNO3/H2SO4 dan

difungsikan dengan APTS yang berkepekatan 60%, masing-masing telah merekodkan

penjerapan sebanyak 64.99 cm3/g, 93.02 cm

3/g, and 127.61 cm

3/g berbanding dengan

MWCNT asli iaitu sebanyak 2.90 cm3/g. Pengubahsuaian yang paling optimum

diwakili oleh sampel yang dirawat dengan HNO3/H2SO4 dan difungsikan dengan

60% APTS kerana keupayaan sampel ini untuk menjerap CO2 dengan kapasiti yang

paling tinggi iaitu sebanyak 127.61 cm3/g disebabkan kelebihannya yang mempunyai

pelekatan kumpulan berfungsi amina yang paling banyak.

In compliance with the terms of the Copyright Act 1987 and the IP Policy of the

university, the copyright of this thesis has been reassigned by the author to the legal

entity of the university,

Institute of Technology PETRONAS Sdn Bhd.

Due acknowledgement shall always be made of the use of any material contained

in, or derived from, this thesis.

© Syuhaidah bt Rahmam, 2014

Institute of Technology PETRONAS Sdn Bhd

All rights reserved.

TABLE OF CONTENT

ABSTRACT ............................................................................................................................. vii

ABSTRAK .............................................................................................................................. viii

LIST OF FIGURES ................................................................................................................. xii

LIST OF TABLES .................................................................................................................. xiv

CHAPTER 1 INTRODUCTION ................................................................................... 1

1.1 Background study .............................................................................................. 1

1.2 Motivation of study ........................................................................................... 3

1.3 Problem statement ............................................................................................. 4

1.4 Objectives .......................................................................................................... 6

1.5 Scope of study.................................................................................................... 6

1.6 Thesis outline ..................................................................................................... 8

CHAPTER 2 LITERATURE RIVIEW ....................................................................... 11

2.1 Introduction of carbon dioxide ........................................................................ 11

2.2 Greenhouse effect ............................................................................................ 13

2.3 Global warming and climate change ............................................................... 15

2.4 Technique in capturing CO2 ............................................................................ 18

2.5 Adsorption ....................................................................................................... 19

2.5.1 Theory of adsorption ........................................................................... 19

2.5.2 Adsorption isotherm ............................................................................ 21

2.6 MWCNT as a potential adsorbent ................................................................... 22

2.6.1 Technique in synthesizing MWCNT ................................................... 22

2.6.2 Hydrophobicity of MWCNT ............................................................... 23

2.6.3 CO2 adsorption mechanism by MWCNT ............................................ 25

2.7 Related research work by other researchers .................................................... 26

2.8 Hypothesis ....................................................................................................... 29

2.9 Summary .......................................................................................................... 30

CHAPTER 3 METHODOLOGY ................................................................................ 31

3.1 Material ............................................................................................................ 31

3.2 Flow chart of the research project ................................................................... 32

3.3 Modification of MWCNT ................................................................................ 32

3.3.1 Treatment of MWCNT ........................................................................ 33

3.3.2 Functionalization of MWCNT ............................................................ 33

3.3.3 Overall procedure ................................................................................ 34

3.4 Characterization ............................................................................................... 35

3.4.1 SEM-EDX ........................................................................................... 35

3.4.2 FTIR .................................................................................................... 36

3.4.3 SAP ...................................................................................................... 36

3.4.4 Raman spectroscopy ............................................................................ 36

3.5 CO2 adsorption measurement .......................................................................... 37

3.5.1 Pre treatment ....................................................................................... 38

3.5.2 Dead volume measurement ................................................................. 38

3.5.3 Adsorption measurement ..................................................................... 39

3.5.4 Desorption measurement ..................................................................... 41

3.6 Summary .......................................................................................................... 42

CHAPTER 4 RESULTS AND DISCUSSION ............................................................ 43

4.1 SEM-EDX........................................................................................................ 43

4.2 FTIR ................................................................................................................. 46

4.3 Surface area and porosity analyzer (SAP) ....................................................... 53

4.4 Raman Spectroscopy ....................................................................................... 57

4.5 CO2 adsorption................................................................................................. 60

4.5.1 CO2 isotherm ....................................................................................... 62

4.6 Comparison with other research work ............................................................. 64

4.7 Optimization of the material ............................................................................ 65

CHAPTER 5 CONCLUSION...................................................................................... 67

5.1 Conclusion ....................................................................................................... 67

5.2 Recommendations............................................................................................ 68

RFERENCES…………………………………………………………………………………69

APPENDIX A KYOTO PROTOCOL ......................................................................... 79

APPENDIX B OVERALL PROCEDURE ................................................................ 811

APPENDIX C LIST OF PUBLICTIONS, CONFERENCES, AND EXHIBITION 855

LIST OF FIGURES

Figure 1.1: Keeling curve [1] ......................................................................................... 2

Figure 1.2: (a) Multiwalled carbon nanotubes (b) Single walled carbon nanotubes ..... 3

Figure 1.3: Overall flow chart of the study .................................................................... 8

Figure 2.1: Composition of CO2 [16] ........................................................................... 11

Figure 2.2: CO2 emission by countries [27]................................................................. 12

Figure 2.3: Harshest area in Antarctica ........................................................................ 13

Figure 2.4: The cycle of greenhouse effect [35] .......................................................... 14

Figure 2.5: Correlation between temperature and mortality [49] ................................ 17

Figure 2.6: a) Monolayer adsorption for chemisorption b) Multilayer adsorption for

physisorption [69] ........................................................................................................ 20

Figure 2.7: Type of isotherm [69] ................................................................................ 21

Figure 2.8: Adsorption sites for MWCNT described as (1) Internal, (2) Interstitial

channel, (3) External grooves sites, (4) External surface [10] ..................................... 26

Figure 3.1: Process flow of the research project ........................................................ 322

Figure 3.2: Overall procedure of modification of MWCNT ...................................... 344

Figure 3.3: Summarized procedure for CO2 adsorption using BELSORP-mini ....... 388

Figure 3.4: Diagram for calculating the dead volume ............................................... 399

Figure 3.5: Diagram for calculating adsorption measurement ..................................... 40

Figure 4.1: Morphology of pristine MWCNT (A), treated MWCNT with H2SO4 (B),

treated MWCNT with HNO3 (C), and treated MWCNT with HNO3/H2SO4 (D) ..... 444

Figure 4.2: FTIR spectra for pristine MWCNT (A), treated MWCNT with H2SO4 (B),

treated MWCNT with HNO3 (C), treated MWCNT with HNO3/H2SO4 (D). ........... 477

Figure 4.3: The attachment of amine group by chemical reaction and through defect

sites group [106]. ....................................................................................................... 499

Figure 4.4: FTIR analysis after functionalized with 30% APTS and treated with

H2SO4 (E), HNO3 (F) and HNO3/H2SO4 (G) ............................................................... 50


H2SO4 (H), HNO3 (I) and HNO3/H2SO4 (J)............................................................... 511

Figure 4.6: Isotherm plot on the adsorption/desorption of nitrogen .......................... 544

Figure 4.7: Pore size distribution for sample A, B, C, D, E, F, G, H, I, and J ........... 566

Figure 4.8: Raman spectra for sample A, B, C, D, E, and F ...................................... 588

Figure 4.9: Raman spectra for sample G, H, I, and J ................................................. 599

Figure 4.10: Maximum uptake for CO2 adsorption from sample A, E, F, G, H, I, and J.

Result are expressed in mean ± standard error (n=3) .................................................. 60

Figure 4.11: CO2 adsorption for sample A, E, F, and G ............................................ 622

Figure 4.12: CO2 adsorption for sample H, I, and J ................................................... 622

Figure 4.13: Adsorption and desorption graph of CO2 by sample J .......................... 655

LIST OF TABLES

Table 3.1: Labeling for pristine, treated, and functionalized samples ....................... 355

Table 4.1: Elements in MWCNT as analyzed by EDX ............................................. 455

Table 4.2: Percentage difference of surface area and pore volume ........................... 555

Table 4.3: Degree of functionalization ...................................................................... 577

Table 4.4: Maximum uptake for CO2 adsorption and the percentage difference of

adsorption relative to Sample A ................................................................................... 60

NOMENCLATURE

P (ref) 1 Pressure of dead volume reference cell when the liquid nitrogen up to level

P (ref) 2 Pressure of dead volume reference cell when the liquid nitrogen decreases to

level 2

P1i (n) Pressure of adsorptive that was dosed from valve A to Vs at nth

measurement point

P2 Pressure detected by sensor P2 at adsorption equilibrium after valve C was

opened

T Absolute temperature of Vs

Vd Dead volume

Vd (ref) (1) Dead volume reference cell when the liquid nitrogen up to level 1

Vd (ref) 2 Dead volume of reference cell when the liquid nitrogen decreases to level 2

Vd (smp) 1 Dead volume of sample cell when the liquid nitrogen up to level 1

Vd (smp) 2 Dead volume of sample cell at level 2

Vs Standard volume

Ws Mass of adsorbent

V1 The change of gas amount inside Vs

V2 The amount that was existed in Vd before adsorption

ADS Adsorption

APTS 3-Aminopropyl triethoxysilane

FNCLZTN Functionalization

FR For

CNF Carbon nanofiber

CNT Carbon nanotubes

CO2 Carbon dioxide

HNO3 Nitric acid

H2SO4 Sulfuric acid

HCl Hydrochloric acid

H2O2 Hydrogen peroxide

MWCNT Multiwalled carbon nanotubes

OPT Optimization

SRFC Surface

TRTMT Treatment

CHAPTER 1

INTRODUCTION

This chapter explains the background of study and the issues that motivate the

research to be conducted in this area. The problem statement, objectives, and scope of

study are also discussed throughout Chapter 1. This chapter ends by summarizing the

information that will be presented in the following chapters.

1.1 Background study

The concentration of carbon dioxide (CO2) in the air is at the alarming rate and

this situation will contribute to the negative implications if there is no action taken to

solve or at least control this problem. The concentration of CO2 that is continuously

measured in dry air at the Hawaiian volcano Mauna Loa since March 1958 until 2012

shows the increasing of CO2 concentration in atmosphere from 315 ppm to 395 ppm

[1] . The trend of the increasing CO2 is represented in the Keeling curve as shown in

Figure 1.1 below. However, the recent data reported that the monthly average of CO2

concentration in atmosphere for June 2013 and June 2014 was 398.58 ppm and

401.14 ppm, respectively [2]. It almost hit the unsafe level of CO2 concentration in

atmosphere which is 426 ppm [3]. The pattern keep increasing year by year just like

the one shown in Keeling curve in Figure 1.1. This terrifying pattern will contribute to

the disasters and calamity. Development that constantly arise, expedite the

concentration of CO2 in atmosphere that is surely will give bad implications to the

world and living organism.

2

To date, there are several technologies that are available for CO2 capture such as

absorption [4, 5], cryogenic separation [5, 6], membrane separation [5-7], and

adsorption [6, 7] . Among those technologies, adsorption has been recognized as the

most feasible method due to the less energy intensive, high CO2 removal efficiency,

simple in design, and ease of handling [7]. Selection of adsorbent is necessary to

ensure that CO2 can be adsorbed at its maximum capacity. Thus, carbon adsorbent is

preferable and widely used because of the easy accessibility, low densities,

interesting recycling characteristics, pore structures, material chemical stability,

possibility to synthesize diverse carbon structures and activate their surface by post-

synthesis processes [8] .

Possible carbon adsorbents based are like activated carbon (AC) [9], carbon

nanotubes (CNT) [9], and carbon nanofiber (CNF) [10]. CNT has been identified as

the promising adsorbent because of its nanosized structure with high surface area [11]

and high porosity [11]. These elements are very essential in adsorption to ensure high

capacity of CO2. Plus, CNT can provide optimum performance in adsorption due to

the existence of wide spectrum of surface functional groups which can be achieved by

chemical modification [12]. There are two recognized types of CNT which are single-

walled carbon nanotubes (SWCNT) [13] and multiwalled carbon nanotubes

Figure 1.1: Keeling curve [1]

Conce

ntr

atio

n (

ppm

)

Year

3

(MWCNT) [13] as depicted in the Figure 1.2 below. However, this research work is

focusing on the modification of MWCNT only with the several reasons such as

MWCNT is easier to be synthesized in a bulk compared to SWCNT. Synthesizing

bulk SWCNT requires proper control over growth and atmospheric condition [14].

The other reasons are MWCNT is higher in purity as compared to SWCNT [14] and

the properties of MWCNT is relatively unaffected during functionalization because of

the multi layered structure of MWCNT [15].

