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University of Calgary
PRISM: University of Calgary's Digital Repository
Graduate Studies The Vault: Electronic Theses and Dissertations
2019-12-13
Conversion of Petroleum Coke to Porous Materials
Wu, Jingfeng
Wu, J. (2019). Conversion of Petroleum Coke to Porous Materials (Unpublished doctoral thesis).
University of Calgary, Calgary, AB.
http://hdl.handle.net/1880/111352
doctoral thesis
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UNIVERSITY OF CALGARY
Conversion of Petroleum Coke to Porous Materials
by
Jingfeng Wu
A THESIS
SUBMITTED TO THE FACULTY OF GRADUATE STUDIES
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
GRADUATE PROGRAM IN CHEMICAL AND PETROLEUM ENGINEERING
CALGARY, ALBERTA
DECEMBER, 2019
© Jingfeng Wu 2019
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Abstract
Petroleum coke (petcoke) is a low value by-product from oil and gas refinery. The production of
oil sand petcoke has been continually increasing over the last 20 years. However, only 11-20%
of the petcoke produced has been utilized as site fuel. The remainder has been largely stockpiled
in northern Alberta. As oil sand petcoke contains higher carbon but a lower ash content
compared to conventional crude oil petcoke, this project was designed to prepare porous carbon
materials from petcoke.
The aim of this thesis was to develop methods to convert by-products from oil refinery (eg.
petcoke and asphaltenes) to value-added porous carbon materials. In order to combine nanoscale
pores and macroscale pores into one monolithic structure, activation was proposed to develop
micro and mesopores on oil sand petcoke as a first step. Both chemical activation (using
KOH/NaOH) and chemical steam co-activation were studied to prepare activated carbon (AC)
from petcoke. A salt template was then utilized to form macroscale pores between AC particles
for hierarchical porous carbon (HPC) preparation.
The co-activation of KOH and steam reduced the chemical agent amount without compromising
pore volume. Before steam was introduced into the system, a molten phase around petcoke
particles is presumed to be formed. A greater amount of chemical agent corresponded to a
thicker molten chemical layer, which restricted the rate of steam gasification. By lowering the
activation temperature to 500 ˚C, a 0.34 cm3/g pore volume and 800 m2/g surface area were
obtained with an AC yield of 94%. Since there was almost no carbon consumption, the pores
developed at 500 ˚C were most likely due to the opening of initial closed pores of petcoke.
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Finally, by using asphaltenes as natural binders to connect non-washed AC particles, HPC was
fabricated with multiple scale pores after washing away the salts.
The experimental results in this thesis provide feasible approaches to prepare porous materials
from petcoke and asphaltenes. A better understanding of pore development during the activation
process will help to optimize the process and control the properties of the final product.
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Acknowledgements
I would like to acknowledge my supervisor, Dr. Josephine M. Hill, for her guidance and support
throughout my graduate program. Thank you for your patience and hard work to train me to
become an independent researcher. I have gradually developed strategic experiment design and
analytical skills. I’m also grateful to the help of my supervisory committee. Dr. Maen Husein.
and Dr. Qingyue (Gemma) Lu, for their assistance and mentorship in experiment design. Also, I
would like to thank Dr. Milana Trifkovic, Dr. Kevin B Thurbide to be part of my candidacy
exam committee, and thanks to Dr. Ron Chik-Kwong Wong, Dr. Nicolas Abatzoglou for being
part of my final exam committee.
This project was funded by the Natural Sciences and Engineering Research Council of Canada
(NSERC: STPGP 447411-13) through a Strategic Grant, and Shell Canada. I am grateful for the
financial support to attend 24th Canadian Symposium on Catalysis from Faculty of Graduate
Studies, the Canadian Catalysis Foundation and the North American Catalysis Society.
I would like to thank all of the members in Laboratory for Environmental Catalytic Applications
(LECA) for their valuable advices and accompany throughout all the years as a graduate student.
My sincere thank to Dr. Stephanie Keptep, Dr. Luis Virla, Dr. Vicente Montes, Dr. Ye Xiao, Dr.
Ross Arnold, Dr. Melanie Hazlett, Sip Chen Liew and Qing Huang for their guidance and
support to my learning and research journey.
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Special thanks to the advisors and mentors to motivate me to explore myself and discover my
curiosity in learning. Thanks for all your help Dr. Karen Quinn, Dr. Paul Papin, Dr. Anna-Lisa
Ciccocioppo, Dr. Ann Laverty, Dr. Stephanie Warner, Liliana Gonzalez and Renata Gordon.
I am thankful to my family and friends around me. You are my pillars to succeed in graduate
study and daily life in Canada. I’ve appreciated the support from every members in my big
family back home. I can feel your caring love though far away from hometown. An extreme
thank for my parents and boyfriend, Jundong, for your patience with me and accompany on the
phone. I’m very grateful to have all of you share my moment to grow up and experience
Canadian work and life together. Though I may not be the talent and excellent one as you think I
would be, you are always believe in me and encourage me to keep moving forward.
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Table of Contents
Abstract ............................................................................................................................... ii
Acknowledgements ............................................................................................................ iv Table of Contents ............................................................................................................... vi List of Tables ................................................................................................................... viii List of Figures and Illustrations ......................................................................................... ix
CHAPTER ONE: INTRODUCTION ..................................................................................1
1.1 Overview ....................................................................................................................1 1.2 Objective ....................................................................................................................2
1.3 Organization of the thesis ..........................................................................................3
CHAPTER TWO: LITERATURE REVIEW ......................................................................5 2.1 Waste carbon source ..................................................................................................5 2.2 Activation .................................................................................................................10
2.3 Mixing impact ..........................................................................................................24 2.4 Hierarchical porous carbon ......................................................................................26
CHAPTER THREE: EXPERIMENTAL METHODS ......................................................35
3.1 Materials ..................................................................................................................35 3.2 Preparation of activated carbon ...............................................................................36
3.3 Mixing impact ..........................................................................................................40 3.4 Preparation of carbon foam ......................................................................................41
3.5 Characterization .......................................................................................................42 3.6 Error source ..............................................................................................................45
3.7 Repeatability of the experiments .............................................................................46
CHAPTER FOUR: THE IMPACT OF THE AMOUNT OF CHEMICAL AGENT ........49 4.1 Reduction of chemical ratios by adding steam as another source of oxygen ..........50
4.2 Impact of steam exposure time ................................................................................62 4.3 Discussion of pore development ..............................................................................69
4.4 Economic estimation of petcoke activation .............................................................74
CHAPTER FIVE: PORE DEVELOPMENT DURING CHEMICAL ACTIVATION ....76
5.1 Experimental methods – Cross-sectioning ..............................................................77 5.2 Results ......................................................................................................................77 5.3 Pore development with chemical activation of high sulfur petcoke ......................115 5.4 Summary ................................................................................................................117
CHAPTER SIX: HIERACHICAL POROUS CARBON FROM ASPHALTENES AND
PETCOKE...............................................................................................................119 6.1 Initial Experiments .................................................................................................120 6.2 Parameters in salt template method .......................................................................122
6.3 HPC preparation ....................................................................................................128
CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS ........................137
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7.1 Conclusions ............................................................................................................137 7.2 Recommendations ..................................................................................................138
REFERENCE ...................................................................................................................142
APPENDIX A: MIXING EFFICIENCY ON ACTIVATED CARBON FROM
PETROLEUM COKE .............................................................................................153
APPENDIX B: PREPARATION OF ACTIVATED CARBON FROM PETCOKE .....155
APPENDIX C: LOW TEMPERATURE ACTIVATION ...............................................157
APPENDIX D: CARBON FOAM BY EMULSION TEMPLATE ................................158
APPENDIX E. REPRINT PERMISSION LETTERS .....................................................159
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List of Tables
Table 2-1. Yields and surface areas of AC prepared from various carbon precursor by KOH
activation and steam activation. Data revised from [25]. ....................................................... 7
Table 2-2. Properties of KOH activated carbon from natural materials. Revised from [41] ........ 13
Table 2-3. Standard Gibbs Energy calculations (∆G0) (for Equations 2-1 and 2-2) during
KOH and NaOH activation. .................................................................................................. 17
Table 2-4. Diffusion mechanism in porous media. [85-87] .......................................................... 19
Table 2-5 Summary of AC properties prepared from high sulfur petcoke (4-8 wt% sulfur). ...... 23
Table 2-6 Summary of AC properties prepared from low sulfur petcoke (< 1 wt% sulfur)......... 23
Table 3-1. Proximate, ultimate, and ash analysis of petcoke. ....................................................... 36
Table 3-2. Equivalent molar ratio for mass ratio of NaOH and KOH. ......................................... 37
Table 3-3. The physical properties selected AC. .......................................................................... 48
Table 4-1. Physical properties of activated carbon produced from the activation of petcoke at
800 ˚C with KOH or NaOH. ................................................................................................. 55
Table 5-1 KOH concentration after separating zirconia balls measured by titration. .................. 81
Table 5-2. Physical properties of the pure K and Na species in activation process [157, 158]. ... 85
Table 5-3. Estimated and actual pore volumes for petcoke activated with KOH. ........................ 97
Table 5-4. Estimated and actual pore volumes for petcoke activated with NaOH. ...................... 98
Table 5-5. Total pore volume and yield versus temperature for same activation time. .............. 106
Table 5-6. Surface area from N2 adsorption and CO2 adsorption. .............................................. 112
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List of Figures and Illustrations
Figure 2-1. 3D cubic model of Qingdao petcoke. ........................................................................... 9
Figure 2-2. The proposed structure of asphaltene molecule. ........................................................ 10
Figure 2-3. IR-active functional groups on an activated carbon surface. ..................................... 11
Figure 2-4. Schematic graph to illustrate diffusion in hierarchical zeolite (a porous material
with both micropores and mesoporous). ............................................................................... 19
Figure 2-5. The schematic representation of hierarchical graphitic carbon. ................................. 26
Figure 2-6. A scheme of the HPC prepared from pitch-based carbon. ......................................... 28
Figure 2-7. A scheme for ice-segregation-induced self-assembly (ISISA). ................................. 29
Figure 2-8. Synthetic pathway for the HPC monolith by using salt templating and self-
binding. ................................................................................................................................. 30
Figure 2-9. (a) Illustration of the synthesis process of the HPC; (b) SEM image of HPC with
salt template; (c) SEM image of HPC without salt template. ............................................... 32
Figure 3-1. Activation setup for converting petcoke to porous materials..................................... 39
Figure 4-1 Impact of amount of KOH on (a) total pore volume per gram of AC, (b) yield and
(c) pore volume per gram of petcoke during the activation of petcoke (800 ˚C, 30 min)
with KOH () and KOH/St () activation .......................................................................... 51
Figure 4-2 Impact of amount of NaOH on (a) total pore volume per gram of AC, (b) yield
and (c) pore volume per gram of petcoke during the activation of petcoke (800 ˚C, 30
min) with NaOH () and NaOH/St () activation ............................................................. 51
Figure 4-3. Total pore volume of KOH activated carbon at 800˚C for 0 min () and 30 min
() with KOH to carbon ratios of 0.11, 0.21, 0.43, 0.64 without steam addition. .............. 52
Figure 4-4. Cross-section of (a) raw petcoke and (b) petcoke after heated up to 800 ˚C in N2
atmosphere for 30 min. ......................................................................................................... 53
Figure 4-5. SEM analysis of 0.43KOH/St (a) before washing (b) with elemental mapping. ....... 57
Figure 4-6. Scanning electron micrographs and particle size distribution for raw petcoke and
AC activated with KOH with or without steam. ................................................................... 59
Figure 4-7. Scanning electron micrographs and particle size distribution for petcoke and AC
activated with NaOH with or without steam. ........................................................................ 60
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Figure 4-8. (a) N2 isotherms and (b) pore size distributions from 2D-NLDFT-HS for petcoke
activated with KOH with/without steam at 800 ˚C for 30 min. ............................................ 62
Figure 4-9. (a) N2 isotherms and (b) pore size distributions from 2D-NLDFT-HS for petcoke
activated with NaOH and/or steam at 800 ˚C for 30 min. .................................................... 62
Figure 4-10. Impact of steam activation time on total pore volume per gram of AC() and
yield (×) at 800 ˚C with different amounts of KOH. ............................................................ 64
Figure 4-11. The influence of steam activation time on pore size distributions of petcoke
activated at 800 ˚C with different KOH to petcoke ratios. ................................................... 66
Figure 4-12. Impact of steam activation time on molar ratio H2O:Carbon consumed when 2.5
g petcoke activated in the presence of steam at 800 ˚C with different amounts of KOH. .... 67
Figure 4-13. Impact of steam activation time pore volume created by 2.5 g petcoke activated
in the presence of steam at 800 ˚C with different amounts of KOH. .................................... 67
Figure 4-14. Impact of steam activation time at 800 ˚C on the change of pore volume from
pores smaller than 1.4 nm() versus pores between 1.4-5 nm(). ..................................... 68
Figure 4-15. Impact of steam time on mesopore volume change during KOH/St activation at
800 ˚C with various chemical ratios ..................................................................................... 69
Figure 4-16. Yields obtained for the activation of petcoke at 800 ˚C for 30 min with different
amounts of KOH () or NaOH(). .................................................................................... 71
Figure 4-17. The relationship between steam time and the pore volume change in the
presence of steam at 800 ˚C with different amounts of KOH. .............................................. 72
Figure 4-18. Schematic graph for KOH activation and co-activation of petcoke. ....................... 75
Figure 5-1 Particle size change before and after ball-milling petcoke. ........................................ 78
Figure 5-2. The impact of mixing method for KOH and petcoke on pore volume (micropore
volume indicated by black bar, mesopore volume by grey bar) and yield(solid red circle)
of AC. .................................................................................................................................... 79
Figure 5-3. The impact of petcoke particle size (0-500 µm) on (a) pore volume(grey bars are
micropores, black bars are mesopores) and yield(solid red circle). ...................................... 83
Figure 5-4. The impact of mixing method for NaOH and petcoke on pore volume (micropore
volume indicated by black bar, mesopore volume by grey bar) and yield(solid red circle)
of AC prepared from NaOH to petcoke molar ratio of 0.71at 800 ˚C for 30 min. ............... 86
Figure 5-5. Mixing impact on porosity and yield with the same particle size range (0-150
µm). ....................................................................................................................................... 88
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Figure 5-6. The impact of mixing methods and amount of NaOH on pore volumes
(represented by bars) and yields (represented by solid red circles) during NaOH
activation. .............................................................................................................................. 92
Figure 5-7. Particle size distribution of raw petcoke and ball-milled samples mixed with
NaOH to petcoke molar ratios of 1.06/0.71/0.35 after washing but before activation. ........ 93
Figure 5-8. SEM images of (a) physically mixed petcoke, (c, e, g)ball-milled petcoke with
NaOH to petcoke molar ratios at 0.35/0.71/1.06, and(d, f, h) AC after NaOH activation.
(b) AC prepared by physically mixed with NaOH to petcoke at molar ratio of 0.71. .......... 94
Figure 5-9. Large particles of petcoke mixed with (a) KOH, (c) NaOH, and (e) K2CO3, and
corresponding images (b, d and f) after activation at 800 ˚C for 30 min with a chemical
to petcoke mass ratio of 2. .................................................................................................... 96
Figure 5-10. SEM images and potassium, sodium and sulfur mapping of non-washed AC
from petcoke with (a, b) 0.43KOH, and (c, d) 0.71NaOH.................................................... 99
Figure 5-11. SEM analysis of petcoke activated with KOH before wash.a-c) SEM images of
0.43KOH with different magnifications, (d) potassium mapping and (e) sulfur mapping. 100
Figure 5-12. SEM images of (a, b) raw petcoke and (c-h) AC prepared from KOH activation
at 800 ˚C for 30 min with various KOH to petcoke molar ratios of 0.21/0.43/0.64. .......... 102
Figure 5-13. The SEM images of (a, b) KOH activated petcoke at 800 ˚C for 30 min, and (c,
d) KOH steam co-activated petcoke at 800 ˚C for 60 min.................................................. 103
Figure 5-14. Pore volume (●) and yield (×) of AC prepared from petcoke with increasing
temperatures. ....................................................................................................................... 104
Figure 5-15. Pore size distributions of raw petcoke and AC (0.43KOH) prepared from
petcoke activated with increasing temperatures.................................................................. 105
Figure 5-16. XRD patterns of (a) raw petcoke and (b) AC activated with KOH to petcoke
molar ratio of 0.43 at 800 ˚C without any holding time. .................................................... 107
Figure 5-17. Evolution of the Raman spectra of (a) raw petcoke and AC (0.43KOH) activated
at (b) 500 ˚C and (c) 800 ˚C. ............................................................................................... 108
Figure 5-18. FTIR spectra (ATR detector) of raw petcoke and AC (0.43KOH) prepared with
increasing activation temperature. ...................................................................................... 110
Figure 5-19. Pore volume (●) and yield (×) of AC (0.43KOH) prepared with extended
holding time at various activation temperatures from 500 to 800 ˚C ................................. 112
Figure 5-20. The pore size distribution of AC (0.43KOH) activated at (a) 500 ˚C, (b) 600 ˚C,
(c) 700 ˚C and (d) 800 ˚C with holding time from 0-240 min. ........................................... 115
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Figure 6-1. (a) A pellet of ball-milled petcoke, asphaltene and NaCl (mass ratio is 1:1:4), and
(b) mixed powder of ball-milled petcoke and NaCl (mass ratio of 1:4) after applying a
hydraulic force of 5 MPa for 20 s. ...................................................................................... 121
Figure 6-2. Carbon materials after carbonizing (a) ball-milled petcoke, asphaltenes, NaCl ...... 122
Figure 6-3. SEM images of carbon foam prepared by salt template with ball-milled petcoke,
asphaltenes and NaCl mixing at a mass ratio of 1:1:4 in (a, c) wet ball-mill (b, d) dry
ball-mill. .............................................................................................................................. 124
Figure 6-4. SEM image of carbon foam using (a, c, e) carbon black as a carbon source, or (b,
d, f) ball-milled petcoke as a carbon source........................................................................ 126
Figure 6-5. SEM image of carbon foam by carbonizing carbon black, asphaltenes and NaCl
(a, c) at 600 ˚C for 2 h, or (b, d) 400 ˚C for 2 h .................................................................. 127
Figure 6-6. SEM image of HPC created by carbonizing 400 ˚C for 2 h (a, c, e) ball-milled
petcoke, asphaltenes and NaCl (mass ratio = 1:1:4) or (b, d, f) petcoke-derived AC
(0.43KOH/St) with asphaltenes and NaCl (mass ratio = 1:1:4). ......................................... 129
Figure 6-7. Pore size distribution of AC (●) prepared from petcoke through co-activation of
KOH and steam (0.43KOH/St), and the corresponding HPC (○) by using the same AC
as carbon precursor. ............................................................................................................ 130
Figure 6-8. Digital photos (a-c) and SEM images (d-f) with EDX mapping (g-i) of HPC
prepared from non-washed AC (0.43KOH) with 1:1 mass ratioof AC and asphaltenes. ... 132
Figure 6-9 Digital photos and SEM images of HPC prepared from non-washed AC
(0.43KOH) and asphaltene at mass ratios of 1:1 (a, d) and 1:0.1 (b, e). ............................. 133
Figure 6-10. Pore size distribution of HPC prepared from non-washed AC (0.43KOH) and
asphaltenes at mass ratios of 1:1 (● for water wash, ○ for water and acid wash) and
1:0.1( for water wash). .................................................................................................... 135
Figure 6-11. The schematic graph of pore development for HPC structure. .............................. 136
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List of Symbols, Abbreviations and Nomenclature
Abbreviations Definition
AC Activated carbon
BM Ball-milling
DFT Density functional theory
EDX Energy-dispersive X-ray spectroscopy
FTIR Fourier-transform infrared spectroscopy
HPC Hierarchical porous carbon
Petcoke Petroleum coke
PM Physically mixing
PSD Particle size distribution
SEM Scanning Electron microscopy
XRD X-ray Diffraction
Symbols Definition
n sample size
s sample standard deviation
�̅� sample mean
𝛼 significance level
La crystallite size
ID Integrated intensity of D Raman band
IG Integrated intensity of G Raman band
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Chapter One: INTRODUCTION
1.1 Overview
Over recent decades, the search for innovative methodologies to convert waste by-products from
oil refineries into value-added materials has been a significant challenge. The production of
bitumen crude oil increased to 4.6 million barrels per day in 2018 from Western Canadian
reserves [1]. Among the various types of waste by-products from oil refineries, petcoke and
asphaltenes are appealing raw materials for further exploration. More than 50% of the petcoke
was stockpiled in oil sand fields in Northern Alberta because of the relative high cost of
transportation but low utilization of petcoke either as itself or other petcoke-derived materials
[2]. First asphaltenes are generally defined as the fraction insoluble in n-heptane. More
specifically, asphaltenes have a polar and high molecular weight (250 g/mol to 2000 g/mol)
fraction in bitumen [3]. Asphaltene deposition resulting in plugging causes severe problems in
oil reservoirs. Therefore, the majority of researchers have focused on the diagnosis, prevention
and mitigation of the asphaltene issues [4], but not the reuse of asphaltene after removal from the
reservoirs.
In contrast to waste materials, such as biomass or coal, used as common carbon precursors in the
preparation of activated carbon, petcoke produce AC with high yields (50-80 % by KOH
activation) which is benefited from its high amount of fixed carbon (> 80 wt%). However, non-
porous carbon surface and releasing of sulfur during petcoke activation hindered the
investigation of pore development mechanism and further application. Among the limited
research studies, petcoke has been activated either physically with CO2/N2/steam or chemically
with NaOH/KOH to prepare porous carbon materials since the early 1990s [5-8]. A chemical to
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petcoke mass ratio of 3 was normally used to develop significant porosity during petcoke
activation. However, the extensive use of chemical agents resulted in a higher production cost
and chemical waste. The co-activation of KOH and steam was attempted for petcoke activation
for the first time by Wu et al. [9] in 2005. The resulting activated carbon (AC) had a high surface
area of 2500-3000 m2/g by only applying KOH: petcoke mass ratio of two. However, the greatest
challenge during the co-activation was the low yield of 25-30%. A better understanding of pore
development during petcoke activation is required for designing AC with high porosity without
compromising yield.
Porous carbon material consisting of multiple scales of pores has been widely applied in the
fields of catalysis, adsorption, separation, energy conversion and storage [10-12]. These types of
materials are called hierarchical porous carbons (HPCs), which has become increasingly
attractive in recent decades because of their electrical conductivity, chemical stability and low
cost, in combination with various functional hierarchical levels of pore sizes spanning from
nanoscales to macroscales [10]. An investigation of HPC synthesis with waste by-products from
oil refineries is a worthwhile in the attempt to design an environmental friendly and cost-
effective approach.
1.2 Objective
The objective of this thesis is to propose methodologies to convert waste by-products from
Athabasca oil sands to porous carbon materials. Petcoke was used as a carbon precursor to
prepare AC with high microporosity, and activation experiments were performed either with
only chemical agents or co-activated with chemical and steam. The advantages of high sulfur
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petcoke for developing pores at low temperature emphasized. The correlation between petcoke
structure (partially ordered) and pore development on petcoke was studied for the first time.
