Synthesis of novel and tunable Micro Mesoporous carbon nitrides for Ultra High CO2 and H2S capture.pdf

Chemical Engineering Journal 456 (2023) 140973
Available online 13 December 2022
1385-8947/© 2022 Elsevier B.V. All rights reserved.
Synthesis of novel and tunable Micro-Mesoporous carbon nitrides for
Ultra-High CO2 and H2S capture
Farnaz Tabarkhoon a
, Hossein Abolghasemi a,*
, Alimorad Rashidi b,*
, Mohammad Bazmi c
,
Masood S. Alivand d,e
, Farnoush Tabarkhoon f
, Mehran Vashaghani Farahani b
,
Mehdi D. Esrafili g
a
School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box 11365-4563, Iran
b
Nanotechnology Research Center, Research Institute of Petroleum Industry, Tehran, Iran
c
Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, University Park, Los Angeles, CA 90089-1211, USA
d
Department of Chemical Engineering, Monash University, Victoria 3800, Australia
e
Department of Chemical Engineering, The University of Melbourne, Victoria 3010, Australia
f
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), No. 424 Hafez Ave., Tehran, Iran
g
Department of Chemistry, Faculty of Basic Sciences, University of Maragheh, Maragheh, Iran
A R T I C L E I N F O
Keywords:
CO2 capture
H2S removal
Carbon nitride
Adsorption
Tunable porosity
DFT
A B S T R A C T
Carbon nitride (CN) materials with intrinsic high nitrogen content are potential candidates for acidic gas
adsorption. However, these nanomaterials should be further treated to achieve tunable textural properties for
ultra-high gas adsorption. Herein, we synthesized dual-pore carbon nitride materials (DP-CN) with a series of
ethylenediamine to carbon tetrachloride ratios with different amounts of potassium hydroxide (KOH) as a
chemical activator using nanosilica (SiO2) as a hard template to tune the physicochemical properties of the
materials. The prepared DP-CN adsorbents had a large surface area (up to 2036.9 m2
/g), great pore volume (up
to 1.15 cm3
/g), and high nitrogen content (10.6 to 15.1 wt%). The best DP-CN displayed ultra-high CO2 and H2S
adsorption capacity at 1 bar (8.3 and 13.8 mmol/g, respectively), 10 bar (16.9 and 23.1 mmol/g, respectively),
and 30 bar (22.9 mmol/g for CO2) at 25 ◦
C, which was significantly higher than those of other pure mesoporous
carbon nitrides (M-CN) and carbon-based adsorbents. Moreover, the best adsorbent exhibited good CO2/N2,
CO2/CH4, H2S/N2, and H2S/CH4 selectivity, suitable heat of adsorption, and excellent cyclic stability. According
to density functional theory calculations, H2S adsorbs more strongly than CO2 on carbon nitride surfaces, and the
adsorption energies of CO2 and H2S are related to charge-transfer values from the surface to the adsorbed species.
The results revealed that the exceptional textural properties and high nitrogen content of the materials could play
the main role in the superior adsorption of CO2 and H2S. This generation of CN materials is expected to be
practical for a various range of separation processes, catalysis, capacitors, and energy storage.
1. Introduction
Over the last decades, the demand for energy has dramatically
increased due to human activities, population growth, and industriali
zation [1]. Burning fossil fuels, as one of the predominant sources of
energy, has always had major environmental implications. Carbon di
oxide (CO2) and hydrogen sulfide (H2S) are two critical pollutants that
are released into the environment during fossil fuel consumption [2].
CO2 is one of the main components of greenhouse gas (GHG), causing
severe natural disasters, such as global warming, rising seawater levels,
and desertification of green lands [3]. H2S is another hazardous gas
mainly coming from the beginning steps of oil and gas processing. H2S is
toxic, malodorous, and detrimental to living species and can be a serious
issue for industrial facilities owing to its corrosive nature [4,5]. Thus,
CO2 and H2S capture technologies have to be promptly taken into
consideration.
There are several methods to harness the progressive emission of CO2
and H2S, including physical and chemical absorption [6,7], adsorption
[8,9], membrane separation [10,11], catalytic oxidation [12,13], and
cryogenic distillation [14]. Among all the methods, chemical absorption
* Corresponding authors.
E-mail addresses: hoab@ut.ac.ir (H. Abolghasemi), rashidiam@ripi.ir (A. Rashidi).
Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
https://doi.org/10.1016/j.cej.2022.140973
Received 23 August 2022; Received in revised form 26 November 2022; Accepted 12 December 2022

2
by alkanolamines is the most common technique that has been widely
investigated [15,16]. However, this method has some drawbacks, such
as low thermal stability, high operating cost, solvent loss, and corrosion
[17,18]. Contrarily, the adsorption method can cover most of these is
sues, making it a promising approach for CO2 and H2S capture [19].