1.2 Motivation of study

Since the declaration of Kyoto protocol in February 2005, 37 industrialized

countries and the European Community committed to reduce greenhouse gases

(GHG) emissions to an average of 5% against 1990 levels [16]. During the second

commitment period, they committed to reduce GHG emissions by at least 18% below

1990 levels for eight years period starting from 2013 to 2020 [16]. There are 43

countries (Appendix A) that agreed to sign for the second commitment period of

Kyoto Protocol.

In order to achieve the target, various methods in capturing CO2 are extensively

researched to satisfy the requirement of the protocol. Concerning on the global

warming and climate change issue like rising of sea level and extreme weather events,

Figure 1.2: (a) Multiwalled carbon nanotubes (b) Single walled carbon nanotubes

4

causes many researchers to passionately explore the best method for CO2 capture. So

far, adsorption method is recognized as the most promising method in adsorbing CO2

due to the less energy consumption, high CO2 removal, and ease of handling and

operating. Anticipating for the disaster to occur due to the high emission of CO2 that

will affect the earth, people, and living organism, motivate the continuity search for

the best material that can adsorb CO2 efficiently. Here, the work is focuses on finding

the suitable material that can adsorb CO2 at the maximum capacity through chemical

modification.

MWCNT is one type of adsorbent that can adsorb CO2 due to the nanosized

structure of MWCNT that will provide high porosity and high surface area [17].

These two criteria are very important to ensure the ability of MWCNT to adsorb high

efficiency of CO2 through physical adsorption. In addition, MWCNT can adsorb more

CO2 with the assistance of functional group that can be obtained through chemical

modification. The main focus of functional group that needs to be obtained is amine

group that can adsorb CO2 through chemisorption. The attraction of CO2 and amine

group is based on the concept of acid-base reaction, where CO2 acts as acid while

amine group acts as base [18]. The functionalization of MWCNT to get amine group

can be done with amine precursor like 3-Aminopropyl triethoxysilane (APTS). More

discussion on the success of modifying MWCNT to be a good adsorbent in adsorbing

CO2 by other researchers will be discussed deeper in literature review.

The situation above motivates the author to study deeper on the CO2 adsorption

and the ability of the adsorbent (MWCNT) to adsorb high capacity of CO2. With the

successful in modifying MWCNT to be a good adsorbent, it can helps to reduce the

concentration of CO2 in atmosphere and prevent the global warming and climate

change to occur rapidly due to the greenhouse effect.

5

1.3 Problem statement

In-depth study and investigation should be carried out to reduce the emission of

CO2 to the atmosphere. Disadvantages of other techniques such as absorption,

membrane separation, and cryogenic distillation in capturing CO2, make adsorption as

a promising method to adsorb CO2 and MWCNT is the optimum adsorbent in

adsorbing CO2. However, pristine MWCNT is not able to adsorb high capacity of

CO2 due to its hydrophobic nature and the bundled, agglomerated, and entangled

nanotubes.

In order for MWCNT to adsorb high capacity of CO2, it needs assistance from

functional groups. The main focus of functional group here is amine group that can be

obtained through the functionalization with APTS. With the assistance from amine

group, CO2 can be adsorbed into MWCNT through two mechanisms which are

physical and chemical adsorption. However, due to the hydrophobic and

agglomeration of nanotubes issue, it will create lack of dispersibility of MWCNT in

APTS solution during the functionalization and limit the space for amine group to be

attached on the surface of MWCNT. As a result, only less amine group is allowed to

be attached on MWCNT, and will not give good result in adsorbing high capacity of

CO2.

Some researchers had done studies on the capability of acids to reduce the

hydrophobic problem and the agglomeration of nanotubes, but none of the researchers

had studies on the effect of different acids towards the attachment of amine group. In

addition, there are also studies on the ability of amine group that has been

functionalized with APTS to assist MWCNT in adsorbing CO2. However, there are no

studies yet on the effect of different concentration of APTS towards the attachment of

amine group onto the surface of MWCNT after treating with different types of acids,

hence the ability of MWCNT to adsorb high capacity of CO2.

Therefore, this study is crucial to be carried out in order to investigate the effect of

different types of acids towards the reduction of hydrophobic problem and the

agglomeration of nanotubes. The experiment on different concentration of APTS will

be conducted as well to observe the attachment of amine group, and the ability of

6

amine group in assisting MWCNT for CO2 adsorption. Further and systematic

investigation is necessary to explore the special characteristics and the capability of

pristine and modified MWCNT as the potential adsorbent. This study will provide a

solution to effectively capture CO2, and leading to a better environment for mankind.

1.4 Objectives

There are several objectives that need to be achieved in this research work. They

are:

1) To resolve the hydrophobic nature and the bundling issue of MWCNT by

surface treatment.

2) To investigate the characteristics of MWCNT for before and after

modification by characterizing them with analytical instruments.

3) To increase the efficiency of CO2 adsorption by functionalizing MWCNT with

APTS.

4) To find the optimum modification’s condition that can adsorb high capacity of

CO2.

The research work is considered to be successful if all the objectives can be

achieved.

1.5 Scope of study

The study is started by understanding the properties and the nature of MWCNT,

together with the related research work done by other researchers before proceeding

with the experimental work. Literature review study was carried out at the initial state

as it will provide the information and theory related to the research work. However,

the utilization of literature review did not stop at the initial state only, but

continuously applied for knowledge, reference, and guideline.

7

For the experimental work, it can be divided into three parts involving

modification of MWCNT, characterization of pristine and modified MWCNT and the

testing of CO2 adsorption. In modification part, it can be divided into two tasks

namely surface treatment and functionalization of MWCNT. The purpose of surface

treatment is to reduce the hydrophobic problem and loosen the agglomerated and

entangled nanotubes of pristine MWCNT, whilst the purpose of functionalization is to

have the attachment of amine group that will assist MWCNT in adsorbing CO2. For

surface treatment, different types of acid were used such as sulfuric acid (H2SO4),

nitric acid (HNO3), and the mixture of nitric and sulfuric acid (HNO3/H2SO4), while

for functionalization, 3-Aminopropyl triethoxysilane (APTS) was used.

After modifying MWCNT, the samples of pristine and modified MWCNT were

characterized using scanning electron microscopy – energy dispersive X-ray (SEM-

EDX), surface area and porosity (SAP) analyzer, Fourier transform infrared (FTIR),

and Raman Spectroscopy. The outcome from the characterization will allow for

identification of the best condition required to give the highest amount of CO2

adsorbed.

For the last part, pristine and modified samples will be tested for CO2 adsorption

using BELSORP-mini. The overall flow chart for the work study is summarized in

Figure 1.3.

8

1.6 Thesis outline

There are five chapters starting from chapter 1 on introduction, followed by

chapter 2 on literature review, chapter 3 on research methodology, chapter 4 on result

and discussion, and finally chapter 5 on conclusion and recommendation for future

work.

Chapter 1 discusses on the current phenomena related to high emission of CO2

and the effect of high emission of CO2 that lead to the overall idea of this research. As

the emission of CO2 is keep increasing over the years, an effective method must be

used to solve this problem. Past studies have shown that adsorption method, together

Literature review

Modification of MWCNT

Surface treatment

Functionalization

Characterization of MWCNT

SEM – EDX

FTIR

SAP

Raman Spectroscopy

Testing on CO2 adsorption

Figure 1.3: Overall flow chart of the study

9

with the modified MWCNT as adsorbent is found out to give significant result in

adsorbing CO2. This chapter also highlighted the problem statement, objective, and

scope of study.

In the literature review of Chapter 2, discussion is on the related research work

done by other researchers. This chapter also provides information on the

environmental issue that need to be addressed and understood before proceeding with

research hypothesis. Introduction of CO2, the effect due to the high emission of CO2 in

atmosphere, available techniques in capturing CO2, and characteristics of MWCNT,

are some of crucial parts that are necessary to be discussed to give the overall picture

of the research. It also discusses on the treatment and functionalization method

conducted by other researcher as the reference and comparison.

Chapter 3 on research methodology part explains the experimental methods to be

carried out. The experiment is divided into four steps:

1. Treatment of MWCNT by different types of acids such as H2SO4, HNO3, and

the mixture of H2SO4 and HNO3

2. Functionalization of MWCNT using APTS to get amine functional group for

assisting MWCNT to adsorb CO2

3. Characterization of MWCNT to know the characteristics of pristine and

modified MWCNT using SEM-EDX, SAP, FTIR, and Raman spectroscopy

4. Testing for CO2 adsorption by using BELSORP-mini

Detailed procedure for each step is presented in this chapter.

All results from SEM-EDX, SAP, FTIR, and Raman spectroscopy are presented,

discussed and analyzed in Chapter 4. From the analysis, characteristics between the

pristine and modified MWCNT samples can be compared. Prediction can be made on

which of the samples give the highest efficiency of CO2 adsorption based on the

characteristics obtained. To validate the predicted behavior based on the

characteristics of MWCNT, testing for CO2 adsorption will be carried out using

BELSORP-mini equipment.

10

In the conclusion part of Chapter 5, the research work is summarized and some

recommendations for future study are given. In addition, the benefits that can be

gained from the findings of this research work are elaborated in this chapter.

CHAPTER 2

LITERATURE RIVIEW

Chapter two emphasizes the definition, theory, and research work form others.

Basically this chapter will be divided into several sections beginning with the

introduction of CO2, followed by the effect of high concentration of CO2 in

atmosphere, techniques for capturing CO2, adsorbent available for CO2 adsorption,

and finally the functionalization of MWCNT as the adsorbent.

2.1 Introduction of carbon dioxide

Carbon dioxide (CO2) consists of one carbon atom and two oxygen atoms [19] as

shown in the Figure 2.1 below.

CO2 is colorless, odorless, non-flammable, heavier than air, and soluble in water

[20]. It has been applied in many sectors and applications like food [21],

pharmaceutical [21], chemical industry [22], fire extinguisher [23], as a solvent [24],

and as a fumigation [25]. CO2 can be produced naturally from the respiration process

and volcanic outgassing and from the human’s activities as well. CO2 emission

Figure 2.1: Composition of CO2 [16]

12

contributed by human activities can be classified into four main categories which are

transportation [26, 27], the combustion of fossil fuel such as coal, natural gas or

petroleum, industrial process like oil refining, power plant, the production of cement,

iron, and steel [28, 29] and lastly is the land-use change [26]. The land-use change

means the conversion of land from forested to agricultural land and this phenomena is

estimated to contribute from 10 - 30% of the current emission of CO2 [26]. Release of

CO2 due to the human’s activity becomes the main factor of the increasing CO2

concentration in the atmosphere leading to the greenhouse effect that will expedite the

global warming phenomena. Figure 2.2 below shows the data on the CO2 emission by

countries from fossil fuel use and cement production [30].

Two points can be concluded based on the data in Figure 2.2. The first one is the

high CO2 emission is released by developed countries like Australia, Canada, Russia,

and United States compared to the other countries that can be considered as

developing countries. The second point is that most of the countries show higher

emission of CO2 in 2011 compared to the years before due to the development done

by the respective countries. There is no doubt that high emission of CO2 will give bad

implication to the life and environment. Overloaded emission of CO2 will disturb the

normal cycle of greenhouse effect in the earth.

Figure 2.2: CO2 emission by countries [27]

0

5

10

15

20

25

Au

stra

lia

Bra

zil

Can

ada

Ch

ina

Eu2

7

Fran

ce

Ger

man

y

Ind

ia

Ind

on

esi

a

Iran

Ital

y

Jap

an

Me

xico

Net

her

lan

ds

Po

lan

d

Russian…

Sau

di A

rab

ia

Sou

th A

fric

a

Sou

th K

ore

a

Spai

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Taiw

an

Thai

lan

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UK

Ukr

ain

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US

Sum of 1990

Sum of 2000

Sum of 2011

13

2.2 Greenhouse effect

CO2 can be considered as the main anthropogenic contributor to the global

warming, and is identified as a greenhouse gasses other than methane, nitrous oxide

[31, 32] and water vapor [32]. Greenhouse gases are the gases that absorb the

radiation emitted by the earth and reradiate the energy again in the form of heat [33].

The concept of greenhouse effect on earth is based on the greenhouse nursery for

plants where heat will be trapped in the house to make it warm [34] . Similar to the

earth, natural greenhouse effect is necessary to provide a comfort condition for the

livings to live comfortably; not too cool, and not too hot. Earth without greenhouse

effect is something that is impossible and unimaginable. If greenhouse effect does not

occur, the temperature of this planet will be very cold, about -0.4oF or -18

oC and

most likely with very little liquid available [34]. The ecosystem of the entire earth

will be similar to the harshest area located in Antarctica [34]. Figure 2.3 below shows

phenomena of harshest area in Antartica. Most of the living organism surely cannot

adapt and survive with the coldness of the earth with very little liquid available.

