Characteristics of Activated Carbon Fibers by
Chemical Modifications and Thermal Treatments
Paper # 1190
Yu-Chun Chiang and Chao-Hsuan Su
Department of Mechanical Engineering, Yuan Ze University. 135 Yuan-Tung Rd.,
Chung-Li, Taoyuan 320, Taiwan, ROC.
ABSTRACT
Activated carbon fibers (ACFs) are widely used in air pollution control because of their
extended surface area, high adsorption capacity, fast adsorption/desorption rates,
microporous structure, and specific surface reactivity. Several studies have been
attempted to improve the adsorption performance of ACFs. Therefore, the objective of
this study is intended to understand the properties of ACFs after chemical oxidations
and thermal treatments, and further to evaluate their adsorption capacities. One
rayon-based ACF was selected in this study. The fractions of as-received ACF were
immersed in H2O2, H3PO4 or HNO3, under room temperature or 60 oC for 1 hour. The
thermal treatments were operated in a tubular furnace, carried out at 400, 600 and 850
o
C for 30 or 60 min. The properties of ACFs were analyzed, including the surface
morphology, the specific surface area, the pore volume, the surface oxides, the ash
content and the pH value. The amounts of toluene adsorption on ACFs at 25 oC were
determined for evaluation of the effect of chemical modifications and thermal
treatments. The results show that the chemical oxidations decreased in specific surface
area and pore volume with temperature because parts of the occupation of surface
oxides or the collapse of graphitic sheets. Nevertheless, the oxidation followed by
thermal treatment would extend the specific surface area and the toluene adsorption of
ACFs. But the soaking period of 60 min at 850 oC produced the closure of micropores.
The ACFs treated by HNO3 followed with heat treatment possessed much narrow fibers,
lower pH values, less ash contents, and better toluene adsorption. The increase of
1
micropore volume after thermal treatment primarily occurred in the neighborhood of
indentations or the fiber surface, observed from FE-SEM images.
INTRODUCTION
Because of their extensive surface area, high adsorption capacity, rapid rates of
adsorption/ desorption, microporous structure, and special surface reactivity, activated
carbon fibers (ACFs) have been widely used in diverse applications, such as adsorption
for pollution control, catalyst and catalyst support, and gas storage. ACFs can be
prepared from various precursors like polyacrylonitrile (PAN), rayon, resins and
pitches.1 Different precursors would make their final products possess different
properties.2
The textural structure and the surface chemistry of ACFs play an important role in ACF
applications.3 The pore texture of ACFs depends on the nature of the precursors, the
impregnants, flow rate of the reacting gas, maximum heat treatment temperature (HTT)
and heating rate.4 Needle-shaped voids have been observed to exist between crystallites
in the surface of the PAN fiber.5 The surface chemistry of carbons is determined by the
distribution and the nature of the surface functional groups (SFGs) and the heteroatoms.
Carbon–oxygen complexes, the most important SFGs,6 are thought to be located near
the edges of the polyaromatic sheets in carbon7 or the internal pore/slit/void surfaces
and not within the graphitic sheets.8 Lahaye9 and Boehm10 summarized the common
methods for characterizing surface oxides, such as acidimetric titration techniques, IR
spectroscopy, XPS, thermal desorption spectroscopy, and electrokinetic measurements.
It is known that surface oxides on carbon surfaces can be formed by means of thermal
treatment at high temperature, ozone treatment, and liquid treatment of the chemical
aqueous solution.1 The existence of surface functional groups on carbons such as
carboxyl, lactone, phenolic and carbonyl has been postulated as constituting the source
of surface acidity.3
A number of treatment methods, including chemical modifications and heat treatments,
have been investigated to modify the carbon. For instance, surface oxygen functional
groups can be formed on carbons by treatments such as electrochemical oxidation,11
plasma treatment,12 chemical oxidation in HNO3, H2SO4 or inorganic chloride
2
solution,13 and gaseous oxidation with oxidizing gases.13 The gaseous oxidation
methods have the advantages of convenience and no significant impacts on physical
characteristics of the carbons. Porous carbon materials should be treated at an
appropriate temperature to avoid the loss of porosity.14 As HTTs increased, the amount
of SFGs was reduced; as a result, the ACFs became more hydrophobic.15 Besides that,
the average micropore diameter increased due to the loss of oxygen complexes.8 After
ACF was treated above 1100 oC, the degree of graphitization increased significantly.