Zeolites [20–22], polymers [23], metal–organic frameworks (MOFs)
[24–26], covalent organic frameworks (COFs) [27], and carbon-based
materials [28–32] have been widely utilized for CO2 and H2S removal.
Carbon-based materials have attracted great attention over time owing
to their adjustable pore structures, good thermal and mechanical sta
bility, and low production cost [33]. However, CO2 and H2S adsorption
on the surface of carbon-based nanomaterials is usually governed by
weak Van der Waals interactions leading to some limitations on the
acidic gas adsorption capacity. One of the techniques to overcome this
challenge and improve the interaction of acidic gas molecules with the
surface of carbon-based nanomaterials is the introduction of basic het
eroatoms, such as nitrogen groups, through the structure, mainly by
choosing a nitrogen-containing precursor for the synthesis of nano
porous carbons [34]. This can be carried out in two approaches: i) post-
synthesis treatment with nitrogen-rich compounds and ii) one-step
synthesis in the presence of nitrogen-rich precursors [35,36]. The first
method is less favorable due to the involvement of multi-step sample
preparation to create nitrogen functional groups on the surface and pore
blockage, causing a decrease in the surface area and gas adsorption
capacities. Moreover, the materials obtained from this method often lack
stability [37,38]. Therefore, adsorbents synthesized by the latter method
have been widely studied.
Nanoporous carbon nitride prepared by one-step synthesis is a
nitrogen-rich material with adjustable textural properties and good
mechanical and thermal stability that can be a potential candidate for
CO2 and H2S adsorption [39,40]. Vinu [41] showed that the nitrogen
content of mesoporous carbon nitride (MCN) could be easily adjusted by
changing the ratio of precursors (ethylenediamine and carbon tetra
chloride). Moreover, it was suggested that the textural properties of
MCN can be controlled by using different templates. Accordingly,
various hard templates, such as SBA-15 [37,38], KIT-6 [42,43], FDU-12
[44], and mesoporous cellular silica foams (MCF) [45], have been uti
lized to improve the textural properties of MCNs. Among all the studies,
the highest CO2 adsorption capacity is about 2.35 and 13.12 mmol/g at
1 and 30 bar at 25 ◦
C, respectively [37,38]. In a different research study,
Deng et al. [37] succeeded in introducing micropores through the
structure of MCN using carbon coating and showed that the presence of
microporosity is highly favorable for CO2 adsorption. Although there are
a handful of researches that study the modification of MCN for superior
CO2 adsorption, there is no study so far focusing on the capability of
these nanomaterials for H2S adsorption. Moreover, none of the before-
mentioned studies could combine the intrinsic basicity of carbon
nitride structure with a perfect textural property, including both micro
and mesoporosity for ultra-high gas adsorption.
Herein, for the first time, we report the fabrication of dual-pore
carbon nitride (DP-CN) nano-materials with tunable textural proper
ties and adjustable nitrogen content for ultra-high CO2 and H2S
adsorption. In this method, we introduce a hybrid preparation technique
using a new silica nanoparticle for mesoporosity formation and low-
temperature chemical activation to effectively create microporosity
through the structure. This hybrid synthesis methodology enables the
effective engineering of structural defects by simply changing the ratio
of precursors or chemical activation agents. Despite most of the carbon-
based adsorbents for CO2 and H2S capture that lack either an excellent
textural property or high nitrogen content, the DP-CN adsorbents secure
these two essential characteristics at the same time. Moreover, the
presence of micro and mesopores in the structure guarantees a satis
factory adsorption performance in the entire pressure range. Due to the
wide variety of textural properties and nitrogen content in different
synthesized DP-CN samples, we expect this generation of carbon nitride
materials can be interesting for a wide range of separation processes,
catalysis, capacitors, and energy storage.
2. Materials and methods
2.1. Chemicals
All chemicals were purchased from commercial vendors and utilized
without any purification. Carbon tetrachloride (CTC, 99.9 %) and eth
ylenediamine (EDA, >99 %) were purchased from Sigma-Aldrich.
Methanol (MeOH, 99.99 %), hydrochloric acid (HCl, 37 %), hydroflu
oric acid (HF, 48 %), and potassium hydroxide (KOH, >85 %) were
obtained from Merck. Mesoporous nanosilica (SiO2) was provided from
Fadak Co. Nitrogen (N2, 99.99 %), helium (He, 99.99 %), carbon dioxide
(CO2, 99.99 %), methane (CH4, 99.99 %), and hydrogen sulfide (H2S,
99.99 %) were purchased from Farafan Gas Co.
2.2. Synthesis of mesoporous carbon nitride (M-CN)
The preparation of M-CN materials was accomplished according to
the literature [41]. First, SiO2 (0.5 g) as a hard template was added to a
Scheme 1. Schematic diagram of the synthesized DP-CN-X-Y.