Greenhouse effect occurs with the concept of heat transfer where the solar energy

reaches the atmosphere through radiation and reflected back to the space. The

remaining energy will pass through to the earth’s surface and some of them will be

reflected back to the space and some of them will be absorbed and converted into heat

[35]. About 30% of the radiation is reflected back to the space by cloud, snow fields,

and other reflective surfaces and about 70% is passed through to the earth’s surface

Figure 2.3: Harshest area in Antarctica

14

and absorbed by ocean, land, plants, and other things [36]. This phenomena creates

‘partial blanket effect’ [37] that traps the heat and makes this planet warm. Figure

2.4 explains how the greenhouse effect occurs on earth [38].

Figure 2.4: The cycle of greenhouse effect [35]

Natural greenhouse effect is good but the unnatural greenhouse effect does not

give any benefits at all. The unnatural greenhouse effect is referring to the situation

where too much heat is absorbed by greenhouse gas and makes the earth hotter.

According to the National Oceanic and Atmospheric Administration (NOAA) Earth

Research System Laboratory, the concentration of CO2 has been increased by about

40% from the pre-industrial era [39] and contributes to the unnatural greenhouse

effect to the earth.

One of the French mathematician-physicist named Fourier made a hypothesis that

the atmosphere experienced decrease reflection from the earth and this theory was

supported by Tyndall in 1861 [40]. Water vapor and CO2 which are basic components

in the atmosphere absorb part of the energy that departs from the surface in a form of

infrared radiation [40]. After that, part of the absorbed energy will be reradiated back

15

to the surface and increase the temperature of the earth [40]. As the concentration of

CO2 is getting higher due to human’s activity, the heat that reradiated back to the earth

become higher, thus contributes to the hotness of the earth. As a result of the

increasing of CO2 that is also greenhouse gas, global warming and climate change

will occur and disturb the natural ecosystem. Among the greenhouse gases, CO2 has

the highest portion in term of the amount of gases present in atmosphere and

contributing 60% of global warming effect [41].

2.3 Global warming and climate change

Global warming and climate change like rising of sea level [42], sea-surface

warming [43], and extreme weather events [44] are some examples that attributed by

the anthropogenic emission of CO2 .This is a result on the increasing amount of heat

trapped by CO2 in atmosphere that makes the earth getting hotter from time to time.

When the temperature keeps rising more than it supposed to be, it disturbs the

ecosystem and contributed to the calamity. One of the calamity occurred due to the

climate change is the occurrence of killer storm. In May 2008, Irrawaddy delta which

is located in Myanmar was hit by a tragedy called Cyclone Nargis that had killed

about 100,000 of the Mynmar’s people [45]. Not only that, three years before

Myanmar’s tragedy, New Orleans was attacked by Hurricane Katrina in August that

showed the most significant virulence among other places that experienced Hurricane

Katrina before [46]. This natural disease had caused death to more than 1000 people

at New Orleans area and with an additional about 306 death in nearby southern

Mississippi [47] . In addition, more than 300 people were reported missing and the

damage cost was about $100 to $200 billion [47]. Even though there are no significant

value of the increasing average number or intensity of the storm, but statistic shows

the significant shift in distribution towards stronger storm that wreak the greatest

havoc. The shift observed is due to the rising ocean temperature as the effect of the

global warming [48].

Besides that, global warming impacts our life and health. Obvious effects are on

the death and injury caused by extreme weather such as Cyclone Nargis and

16

Hurricane Katrina. People will also suffer from the illness aftermath the havoc due to

the infection of virus and bacteria. Some infectious agents are very sensitive to the

climatic change. For example, salmonella in animal gut and food, and cholera bacteria

in water, both of them proliferate more rapidly in higher temperature [49]. However,

the degree of infectious of cholera in the water is depends on the geographical

structure of the respective country [50]. For a country that has a good and well

established water, food and sewage system, the bad effect on the infection will be less

[50] and the amount of people effected by the cholera bacteria will be low as well.

In addition, global warming put up a direct effect disease which is respiratory

illness due to the worsening of air quality related to change in temperature [51] and

heat shock [50]. Heat shock is a problem influenced most directly by the ambient

temperature. A study was carried out in Japan that discusses on the relationship

between the temperature and the number of heat shock cases that involve most of the

major cities in the Japan. It was found that the number of heat shock will increase

drastically when the temperature become 32oC and higher [50]. Based on Japan study,

it can be assumed that the global warming problem will increase the number of heat

shock patient.

The event of extremely hot and cold because of the climate change affect the

mortality rate of the people. Especially the elderly, who are considered to be

vulnerable, lesser antibody, and susceptible to disease, the value of mortality is

higher. A research done by Yu, 2012 [52] reported that as a whole, there were 2–5%

increase in mortality for 1°C increment during hot temperature intervals, and 1–2 %

increase in mortality for 1°C decrease during cold temperature intervals. The relation

between temperature and mortality is varying very much on the latitude and climate

zone aspects. People in colder place get influence easily towards hot temperature

compared to people that lives in hotter area. Figure 2.5 below shows on the

correlation between temperature and mortality.

17

Figure 2.5: Correlation between temperature and mortality [49]

The graph above explains the daily temperature and the number of daily death

based on 2005 and 2050 annual temperature range. It is expected that by 2050, there

will be many mortality cases that are related to the high heat condition due to the

global warming. Favre, 2011 [53] reported that the concentration of CO2 is predicted

to be doubled in 2050 if there is no comprehensive actions taken to solve this

problem.

In a nutshell, there are many bad impacts from the climate change and global

warming as discussed above. It just not causes the damage of material and the waste

of money, but the worst thing is it causes death to human and other living organism.

As a whole, it will change and disturb the harmony biodiversity of this planet.

Without awareness of this issue, the horrible scenario will become worse from time to

time. In order to prevent the global warming and climate change become worse, the

comprehensive prevention in adsorbing CO2 must be taken.

18

2.4 Technique in capturing CO2

It is crucial to control the emission of CO2 in the industrial sectors such as power

plant, petroleum industry, food and beverage processes, polymer industries, and many

more. Extensive works have been carried out to develop the best material and method

that can capture CO2 at the optimum capacity. There are various techniques available

to capture CO2 for example absorption [54], adsorption [55], membrane separation

[56], and cryogenic separation [57].

Absorption with aqueous amine is a matured technology and widely established in

the industry [41, 58, 59]. The normal practice of this technique is by using

monoethanolamine (MEA) as a solvent. However, the use of MEA gives many

disadvantages throughout the process such as corrosion problem [41, 60], large

amount of energy required during regeneration [41, 59, 60], large equipment [41, 60],

large solvent consumption [60], high operating cost [59, 60], amine degradation by

sulfur dioxide (SO2), nitrogen dioxide (NO2), hydrochloric acid (HCl), hydrogen

fluoride (HF) and oxygen (O2) when using in natural gas industry [41], and low

carbon dioxide loading capacity (g CO2 adsorbed/g adsorbent) [41].

Another well-established CO2 capture technique in industry are cryogenic

separation [58] and membrane separation [59] . For cryogenic separation, it has an

advantage where it enables recovery of pure component in the form of liquid CO2

which is needed for transportation by ship or pipeline [7, 60]. Unfortunately, there are

some significant disadvantages for cryogenic distillation like complicated flow

streams, low thermal efficiency, and high capital cost and utility requirement [7] that

make this technique unfavorable. Membrane separation can be an efficient method if

the species that need to pass through the membrane (in this case, the species is CO2) is

in high concentration, otherwise it could not be a feasible separation technique [58].

Besides that, it is necessary for the membrane separation to have multiple stages of

separation because it might not achieve high degree of separation at one stage only,

and the multiple stages process will contribute to high energy consumption and high

cost [60].

19

While the techniques discussed above (absorption, cryogenic separation, and

membrane separation) provide many drawbacks especially in the aspect of high

energy consumption, high cost, and large equipment, adsorption method seems to be

feasible for CO2 capture. With the right combination of adsorption technique with

adsorbent, this method can be an excellent way to adsorb CO2. Even though

adsorption is still considered under research [41] but the advantages of the adsorption

methods must be highlighted as well. Less energy intensive, high CO2 removal

efficiency, simple in design and easy to handle and operate are the advantages owned

by adsorption technique [7]. The feasibility of the adsorption method is supported by

the Intergovernmental Panel on Climate Change (IPCC) that reported on the

feasibility of adsorption [12, 61] with the advancement of new generation of material

adsorbents that will adsorb CO2 efficiently like activated carbon [6, 29], zeolites [6,

29], single walled carbon nanotubes (SWCNT) [62] and multi walled carbon

nanotubes (MWCNT) [12, 63].

2.5 Adsorption

2.5.1 Theory of adsorption

Adsorption is a chemical process that takes place when a gas or liquid, most

commonly gas accumulated on the surface of solid (adsorbent) [64], forming a

molecular or atomic film (adsorbate). Molecules and atoms can be attached onto the

surface in two ways which are physical adsorption (physisorption) or/and chemical

adsorption (chemisorptions) [65]. Physisorption involves van der Waals attraction in

which the adsorbate is attracted to the surface of adsorbent [66], whilst chemisorption

is a type of chemical reaction adsorption that involves the exchange and sharing of

electrons between the adsorbate molecule and the surface of adsorbent [67]. The

chemisorption interaction is much stronger than physisorption because of the

chemical bond formed [67], compared to the pysisorption that only involves weak

intermolecular interaction.

20

In general, the heat of adsorption for physisorption is lower than chemisorption,

physisorption can form more than one layer compared to chemisorption that can form

only one layer, and the physisorption process is reversible but chemisorption process

is irreversible [68, 69].

The amount of molecules that are physically adsorbed into the adsorbent is

measured by the relative pressure p/po where p is the partial pressure of the vapor and

po is the vapor pressure of the pure adsorbent at the same temperature. The amount of

physical adsorption is considered to be negligible when p/po is <0.01, and the amount

adsorbed corresponds to monolayer adsorption when p/po is about 0.1. As the

adsorption keeps increasing, the multilayer adsorption will occur until the bulk liquid

reached p/po = 1. For chemisorption, the adsorption is similar to the chemical reaction,

but the process is always slower than physisorption. Only monolayer adsorption takes

place for chemisorption based on its accessibility and capability of forming a

chemical bond with the adsorbent [70].

Figure 2.6 below shows the diagram of monolayer adsorption of chemisorption

and multilayer adsorption for physisorption [71].

Figure 2.6: a) Monolayer adsorption for chemisorption b) Multilayer

adsorption for physisorption [69]

a) b)

21

There are several factors that will affect the adsorption process such as [72]:

1. Nature of the gas

2. Nature of the solid adsorbent

3. Specific area of the solid adsorbent

4. Activation of the solid adsorbent

5. Effect of pressure

6. Effect of temperature

2.5.2 Adsorption isotherm

Usually, physical adsorption is described through the isotherm, the relationship

between pressure (for vapour) or concentration (for liquid), and the capacity adsorbed

by adsorbent. The type of isotherm for vapour/solid equilibria [73] is determined by

International Union of Pure and Applied Chemistry (IUPAC) classification based on

an earlier classification by Brunauer which have five types of isotherm and had now

extended to six types [74] .

Figure 2.7: Type of isotherm [69]

22

The type I isotherm suggests on the ‘approaches a limiting value’ and usually is

used to describe microporous adsorbents [73]. Macroporous adsorbents with strong

and weak adsorbate-adsorbent interactions are described by isotherm type II and III

respectively [75]. Both type IV and V represent adsorption isotherm with hysteresis

behavior [76]. Type IV isotherm is typical for mesoporous adsorbents while type V

hysteresis loop is a typical sign of a weak fluid-wall interaction [77]. Finally, the type

VI isotherm that was not included in Brunauer illustrates that the adsorption isotherm

can have one or more steps [76].

2.6 MWCNT as a potential adsorbent

After discussing on the adsorption process and the advantages of adsorption, it is

important to discuss about the adsorbent that will be used in this research project.

MWCNT was chosen as the potential adsorbent in adsorbing CO2 because of its

remarkable characteristics which has been mentioned in Chapter 1. In this section,

characteristics on MWCNT will be given in order to give a clearer understanding on

the function of MWCNT as adsorbent in adsorbing CO2.

2.6.1 Technique in synthesizing MWCNT

There are several methods to synthesis MWCNT such as chemical vapor

deposition (CVD), arc-discharge, laser ablation [78-80], low temperature solid

pyrolysis, plasma, and electrolysis [81]. Among those techniques, CVD is proven to

be the most suitable method for production of large quantity of high quality MWCNT

at relatively low cost [82]. In addition, the diameter, number of layers, and the growth

of MWCNT is easily control by using the CVD technique [83].

