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Adsorption of carbon dioxide by sodium hydroxide-modied granular
coconut shell activated carbon in a xed bed
Y.L. Tan a , Md. Azharul Islam a , b, M. Asif c , B.H. Hameed a , *
a School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysiab Foretsry and Wood Technology Discipline, Khulna University, Khulna 9208, Bangladeshc Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
a r t i c l e i n f o
Article history:
Received 2 May 2014
Received in revised form
19 August 2014
Accepted 29 September 2014
Available online 29 October 2014
Keywords:
Fixed-bed
Adsorption
CO2Breakthrough
NaOH-modied activated carbon
a b s t r a c t
In the present work, commercial coconut shell activated carbon was impregnated with alkaline NaOH to
investigate the ef ciency of modied activated carbon for CO2 adsorption in a xed-bed column
adsorption system. The modication parameters, such as the NaOH concentration (24e48%) and
dwelling time (1e4 h), were also investigated. The results showed that a 32% NaOH concentration with a
3 h dwelling time provided the best CO2 adsorption capacity. Later, the modied activated carbon was
characterized by nitrogen adsorptionedesorption, scanning electron microscopy and Fourier transform
infrared spectroscopy. The effects of the CO2 % in the feed, the adsorption temperature, the feed ow rate
and the amount of adsorbent in the column were investigated in the adsorption experiments. The
maximum CO2 adsorption capacity in this study was 27.10 mg/g at 35 C. This study also suggests that
NaOH-modied activated carbon is a state-of-the-art adsorbent for CO2 adsorption.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
GHGs (global warming and greenhouse gases) are highly dis-
cussed topics around the world. Of all the GHGs, CO2 has been
regarded as a major contributor to the present global warming
trend and is mainly emitted from anthropogenic sources. Thus,
global awareness about CO2 emissions has increased during the last
decades, leading to increasing efforts to reduce their environmental
impact, including preventive and remediation methods. To combat
CO2, innovative CO2 capturing technologies are needed. Several
techniques have been proposed to capture CO2, including chemical
absorption, physical adsorption and membrane separation [1e5].
Of these techniques, chemical absorption with an aqueous solution
of alkanolamines, such as monoethanolamine and diethanolamine,is the most popular [6e8]. However, there are some problems
associated with this technique, such as high corrosion, oxidative
degradation of absorbents, and foaming in the gaseliquid interface
[9]. Adsorption is a widely used technology for gas treatment due to
its versatility and ef ciency. For this method, solid porous adsor-
bents, such as activated carbon, can act as suitable alternatives for
capturing CO2 because activated carbons are insensitive to
moisture, have large surface areas and have distinct porosities, ul-
timately leading to high adsorption capacities. Moreover, the
regeneration of ACs (activated carbons) is possible at a low cost.
The surface chemistry of the activated carbon can be modied
by chemical activation to increase the adsorption capacity.
Recently, several excellent research initiatives have been taken to
removal of CO2 by modied activated carbon as an adsorbent.
Caglayan and Aksoylu [10] observed the substantial adsorption
capacity of CO2 by commercial activated carbon through modied
with HNO3 oxidation, air oxidation, alkali impregnation and heat
treatment under helium gas atmosphere. In another study, by Yin
and other coauthors [11] reported that the effect of N2 has little
inuence of the CO2 adsorption on activated carbon. Three types of
sterically hindered amines were successfully impregnated onto thesurface of palm shell-based activated carbon and found higher
adsorption capacity than virgin activated carbon [12]. Activated
carbonwas modied by washing with acid mixture of concentrated
HNO3 and H2SO4, and it was found that the adsorption capacities of
the modied samples were increasedupto 70% [13]. Inonestudyby
Shafeeyan et al. [14] reported that the commercial GAC (granular
activated carbon) became an ef cient adsorbent for CO2 when
modied by using thermal treatment with ammonia. But activated
carbon modied with NaOH for CO2 adsorption is scarce in the
literature. Because CO2 is an acidic gas, treatment with an alkaline
compound can provide more active functional sites for CO2.* Corresponding author. Tel.: þ60 45996422; fax: þ60 45941013.