F. Tabarkhoon et al.

3
mixture of EDA (1.35 g) and CTC (3 g) while EDA to CTC mass ratio was
kept constant at 0.45. The above mixture was placed under continuous
reflux and stirred at 90 ◦
C for 6 hr to complete the polymerization re
action. The resultant dark-brown solid mixture was dried under vacuum
at 60 ◦
C overnight and ground into a fine powder. The prepared poly
meric nanocomposite was carbonized at 600 ◦
C (3.0 ◦
C.min− 1
heating
rate) under a constant N2 flow (50 mL.min− 1
) for 5 hr. Afterward, the
sample was washed with HF (5 wt% in water) to remove the SiO2
template. Finally, the obtained material was rinsed several times with
MeOH, dried at 80 ◦
C under vacuum overnight, and labeled as M-CN. To
evaluate the role of amine, the whole synthesis procedure was repeated
with different X ratios of 0.45, 0.5, 0.55, and 1 resulting in the prepa
ration of M-CN-X, where X is the mass ratio of EDA to CTC.
2.3. Synthesis of dual-pore carbon nitride (DP-CN)
To improve the textural properties of the pre-prepared M-CN-X
samples and create microspores through the structure, a chemical acti
vation methodology was utilized (Scheme 1). Briefly, M-CN-X samples
were mixed with different amounts of KOH (KOH/M-CN-X weight ratios
of 2, 3, and 4), transferred to a stainless steel (grade 310) boat, placed at
the center of another stainless steel tubular reactor in a tubular furnace,
and then activated at 600 ◦
C (5.0 ℃. min− 1
) for 1.5 hr under a constant
N2 flow of 50 mL.min− 1
. Then, the samples were allowed to naturally
cool to the ambient temperature, treated with 1 M HCl solution, filtered,
washed with a copious amount of DI water, and dried at 80 ◦
C under a
severe vacuum for 12 hr. The obtained samples were denoted as DP-CN-
X-Y where X represents the EDA to CTC mass ratio and Y represents the
KOH to M-CN-X mass ratio at the beginning of the activation process (Y
= 2, 3, and 4).
2.4. Characterization
To get an insight into the physicochemical properties of the syn
thesized nanomaterials, different characterization techniques were
employed. Low-angle and wide-angle X-ray diffraction (XRD) patterns
were collected on a Bruker D8 Advance X-ray diffractometer via Cu Kα
radiation (λ = 1.54056 Å) operating at 30 mA and 40 kV with 2 θ from
Fig. 1. Representative of a) low-angle XRD of SiO2 and M-CN-0.45; b) wide-angle XRD of M-CN-0.45, M-CN-1, and DP-CN-1-4; c) FT-IR spectra of M-CN-0.45, M-CN-
1, and DP-CN-1-4;d) TGA curves of M-CN-0.55, M-CN-1, DP-CN-1-4 under air atmosphere, along with showing graphic of defect no defect structure of samples; e)
TGA weight reduction in two different stages A: 380–470 ◦
C, B: 470–620 ◦
C; f) TGA curves of M-CN-0.55, M-CN-1 under Ar atmosphere; g) XPS survey spectrum
showing elemental composition of M-CN-1 with high resolution spectra of C1s and N1s; h) XPS survey spectrum showing elemental composition of DP-CN-1-4 with
high resolution spectra of C1s and N1s.

4
0.85 to 10 for low-angle and 10 to 80 for wide-angle. Fourier-transform
infrared spectroscopy (FT-IR) was collected on a Rayleigh WQF-540
instrument in the range of 400 to 4000 cm− 1
using KBr pellets. The
field emission scanning electron microscopy (FE-SEM) images were
taken on a Tescan Mira II under 15 kV voltage. Transmission electron
microscopy (TEM) images were obtained on a Philips/FEI Bio Twin
CM120. Carbon, hydrogen, and nitrogen (CHN) elemental analysis were
carried out with a FLASH EA 1112 elemental analyzer. X-ray photo
electron spectroscopy (XPS) was employed on a Kratos Axis Supra using
an Al-Kα X-ray source (1486.58 eV) at 15 mA emission current and a
pass energy of 40 eV. Thermogravimetric analysis (TGA) was carried out
on a Mettler Toledo TGA/SDTA851 instrument. Samples were heated
from room temperature to 900 ◦
C with a heating ramp of 10 ◦
C.min− 1
under air and argon atmosphere (separately). N2 adsorption–desorption
analysis was employed using a Micrometric ASAP 2020 instrument at −
196 ◦
C. Barrett-Joyner-Halenda (BJH) and Burnauer Emmett-Teller
(BET) methods were used to measure the pore size distribution and
the specific surface area of the samples.