23

2.6.2 Hydrophobicity of MWCNT

MWCNT is a material that are made of a hexagonal lattice of carbon atoms [84]

and consists up to several tens concentric cylindrical graphitic shells [11, 84]. Pristine

MWCNT cannot act as a better adsorbent than modified MWCNT because it faced

two critical issues which are hydrophobic problem that creates lack of dispersibility

and the bundling of nanotubes due to the attraction of van der Waals [85]. These two

problems can be reduced by surface treatment, [86, 87] by using several types of

acids.

The surface treatment will introduce carboxyl group [88-90], that is also

hydrophilic functional group [91] to overcome the hydrophobic problem. For the

treatment that using HNO3, the introduction of surface oxide is started with the attack

of nitronium ion (NO2+) to the aromatic compound of MWCNT [88]. It is called

surface oxide due to the presence of oxygen functional group on the surface of

MWCNT. If the treatment were using H2SO4, the introduction of carboxyl group can

occur as well due to the strong interaction between MWCNT and acid [88]. The

attachment of carboxyl group is also known as carboxylation and it will always be

accompanied by the cutting and etching of nanotubes [89]. The experimental result is

very useful in finding the balance between increasing the solubility and destroying the

nanotubes [89]. In addition, the presence of oxygen-containing functional group

assists the exfoliation of CNT bundle and increase the solubility of MWCNT in polar

solution [92]. Thus, the treated MWCNT has more chances and opportunity for

further functionalization or modification depending on its application.

Besides that, other hydrophilic group that is expected to be attached on the surface

of MWCNT is amino group [93] due to the presence of nitrogen element in HNO3.

Further modification to functionalize MWCNT with APTS will be easier after the

treatment because the nanotubes can disperse better in APTS and the loosely packed

nanotubes will allow more amine functional group to be attached on the surface of

MWCNT.

Moraes et al. [94] had conducted an experimental work to analyze the effect of

the chemical treatment by using several types of acids such as HNO3, H2SO4,

24

hydrogen peroxide (H2O2), HNO3/H2SO4, and HNO3/hydrochloric acid (HCl) on the

properties of MWCNT. Based on the result obtained, it was proven that the chemical

treatment has effectively removed the non-nanotubes carbonaceous in the sample,

offered good removal of free iron-species, increased the homogeneity, and promoted

the increase of carboxyl and hydroxyl group on the surface of MWCNT [94].

Yudianti et al. [95] who used the mixture of HNO3/H2SO4 as a method of

treatment concluded that the treatment caused defects on the surface of MWCNT and

allowed the functional group to attach on it. The carboxyl group was dominantly

attached to the surface of MWCNT, and the density of carboxyl group is strongly

dependent on the strength of acid solution [95].

In general, MWCNT is a chemical inert material that cannot be used in further

application without purifying or treating it first. Remarkably, HNO3, H2SO4, and the

mixture of HNO3/H2SO4 are the commonly used reagents for purification. Other acids

had also been used, but they were not as commonly used as HNO3 and H2SO4. The

versatility, efficiency, and potential to scale up are among the reasons why acids are

commonly and widely used as the treatment and purification agent [96].

In addition, since the nanotubes will be loosen up due to etching by acids; one of

the characteristics that need to be observed is the occurrence of defects on nanotubes

by using Raman spectroscopy analysis and the attachment of functional group on the

defective parts by using FTIR. High stirring speed and long duration of stirring is

expected to give more opportunity for the acids to react with MWCNT through

etching along the nanotubes, producing defects nanotubes with the attachment of

functional groups. Gui et al. had conducted the experiment by using different stirring

duration under reflux conditions such as 1, 5, and 10 hours. It was found that the

sample refluxed for 10 hours gave the highest APTS loading compared to others

duration [97] due to more time for the attachment of amine onto MWCNT. It can be

concluded that longer time for the stirring contribute to more attachment of functional

group. With the help of high speed stirring it is expected that more functional group

can be attached onto the surface of MWCNT.

25

Besides using acid as a medium to reduce the hydrophobicity of MWCNT and

loosen up nanotubes, it was used as a metal removal as well. As reported by Shahidah

et al. [98], acid oxidation is one of the purification methods that will remove

carbonaceous and metal particle impurities [99]. The same technique which is acid

etching that was applied for reduction of hydrophobicity of MWCNT and loosens up

the nanotubes is applied for metal removal as well. Ultimately, acid etching method is

considered as an effective technique to remove metal particles [100] that was

deposited during the production of MWCNT.

After the treatment and purification, the next step, which is to functionalize

MWCNT with APTS will be easier since the MWCNT can now be uniformly or well

dispersed in APTS. The application of MWCNT as a CO2 adsorbent is expected to be

much better than pristine MWCNT due to the favorable characteristics acquired by

the treated and functionalized MWCNT.

2.6.3 CO2 adsorption mechanism by MWCNT

After the treatment of MWCNT and the introduction of hydrophilic group to

overcome the critical problems mentioned above, the functionalization process to get

amine groups that can assist MWCNT in adsorbing CO2 can be proceeded. The

reaction between amine group and carbon dioxide is like acid-base reaction where

amine acts as basic site and CO2 as acid species [18]. There are two mechanisms for

CO2 to be adsorbed onto MWCNT after functionalization. The first method is through

physisorption and the second one is through chemisorptions. For physisorption, CO2

can be adsorbed physically to the bundled of MWCNT at several locations such as

internal (1), interstitial channel (2), external groove sites (3), and external surfaces (4)

[11] as shown in Figure 2.8, meanwhile for chemisorption, CO2 can be adsorbed to

amine group through chemical bonding [63]. The mechanism in adsorbing CO2

through chemisorption will be depicted in Figure 4.3 in Result and Discussion section.

26

2.7 Related research work by other researchers

Van der Waals forces attract the nanotubes of MWCNT to be together [101].

However, the van der Waals force is very weak, and in fact it is weaker than covalent

bond [101]. This force binds the nanotubes and creates agglomeration and bundled

nanotubes. It can be unbundled with the acids that etch along the nanotubes. Acid

etched the nanotubes, create defects, and allow the attachment of carboxyl that is also

a hydrophilic group to overcome the hydrophobic problem.

Many researches have been done on the acid treatment to treat the surface of

MWCNT. Some of them are like Moraes et al., and Yudianti et al. as discussed in

section 2.6.2. Besides them, Aviles et al. have also treated the surface of MWCNT

using various types of acid which are HNO3, H2SO4, and H2O2 at relatively low

concentration, short time, and low sonication power. MWCNT was dissolved in the

ethanol to observe the dissolution of MWCNT after the treatment. It can be observed

that the MWCNT become more disperse in ethanol after the treatment due to the

attachment of functional group that led to a reduction of van der Waals interaction

[96].

Upon the treatment of MWCNT, further functionalization can be done, depending

on the desired application. For the purpose of CO2 adsorption, the main functional

group required is amine group that will act as the base and CO2 as the acid. Therefore,

they will attract to each other based on acid base concept.

Figure 2.8: Adsorption sites for MWCNT described as (1) internal, (2) interstitial

channel, (3) external grooves sites, (4) external surface [10]

27

Work on ‘Capture of CO2 from flue gas via multi walled carbon nanotubes’ was

reported by Su et al. in 2009 [12]. MWCNT sample was modified with APTS and the

CO2 adsorption was tested at multiple temperatures starting from 20oC to 100

oC.

After purification with sodium hydroxide (NaOH), MWCNT was modified with 3

types of amine precursor which are APTS, N-(3-(trimethoxysilyl)propyl)

ethylenediamine (EDA), and poltethylenimine (PEI). An amount of 200 mg of

MWCNT was dispersed in a solution containing APTS, EDA, and PEI. For APTS and

EDA mixture, both were refluxed at 100oC for 10 h and 2 h respectively, whilst the

PEI mixture was stirred for 30 min. The mixture was cooled to room temperature, and

then the mixture was filtered and heated. In order to cover the range of temperature

between 27oC to 77

oC where most flue gas desulfurization adiabatic saturation

temperature lies, the temperature range for CO2 adsorption testing was set between

20oC to 100

oC. To observe the moisture effect, the range of water content in air was

varied from 0 to 17.8%. The CO2 influence was controlled between 5 to 50% and the

system flow rate was controlled at 0.08 L/min. As a result, the CO2 adsorption

increased as the influent of CO2 increasing, but decreased with increasing temperature

due to the exothermic nature of adsorption. The highest reading for CO2 adsorption

was 114 mg/g.

A paper entitled ‘Adsorption of low concentration carbon dioxide on amine

modified carbon nanotubes at ambient temperature’ was reported by Ye et al. in 2012

[102]. CNT was impregnated with tetraethylenepentamine (TEPA) to adsorb low

concentration of CO2 (2.0 vol%) in a fixed bed column. Adsorption experiment was

done by placing 0.5 g of functionalized CNT in a reactor, and the adsorption testing

was done under 298 K by flowing the mixture of N2 and CO2 gas. The adsorbent

achieved high CO2 concentration capacity with the amount of 3.56 mmol/g at the

temperature of 313 K, and the CO2 capacity become higher (3.87 mmol/g) in the

presence of moisture (2.0% H2O). The variation in CO2 adsorption result was reported

to be dependently related to the amount of amine loading.

A research paper on ‘Amine-functionalization of multiwalled carbon nanotubes

for adsorption of carbon dioxide’ was published by Gui et al. in 2012 [103]. The

effect of solvent, thermal treatment, functionalization duration during the

28

functionalization process of MWCNT was investigated. The sample was tested for

CO2 adsorption in order to determine the best sample that can adsorb CO2 at high

capacity. MWCNT was treated with the mixture of acid that contained 30 ml of HNO3

and 90 ml of H2SO4 with the concentration of 5 M each. For functionalization, the

treated MWCNT was continuously stirred with APTS and toluene with the volume

ratio of 1:9 under various conditions. The adsorption testing was carried out at the

temperature of 60oC, with 0.3 g of adsorbent. The gaseous used were CO2 and N2 with

the ratio of 5:95 (v/v), giving a total flow rate of 200 ml/min. The highest CO2 uptake

was 75.4 mg/g represented by the sample that used toluene, had stirring and reflux

duration of 5 h, and reflux temperature of 105oC.

The exothermic characteristic of adsorption requires low temperature of

adsorption to ensure the capability of adsorbent to adsorb high capacity of CO2. In

this case, several research work that studied the modified carbon adsorbent for CO2

adsorption concluded a similar theory based on the experimental findings. For

example, Su et al. [12] that reported work on capturing CO2 from flue gas via

multiwalled carbon nanotubes, had done experiments on different temperature of

adsorption. The adsorbent used was CNT that had been functionalized with APTS, N-

(3-(trimethoxysilyl)propyl) ethylenediamine (EDA), and poltethylenimine (PEI). The

four samples including pure CNT were tested for the adsorption of CO2 at multiple

temperatures ranging from 20oC to 100

oC. As a result, the adsorption capacities for all

samples were decreasing as the temperature increasing indicating the exothermic

nature of adsorption process [12].

Another researcher that reported on the high capacity of CO2 adsorption can be

done by applying lower temperature was Pevida et al. that reported it in ‘Surface

modification of activated carbons for CO2 capture’, published in 2008. The main

objective of his research work is to improve selectivity and capacity of the sorbents to

capture CO2 by the introduction of basic nitrogen-functionalities into the carbons by

using two types of activated carbon which were C (a wood based granular carbon

manufactured by phosphoric acid activation process) and R (peat-based steam

activated extruded carbon with a diameter of 3 mm that possess a superior mechanical

hardness). These two samples were prepared by heat treatment with ammonia gas in

29

the temperature range between 200 - 800oC. Both samples were found to adsorb

highest capacity of CO2 under the condition of 25oC and 800

oC of ammonia heat

treatment’s temperature.

2.8 Hypothesis

As described above, a lot of research had been conducted on the amine

modification of MWCNT. However, there are some parameters that have not been

extensively studied such as the effect of various acids for surface treatment towards

the functionalization and the effect of APTS concentration on the attachment of amine

group. Therefore, this research will begin with various types of acid for the treatment

before proceeding with the functionalization. The hypotheses in the research work

are:

1) Different type of acids will have different effect on the attachment of carboxyl

and hydrophilic group hence directly affecting the attachment of amine group as well.

2) Optimum duration and speed of stirring is required in order to have the

attachment of carboxyl and hydrophilic group for the surface treatment and to get the

amine functional group during the amine functionalization.