Asphaltenes, being another waste material from refineries, were used for the first time as binders
for the formation of carbon foam and HPC structure. There are three main topics related to
prepare porous material from these by-products in this thesis:
The impact of the amount of chemical agents (KOH or NaOH) for the activation of
petcoke
Pore development of high sulfur petcoke during chemical activation
Carbon foam and HPC structure formation by salt template with petcoke and asphaltenes
1.3 Organization of the thesis
This thesis contained seven chapters. Chapter Two included the literature review of petcoke and
asphaltenes, and the approaches of preparing AC and HPC were also summarized with
corresponding physical and chemical properties. Some parts of the literature review were
previously published by Wu el al. in Fuel Processing Technology [13]. Chapter Three introduced
the experimental procedures and specific characteristic techniques. Chapter Four discussed the
co-activation of chemical and steam for the reduction of chemical amount. Most results in
Chapter Four were previously published by Wu et al. in Fuel Processing Technology [13]. My
role in this paper was designing and performing the experiments, analyzed data, and wrote the
manuscript. Dr. Vicente Montes Jiménez performed SEM, organized the data to discuss the
mechanism of co-activation. Dr. Luis Daniel Virla Alvarado contributed with the discussion of
data analysis and the explanation of the mechanism of co-activation of petcoke. Chapter Five
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reported on the low temperature activation of high sulfur petcoke. Chapter Six covered carbon
foam and HPC synthesis by using petcoke and asphaltene with a salt templating approach.
Chapter Seven outlined a summary and recommendations for further work from the project.
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Chapter Two: LITERATURE REVIEW
This chapter introduces two types of waste materials from oil refineries, petroleum coke and
asphaltenes, and their potential applications in various fields. The activation of petcoke is one of
the applications described in detail in this chapter, especially focused on various mixing
approaches and activation methods. The synthesis of HPC is also introduced as another
application.
2.1 Waste carbon source
2.1.1 Petroleum coke
Petroleum coke (petcoke) is a carbon-rich solid by-product from crude oil refineries. Crude
bitumen production was 4.6 million barrels per day in 2018 from Western Canadian reserves,
and is estimated to grow up to 5.9 million barrels per day in 2035 [1]. A coker unit in oil
refineries is designed for the thermal cracking process to upgrade hydrocarbons with high value
overhead and leave solid petcoke at the bottom. Depending on the reactor selected for the coking
process, there are two different types of petcoke in the market: delayed coke produced in a semi-
batch reactor and a fluid petcoke from fluid reactor. Since the volatiles can not escape from
semi-batch reactor, delayed petcoke usually contains a higher number of volatiles (8.1 wt%) than
fluid petcoke [14, 15]. In this thesis, delayed coke was used as our activating and characterizing
material. Syncrude produces delayed coke and is based in Fort McMurray.
High grade petcoke is widely used in the metallurgical industry and the production of electrodes.
Over 80% of the petcoke produced worldwide, however, is low grade (i.e., 5-7 wt% sulfur and
heavy metals including nickel and vanadium), which prevents it from being used as a traditional
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fuel due to environmental concerns [16]. Currently, the surplus petcoke is stockpiled near
refinery and upgrading facilities [17]. These stockpiles represent a potential hazard for public
health since petcoke particulates can be transported in the form of airborne dust [18, 19], and the
stockpiles occupy considerable space that could be otherwise utilized. Given these issues, there
is interest in valorizing this by-product. Previous studies have shown the potential of using
petcoke as a starting material to produce adsorbents and catalyst supports [5, 15, 20, 21]
Petcoke is a non-porous solid, which generally must be activated before being used for the
above-mentioned applications [15, 20, 22-24]. An economic evaluation of minimizing
production costs of activated carbon (AC) found that petcoke is the most promising raw material
among several carbonaceous materials including wood, used tires, carbon black, charcoal, and
lignite, its biggest challenge is its non-porous nature. The major reason is that petcoke has a
relatively high yield and surface area compared to the other carbonaceous materials (Table 2-1).
Therefore, the production cost of petcoke is the lowest (1.08 US$/kg) among the other carbon
materials (1.22-2.49 US$/kg). In addition, a higher surface area through KOH activation of
petcoke (Table 2-1) also reduces the production cost compared to steam activation.
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Table 2-1. Yields and surface areas of AC prepared from various carbon precursor by KOH
activation and steam activation. Data revised from [25].
Raw material
KOH activation Steam activation
AC yield (wt%) N2 BET surface
area (m2/g)
AC yield
(wt%)
N2 BET
surface area
(m2/g)
Wood 22 800 13 800
Used tires 20 700 15 500
Petcoke 45 3000 63 1000
Carbon black 60 500 48 500
Charcoal 44 2000 45 900
Lignite 25 2200 16 800
Another challenge when utilizing petcoke is its impurities such as heavy metals and sulfur
content. The sulfur content of petcoke ranges from 0.5 wt% to 10 wt%, which strongly depends
on the specific feedstock [26]. High sulfur content is a major problem for low grade petcoke. A
recent study investigated the sulfur species of Zibo petcoke and American petcoke through X-ray
photoelectron spectroscopy. The results showed that almost half of the sulfur was thiophene, and
the smallest amount of sulfur was inorganic sulfur (< 20%). The remaining sulfur was in the
form of sulfoxide sulfur [27]. However, it should be mentioned that XPS is a surface sensitive
technique, and the bulk composition may be different. Many researchers have established both
physical and chemical approaches to removing sulfur from petcoke [26, 28, 29]. Among all the
desulfurization approaches, chemical activation with alkali metals has proved to remove over
90% of sulfur at activation temperatures lower than 827 ˚C [26, 27, 30]. Nonetheless, an abrupt
yield drop (about 10% - 40%) and the excessive amounts of chemicals involved are barriers to
considering chemical activation as a desulfurization approach in scale-up industries. However,
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the porosity developed in carbonaceous materials during activation has inspired researchers to
prepare AC from petcoke.
The structure of petcoke is still a mystery in scientific research fields since petcoke is a
complicated mixture of carbon, hydrogen, nitrogen, oxygen, sulfur and other impurities [15].
Moreover, petcoke composition varies from region to region [26, 28]. As well, the challenges
with the development of analysis technology hinder progress in the structural illustration of
petcoke [31]. In the late 1990s, a group of researchers studied the microstructure of petcoke from
five different regions by using X-Ray diffraction (XRD) and electron microscopy [32].
Differences appeared in the line broadening of XRD patterns for the five different petcoke
samples from 5 different regions, which implied that the microstructure of petcoke is heavily
dependent on its region. The researchers also found the presence of turbostratic solids for certain
types of petcoke. This indicated the co-existence of crystalline graphitic carbon layers, whose
basal planes are in a random orientation. In 2010, another research group studied the
microstructure of petcoke using XRD, and similar microcrystalline and interlayer spacing were
observed [33]. With the advanced characteristic techniques of high-resolution transmission
electron micrographs (HRTEM) and selected area diffraction (SAED), the distorted crystals in
amorphous layers of carbon were observed to have a graphitic lattice fringe of 2.85 nm.
Recently, Zhong et al. investigated the structural features of Qingdao petcoke through high-
resolution transmission electron micrographs (HRTEM), X-ray powder diffraction (XRD) and
nuclear magnetic resonance (NMR) [34]. An atomistic structural representation (Figure 2-1) was
proposed by integrating experimental data with an image-guided construction strategy (Fringe3D
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and Vol3D). Image analysis was first used to gather information about fringe length and
orientation degree. A ‘stack’ analysis was then performed to obtain the statistical stacking data
for all the lattice fringe micrographs. The structural constraints were determined by fringe
curvature statistical analysis through identifying the location, length, and angles of each fringe.
Figure 2-1. 3D cubic model of Qingdao petcoke. Green for carbon, red for oxygen, yellow for
sulfur, blue for nitrogen, and grey for hydrogen atoms. Reprinted from Carbon, 129, Q. Zhong,
Q. Mao, L. Zhang, J. Xiang, J. Xiao, J.P. Mathews, Structural features of Qingdao petroleum
coke from HRTEM lattice fringes: Distributions of length, orientation, stacking, curvature, and a
large-scale image-guided 3D atomistic representation, 790-802 [34], Copyright (2018), with
permission from Elsevier.
2.1.2 Asphaltenes
Asphaltenes are the heaviest fraction of crude oil. Asphaltenes are a mixture defined as toluene-
soluble, n-heptane-insoluble fraction from crude oil. The polyaromatic rings surrounded by
aliphatic tails contain heteroatoms (O, N, S), which comprise asphaltene molecules. Both the
‘continental’ and ‘archipelago’ model (Figure 2-2) have been widely reported in the literature
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[35]. There is still a debate on the molecular structure of asphaltenes because of their chemical
complexity as a group of chemical materials.
Figure 2-2. The proposed structure of asphaltene molecule. A. Continental; B Archipelago.
Reprinted from Journal of Petroleum Science and Engineering, 158, K. Gharbi, K. Benyounes,
M. Khodja, Removal and prevention of asphaltene deposition during oil production: a literature
review, 351-360 [35], Copyright (2017), with permission from Elsevier.
2.2 Activation
2.2.1 Activated carbon (AC)
Activated carbon (AC), also called activated charcoal, refers to the highly porous carbonaceous
materials which are utilized as adsorbents [5, 36, 37] , catalyst supports [38-40] and electrodes
[41-43]. Depending on the pore width, AC can be classified into micropores (< 2 nm), mesopores
(2-50 nm) and macropores (>50 nm). Since activated carbon is a black solid without a distinctive
chemical formula, activated carbon could also be classified based on its shape and industrial
applications: powered activated carbon [44], granular activated carbon [45], extruded activated
carbon [46], activated cloths [47], and others. Several heteroatoms, such as oxygen, nitrogen,
hydrogen, sulfur and other elements, are attached to carbon atoms as functional groups [48, 49].
The amount of heteroatoms depends on the types of carbon precursor. For instance, carbon black
has over 99 wt% carbon content [50], however, biomass has only 40-50 wt% carbon [51]. It has
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been reported that the surface oxygen groups on the carbon precursors before activation, acted as
‘active sites’ in the continuous activation process [52, 53]. A representative of IR-active
functional groups after chemical modification is shown in Figure 2-3. The functional groups
include carboxylic, lactonic, carbonyl, quinone and others.
Figure 2-3. IR-active functional groups on an activated carbon surface. (a) aromatic C=C
stretching; (b, c) carboxyl-carbonates; (d) carboxylic acid; (e, f) lactone; (g) ether bridge; (h)
cyclic ether; (i, j) cyclic anhydride; (k) quinine; (l) phenol; (m) alcohol; and (n) ketene.
2.2.2 Carbon precursor for activation
Although commercial AC has been used to purify water for 150 years, it is still challenging to
design a cost-effective and environmentally-friendly AC. Therefore, waste materials with high
carbon content have become one of a growing number of carbon precursors for AC preparation
[54]; for instance, wood [55], vegetables [36, 56], coal [57, 58] and petroleum residues [30, 59].
The type of carbon precursor determines the properties of the resulting AC, because the strength
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and structure of the raw materials has been shown correlation to the physical and chemical
properties of AC product [60].
As materials from nature, such as biomass, vegetables or animal bones, are abundant, many
researchers have preferred to utilize the inherently porous structure of these materials to develop
further porosity through activation. These materials are also regarded as renewable and economic
carbon precursors for activation, because the porosity can either be developed with or without
chemical agents [61]. Table 2-2 is a summary of KOH activated carbon prepared from natural
materials. Most produced AC have surface area between 1000 and 2500 m2/g and pore volumes
above 1.2 cm3/g. However, the low yield (< 50%) is mostly neglected in the literature, even
though it is a negative factor when considering natural materials for AC preparation. Among
natural materials, the porosity and yield of AC reflects the specific raw material used. Minkova
prepared AC from five different biomass materials under the same activation conditions, and
found relative high yields with larger adsorption capacities obtained with olive wastes, birch and
bagasse compared to straw and miscanthus [62].
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Table 2-2. Properties of KOH activated carbon from natural materials. Revised from [41]
Carbon precursor BET surface area (m2/g) Pore volume (cm3/g) Reference
Cherry stones 1273 __ [63]
Fish scale 2273 2.74 [64]
Pig bone 2157 2.26 [65]
Sunflower seed shell 2585 1.41 [66]
Waste paper 526 __ [67]
Wood sawdust 2967 1.35 [68]
Wheat straw 2316 1.50 [69]
Potato starch 2342 1.24 [70]
In the literature, coal is another abundant carbon precursor for AC production. Since Turkish
coal has fewer volatiles (~50 wt%) than biomass materials (60-80 wt%), the AC yield from four
Turkish deposits ranged from 50-60%, which is slightly higher than the AC yield from biomass
(<50%) under similar experimental conditions [7]. Extensive research on chemical activation of
Spanish coal showed that the chemical reaction between carbon and hydroxides for lowest rank
coal (less fixed carbon, more fractions of moisture and volatiles) started at a lower activation
temperature than the highest rank coal [8]. This indicated that the material purity also influences
the carbon reactivity for the oxidation reactions.
As high temperatures (> 1000 ˚C) were applied when producing petcoke, fewer volatiles (< 10
wt%) remain on petcoke after thermal treatment. Therefore, high yields (50-80 %) of petcoke
derived AC were reported in the literature [13, 39]. Except for more fixed carbon on petcoke,
other significant differences to distinguish petcoke from other carbon precursors are its non-
porous surface and the partially ordered structure, respectively: one, petcoke is a non-porous
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material (surface area < 2 m2/g) [15] while coal and biomass are porous materials (surface are ~
12 m2/g for coal [71], 100-250 m2/g for biomass [72] ); two, coal has amorphous amorphous
structure [73], while petcoke has crystalline graphitic carbon layers with a random orientation
within the structure [33].
2.2.3 Pore development approach
Activation generally involves oxidation and/or intercalation of species within the structure [74,
75]. In the former process, solid carbon is transformed into CO and CO2, creating pores, while in
the latter process, a species, generally a metal, acts as a spacer and increases the pore volume by
widening the distance between the carbon layers [76]. In either method, the product AC is
generally washed to remove excess reactants and to ensure that the pores are accessible.
The advantage of intercalation during activation is the increase in porosity without carbon
consumption, resulting in higher yields and lower emissions than oxidation. Metallic potassium
(K) and metallic sodium (Na) were the most accepted intercalation species in the literature. The
metallic potassium was widely reported as a more active species for intercalation when
compared to metallic sodium. The active intercalation could be explained by the reactions
between distilled metal and carbon precursors of various structural orders [75]. Distilled K (K
vapor) was capable of reacting with highly ordered carbon precursors, as indicated by the
increase of interplanar distance from 0.35 to 0.4 nm (calculated from XRD). For distilled Na,
similar reactions could only happen with a less ordered carbon precursor. The study also found
that the interplanar distance from K vapor reactions was similar to the one produced from KOH
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activation of the same carbon precursor. Therefore, it is clear that the increase in interplanar
distance during KOH activation is related to the formation of metallic potassium.
However, it was difficult to know the exact temperature to form these active species since these
metals are sensitive to temperature and are highly reactive with oxygen when exposed to air.
Since at 25 ˚C and 1 atm the standard reduction potential for K and Na are -2.93 and -2.71,
respectively [77]. It indicated a high tendency for metallic K or Na to be oxidized even at room
temperature. Maintaining the metallic forms of K or Na is even harder for chemical activation at
800 ˚C.
Additionally, the presence of graphitic structure in petcoke has been reported as a benefit for
lithium intercalation, leading to a larger capacity when using petcoke as anode [32, 33].
However, the correlation between petcoke structure and intercalation during petcoke activation
has not been understood as yet.
In order to understand the oxidation reactions involved in the chemical activation process, many
researchers have studied the product of activation by determining the species in the solid and gas
phase after AC samples are cooled down (e.g. [78] [8]). Table 2-3 shows the experiment results
in comparison with thermodynamics data calculated for each proposed reaction. Similar
oxidation reactions (Equation 2-1 and 2-2) during NaOH activation and KOH activation were
proposed by Lillo-Ródenas et al.[8] and Yuan et al. [78], respectively. During KOH activation,
the major product for both reactions (Equation 2-1 and 2-2) is K2CO3. The XRD showed that
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over 80% of potassium species were in the form of K2CO3 after petcoke was activated at 800 ˚C
before washing with HCl and water [78].
A group of researchers studied the chemical bonding of carbon (C 1s), oxygen (O 1s) and
potassium (K 2p3/2) using the XPS on the surface of lignocellulose AC (3-4 nm) before the
washing process [79]. The deconvolution of O 1s implied the presence of K-O bond on non-
washed AC. However, if the K species was washed away after activation, the O/C ratio also
reduced to a very low value, which indicated that all the oxygen on the surface is attached to
potassium rather than carbon. K/O ratio on the AC surface was lower than the theoretical ratio as
when forming pure K2CO3 or a mixture of K2CO3 and K2O, other K species, such as metallic K
or KO2, may also exist on the surface.
The possible oxidation reactions during the chemical activation with metal hydroxides were
reported as following [5, 78, 80]:
6𝑀𝑂𝐻 + 2𝐶 → 2𝑀 + 3𝐻2 + 2𝑀2𝐶𝑂3 Equation 2-1
2𝑀𝑂𝐻 + 𝐶𝑂2 → 𝑀2𝐶𝑂3 + 𝐻2𝑂 Equation 2-2
𝑀𝑂𝐻 + 𝐶 → 𝑀 + 𝐶𝑂 +1
2𝐻2 Equation 2-3
where M refers to either potassium (K) or sodium (Na).
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Table 2-3. Standard Gibbs Energy calculations (∆G0) (for Equations 2-1 and 2-2) during KOH
and NaOH activation.
Temperature
(˚C)
KOH activation [78] Temperature
(˚C)
NaOH activation [8]
∆G0 (kJ/mol)
Equation 2-1
∆G0 (kJ/mol)
Equation 2-2
∆G0 (kJ/mol)
Equation 2-1
∆G0 (kJ/mol)
Equation 2-2
25 182.4 -122.8
530 27.3 -101.2
600 -2.6 -103.6 630 7.14 -93.3
700 -17.5 -93.7 730 -12.3 -85.4
800 -36.9 -83.8 830 -31.3 -77.3
900 -66.6 -74.1
Lu et al. further investigated the role of K2CO3 during KOH activation of petcoke [81]. The
formation of K2CO3 in the initial pores through the reactions proved to improve the porosity of
AC. Further reactions on converting K2CO3 to K2O or K (see Equation 2-4 to Equation 2-7) were
likely because they were thermodynamically favored at temperatures above 430 ˚C. FTIR spectra
also indicated the formation of -CH-, -CH2- as ‘active carbons’ at 500 ˚C, the active carbon
species disappeared as the temperature increased to 730 ˚C. Meanwhile, the XRD pattern of AC
before the washing process clearly showed the formation of metallic K and K2O at 500 ˚C and
730 ˚C, respectively. Other research groups also found the decomposition of K2CO3 (Equation 2-
4 and 2-5) and the reduction of potassium compounds (Equation 5-6 and 2-7) at temperatures
above 700 ˚C. [5, 75, 82]. However, all the sample analysis was performed after cooled down to
room temperature and exposed to air. Thus, the conclusions should be considered cautiously.
𝐾2𝐶𝑂3 + −𝐶𝐻2− → 𝐾2𝑂 + 2𝐶𝑂 + 𝐻2 Equation 2-4
𝐾2𝐶𝑂3 + 2 − 𝐶𝐻− → 𝐾 + 3𝐶𝑂 + 𝐻2 Equation 2-5
𝐾2𝑂 + −𝐶𝐻2− → 2𝐾 + 𝐶𝑂 + 𝐻2 Equation 2-6
2𝐾2𝑂 + 2 − 𝐶𝐻− → 4𝐾 + 2𝐶𝑂 + 𝐻2 Equation 2-7
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2.2.4 Diffusion into porous media
As activation produced porous carbon materials, a large number of chemical species are
produced and then co-existed within the porous structure during the activation. These chemical
species (listed in Equation 2-1 to Equation 2-7), either in the gas, solid or liquid phases,
spontaneously transport into the pores of carbonaceous materials. The movement is called
diffusion, which refers to a net flow of molecules from places of the high concentrations to
places of the low concentrations, resulting in an equilibrium system [83]. Diffusion is normally
driven by the concertation gradient, chemical potential, temperature and pressure [84].
The pore diffusion is governed by the diameter of pores (shown in Table 2-4). The molecular
diffusion is the ordinary diffusion happened when two components are mixed with a
concentration gradient [85]. The pore diameter (> 10 nm) is larger than the mean free path of
molecules (the average distance for a molecule to travel from one place to another). Knudsen
diffusion, sometimes refer to surface diffusion, occurs when the mean free path of molecules is
larger than the pore diameter (2-100 nm) [86]. The molecules collide with the pore wall instead
of with other molecules. As the interaction is between molecules and pore walls, both surface
force and capillary force made contribution to the diffusion process [87]. An example of
Knudsen diffusion is that gas molecules diffuse into capillaries or through porous solids [85].
There is a transition region between molecular diffusion and Knudsen diffusion, which occurs
when the mean free path is similar to the pore diameter. Configurational diffusion is the
dominant transport mechanism when pore sizes are smaller than 1.5 nm [86]. The overlapping
surface force from the opposite pore walls is the driving force in this stage.
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Table 2-4. Diffusion mechanism in porous media. [85-87]
Diffusion mechanism Pore size (d) Driving force
Molecular diffusion d > 10 nm Concentration gradient
Knudsen diffusion (or
Surface diffusion)
2 nm < d < 100 nm Surface force and Capillary
force
Configurational diffusion d < 1.5 nm Overlapping surface force
Vattipalli et al. studied the mass transport on a hierarchical zeolite (a porous material with both
micropores and mesoporous). A longer diffusion path was observed in comparison to the
physical length of an adsorbent. The result showed that combined surface and configurational
diffusions created longer path for the adsorbent to travel in and out between the external surface
and the internal micropores (simple scheme illustrated in Figure 2-4), which reduced the mass
transfer limitation within the interconnected pore structure.
Figure 2-4. Schematic graph to illustrate diffusion in hierarchical zeolite (a porous material with
both micropores and mesoporous).[87]. Adapted with permission from V. Vattipalli, X. Qi, P.J.
Dauenhauer, W. Fan, Long Walks in Hierarchical Porous Materials due to Combined Surface
and Configurational Diffusion, Chemistry of Materials, 28 (2016) 7852-7863. Copyright (2016)
American Chemical Society.
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2.2.5 Activation method
Chemical Activation
Lillo-Ródenas et al. compared the activation of non-porous anthracite with KOH and NaOH at
730 ˚C, and they found KOH produced more pore volume than NaOH with a higher than one
mass ratio of KOH and anthracite [58]. Thus, the nature of chemical agents has been proved to
be an essential factor during chemical activation. KOH was more effective to develop pores for
ordered carbon precursors like anthracite while NaOH produced higher pore volume and surface
area for less ordered lignocellulose materials [88].
In other studies involving the NaOH activation with different feeds, including non-porous
anthracite [89], plum kernels [90], and corncobs [91], higher pore volumes and pore sizes were
obtained as the chemical agent to feed mass ratio increased to above one, but none of these
studies discussed the process of activation in depth.
Co-activation of chemical and steam
While there are many studies on gasification, in which the goal is to consume all of the feed, co-
activation (e.g., simultaneous chemical - H3PO4, KOH, ZnCl2 - activation and physical activation
- CO2, steam, air) has only been studied by a few groups. The synergistic effect of chemical and
physical agents (2-5 h co-activation from 600 – 900 ˚C) enhanced the pore volume and surface
area on various natural materials including coconut shells [92], palm stones [92], Zizania
latifolia leaves [93], and woody biomass [55] but resulted in yields less than 30%. The
interaction between the chemical and physical agents is not well understood, especially as most
studies chose a fixed amount of the agents and varied the other parameters (temperature, time,
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etc.). In addition, the porosity of the feedstock influences pore development [81, 94], so the
results that have been obtained with porous lignocellulosic materials may not be applicable to
other feedstocks.