2.5. Density functional theory (DFT) calculations
DFT calculations were also performed to explore the adsorption
behavior of CO2 and H2S using several models of carbon nitride with
varied pore sizes. All electronic structure computations were carried out
using the PBE [46] density functional and a double numerical basis set
supplemented with polarization functions (DNP). Grimme’s DFT-D2
[47,48] scheme was adopted because of the importance of dispersion
effects in the adsorption of CO2 and H2S molecules. The tolerance for
energy convergence was set at 10− 5
Ha, while the maximum allowable
force and displacement were set at 0.001 Ha/Å and 0.005 Å, respec
tively. To avoid interactions between two nearby carbon nitride
monolayers, a 20 Å vacuum spacing was introduced. To generate quick
yet reliable electronic convergences, a smearing of 0.005 Ha to the
orbital occupancy was applied. The Monkhorst-Pack approach was used
to sample the reciprocal space using a 3 × 3 × 1 k-point grid for the
geometry relaxations. The adsorption energy was determined as follows:
Eads(X) = Ecomplex − EX − ECN (1)
where ECN and Ecomplex are the total energies of the pure carbon nitride
surface and that with the adsorbed species, respectively, and EX is the
total energy of the free CO2 or H2S molecule. By this definition, an Eads <
0 value denotes an exothermic adsorption process. All the DFT calcu
lations were conducted by the DMol3
electronic structure code [49].
3. Results and discussion
3.1. Characterization of nanoadsorbents
To reveal the crystallinity and phase purity of nanoadsorbents and
the SiO2 template, XRD patterns were acquired. Fig. 1a shows that the
SiO2 template has three peaks related to (1 0 0), (1 1 0), and (2 0 0)
reflections [41]. Similarly, M-CN-0.45 has the same peaks as the parent
SiO2 template, inferring that the structure of the template is well-
replicated in the carbon nitride samples [50]. However, the intensity
of (1 1 0) and (2 0 0) reflections in M-CN-0.45 is weaker than that of the
parent SiO2 template, possibly because of incomplete filling of meso
pores with carbon and nitrogen precursors or partial disorientation of
the mesochannels of the template [45]. Fig. 1b reveals the wide-angle
XRD patterns of carbon nitride before and after the chemical activa
tion. As seen, all the samples have two peaks at 2θ = 26◦
and 46◦
,
assigned to (0 0 2) and (1 0 0) diffraction plans of graphitic layers [51].
It should be noted that the peak intensity of M-CN-1 is lower than that of
M-CN-0.45. This might be due to the presence of extra nitrogen-rich
compounds in the pores and on the surface of the structure, creating
defects by incomplete polymerization and decreasing the crystallinity of
the M-CN-1 sample. Moreover, the drop in the peak intensity of the DP-
CN-1-4 sample suggests that the chemical activation process using KOH
impacted the crystalline structure of the sample by creating more cav
ities in the structure [9,52]. In fact, the presence of irregular cavities and
pores makes the diffracted X-ray beam, to some extent, unorganized
resulting in a decrease in peak intensity.
To identify the functional groups present in carbon nitride samples,
FT-IR spectroscopy was employed. Fig. 1c and Fig. S1 show that there
are three major peaks in M-CN-45, M-CN-1, DP-CN-1-4, and DP-CN-
0.45-2 at 1260, 1570, and 3400 cm− 1
. The band at 1260 cm− 1
is
ascribed to aromatic C–N stretching bonds, while 1570 cm− 1
is related
to C–
–N stretching vibrations (aromatic ring modes) of the carbon
nitride wall structure [53,54]. The broad band at 3400 cm− 1
is assigned
to the stretching mode of N–H groups in the aromatic ring [41]. All
three peaks confirm the presence of a 1,3,5- triazine ring in the samples
[55]. As observed, the intensity of peaks of the M-CN-1 is greater than
those of M-CN-0.45, owing to the greater amount of nitrogen through
the structure. Also, similar to XRD spectra, the KOH activation reduced
the intensity of FT-IR peaks in DP-CN-1-4 sample. This can be related to
the damaging effect of KOH activation on carbon–nitrogen and car
bon–carbon bonds in the structure.
To determine the thermal stability of nanoadsorbents, thermog
ravimetry analysis (TGA) was utilized in the air (Figs. 1d and S2) and
argon (Fig. 1f) atmospheres. Fig. 1d reveals that there are three major
weight loss regions. The first region was observed at around 70–200 ◦
C,
attributed to the evaporation of the adsorbed water and other volatile
impurities [56]. The mass loss of DP-CN-1-4 in this region is relatively
higher than that of M-CN-1 and M-CN-0.55, which can be ascribed to the
considerably higher porosity of this sample and, subsequently, its higher
moisture adsorption capacity. The second mass loss region was found
between 380 and 470 ◦
C (region A). Based on the literature, carbon
nitride materials without any significant structural defects do not
display any weight reduction in this region [45]. However, for M-CN-
0.55 and M-CN-1 samples, this weight loss is clearly seen, which is
owing to the fact that by increasing the ratio of EDA/CTC in the reactant
mixture, a greater amount of nitrogen-rich compounds can penetrate
into the SiO2 pores with a faster diffusion rate. This results in a change in
the stoichiometry of the carbon nitride polymerization reaction [41],
hindering a complete polymerization of the matrix and leaving some
defects and weak carbon–nitrogen bonds as the amount of nitrogen in
the pores is beyond the stoichiometric value. M-CN-0.55 and M-CN-1
samples exhibited 15.7 % and 35.3 % weight loss in Region A, respec
tively, while no significant weight loss was observed in DP-CN-1-4
(Fig. 1e). This is because a part of the whole nitrogen compound has
already been decomposed in the chemical activation process and gone
away. The higher thermal stability of the DP-CN-1-4 sample can make
this material compatible with a wide range of operating conditions.