E-mail address: [email protected] (B.H. Hameed).
Contents lists available at ScienceDirect
Energy
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m/ l o c a t e / e n e r g y
http://dx.doi.org/10.1016/j.energy.2014.09.079
0360-5442/©
2014 Elsevier Ltd. All rights reserved.
Energy 77 (2014) 926e931
mailto:[email protected]://www.sciencedirect.com/science/journal/03605442http://www.elsevier.com/locate/energyhttp://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://dx.doi.org/10.1016/j.energy.2014.09.079http://www.elsevier.com/locate/energyhttp://www.sciencedirect.com/science/journal/03605442http://crossmark.crossref.org/dialog/?doi=10.1016/j.energy.2014.09.079&domain=pdfmailto:[email protected]
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Activation with sodium hydroxide (NaOH) is more effective in the
preparation of activated porous carbon because NaOH is inexpen-
sive, minimally corrosive and environmentally friendly [15,16].
In the present work, granular coconut shell activated carbonwas
modied with NaOH for CO2 adsorption. The effects of different
NaOH concentrations, contact times, gas ow rates, temperatures,
and CO2 concentrations on the adsorption process were also
studied in a xed-bed adsorption mode.
2. Materials and methods
2.1. Materials
Coconut shell-based activated carbon was purchased from Laju
Carbon Products Sdn. Bhd., Malaysia. Granular activated carbons
with particle sizes ranging from 0.85 to 0.425 mmwere used in this
study. Sodium hydroxide (NaOH) was purchased from Merck
Chemical Company, and puried carbon dioxide (99.98%) and ni-
trogen (99.995%) as carrier gases were supplied by Wellgas Sdn.
Bhd., Malaysia.
2.2. Preparation of the NaOH e AC adsorbent
Commercial coconut shell-based activated carbon was modied
by sodium hydroxide (NaOH) impregnation. For this purpose, 10 g
of AC was soaked in 100 mL of different strength of aqueous NaOH
solutions (24e48%) and shaken in a water bath shaker for a pre-
determined dwelling time (1e4 h) at room temperature and
40 rpm. The sample was then ltered and washed with deionized
water. After washing, the sample was dried overnight in an oven at
105 C. The prepared samples were named according to the usage
of NaOH% and dwelling time as 24ACSH3, 32ACSH3, 40ACSH3 and
48ACSH3 for 24%, 32%, 40% and 48% NaOH, respectively.
2.3. Carbon dioxide xed-bed column adsorption procedures
A known amount of adsorbent was placed into the xed-bedcolumn (l ¼ 42 cm and d ¼ 1.1 cm), and the column was heated
to 110 C and held for 1 h under a constant nitrogen ow rate of
90 mL/min to remove excess moisture. At the same time, nitrogen
gas and carbon dioxide gas were mixed in a steel mixer and owed
through the analytical system. The inow of each gas from the
cylinders were controlled by a calibrated mass ow controller
(AALBORG, model AFC26 NY, USA). After heating, the column was
cooled to 35 C. The nitrogen gas purging was stopped, and the
adsorption columnwas vacuumed. Then, the mixed gas waspurged
upward into the column. The CO2 concentration was measured and
recorded every 10 s by an online carbon dioxide analyzer model
906 (Quentek instrument, USA). A blank experiment was also
performed following the same procedure as described above
without the adsorbent in the column and found no CO 2 adsorptionby the column.
The experiment was repeated with different parameters, such as
varying reaction temperatures, total gas ow rates, adsorbent
loadings and carbon dioxide concentrations, to optimize the reac-
tion conditions. All experiments were conducted under conditions
of 0.1 Mpa (atmospheric) pressure and the standard deviation was
used as the value of the experimental error.