2.2.6 Activation of petcoke
In 1993, the Kansai Coke and Chemicals Company developed MAXSORB, a high-surface-area
(3100 m2/g) AC derived from petcoke[5] by KOH activation with a chemical (KOH) to petcoke
weight ratio of 5. Even though a pore volume of 1.76 cm3/g was obtained using a ratio of 5, the
higher chemical usage led to low yields (< 30%), higher production costs, and more waste.
Generally a ratio of at least 3 is required to develop significant porosity (pore volume above 0.8
cm3/g) at 800 ˚C on AC from petcoke [22, 38, 52, 81, 95, 96]. Various approaches have been
tried to reduce the amount of chemical agent required. Deng et al. used intercalation with HClO4
to produce an AC from petcoke with a surface area (~3000 m2/g) that was similar to the AC
prepared using chemical activation with a KOH to petcoke weight ratio of 5 [97]. Wu et al. used
the weight ratio of 2 by co-activation with steam at 800 ˚C for only 25 min, and the surface area
of AC reached 2500-3000 m2/g [9]. The porosity created during the catalytic steam gasification
promoted the diffusion of steam. Other gasification catalysts such as alkali metal carbonate (e.g.
K2CO3) have also promoted porosity development [81]. In many other studies, the yields
obtained and the wastes created by the methods used have been ignored. On an industrial scale,
washing away 75% of the reactants (i.e., for a 3:1 mass ratio of chemical agent to carbon feed)
may not be economically feasible.
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Most raw petcoke used for activation is green petcoke without desulfurization treatment. Thus, a
variety of research groups in the past 10 years have been working on converting high sulfur
petcoke to AC, and applied it as a catalyst base or an adsorbent [21, 39, 98-101]. There are only
limited publications on using low sulfur petcoke as carbon precursor for activation [22, 102,
103]. Considering the summarized data in Table 2-5 (AC from high sulfur petcoke) and Table 2-
6 (AC from low sulfur petcoke), it was difficult to compare AC properties prepared from high
sulfur with low sulfur petcoke, since the petcoke composition and activation conditions,
including chemical to petcoke mass ratio (R), activation temperature, and holding time varied
significantly.
However, no matter what activation condition was applied, pore volume of 0.25 cm3/g started to
be developed at 500 ˚C for high sulfur petcoke (6.5 wt%) [21]; similar pore volume (0.28 cm3/g)
could be only achieved at 600 ˚C for low sulfur petcoke (0.48 wt%) [102]. The different
temperatures for initial pore formation indicated that the breakage of the C-S bond in high sulfur
petcoke promoted pore development at low activation temperatures. Only a few papers recorded
corresponding yields of AC prepared from petcoke. Considering similar activation temperature
above 800 ˚C and the same chemical to carbon ratio of three, AC prepared from higher sulfur
petcoke (4.8 wt%, [101]) produced a higher yield (27% higher) than AC derived from low sulfur
petcoke (0.3 wt%, [22]). The higher yields and lower activation temperatures are two key points
to understand the pore development mechanism for high sulfur petcoke.
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Table 2-5 Summary of AC properties prepared from high sulfur petcoke (4-8 wt% sulfur). The
activation temperature from 500 to 850 ˚C.
Sulfur
content
(wt%)
Activation
condition*
Activation temperature (˚C)
500 600 700 750 800
Pore volume (cm3/g) / Surface area (m2/g)
7.7 [98] R=2, Tc=300-
600 ˚C, tc = 1
h, ta =1 h
0.38-0.67
/936-1673**
7.5 [99] R=2, ta =1 h 0.22/538 0.6/1433 0.7/1600
7.7 [39] R=2, Tc = 400
˚C, tc = 1 h, ta
= 1.5 h
0.65/1174
5.3
[100] R=2 0.73/1204 0.83/1409 0.86/1471
4.8
[101] R=3, Tc = 400
˚C, ta = 1 h
0.72/1281
/69.4***
6.5 [21] R=3, Tc = 300
˚C, tc = 1 h
0.254/
692
* R refer to mass ratio of chemical agent to petcoke.
Tc is carbonization temperature.
tc and ta are the holding times at either carbonization or activation temperatures.
If there is no carbonization temperature or holding time shown in the table, no specific
condition was reported in the literature. ** Maximum pore volume and surface area were carbonized at 450 ˚C ***Yield of AC
Table 2-6 Summary of AC properties prepared from low sulfur petcoke (< 1 wt% sulfur). The
activation temperature was from 500 to 850 ˚C.
Sulfur
content
(wt%)
Activation
condition
Activation temperature (˚C)
600 700 800 850
Pore volume (cm3/g) / Surface area (m2/g) / Yield (%)
0.5 [102]** R=2, ta* 0.283/867 0.437/1265 0.477/1070
0.8 [22] R=2, Tc = 500 ˚C, tc
= 1h, ta =2h 0.68/1841/64
0.3 [103] R=3, Tc = 500 ˚C, tc
= 1h, ta = 2h 0.05/864/65 0.11/1063/59 0.31/750/51 0.44/577/42
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* ta varied according to different temperature. Holding time for 600 and 800 ˚C were 1 h, but for
700 ˚C was 2 h ** Yield was not reported
2.3 Mixing impact
2.3.1 Physically mixing and Impregnation
There are a limited number of publications discussing the impact of different mixing methods on
AC properties. Researchers in this area focused on the different impact of physically mixing and
impregnation. Physically mixing of chemical agents and carbon precursors were accomplished in
a mortar at room temperature, and the entire process was in a dry environment without water
being involved [104, 105]. The impregnation was in wet environment of concentrated activating
agent solution [106]. The chemical agents were impregnated into the carbon precursor by stirring
at given temperature (mostly around 60 ˚C) for several hours, and then samples were dried in the
oven overnight. As described above, impregnation took a longer time and greater energy than
physically mixing.
From the literature, the impact of these two mixing methods was highly related to the nature of
hydroxide and the structure of the carbon precursor. Lillo-Rodenas et al was the first research
group who suggested physically mixing as an easy and effective way to develop porosity [58].
Additionally, their results showed that physically mixing NaOH and Spanish anthracite with a
mass ratio of 2 produced double the amount of micropore volume compared to impregnated AC.
Similar to Spanish anthracite, the physical mixture of NaOH and Mongolian anthracite produced
35% higher pore volume than the impregnated mixture [89]. However, KOH impregnation on
Spanish anthracite created similar micropore volume, and a slightly higher surface area (60 m2/g)
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than physically mixed AC [58]. However, the researchers did not provide further explanation for
the reason NaOH and KOH had different effect on pore development when using different
mixing methods.
2.3.2 Ball-mill mixing
The ball-milling technique is a mechanical grinding or blending process. The grinding material
and grinding balls are mixed in a container for a certain rotation speed and a certain period of
time. For the size reduction in a large scale production, ball-milling is an easy operating method
[106-108]. Homogenous mixture is another advantage of ball-milling technique. By adding
liquid (such as ethylene glycol, isopropanol alcohol, water, acetone, etc.) into the container, wet
ball-mill is used for a better dispersibility in liquid phase [108, 109]. The poor dispersion of
grinding materials in dry ball-milling led to the agglomerates with a much rougher surface of the
particles in SEM images when compared to wet ball-milled boron particles [106]. The thermal
problem caused by operating the rotary tumbler is also reduced with extra liquid in contrast to
the dry ball-mill process. The surface area was increased after wet and dry ball-milling biochar
[110], graphite (dry ball-mill) [107, 111], titanium monoxide (wet ball-mill) [109] and composite
nanoparticles like ZnFe2O4-C (dry ball-mill) [108]. New chemical bonds are formed during high-
energy ball-milling [106, 112]. However, no previous study has investigated the impact of ball
milling mixing between chemical agents and carbon precursor before activation for the final AC
property. With consideration of less energy consumption, low-energy dry ball-milling was used
for the first time to obtain a homogenous mixture before activation. In order to get more
dispersed mixture with smaller particles, low-energy wet ball-milling was also applied in the
synthesis of carbon foam and hierarchical porous carbon (HPC) through a salt template process.
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2.4 Hierarchical porous carbon
Many biological materials are built in hierarchical components over large length scales. The
structure is related to diverse functionalities, such as mechanical protection, chemical barriers,
energy transfer and storage. [113]. The functional adaptability from natural structures has
inspired scientists to solve engineering problems by man-made hierarchical materials. The
advanced bio-inspired materials have been widely exposed to research fields including catalysis,
adsorption, separation, drug delivery, energy conversion and storage [114]. Among these
materials, HPC has become increasingly attractive in recent decades. A representation of 3D
hierarchical graphitic carbon is shown in Figure 2-5. The nanoscale pores with interconnected
macroscale pores within the carbon wall is crucial for the synthesis of HPC materials [115].
Because the electrical conductivity, chemical stability and production cost are combined with
various functional hierarchical levels of pore size, spanning from nanoscales to macroscales.
Figure 2-5. The schematic representation of hierarchical graphitic carbon. Reprinted from S.
Dutta, A. Bhaumik, K.C.W. Wu, Energy & Environmental Science, 2014, 7, 3574-3592 [115]
with the permission from The Royal Society of Chemistry.
2.4.1 HPC synthesis by template
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Template synthesis is an easy approach to operating and controlling the pore structure of HPCs.
It is usually classified as hard template or soft template depending on the structure of template.
The hard templates need to be removed at the end because of either chemical or physical
differences between the template and the final product [116]. However, the soft templates are
thermally decomposable polymers, which lowers the risk of environmental pollution caused by
the corresponding step of removing the templates [115, 116]. Moreover, a sustainable method by
which the template could be recycled is more competitive for large scale production.
Dual-templating has been explored to create pores on two length scales – small mesopores and
large mesopores or macropores. The templates can be both hard templates, both soft templates or
combined hard/soft templates. However, two hard templates make the procedure tedious and
time-consuming [115]. In-situ one-step templating has become an advanced technique to prepare
HPCs. Li et al. reused petroleum pitch as a carbon precursor [117]. The detailed experiment
procedure was shown in Figure 2-6. Silica gel and silica nano-aggregates were chosen to be dual-
templates in one-step templating. 3D-HPCs were fabricated with wormhole-like small mesopores
(3-4 nm) and large particle-like mesopores (about 17 nm).
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Figure 2-6. A scheme of the HPC prepared from pitch-based carbon. Reprinted from Carbon,
48, S. Li, Y. Liang, D. Wu, R. Fu, Fabrication of bimodal mesoporous carbons from petroleum
pitch by a one-step nanocasting method, 839-843 [117], Copyright (2010), with permission from
Elsevier.
Three well-used polymers in soft templating are trilock copolymer surfactant template (CEO-
PPO-PEC), colloidal crystal and polyurethane foams. The former is accepted as a template for
ordered mesoporous structure. The latter two are used for macropores. An example of a dual-
template of polyurethane foam and triblock copolymer was produced by Xue et al. [118]. An
evaporation induced self-assembly (EISA) procedure facilitated the organic-organic self-
assembly between mesoporous coating layer and polyurethane foam. The large internal surface
area of polyurethane facilitated evaporation and assembly.
Limited soft templates are available for mesopores. A tunable pore size of HPCs is obtained by
the combination of hard and soft templates. Liang et al. proposed an approach for 3D
interconnected HPCs with three different levels of pore sizes. Macropores were developed from
PMMA colloidal crystal. After removing the templates, the wormhole-like mesoproes (2.7 nm)
and large spherical mesopores (10 nm) were created from SiO2 nanoparticles.
Ice-segregation-induced self-assembly (ISISA in Figure 2-7) is an environmentally friendly
method because ice rather than polymers is selected as hard template [115, 119]. The aqueous
phase is segregated by ice for macroporous channel formation. After the sublimation of ice, the
porous structure develops.
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Figure 2-7. A scheme for ice-segregation-induced self-assembly (ISISA). Ice channels were
formed with particle segregation during the freezing procedure. Reprinted with permission from
M.C. Gutiérrez, M.L. Ferrer, F. del Monte, Ice-Templated Materials: Sophisticated Structures
Exhibiting Enhanced Functionalities Obtained after Unidirectional Freezing and Ice-Segregation-
Induced Self-Assembly, Chemistry of Materials, 20 (2008) 634-648 [119]. Copyright (2008)
American Chemical Society."
The salt template method is another environmentally friendly approach because the salt (eg.
NaCl) could be easily removed by water [120]. The recycling and reuse of the salt are feasible as
well [121]. This lowers the production cost as well as reducing the risk of environmental hazards
if the salts require pre-treatment before disposal. Furthermore, the size of salts can be controlled;
therefore, the HPC is likely to be synthesized with a expected structure [122]. Lu et al.
established a simple approach to synthesize the HPC monolith using a salt template and a self-
binder [120]. NaCl was the salt template for the macropores structure within the monolith. By
impregnating furfuryl alcohol into SBA-15 (mesoporous silica sieves with uniform hexagonal
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pores ranging from 5 to 15 nm), polymerization reactions occurred to bind the templates (both
NaCl and SBA-15) with furfuryl alcohol together for the formation of the HPC monolith.
Furfuryl alcohol in this study acted as both the carbon source and the binder. The NaCl template
and SBA-15 template were removed by washing with water and HF solution (~10%),
respectively at the end .The pathway to synthesize the HPC monolith is illustrated in Figure 2-8.
Figure 2-8. Synthetic pathway for the HPC monolith by using salt templating and self-binding.
Reprinted from Microporous and Mesoporous Materials, 95, A.-H. Lu, W.-C. Li, W. Schmidt, F.
Schüth Fabrication of hierarchically structured carbon monoliths via self-binding and salt
templating, 187-192. [120], Copyright (2006), with permission from Elsevier.
A recent study by Qiu et al. also used salt templating to precisely control carbon structure of
pitch [123]. NaCl was selected to be a salt template for macropores formation due to its stability
and low cost. Both phenolic resin (hard carbon precursor) and pitch (soft carbon precursor) are
carbon precursors in the study. Three components mixed in a wet ball milling container (Figure
2-9a) to obtain a homogenous mixture before carbonization. After pre-treatment at 750 ˚C and
then carbonization at 1500 ˚C, the HPC with nanoscale and macroscale pores was successfully
produced (See Figure 2-9b). Without the NaCl template, the produced HPC was not able to
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develop macropores as shown in Figure 2-9c. This implied the presence of NaCl has an essential
role in the formation of macrpores during HPC synthesis.
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Figure 2-9. (a) Illustration of the synthesis process of the HPC; (b) SEM image of HPC with salt
template; (c) SEM image of HPC without salt template.. Reprinted from Journal of Energy
Chemistry, 31, D. Qiu, T. Cao, J. Zhang, S.-W. Zhang, D. Zheng, H. Wu, W. Lv, F. Kang, Q.-H.
Yang, Precise carbon structure control by salt template for high performance sodium-ion storage,
Journal of Energy Chemistry, 101-106 [123]. Copyright (2019), with permission from Elsevier.
The binder during monolith preparation should: 1) Increase the amount of carbon to binder ratio
to improve the mechanical strength of the monolith; 2) Ensure nanoscale pores on the particles
b c
a
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are not blocked. Though binders work to form a monolith, most of binders reported in the
literature have an adverse impact on AC property, resulting in the blockage of micropores during
the carbonization process [124]. Pitches have been proved to be a suitable carbon precursor as
well as a binder due to their thermoplastic properties when they are heated [125, 126].
2.4.2 HPC synthesis by activation
Activation is a traditional way for pores to develop without any template. Targets for HPCs,
natural materials with hierarchical structure are considered as economical carbon precursors,
such as luffa sponge [127], animal bone [65, 128], silk cocoon [129], and rice straw [130]. The
pore size distribution is restricted by the pore structure of the raw material. Since the silk cocoon
is made up of natural microfibers, Liang et al. transformed silk cocoon into microporous
carbonaceous material via a simple carbonization process [129]. The activation via chemical
agents (KOH, NaOH, H2SO4) enhanced the porosity and shortened the activation time. A luffa
sponge has four-level hierarchical cellular structure. Fan et al. have suggested KOH activated
carbon derived from luffa sponge combines micrometer channels with micro/mesoporosity
[127]. For the convenience of large-scale application, the useful biopolymers are also extracted
from the industries. A lab scale experiment has been completed by Zhao et al. They prepared
HPCs using phenolic resin synthesized by replacement phenol derived from bark [131].
Commercial polymer is another carbon precursor. Through one-step chemical activation, well-
structured HPCs were fabricated from commercial polymers including phenol-formaldehyde
resin [132] and poly(vinylidene fluoride) [133, 134].
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As a low-cost operation procedure, the 3D hierarchical porous net was constructed via bottom-up
technique. Macro/mesopores were formed by aggregated carbon spheres. Gong et al. developed
two ranges of pore size (macro-mesopores and micro-mesopores) in respective hydrothermal
carbonization and air activation [135]. The carbon sphere of 60-80 nm aggregated slightly with
the help of a structure-directing agent (commercial kayexalate) via hydrothermal carbonization,
and the interparticle void contributed to the macro-mesopore. The consequent air activation
improved the micropore volume. In addition, loose agglomeration of carbon nanoparticles for
macropore formation was reported by KOH activation process [136] or only carbonization [137].
From the reviewed literature, simple chemical activation or carbonization is an approachable
method to fabricate HPCs.
Most HPCs only focus on the micro and mesopores range applied in the adsorption, catalysis and
electrochemical fields. However, there are few research papers on HPCs integrated micropores
and macropores together.
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Chapter Three: EXPERIMENTAL METHODS
This chapter describes the production of AC from petcoke, and the fabrication of hierarchical
porous carbon by using asphaltenes as a binder in between the prepared AC particles. The
characterization techniques include N2 adsorption, CO2 adsorption, scanning electron
microscopy, and Fourier-transform infrared spectroscopy. In addition, the error analysis of AC
preparation is discussed in this chapter.
3.1 Materials
Delayed petcoke provided by Suncor Energy Inc. was ground with a mortar and pestle manually.
By using nesting 8 inches stainless steel standard sieves of No. 100 and No. 50 (W.S. Tyler, ON,
Canada), the petcoke particles were shaken and sieved for 2 min to obtain the sizes between 150-
300 µm. The composition of petcoke is listed in Table 3-1. NaOH (97%, Sigma-Aldrich Inc.,
USA) and KOH (85%, Alfa Aesar, MA, USA) were used for the chemical activation. NaCl
(99%, EMC Chemicals Inc., Germany), isopropanol (>99.5%, Sigma-Aldrich Inc, USA) and
Athabasca Asphaltenes precipitated in n-pentane (C5) were prepared for HPC synthesis. N2
(99.999%, Praxair Canada Inc, Mississauga, ON, Canada) gas was utilized to provide an inert
atmosphere during activation and carbonization.
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Table 3-1. Proximate, ultimate, and ash analysis of petcoke.
Delayed petcoke [15] Athabasca
asphatlenes[138]
Proximate analysis (wt%)
Moisture 0.3 0.1
Ash (dry basis) 3.7 0.6
Volatile (dry basis) 15 63
Fixed carbon (dry basis) 82 36.3
Ultimate analysis (wt%), dry and ash free
Carbon, C 84 83
Hydrogen, H 3.8 8.3
Nitrogen, N 1.8 1.2
Sulfur, S 6.5 7.5
Oxygen, O (by difference) 3.8 0
Ash Analysis (wt%)
Si 17
Al 14
Ti 2.5
Fe 5.4
Ca 3.4
Mg 1.2
Na 1.4
K 2.9
P 0.1
S 0.8
Balance 51
3.2 Preparation of activated carbon
The mass ratios are commonly reported in the literature. However, the molar ratios are more
instructive when considering the chemical reactions. In this set of experiment, AC was prepared
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from petcoke by chemical activation or chemical steam co-activation. In a mortar and pestle, 2.5
g of petcoke was hand mixed with NaOH or KOH (the chemical agents) with mass ratios of
chemical agent to petcoke from 0.5-3. In the following chapters, the discussion was based on the
molar ratios of NaOH or KOH to carbon calculated according to the purities of petcoke (84%
carbon), KOH (85%) and NaOH (97%). As shown in Table 3-2, the equivalent molar ratios of
NaOH to carbon were from 0.18-1.06, and for KOH to carbon were 0.11-0.64. The samples are
named according to the molar ratio and activation method (eg. KOH stands for KOH activation;
KOH/St indicates KOH steam co-activation) used, then followed by activation temperature and
holding time at that temperature. (eg. 0.43KOH_600_40 refers to a sample activated with a
molar ratio of KOH to carbon of 0.43 at 600 ˚C with 40 min holding time). The default activation
temperature and holding time were 800 ˚C and 30 min, respectively.
Table 3-2. Equivalent molar ratio for mass ratio of NaOH and KOH.
Mass Ratio Molar Ratio
NaOH KOH
0.5 0.18 0.11
1 0.35 0.21
2 0.71 0.43
3 1.06 0.64
3.2.1. Chemical activation
Chemical activation is a traditional approach to obtain highly microporous and high surface area
(> 950 m2/g) AC from petcoke [5, 8, 30, 99]. The mixture of petcoke and chemical agent was
placed in a ceramic boat, which was then placed inside a horizontal furnace and heated under
flowing nitrogen (100 cm3/min) at a rate of 5 ˚C/min from room temperature to the activation
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temperature of 800 ˚C. The mixture was then held at this temperature for 30 min. The produced
AC was cooled down to room temperature, then washed with 250 cm3 of 1 molar HCl solution
and de-ionized (DI) water until the pH of the washing solution was neutral. The sample was
dried in oven at 120 ˚C overnight before further analysis. Thermogravimetric analysis (TGA)
was completed by another PhD student in our group Ross Arnold. 10 mg AC was placed in an
alumina crucible in TGA unit (Q600, TA Instruments, New Castle, DE). Oxygen was flowed
into the system at 100 mL/min to burn the sample, and the changes in sample mass were
measured as the temperature increased from room temperature to 800 ˚C at the rate of 50 ˚C/min.
The result showed that samples after washing contained less than 1 wt% ash.
The yield obtained after activation was calculated according to Equation 3-1 in which mi and mf
are the initial and final masses, respectively. The pore volume per gram of petcoke was obtained
by multiplying the total pore volume by the yield.
𝑌𝑖𝑒𝑙𝑑 (%) =𝑚𝑓
𝑚𝑖× 100 Equation 3-1
Different temperatures from 400-800 ˚C were also studied for KOH activation. KOH and carbon
molar ratio of 0.43 (equivalent to a 2:1 mass ratio of KOH:petcoke) was selected for all the
experiment in this section. A similar procedure was followed as the previous chemical activation
of petcoke, however, the temperature profile was different. The furnace was heated up from
room temperature to certain activation temperatures (400 ˚C, 500 ˚C, 600 ˚C, 700 ˚C and 800
˚C), and there is no holding time (0 min) applied at these activation temperatures.
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In order to understand the impact of total activation time, the activation holding time was
extended from 0 to 40 and 240 min at various activation temperatures.
The activation setup is displayed in Figure 3-1. Petcoke was activated in a horizontal ceramic
tube reactor (L: 70 cm, OD: 4 cm). The nitrogen was introduced into the reactor, and the nitrogen
flow was modulated with a mass flow controller (Brooks Instrument, Hatfield, PA, USA). The
steam was generated before activation started at 800˚C by heating up the temperature above 110
˚C in steam generator. A two-way valve was used to control the activation atmosphere, either
pure N2 or N2 mixed with steam. The actual temperature of the furnace was measured with a
thermocouple in the center of the furnace, and it was compared with the programmed
temperature and then controlled by a temperature controller. Water and KOH solution were used
to remove toxic emissions such as CO and H2S. The fine charcoal particles in the gas flow were
filtered before the exhaust gases were vented to a fume hood.