Finally, at the temperature range between 470 and 620 ◦
C (region B), all
samples exhibited a remarkable weight loss due to the decomposition of
hepatize rings [57]. Furthermore, TGA analysis in Ar atmosphere was
performed for M-CN-0.55 and M-CN-1 up to 900 ◦
C (Fig. 1f). An inter
esting behavior can be seen after 400 ◦
C, where M-CN-1 decomposes at
approximately 520 ◦
C while the starting point for the decomposition of
M-CN-0.55 is at about 580 ◦
C. This further confirms that increasing the
EDA/CTC ratio creates defects and weak carbon–nitrogen bond in the
structure, as explained before, which decreases the thermal stability of
the M-CN-1.
The surface properties and the nature of M-CN-1 and DP-CN-1-4
samples were analyzed using XPS, shown in Fig. 1g, h. According to
the survey spectrum showing the elemental composition, both adsor
bents are composed of carbon, nitrogen, and oxygen, whereas there is no
peak related to Si due to the complete removal of silica during HF
treatment. The presence of oxygen atoms on the surface of the samples
could be attributed to the atmospheric O2 or CO2 adsorbed on the sur
face, methanol washing, or oxidation through the KOH activation
(related to DP-CN-1-4). A comparison between the XPS survey spectrum

5
of the adsorbents shows that activation of M-CN-1 decreases the peak
intensity of nitrogen atoms while increasing the peak related to oxygen
atoms in the structure of DP-CN-1-4. The deconvolution of high-
resolution C1s reveals four peaks with the binding energy of 284.2
(C1), 285.6 (C2), 287.6 (C3), and 289.1 (C4) eV for both adsorbents. The
lowest energy binding (284.2 eV) with the highest intensity corresponds
to the pure graphitic site in the amorphous carbon nitride matrix while
the peak at 285.6 eV is ascribed to the sp2
C atom bonded to nitrogen
inside the aromatic ring [41]. The peak at 287.6 eV is a characteristic of
the sp3
-hybridized carbon atom bonded to nitrogen (C–N) [42,58,59].
Moreover, the highest energy binding (289.1 eV) is attributed to the sp2
-
hybridized C atom in the aromatic ring connected to the NH2 group. It is
clear that KOH activation decreases the relative content of C2, C3, and
C4, which are related to the carbon bonded to nitrogen. The deconvo
lution of N1s spectra reveals that there are three peaks at 397.9 (N1),
400.3 (N2), and 401.8 (N3) eV, confirming the FT-IR results. The peak at
397.9 eV is associated with nitrogen atoms bonded to sp2
carbon (pyr
idinic-N), whereas the peak at 400.3 eV is assigned to the N atoms
strongly bonded to sp2
carbons (graphitic-N) [60]. The last peak at
401.8 eV is attributed to terminal amino functions [61]. As seen, KOH
activation decreases the relative content of N1 while increasing the
relative content of N2 and N3. Based on the previous studies [62,63],
pyridinic nitrogen (N1) located at the edges of graphene layers is
affected more by the activation process. The relative N atomic content of
the surface of the DP-CN-1-4 adsorbent evaluated from XPS analysis is
9.7 %, highlighting that the low-temperature chemical activation suc
cessfully preserved a considerable amount of nitrogen content in the
adsorbent.
To investigate the physical properties of the synthesized nano
materials, N2 adsorption–desorption was employed (Fig. 2a and S3).
Fig. 2a shows the N2 adsorption–desorption of M-CN-1 and DP-CN-1-4.
According to IUPAC classification, M-CN-1 displays an IV-type adsorp
tion isotherm with a sharp capillary condensation step and an H1 hys
teresis loop in 0.70–0.95 relative pressure (p/p0) range indicating the
dominance of relatively large mesopores within the structure. On the
other hand, the adsorption isotherm for DP-CN-1-4 is type-I with a high
adsorption rate at low pressure (p/p0 < 0.01), representing that DP-CN-
1-4 sample has a microporous structure while a slight H1 hysteresis loop
at p/p0 ≈ 0.3–0.9, signaling the presence of small mesopores in the
structure. In detail, the noticeable N2 uptake of DP-CN-1-4 at both low
and high relative pressures reveals the presence of relatively large mi
cropores (near 2 nm) in its structure. This interpretation can be further
supported by the pore size distribution curve (Fig. 2b). Additionally, the
textural properties of the SiO2 and nanoadsorbents are provided in
Table S1.