2.4. Regeneration
The regeneration study was carried out by performing the car-
bon dioxide adsorption with the sample that gave the optimum
result. The rst adsorption cycle was performed as mentioned in
section 2.3. When the
rst cycle was completed, the furnace
temperature was increased to 110 C and held for a half hour with
nitrogen gas purging. After heating, the temperature was reduced
to 35 C and vacuumed. Then, a gas mixture with the same con-
centration and ow rate as the rst cycle was purged upward from
the bottom of the column. The concentration of the outlet gas was
determined and recorded. The procedures were repeated for a few
cycles to determine the reusability of the adsorbent.
2.5. Characterization of the adsorbents
The surface area and porosity properties of the AC and NaOH-
modied AC were determined by nitrogen (N2) adsorp-
tionedesorption at 196 C (77 K) with a saturation pressure of
106.65 kPa using an automated gas sorption system (Micromeritics,
Model ASAP 2020, USA). The BET surface area, Langmuir surface
area, cumulative pore volume, micropore volume and average pore
width were obtained from the N2 adsorption isotherms. The
external surface area, micropore surface area and micropore vol-
ume were evaluated using the t-plot method. The cumulative pore
volume was calculated by measuring the amount of N2 adsorbed at
a relative pressure of 0.984. The BJH (Barrette JoynereHalenda)
method was used to determine the average pore width and pore
size distribution.FT-IR (Fourier transform infrared) spectroscopy analysis was
performed using an FT-IR spectrometer (Perkin Elmer, Model 2000
FTIR, USA) to identify the surface functional groups on the AC and
NaOH-modied ACs. The surface morphologies of the unmodied
and modied AC were examined, and their porosities were veried
using a scanning electron microscope (SEM, JEOL JSM- 6460LV,
Japan).
3. Results and discussion
3.1. Characterization of the adsorbent
The BET surface areas and average pore sizes of selected ad-
sorbents are presented in Table 1. The values of the BETsurface areaand micropore area after modication by 32% NaOH decreased
drastically compared to the unmodied AC. This observation is
attributed to the structural changes of the adsorbents due to the
entrapment of NaOH in the micropore area, which reduced the
ultimate surface area of the adsorbents during modication with
high temperatures [17]. On the other hand, after the modication,
the poresizeof AC increased from2.89 nm to 4.12nm. The probable
mechanism behind the large pore size due to NaOH activation is as
follows.
2Cþ 6NaOH/2Naþ 2Na2CO3 þ 3H2 (1)
with elevated temperature, Na2CO3 degraded into CO2 and H2O,
which were entrappedand created wider holes and surface become
less acidic in presence of Naþ by replacing carboxylic and phenolsions [18].
Fig. 1 shows the SEM images of the unmodied and modied
granular activated carbon. The morphological structures appear as
Table 1
Surface area and average pore size of the prepared adsorbent.
Sample BET surface
areaa (m2/g)
Pore sizeb
(nm)
Micropore areac
(m2/g)
AC 787.65 2.89 555.80
32ACSH3 378.23 4.12 277.42
a Obtained by BET measurement.b Calculated by the BJH (desorption) method using N2 adsorption isotherms.c
Obtained by the t-plot method.
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wavy ripple marks (Fig.1a) beforeNaOH modication, but after 32%
NaOH modication with a 3 h dwelling time, the morphological
structure changed. The 32ACSH3 adsorbents showed highly
cracked surfaces with a distinct pore size, indicating the suitability
of CO2 adsorption onto these pores.
The FT-IR spectrum of the adsorbent is important for evaluating
the active functional groups on the surface of the raw material and
on the adsorbent. The spectra of the different NaOH-modied ACs
are displayed in Fig. 2. All of the FT-IR curves were similar. The
broad band at approximately 3500 cm1 is usually ascribed to
hydroxyl groups (eOH). The band located at approximately
1650 cm1 corresponds to amide (eNeH) groups. Some distinct
strong peaks at 1400 cm1 were attributed to the presence of some
carboxylates (C]O) and asymmetric nitro functional groups on all
modied adsorbents.