Figure 3-1. Activation setup for converting petcoke to porous materials. (Revised from [139])
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3.2.2. Co-activation (Chemical and steam activation)
In order to reduce the chemical amount utilized during activation, steam as another source of
oxygen was introduced to the chemical activation system. Steam was injected into the furnace, at
a rate of 5 cm3/h, when the sample temperature reached 800 ˚C. In the presence of steam,
activation times of 15 min, 30 min, 45 min, and 60 min were tested. Samples produced from co-
activation have “St” added to the name. For example, 0.43KOH/St refers to a sample co-
activated with KOH and steam with a molar ratio of KOH to carbon of 0.43. After cooling down
to room temperature, the products were washed and dried in the same way described in 3.2.1.
3.3 Mixing impact
In order to understand the mixing impact on AC properties, petcoke and chemical agent
(NaOH/KOH) were combined in three different ways before activation.
3.3.1 Physically mixing
Physically mixing is a common mixing approach for AC preparation due to its simple process as
well as effective porosity development. The physically mixing of petcoke and chemical agent
was achieved manually using a mortar and a pestle. In this study, chemical agent pellets
(KOH/NaOH) were ground into powder in the mortar for 2 minutes. Then, 2.5 g petcoke (150-
300 µm) was mixed with the chemical powders in the mortar for another minute to obtain a
uniform mixture.
3.3.2 Ball-milling
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Ball-milling was performed with a Rotary Tumble (LORTONE Model 33B, WA, USA) at a
rotational speed of 400 rpm for 14 h. Ball-milling mixed 2.5 g petcoke and chemical agent
(NaOH/KOH) using zirconia grinding balls (with diameter of 5 mm) in a dry plastic milling jar
(200 mL). The mass of grinding ball was ten times of the total mass of petcoke and chemical
agent.
3.3.3 No mixing
In these experiments, chemical pellet was placed below or above 2.5 g petcoke in the ceramic
boat. This method was used to observe the bulk mobility of the chemical agents.
In the mixing methods mentioned above, the mass ratio of chemical agent to petcoke was 2,
which corresponds to molar ratio of 0.71 for KOH or 0.43 for KOH (See Table 3-2). The
chemical activation was performed with either KOH or NaOH at 800 ˚C for 30 min.
3.4 Preparation of carbon foam
Carbon foam was synthesized by a salt template approach. Asphaltenes were attempted to be
binder in this study to form carbon foam structure. The carbon source in this set of experiments
is either carbon black (CB, Monarch 120 from Cabot Cop, Boston, MA) or ball-milled petcoke.
The ball-milling process in 3.3.3 was followed to further reduce the size of petcoke (150-300
µm). In salt template process, wet ball-milling was utilized to mix 1 g carbon precursor (carbon
black or ball-milled petcoke), 1 g binder (asphaltenes), 4 g NaCl and 60 g zirconia grinding balls
in 60 mL isopropanol for 9 h. After removing grinding balls, the mixture was dried in a muffle
furnace at 70 ˚C for 12 h. The powder that resulted was pressed with a hydraulic mounting press
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(Buehler LTD, IL, USA). A hydraulic pressure of 5 MPa was applied to the powder for 20 s,
which led to the formation of a pellet. In order to fix the carbon structure, the pellet was then
carbonized in N2 atmosphere at certain temperature (400 ˚C or 600 ˚C) for 2 h. After the pellet
was cooled down, the salt was removed by washing alternatively with hot water and in an
ultrasonic bath for 1 h consecutively five times. Finally, the sample was dried in the oven at 40
˚C overnight.
3.5 Characterization
3.4.1. N2 adsorption
Surface area, pore volume and pore size distribution were determined by N2 adsorption on the
TriStar II Plus apparatus (Micromeritics Instrument Corp., Norcross, GA, USA) at -196°C.
Before the physisorption experiments, all samples were degassed in a separate unit under
vacuum at 150 °C for 5 h to remove the remaining moisture and volatiles on the carbon surface.
The typical sample mass used was 0.1 g [140]. The dry sample mass was measured immediately
after completing the TriStar analysis. The relative pressure (P/P0) for N2 adsorption was from
0.01-0.995.
The total pore volume was obtained at a relative pressure of 0.97 on the isotherms, and the pore
size distributions were determined with the SAIEUS program (Micromeritics). The best fits to
the isotherms were obtained with the 2-Dimensional Non-Local Density Functional Theory with
heterogeneous surface (2D-NLDFT-HS) model for samples activated with KOH, and the 2D-
NLDFT model for samples activated with NaOH. More details on these methods and their
applications to carbon-based materials are available elsewhere [140]. The micropore (< 2 nm)
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and mesopore volume (2-50 nm) were the cumulative pore volume read from the pore size
distributions.
3.4.2 CO2 adsorption
CO2 adsorption was proposed as a standard adsorbate for microporous material, especially for
pore size smaller than 1 nm, since N2 molecule has diffusion problems into micropores smaller
than 0.45 nm at low temperature (-196 ˚C). CO2 adsorption conducted at higher temperature (0
˚C) and higher saturation pressure (34.5 atm), which resolve the diffusion issues into narrow
micropores. Therefore, the use of CO2 adsorbate was suggested as a complement for N2
adsorption in several publications [141-143].
In this study, CO2 adsorption was performed when N2 adsorption did not complete after 24 h,
which is an indication of the existence of ultra-micropores (pore size < 0.7 nm). The pore volume
and surface area determined through CO2 adsorption were collected by TriStar II Plus apparatus.
The same degassing procedures (in 3.4.1) were followed for all CO2 adsorption experiments. Ice
water was required to keep the adsorption temperature at 0 ˚C for the entire CO2 adsorption
process (around 9 h depending on the sample properties). The ice water was prepared by adding
ice cubes into in the dewar with DI water. An adequate amount of ice was needed, but the liquid
phase must remain in the dewar. The mobile mixture lowered the risk to break the glass sample
holders. CO2 adsorption was suggested to collected data up to P/P0 at 0.3 with NLDFT model
[140]. The saturation pressure at 0 ˚C for CO2 is 26400 torr (equivalent to ~35 atm) [144]. It is
noted that the relative pressure (p/p0) should be carefully chosen when analyzing with CO2
adsorption (See Appendix A).
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3.4.3. Scanning Electron Microscopy (SEM)
For scanning electron microscopy (SEM) analysis, samples were placed on a holder using carbon
tape and then analyzed with an SEM (FEI Quanta 250 field emission SEM, Hillsboro, Oregon,
USA). In order to obtain sample cross-sections, the samples were embedded in an epoxy resin
(Devcon). A glass Pasteur pipette was filled with the mixture and then allowed to solidify
overnight. The pipette was dipped into liquid N2 for 10 s, and then broken to obtain the sample
cross-section after removing the glass pipette. The sample was mounted on carbon tape in a
sample holder for SEM analysis. The Energy Dispersive X-ray (EDX) mapping in SEM analysis
is a surface technique to measure the elemental composition of samples. However, the sulfur
composition measured by the benchtop SEM may not accurate since the samples were not under
vacuum (See Appendix A). An elemental analyzer is suggested to be utilized for sulfur
composition in the future research.
3.4.3 Fourier-transform infrared spectroscopy (FTIR)
The surface groups were determined by Fourier transform infrared spectroscopy (FTIR, Nicolet
iS 50 Spectrometer, Thermo Fisher Scientific, Madison, US). A DRIFTS accessory was selected
for the measurement. The sample was mixed with KBr (0.3-0.5 wt%), and then placed in a
cylindrical holder for the analysis. Spectra were collected between 400 and 4000 cm-1 with a
resolution of 4 cm-1, and 120 scans.
3.4.4 Raman spectroscopy
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Raman spectroscopy measures the rotational and vibrational modes of molecules. Once an ultra
violet-visible beam is excited and interact with the tested molecule, a shift in energy provides
information on chemical structure, crystallinity and molecular interactions. In this study, Raman
analysis was used to gather structural information for petcoke and AC samples. Raman
spectroscopy was conducted by a Research Associate Nael Yasri in Dr. Robert’s group at
University of Calgary, using WITec Raman Microscope (alpha300 RA, WITec, Germany)
3.4.5 X-ray Diffraction (XRD)
XRD was also used to gather information of the crystallographic structure of petcoke and AC
samples. A Multiflex X-ray diffractomer (Figaku, Woodland, Texas, US) in Department of
Geoscience at the University of Calgary was operated by previous PhD student Luis Virla. The
experiment was performed by using Cu/Kα radiation, with 24kV, 20 mA and a scan rate of 0.5
˚/min from 5-90˚ of 2θ.
3.6 Error source
Variability in the data could be from variations in the carbon precursor, sample preparation and
sample characterization. Since the particle size of petcoke has a significant influence on pore
volume, surface area and AC yield [10], the petcoke was ground with a mortar and pestle
manually. By using standard sieves of No. 100 and No. 50 (W.S. Tyler, ON, Canada), the
petcoke particles were shaken and sieved 2 min to obtain the sizes between 150-300 µm. At least
three times of alternative sieving particles between 150 µm and 300 µm were applied to obtain
final petcoke particles for the experiments. In order to get an accurate N2 adsorption result, the
sample was degassed for 5 h at 150 ˚C after analysis was completed. Since the sample mass is
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very sensitive to the N2 adsorption, fingerprints should be avoided by using cotton gloves while
transferring the sample to the balance. At least two times of mass measurements were recorded
until the variation was within + 0.2 mg. Since precision of scale for the analytical balance
(MS304S, Mettler Toledo, QC, Canada) in our lab is + 0.2 mg.
3.7 Repeatability of the experiments
To determine the repeatability of AC preparation, four AC samples were prepared with petcoke
at activation temperature of 800 ˚C for 30 min with a molar ratio of KOH to carbon of 0.43.
Since the AC properties in Table 3-3 (total pore volume, DFT surface area and yield) did not
have any outliers, it is appropriate to use t-test to find out if the difference between the means for
each AC property is significant. Most researchers select 95% confidence interval [145], t-
distribution critical value at α/2 of 0.025 and degree of freedom of 3 was 3.182 [145]. The
calculated confidence interval (based on Eq 3-4) for total pore volume, DFT surface area and
yield are 1.04 + 0.08 cm3/g, 1590 + 80 m2/g, 47 + 1.5%. Thus, there isn’t significance difference
between the sample means within 95% confidence interval.
The average was calculated by Eq. 3-2
�̅� =∑ 𝑥𝑖
𝑛𝑖=1
𝑛 Equation 3-2
The standard deviation was calculated by Eq. 3-3
s = √1
𝑛−1∑ (𝑥𝑖 − �̅�)2𝑛
𝑖=1 Equation 3-3
The confidence interval was calculated by Eq 3-4
�̅� ± 𝑡𝑛−1,𝛼/2𝑠
√𝑛 Equation 3-4
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In the equations above, n is sample size, s is sample standard deviation, �̅� is sample mean, 𝛼 is
the significance level (calculated by 95% confidence interval)
Raw petcoke is not a homogenous material. AC produced from petcoke may not be homogenous
as well. In order to check if AC properties analyzed by N2 adsorption is stable and uniform, two
samples of a highly porous AC (0.43KOH_800_240) were selected and then analyzed in two
different ports of TriStar Plus II in one experiment. Two sample properties were listed in Table
3-3. The results were the same within instrumental error. If compared to the properties of same
AC sample analyzed by N2 adsorption two months earlier, the similar total pore volumes were
obtained but surface area decreased ~330 m2/g. The decrease of surface area may be related to
the adsorption of small molecules in the air, such as H2O and CO2 when AC samples were stored
not under vacuum. Therefore, if AC was not newly-prepared samples, they were suggested to be
degassed at 250 ˚C for at least 3 h under vacuum before the physisorption tests and the future
applications for these materials [10]. The same sample was degassed at 250 ˚C overnight after 5
months preparation, both pore volumes (0.94 cm3/g) and surface areas (1800 m2/g) were the
similar as newly prepared AC (See Table 3-3).
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Table 3-3. The physical properties selected AC.
Date Sample Pore volume
(cm3/g)
DFT surface area
(m2/g)
Yield
(%)
2016.08.03 0.43KOH/St_800_30 1.07 1550 48
2016.08.24 0.43KOH/St_800_30 1.09 1600 46
2017.03.27 0.43KOH/St_800_30 1.04 1650 47
2018.09.11 0.43KOH/St_800_30 0.98 1540 46
Average 1.04 1590 47
Standard derivation 0.048 50 0.957
2019.06.08 0.43KOH_800_240 0.932 1440 71
2019.06.08 0.43KOH_800_240 0.929 1520 71
Average 0.931 1480 71
Standard derivation 0.002 54 0
2019.04.02 0.43KOH_800_240 0.944 1810 71
2019.09.27 0.43KOH_800_240 0.938 1800 71
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Chapter Four: THE IMPACT OF THE AMOUNT OF CHEMICAL AGENT
An investigation of reducing chemical amount without compromising pore volume will be
focused in the next section (4.1). Steam was added as another source of oxygen to a chemical
activation system, which is called chemical steam co-activation. Pore development during
chemical activation, with and without steam, will be discussed (4.2). This chapter has been
adapted with the permission from Jingfeng Wu, Vicente Montes, Luis D. Virla, Josephine M.
Hill (2018) Impact of amount of chemical agent and addition of steam for activation of
petroleum coke with KOH and NaOH, Fuel Processing Technology [13]. Copyright (2018), with
permission from Elsevier (See Appendix E). My role in this paper was designing and performing
the experiments, analyzing data, and writing the manuscript.
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4.1 Reduction of chemical ratios by adding steam as another source of oxygen
The total pore volumes and yields as a function of the amount of chemical agent are shown in
Figures. 4-1 and 4-2 for KOH and NaOH, respectively. Chemical activation generally produced
microporosity (>70%, the specific pore size distributions will be discussed later). In the absence
of steam, the pore volumes increased (Figures. 4-1a and 4-2a) while the yields decreased - from
83% to 67% (Figures. 4-1b and 4-2b) - as the amount of chemical agent increased. Specifically,
the pore volume of AC activated with KOH increased from 0.16 cm3/g to 1.1 cm3/g as the molar
ratio increased from 0.11 to 0.64 (Figure 4-1a). There was a linear increase in total pore volume
with the amount of KOH (e.g., doubling the amount of KOH, approximately doubled the total
pore volume) and this linear relationship appears to pass through the origin (Figure 4-1a).
Because the porosity developed was mainly microporous, the trends for surface area are
essentially the same as those for total pore volume. The surface areas of these samples varied
from 380-2000 m2/g (Table 4-1 provides detailed information on the samples). Many researchers
have used the BET model to determine surface area but it is known that the BET model
overestimates the surface area for microporous materials [146]. For comparison to the literature,
however, the surface areas in terms of the BET model varied from 380-2800 m2/g. The
properties (pore volumes and yields) were the same whether the activation time was 0 or 30 min
(Figure 4-3), suggesting that all of the oxygen, hydrogen and intercalating species in the
chemical agents reacted with the carbon before reaching the final activation temperature.
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.
Figure 4-1 Impact of amount of KOH on (a)
total pore volume per gram of AC, (b) yield
and (c) pore volume per gram of petcoke
during the activation of petcoke (800 ˚C, 30
min) with KOH () and KOH/St ()
activation
Figure 4-2 Impact of amount of NaOH on
(a) total pore volume per gram of AC, (b)
yield and (c) pore volume per gram of
petcoke during the activation of petcoke
(800 ˚C, 30 min) with NaOH () and
NaOH/St () activation
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Figure 4-3. Total pore volume of KOH activated carbon at 800˚C for 0 min () and 30 min ()
with KOH to carbon ratios of 0.11, 0.21, 0.43, 0.64 without steam addition. The surface areas
and pore size distributions of the samples activated for 30 min are essentially the same as those
of the corresponding samples activated for 0 min as given in Table 4-1.
It is also noted that similar surface areas were obtained with raw petcoke and petcoke treated at
800 ˚C for 30 min in N2 atmosphere (Table 4-1). The result implied that no thermal
decomposition occurred on petcoke when petcoke was heated up to 800 ˚C under N2 atmosphere.
The SEM images of the petcoke cross-section verified that without chemical agents involved in
the activation, petcoke particles kept similar morphology (See Figure 4-4).
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Figure 4-4. Cross-section of (a) raw petcoke and (b) petcoke after heated up to 800 ˚C in N2
atmosphere for 30 min.
When steam was added for 30 min at 800 ˚C, higher pore volumes were obtained for ratios of
0.21 and higher (Figure 4-1a). In fact with steam, essentially same total pore volumes could be
obtained with significantly less chemical agent. That is, a total pore volume of 1.1 cm3/g was
obtained with a molar ratio of 0.64 (3:1 mass ratio) without steam or 0.43 (2:1 mass ratio) with
steam. Similarly, a total pore volume of 0.67 cm3/g was obtained with a molar ratio of 0.43 (2:1
mass ratio) without steam or 0.21 (1:1 mass ratio) with steam. The yield was much lower with
steam and relatively constant (~45%) regardless of the amount of chemical agent used (Figure 4-
1b). For comparison, Wu et al. reported yields of 25 – 30% for co-activation of petcoke at 800
˚C for 25 min with a KOH to carbon molar ratio of 0.43 (no pore volume was reported) [9]. The
lower yields during co-activation are consistent with catalytic gasification of the petcoke. Note,
the washing step removed all potassium (and sodium) added and so the yield refers to the amount
of carbon (and less than 1 wt% ash) remaining in the product. Normalizing the pore volume by
the initial amount of petcoke, rather than by the amount of AC produced showed that the
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increased pore volume obtained in co-activation was offset by the lower yields (Figure 4-1c).
Other analyses of the samples showed no differences in the surface functional groups (diffuse
reflectance infrared Fourier transform spectroscopy, DRIFTS) or the combustion behaviour
(thermogravimetric analysis, TGA) whether the AC was produced with or without steam.
The results for activation with NaOH are shown in Figure 4-2. Similar to activation with KOH,
the total pore volume achieved increased with increasing amount of chemical agent (Fig. 4-2a).
The total pore volumes, however, were lower than that achieved with KOH, ranging from 0.01
cm3/g to 0.73 cm3/g over the range of ratios studied, and the change in yield was greater – from
91% to 54% (Fig. 4-2b). The relationship between the total pore volume and amount of chemical
agent was linear but only between ratios of 0.35 and 1.06 (i.e., starting at a mass ratio of one),
similar to that reported for the activation of non-porous anthracite [58] and plum kernels [91]. In
contrast to co-activation with KOH, the addition of steam did not improve co-activation with
NaOH (Figure 4-2). Both lower pore volumes and yields were obtained, suggesting that the
particles were consumed rather than activated to develop porosity.
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Table 4-1. Physical properties of activated carbon produced from the activation of petcoke at 800
˚C with KOH or NaOH.
Sample Steam Time
(min)
Total pore
volume(cm3/g)
DFT
SA(m2/g)
BET
SA(m2/g)
Yield
(%)
Petcoke* 0 < 0.01 < 0.1 < 0.1 98
0.11KOH 0 0.16 380 377 83
0.11KOH/St 15 0.15 259 321 50
0.11KOH/St 30 0.15 306 389 46
0.11KOH/St 45 0.1 149 180 7
0.21KOH 0 0.37 936 885 80
0.21KOH/St 15 0.64 1027 1332 51
0.21KOH/St 30 0.57 852 1115 44
0.21KOH/St 45 0.71 991 1296 19
0.21KOH/St 60 0.69 961 1212 7
0.43KOH 0 0.67 1867 1825 77
0.43KOH/St 15 0.99 1636 2221 61
0.43KOH/St 30 1.08 1576 2197 47
0.43KOH/St 45 1.17 1605 2289 38
0.43KOH/St 60 1.29 1739 2392 32
0.43KOH/St 90 1.23 1608 2183 14
0.43KOH/St** 30 0.6 951 1227 24
0.64KOH 0 1.1 1872 2411 67
0.64KOH/St 15 1.37 1998 2942 56
0.64KOH/St 30 1.4 1987 2880 48
0.64KOH/St 60 1.51 1945 2745 34
0.64KOH/St 120 1.53 1816 2464 6.5
0.18NaOH 0 <0.01 - - 91
0.18NaOH/St 30 0.07 33 37 58
0.35NaOH 0 0.27 396 390 74
0.35NaOH/St 30 0.05 28 27 56
0.71NaOH 0 0.6 1034 1064 59
0.71NaOH/St 30 0.45 560 614 52
1.06NaOH 0 0.73 1277 1351 54
1.06NaOH/St 30 0.69 907 995 36
1.06NaOH/St** 30 0.22 166 169 22
*Petcoke was activated in N2 at 800 ˚C for 30 min.
**For these samples, steam was introduced at 120 ˚C and then the sample was heated to 800 ˚C for extra
30 min in humidified N2.
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These results may be explained by 1) the difference in the initial porosity (pore volume and
possibly pore shape) created by KOH and NaOH before steam was introduced; larger porosity
facilitates diffusion and gas/solid interactions, 2) the relative activities for gasification; K is a
more active catalyst than Na [147-149], and 3) the relative diffusivities of the active species.
Analysis of the cross-sections of particles after activation but before washing was attempted but
was difficult. In Figure. 4-5a, it is clear that there was not a clean break through the particles.
Based on the potassium and sulfur mapping shown in Figure. 4-5b, a layer rich in potassium was
evident around the particles. The smooth areas in Figure. 4-5a correspond to the resin while the
rough areas correspond to areas in which the carbon particles have separated from this layer. The
surface areas of all AC samples (with KOH and NaOH) before washing were less than 5 m2/g
compared to >380 m2/g after washing, which is consistent with the pore volume being occupied
by the chemical agent and/or species formed during activation. Sulfur is visible throughout the
particles.
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Figure 4-5. SEM analysis of 0.43KOH/St (a) before washing (b) with elemental mapping. The
distribution of sulfur (orange) and potassium (green) showed in mapping with the grey areas
being the remaining elements (mainly carbon with oxygen).
Once chemical layers were washed away, cracks previously occupied by chemical agents
remained on AC particles. The SEM images in Figure 4-6 showed more cracks on AC activated
with higher KOH (Figure 4-6g and h). There was an increasing number of cracks formed by co-
activation of KOH and steam compared to KOH activation. Especially by co-activation with
highest chemical ratio of 0.64, intense reactions resulted in expanding of the particles and
burning them into several tiny particles, which corresponded to the peak at around 50 µm on
particle size distribution (Figure 4-6p). Except for 0.64KOH/St, other particles kept similar
particle size distribution as petcoke. The difference could be explained by 1) a higher number of
cracks filled with K species before steam addition provided less diffusion limitation for steam; 2)
K species changed to more active species when steam was added. For instance, the intercalated-
like compounds were reported as an essential intermediate to increase surface area during steam
gasification of bitumen coke [150] and AC [151].
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A similar experiment was completed with NaOH activation and NaOH steam co-activation.
Cracks were observed on SEM images of AC with higher amount of NaOH (Figure 4-7 g, h). In
contrast to KOH steam activation, co-activated NaOH and steam did not increase the amount of
cracks when NaOH to petcoke molar ratio of 0.64 was applied. Less intense reactions may be
involved in the co-activation process with NaOH and steam than KOH and steam. Less activity
of Na compared to KOH may also be reflected by similar particle size distributions among AC
prepared from all experiment conditions shown in Figure 4-7.
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Figure 4-6. Scanning electron micrographs and particle size distribution for raw petcoke and AC
activated with KOH with or without steam. KOH:petcoke molar ratios were 0.21/0.43/0.64.
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Figure 4-7. Scanning electron micrographs and particle size distribution for petcoke and AC
activated with NaOH with or without steam. NaOH:petcoke molar ratios were 0.34/0.68/1.02.