3) Sufficiently low concentration of acid and amine group is needed to get the

optimum amount of functional group.

4) More attachment of carboxyl and hydrophilic group upon surface treatment

will result in more presence of amine group upon functionalization, hence

contributing to high amount of CO2 adsorption.

The ability of amine group on the surface of MWCNT in assisting pristine

MWCNT in adsorbing CO2 was proven by a number of research. However, there are

no extensive studies yet on the effect of amine’s concentration on the attachment of

amine group and the effect of different types of acids for the treatment. Thus, this

study will focus on the treatment or purification of MWCNT by using various types of

30

acids such as H2SO4, HNO3, and HNO3/H2SO4, and the functionalization of MWCNT

with different concentration of APTS solution.

2.9 Summary

This chapter had discussed the theory aspect and the related research work done

by other researchers. The next chapter, Chapter 3 will discuss on the methodology

part, which is the steps involved in modifying MWCNT to be a good adsorbent in

adsorbing CO2.

CHAPTER 3

METHODOLOGY

This chapter describes the procedure involved in running the experimental works.

It starts with the experimental procedure on the modification of MWCNT that consists

of treatment and functionalization, followed by the characterization of MWCNT, and

the testing on CO2 adsorption. The following sections explain deeper on the

experimental procedure for each parts of the experiment.

3.1 Material

The main and raw material for this project is MWCNT. Commercial MWCNT

was purchased from CheapTubes.com, fabricated by chemical vapor deposition

(CVD) technique, and have the purity of 95%. The outer diameter of this MWCNT is

less than 8 nm, while the inside diameter is between 2 nm to 5 nm. It has the bulk

density of 0.27 g/cm3, true density of ~2.1 g/cm

3 and the length between 10 µm to 30

µm.

For the treatment of MWCNT, several acids were used such as H2SO4, HNO3, and

the mixture of HNO3 and H2SO4. All acids were purchased from Sigma-Aldrich and

the concentration used was 4M. The functionalization process was used APTS that

was purchased from Sigma-Aldrich as well, and the concentration used was 30% and

60%.

32

3.2 Flow chart of the research project

The flow chart in Figure 3.1 describes briefly on the overall process flow of the

research work.

3.3 Modification of MWCNT

There are two main steps in modification of MWCNT which are the surface

treatment and functionalization of MWCNT. These two steps are necessary to

improve the characteristics of pristine MWCNT to be functioning as a good adsorbent

in adsorbing CO2.

Surface treatment of MWCNT using various types of

acids such as H2SO4, HNO3, HNO3/H2SO4

Functionalization of MWCNT using 30 and 60%

APTS

Characterization of MWCNT using SEM-EDX,

FTIR, SAP, and Raman spectroscopy

Adsorption testing of CO2 using Belsorp - mini

Figure 3.1: Process flow of the research project

33

3.3.1 Treatment of MWCNT

The main purpose of the treatment is to get the main functional group which is

carboxyl group and other hydrophilic functional group like amino group to reduce the

hydrophobic nature possessed by pristine MWCNT. Besides that, the treatment is

necessary to loosen the agglomerated and entangled nanotubes as a result of van der

Waals attraction. Besides that, surface treatment will remove metal impurities like

cobalt (Co) and molybdenum (Mo) that were introduced into nanotubes during the

production process of pristine MWCNT.

An amount of 1 g of MWCNT was put into a beaker that contained 4 M of H2SO4,

and the mixture was then stirred vigorously at 500 rpm for 15 hours at room

temperature. Rotation of 500 rpm was chosen to ensure the uniformity of the mixing

during the stirring process. The mixture was then filtered by using 0.45 µm membrane

filtration paper, and the filtered MWCNT was dried in the oven at 80oC overnight

until it became completely dried. The experimental procedure was repeated by using 4

M of HNO3 and 4 M of HNO3/H2SO4 (v:v;1:1), to get 3 different samples that were

treated with 3 different acids. Different acid would affect the presence of carboxyl and

hydrophilic group, hence influence the attachment of amine group during the

functionalization process of MWCNT with APTS.

3.3.2 Functionalization of MWCNT

Functionalization of MWCNT was carried out to capture the amine group that can

assist on the CO2 adsorption application. In order to functionalize MWCNT, 0.2 g of

treated MWCNT with H2SO4 was put into a beaker that contained 100 ml of 30%

APTS. The mixture was stirred vigorously at 500 rpm for 24 hours at room

temperature. According to Gui et al. [103] who had conducted experiment to observe

the effect of stirring time on functionalization, 24 hours of stirring was considered the

best period of time to ensure the most attachment of amine group on the surface of

MWCNT. The sample was dried at 80oC in the oven overnight until the sample was

completely dried. The experiment was repeated by using the treated MWCNT with

HNO3 and the treated MWCNT with HNO3/H2SO4. Lastly, all treated samples were

functionalized with 60% APTS with the same method as the 30% APTS tin order to

34

investigate the effect of APTS’s concentration on the presence of MWCNT,

consequently the capability of the samples to adsorb CO2.

3.3.3 Overall procedure

The overall procedure for the experimental work is presented in Figure 3.2 below:

Figure 3.2: Overall procedure of modification of MWCNT

35

The list of samples is tabulated in Table 3.1:

Table 3.1: Labeling for pristine, treated, and functionalized samples

Label Sample

A Pristine MWCNT

B MWCNT + H2SO4

C MWCNT + HNO3

D MWCNT + HNO3/H2SO4

E MWCNT + H2SO4 + 30%APTS

F MWCNT + HNO3 + 30% APTS

G MWCNT + HNO3/H2SO4 + 30% APTS

H MWCNT + H2SO4 + 60%APTS

I MWCNT + HNO3 + 60% APTS

J MWCNT + HNO3/H2SO4 + 60% APTS

3.4 Characterization

After completing the experimental procedure, the pristine and modified MWCNT

were characterized in order to determine the characteristics of MWCNT before and

after the modification. The analytical tools used for characterization were SEM-EDX,

FTIR, SAP, and Raman spectroscopy.

3.4.1 SEM-EDX

Field emission SEM ( Zeiss Supra55 variable pressure (VP) ) was used to observe

the morphology of pristine and modified MWCNT before and after the surface

treatment. It is expected to see the bundled, agglomerated and entangled nanotubes of

pristine MWCNT and loose nanotubes of treated MWCNT. The morphology was

taken at 100Kx magnification and operated at 4 to 7 kV. The percentage of elements

present such as carbon, oxygen, nitrogen, and metal can be quantified with EDX.

36

3.4.2 FTIR

The presence of functional groups on the surface of MWCNT was verified by

using FTIR (Perkin Elmer). The analysis was done on KBr disc containing MWCNT

and was conducted at the range of 400 to 4000 cm-1

. FTIR is based on the working

principle of finding the absorption of energy in a wavelength to investigate the

chemical structure of the material being tested [104]. The chemical structure at the

particular wave number can be found from infrared absorption table.

3.4.3 SAP

In order to observe the surface area, pore volume, and pore size distributions of the

samples, characterization by using SAP analyzer (TriStar II 3020 (micrometrics)) was

carried out. The surface area of the powder sample was determined by physical

adsorption of nitrogen onto the surface at liquid nitrogen temperatures of 77

K. Nitrogen adsorption/desorption isotherm was measured within the partial pressure

range of 0.0001 to 0.99 and then used to determine the surface area that can be

calculated by Brunauer, Emmett and Teller (BET). For pore size distribution, it can be

obtained by plotting a graph of pore volume versus pore width.

3.4.4 Raman spectroscopy

Raman spectroscopy (HoribaJobin Yvon HR 800) was used to investigate the

degree of functionalization for all samples. It can be determined by the ratio of

intensity between the D (disordered) band and intensity of G (graphitic) band when

operating in the wave number range of 1000 cm-1

to 2000 cm-1

. D band is related to

the presence of defects in the graphitic structure of MWCNT and the peak is located

at ~1320cm-1

and G band located at the peak of ~1580 cm-1

represented the

crystallized graphitic structure of the material [105].

37

3.5 CO2 adsorption measurement

CO2 adsorption is the last part of the research work and is very important to

determine the capability of modified MWCNT to adsorb MWCNT. The equipment

used for the testing is BELSORP-mini. The gas used for the adsorption is the mixture

of 80% of nitrogen, and 20% of CO2. Two reasons for the selection of the mentioned

gases are to represent the environmental condition, and prevent the corrosion to occur

in a pipeline due to the acidic characteristic of CO2.

In order for nitrogen to settle or adsorb in any materials, the condition must be at -

196.15oC or 77 K because nitrogen at 77 K is the standard adsorptive [106]. That is

the reason why in determining the characterization of material using nitrogen, the

temperature must be at 77 K [107] otherwise, the characterization cannot be done.

Therefore, since the testing for CO2 adsorption measurement is taken place at 25oC,

only CO2 will be adsorb by the MWCNT. In addition, the role of amine group is to

assist MWCNT to adsorb CO2 only, based on the acid and base concept [18]. Amine

acts as base while CO2 acts as acid and these two compounds will attract to each

other. It is very crucial to get the most attachment of amine group on the surface of

MWCNT to ensure that more CO2 can be adsorbed through chemisorption process.

There is a limitation in using BELSORP-mini where the highest allowable

pressure is at one bar only. Due to the limitation, CO2 adsorption testing could only be

conducted until the maximum pressure is 1 bar. However, the pressure of 1 bar is

sufficient since the aim of the adsorption is to adsorb CO2 at atmospheric condition.

Besides that, the most important analysis in this study is to compare the performance

of the pristine and modified MWCNT in their ability to adsorb CO2. Therefore, the

limitation of BELSORP-mini should give no problem for the experiment to be

conducted.

There are several procedures to follow for the CO2 adsorption measurement. The

measurement was started with the pretreatment, dead volume measurement,

adsorption measurement and desorption measurement. Figure 3.3 shows the

adsorption procedure [108]:

38

3.5.1 Pre treatment

Before proceeding with CO2 adsorption, pretreatment was necessary to vaporize

all the moisture inside the sample and to ensure the CO2 adsorption measurement can

be done accurately. The gas adsorption measurement was measured to the sample

weight after pretreatment. If there was even 1% error of the mass, it would directly

affect the accuracy of CO2 measurement. The pretreatment was done by heating the

sample at 80oC for 4 hours under the flow of nitrogen. Sample weight must be

measured precisely by comparing the blank sample cell weight (measured before

pretreatment) and the sample cell weight with the sample inside (after pretreatment).

Final weight for sample = Blank sample cell before treatment – Blank sample cell

with sample inside (after treatment)

3.5.2 Dead volume measurement

Dead volume is a measurement for the space inside the sample cell. For

BELSORP mini, the dead volume (Vd) changes as the surface level of liquid nitrogen

changes. The dead volume calculation can be done by calculating the dead volume

reference Vd (ref) first. Both reference cell and sample cell were submerged into the

liquid nitrogen as shown in Figure 3.4. When the liquid nitrogen up to level 1, dead

volume of sample cell was expressed by Vd (smp) 1 while dead volume and pressure

of dead volume reference cell were denoted by Vd (ref) (1) and P (ref) 1 respectively.

Then, when the liquid nitrogen decreases to level 2, dead volume of reference cell and

Figure 3.3: Summarized procedure for CO2 adsorption using

BELSORP-mini

Pre treatment Dead volume measurement

Adsorption measurement Desorption measurement

39

pressure of dead volume reference cell can be denoted as Vd (ref) 2 and P (ref) 2,

respectively. Thus, ΔVd (ref) can be obtained as follows [108]:

Vd (ref) 2 = Vd (ref) 1 x P (ref) 1 / P (ref) 2 (3.1)

ΔVd (ref) = Vd (ref) 2 – Vd (ref) 1 (3.2)

Thus, the dead volume of sample cell at level 2, Vd (smp) 2 can be expressed as

follows [108]:

Vd (smp) 2 = Vd (smp) 1 + ΔVd (ref) (3.3)

3.5.3 Adsorption measurement

The adsorption amount was measured by calculating the difference between

pressure before and after adsorption. The adsorption measurement can be obtained by

following the steps with the help of a diagram in Figure 3.5:

Figure 3.4: Diagram for calculating the dead volume

40

a) The change of gas amount (V1) inside Vs before and after adsorption [108]:

( ( ) ( ))

Where;

Vs = standard volume

P1i (n) = pressure of adsorptive that was dosed from valve A to Vs at nth

measurement point.