3.2. Parameters affecting the modi ed adsorbents
The granular coconut shell activated carbon was modied by
different strengths of 24%, 32%, 40% and 48% NaOH at a constant
dwelling time (3 h). The prepared NaOH-modied activated car-
bons were later used as adsorbents to investigate their perfor-
mance for CO2 adsorption. As shown in Fig. 3 the 32% NaOH-modied adsorbent (32ACSH3) showed the best performance for
CO2 adsorption. The unmodied AC (without NaOH loading) can
adsorb 17.52 mg/g CO2, whereas the 24%, 32%, 40% and 48% NaOH-
modied adsorbents can adsorb 24.10, 27.10, 21.69, and 23.24 mg/g
CO2, respectively. Again, the 32% NaOH-enriched activated carbon
modication was performed with various dwelling times (1e4 h).
As shown in Fig. 4 a 3 h dwelling time provides better CO2adsorption compared to the other contact times. Thus, the
32ACSH3 adsorbent was used for further studies. Moreover, the
longest breakthrough time corresponds to the 32ACSH3, indicating
the porous structure of the adsorbent. The CO2 adsorption was also
increased due to the formation of carbonates between the carbon
dioxide molecules and the sodium on the surfaces of the modied
activated coconut shell (32ACSH3) [19].
Fig. 1. SEM micrographs of the prepared adsorbents: (a) unmodied AC (magnication ¼ 1000), (b) 32ACSH3 (magnication ¼ 1000), (c) unmodied AC
(magnication ¼ 10000) and (d) 32ACSH3 (magnication ¼ 10000).
40080012001600200024002800320036004000
% T r a n s m i t t a n c e
wavenumber (cm-1)
(a)
(b)
(c)
(d)
(e)
Fig. 2. FT-IR spectra for (a) unmodied AC, (b) 24ACSH3, (c) 32ACSH3, (d) 40ACSH3,
and (e) 48ACSH3.
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3.3. Factors affecting xed-bed column study
3.3.1. Effect of total gas ow rate on CO 2 adsorption
Three gas ow rates of 90, 120 and 150 mL/min into the
adsorption column were compared against the 32ACSH3 adsorp-
tion capacity. As shown in Fig. 5, the slowest ow rate of 90 mL/min
produced a longer breakthrough time compared with the highest
ow rate of 150 mL/min. The longer dwelling times at lower ow
rates allow for slower CO2 diffusion, which leads to a higher
adsorption capacity of 27.10 mg/g in comparison with 25.38 and
25.73 mg/g for 120 and 150 mL/min, respectively. The high ow
rate of gases saturated the adsorption column very quickly, which
was associated with higher mass transfer coef cients [20].
3.3.2. Effect of the adsorption temperature on CO 2 adsorption
The effects of different adsorption temperature, including 35 C,
45
C and 55
C, on CO2 adsorption by 32ACSH3 are presented inFig. 6. The adsorption capacity at 35 C is 27.10 mg/g, and it grad-
ually decreases from 24.3 mg/g to 16.62 mg/g with increasing
temperature from 45 C to 55 C. Shorter breakthrough times and
consequently lower breakthrough values at higher adsorption
temperatures may be attributed to the fact that the adsorption
capacity of the adsorbent is reduced at elevated temperatures. This
trend of lower adsorption capacity with high temperature indicates
that the adsorption of CO2 on 32ACSH3 is physisorption. Moreover,
according to the FT-IR analysis, the surface of the 32ACSH3
contained some N2 functional groups, which may be responsible for
the CO2 adsorption at low temperature [14]. The presence of so-
dium molecules on the surface of 32ACSH3 might absorb CO2 and
form Na2CO3 at low temperature, but if the temperature increases,
then Na2CO3 decomposes and converts into CO2 and H2O, which
ultimately reduce the CO2 adsorption.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
C / C 0
Time (s)
32ACSH1
32ACSH2
32ACSH3
32ACSH4
Fig. 4. Effect of the contact time of the 32% NaOH solution with AC on CO 2 adsorption
(Reaction condition: gas ow rate: 90 mL/min; reaction temperature: 35 C; concen-
tration of CO2: 10%; adsorbent loading: 3 g).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
C / C 0