To better understand the pore structure developed at different conditions, the isotherms and
corresponding pore size distributions were examined. The shapes of the isotherms depended on
the specific chemical agent used but not significantly on the amount of that agent. Representative
isotherms and pore size distributions are shown in Figures 4-8 and 4-9 for samples 0.43KOH(/St)
and 1.06NaOH(/St), respectively. With chemical activation, these samples had similar total pore
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volumes - 0.67 cm3/g (0.43KOH) and 0.73 cm3/g (1.06NaOH). According to the IUPAC
classifications [152], the isotherm of sample 0.43KOH (Figure 4-8a) is Type I, typical of
microporous materials. The pores filled with N2 at a low relative pressure (~0.01) consistent with
a narrow micropore range (0.5-1.5 nm) as shown in Figure 4-8b. The addition of steam increased
the total pore volume by over 60% (0.67-1.08 cm3/g) (Figure 4-8a) through the creation of larger
pores (1.5-3 nm, Figure 4-8b). Steam would be able to diffuse into the micropores created by
potassium and enlarge the pores. Steam by itself did not activate petcoke under these conditions
(800 ˚C, 30 min)[96].
Although activation with NaOH produced less pore volume (Figure 4-9a), the pores produced
were larger than with KOH – 24% of the pore volume was related to the pores larger than 4 nm
(Figure 4-9b). Hysteresis loops are clearly visible in the isotherms, which resemble Type IV
isotherms typical of micro-mesoporous materials. The pore size distributions were similar for the
samples activated with and without steam, and contained peaks up to ~30 nm from 5 to 30 nm
(see inset in Figure 4-9b). Similar trend was followed with other NaOH to petcoke molar ratios
(See Appendix B).
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Figure 4-8. (a) N2 isotherms and (b) pore size distributions from 2D-NLDFT-HS for petcoke
activated with KOH with/without steam at 800 ˚C for 30 min. ( 0.43KOH, 0.43KOH/St).
Figure 4-9. (a) N2 isotherms and (b) pore size distributions from 2D-NLDFT-HS for petcoke
activated with NaOH and/or steam at 800 ˚C for 30 min. ( 1.06NaOH, 1.06NaOH/St).
4.2 Impact of steam exposure time
As shown in Figure 4-1, the addition of steam increased the pore volume obtained with KOH,
but decreased the yield. Surprisingly, the yield was constant (~45%) regardless of the amount of
KOH. Further experiments were done varying the activation time (0-120 min) in the presence of
steam and the results are shown in Figure 4-10 (total pore volumes and yields) and Figure 4-11
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(pore size distributions) below. The yield decreased with increasing steam exposure time,
essentially linearly, but with different slopes depending on the KOH:carbon ratio (slopes of -
0.039%/min, -0.030%/min, -0.017%/min, -0.012%/min for KOH:carbon ratios of 0.11, 0.21,
0.43 and 0.64, respectively). Thus, the amount of carbon consumed by the addition of steam
decreased with increasing amount of chemical agent, consistent with a thicker layer forming over
the petcoke particles that reduced the interaction of steam with the surface. Based on the widened
pores (Figures. 4-8 and 4-11 below), however, the presence of steam did result in a modification
of the pores.
Further experiments with varying amounts of sample (between 4.5 g and 7.5 g total) but the same
steam flow rate and same chemical to carbon mass ratio produced AC with the same properties
(yield, pore volume, and pore size). The SEM analysis (not shown) showed that the particles
were relatively uniform for specific activation conditions. These results indicate that the amount
of steam was not the limiting factor for most conditions used (with the lowest chemical amounts
where the most carbon was consumed, the amount of steam may have limited the reaction, see
Figure 4-12) and that the reaction atmosphere reached particles throughout the bed. Once the
steam reached the particles, however, there was likely diffusion resistance through the chemical
layer to the carbon. The least amount of porosity was presented on petcoke activated with the
lowest chemical ratio (0.11KOH). The solid carbon restricted the steam diffusion, therefore,
higher amount of steam was required to consume one mole of carbon when introducing steam at
800 ˚C (Figure 4-12).
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Figure 4-10. Impact of steam activation time on total pore volume per gram of AC() and yield
(×) at 800 ˚C with different amounts of KOH. a. 0.11KOH/St, b. 0.21KOH/St, c. 0.43KOH/St, d.
0.64KOH/St.
The pore volume decreased slightly with steam exposure time for the KOH:carbon ratio of 0.11
(Figure 4-10a). For the ratios of 0.21 and 0.64, there was a significant increase within the first 15
min of steam exposure and then the pore volumes remained relatively constant (Figure 4-10b and
d). The results were different for the ratio of 0.43 with the pore volume continuing to increase up
to 60 min of steam exposure (Figure 4-10c). When the total pore volumes are normalized by the
initial amount of petcoke (rather than the amount of AC produced), the maximum total pore
volumes occurred within 15 min of steam exposure for the chemical ratios of 0.43 or 0.64 (see
Figure 4-13).
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Although the total pore volumes plateaued, the pore sizes continued to increase with steam
exposure (Figure 4-11). Without steam (0 min exposure), there was one peak at ~0.7 nm for the
ratios of 0.11 and 0.21 (Figure 4-11a and b). For a ratio of 0.43 (Figure 4-11c), a shoulder has
developed just above a pore width of 1 nm, and for the highest ratio (Figure 4-11d), there is a
second peak at ~1.7 nm. When steam is added, the previous peak at 0.7 nm has shifted to ~1 nm
and there is a second peak at 1.7 nm in the pore size distributions for all samples (Figure 4-11a-
d). The highest intensities of the peaks at 1 nm were reached after 15 min of steam exposure,
after which the relative number of wider pores (i.e., pores > 2 nm) increased with a
corresponding decrease in the number of smaller pores. A quantitative analysis of the change in
pore sizes below 1.4 nm and those between 1.4-5 nm is given in Figure 4-14.
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Figure 4-11. The influence of steam activation time on pore size distributions of petcoke
activated at 800 ˚C with different KOH to petcoke ratios. a. 0.11KOH/St, b. 0.21KOH/St, c.
0.43KOH/St, and d. 0.64KOH/St.
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Figure 4-12. Impact of steam activation time on molar ratio H2O:Carbon consumed when 2.5 g
petcoke activated in the presence of steam at 800 ˚C with different amounts of KOH.
0.11KOH/St (), 0.21KOH/St (), 0.43KOH/St (), 0.64KOH/St ().
Figure 4-13. Impact of steam activation time pore volume created by 2.5 g petcoke activated in
the presence of steam at 800 ˚C with different amounts of KOH. 0.11KOH/St (), 0.21KOH/St
(), 0.43KOH/St (), 0.64KOH/St ().
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Figure 4-14. Impact of steam activation time at 800 ˚C on the change of pore volume from pores
smaller than 1.4 nm() versus pores between 1.4-5 nm(). a. 0.11KOH/St, b. 0.21KOH/St, c.
0.43KOH/St, d. 0.64KOH/St.
Considering the mesopore instead of total pore volume, the mesopore volume in Figure 4-15 of
AC prepared with same chemical ratios increased linearly with steam time. The higher chemical
ratio was applied, the higher increment of mesopore volume was observed.
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Figure 4-15. Impact of steam time on mesopore volume change during KOH/St activation at 800
˚C with various chemical ratios ( 0.64KOH/St, 0.43KOH/St, 0.21 KOH/St, 0.11
KOH/St).
4.3 Discussion of pore development
There are multiple reactions and phase changes occurring during the activation of carbon with a
chemical agent and steam. During heating, the chemical agent melts - the melting points of KOH
and NaOH are 360 ˚C and 318 ˚C, respectively - and/or decomposes, which releases the oxygen
to react with the carbon and/or the metals to intercalate in the structure, both of which could
create the porosity evident at the activation temperature (800 ˚C, Figures. 4-1, 4-2 and Table 4-
1). The carbon-sulfur bonds in the petcoke break first [27, 30], releasing the sulfur into the
molten phase around the particles (Figure. 4-5). Previous studies on the activation of low grade
petcoke without steam (N2 atmosphere) with chemical agent to carbon molar ratios between 0.43
and 0.64 examined the cooled samples by XPS, XRD and XANES. The species present after
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activation with KOH included K2S, K2SO4, and K2O [27, 78]. Analysis of the tubing at the
exhaust after activation at 800 ˚C revealed potassium deposition. The species present after
activation with NaOH were Na2CO3 as detected by XRD and Na2S was assumed to be formed as
sulfur had been removed [78]. None of the analysis was done at the activation conditions. The
melting points of K2CO3 and Na2CO3 are 891 ˚C and 851 ˚C, respectively, but mixtures of
carbonates and hydroxides have melting points between those of the pure substances [153].
A previous study in our group [154] on gasification examined ash free coal with K2CO3
(potassium to carbon molar ratios < 0.14) using in-situ XRD at room temperature and 700 ˚C.
The peaks associated with K2CO3 disappeared at 700 ˚C and reappeared when the sample was
cooled. XRD only detects crystalline phases and so this analysis is also not direct evidence that
carbonate species are not present at activation conditions.
Metal hydroxides can react with carbon as shown in Equations 4-1 and 4-2 [5, 78, 80]:
6𝑀𝑂𝐻 + 2𝐶 → 2𝑀 + 3𝐻2 + 2𝑀2𝐶𝑂3 Equation 4-1
𝑀𝑂𝐻 + 𝐶 → 𝑀 + 𝐶𝑂 +1
2𝐻2 Equation 4-2
where M refers to either potassium (K) or sodium (Na). Based on these reactions, the theoretical
yields were calculated, and as shown in Figure 4-16, the actual yields were between the limits
provided by the two reactions. Carbonates react with carbon by first reducing to the metal [155].
This metal may react through a redox cycle to consume carbon and produce CO or intercalate
into the carbon matrix. Thus, the porosity developed during heating may be from Reaction (4-1),
Reaction (4-2), gasification, and/or intercalation. The literature is in agreement that potassium is
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a more active gasification catalyst [147-149] as well as better able to intercalate than sodium [75,
88].
Figure 4-16. Yields obtained for the activation of petcoke at 800 ˚C for 30 min with different
amounts of KOH () or NaOH(). The lines are the theoretical yields based on Equation 4-1
(solid line) and Equation 4-2 (dashed line).
When steam is introduced, further porosity is developed by steam gasification, conversion of the
chemical agent to a more active form either by reactions with steam (e.g., H2O + K2CO3 ⇌ K2O
+3CO + H2 [155]) and/or a change in the environment around the particles shifting the
equilibrium composition. We were unable to find any data on the diffusion of steam through
liquid hydroxide. If the chemical layer around the particles is very thin or incomplete (i.e., for the
lowest chemical ratios), the steam could more easily gasify the particles (Figures 4-10a and b).
The higher the ratio of chemical agent to carbon, the thicker the layer and the lower the
gasification rate of the particle that reduces the yield with little to no increases in the pore
volume. With the exception of the 0.43 molar ratio (Figure 4-10c), the pore volume did not
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increase significantly after 15 min of steam exposure. At this point, it is not clear why there were
differences with a molar ratio of 0.43.
Figure 4-17. The relationship between steam time and the pore volume change in the presence of
steam at 800 ˚C with different amounts of KOH. The change of total pore volume equals to (pore
volume at steam time X) – (pore volume at steam time 0). 0.11KOH/St (), 0.21KOH/St (),
0.43KOH/St (), 0.64KOH/St ().
The pore sizes increased with steam exposure after KOH activation (Figures 4-8 and 4-11) with a
corresponding decrease in yield suggesting that carbon from the pore walls was consumed.
Introducing steam at a lower temperature of 120 ˚C (Table 4-1) resulted in both a lower yield
(24%) and a lower total pore volume (0.6 cm3/g) than introducing steam at 800 ˚C (48% yield
and 1.1 cm3/g total pore volume), suggesting that porosity and/or the appropriate species/phase
of chemical was not established before steam introduction. A similar trend was observed when
introducing steam at 120 ˚C with NaOH (Table 4-1).
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After NaOH activation, steam exposure decreased the pore volume (Figure 4-2) without
changing the pore size distribution (Figure 4-9) suggesting that the particles were gasified
(consistent with particle size distributions seen in SEM [96]). As sodium is a less effective
activation agent, the porosity likely extended less far into the particles, and exposure to steam
would remove relatively more of the developed pores than when potassium was used. At this
point, however, we do not have direct evidence for the actual extent of porosity development
within the particles. In a previous study [10], petcoke particles less than 45 µm developed
significantly more microporosity than particles of 300-600 µm size activated with CO2 at 900 ˚C,
suggesting the porosity was developed following a shrinking core model. Further investigations
are underway to better understand the pore development within the particles including
interconnectivity of pores and distribution of pores throughout the particles.
In summary, there is a not simple relationship between the amount of chemical agent used and
the properties of the resulting AC. As steam is not effective at activating non-porous materials
[96], activation with a chemical agent is required. If introduced together, significant gasification
of the carbon occurs. Thus, introducing the chemical agent first is a better approach. Before
steam addition, the amount of porosity developed is limited by the amount of oxygen in the
chemical agent (Figure 4-16) and the mobility of the chemical agent - potassium is more mobile
than sodium and hence develops more porosity (Figure 4-1) and also pores of different sizes and
shapes (compare Figure 4-8 and 4-9). When steam is introduced after KOH activation, the pores
are widened (Figure 4-8 and 4-11), but the particles are also consumed (Figure 4-1b and 4-10). A
potassium rich layer around the particle (Figure 4-5) slows this consumption. An in situ
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technique is required to determine the thickness at activation conditions, as well as the particle
size distribution in order to calculate the appropriate chemical agent to petcoke ratio.
4.4 Economic estimation of petcoke activation
As summarized in Figure 4-18, similar pore volume (1.1 cm3/g) were achieved with a lower
chemical amount by co-activation (0.43KOH/St) and a higher chemical amount by KOH
activation (0.64KOH). However, co-activated sample has a slightly smaller yield (47%) than the
chemical activated sample (67%). With the consideration of lower yield by co-activation, the
increased pore volume of co-activated sample was offset by the lower yield. Regarding to
porosity, there is no need to consume additional energy by injection of steam for 30 min (5 cm3
steam). However, the extra cost to generate steam (3 g steam) should be compared with the
saving from less chemical agents (2.5 g KOH). Steam cost is difficult to estimate because there
are multiple ways to generate steam. All parameters in the entire process (e.g. condensate, power
generator, cooling system) need to be considered carefully [156]. The net low-pressure steam
cost generated from conventional methods range from 1 USD/Klb to 3 USD/Klb [156]. So the
cost of extra 30 min steam is from 1.36 USD to 4.08 USD. 1 kg KOH (>84%) is 45.8 USD,
therefore, using 2.5 g less KOH saves 0.11 USD. The cost of steam is over 10 to 40 times of the
savings from reduction of the chemical amounts. Thus, co-activation of steam and KOH require
higher expense in the entire process.
However, comparing between the same amounts of chemical agents in Figure 4-13, the
maximum pore volume was developed by introducing steam for only 15 min with KOH to
petcoke ratios of 0.43 or 0.64. In both case, an additional 0.1 cm3/g pore volume per gram of
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petcoke were formed when introducing 15 min steam. The cost of 15 min steam ranged from
0.68 USD to 2.04 USD. However, the value of 0.1 cm3/g pore volume increasing is heavily
dependant on the sale price of the final AC. As petcoke-derived AC has not been ultilized as a
commercial AC product yet, it is not easy to evaluate the value of 0.1 cm3/g pore volume
increasing, and then compare with the expense of steam generation.
Figure 4-18. Schematic graph for KOH activation and co-activation of petcoke.
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Chapter Five: PORE DEVELOPMENT DURING CHEMICAL ACTIVATION
The development of pores in petcoke is explained in more depth in this chapter. The impact of
mixing methods before activation was investigated at the beginning. The diffusivity of different
chemical agents (KOH/NaOH) was compared with cross-sectional areas of non-washed AC, in
order to understand the pore development at a high activation temperature of 800 ˚C. For lower
energy requirement and increased yield, activation was also performed at lower activation
temperatures with various holding times. A calculated bulk density of petcoke was compared
with the measured value to study the pore structure of raw petcoke. Finally, a pore development
mechanism was proposed.
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5.1 Experimental methods – Cross-sectioning
A similar approach as that described in Chapter 3 was applied for non-washed AC samples
initially but the approach was not successful. The particles did not break and so cross-sections
were not exposed. Therefore, an extra polishing step was required. The polishing used an
automatic polishing machine (LaboSystem, Struers Ltd, Ontario, Canada), operated by a
technician in the Univeristy of Calgary’s Department of Geoscience. Non-washed AC samples
were fixed into a resin as described in Chapter 3, and then samples were polished with the
machine for ~ 5 min.
5.2 Results
5.2.1 Impact of mixing method – ball-milling
As shown in Figure 5-1, the size of the petcoke particles decreased from 50-250 µm to mostly <
10 µm after ball-milling. The peak of particle size distribution of raw petcoke was approximately
150 µm, while the peak decreased to 5 µm after ball-milling. The ball-milled petcoke particles in
Figure 5-1 (b) included not only tiny particles but also a few large particles, compared to raw
petcoke (Figure 5-1 (c)). Thus, ball-mill technique reduced the particle size for the majority of
the petcoke particles.
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Figure 5-1 Particle size change before and after ball-milling petcoke. (a) particle size of ball-
milled petcoke for 14 h in a rotary tumbler and raw petcoke. The corresponding SEM images are
displayed in insets (b) and (c), respectively.
5.2.2 Physically mixing and ball-milling with KOH activation
Different mixing methods were used to determine the impact of the initial dispersion of KOH
with petcoke (Figure 5-2). The least dispersed sample contained KOH pellets with petcoke (150-
300 µm), followed by physically mixed KOH and petcoke (150-300 µm), and then ball-milled
KOH and petcoke (0-150 µm) which was ball-milled together for 14 h with zirconia balls (5
mm).
Particle Size (µm)
0 50 100 150 200 250 300
Fre
qu
en
cy (
%)
0
10
20
30
40
50
60
P-Petcoke B-Petcoke
c b a
Raw
petcoke
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The results in Figure 5-2 showed that directly using KOH pellet and petcoke produced similar
pore volume (~0.88 cm2/g) and yield (72%) as physically mixed KOH powder with petcoke, but
less time and energy consumed when placing the pellets directly on the top/bottom of petcoke.
Figure 5-2. The impact of mixing method for KOH and petcoke on pore volume (micropore
volume indicated by black bar, mesopore volume by grey bar) and yield(solid red circle) of AC.
The total height of the bar in the figure represents total pore volume. AC was prepared from
KOH to petcoke molar ratio of 0.43at 800 ˚C for 30 min. The samples from left to right were AC
from KOH pellet, KOH powder by physically mixing (PM) and ball-milling (BM). The digital
photos show the corresponding mixture of KOH with petcoke before activation.
Although AC produced by ball-milling had the highest pore volume (1.3 cm3/g), the yield was
only 53%. The low yield was mainly from the loss of fine particles by explosion during
activation (similar photo as Figure 5-3c). As KOH melted at 360 ˚C, the carbon oxygen reaction
Pellet PM BM
Po
re V
olu
me (
cm
3/g
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Yie
ld (%
)
0
20
40
60
80
100
Page 94
80
transferred from solid-solid reaction to liquid-solid. As petcoke has a smaller density (~1 g/cm3)
than KOH (2.12 g/cm3), petcoke after ball milling became fine particles which floated on the top
layer of melting KOH. The oxidation reaction released gases, such as H2 and CO, resulting in
sample explosion. Therefore, ball-milling is not required to mix petcoke with KOH. Aside from
being time consuming, the separation of the sample for the zirconia balls is difficult, creating a
higher possibility of sample loss.
In order to quantify the KOH amount in the ball-milled mixture (5 g KOH and 2.5 g petcoke),
the sample was titrated with HCl (0.1g of mixture was put in 20 mL of DI water). The KOH
concentrations of the solutions from these samples were 0.028 mol/L, 0.036 mol/L and 0.039
mol/L. Each sample was titrated at least twice to obtain reliable data (the replicates were within
0.1 mL difference at the end point). The variation indicated ball-balling mixing could not result
in a homogenous mixture of KOH and petcoke. These KOH concentrations by titration were
compared with theoretical KOH concentrations when KOH and petcoke were physically mixed
at mass ratios of 1 and 2. The theoretical values were calculated with the same experimental
conditions as the titration solution (0.1 KOH [85%] in 20 mL DI water). The KOH
concentrations by titration were in the range of 0.037-0.05 mol/L (theoretical values in Table 5-
1). Additionally, the corresponding pore volume (Table 5-1) of AC prepared from ball-milled
KOH and petcoke at a mass ratio of 2 was 0.55 cm3/g, which was in the range of pore volumes
by AC from physically mixed KOH and petcoke with mass ratios of 1 and 2. As discussed in
Chapter 4, the pore volumes were increased with higher amount of KOH. Both titration data and
pore volumes indicated the separation process after ball-milling reduced the initial KOH to
petcoke mass ratio from 2 to nearly 1. The powder mixture of KOH and petcoke stuck to the
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zirconia balls after ball-milling because the washing solution to clean zirconia balls was basic
(pH >10). Also there were a great number of KOH pellets remaining with the zirconia balls after
separating through sieving (mesh size of the sieve is No. 35). A better separation method is
required for further investigation of ball-milling mixing method.
Table 5-1 KOH concentration after separating zirconia balls measured by titration.
Samples Ball-milling (KOH:PC = 2) Ball-milling (KOH:PC = 3) Theoretical values
#1 #2 #3 #1 #2 #3 KOH:PC = 1 KOH:PC = 2
KOH amount in
solution (mol/L) 0.028 0.036 0.039 0.042 0.054 0.049 0.037 0.05
AC pore volume
(cm3/g) 0.55 1.33 0.37 0.74
To increase KOH to petcoke ratio after ball-milling, 10 g of mixture (7.5 g KOH and 2.5 g
petcoke, KOH to petcoke mass ratio of 3) was initially put into the milling jar. However only 6 g
of the mixture was collected after removing the zirconia balls. Therefore, the sample loss was
40% after separating out the zirconia balls. The same titration method was used to measure KOH
amount in the ball-milled mixture. The average KOH concentration in 20 mL DI water for three
samples was 0.048 mol/L, which was similar to the theoretical KOH concentration (0.05 mol/L)
when physically mixed KOH to petcoke at a mass ratio of 2. Thus, this set of data was used for a
fair comparison of different mixing methods with a similar KOH amount.
5.2.3 Physically mixing and ball-milling with NaOH activation
In order to discuss the impact of particle size for pore development, petcoke was ground
physically in a mortar, and then sieved for different particle sizes ranging from 20 to 500 µm
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(20-63 µm, 63-150 µm, 150-300 µm, 300-500 µm). Only few particles smaller than 20 µm could
be obtained by physical grinding and sieving. Therefore, particles smaller than 20 µm in this set
of experiment were obtained via ball-milling, but it was noted that the presence of a few large
particles (63-300 µm) after ball-milling (see Figure 5-3b). The sample was not sieved after ball-
milling because of the limited amount of petcoke obtained.
The pore volumes and yields obtained by chemical activation of NaOH and petcoke with a molar
ratio of 0.71 are given in Figure 5-3a. The change of particle sizes of petcoke did not influence
the mesopore volume when particles were smaller than 300 µm. When the particle size exceeded
300 µm, the mesopore volume decreased slightly by 0.03 cm3/g. The change in particle size,
however, had a significant impact on the micropore volume. It reached a maximum value of 0.46
cm3/g at the particle size range of 20-63 µm. The results were identical to the previous findings
by Karimi who activated petcoke with CO2 at 900 ˚C for 2 - 15 h [10]. That is, higher surface
areas (175 - 650 m2/g) were achieved for 20 - 45 µm particles compared to 50 - 125 m2/g for 300
- 600 µm particles.