Fig. 2b shows the pore size distribution of M-CN-1 and DP-CN-1-4. As
it was mentioned in the previous discussion, the pore size distribution of
M-CN-1 is predominantly in the range of mesopores. However, the pore
size distribution for DP-CN-1-4 is mostly in the boundary region between
micropores and mesopores. This result indicates that KOH activation has
an incredible effect on the textural properties of the sample, causing
remarkable changes in the pore structure of the carbon nitride materials
and creating extra micropores through the structure, which are favor
able for CO2 and H2S adsorption. Moreover, Fig. S4 is presented to
further analyze the pore size distribution of other samples.
Fig. 2c shows the comparison between the surface area of different
prepared CN materials using various EDA/CTC ratios with and without
chemical activation. Among non-activated samples, M-CN-0.55 has the
highest surface area (329.1 m2
/g) owing to the high amount of nitrogen-
rich precursor, which can effectively create more defects in the sample
during the synthesis process. However, increasing the EDA/CTC ratio
beyond 0.55 (M-CN-1) resulted in a substantial decrease in the surface
area (160.6 m2
/g). This could be due to the partial filling of the entrance
pores and external surface by excessive nitrogen-containing compounds
leading to severe pore blockage [44] and surface area reduction
(Table S1). Among chemically activated carbon nitride materials, DP-
CN-1-4 exhibited the highest surface area of 2036.9 m2
/g, while its
corresponding non-activated precursor (i.e., M-CN-1) had the lowest
surface area. This might be related to the fact that the presence of
excessive nitrogen compounds in the structure is highly favorable for the
Fig. 2. A) n2 adsorption–desorption and b) pore size distribution of M-CN-1 and DP-CN-1-4. c) The comparison of surface area among M-CN-X and DP-CN-X-Y
(before and after activation with KOH). d) The surface area and micropore volume of M-CN-1, and DP-CN-1-Y prepared with different activation ratios (2, 3, and
4). The effect of e) EDA/CTC ratio on the nitrogen content of M-CN-X samples, and f) chemical activation on the nitrogen content of M-CN-1 and DP-CN-1-Y.

6
chemical activation process [64]. To further scrutinize this observation,
the impact of KOH activation on the surface area and micropore volume
of M-CN-1 is illustrated in Fig. 2d. In detail, M-CN-1 has the highest ratio
of nitrogen compounds, which is beneficial for reacting with KOH to
improve the total pore volume, surface area, and micropore volume. It is
owing to the fact that during the reaction of KOH with nitrogen, some
gases like NOx and NH3 are released to provide more pores in the
structure. In other words, during the KOH activation, a portion of ni
trogen elements is liberated as nitrogen-containing gases, and the rest is
maintained in the sample’s structure [64]. The other important factor
that can affect the structure of DP-CN-X-Y is the ratio of KOH to M-CN-1.
As seen in Fig. 2d, the specific surface area and micropore volume are
increased by increasing the amount of KOH. In detail, based on the
6KOH + 2C → 2 K + 3H2 + 2K2CO3 reaction (the comprehensive KOH
activation mechanism is provided in supplementary information, sec
tion 1.1. and 1.2.), adding more KOH will form more pores and cavities
in the structure [65,66]. The last vital parameter that influences the
structure of nanoadsorbents is the activation temperature. It should be
noted that the moderate activation temperature (600–700 ◦
C) can pro
duce sufficient micropores and a high surface area. On the opposite,
harsh activation temperature (>700 ◦
C) mostly destructs microporous
structures to create larger mesopores [67]. Therefore, in this research,
we chose the mild activation temperature of 600 ◦
C to improve the
textural properties. As a result, as seen in Fig. 2d, by changing the ratio
of KOH to M-CN-1 and choosing 600 ◦
C for the activation, we obtained
tunable DP-CN-1-Y with different surface areas (varying from 647.9 to
2036.9 m2
/g) and micropore volume (changing from 0.25 to 1.09 cm3
/
g).
In order to study the effect of EDA/CTC and KOH/M-CN-1 ratios on
the amount of nitrogen content, CHN analysis was utilized. Fig. 2e, f
show that by raising the EDA/CTC ratio from 0.45 to 1, the nitrogen
content of the synthesized M-CN-X adsorbents increased from 17.6 to
22.7 wt%, while increasing the KOH/M-CN-1 ratio decreased the
amount of nitrogen through the DP-CN-1-Y samples from 15.1 to 10.6 wt
%. These results are compatible with those observed in N2 adsorp
tion–desorption analysis, as using more KOH can consume more nitro
gen (less nitrogen available in the structure) and provide more defects
and pores. More importantly, it should be noted that although the ratio
of KOH to M-CN-1 is relatively high, there is still a notable amount of
nitrogen in the structure of DP-CN-1 adsorbents due to the mild acti
vation temperature and the stability of nitrogen groups in the DP-CN
samples.