This is partially attributed to the release of the C=O groups.15 Kumar et al.4 suggested
that the increase in micropore volume and BET surface area of ACFs under N2 at HTT
above 850 oC may be due the formation of new pores.
The lower surface area ACFs with smaller pore size possessed higher adsorption
capacities for low boiling point alkanes at low concentrations.16 ACFs with different
pore texture and surface composition had different adsorption/desorption behavior for
polar and non-polar vapors.17 The effects of VOC polarity on the adsorption capacity at
lower concentrations were more significant on ACFs with smaller surface areas.17 In
addition, SFGs may partially block the pore entrances, altering the rate of diffusion of
vapor molecules through the pore system.18
In order to extend their applications, ACFs are usually modified by a variety of surface
treatments. There is little information in the literature on how post-treatments, i.e.,
chemical oxidation followed with heat treatments, affect the physicochemical properties
and adsorption capacity of ACFs. Therefore, this study is intended to increase the
understanding of the influence of chemical oxidation and thermal treatment on the
physicochemical properties of ACFs. A number of techniques have been used to
characterize the changes in the carbon surface, and then the adsorption capacities of
ACFs have also been determined.
EXPERIMENTAL METHODS
Activated carbon fibers
One rayon-based activated carbon fiber (ACF) sample was used in this study, which was
supplied by Taicarbon Inc., Taiwan. It is a non-woven felt with a density of 250 g/m2, a
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thickness of 2.75 mm, a BET surface area of about 1100 m2/g, a pore volume of
0.45-0.55 cm3/g, and a mean pore diameter of 19-20 Å. The applications suggested by
the producers are VOC recovery and waste water treatment. The data of BET surface
area, pore volume and pore diameter from the manufacturers are approximate estimates,
which are proven in the results section. Carbon (~ 58.42 wt %) is the most abundant
element in the as-received ACF samples, and the contents of N and H are 2.57 and 3.58
wt %, respectively.
Chemical oxidation and thermal treatment
Fractions of as-received ACF, about 20 g in each of the batch, were immersed in H2O2,
H3PO4 or HNO3, under room temperature or 60 oC for 1 hour. After that, rinse out the
residuals from the ACFs and make ACFs dried. Then the ACF samples were
heat-treated inside an alumina combustion tube enclosed in a horizontal tubular furnace
under flowing industrial-grade nitrogen gas. After loading the samples the tubular
furnace was purged with nitrogen gas for 30 minutes to expel the air inside the furnace
tube. Then the samples were heated in an atmosphere of N2 to 400, 600, and 850 oC at a
rate of about 15 oC /min. The soaking period was 30 or 60 minutes. After thermal
treatment, the samples were cooled in flowing nitrogen gas until they reached ambient
temperature. The ACF samples treated are denoted in the text following the rule. The
chemical oxidation using H2O2, H3PO4 or HNO3 is denoted as O, P, or N. The time of
chemical oxidation treatment at room temperature or 60 oC is denoted as R or H. For
example, the ACF samples oxidized by H2O2 at 60 oC for 1 hour and heat-treated at 400
o
C for 30 minutes is denoted as OH400-30. Before conducting analyses of surface
physicochemical properties and adsorption capacities, the samples were preheated at
103 oC overnight in vacuo to eliminate the contaminants on the surface.
Analysis of physicochemical properties
Field emission-scanning electron microscope (FE-SEM) images
Surface microstructures of ACF samples were observed using the field emission scanning electron microscope (FE-SEM, Hitachi, S-4700, Type II). The specimens of
ACFs were coated with a layer of gold or platinum film to enhance the analytical
resolution. For each carbon, surface microstructure was inspected with 1000, 5000,
4
10,000 and 100,000 magnifications.