As seen in the SEM image (Figure 5-3b), ball-milled petcoke had a mixture of fine particles
(particle size < 20 µm) and larger petcoke particles (63 µm < particle size < 300 µm). The pore
volume measured by N2 adsorption was the average pore volume of the mixture, which led to
similar pore volumes for ball-milled AC and physically ground AC (20-63 µm). AC prepared
from ball-milled mixture was shown in Figure 5-3c. It was likely that a portion of fine particles
exploded when heating up. During activation of the ball-milled sample, a certain amount of fine
particles may remain on the wall of the ceramic tube, be deposited down stream, or blow away
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with the N2 flow. So yield for ball-milled AC reduced by ~ 20% compared to physical ground
AC (20-63 µm). The yields for the physically-ground petcoke samples were similar (~ 40%)
regardless of particle size range.
Figure 5-3. The impact of petcoke particle size (0-500 µm) on (a) pore volume(grey bars are
micropores, black bars are mesopores) and yield(solid red circle). (b) the corresponding SEM
images of five different petcoke particle sizes ranging in size from 0 to 500 µm. (c) digital photo
of ball-milled sample after taking out of furnace.
20-63 µm Ball-milled 63-150 µm 150-300 µm 300-500 µm
b
Ball-milled (majority < 20 µm)
mic
rop
ore
s
me
so
po
res
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84
Similar experiments with NaOH and petcoke were also done with various mixing methods. The
results in Figure 5-4 showed that despite better mixing, the pore volumes (0.55 cm2/g) and yields
(56%) were similar for all three mixtures. These results suggest that diffusion of the Na to the
petcoke was not the limiting step in the activation. Compared to results of KOH mixing in Figure
5-2, pore volumes of AC activated by NaOH had larger variations (larger error bars). The
difference may be related to different phase behavior of Na and K species during activation. As
shown in Table 5-2, KOH and NaOH melt at 360 ˚C and 400 ˚C, respectively. They are liquid at
activation temperature of 800˚C. As the carbonates (Na2CO3/K2CO3) were produced, the
activation system become complex which includes multiple chemical species listed in Table 5-2.
According to the phase diagrams for the hydroxide and the carbonate (Na2CO3/NaOH and
K2CO3/KOH) [153], the petcoke is in a liquid mixture for both NaOH and KOH activation if
there is hydroxide and carbonate in the activation system. Other products in NaOH activation,
such as Na2S and Na2O, are solid substances at 800 ˚C. While for KOH activation, K2O and K2S
are likely to be liquid phase. Therefore, more solid species (Na2S, Na2O) were possibly presented
in NaOH activation system, which limited the diffusivity of chemical species during activation.
However, it is noted that K2O and Na2O were not stable during activation, and they are
intermediate for the formation of metallic K or metallic Na reported in the literature [5, 157].
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85
Table 5-2. Physical properties of the pure K and Na species in activation process [158, 159].
Chemical
species
Melting
point (˚C)
Boiling
point (˚C)
Decomposition
temperature
(˚C)
KOH 360 1594 High
temperature
K2S 840 912
K2O 740 300
K2CO3 891 1200
K 63.5 759
NaOH 320 1388 High
temperature
Na2S 1176 920
Na2O 1132 700
Na2CO3 851 >1700
Na 98 883
The surface areas of raw petcoke measured by N2 adsorption were similar before (< 2 m2/g) and
after (< 4 m2/g) ball-milling. A different trend was observed for porous corn-stover as the
feedstock. By controlling the factors during the planetary ball-milling process, the surface area of
biochar from corn-stover feedstock increased from 60 to 194 m2/g [110]. In a different study,
seven commercial AC samples from several types of bituminous coal and biomass were ball-
milled in various conditions, and both surface area and pore volume increased after ball-milling
[160]. The different trend between petcoke and other materials mentioned above are their initial
porosity before ball-milling. As petcoke is a non-porous material while commercial AC in the
aforementioned study had at least 0.35 cm3/g pore volume and 500 m2/g surface area.
Page 100
86
Figure 5-4. The impact of mixing method for NaOH and petcoke on pore volume (micropore
volume indicated by black bar, mesopore volume by grey bar) and yield(solid red circle) of AC
prepared from NaOH to petcoke molar ratio of 0.71at 800 ˚C for 30 min. The samples from left
to right are AC from NaOH pellet, NaOH powder by physically mixing (PM) and ball-milling
(BM) with the molar ratio of 0.71 NaOH to petcoke. The digital photos are the mixtures of
NaOH with petcoke before activation.
Ball-milling reduced the particle size and improved the mixing at the same time. In order to
exclude the factor of the change in particle size, a similar particle size range (0-150 µm in Figure
5-5a) was controlled for physically mixed and ball-milled samples. A mixture of NaOH and
petcoke (150-300 µm) was ball-milled together in rotary tumbler for 14 h with a NaOH to
petcoke molar ratio of 0.71. Particle size of petcoke after ball-milling was measured once NaOH
was washed away with DI water. The particle size distribution in Figure 5-5a (blue dash curve)
showed petcoke particles after ball-milling were in a range of 0-150 µm. Therefore, a similar
particle size range (0-150 µm) was controlled for physically mixed petcoke and NaOH sample.
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87
Thereby, raw petcoke (150-300 µm) was manually ground in a mortar with a pestle for 5 min,
and the ground petcoke was then sieved to particles smaller than 150 µm. Extra NaOH with a
molar ratio of NaOH to petcoke at 0.71 was mixed with the petcoke (0-150 µm) for ~1 min.
Particle size distribution of the petcoke after sieving (black solid curve) in Figure 5-5a confirmed
that a similar particle size range (0-150 µm) was achieved for both ball-milled and physically
mixed samples. However, AC prepared after ball-milling had a lower pore volume (both
micropore and mesopore volume) and yield compared to physically mixed AC in Figure 5-5b.
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88
Particle size (µm)
0 50 100 150 200 250 300
Fre
qu
en
cy (
%)
0
5
10
15
20
25
30
PM_0-150 µm
BM_0-150 µm
a
AC_B_0.68Na AC_P_0.68Na
Po
re v
olu
me
(c
m3
/g)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Yie
ld (%
)
0
20
40
60
80
100b
Figure 5-5. Mixing impact on porosity and yield with the same particle size range (0-150 µm).
(a) particle size distribution of petcoke by physically mixed and ball-milled mixture with NaOH
at a molar ratio (NaOH:petcoke) of 0.71 before activation; (b) micropore volume (dark blue bar
for ball-milled AC, black bar for physically mixed AC), mesopore volume (light blue bar for
ball-milled AC, grey bar for physically mixed AC) and yield (●) of corresponding AC activated
at 800 ˚C for 30 min.
BM_0.71NaOH PM_0.71NaOH
Page 103
89
Previous findings of Raman analysis performed before and after ball-milling treatment indicated
that the mechanical energy during ball-milling introduced defects and created disordered
structure on petcoke [161, 162]. These newly developed defects were easier to adsorb oxygen
atoms and become carbon active site C(O), resulting in an improvement of CO2 gasification rate
of petcoke [161]. However, the pore volume of ball-milled AC was lower than physically mixed
AC in Figure 5-5b. An increasing number of defects after ball-milling did not improve the
reactivity of carbon oxygen reactions for higher pore volumes obtained. Therefore, a possible
explanation for a lower pore volume obtained with ball-milled AC could be less NaOH collected
after separating out zirconia balls from the ball-milled mixture. A similar discussion was covered
with KOH mixing (see Figure 5-2 and Table 5-1). But further investigation with NaOH mixing
need to be carried on.
5.2.4 Physically mixing and ball milling with different chemical ratios
Consistent with earlier findings [96] reported by Virla et al, more mesopores were created by
physically mixing when NaOH was used as the chemical agent compared to KOH. However, the
mechanism of pore development is still unknown when activating petcoke with different mixing
methods and various chemical amount. The following experiment was performed to understand
the impact of the amount of NaOH on AC porosity and yield when using either method of
physically mixing or ball-milling before activation.
The possible reactions between NaOH and petcoke are [5, 8, 81]：
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90
𝑁𝑎𝑂𝐻 + 𝐶 → 𝐶𝑂 + 𝑁𝑎 +1
2𝐻2 Equation 5-1
6𝑁𝑎𝑂𝐻 + 𝐶 → 2𝑁𝑎 + 3𝐻2 + 2𝑁𝑎2𝐶𝑂3 Equation 5-2
4𝑁𝑎𝑂𝐻 + 𝐶𝐻2 → 𝑁𝑎2𝐶𝑂3 + 𝑁𝑎2𝑂 + 3𝐻2 Equation 5-3
𝑁𝑎2𝑂 + 𝐶 → 2𝑁𝑎 + 𝐶𝑂 Equation 5-4
𝑁𝑎2𝐶𝑂3 + 2𝐶 → 2𝑁𝑎 + 3𝐶𝑂 Equation 5-5
The mesopore volumes in Figure 5-6 decreased linearly with sodium to petcoke ratios. However,
the micropore volumes and yields were not linearly related to the ratios. Assuming the
occurrence of an ideal reaction during activation process, one mole of carbon consumes one
mole of sodium hydroxide (Equation 5-1). The theoretical yields should be 7%, 37% and 67%
with decreasing NaOH to petcoke molar ratios. However, the experimental yields were 53%,
57% and 74% respectively. The experimental yields were higher than the theoretical yields for
all of the samples, and the difference between experimental and the corresponding theoretical
yields became smaller (i.e., 46%, 20%, 7%) with decreasing NaOH amount. This indicated
undesired reactions occurred and reacted with NaOH rather than consuming carbon for pore
formation. For instance, one mole of carbon required two mole of NaOH to produce CO2
(Equation 5-2). In this case, less carbon was consumed with a same amount of NaOH as that
used in the ideal reaction. In addition, Na2O and Na2CO3 were considered as intermediates. The
incomplete reactions (from Equation 5-3 to Equation 5-5) reacted with a greater amount of
NaOH, which also wasted NaOH for the purpose of developing porosity. Since the biggest
difference between theoretical and experimental yields was presented at the highest NaOH to
petcoke molar ratio of 1.06, a greater number of undesired reactions were likely involved during
its activation process.
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In order to make full use of the NaOH, ball-milling in a rotary tumbler was applied to mix
chemicals and petcoke more homogenously with various NaOH to petcoke molar ratios (ranging
from 0.35-1.06). However, for all the NaOH to petcoke molar ratios tested, the pore volumes and
yields of ball-milled samples did not change significantly compared to physically mixed
samples. Since the color of mixture became darker after ball-milling, the homogenous mixture of
NaOH and petcoke was achieved by ball-milling NaOH and petcoke together. However, when
petcoke was ball-milled with different amounts of NaOH, the petcoke particle size changed at
the same time.
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92
PM-1.06NaOH
BM-1.02NaOH
PM-0.71NaOH
BM-0.71NaOH
PM-0.35NaOH
BM-0.35NaOH
Po
re V
olu
me
(c
m3
/g)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Yie
ld (%
)
0
20
40
60
80
100
Figure 5-6. The impact of mixing methods and amount of NaOH on pore volumes (represented
by bars) and yields (represented by solid red circles) during NaOH activation. AC prepared with
1.06/0.71/0.35 NaOH to petcoke molar ratios by physically mixing (PM) or ball-milled (BM).
The black bar is micropore volume, and the grey bar is mesopore volume.
After 14 h ball-milling treatment, DI water was used to remove NaOH in the mixture to measure
particle size of petcoke. Summarized in Figure 5-7, after ball-milling with NaOH to petcoke at
molar ratios of 1.06/0.71/0.35, the majority of the particles became smaller. However, the peaks
of particle size distribution at highest (1.06) and lowest (0.35) NaOH to petcoke molar ratios
shifted from 150 µm to 40 µm, while less particle size reduction was observed for NaOH to
petcoke molar ratio of 0.71. The reduction of particle size before and after ball-milling could also
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93
be clearly observed in SEM images by comparing between them (Figure 5-8) and with raw
petcoke (Figure 5-8a). Further activation (Figure 5-8d, f, h) did not change the particle size
significantly, but created more fine particles. In contrast to AC prepared from ball-milled
mixture with the same NaOH to petcoke ratio of 0.71, physically mixed AC had much larger
particles with distinct cracks on those particles.
Figure 5-7. Particle size distribution of raw petcoke and ball-milled samples mixed with NaOH
to petcoke molar ratios of 1.06/0.71/0.35 after washing but before activation.
Particle Size (µm)
0 50 100 150 200 250 300
Fre
qu
en
cy (
%)
0
10
20
30
40
50
60
B-Na
B-2Na
B-3Na
P-Petcoke
Raw petcoke
BM-0.71NaOH
BM-0.35NaOH
BM-1.06NaOH
Page 108
94
Figure 5-8. SEM images of (a) physically mixed petcoke, (c, e, g)ball-milled petcoke with NaOH
to petcoke molar ratios at 0.35/0.71/1.06, and(d, f, h) AC after NaOH activation. (b) AC prepared
by physically mixed with NaOH to petcoke at molar ratio of 0.71.
a. Petcoke
c. BM_0.34NaOH
e. BM_0.71NaOH
g. BM_1.06NaOH h. AC_BM_1.06NaOH
b. AC_PM_0.71NaOH
f. AC_BM_0.71NaOH
d. AC_BM_0.34NaOH
Page 109
95
5.2.5 Measurement of chemical diffused into petcoke cores
In order to study if the chemical agent reached the core of the AC particles, the activation of
large chunk of petcoke was completed by previous postdoc Dr. Montes in our group. The digital
photos in Figure 5-9 (b and d) clearly showed that petcoke activated with KOH and NaOH
formed a grey layer around the petcoke, and less chemicals remained at the bottom of ceramic
boat. In contrast, the K2CO3 powder stayed beneath the petcoke after activation, and there was a
distinct boundary between the chemicals and petcoke. The different phenomena may result from
the formation of a molten phase when increasing temperature during KOH and NaOH activation.
Therefore, petcoke at the top layer adhered with fluid chemicals, but the solid phase in K2CO3
prohibited the interaction with petcoke at the top surface. K2CO3 was recognized as a less
effective chemical agent to create porosity on petcoke compared to KOH or NaOH.
The pore volume of AC core and edge was determined by N2 adsorption. Only big chunks of
petcoke activated by KOH developed porosity on both edge and core. The average pore volume
on the edge was 0.5 cm3/g, which was higher than pore volume at the core (0.1 cm3/g). As
petcoke is a non-porous material, the porosity at the core of petcoke was developed by KOH.
Thus, KOH was able to diffuse and reach the core. However, it was hard for NaOH and K2CO3
to reach the core. The surface area was similar to raw petcoke (< 2 m2/g) both on the edge and at
the core for either NaOH or K2CO3 activation.
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Figure 5-9. Large particles of petcoke mixed with (a) KOH, (c) NaOH, and (e) K2CO3, and
corresponding images (b, d and f) after activation at 800 ˚C for 30 min with a chemical to
petcoke mass ratio of 2.
To understand whether or not the pore formed without carbon consumption during activation, the
difference of the experimental pore volume and the pore volume by carbon oxygen reactions was
calculated in Table 5-3 (KOH) and Table 5-4 (NaOH). By assuming carbon consumption was the
only way to create porosity, the pore volumes through carbon oxygen reactions were calculated
by yield (mass of carbon consumed) according to Equation 5-6. The same bulk density of AC (1
g/cm3 petcoke-derived AC prepared at default activation condition) was used for all samples. The
experimental pore volumes in Table 5-3 were higher than estimated pore volumes for both KOH
activated and co-activated AC prepared from higher molar ratios of 0.43 and 0.64. The larger
difference between estimated and experimental pore volume indicated a greater possibility of
developing porosity without consuming carbon. However, all NaOH activated carbons either
with or without steam addition have a higher estimated pore volume than experiment pore
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volume (Table 5-4). It may be an indication of NaOH developed pores in shrink core model. The
different results of KOH and NaOH suggested 1) NaOH developed pores based on the shrink
core model, which was also indicated by the data in Figure 4-2 and Figure 4-9; 2) K species were
more active than Na species when developing porosity without consuming carbon during
activation, which may also be a reason for KOH to reach the core but not for NaOH as the
discussion mentioned above (in Figure 5-9).
𝐸𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑝𝑜𝑟𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 = 1𝑔∗𝑌𝑖𝑒𝑙𝑑
𝜌𝐴𝐶 Equation 5-6
Table 5-3. Estimated and actual pore volumes for petcoke activated with KOH.
Sample
Estimated pore
volume by carbon
oxygen reactions
(cm3/g)
Experimental pore
volume (cm3/g) Difference
0.11KOH 0.43 0.18 -0.25
0.21KOH 0.50 0.37 -0.14
0.43KOH 0.58 0.67 0.09
0.64KOH 0.83 1.10 0.27
0.11KOH/St 1.35 0.15 -1.20
0.21KOH/St 1.40 0.57 -0.83
0.43KOH/St 1.33 1.08 -0.25
0.64KOH/St 1.30 1.38 0.08
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Table 5-4. Estimated and actual pore volumes for petcoke activated with NaOH.
Sample
Estimated pore
volume based by
carbon oxygen
reactions (cm3/g)
Experimental pore
volume (cm3/g) Difference
0.18NaOH 0.23 0.01 -0.22
0.35NaOH 0.65 0.14 -0.51
0.71NaOH 1.03 0.55 -0.48
1.06NaOH 1.15 0.73 -0.42
0.18NaOH/St 1.05 0.05 -1.00
0.35NaOH/St 1.10 0.10 -1.00
0.71NaOH/St 1.20 0.43 -0.77
1.06NaOH/St 1.60 0.71 -0.89
5.2.6 Cross-section analysis
Cross-sections of non-washed sample were obtained by fracturing the solidified resin without the
extra polishing step. However, the cross-section has a tendency to break through the chemical
layers instead of the middle of AC particles as shown in Figure 5-10a and b. Therefore, no cross-
section was successfully made.
The EDX mappings (in Figure 5-10b and d) found chemicals (either K species or Na species)
around the particles. However, sulfur distribution on cross-sections of NaOH and KOH activated
carbon were different in Figure 5-10b and d. KOH activated carbon had a homogenous S cross-
section (Figure 5-10b), but appeared to be more rod-shaped with NaOH activated carbon (Figure
5-10d). The co-existence of Na and S could also be an indication of the formation of Na2S. A
higher melting temperature of Na2S (1176˚C) compared to K2S (840˚C) could be a possible
explanation for different sulfur distribution between the cross-sections of NaOH and KOH
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activated carbon since the solid phase during NaOH activation limited the diffusivity of active
chemical species.
Figure 5-10. SEM images and potassium, sodium and sulfur mapping of non-washed AC from
petcoke with (a, b) 0.43KOH, and (c, d) 0.71NaOH. Note: the elements in image (b) were
alumina, sulfur and potassium, but (c) involved alumina, sulfur and sodium. The activation was
performed at 800 ºC for 30 min with chemical to petcoke mass ratio of 2 without washing step.
From the SEM images of the cross-section of non-washed AC activated with KOH in Figure 5-
11, the chemicals (in grey) were present in the cracks through the cross-section or around the
particles. However, not all of the cracks were occupied with chemicals since a certain amount of
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chemicals were likely to be removed or dissolved into the oil solution while polishing with an
automatic machine. As the potassium mapping in Figure 5-11d was highly related to the grey
area in SEM image in Figure 5-11c, the evidence of K diffusion into the core of the particles was
present. Moreover, the presence of K was also related to S when comparing Figure 5-11d and e.
This result could imply the formation of chemical compounds of K and S. For instance, K2S was
reported as a product during KOH activation of high sulfur petcoke [27, 30].
Figure 5-11. SEM analysis of petcoke activated with KOH before wash.a-c) SEM images of
0.43KOH with different magnifications, (d) potassium mapping and (e) sulfur mapping. Petcoke
was activated with KOH to petcoke molar ratio of 0.43 at 800 ºC for 30 min.
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After removing chemical layers with HCl (1M) and DI water, the cross-section of AC was
achieved by fracturing solidified resin. Compared to the smooth cross-section of raw petcoke
(Figure 5-12b), AC developed large cracks on the cross-section when activation with KOH to
petcoke molar ratios at 0.21 (Figure 5-12c and d) and 0.43 (Figure 5-12 e and f). As chemical
ratio reached 0.64, a distinct ‘onion-like’ structure could be observed in Figure 5-12g and h. A
similar structure was also discovered by Niasar et al. when activating petcoke at 750 ˚C for 3 h
with KOH to petcoke mass ratio of 3 (corresponding molar ratio of KOH to petcoke was 0.64)
[102]. The author considered that those cracks and the ‘onion-like’ structure benefitted pore
development at higher chemical ratios.
As shown in Figure 5-13b, the large cracks on cross-section of AC prepared during KOH
activation transformed into ‘onion-like’ structure when adding 60 min steam into the same
system. The change to an ‘onion-like’ structure is related to pore volume and pore size increment
with the addition of steam, since the addition of steam is like to widen the cracks and form the
‘onion-like’ structure. However, the structure did not change in this way at lower chemical
ratios; the structure transformation only occur if there were a sufficient number of cracks before
steam addition, in which case the expansion of the whole particle size led to the formation of an
‘onion-like’ structure. The structure transformation may be explained by the release of gases
through the reactions of steam with carbon (C + H2O → CO + H2). Gases trapped in the thicker
chemical layers; led to expanding particles. The expansion of particles exposed larger contact
surface area for K species to develop further porosity, leading to higher possibility for pore
formation.
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Figure 5-12. SEM images of (a, b) raw petcoke and (c-h) AC prepared from KOH activation at
800 ˚C for 30 min with various KOH to petcoke molar ratios of 0.21/0.43/0.64.
a. Petcoke b. Petcoke
c. 0.21KOH d. 0.21KOH
e. 0.43KOH f. 0.43KOH
g. 0.64KOH h. 0.64KOH
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Figure 5-13. The SEM images of (a, b) KOH activated petcoke at 800 ˚C for 30 min, and (c, d)
KOH steam co-activated petcoke at 800 ˚C for 60 min. The KOH to petcoke molar ratio was of
0.43 for both KOH and KOH steam activation.
5.2.7 Activation at lower temperatures
The total pore volumes and yields of the petcoke samples activated at different temperatures are
shown in Figure 5-14. At 400 ˚C, there was essentially no activation. As the activation
temperature increased from 400 to 800 ˚C, the yield appears to decrease linearly from 98% to
75%. The total pore volume, however, increased more rapidly to a temperature of 600 ˚C
(0.01375 cm3/g*min) than between 600 and 800 ˚C (0.00475 cm3/g*min). The pore volumes
a. 0.43KOH b. 0.43KOH
c. 0.43KOH/St_60 min d. 0.43KOH/St_60 min
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were 0.55 cm3/g and 0.74 cm3/g at 600 ˚C and 800 ˚C, respectively. As shown in Figure 5-15, the
AC activated at temperature lower than 600 ˚C contained only micropores, mainly below 1.5 nm
in size. When the activation temperature increased to 700 ˚C and 800 ˚C, pores between 1.5 and
2 nm in size appeared.
Figure 5-14. Pore volume (●) and yield (×) of AC prepared from petcoke with increasing
temperatures. KOH activation of petcoke was performed with KOH to petcoke molar ratio of
0.43 at certain temperature without any holding time.