P1e (n) = pressure that were detected by sensor P1 at adsorption equilibrium

after valve C was opened

Ws = mass of adsorbent

T = absolute temperature of Vs

(3.4)

Figure 3.5: Diagram for calculating adsorption measurement

41

b) The gas amount (V2) that was existed in Vd before adsorption [108]:

( )

Where P2 was the pressure detected by sensor P2 at adsorption equilibrium after

valve C was opened.

c) Amount of the gas that remains in Vd at adsorption equilibrium [108]:

( )

d) The increased amount of adsorbed gas (ΔV) at nth

measurement [108]:

e) Adsorbed amount at nth

measurement [108]:

( ) ( )

3.5.4 Desorption measurement

For desorption measurement, there were several steps to be followed:

1. The valve C was closed and Vs was depressurized from valve B

2. The equilibrium pressure was measured after the opening of valve C and the

closing back after a few seconds.

3. To get the amount of desorption measurement, the steps to be followed is the

same steps for the adsorption measurement.

The result produced by BELSORP-mini was a graph of partial pressure versus the

volume adsorb by 1 g of adsorbent (cm3/g). From the graph, the amount of CO2

adsorbs by pristine and modified MWCNT can be known and the best adsorbent that

can adsorb highest amount of CO2 can be determined as well.

(3.5)

(3.7)

(3.6)

(3.8)

42

3.6 Summary

This chapter described the methodology used for the research work. It started with

the surface treatment, followed by functionalization, and then characterization of

pristine and modified MWCNT by using analytical methods such as SEM-EDX,

FTIR, SAP, and Raman spectrometer. Lastly, both pristine and modified MWCNT

were tested for CO2 adsorption, to investigate the performance of MWCNT in

adsorbing CO2.

CHAPTER 4

RESULTS AND DISCUSSION

Chapter 4 will discuss on the results obtained from the characterization (SEM-

EDX, FTIR, SAP, and Raman spectroscopy) of the modified and functionalized

MWCNT and CO2 adsorption measurement (BELSORP-mini). Observation and

results obtained are discussed and analyzed in order to verify the theory and

hypothesis made earlier. Tests conducted on various samples would allow for

identification of the optimum adsorbent that functions as an effective adsorbent to

adsorb CO2.

4.1 SEM-EDX

SEM-EDX was used to characterize the pristine and treated MWCNT. With SEM,

the morphology of the nanotubes can be determined. On the other hand, EDX can

quantify the percentage of elements in pristine and treated MWCNT.

Morphology’s observation using SEM is not necessary for functionalized

MWCNT because the morphology will not much affected upon functionalization

unlike the treated MWCNT. The most important characterization for functionalized

MWCNT is to study the attachment of amine functional group after functionalization.

Therefore, SEM-EDX was used for the pristine and the treated MWCNT only to

observe the morphology and the percentage of elements.

Figure 4.1 shows the morphology of pristine MWCNT (A), treated MWCNT with

H2SO4 (B), treated MWCNT with HNO3 (C), and treated MWCNT with

HNO3/H2SO4 (D). One of the purposes of surface treatment is to unbundle and loosen

the nanotubes from the bundled, agglomerated, and entangled nanotubes. Referring to

Figure 4.1, sample A shows the most bundled, agglomerated, and entangled

44

nanotubes, believed to be due to the van der Waals attraction [109]. Van der Waals

However, the unbundled and loose nanotubes can be observed from the SEM image

of sample B, C, and D due to the oxidative etching along the walls of MWCNT by

acids [110].

The morphology above verified the ability of acids to change the arrangment of

MWCNT from bundled, agglomerated, and entangled nanotubes to the loose and

unbundled nanotubes. The loose and unbundled nanotubes are very important

characteristics because it will open up more space for amine group to be attached on

the surface of MWCNT during functionalization. For example, sample A that has low

and limited surface area will allow only a few functional groups to be attached on the

surface of MWWCNT, while other samples that were treated with acids, will have

higher surface area and allowing more attachment of functional group on the surface

A B

C

D

Figure 4.1: Morphology of pristine MWCNT (A), treated MWCNT with H2SO4 (B),

treated MWCNT with HNO3 (C), and treated MWCNT with HNO3/H2SO4 (D)

45

of MWCNT. Besides that, acid treatment is expected to create defects in the

hexagonal or pentagonal structure of MWCNT which act as ‘active sites’ for the

amine group to be attached on the surface of MWCNT during the functionalization

[103]. They are called ‘active site’ because the defects will provide sites for the amine

groups to attach on the surface of MWCNT. The schematic diagram of the attachment

of amine group can be seen in Figure 4.3. The defects created during treatment will

change the sp2 hybridization of MWCNT to sp3 hybridization [111].

In addition, surface treatment will allow the oxygen-containing functional group

to attach covalently to MWCNT due to the strong interaction between MWCNT and

acid [88]. The surface treatment will introduce carboxyl [88-90] that belong to

hydrophilic group to overcome the hydrophobic problem. The carboxyl and other

hydrophilic group is represented by oxygen functional group such as O-H, C=O, and

C-O [112]. The oxygen-containing functional group that represented the carboxyl and

hydrophilic group can be preliminary observed from EDX. Table 4.1 provides the

information on the elements presence before and after surface treatment.

Table 4.1: Elements in MWCNT as analyzed by EDX

Sample Carbon (C)

(weight %)

Oxygen (O)

(weight %)

Cobalt (Co)

(weight %)

Molybdenum

(Mo) (weight %)

A 93.73 4.85 0.49 0.93

B 93.99 5.53 0.49 -

C 93.55 6.10 0.35 -

D 88.91 10.8 0.29 -

According to the data above, oxygen element is increased from pristine MWCNT

(A) to treated MWCNT with HNO3/H2SO4 (D), and sample D shows the highest

percentage of oxygen. It is believed that the nitronium ion (NO2+) from sample C

(treated MWCNT with HNO3) and D that attacks the aromatic compound of

MWCNT will be the first step for the introduction of surface oxide [88]. It is called

surface oxide due to the presence of oxygen element on the surface of MWCNT.

However, for H2SO4 that does not have nitronium ion, the attachment can still occur

46

due to the strong interaction between MWCNT and acid [88]. The increasing amount

of oxygen from sample A to D explains the increasing presence of carboxyl group on

the surface of MWCNT. This implies that the presence of carboxyl group from the

acid treatment is the order of H2SO4 > HNO3 > HNO3/H2SO4.

In addition, significant removal of metal impurities after acid treatment can be

observed in Table 4.1, which indicates the role of H2SO4 and HNO3 as the purification

agent. Mo element is completely diminished from all samples while there are still

some traces of Co in sample B (treated MWCNT H2SO4), C (treated MWCNT with

HNO3), and D (treated MWCNT with HNO3/H2SO4). Only sample B shows no

significant removal of Co, but the other two samples, C and D show significant

removal of Co from 0.49% to 0.35% and 0.29% respectively.

4.2 FTIR

In order to investigate the existence of the functional group in the pristine and

modified MWCNT, FTIR analysis is employed as it can detect the chemical structure

based on the absorption of energy in a wavelength. Sample B, C, and D are expected

to have the attachment of carboxyl group after the treatment with H2SO4, HNO3, and

HNO3/H2SO4, respectively while sample C and D are expected to have additional

attachment of hydrophilic group such as amino group due to the existence of nitrogen

element in the acids compound.

Figure 4.2 shows four images from sample A, B, C, and D for pristine MWCNT

and MWCNT that had been treated with H2SO4, HNO3, and HNO3/H2SO4,

respectively.

47

Hydroxyl group (OH) is observed at the wave number of 3384.61cm-1

, 3417.48

cm-1

, 3406.59 cm-1

, and 3406.59 cm-1

for sample A, B, C, and D, respectively. CH

stretching is also observed for all samples at the band of ~2800 to ~2900 cm-1

[113]

that identical for MWCNT [103]. Peak of ~1550 cm-1

confirmed the back bond and

hexagonal structure of MWCNT with the existence of carbon double bonding (C=C)

for sample A, B, and C [114]. Peak observed at ~500 cm-1

for sample A, C, and D,

correspond to the C-Br stretching, coming from potassium bromated (KBr) that was

used for FTIR testing. The peak at 1152.30 cm-1

corresponding to the C=O bonding

for sample A indicates the presence of carboxyl group on the surface of MWCNT

[115]. The presence of various functional groups in pristine MWCNTs implies that

there were introduced during the production and purification process [96].

For sample B, the existence of C=O bonding at the wave number of 1717.43 cm-1

[109] and 1155.04 cm-1

[115] implied that the carboxylic groups have successfully

attached on the surface of MWCNT after the treatment with H2SO4. For sample C,

Figure 4.2: FTIR spectra for pristine MWCNT (A), treated MWCNT with H2SO4

(B), treated MWCNT with HNO3 (C), treated MWCNT with HNO3/H2SO4 (D).

Sample A

Sample B

Sample

Sample D

48

carboxyl groups (C=O) also appeared at the wave number of 1157.78 cm-1

[115] and

1714.69 cm-1

[109]. Other hydrophilic group existed in this sample is represented by

OH stretching at the wave number of 2380 cm-1

indicating the presence of amino

group. According to Abeden (1996), amino group is considered as hydrophilic group

[93] and it is found to be present in sample C due to the HNO3 chemical compound

that has nitrogen element in the structure.

Another important group, carboxyl is exhibited in sample D by the band at

1048.05 cm-1

, 1185.22 cm-1

, and 1730 cm-1

, represented by C=O stretching Higher

intensity of carboxyl group can be obtained in sample D as compared to sample B and

C, implying more carboxyl group present in sample D than other samples. Peak at

2401.09 cm-1

indicates the existence of amino group from the chemical compound of

HNO3 represented by O-H stretching. Referring to sample B, C, and D, which have

been treated with H2SO4, HNO3, and HNO3/H2SO4 respectively, sample D shows

better appearance of carboxyl and hydrophilic group as compared to sample B and

sample C.

The treatment of MWCNT should allow better dispersibility of MWCNT in the

APTS solution since the hydrophobic problem had been reduced. There are two

mechanisms for amine group to be attached on the surface of MWCNT. The first

mechanism is by the reaction between carboxyl group with APTS, and the second

mechanism is by the attachment of amine group on the defects side or active sites that

had been created during the treatment. Figure 4.3 shows the schematic diagram of the

attachment of carboxyl and amine group on MWCNT [116].

49

The FTIR result after functionalization with 30% and 60% APTS are shown in

Figure 4.4 and Figure 4.5 respectively.

Attachment by

chemical reaction

Attachment through

defect sites

Figure 4.3: The attachment of amine group by chemical reaction

and through defect sites group [106].

50


H2SO4 (E), HNO3 (F) and HNO3/H2SO4 (G)

51


H2SO4 (H), HNO3 (I) and HNO3/H2SO4 (J)

52

Again, C=C was identified in sample E (treated MWCNT with H2SO4 and

functionalized with 30% APTS), indicating the back bond or hexagonal structure of

MWCNT. Attachment of –C-NH2 was found at the wave number of 1094.69 cm-1

[103] and C-N stretching at 1187.96 cm-1

, implying the existence of amine group after

functionalization with 30% APTS. For sample F (treated MWCNT and functionalized

with 30% APTS), more amine group can be observed compared to sample E, shown

at the wave number of 3384.61 cm-1

and 2340.65 cm-1

for N-H stretching, 1541.85

cm-1

for N-H2 [103], and 1097.43 cm-1

for C-N stretching [117]. Other peaks that can

be observed from sample F are at 2917.58 cm-1

and 2846.15 cm-1

for CH, exhibited by

MWCNT. The last sample which is sample G that was treated with HNO3/H2SO4 and

functionalized with 30% APTS, more amine group can be observed compared to

sample E and F. N-H bending can be identified at the band of 1635.13 cm-1

and

688.67 cm-1

while C-N stretching can be observed at 1187.96 cm-1

, 1094.69 cm-1

, and

1023.36 cm-1

[117]. Primary amine, NH2 is detected in this sample as well, at the

wave number of 1541.85 cm-1

[103].

Sample H, I, and J are samples of MWCNT that have been functionalized with

60% APTS and treated with H2SO4, HNO3, and HNO3/H2SO4, respectively. For

sample H, OH bonding appeared at the wave number of 3406.59 cm-1

and C-H

stretching at the wave number of 2912.08 cm-1

and 2846.15 cm-1

. The appearance of

amine group can be identified at several wave number such as at 2400 cm-1

and 2300

cm-1

to 2035 cm-1

corresponding to N-H stretching, and 1566.54 cm-1

corresponding to

NH2 [97]. At 1127.61 cm-1

and 729.82 cm-1

, another amine group appeared

representing by C-N stretching, and N-H bending respectively. The existence of

amine group for sample I such as N-H stretching can be confirmed at 2600 cm-1

and

2300 cm-1

to 2082.41 cm-1

, N-H bending at 877.96 cm-1

and 703.38 cm-1

[117], and

lastly C-N stretching at the wave number of 1105.66 cm-1

[118].