Specific surface area and pore texture
Adsorption isotherms of nitrogen gas on selected ACF samples were measured with an
Accelerated Surface Area and Porosimetry System (ASAP 2010 V4.02), and these
isotherms were used to estimate the surface area and the pore texture of ACFs. A small
portion of the carbon sample was weighed into a sample tube. Net weight of the carbon
sample was about 100 mg after degassing procedures. The purpose of degassing is
removal of contaminants from sample surface. During this step the sample was
controlled at 250 oC and degas vacuum level of 500 m Hg. The adsorption–desorption
isotherm of the nitrogen gas at 77 K was recorded against relative pressures from 0 to
1.0 for a period of about 7–12 hours. Surface areas of the samples were determined by
applying the BET and the Langmuir equations to the isotherm data. The amount of
nitrogen gas adsorbed at relative pressure near unity was employed to determine the
total pore volume. On the assumption that the pores were cylindrical and parallel, the
average pore diameter was obtained according to the BET surface area and total pore
volume.
Surface functional groups (XPS or ESCA)
The XPS spectra were obtained with a photoelectron spectrometer (Thermo
VG-Scientific, Sigma Probe), also denoted as Electron Spectroscopy for Chemical
Analysis (ESCA). A micofocus monochromator Al anode X-ray source was used (h =
1486.68 eV), The survey scans were collected from the binding energy of 0 to 1400 eV,
and the high resolution scans were performed over 275-295 eV. For calibration purposes,
the C 1s electron bond energy corresponding to graphitic carbon was referenced to
284.5 eV. A Shirley type background was chosen to be subtracted. After the base line
was subtracted, the curve-fitting was performed using the non-linear least-squares
algorithm under an optimized peak shape. This peak-fitting procedure was repeated
until an acceptable fit was obtained.
Total ash content
The total ash content of ACFs was determined by burning a portion of each carbon in
5
air at 650 oC until constant weight of the residue. The detailed procedures followed the
ASTM D2866-94 method.19 The pretreated ACF samples, about 300 mg, were put in the
crucible and placed in the muffle furnace at 650  25 oC for about 16 h. The weight
percentage of the residue from the original sample is defined as the total ash content.
Surface pH value
The determination of the pH value of ACFs followed the procedures suggested by the
ASTM D3838-80 method.20 A 20-mL portion of boiled de-ionized water was added to a
600-mg ACF sample in the flask equipped with a reflux condenser. The water
containing the ACF sample was heated to a boil on a hot plate and boiled gently for 900
 10 s. The contents were then filtered immediately. The pH value was determined after
the filtrate (10 mL) had been cooled to below 50  5 oC. A triplicate analysis was done
for each carbon sample.
Adsorption experiments
Toluene (C7H8) was selected to evaluate the adsorption capacities of as-received and
treated ACF samples. The saturated adsorbed amounts of adsorbate on ACFs were
determined according to the ASTM D3467-94 method.21 A small amount of ACF (about
300 mg) was weighed and placed into a U-shaped glass tube. The adsorption
temperature was controlled to 25 oC using a water bath. A nitrogen gas purge was
blown over the pure liquid VOC solvent such that an expected concentration of VOC
was generated continuously. The VOC concentration was controlled between about
1500 ppmv quantified by gas chromatography. The nitrogen gas carried the vapor into a
U-shaped sample tube containing a weighed ACF sample. The equilibrium adsorption
capacity of ACFs was evaluated after proceeding for 24 hrs.
RESULTS AND DISCUSSION
Morphology
Figure 1 shows the field emission-scanning electron microscope (FE-SEM) images of
parts of ACFs samples with a 100,000 magnification. Compared to as-received ACF
(in Figure 1a), the microporosity of ACFs can be improved after chemical oxidation or
6
thermal treatment. Especially, for as-received ACF, the micropores probably only occur
at the joins of the spunbonded fibers. The post-treatment, oxidation or heat treatment,
could generate more micropores at the joins or even on the surface of the fiber (in
Figure 1b). The longer period exposed at a specific heating temperature might make the
closure of micropores (e.g., Figure 1c and 1d) or the collapse of the microtexture, which
was usually observed for the samples treated at room temperature by chemical oxidation.