Time (s)
90 mL/min
120 mL/min
150 mL/min
Fig. 5. Effect of the total gas ow rate on CO2 adsorption by 32ACSH3. (Reaction
condition: adsorption temperature ¼ 35 C; concentration of CO2 ¼ 10%; adsorbent
loading ¼ 3.0 g).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
C / C 0
Time (s)
35 C
45 C
55 C
Fig. 6. Effect of the adsorption temperature on CO2 adsorption by 32ACSH3 (reaction
condition: gas ow rate ¼ 90 mL/min; concentration of CO2 ¼ 10%; adsorbent
loading ¼ 3.0 g).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
C / C 0
Time (s)
Unmodified AC
24ACSH3
32ACSH3
40ACSH3
48ACSH3
Fig. 3. Effect of the NaOH concentration on the CO2 adsorption (Reaction condition:
gas ow rate: 90 mL/min; reaction temperature: 35 C; concentration of CO2: 10%;
adsorbent loading: 3 g).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600 700 800 900
C / C 0
Time (s)
3.0 g
4.5 g
6.0 g
Fig. 7. Effect of adsorbent loading on CO2 adsorption by 32ACSH3 (reaction condition:
gas ow rate ¼ 90 mL/min; adsorption temperature ¼ 35 C; concentration of
CO2 ¼
10%).
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3.3.3. Effect of adsorbent loading on CO 2 adsorption
The effects of different 32ACSH3 loadings on CO2 adsorption are
illustrated by the breakthrough curve shown in Fig. 7. Three
different amounts of 32ACSH3 adsorbents, 3, 4.5 and 6 g, were
selected for CO2 adsorption. According to the data presented in
Table 2, increasing the adsorbent loading did not increase the
adsorption capacity of CO2. The highest adsorption capacity of
27.01 mg/g was found for the lowest 32ACSH3 loading of 3 g, and
further loading from 4.5 to 6 g did not substantially increase the
CO2 adsorption capacity (17.33 and 14.07 mg/g). The higher
adsorbent loading means higher total surface area of 32ACSH3, but
fewer CO2
molecules were adsorbed onto the surface per gram unit
of 32ACSH3, which led to lower adsorption capacity at high
adsorbent loading.
3.3.4. Effect of the CO 2 concentration on CO 2 adsorption
The effect of various CO2 concentrations (10, 15 and 20%) in the
feed with N2 were studied to determine the adsorption capacity of
32ACSH3. The breakthrough curves for different CO2 concentration
in the adsorption are presented in Fig. 8. The percentage increase in
the CO2 feed ow also increased the adsorption capacity. For
instance, the 10, 15 and 20% CO2 concentrations contributed to
27.10, 29.69 and 34.18 mg/g CO2 adsorption, respectively. The as-
prepared and modied 32ACSH3 adsorbents have highly porous
active sites that can easily accommodate the higher number of CO2molecules on its surface. Moreover, an increase in the CO 2 feed also
increases the concentration gradient, which overcomes the mass
transfer resistance and allows higher adsorption capacity on the
32ACSH3 [9].
3.4. Regeneration of adsorbents
The possibility of regenerating the adsorbent after the rst cycle
of CO2 adsorption was studied. For this purpose, the spent adsor-
bent (32ACSH3) was regenerated by heating the reactor at 110 C,
which was similar to the adsorbent pretreatment column temper-
ature (110 C) for 1 h under N2 gas purge at a ow rate of 90 mL/
min. The breakthrough curve for the cyclic CO2 adsorption by32ACSH3 is shown in Fig. 9. The desorption of the attached CO2molecules on the surface of 32ACSH3 was attributed to initial
physical adsorption into the pore volume of the CO2. According to
the experimental data presented in Fig. 10, the adsorption capacity
of fresh sample was 27.10 mg/g, and the adsorption capacity later
decreased to 19.30 mg/g and 17.33 mg/g at the second and third
cycles of regeneration. Similar performance of the adsorbents
continues until ten cycles. The variation in the adsorption capacity
of the 32ACSH3 adsorbent at the rst cycle compared to the other
Table 2
Breakthrough time and adsorption capacity for the different investigated factors.