Temperature (˚C)
400 600 800
To
tal p
ore
vo
lum
e (
cm
3/g
)
0.0
0.2
0.4
0.6
0.8
Yie
ld (%
)
0
20
40
60
80
100
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Figure 5-15. Pore size distributions of raw petcoke and AC (0.43KOH) prepared from petcoke
activated with increasing temperatures.KOH activation of petcoke was performed with KOH to
petcoke molar ratio of 0.43 at certain temperature without any holding time.
Within the same activation time (134 min) in Table 5-5, total pore volume was 0.27 cm3/g higher
but yield was 7% lower for AC prepared at 700 ˚C in contrast to that activated at 500 ˚C.
However, with the same activation temperature (500 ˚C), the similar pore volume (~0.36 cm3/g)
and yield (~94%) were obtained with 0 min (Figure 5-14) and 40 min holding time (Table 5-5) at
500 ˚C. Similar results were observed with AC activated at 600 and 800 ˚C within the same
activation time (Table 5-5). Therefore, the pore volumes and yields are more dependent on
activation temperature than activation time.
Pore width (nm)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
dV
/dw
po
re v
olu
me (
cm
3/(
g*n
m))
0.0
0.5
1.0
1.5
2.0
Petcoke0.43KOH_400_00.43KOH_500_00.43KOH_600_00.43KOH_700_00.43KOH_800_0
0.0 0.5 1.0 1.5 2.0 2.5 3.0
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
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Table 5-5. Total pore volume and yield versus temperature for same activation time.
Temperature
(˚C)
Holding time
(min)
Total activation time*
(min)
Pore volume
(cm3/g)
Yield
(%)
500 40 134 0.37 93
700 0 134 0.64 86
600 40 154 0.59 85
800 0 154 0.74 75
*Total activation time indicates the entire experiment time, which includes the time for
increasing the temperature to activation temperature as well as the time for holding at certain
activation temperatures
Low temperature activation is a more efficient way to create pores for a limited time period. To
understand how the pores were developed on non-porous material petcoke at lower activation
temperature, XRD, Raman and FTIR analysis were performed for structural and surface analysis.
The XRD spectra in Figure 5-16 confirmed that raw petcoke had a graphitic structure, since there
is a sharp peak at ~25˚in the spectra, which is related to the (002) face of graphite. The graphite
peak disappeared after activation at 800 ˚C. The absence of the graphite peak can not confirm
AC became amorphous, since XRD has a limitation of crystallite size.
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Figure 5-16. XRD patterns of (a) raw petcoke and (b) AC activated with KOH to petcoke molar
ratio of 0.43 at 800 ˚C without any holding time.
Raman spectra for petcoke and AC activated at 500 ˚C and 800 ˚C (Figure 5-17) had G band
located at 1581 cm-1 and D band located at 1350 cm-1. G band is also called single band, which is
a common feature for graphitic materials [163]. D band is for disordered (defects) materials
when a sample has a small crystallite size (La < 0.5 µm). The intensity of D band is proportional
to the amount of disorder in the sample, thus the ratio of ID/IG can be used for disorder [101] and
defect quantity in graphitic materials [102]. The ID/IG ratios beside Figure 5-17 were calculated
by using the peak area of these two bands. The maximum value of 4.74 appeared when activating
AC at 500˚C without any holding time. The highest ratio of ID/IG means the most disordered
0 10 20 30 40 50 600
5000
10000
15000
20000
25000
30000
b. 0.43KOH_800_0
In
ten
sit
y (
a.u
.)
2 Theta (degree)
a. Raw petcoke
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structure and defects were present on the sample (0.43KOH_500_0). ID/IG = A/La, A is a constant
for a fixed laser excitation energy by using Tuinstra Koenig law [164]. An increased ratio also
indicates a smaller La (crystallite size). Therefore, when petcoke was activated at 500 ˚C, the
structure became disordered and crystallite size decreased at the same time. As reported in the
literature [43, 165], a disordered structure developed pores more easily during KOH activation
than an ordered structure. When higher activation temperature were applied, a more ordered
structure was formed with larger crystallite size. Thus, the pore volume increased in a slower rate
at higher activation temperatures (600-800 ˚C) in contrast to lower activation temperatures (400-
600 ˚C) in Figure 5-14.
Figure 5-17. Evolution of the Raman spectra of (a) raw petcoke and AC (0.43KOH) activated at
(b) 500 ˚C and (c) 800 ˚C. KOH activation of petcoke was performed with KOH to petcoke
molar ratio of 0.43 at certain temperature without any holding time.
500 1000 1500 2000 2500 3000 3500
0
400
800
1200
1600
2000
2400
2800
3200
3600
c. 0.48K_800
a. Raw petcoke
Inte
nsit
y (
a.u
.)
Raman shift (cm-1)
b. 0.48K_500
c. 0.43K_800_0
b. 0.43K_500_0
a. Raw petcoke
3.72
4.74
3.79
ID/IG ratios
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109
Both Raman and XRD are able to observe a crystalline structural change of AC from petcoke.
The petcoke had a graphtic structure which was characterized by both Raman and XRD.
However, the graphitic structure was damaged when petcoke was activated at 500 ˚C. By
increasing the activation temperature to 800 ˚C, the structure became more ordered on the
Raman spectra, but the ordered structure was not seen on the XRD pattern.
The change of surface functional groups while activating petcoke with increasing temperature
(400-700 ˚C) was analyzed by FTIR. C-H groups (peaks at 700-900 cm-1) disappeared when
activation temperature increased from 400 ˚C to 500 ˚C. These groups were reported as very
reactive peripheral hydrocarbons in coal and coke. The absence of C-H groups indicated that
carbonization occurred at 400-500 ˚C [30]. The oxygen groups (broad peak at 1200-1300 cm-1)
were formed at lower activation temperatures (400-600 ˚C) [30]. Also within the same activation
temperatures from 400-600 ˚C, C=C in aromatics (peak at 1600 cm-1) become intense [166]. The
existence of these functional groups provided active sites during KOH activation. As the
intensity of each peak reduced when activation temperature was over 600 ˚C, the corresponding
functional groups disappeared. The lack of surface functional groups as active sites could be one
of the reasons why pores volumes increased slower when activating at higher temperature from
600-800 ˚C (Figure 5-14). Another reason for the slower rate of the pore volume growth may be
explained by the structure transformation from a disordered one to an ordered one, which slows
down the carbon oxygen reaction. Also, the reduction of surface groups at activation
temperatures from 600 to 800 ˚C resulted in an increase in carbon composition (80 wt% to 90
wt%) of AC (See Appendix C).
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4000 3500 3000 2500 2000 1500 1000
0.00
0.05
0.10
0.15
0.20
Raw petcoke
0.43KOH_400_0
0.43KOH_500_0
0.43KOH_600_0
A
bs
orb
an
ce
Wavenumbers (cm-1)
0.43KOH_700_0
Figure 5-18. FTIR spectra (ATR detector) of raw petcoke and AC (0.43KOH) prepared with
increasing activation temperature. KOH activation of petcoke was performed with KOH to
petcoke molar ratio of 0.43 at certain temperatures without any holding time.
5.2.7 Density calculations
Porosity may be developed at lower activation temperatures by opening existing closed pores on
petcoke. In order to assess this hypothesis, the density of the petcoke was measured in the lab,
and compared it to calculated values. The bulk density of petcoke was calculated by Equation 5-
6 using pore volume, yield and the density of graphite (2.16 g/cm3). For AC activated at 500 ˚C
with KOH to petcoke molar ratio of 0.43, 0.3 cm2/g pore volume and 94% yield were obtained.
Assuming all the pores developed at 500 ˚C were by opening the initially closed pores of
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petcoke, in other words, 1 g petcoke had 0.3 cm2 initial pore volume and graphite structure
occupied the rest of space. The calculated bulk density of petcoke is 1.36 g/ cm3. A similar
assumption was made for pores developed at 600 ˚C. The calculated bulk density of petcoke is
1.05 g/ cm3.
Equation 5-7
The measured bulk density of petcoke is 1.26 g/cm3. Since the measured petcoke density is
closed to the average of calculated bulk density (1.21 g/cm3), the pores developed at 500-600 ˚C
were mainly by opening the initial closed pores of petcoke.
As reported in literature [167], petcoke has a structure with graphite-like crystallites embedded in
an amorphous matrix. Petcoke in our lab had a density of 1.256 g/cm3, which was lower than the
density of graphite (2.16 g/cm3). In the modeling structure of petcoke proposed by Zhong et al.
[28], the obvious space within the petcoke structure could be observed. These empty spaces may
be the closed pores mainly in ultra-micropore range, which can only be detected by CO2
adsorption. Thus, the surface area of raw petcoke by CO2 adsorption was higher than N2
adsorption (see data in Table 5-6).
Both surface areas and pore volumes detected by CO2 adsorption increased once petcoke was
activated with KOH to 400 or 450 ˚C (See Table 5-6). It implied that ultra-micropores started to
develop even at low temperature before 400 ˚C. It is noted that N2 has a diffusion problem into
micropores smaller than 0.45 nm. It was hard for 0.43KOH_450_0 to get to equilibrium at a low
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relative pressure range. The analysis could not start even after 48 h. Thus, CO2 adsorption was
suggested to detect micropores smaller than 0.7 nm [140].
Table 5-6. Surface area from N2 adsorption and CO2 adsorption.
Sample DFT surface area by N2
adsorption
DA surface area by CO2
adsorption (m2/g)
Total pore volume
by CO2 adsorption
(cm3/g)
Raw petcoke 2 20 0.0006
0.43KOH_400_0 2 55 0.0033
0.43KOH_450_0 N/A 88 0.0056
5.2.8 Total activation time
Figure 5-19. Pore volume (●) and yield (×) of AC (0.43KOH) prepared with extended holding
time at various activation temperatures from 500 to 800 ˚C. Note: AC (0.43 KOH) was prepared
with KOH to petcoke molar ratio of 0.43.
Activation time (min)
0 50 100 150 200 250 300
To
tal p
ore
vo
lum
e (
cm
2/g
)
0.0
0.2
0.4
0.6
0.8
Activation time (min)
0 50 100 150 200 250 300
Yie
ld (%
)
0
20
40
60
80
To
tal p
ore
vo
lum
e (
cm
2/g
)
0.0
0.2
0.4
0.6
0.8
1.0
Yie
ld (%
)
0
20
40
60
80
100
a. 0.43KOH_500 b. 0.43KOH_600
c. 0.43KOH_700 d. 0.43KOH_800
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The total pore volumes and yields changes with extended holding time at various activation
temperatures, as shown in Figure 5-19. As the holding time increased from 0 to 240 min, the
largest pore volume increment (0.18 cm3/g) and biggest yield drop (10%) were achieved at 700
˚C activation. The pore volume increased by ~0.1 cm3/g without reduction of yield at 500 ˚C
activation by extending holding time from 0 to 240 min. A similar trend was followed by AC
activated at 800 ˚C. Pore volume increased ~0.1 cm3/g, but only 4% yield dropped within the
240 min holding time. These results indicated that possible ways of pore formation may exist
without carbon consumption.
One mechanism of pore formation could be intercalation. Intercalation is beneficial for pore
development, because no carbon is consumed with an expanded carbon layer by active species
like metallic K or Na. As a consequence, the pore volume increased with a high product yield.
However, the in-situ techniques are limited to detect these intercalated species formed at certain
temperatures from 400-800 ˚C, especially when considering high sulfur releasing with the
involvement of a corrosive chemical agent like KOH or NaOH. Therefore, there is no direct
evidence that intercalation is actually happening during activation of petcoke. Since I have only
observed sparks while washing AC samples prepared from 800 ˚C, the formation of K or K2O
happened at high temperature activation. Similar results were reported in several research groups
that the intercalated species formed at temperature higher than 700˚C by analyzing XRD of non-
washed AC after cooling down [78, 81].
The other way to develop pores with only a limited amount of carbon consumed is exposing
closed pores on raw petcoke. As reported in literature by Xiao et al [168], the presence of H2 in
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the carbonization process reacted with surface heteroatom species (S, O, N) of petcoke, and then
formed –CH- and –CH2- species which are ‘active sites’ for further developed pore volume.
When activation temperature reached 500 ˚C, the active species were formed for further pore
development. At lower temperatures, pores could develop from opening the initially closed pores
of raw petcoke. When extending holding time at 800 ˚C, the closed pores of petcoke may be
continuously opened from where originally covered by heteroatom species. Another reason for
increasing pore volume without consuming too much carbon is that the adequate cracks provided
more channels for large amount of K active species to diffuse into the petcoke core, therefore, a
growing number of carbon surface were exposed and attached to K species. Then, pores could be
developed by either oxidation reaction or intercalation.
The corresponding pore size distribution was listed in Figure 5-20. For lower temperature
activation (400-600 ˚C), the pore size distribution kept similar shape. But the intensity of the
peak increased with increasing activation holding time. During higher temperature activation
(600-800 ˚C) and extended activation time, pores with sizes larger than 1.5 nm appeared with.
However, the intensity of peak increased within the first 40 min of activation time, and then
decreased when the activation time was extended to 240 min. The smaller pores may transform
to bigger pores with sizes larger than 1.5 nm.
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Figure 5-20. The pore size distribution of AC (0.43KOH) activated at (a) 500 ˚C, (b) 600 ˚C, (c)
700 ˚C and (d) 800 ˚C with holding time from 0-240 min. (without any holding time (), 40 min
holding time (), and 240 min holding time ()).
5.3 Pore development with chemical activation of high sulfur petcoke
As KOH melted at approximately360 ˚C, the oxidation reaction of KOH and carbon were mainly
solid-liquid reactions. Fixed sulfur (S, 6.5 wt%) was removed from the petcoke before ultra-
micropores started developing on the petcoke at 400 ˚C. KOH was first reacted with S to
transform fixed organic sulfur into the inorganic sulfur species (sulfate, sulfite, K2S) or gases
(H2S, SO2). The remaining KOH in liquid phase was then diffused into petcoke particles.
Though petcoke is a non-porous material, the partially ordered structure of petcoke (reported in
Pore width (nm)
0 1 2 3
dV
/dw
po
re v
olu
me (
cm
3/(
g*n
m))
0.0
0.2
0.4
0.6
0.8
1.0
0 min hold time40 min hold time 240 min hold time
Pore width (nm)
0 1 2 3 4
0 hold time40 min hold time240 min hold time
dV
/dw
po
re v
olu
me (
cm
3/(
g*n
m))
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 min hold time 40 min hold time240 min hold time
0 min hold time40 min hold time 240 min hold time
a. 0.43KOH_500 b. 0.43KOH_600
c. 0.43KOH_700 d. 0.43KOH_800
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the literature) may provide some defects acting as ‘active sites’ for the reactions to start from.
Pores (0.3 cm3/g) were developed with almost no carbon consumption at 500 ˚C. The opening of
initial closed pores on petcoke is a possible way develop to porosity on petcoke at low activation
temperature (400-600 ˚C). It should be noted that the existence of initial closed pores on petcoke
was assumed according to the experiment data and calculated petcoke density. There is no direct
evidence in the literature that mentions the closed pores within the petcoke structure. Further
study is required. Functional groups (oxygen groups, C=C) appeared with the development of a
higher disordered carbon structure when activating petcoke at low activation temperatures (400-
600 ˚C). These functional groups could be ‘active sites’ for continuous oxidation reactions. The
disordered carbon structure made it easier for the petcoke to react compared to the ordered
graphitic structure. This explained the reason why AC prepared at low activation temperature
(400-600 ˚C) has a higher rate of pore volume increase than AC from high activation
temperatures (600-800 ˚C).
An obvious yield drop was observed when petcoke was activated at temperatures from 600 to
800 ˚C. The oxidation reactions started to take the lead in this stage. The majority product of
carbon oxygen reactions was K2CO3. Other K species including metallic K, K2S, K2O may be
present as well. From the literature it was reported that the metallic K, as an active intercalation
species, formed at temperature above 700 ˚C. However, the AC samples used for determination
of K species were collected after cooling and exposure to air. The active species are likely to
change under these conditions. There currently is no in-situ technique available to determine the
formation of K species during petcoke activation due to the high experimental temperature and
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the corrosive chemicals (e.g. KOH). The oxidation reactions and the reaction to form metallic K
are thermodynamically favored even at low temperatures, from 500 to 600 ˚C. The standard
reduction potential at 25 ˚C for K is -2.92 V, which implies that metallic K is a very active
reducing agent [77]. Therefore, metallic K may not be stable at these activation temperatures,
and so it is still not clear whether metallic K formed at activation temperatures from 400 to 800
˚C. Furthermore, whether the intercalation occurred through metallic K during petcoke activation
is even more difficult to investigate. As the surface functional groups have already formed at
temperatures from 400 to 600 ˚C, the heteroatoms on the surface may react more easily with
KOH rather than ordered carbon on petcoke. The pores may be continuously exposed by opening
of the closed pores on petcoke which are originally covered by the surface heteroatom species.
The particle expansion, resulting from adequate cracks created on petcoke, is another possible
reason for pore development at 800˚C with high yields (>70%), because the additional carbon
surface increased the possibility for carbon ‘active sites’ to attach active K species.
5.4 Summary
In summary, the C-S bond on petcoke broke before pores started to develop at 400 ˚C, indicated
by the loss of S content on EDX mapping. Meanwhile, the accessible channels were formed.
When KOH melted at around 360 ˚C, KOH became mobile and diffused into these channels.
Higher initial sulfur content on petcoke suggested that more accessible channels could be
created. A greater amount of KOH was likely to be diffused into the petcoke core. When
increasing activation temperature from 400-450 ˚C, the original closed ultra-micropores of
petcoke were gradually exposed, and the surface area was increased and then determined by CO2
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adsorption. Only if the pores were accumulated to sizes larger than 1 nm at the activation
temperature of 500 ˚C, pore volume and surface area could be characterized by N2 adsorption.
However, the pores developed at 500 ˚C were smaller than 1.5 nm. A high yield (94%) was
obtained with high pore volume (0.3 cm3/g) and surface area (800 m2/g). Since almost no carbon
was consumed by carbon oxygen reactions, the most pores develop at 500 ˚C were by opening
initially closed pores on petcoke. At the same activation temperature, the disordered structure
and an increasing number of defects developed, resulting in a smaller crystallite size.
Additionally, when activation temperatures increased from 400-600 ˚C, several types of surface
functional groups (oxygen groups and C=C) appeared. Both functional groups and defects may
perform as ‘active sites’ for reactions to develop more porosity. The interfacial reactions are
possible to adhere onto these ‘active sites’ at a lower temperature, resulting in a rapid increasing
of pore volume in a limited time. Carbon oxygen reactions began to take the lead of the pore
development when activation temperature was increased to 700 ˚C. The pore volume
continuously increased to 0.64 cm3/g, but yield reduced to 82%. As the temperature reached 800
˚C and holding time was extended from 0-240 min, the pore volume increased by 0.1 cm3/g with
only 4% carbon consumed. The pores created during extended holing time may from either
intercalation or continuously opening of the closed pores on petcoke (usually originally covered
by the surface heteroatom species). The particle expansion with highest chemical amount or with
addition of steam increased the possibility for additional carbon surface to attach to K active
species, and thereafter, higher porosity were developed.
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Chapter Six: HIERACHICAL POROUS CARBON FROM ASPHALTENES AND
PETCOKE
In this chapter, the microporous AC prepared from petcoke in Chapter 4 and Chapter 5 were
combined into a carbon foam material with nanoscale and macroscale pores. To develop
macroscale pores in interparticle space, a salt template was used. A specific role for each
component (salt, asphaltene or carbon source) in the salt template method was investigated. The
experimental conditions using a salt template, including ball-milling environment, carbon source
and carbonization temperature, were also studied. Asphaltenes were used as natural binders for
the first time to fabricate carbon foam and hierarchical porous carbon (HPC). The following is
the schematic of preparation procedure.
Ball-mill
in isopropanol 9h
Press
5 MPa 20 s
Carbonization
5 MPa 20 s
Wash
H2O with/without HCl
Carbon foam
Carbon source (ball-milled petcoke/ CB)
Binder (Asphaltene)
Pore former (NaCl)
Mixture
Ball-mill
in isopropanol 9h
Press
5 MPa 20 s
Carbonization
5 MPa 20 s
Wash
H2O with/without HCl
HPC
Carbon source (Petcoke-derived AC)
Binder (Asphaltene)
Pore former (non-washed K salts)
Mixture
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120
6.1 Initial Experiments
Emulsion template was the first synthesis approach for carbon foam preparation (See Appendix
D). The water droplets were suspended in the oil phase which comprised of asphaltenes and
toluene. A stable mixture was then formed in the aqueous phase. However, the mixture became
instable when adding petcoke as the carbon source into the emulsion system. Furthermore, the
carbon structure collapsed after the sample was dried in the vacuum oven. Therefore, the
emulsion template was not a successful approach to prepare carbon foam.
In order to study if asphaltenes could work as a binder, the mixture of 1 g petcoke (after ball-
mill) and 4 g salt (NaCl) was wet ball-milled with zirconia balls in isopropanol for 9 h either
with or without asphaltene (1 g). After removing the zirconia balls, the mixture was dried in a
muffle furnace at 70 ˚C for 12 h. The powder was then applied with a hydraulic pressure of 5
MPa for 20 s. A monolith could only be formed with the addition of asphaltenes (in Figure 6-1a).
Otherwise, the mixture was still powdered (in Figure 6-1b). Therefore, asphaltenes were
introduced as binders for the first time to synthesize the monolith.
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Figure 6-1. (a) A pellet of ball-milled petcoke, asphaltene and NaCl (mass ratio is 1:1:4), and (b)
mixed powder of ball-milled petcoke and NaCl (mass ratio of 1:4) after applying a hydraulic
force of 5 MPa for 20 s.
To study the role of salt in carbon foam preparation, 1 g petcoke and 1 g asphaltenes were mixed
and carbonized in N2 at 400 ˚C for 2 h either with 4 g of NaCl (in Figure 6-2a) or without NaCl
(in Figure 6-2b). Even though both mixtures with or without NaCl formed similar monoliths
after applying a hydraulic pressure of 5 MPa for 20 s, the monolith was only retained after
carbonization when NaCl was present. Thus, salt (NaCl) held the monolithic structure during
carbonization process. As reported by Gray et al., Athabasca asphaltenes were melted at an
average temperature of 224 ˚C, which was lower than asphaltenes decomposition temperature
(350 ˚C) [169]. NaCl remained as solid at 400 ˚C during carbonization. And NaCl did not
decompose because of its chemical stability. Asphaltenes in molten phase provided a mobile
environment for tiny petcoke particles to move around (particle size of 1.5-2.0 µm), but not for
large NaCl particles (initial particle sizes various from 100-500 µm). The monolith structure was
fixed by large NaCl particles in a well-dispersed molten mixture. However, for the system
without NaCl, only a thin layer was formed after releasing gases through asphaltenes
decomposition process.
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Figure 6-2. Carbon materials after carbonizing (a) ball-milled petcoke, asphaltenes, NaCl (mass
ratio is 1:1:4) and (b) ball-milled petcoke and asphaltene(mass ratio of 1:1) in N2 at 400 ˚C for 2
h. The corresponding carbon foams were (c) with NaCl or (d) without NaCl after removing
NaCl in hot water, respectively.
Comparing to the SEM images of carbon foam with NaCl (Figure 6-2c) and without NaCl
(Figure 6-2d) after washing with hot water, the macroscale pores in carbon foam were only
formed with the presence of NaCl after removing the salt template (NaCl).