The attachment of amine group on the surface of MWCNT is more crowded and

intense for sample J (treated MWCNT with HNO3/H2SO4 and functionalized with

60% APTS). The significant peak at 2313.18 cm-1

and 2230.76 cm-1

are

corresponding to N-H stretching and the appearance of N-H bending can be observed

at the wave number of 1585.75 cm-1

and 869.73 cm-1

and 710.61 cm-1

. Aromatic

53

amine group can be observed through C-N stretching represented by the band of 1380

cm-1

and 1135.84 cm-1

[118]. Besides that, there are appearances of C-Br at 504.86

cm-1

for sample I and at 592.65 cm-1

and 510.35 cm-1

for sample J.

As a whole, samples that were functionalized with 60% APTS have more

presence of amine group as compared to the samples that were functionalized with

30% APTS. The presence of more amine group for samples that were functionalized

with 60% APTS can be determined from the packed peak of amine group in the

graph, high intensity, and broader and wider peak of the samples [103]. The highest

intensity of amine group is contributed by sample J because since from the treatment,

this sample has the most existence of carboxyl and hydrophilic functional group that

can assist MWCNT to disperse well in the APTS solution and created active sites for

the attachment of amine group. Hence, the amine group can be attached easily on the

surface of MWCNT through the active site, and through the reaction with carboxyl

group as well (as shown in Figure 4.3)

4.3 Surface area and porosity analyzer (SAP)

SAP analyzer is used to determine the surface area, pore volume, and pore size

distribution of the pristine and modified MWCNT. This characterization is important

to prove that the attachment of functional groups have occurred by analyzing the

surface area and pore volume. The surface area and pore volume are expected to be

higher when the existence of functional groups becomes more intense. For this case,

pristine MWCNT was predicted to have the highest surface area and pore volume,

followed by treated MWCNT, functionalized MWCNT with 30% APTS and

functionalized MWCNT with 60% APTS.

Characterization by using SAP will also provide nitrogen isotherm as shown in

Figure 4.6. The isotherm plot presents the adsorption/desorption isotherm of nitrogen

via pristine and modified MWCNT.

54

It can be observed that the amount adsorbed by the pristine MWCNT (A) is the

highest compared to other modified MWCNT and sample J (treated MWCNT with

HNO3/H2SO4 and functionalized with 60% APTS) shows the lowest adsorption of

nitrogen. Highest adsorption of nitrogen for the pristine MWCNT is attributed to the

highest porosity of pristine MWCNT [102]. Meanwhile, lower amount of adsorption

by modified MWCNT is due to the grafting of functional group on the adsorbent

surface [9]. From the isotherm plot, it can be understood that the high adsorption of

nitrogen for sample A is due to the least attachment of functional group whereas

treated MWCNT samples, B, C, and D that have been treated with H2SO4, HNO3, and

HNO3/H2SO4, respectively experienced lower amount of nitrogen adsorption as a

result of the attachment of carboxyl and hydrophilic group. For sample functionalized

with 30% APTS (E, F, and G), the adsorption of nitrogen is higher compared to the

sample functionalized with 60% of APTS (H, I, and J). This observation can be

explained by the fact that higher APTS concentration will have more attachment of

amine precursor on the surface of MWCNT giving lower nitrogen adsorption since

most of the porosity has been grafted and blocked by functional groups.

0

50

100

150

200

250

300

350

400

450

500

0 0.2 0.4 0.6 0.8 1 1.2

Quan

tity

adso

rbed

(cm

3/g

)

P/Po

A

B

C

D

E

F

G

H

I

J

Sample A

Sample B

Sample C

Sample D

Sample E

Sample F

Sample G

Sample H

Sample I

Sample J

Figure 4.6: Isotherm plot on the adsorption/desorption of nitrogen

55

According to International Union of Pure and Applied Chemistry (IUPAC), the

isotherm for pristine and modified MWCNT show a type IV shape where there is a

rise in nitrogen adsorption capacity with relative pressure [9] and characteristics

mostly of mesopore size between 2 nm to 50 nm. Hysteresis phenomena that occurred

when relative pressure is approaching one explains the presence of capillary

condensation of nitrogen within mesopores [102].

Table 4.2 gives the value of the surface area and pore volume for the pristine and

modified MWCNT. The value of surface area and pore volume is decreasing from

sample A (pristine MWCNT) to sample J (treated MWCNT with HNO3/H2SO4 and

functionalized with 60% APTS) corresponding to the decrease in porosity [119] and

the blockage of pore entrance by the formation of functional group on the surface of

modified MWCNT [12]. The percentage difference for surface area of sample J

relative to sample A is 95.07% and the percentage difference for pore volume of

sample J relative to sample A is also a significant value of 91.05%.

Table 4.2: Percentage difference of surface area and pore volume

Sample

Surface area

(m2/g)

Percentage

difference

relative to

sample A (%)

Pore volume

(cm3/g)

Percentage

difference

relative to

sample A (%)

A 447.31

- 0.69

- B 442.71

1.03 0.66

4.79

C 388.08

13.24 0.52

24.69

D 335.94

24.90 0.42

40.04

E 90.56 79.75 0.24 65.22

F 83.77 81.27 0.23 66.67

G 73.31 83.61 0.22 68.12

H 48.22

89.22 0.14

79.4

I 40.17 91.02 0.12

83.29

J 22.07

95.07 0.06

91.05

A significant percentage difference in surface area and pore volume shown by

modified MWCNT explains the presence of functional group on the surface of

MWCNT that block the introduction of nitrogen into the pore, hence reduces the

56

surface area and pore volume for modified MWCNT. Sample J shows the lowest

surface area and pore volume, indicating the most attachment of functional group in

that sample.

Figure 4.7: Pore size distribution for sample A, B, C, D, E, F, G, H, I, and J

From the pore size distribution (PSD) shown if Figure 4.7, the highest pore size

distribution is shown by the highest peak of the graph. All samples show that the

distribution of the pore is mostly in mesopore size. This result is consistent with the

discussion of isotherm above that the IV shape indicates the mesopore size of

MWCNT. However, from the PSD graph, it can be observed that the peak of the

graph is reduced in term of pore volume because the mesopore particle being grafted

by more functional group after modification.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70 80 90 100110120130140150160170180190200

Pore

volu

me

(cm

3/g

.nm

)

Pore width (nm)

Sample A

Sample B

Sample C

Sample D

Sample E

Sample F

Sample G

Sample H

Sample I

Sample J

57

4.4 Raman Spectroscopy

The Raman spectroscopy can determine the degree of functionalization through

the ratio between intensity of disordered band, D band and intensity of graphitic band,

G band (Id/Ig). This analysis is more to verify the presence of functional group on the

surface of MWCNT. High degree of functionalization signifies that MWCNT

experienced high defect during the treatment and functionalization, thus allow more

functional group to be attached. Table 4.3 listed the value of degree of

functionalization with the peak of D band and G band depicted in Figure 4.8 and 4.9

below.

Table 4.3: Degree of functionalization

Sample Degree of functionalization

(Id/Ig)

A - Pristine MWCNT 0.2

B - Treated MWCNT with H2SO4 0.32

C - Treated MWCNT with HNO3 0.39

D - Treated MWCNT with HNO3/H2SO4 0.45

E - Treated MWCNT with H2SO4 and functionalized

with 30% APTS

0.53

F - Treated MWCNT with HNO3 and functionalized

with 30% APTS

0.59

G - Treated MWCNT with HNO3/H2SO4 and

functionalized with 30% APTS

0.62

H - Treated MWCNT with H2SO4 and functionalized

with 60% APTS

0.74

I - Treated MWCNT with HNO3 and functionalized

with 60% APTS

0.78

J - Treated MWCNT with HNO3/H2SO4 and

functionalized with 60% APTS

0.84

58

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample A

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A.U

)

Raman shift (cm-1)

Sample B

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A.U

)

Raman shift (cm-1)

Sample C

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample D

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A.U

)

Raman shift (cm-1)

Sample E

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample F

D band

G band

Figure 4.8: Raman spectra for sample A, B, C, D, E, and F

59

The degree of functionalization increases from 0.2, contributed by sample A to

0.84, contributed by sample J. The degree of functionalization is increasing from

sample A to sample J. Sample A shows the lowest degree of functionalization due to

the presence of well-structured hexagonal carbon with almost no defects. However,

for sample B to J, since the sample had undergone modification, the degree of

functionalization is increasing indicated by the defects on the nanotubes structure and

the attachment of functional group. The highest degree of functionalization is shown

by sample J, supported by other characterization techniques (FTIR, and SAP) that

provide information on the intense presence of functional group for sample J,

consisting of carboxyl, hydrophilic, and amine group.

0 500 1000 1500 2000 2500

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample G

D band

G band

0 1000 2000 3000

Inte

nsi

ty -

Arb

itar

y U

nit

(A.U

)

Raman shift (cm-1)

Sample H

D band

G band

0 500 1000 1500 2000 2500

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample I

D band

G band

0 500 1000 1500 2000 2500

Inte

nsi

ty -

Arb

itar

y U

nit

(A

.U)

Raman shift (cm-1)

Sample J

D band G band

Figure 4.9: Raman spectra for sample G, H, I, and J

60

4.5 CO2 adsorption

The maximum uptake for CO2 adsorption and the percentage difference of

adsorption relative to sample A is presented in Table 4.4 and the graph of the

maximum uptake for all the samples are presented in Figure 4.10.

Table 4.4: Maximum uptake for CO2 adsorption and the percentage difference of

adsorption relative to Sample A

Sample Maximum uptake of CO2

adsorption (cm3/g)

Percentage difference of

adsorption relative to

Sample A (%)

A 2.90 -

E 6.53 124.98

F 6.79 133.77

G 6.82 134.68

H 64.99 2137.56

I 93.02 3103.30

J 127.61 4294.13

-20

0

20

40

60

80

100

120

140

160

A E F G H I J

Figure 4.10: Maximum uptake for CO2 adsorption from sample A, E, F, G, H, I,

and J. Result are expressed in mean ± standard error (n=3)

61

From Table 4.4 and maximum uptake for CO2 adsorption in Figure 4.10, sample A

(pristine MWCNT) shows the least value of CO2 adsorption which is 2.90 cm3/g

followed by sample E (treated MWCNT with H2SO4 and functionalized with 30%

APTS), F (treated MWCNT with HNO3 and functionalized with 30% APTS), and G

(treated MWCNT with HNO3/H2SO4 and functionalized with 30% APTS) with the

amount of 6.53 cm3/g, 6.79 cm

3/g, and 6.82 cm

3/g, respectively. The drastically

higher amount of CO2 adsorption is exhibited by sample H (treated MWCNT with

H2SO4 and functionalized with 60% APTS), I (treated MWCNT with HNO3 and

functionalized with 60% APTS), and J (treated MWCNT with HNO3/H2SO4 and

functionalized with 60% APTS) with the value of 64.99 cm3/g, 93.02 cm

3/g, and

127.61 cm3/g, respectively. The percentage difference of adsorption between sample

A and the rest of the samples which are sample E, F, G, H, I, and J show significant

value which are 124.98%, 133.77%, 134.68%, 2137.56%, 3103.30%, and 4294.13%,

respectively. The adsorption result is verified by the earlier characterization testing

where more presence of amine group will give higher amount of CO2 adsorption due

to the ability of amine group to adsorb CO2 through chemisorptions. Physical

adsorption also occurred when CO2 is captured at several parts of MWCNT such as

internal, interstitial channel, external groove sites, and external surfaces. The most

important requirement for the physical adsorption to occur is the pore size of the

MWCNT which must be bigger than the kinetic diameter of CO2. The kinetic

diameter of CO2 is 0.33 nm [120], and it can be easily adsorb by MWCNT because

the pore size distribution of all samples is at mesopore size. The biggest pore size

present in the sample is less than 5 nm as depicted by the graph of pore size

distribution in Figure 4.7. Therefore, the physical adsorption of CO2 into the

MWCNT can occur effectively since the kinetic diameter of CO2 is smaller than the

pore width of MWCNT.

62

4.5.1 CO2 isotherm

This section is an extension of the previous analysis on the maximum CO2 uptake

and will present where CO2 adsorption isotherm will be discussed.