The higher the temperature of thermal treatment, the more the microporosity of the
ACFs which have been wet-oxidized (e.g., Figure 1e and 1f). This indicates that the
combination of chemical oxidation and thermal treatment can cause the formation of
new micropores; but if the period treated is too long, the microstructure would be
destroyed and thus to reduce porosity.
Figure 1: SEM images of as-received and treated ACF samples with a 100,000
magnification
(a) As-received ACF
(d) PR400-60
(b) OR600-60
(e) OH850-60
(c) PR400-30
(f) PH850-60
Compare the fiber diameters of the OH-, PH-, NR- and NH-series ACFs, shown in
Figure 2, it concludes that chemical oxidation or thermal treatment could reduce the
average width of the fibers. Although the oxidative activity of HNO3 is inferior to that
7
of H2O2, HNO3 behaviors a strong corrosion which makes the most narrow diameter of
the fibers. For the NH-series samples, chemical oxidations generate a wide variation of
the fiber width. The higher the reaction temperature, the more the active reactions.
Thermal treatments impose a higher impact on NH-series samples, while their effects on
OR- or OH-series samples are quite weakly.
Figure 2: Box plots of fiber width (As-received sample: 14 m)
Specific Surface Area and Pore Volume
The nitrogen adsorption/desorption isotherms at 77 K of selected ACFs samples are
shown in Figure 3. All adsorption isotherms appear to be Type I (Langmuir-type), which
is indicative of microporous materials. Each isotherm reveals that a hysteresis loop
happens, which belongs to the Type H4 of IUPAC classification. Type H4 has been
obtained with samples having slit-shaped pores or platelike particles. As P/Po
approaches to 1, the curve of the isotherms rise which could be attributed to the
occurrence of capillary condensation.
8
Figure 3: Adsorption isotherms of nitrogen gas on selected ACF samples at 77 K
Table 1 summarizes the surface characteristics of as-received and treated ACFs
estimated from the nitrogen adsorption isotherms at 77 K. Chemical oxidations will
eliminate the specific surface area and micropore volume. The reductions are according
to the order of HNO3, H2O2, and H3PO4. In addition, the higher the reaction temperature,
the more the decrease. After oxidations with HNO3, either at room temperature or 60 oC,
followed with thermal treatments would increase the specific surface area and the
micropore volume of the ACF samples. However, for the samples treated with H2O2 or
H3PO4, followed with a heat treatment at 400 oC for 30 min would generate the lowest
specific surface area. In OH-series samples, heat treatments could increase specific
surface area of about 11 %. The highest values of specific surface area and micropore
volume generally occur at the samples treated at 850 oC for 30 min, except for the
NH-series which happens on the sample treated at 850 oC for 60 min. To sum up,
NH850-60 possesses the highest values of specific surface area and micropore volume.
On the contrary, sample NH has the lowest specific surface area and micropore volume,
whose BET is 16 % lower than that of as-received ACF. Any oxidation or thermal
9
treatment process would enlarge the mean pore size.
Table 1. Surface micro-structure of ACF samples.
Langmuir
BET Surface
No.