6.2 Parameters in salt template method
6.2.1 Ball-mill environment (Wet ball-mill vs Dry ball-mill)
Wet ball-mill was achieved by introducing isopropanol in a milling jar for 9 h, whileas dry ball-
mill was achieved with the absence of isopropanol. After pressing the mixtures of ball-milled
petcoke, NaCl and asphaltenes into a monolith, the monolith was then carbonized at 400 ˚C for 2
h. The SEM images of monoliths after washing were carbon foam (Figure 6-3). The macropores
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in the interparticle space of carbon foam was larger via dry ball-mill process (Figure 6-3b) than
wet ball-milling (Figure 6-3a). This indicated that larger NaCl crystals remained after the dry
ball-milling. Jung et al. also found a much broader size reduction by using the dry ball-mill
technique in contrast to the wet ball-mill [106]. The wet ball-mill technique provided a better
dispersion of NaCl and asphaltenes around petcoke particles in the molten phase. The
electrostatic force between particles was weaker when isopropanol presented at the same time,
which avoided the agglomeration of particles. A homogenous mixture was obtained by wet ball-
milling [170]. Additionally, the isopropanol in the wet ball-milling also reduced the local
temperature during the ball-mill process. Thus, wet ball-milling was applied in the following
experiment in this section.
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Figure 6-3. SEM images of carbon foam prepared by salt template with ball-milled petcoke,
asphaltenes and NaCl mixing at a mass ratio of 1:1:4 in (a, c) wet ball-mill (b, d) dry ball-mill.
6.2.2 Carbon source (carbon black vs ball-milled petcoke)
Carbon black (Monarch 120 from Cabot Cop, Boston, MA) was selected as another carbon
precursor compared to petcoke, because it has over 99 wt% carbon content and regular shape.
a b
c d
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Larger macroscale pores between carbon particles were observed on the cross-section SEM
images of ball-milled petcoke (in Figure 6-4 b, e, f), which may be due to the bigger particle
sizes of ball-milled petcoke. Carbon black has a particle size in nanometer scale (8-100 nm);
however, the particle size for ball-milled petcoke is on the micrometer scale (1.5-2.0 µm). Bigger
particle sizes of petcoke also resulted in more cracks (shown in Figure 6-3 b) and less
mechanical strength (tested by hand) of the carbon foam structure.
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Figure 6-4. SEM image of carbon foam using (a, c, e) carbon black as a carbon source, or (b, d,
f) ball-milled petcoke as a carbon source. The carbon foam was prepared from a mass ratio of
carbon precursor, asphaltenes and NaCl of 1:1:4, and then carbonized in N2 at 400 ˚C for 2 h.
6.2.3 Carbonization temperature (400 vs 600 ˚C)
a b
c d
e f
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The variation of carbonization temperature from 400-600 ˚C did not have an impact on carbon
foam structure (Figure 6-5). Therefore, 400 ˚C was selected for further experiments to prepare
the carbon foam with less energy consumption.
Figure 6-5. SEM image of carbon foam by carbonizing carbon black, asphaltenes and NaCl (a, c)
at 600 ˚C for 2 h, or (b, d) 400 ˚C for 2 h The carbon foam was prepared from a mass ratio of
carbon black, asphaltenes and NaCl of 1:1:4.
a b
c d
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6.3 HPC preparation
6.3.1 Carbon foam structure by using petcoke derived AC as carbon precursor
By using petcoke derived AC as a starting material, the hierarchical structure was built according
to the synthesis approach of carbon foam. NaCl was the pore-forming agent for interconnected
porosity. Asphaltenes work as binders to build interparticle space between AC particles.
However, the AC particles (Figure 6-6 b, d, e) were too big (25-250 µm) for a mechanically
strong monolith.
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Figure 6-6. SEM image of HPC created by carbonizing 400 ˚C for 2 h (a, c, e) ball-milled
petcoke, asphaltenes and NaCl (mass ratio = 1:1:4) or (b, d, f) petcoke-derived AC (0.43KOH/St)
with asphaltenes and NaCl (mass ratio = 1:1:4). Note: AC (0.43 KOH/St) was prepared with
KOH steam co-activation at 800 ˚C for 30 min with KOH to petcoke molar ratio of 0.43.
a b
c d
e f
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It was hypothesized that by introducing micro-mesoporous AC derived from petcoke, the
macroscale pore sizes of carbon foam would be integrated with nanometer pores on AC particles.
The initial total pore volume of AC was 0.98 cm3/g with a surface area of 2100 m2/g (DFT).
However, only 0.1 cm3/g pore volume and 180 m2/g surface area remained after the formation of
HPC. When comparing pore size distribution of HPC and the original AC (Figure 6-7), it was
seen that most pores in micropores (< 2 nm) and small mesopores (2-4 nm) on AC were blocked
by asphaltenes during wet ball-milling or carbonization process.
Figure 6-7. Pore size distribution of AC (●) prepared from petcoke through co-activation of
KOH and steam (0.43KOH/St), and the corresponding HPC (○) by using the same AC as carbon
precursor.Note: AC (0.43 KOH/St) was prepared with KOH steam co-activation at 800 ˚C for 30
min with KOH to petcoke molar ratio of 0.43.
6.3.2 HPC by using non-washed AC as carbon precursor
Wu et al. reported that a chemical layer (mainly KOH and K2CO3) was formed around the
microporous AC during KOH activation of petcoke [13]. In order to avoid the asphaltene
Pore width (nm)
0 1 2 3 4 5
dV
/dw
po
re v
olu
me (
cm
3/(
g*n
m))
0.0
0.1
0.2
0.3
0.4
0.5
AC HPC
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blockage and make use of the chemical species inside micropores, non-washed AC was used as
an alternative carbon precursor for HPC preparation because the micropores of AC were
protected by chemical species before HPC structures was built. Since the literature showed that
asphaltene has a preference to adsorb into mesopores rather than micropores [96]. Highly
microporous AC without any mesopores (0.43KOH) was selected for the following experiments.
The exclusion of mesopores on AC lowered the possibility of asphaltene blockage into pores
during ball-milling.
The surface area for non-washed AC (0.43KOH) was lower than 2 m2/g, which confirmed that
all the micropores (Vmicro = 0.84 cm3/g) were blocked with K salts. After wet ball-milling in
isopropanol with asphaltene, the surface area was similar (1.8 m2/g) to what it was before ball-
milling. EDX mapping in Figure 6-8g showed 54 wt% K was dominant on the surface of the
particle. Oxygen and sulfur content were 42 wt% and 4 wt%, respectively. A monolith was
formed when pressing at 5 MPa for 20 s, and the physical shape of the monolith (Figure 6-8b)
did not change after carbonizing at 400 ˚C for 2 h. The additional pressure compressed the
powder mixture into the monolith firmly, which was hard to break manually. The EDX mapping
of the monolith (h) indicated a lower potassium content (46 wt%) but a higher oxygen content
(51 wt%) presented on the surface after carbonization. After 1 h of sonication and hot water
washing, the monolith broke into several small chunks. Only 16 wt% of K remained on the
coarse surface (Figure 6-8f) of chunks after washing. However, a high potassium amount (43
wt%) was detected on the smooth surface (Figure 6-8f), which is an indication of remaining K
salts in the HPC structure. Therefore, by continuous acid washing, almost all K was washed
away (< 0.5 wt%). Over 90 wt% C was exposed on the external surface. The sulfur left was only
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2.2 wt%. The surface area for HPC after the acid wash increased from 3.9 m2/g (only water
wash) to 74 m2/g. However, the surface area after continuous acid washing was still lower than
original AC (2300 m2/g for 0.43KOH).
After wet BM After press and carbonize After H2O wash and dry
Figure 6-8. Digital photos (a-c) and SEM images (d-f) with EDX mapping (g-i) of HPC prepared
from non-washed AC (0.43KOH) with 1:1 mass ratioof AC and asphaltenes. Note: Red
indicates potassium in EDX mapping. AC (0.43 KOH) was prepared with KOH activation at 800
˚C for 30 min with KOH to petcoke molar ratio of 0.43.
5 mm 5 mm 5 mm
200 µm 200 µm 200 µm
50 µm 50 µm 50 µm
a
)
b c
d e f
g h i
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As most of the micropores may have been blocked by asphaltene, the mass ratio of asphaltene to
AC was then reduced from 1:1 to 0.1:1. A similar experiment was performed with lower
asphaltene amount. The result showed that monolith shape stayed the same after carbonization at
400 ˚C for 2 h (Figure 6-9b). With a water wash only, potassium was removed from the surface
of HPC by using lower asphaltene amount (< 2 wt% K detected by EDX mapping). And the
sulfur content was also reduced to 1.3 wt%. The surface area after water washing increased to
830 m2/g. Meanwhile, the total pore volume increased to ~ 0.3 cm3/g.
AC + Asp (R=1:1) AC + Asp (R=1:0.1) Asp without AC (R=0:1)
Figure 6-9 Digital photos and SEM images of HPC prepared from non-washed AC (0.43KOH)
and asphaltene at mass ratios of 1:1 (a, d) and 1:0.1 (b, e). Note: AC (0.43 KOH) was prepared
with KOH activation at 800 ˚C for 30 min with KOH to petcoke molar ratio of 0.43.
Without any AC involved in the entire process, asphaltene was directly activated with KOH at a
mass ratio of 2 (Figure 6-9c and f). But there was no porosity developed after activation at 400
a b c
d e f
50 µm 50 µm 50 µm
1 cm 1 cm 1 cm
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˚C for 2 h (the same experiment condition was applied as HPC preparation). The monolith did
not maintain its physical shape when the temperature increased. Only a layer was left in the
ceramic boat (Figure 6-9c).
The pore size distribution in Figure 6-10 contained extra peaks that appeared at 0.5 nm and 1.3
nm for the lower asphaltene amount (R=1:0.1). As for the high asphaltene amount (R=1:1), there
was only one peak at 0.5 nm after water and acid washing. Therefore, the occupied pores could
be exposed by either continuous acid washing or lowering the asphaltene amount. The pore
exposure was also related to the DFT surface area detected by N2 adsorption. After continuous
acid washing, the DFT surface area increased from 1.8 m2/g to 74 m2/g. And the DFT surface
area for lower asphaltene amount was almost 830 m2/g. A schematic of pore development is
given in Figure 6-11.
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Figure 6-10. Pore size distribution of HPC prepared from non-washed AC (0.43KOH) and
asphaltenes at mass ratios of 1:1 (● for water wash, ○ for water and acid wash) and 1:0.1( for
water wash). Note: AC (0.43 KOH) was prepared with KOH activation at 800 ˚C for 30 min with
KOH to petcoke molar ratio of 0.43.
Pore width (nm)
0.0 0.5 1.0 1.5 2.0
dV
/dw
po
re v
olu
me (
cm
3/(
g*n
m))
0.0
0.2
0.4
0.6
0.8
1.0HPC after water wash (R=1:1)HPC after water and acid wash (R=1:1)HPC after water wash (R=1:0.1)
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Figure 6-11. The schematic graph of pore development for HPC structure.
Salt (K species) AC Aphaltene
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Chapter Seven: Conclusions and Recommendations
7.1 Conclusions
AC was successfully prepared using either NaOH or KOH activation with petcoke (6.5 wt%).
The mixing method for KOH/NaOH and petcoke before activation did not have a significant
impact on produced AC properties (pore volume and yield). Therefore, directly putting
NaOH/KOH pellets (about 6 mm diameter and 2 mm length in the form of a cylinder) on the top
of petcoke particles gave the same results as physically mixing by hand, while ball-milling
resulted in high dispersion, but separation of the material from the zirconia balls was difficult. In
both NaOH and KOH activation processes, the pore volume was positively related to an
increasing amount of chemical agent, but the yield showed a negative trend. The total pore
volumes of KOH activated samples (0.16-1.1 cm3/g) were higher than that activated with NaOH
(0.01-0.73 cm3/g). Also, activation with KOH resulted in highly microporous material, while
activation with NaOH resulted in micro-mesopores. By introducing steam as another oxygen
source, the chemical amount was reduced without compromising pore volume when using KOH,
but was not for NaOH. The AC yields with co-activation decreased to < 45% because of steam
gasification at 800 ˚C. A linear decrease in the yield was observed with increasing steam time (0
to 120 min) for all KOH to petcoke ratios.
A chemical layer was seen on the cross-sections of particles after activation but before washing
through SEM/EDX mapping. Large cracks on both external surface and cross-section of AC
(SEM images) were exposed once chemical layers were washed. An ‘onion-like’ structure was
formed under extensive activation conditions: for instance, chemical activation with highest
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KOH to petcoke ratio of 0.64 or co-activation for 60 min with KOH to petcoke ratio of 0.43. The
transformation from cracks to the ‘onion-like’ structure may be related to the release of gases by
the interactions between steam and chemical species. The thickness of chemical layers around
particles corresponded with the steam gasification rate, and thus the amount of carbon consumed
by the addition of steam decreased with an increasing amount of chemical agent.
The study of KOH activation at various activation temperatures ranging from 400-800 ˚C
showed ultra-micropores started to develop at 400 ˚C on petcoke. An increasing pore volume
with almost no yield drop was achieved at 500 ˚C. At this temperature of activation, pores were
possibly developed by opening initially closed pores of petcoke. Carbon-oxygen reaction started
to take the lead when temperature increased to 700 ˚C. By extending holding time at 800 ˚C,
pores may also be developed continuously either by intercalation or further exposure of the
initially closed pores.
Finally, HPC was successfully fabricated by using asphaltenes as binders between petcoke-
derived AC. The macroscale pores within interparticle space were combined with the already
produced nanoscale pores on petcoke by using a salt template.
7.2 Recommendations
Considering the findings of this study, the following work is suggested
(a) Sulfur evolution of activation process
For petcoke with higher sulfur content, it is always appealing to consider. To begin,
understanding how sulfur species were transformed during the activation process will be crucial.
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Sulfur evolution for the entire activation process is needed. SEM/EDX is only a surface
characteristic technique. Instead, elemental analysis is suggested for an accurate sulfur
determination of bulk materials. The specific sulfur species could be analyzed by X-ray
photoelectron spectroscopy (XPS) on the surface. Additionally, the amount of sulfur in gases
will be trapped in water. The sulfur in filtrates could be detected by inductively coupled plasma
(ICP). In this way, it may be possible to study the approximate temperature for C-S breakage and
possible reactions involving several sulfur species. The understanding of these fundamentals will
guide ways to utilize sulfur in either petcoke or asphaltenes.
(b) HPC with petcoke and asphaltenes
In order to optimize experiment conditions of HPC preparation, systematic experiments should
be designed. For instance, the mechanical strength of the monolith is required to be measured by
tensile test. The washing step needs to be modified in order to wash most of K species but
maintain the monolith shape. As Chapter 6 showed, the amount of asphaltene is also an
important parameter for the exposure of the nanoscale pores on petcoke. Further investigation of
the relationship of the amount of asphaltene and micropore volumes occupied by salts (K
species) will also be required. Other parameters are suggested as well. For instance, lower
activation temperature (200 ˚C).
(c) Life-cycle assessment for KOH activation and KOH and steam co-activation of petcoke
From an economic evaluation on AC production from several feedstocks, petcoke was proposed
as the most cost-effective raw material [25]. However, the most effective petcoke activation
approach is still not clear. Regarding pore volume, KOH and steam co-activation proved to be an
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effective approach to reduce KOH amount. However, the addition of steam consumed a greater
amount of carbon and required more energy as well. Therefore, performing a life-cycle
assessment for different activation methods will provide information on the most effective way
to activate petcoke.
(d) Petcoke structure
As shown in XRD and HRTEM, petcoke is a partially ordered material with no porosity on the
surface. The existence of closed pores on petcoke was suggested in Chapter 5, because high
yields (>86%) of AC were prepared from petcoke activated at low temperatures. However, there
is no direct evidence to illustrate the porous structure in petcoke. Micro-computed tomography
(micro-CT) is a 3D imaging technique with high resolution (~ 2 µm). The internal structure of
petcoke can be observed without destroying the sample. Micro-CT has already been applied in
various geological applications [171, 172]. Several researchers captured micrometer-sized
features in coal using Micro-CT [173-175]. The density and porosity of large petcoke particles
are also suggested in order to understand better the initial porous structure of the petcoke.
(e) K species on AC
Determining the formation of specific K species during petcoke activation is essential for the
understanding of reactions involved and whether intercalation happened in the process to
develop pores. As a result, designing petcoke-derived AC with desirable properties will be
easier. XRD and XPS are suggested techniques to determine K species on petcoke before
washing at each activation temperature.
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(f) Ball milling mixing with NaOH and petcoke
The titration data in Table 5-1 showed that ball milling reduced KOH amount before activation.
Similar trend may be followed with ball milling mixing of NaOH and petcoke as well. The initial
NaOH to petcoke ratio should be increased to 3. The same titration experiment is required to
measure the exact amount of NaOH in ball-milled mixture before activation. Then, AC
properties after ball milling with NaOH to petcoke of 3 can compare with the other two mixing
methods (physical mixing and no mixing) with a lower NaOH to petcoke of 2.
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Appendix A: Mixing efficiency on activated carbon from petroleum coke
Table A-1. Element composition of petcoke and ACs by SEM/EDX and ultimate analysis
Sample Carbon (wt%) Oxygen (wt%) Sulfur (wt%)
Petcoke
Petcoke cross-section from ES 86.60 + 3.77 6.96 + 3.81 6.44 + 2.11
Petcoke cross-section from
common lab 86.43 + 2.11 9.13 + 1.29 4.42 + 0.96
Petcoke exterior from common lab 85.02 + 3.89 6.61 + 1.18 8.37 + 4.48
Ultimate analysis 83.9 1.4 6.4
ACs
0.43K_500 cross-section from
common lab 74.41+ 8.86 23.89 + 7.45 1.70 + 2.39
0.43K/St_500 cross-section from
common lab 78.23+ 4.43 20.71+ 3.26 1.08+ 1.17
The ultimate analysis not only detect carbon, oxygen, sulfur content of petcoke, but
hydrogen (3.6 wt%), nitrogen (2.0 wt%), and ash (2.6%) was included into 100% of the
material composition.
The oxygen content was indirectly calculated in ultimate analysis by the difference:
O = 100 − (C + H + N + S)
I only consider carbon, oxygen, sulfur content for SEM/EDX analysis, since the other
elements (such as hydrogen, nitrogen) peaks were not obvious to detect on the spectra.
Since the analysis process is different with ultimate analysis and SEM/EDX, the data was
not equally comparable within this two measurement.
Considering the variation in sulfur and oxygen content by SEM/EDX, other method to
measure the elemental analysis need to be found.
Elemental analysis in Chemistry department or ICP analysis in common lab could be
considered.
Considering the variation in each element content, the composition was similar for AC
with or without steam addition.
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Table A-2. Surface area and pore volume of raw petcoke by CO2 adsorption
Petcoke particle size
(µm)
P/P0 SA by DA method
(m2/g)
Pore volume by DA method
(cm3/g)
300-600 0-0.035 38 0.014
150-300 (Previous) 0-0.3 20 0.006
150-300 (Corrected) 0-0.035 84 0.08
<150 (Melanie’s
data)
0-0.03 29 0.02
Same relative pressure and Dubinin-Astakhov (DA) method were used to compared with
data in Arash’s paper.
My data after correction was following the tread with Arash’s data.
I also double checked the raw file of Melanie’s data. Since her data was only collected
from 0-0.03 P/P0, she had a lower value of surface area and pore volume for petcoke
particle smaller than 150 µm.
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Appendix B: Preparation of activated carbon from petcoke
Figure B-1. Pore size distribution (PSD) and cumulative pore volume on NaOH ( for PSD,
solid line for cumulative pore volume) and NaOH/St ( for PSD, dash line for cumulative pore
volume) activated samples at different NaOH to petcoke molar ratio (a.1.06, b. 0.71, c. 0.35)
NaOH activated samples are micro-mesoporous materials.
Pore Width (nm)
0 5 10 15 20 25 30
dV
/dlo
g(w
) P
ore
Vo
lum
e (
cm
3/g
)
0.0
0.2
0.4
0.6
0.8 Cu
mu
lativ
e P
ore
Vo
lum
e (c
m3/g
)
0.0
0.2
0.4
0.6
0.8
dV
/dlo
g(w
) P
ore
Vo
lum
e (
cm
3/g
)
0.0
0.2
0.4
0.6
0.8 Cu
mu
lativ
e P
ore
Vo
lum
e (c
m3/g
)
0.0
0.2
0.4
0.6
0.8
dV
/dlo
g(w
) P
ore
Vo
lum
e (
cm
3/g
)
0.0
0.2
0.4
0.6
0.8 Cu
mu
lativ
e P
ore
Vo
lum
e (c
m3/g
)
0.0
0.2
0.4
0.6
0.8a
b
c
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156
Larger mesopores appears at around 4 nm and 20 nm.
The impact of steam addition on NaOH activated samples are similar.
Steam destroyed almost half of the micropore volume.
The lines of cumulative pore volume are parallel at the same NaOH ratio regardless of
the steam addition. Pores can not be enlarged during the process of adding steam.
.
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Appendix C: Low temperature activation
Table C-1. Elemental analysis
Samples C (%) H (%) N (%) Other
0.43KOH_400 78.54 3.38 1.67 16.42
0.43KOH_450 79.59 3.14 1.74 15.53
0.43KOH_500 72.05 2.56 1.76 23.64
0.43KOH_600 77.60 2.09 1.29 19.03
0.43KOH_700 80.95 1.19 0.91 16.96
0.43KOH_800 90.53 0.58 0.83 8.06
The composition of carbon, hydrogen and nitrogen is similar for AC activated at lower
temperature from 400-600.
An increasing of carbon content was observed with slight decreasing content of hydrogen
and nitrogen.
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Appendix D: Carbon foam by emulsion template
Figure D-1. The emulsion template to produce carbon foam
The carbon structure collapse after drying in the vacuum over
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Appendix E. Reprint Permission Letters
Permission for Figure 2-1
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Permission for Figure 2-2
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Permission for Figure 2-3
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Permission for Figure 2-4
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Permission for Figure 2-5
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Permission for Figure 2-6
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Permission for Figure 2-7
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Permission for Figure 2-8
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Permission for Figure 2-9
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Permission for Chapter 4
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Permission letter from co-author
Library
University of Calgary
2500 University Dr. NW, Calgary, AB, T2N 3Y7
Canada
March 05, 2019
RE: Copyright permission
Calgary, Alberta, Canada
I, Vicente Montes Jiménez, hereby provide the permission to Jingfeng Wu (PhD Candidate,
Dept. of Chemical & Petroleum Engineering, University of Calgary) to include the journal paper
'Impacts of amount of chemical agent and addition of steam for activation of petroleum coke
with KOH or NaOH' in her PhD thesis. The paper is published in Fuel Processing Technology
(2018, vol. 181, pp. 53-60). I am the second author of this paper and performed SEM, organized
the data to discuss the mechanism of co-activation. I am happy to authorize her to include this
article in her PhD thesis.
Sincerely
Vicente Montes Jimenez
Córdoba, Spain, 2019/03/05
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Permission letter from co-author
Library
University of Calgary
2500 University Dr. NW, Calgary, AB, T2N 3Y7
Canada
January 25, 2019
RE: Copyright permission
I, Luis Daniel Virla Alvarado, hereby provide the permission to Jingfeng Wu (PhD Candidate, Dept.
of Chemical & Petroleum Engineering, University of Calgary) to include the journal paper ‘Impacts
of amount of chemical agent and addition of steam for activation of petroleum coke with KOH or
NaOH’ in her PhD thesis. The paper is published in Fuel Processing Technology (2018, vol. 181, pp.
53-60). I am the third author of this paper and contributed with the discussion of the data analysis and
the explanation of the mechanism of co-activation of petcoke. I am happy to authorize her to include
this article in her PhD thesis.
Sincerely,
Luis Virla, PhD
Post Doctoral Fellow R&D
Industrial Climate Solutions, Inc.