To demonstrate the structure and morphology of the nano
adsorbents, FE-SEM analysis was utilized (Fig. 3a–d and S5a, b). Fig. 3a-
d show that M-CN-0.45, M-CN-0.55, M-CN-1, and DP-CN-1-4 have a
compact network with a rough surface. Moreover, adding KOH to the M-
CN-1 sample did not result in a considerable morphological change in
DP-CN-1-4. According to Fig. S5a, the predominant part of DP-CN-0.45-
4 morphology consists of spherical particles stemming from the spher
ical SiO2 template. However, the minor flat-like morphology on the
surface of the adsorbent is due to the chemical activation, making the
adsorbent surface heterogeneous. To further analyze the structure of DP-
CN-1-4 and the SiO2 template, TEM analysis was utilized (Figs. 3e, f and
S5c). According to Fig. 3e, DP-CN-1-4 is arranged in a systematic
manner, confirming the presence of intermittent graphitic layers, which
is useful for H2S and CO2 adsorption [65]. Fig. 3f clearly shows the
microporous structure of the DP-CN-1-4 sample. Moreover, Fig. S5c
reveals that the SiO2 template has a spherical structure, the same as M-
CN samples, with an average particle size of approximately 33 nm.
3.2. CO2 and H2S adsorption performance
To evaluate the equilibrium adsorption loading of CO2 and H2S gases
on M-CN-X and DP-CN-X-Y samples, an in-house-built volumetric setup
described in the supplementary information was utilized (Scheme S1).
The experimental adsorption isotherms at 0–30 bar for CO2 and 0–10 bar
for H2S at 25 ◦
C are also provided in Figs. S6 and S7. Figs. 4a–d and S8a
show the CO2 and H2S uptake of M-CN-X and DP-CN-X-Y at low (1 bar),
medium (10 bar), and high pressure (30 bar for CO2) at 25 ◦
C. For M-CN-
X samples, increasing the ratio of EDA/CTC from 0.45 to 0.55 improved
the CO2 and H2S adsorption capacity because of the greater surface area
and optimum amount of nitrogen, causing a strong interaction and co
valent bonds between acidic CO2 and H2S gas molecules and the Lewis
basic surface of M-CN adsorbents. However, M-CN-1 has the lowest CO2
Fig. 3. Representative of FE-SEM images of a) M-CN-0.45, b) M-CN-0.55, c) M-CN-1, and d) DP-CN-1-4. HR-TEM images of e, f) DP-CN-1-4.

7
and H2S uptake among all M-CN materials because of the excessive
amount of amines in the structure of M-CN-1 resulting in severe pore
blockage and poor gas uptake.
To tune the textural properties of carbon nitride, all M-CN-X samples
were chemically activated, and the effect of KOH (as the chemical
porosity agent) and EDA (as the nitrogen-containing precursor) on the
gas adsorption capacity of hierarchical micro-mesopore DP-CN mate
rials were investigated. According to Fig. 4a–d, both CO2 and H2S
adsorption gradually improved with increasing the amount of KOH for
the entire range of EDA/CTC. Interestingly, we observed that the
maximum gas uptake is usually obtained for the DP-CN materials with a
high EDA/CTC ratio of 1, while M-CN-1 (the non-activated version of
DP-CN-1-4) displayed the worst CO2 and H2S gas uptake due to its low
surface area (167.67 m2
/g). Therefore, it was revealed that the excess
amount of nitrogen-containing groups could effectively react with KOH
at the activation temperature and catalyze its penetration through the
structure resulting in the formation of a hierarchical micro-mesopore
structure with extra high surface area and gas adsorption behavior.
Specifically, DP-CN-1-4 with the maximum EDA and KOH recorded the
highest adsorption loading of 8.3 and 16.9 mmol/g at 1 and 10 bar for
CO2 and 13.8 and 23.1 mmol/g at 1 and 10 bar for H2S, respectively. As
seen in Fig. 4a–f, H2S adsorption values were greater than those of CO2
for all the materials. This observation can be primarily ascribed to the
higher polarizability of H2S (38.0 × 10− 25
cm3
) against CO2 (26.3 ×
10− 25
cm3
) molecules[68], causing a stronger electrostatic affinity
towards the nitrogen-rich surface.