ACF
OH
OH400-30
OH400-60
OH850-30
OH850-60
PH
PH400-30
PH400-60
PH850-30
PH850-60
NR
NR400-30
NR400-60
NR850-30
NR850-60
NH
NH400-30
NH400-60
NH850-30
NH850-60
Area (m2/g)
1086.99
995.06
993.52
1103.61
1184.82
1183.96
1036.61
999.76
1030.56
1037.35
945.53
1001.47
1067.00
1116.10
1190.39
1097.70
909.90
1059.50
1002.30
1155.03
1279.78
Surface Area
(m2/g)
1441.67
1317.18
1322.84
1468.21
1582.60
1569.15
1369.32
1321.37
1361.71
1371.65
1248.61
1323.03
1409.97
1477.50
1571.73
1449.90
1204.16
1400.54
1325.26
1527.52
1714.79
Total Pore
Mean Pore Size
Volume (cm3/g)
(Å)
0.5164
0.4760
0.4905
0.5374
0.5842
0.5764
0.5008
0.4775
0.4943
0.4978
0.4477
0.4788
0.5148
0.5385
0.5726
0.5274
0.4382
0.5141
0.4840
0.5571
0.6307
19.0
19.1
19.7
19.5
19.7
19.5
19.3
19.1
19.2
19.2
18.9
19.1
19.3
19.3
19.2
19.2
19.3
19.4
19.3
19.3
19.7
Based on the adsorption isotherms of nitrogen, the distribution of pore volume primarily
concentrates on the micropore-size range. Chemical oxidation could make pore become
larger or generate more surface oxides, thus reduce the specific surface area and
micropore volume. The reduction is related to the oxidant strength or reaction
temperature. Low thermal treatment (400 oC) without oxygen would have a further loss
of specific surface area and micropore volume. On the other hand, high thermal
treatment (850 oC) without oxygen will exhibit a higher specific surface area and
micropore volume, due to the generation of new micropore or the decomposition of
10
surface oxygen-containing functional groups. In this case, it needs to be noted that too
long reaction time could have an adverse effect on the development of pore structure.
Surface Functional Groups
XPS data gives information about the composition of the most external surface of ACFs.
XPS experiments were performed on three selected samples, i.e., as-received ACF, NH,
and NH850-60. The O 1s/C 1s atomic ratios from the survey scans show that the
as-received ACF displays a smaller value (0.13). After HNO3 oxidation at 60 oC for 1
hour, the ratio could increase to 0.17. Once continued a thermal treatment at 850 oC for
60 min, the ratio will go up to 17.23. Figure 4 illustrates the high-resolution XPS
spectra of the C 1s region. With all carbons, the C1s signal exhibited an asymmetric
tailing. This is partially due to the intrinsic asymmetry of the graphite peak and to the
contribution of oxygen surface complexes. Deconvolution of the C 1s spectra gives five
peaks that represent graphite carbon (peal I, 284.5 eV), carbon present in phenolic,
alcohol, ether or C=N groups (peak II, 286.3 eV), carbonyl or quinine groups (peak III,
287.9 eV), carboxyl or ester groups (peak IV, 289.3 eV), and shake-up satellite peaks
due to -* transitions in aromatic rings (292.7 eV). The calculated percentages of
graphitic and functional carbon atoms are shown in Table 2. There is a slightly increase
in the relative content of graphitic carbon and a significant rise in the relative content of
carbonyl or quinine groups after HNO3 oxidation at 60 oC for 1 hour. Correspondingly,
the relative contents of phenolic, carboxyl, or the carbons present with -electrons are
decreased. A dramatic change occurred at sample NH850-60. Compared to as-received
ACF, the relative contents of graphitic carbon, and carbonyl or quinine groups increase
27 and 265 %, respectively. The percentage of carboxyl groups decreases 65 %, and the
contents of phenolic and carbon with -electrons have been eliminated completely.
11
Figure 4: XPS C1s spectrum of ACFs
(a) ACF
(b) NH
(c) NH850-60
Table 2. The percentages of surface functional groups on ACFs by deconvolution of the
C 1s spectra.
Surface functional groups
Binding
energy (eV)
As-received
ACF
NH
NH850-60
Graphitic carbon
284.5
68.65 %
68.79 %
86.88 %
Phenolic, alcohol, ether or C=N
groups
286.3
14.57 %
13.23 %

Carbonyl or quinine groups
287.9
2.30 %
4.6 %
8.4 %
Carboxyl or ester groups
289.3
13.39 %
12.85 %
4.72 %
Carbon present with -electrons
292.7
1.08 %
0.54 %

12
Total Ash Contents
Figure 5 gives the total ash content of ACFs. It should be noted that the total ash content
of as-received ACF is about 3.75 %; however, the chemical oxidations and thermal
treatments will significantly reduce the amount of total ash content. The average ash
contents of OH- and NH-series samples are higher than the corresponding OR- and
NR-series. This implies that the surface oxides generated during oxidation could prevent
the structure from attack during thermal treatment. Aside from the samples treated at
600 oC, the amounts of total ash content of ACFs heat-treated will increase as the
reaction temperature or the retention time increases. This could be attributed to the loss
of CO, CO2 or other volatiles during thermal treatments; thus, increasing in the content
of mineral materials in the residue.