Figure 4.11: CO2 adsorption for sample A, E, F, and G

Figure 4.12: CO2 adsorption for sample H, I, and J

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1

Am

ount

of

CO

2 a

dso

rbed

(cm

3/g

)

P bar

Sample H

Sample I

Sample J

0

1

2

3

4

5

6

7

0 0.2 0.4 0.6 0.8 1

Am

ou

nt

of

CO

2 a

dso

rbed

(cm

3/g

)

P bar

Sample A

Sample E

Sample F

Sample G

63

Figure 4.11 and 4.12 above show the relationship between the pressure and the

amount of CO2 adsorbed by pristine MWCNT, modified MWCNT with 30% APTS,

and modified MWCNT with 60% APTS. It is observed that the plot of CO2 isotherm

is different between the adsorption isotherm produced by pristine MWCNT and the

ones modified with 30% APTS (Figure 4.11) and 60% APTS (Figure 4.12)

For pristine and modified MWCNT with 30% APTS as shown in Figure 4.11, a

straight line graph is produced. The amount adsorb by pristine and modified MWCNT

with 30% APTS is increased as the pressure is increased. It also indicates that pristine

and modified MWCNT with 30% APTS experience very consistent of CO2 adsorption

since the early stage of adsorption to the end of adsorption. However, for Figure 4.12,

it shows nonlinear graph and more to exponential shape. The adsorption is quite slow

at the beginning, but increases drastically at the pressure of 0.7 onwards.

The different graph produced for Figure 4.11 and Figure 4.12 are clearly because

of the concentration of APTS used which are 30% and 60%, and the presence of

amine group on the surface of MWCNT. For pristine MWCNT and MWCNT that

modified with 30% APTS, the attachment of amine group is less, hence allow

physisoprtion to take place more than chemisorptions. In fact, the rate of

physisorption is faster than chemisorptions [121], thus justify the linear trend of

samples in Figure 4.11 that can adsorb CO2 at faster rate and consistent.

Comparing to MWCNT that was functionalized with 60% APTS, more presence

of amine groups make the chemisorption to occur more than physisorption. In the

range of 0 to 0.6, chemisorption is expected to occur that involved the chemical

bonding between amine group and CO2 whilst in the range of 0.7 onwards,

physisorption is expected to occur and explains on the drastic increment of CO2

adsorption in that range. Due to the process of chemisorptions that is similar to a

chemical reaction, the chemisorptions process is always slower than physisorption

[70]. It justified the trend of the graph that experience low adsorption due to the

occurrence of chemisorption in the range of 0 to 0.6

As a whole, the adsorption of CO2 is related closely with the presence of amine

group in MWCNT. More attachment of amine group on the surface of MWCNT will

64

yield a high amount of CO2 adsorption and less attachment of amine group will yield

a low amount of CO2 adsorption. Chemisorption will take place when it involves the

reaction between amine group and CO2 while physisorption takes place when the CO2

is adsorbed physically onto MWCNT.

4.6 Comparison with other research work

In order to determine how significant is this research work in terms of its

contribution as an improvement from the previous work, the result is compared with

the recent literature that discuss on the modification of MWCNT with amine for CO2

adsorption as well.

From this research work, the highest capacity of CO2 adsorption is contributed by

sample J with the amount of 127.61 cm3/g whilst the research works done by Gui et

al. [97] gave the highest amount of CO2 adsorption to be 42.25 cm3/g. The percentage

difference between this work and Gui et al. is 202.04% which can be considered as

significant. Even though both works used almost similar process which is

functionalizing MWCNT with APTS to get high capacity of CO2 adsorption, but the

difference in the CO2 adsorption testing condition might lead to the huge percentage

difference.

In this research work, temperature used for CO2 adsorption is 25oC while the work

by Gui et al used 60oC for the CO2 adsorption testing. Study done by Su et al. [12]

claimed that the adsorption capacity is decreasing with the increasing temperature due

to the exothermic nature of adsorption process. Therefore, CO2 adsorption at

temperature of 60oC produce lower result compared to the adsorption at temperature

of 25oC. That study justified the huge percentage difference for CO2 adsorption

between this research work and the work done by Gui et al.

As a conclusion, even though amine group affect the CO2 adsorption and give

better result compared to pristine MWCNT, but the condition set up for CO2

adsorption measurement must be given careful consideration as well. Optimum

65

modification on MWCNT and the accurate parameter for CO2 adsorption

measurement will contribute to the high capacity of CO2 adsorption.

4.7 Optimization of the material

Optimization in engineering is about producing a product that has high efficiency

with the consumption of low cost. Among the entire sample that is tested for CO2

adsorption, sample J (treated MWCNT with HNO3/H2SO4 and functionalized with

60% APTS) is identified as the best adsorbent that can adsorb CO2 at optimum

capacity. As discussed previously, Sample J was treated with the mixture of HNO3

and H2SO4, resulting in the most presence of carboxyl and other hydrophilic group,

and functionalized with 60% APTS resulting in the most attachment of amine groups.

Sample J contributes to the highest amount of CO2 adsorption compared to other

samples with the value of 127.61 cm3/g.

In order to ensure the sample is good for adsorption process, it must have the

ability to desorb as well so that the regeneration process can be done. The adsorbent

can be used for several times if it has the ability to be regenerated and this condition is

directed to the low cost consumption. Figure 4.13 shows the adsorption and

desorption graph for CO2 by sample J.

Figure 4.13: Adsorption and desorption graph of CO2 by sample J

0

20

40

60

80

100

120

140

160

180

0 0.2 0.4 0.6 0.8 1 1.2

Am

ount

of

CO

2 a

dso

rbed

(cm

3/g

)

P

66

The graph shows the ability of sample J to undergo the desorption process. The

adsorption of Sample J is not only involved chemical adsorption, but also physical

adsorption that make the desorption process feasible at low temperature (25oC) [12].

Besides that, with the advancement of the equipment and techniques in

synthesizing MWCNT, bulk production of MWCNT become feasible and the cost

consumption can be maintained low [79]. For example, production of high quality and

MWCNT by using CVD technique is claimed to be at relatively low cost for a large

scale production [82]. In addition, Lu et al. also reported that the cost of commercial

available CNT is continuously decreasing, making it possible to be used constantly

[9].

Furthermore, the technique used in the treatment and functionalization of

MWCNT is simple and also low cost. It requires the stirring method only, without the

use of heat. Stirring method consumes lower energy compared to the heat and

consequently leads to the lower cost as well.

Based on the finding above, sample J is considered to be the most optimum

adsorbent in adsorbing CO2, due to the ability of this sample to adsorb CO2 effectively

and at the same time consumed low energy and cost for processing.

CHAPTER 5

CONCLUSION

5.1 Conclusion

The research work has been successfully carried out because all the objectives

stated have been achieved.

The hydrophobic and bundling issue of MWCNT has been successfully resolved

by the surface treatment. Three types of acids were used which are HNO3, H2SO4, and

HNO3/H2SO4. Amongst all the acids, the mixture, HNO3/H2SO4 shows the best

reagent because the treated samples have the most attachment of carboxyl and

hydrophilic groups compared to others that were treated with H2SO4 and HNO3 only.

Therefore, HNO3/H2SO4 is identified as the optimum reagent to be used in order to

resolve the mentioned issues.

For second objective which is to increase the efficiency of CO2 adsorption, the

sample functionalized with 60% APTS gives the highest result in CO2 adsorption with

the value of 127.61 cm3/g due to the most attachment of amine group in this sample.

Characteristics of MWCNT were investigated by using several analytical

instruments. From SEM images, the samples treated with acids show the unbundled

and less agglomerated nanotubes whilst the reduction of impurities from pristine

MWCNT to treated MWCNT is shown clearly in the EDX result. The most

attachment of carboxyl and hydrophilic group is found in the sample treated with

HNO3/H2SO4, and the most attachment of amine group is found in the sample

functionalized with 60% APTS. Those attachments were evaluated from FTIR, where

the sample that has the most appearance and high intensity of respected peak is

considered to be the optimum condition of modification. From SAP result, it can be

68

observed that the surface area and pore volume are decreasing from pristine MWCNT

to the sample treated with HNO3/H2SO4 and functionalized with 60% APTS (sample

J) due to the grafting of functional groups onto the surface of MWCNT. From Raman

spectroscopy, the highest degree of functionalization is represented by sample J with

the ratio of 0.84, indicated the most functional groups in this sample.

Lastly, it was proven that treatment with HNO3/H2SO4 and functionalization with

60% APTS are the optimum modification condition which allow the sample to adsorb

CO2 with high efficiency, typically ~ 127.61 cm3/g.

5.2 Recommendations

In future, more studies can be done to enhance the research work such as:

1. More values for APTS concentration can be tested if the researcher would like

to observe the highest concentration of APTS that will result in maximum capacity of

CO2.

2. This research work was using stirring method and observing the result obtained

after the treatment with acids and functionalized with APTS. In future, other methods

can be used as well such as ultrasonic bath by applying the similar parameter with this

work. The result between these two methods can be compared.

3. The future study also may find a better way to produce an adsorbent with

similar properties at low cost but high efficiency.

69

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APPENDIX A

KYOTO PROTOCOL

80

The countries that signed for second commitment period of Kyoto protocol:

1. Australia 18. Iceland 35. Slovakia

2. Austria 19. Ireland 36. Spain

3. Belarus 20. Italy 37. Sweden

4. Belgium 21. Japan 38. Switzerland

5. Bulgaria 22. Latvia 39. Turkey

6. Canada 23. Liechtenstein 40. Ukraine

7. Croatia 24. Lithuania 41. United Kingdom

8. Cyprus 25. Luxembourg 42. Northern Ireland

9. Czech Republic 26. Malta 43. United States

10. Denmark 27. Monaco

11. Estonia 28. Netherlands

12. European Union 29. New Zealand

13. Finland 30. Norway

14. France 31. Poland

15. Germany 32. Portugal

16. Greece 33. Romania

17. Hungary 34. Russian Federation

81

APPENDIX B

OVERALL PROCEDURE

82

1) 1 g of MWCNT was weighed on the

weighing scale

2) 1 g of MWCNT was poured into a

beaker that contained 150 ml of 4M

H2SO4 and stirred vigorously at 500 rpm

for 15 hours at room temperature

3) The mixture was filtered using the

filtration setup

4) The filtered MWCNT was put into a

container, ready to be dried in an oven

83

5) The filtered MWCNT was dried in the

oven at 80oC overnight

6) Step 1 until 5 was repeated by using

150 ml of 4M HNO3 and 150 ml of 4M

HNO3/H2SO4 (1:1)

(Dried MWCNT from oven)

7) 0.2 g of treated MWCNT with

H2SO4was weighed by using weighing

scale

8) The sample of MWCNT was poured

into a beaker that contained 100 ml of

30% APTS and stirred vigorously at 500

rpm for 24 hours at room temperature

84

9) The mixture was filtered using the

filtration setup

10) The filtered MWCNT was put into a

container, ready to be dried in an oven

11) The filtered MWCNT was dried in

the oven at 80oC overnight

12) Steps 7 to 11 were repeated for

sample treated with HNO3 and

HNO3/H2SO4. After completing with the

30% APTS, the concentration was

increased to 60% APTS and similar

procedure was carried to obtain

functionalized MWCNT.

85

APPENDIX C

LIST OF PUBLICTIONS, CONFERENCES, AND EXHIBITION

86

PUBLICATIONS

1. Syuhaidah Rahmam, Norani Muti Mohamed, Sufian Sufian, “Characterization

of Modified Multiwalled Carbon Nanotubes”, Advanced Materials Research

Vol. 925 (2014) pp 369-373.

2. Syuhaidah Rahmam, Norani Muti Mohamed, Sufian Sufian, “The Effect of

Surface Area, Pore Volume, and Pore Size Distribution on the Modified

Multiwalled Carbon Nanotubes”, Applied Mechanics and Materials Vol. 625

(2014) pp 148-151.

3. Paper entitled ‘Effect of Acid Treatment on the Multiwalled Carbon

Nanotubes’ is accepted to be published in Material Research Innovations.

CONFERENCES AND PUBLICATIONS

1. Joint International Conference on Nanoscience, Engineering, and Management

2013, held in Bayview Beach Resort, Batu Ferringhi, Pulau Pinang on 19th

–

21st August 2013.

2. International Conference on the Science and Engineering of Materials held in

Sunway Putra Hotel, Kuala Lumpur on 13th

to 14th

November 2013.

3. International Conference on Process Engineering and Advanced Materials will

be held in Kuala Lumpur Convention Center (KLCC), Kuala Lumpur on 3rd

to

5th

June 2014.

EXHIBITION

Nano Malaysia Summit & Expo 2012 held in Kuala Lumpur Convention Center

(KLCC), Kuala Lumpur, on 3rd

– 5th

Nov. 2012.

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