Parameters Values Adsorption capacity,
mg/g
NaOH loading,% strength 0 17.52 (±0.310)
24 24.10 (±0.302)
32 27.10 (±0.362)
40 21.69 (±0.320)
48 23.24 (±0.354)Contact time, h 1 21.53 (±0.319)
2 22.03 (±0.319)
3 27.10 (±0.362)
4 21.91 (±0.330)
Adsorption temperature, C 35 27.10 (±0.362)
45 24.03 (±0.266)
55 16.62 (±0.329)
Flow rate, mL/min 90 27.10 (±0.306)
120 25.38 (±0.440)
150 25.73 (±0.567)
Adsorbent loading, g 3.0 27.10 (±0.217)
4.5 17.33 (±0.206)
6.0 14.07 (±0.156)
CO2 concentration (feed), % 10 27.10 (±0.008)
15 29.69 (±0.016)
20 34.18 (±0.047)
Figure in parentheses is the respective standard deviation.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 100 200 300 400 500 600
C / C 0
Time (s)
10%
15%
20%
Fig. 8. Effect of the CO2 concentration on CO2 adsorption by 32ACSH3 (reaction con-
dition: gas ow rate ¼ 90 mL/min; adsorption temperature ¼ 35 C; adsorbent
loading ¼
3.0 g).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50
C / C 0
Time (min)
Fig. 9. Breakthrough curve for cyclic CO2 adsorption by 32ACSH3 (reaction condition:
gas ow rate ¼ 90 mL/min; adsorption temperature ¼ 35 C; CO2 concentration: 10%;
adsorbent loading ¼ 3.0 g).
0
10
20
30
1 2 3 4 5 6 7 8 9 10
A d s o r p t i o n c a p a c i t y ( m g / g )
No. of cycles
AC
32ACSH3
Fig. 10. Comparison of the CO2 adsorption capacity on the regenerated 32ACSH3
adsorbent with non-modied AC (reaction condition: gas ow rate ¼ 90 mL/min;
adsorption temperature ¼
35
C; CO2 concentration: 10%; adsorbent loading ¼
3.0 g).
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cycles was attributed to the occupying of the 32ACSH3 pores by
some chemically bonded CO2 molecules with existing residual so-
dium on the surface of the adsorbent. The performance of the
adsorbent after different cycles of regeneration also depends on
different adsorption parameters, such as the ow rate, CO2 con-
centration, and temperature. Therefore, this study suggests that the
adsorbent can be reused for several cycles successfully under mild
regeneration conditions.
4. Conclusion
The coconut shell activated carbon was modied by NaOH
(32ACSH3) with 3 h of dwelling time and was studied for CO2adsorption in a xed bed under different experimental conditions,
including ow rate, adsorption temperature and feed concentra-
tion. The xed-bed column adsorption experiment revealed that
the feed ow rate of 90 mL/min, adsorbent loading of 3.0 g, CO2concentration (feed) of 15%, and adsorption temperature of 35 C
were optimum for CO2 adsorption. Furthermore, the 32ACSH3 can
be completely regenerated under moderate conditions (i.e., 1 h at
110 C under N2 purge at a ow rate of 100 mL/min). Therefore,
activated carbon modied with alkaline NaOH, i.e., 32ACSH3, offers
outstanding properties in terms of high CO2 capacity and regen-eration, and this system can provide further insight into other
modied activated carbons for ef cient CO2 adsorption.
Acknowledgments
The support of Distinguished Scientist Fellowship Program
(DSFP) at the King Saud University is greatly appreciated by Prof. M.
Asif.
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