Figure 5: Box plots of total ash contents in various series of ACF samples
pH Values
The as-received ACF has a pH value of 3.27, and after oxidation with H2O2, the pH
values will be increased. However, as the samples were oxidized with HNO3 or H3PO4
at 60 oC rather than at room temperature, they could have a higher pH value. The box
plots of measured pH values are summarized in Figure 6. As the reaction temperature
increases, the pH value increases at first and then decreases, with a maximum value
occurs at 600 oC. The increase in the period of thermal treatments generally reduce the
13
pH value, which could be attributed to the occurrence of unstable acidic hydrogen. But
when the samples treated at 850 oC, the surface pH values become higher as the
treatment duration increases. This finding is speculated on the fact that most of the
surface acidic functional groups could have been exhausted under treated at 850 oC.
Figure 6: Box plots of pH values of ACF samples after various HTT conditions
Adsorption Capacity
The equilibrium adsorption amounts of as-received and selected treated ACFs for
toluene (C7H8) of about 1500 ppmv at 25 oC were performed. The adsorption amount of
toluene on as-received ACF is 30.8 g/100g, but there is a wide variation with
progressive treatments. For H2O2-treated ACFs, the adsorption amounts of toluene vary
from 5.4 to 42.2 g/100g. For H3PO4-treated ACFs and HNO3-treated ACFs, the values
range from 11 to 41.3 and from 14.2 to 48.5 g/100g, respectively. The maximum
amount of toluene adsorption occurred on the sample treated with HNO3 at 60 oC for 1
hour and followed with thermal treatment at 850 oC for 60 min. If followed a thermal
treatment at low temperature merely after the chemical oxidation, the adsorption
capacity of ACFs could not be able to be improved. The adsorption capacities of the
samples treated with H3PO4 are compatible with those treated with H2O2 at 60 oC for 1
hour. But the capacities of the samples treated with H3PO4 would be worse when
treatments occur at room temperature. The adsorption amounts of toluene on selected
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ACF samples seem to be related to the calculated BET surface area. Figure 7 illustrates
the relationship between the BET surface areas and the adsorption amounts of toluene
on the corresponding ACFs samples. It displays a middle degree of correlation between
these two parameters.
Figure 7: The relationship of BET surface area and adsorption capacity of toluene
CONCLUSIONS
The results show that the chemical oxidations would decrease in specific surface area
and pore volume with reaction temperature. Nevertheless, the oxidation followed by
thermal treatment would extend the specific surface area of ACFs. For instance, the
specific surface areas of the NR series and NH series increased 18 and 41 %. But the
soaking period of 60 min at 850 oC produced the closure of micropores. The increase of
micropore volume after thermal treatment primarily occurred in the neighborhood of
indentations on the fiber surface, which was observed from the FE-SEM images.
Because of high reactivity of HNO3, the ACF samples treated by HNO3 possessed a
thinner fiber, a lower pH value, and a less ash amount; but the effect on the amount of
15
surface functional groups was insignificant. The higher the temperature of chemical
oxidation, the higher the pH value, and total ash content. With HNO3 oxidation
followed by 850oC treated for 60 min, there is an increase in the amount of carbonyl
groups on ACFs; however, a significant decrease in the fraction of carboxyl groups has
been observed and the amounts of phenolic groups has been completely exhausted.
Finally, the amounts of toluene adsorption revealed that the adsorption capacities of
ACFs were consistent with the specific surface area and pore volume.
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KEYWORDS
Activated carbon fiber; Physicochemical property; SEM; XPS; BET; Adsorption
17