To better understand the effect of textural properties of DP-CN-1-Y
samples on gas adsorption, the correlations between the surface area
and gas uptake are shown in Fig. 4e, f (see also Fig. S8b–e). The results
revealed that both surface area and micropore volume have a linear
impact on the adsorption capacity. As seen, DP-CN-1-4 has the highest
surface area (2036.9 m2
/g) and micropore volume (1.09 cm3
/g),
resulting in notably improved CO2 and H2S uptake at different operating
conditions. A comparison between the gas adsorption capacity of DP-
CN-1-4 with other MCN and nitrogen-doped carbon adsorbents
(Table S2) displayed that DP-CN-1-4 has remarkably higher CO2 and H2S
uptake at 1 bar and 25 ◦
C compared to those of other samples. The
possible reason could be attributed to the unique physicochemical
properties, including extra high surface area, tailored pore structure,
and high nitrogen content.
To scrutinize the gas adsorption mechanism on M-CN and DP-CN
adsorbents, the experimental data were fitted by Langmuir (Equation
S6), Freundlich (Equation S7), Sips (Equation S8), and Toth (Equation
S9) equations [18,69,70]. The fitting parameters and regression coeffi
cient (R2
) for each isotherm equation are listed in Tables S3–S10.
Moreover, the parity plots for all isotherm models are illustrated in
Fig. S9a–h to exhibit the deviation of the predicted values from the
experimental data. Based on the results, the Sips model displayed the
best accuracy among all the models for both CO2 and H2S adsorption.
Given that a wide variety of textural properties are available among the
Fig. 4. Bar chart of equilibrium a, b) CO2, and c, d) H2S adsorption capacity of M-CN-X and DP-CN-X-Y at low (1 bar) and medium pressure (10 bar) at 25 ◦
C e, f) The
effect of total surface area on CO2 and H2S uptake at 1 and 10 bar (25 ◦
C).

8
synthesized adsorbents, neither Langmuir nor Freundlich are reliable
candidates for predicting DP-CN materials’ gas behavior. As Sips is the
combination of Langmuir and Freundlich models, the homogenous-
heterogeneous adsorption mechanism, predominant in DP-CN adsor
bents, is estimated for all the samples [71,72].
3.3. Isosteric heat of adsorption (ΔH)
In order to analyze the strength of interaction between the best
adsorbent (DP-CN-1-4) and CO2/H2S gases, the isosteric heat of
adsorption using the Clausius-Clapeyron equation (Equation S10) was
calculated [73]. To calculate ΔH for each point, CO2 and H2S adsorption
isotherms of DP-CN-1-4 were fitted on the Sips model at 0, 10, and 25 ◦
C
(Fig. S10a, b). Fig. 5a shows the isosteric heat of adsorption of CO2 and
H2S on DP-CN-1-4 as a function of gas adsorption loading (qe). The heat
of CO2 adsorption was measured to be in the range of 21.7–34.6 kJ.
mol− 1
. Likewise, for H2S, it was in the range of 24.8–42.5 kJ.mol− 1
. As it
can be observed, the high value of ΔH at lower loading is due to the
interaction between the strong adsorption site with the adsorbates
related to the availability of active nitrogen sites while by increasing qe
the active amino sites become fully used for both CO2 and H2S adsorp
tion; thus further adsorption might be occurred by weak van der Walls
interaction or pore filling. Furthermore, based on the higher polariz
ability of H2S versus CO2 molecule, leading to a stronger adsorbent-
adsorbate interaction, the heat of H2S adsorption is more than that of
CO2 in the whole range of adsorption capacities.
3.4. CO2/N2, CO2/CH4, H2S/N2, and H2S/CH4 selectivity
The selectivity of CO2 and H2S over N2 and CH4 is the other impor
tant factor that should be evaluated. For this purpose, the ideal adsorbed
solution theory (IAST) was employed to calculate CO2/N2, CO2/CH4,
H2S/N2, and H2S/CH4 binary adsorption selectivity based on single gas
isotherms fitted on the Sips model (Fig. S10c). The binary gas selectivity
is provided in Equation S11 [74].
Fig. 5b, c show the adsorption selectivity of CO2/N2, CO2/CH4, H2S/
N2, and H2S/CH4 for DP-CN-1-4 nanoadsorbent. According to Fig. 5b, c,
the adsorption selectivity of CO2/N2 reached 45.78 (15:85) and 62.12
(50:50), and CO2/CH4 reached 28.40 (15:85) and 37.30 (50:50) at 1 bar
and 25 ◦
C. Similarly, H2S/N2 selectivity was 62.64 (1:99) and 116.57
(10:90), and H2S/CH4 was 45.70 (1:99) and 68.20 (10:90). A compari
son between the gas selectivity of DP-CN-1-4 and those of carbon-based
materials reported in the literature disclosed the superiority of nano
porous CN materials for selective gas adsorption [30,75–82]. This high
selectivity of DP-CN-1-4 for all the mixtures can be explained by two
reasons: i) the presence of nitrogen compounds in the structure, which
increases the interaction of the basic adsorbent surface and acidic gases
and ii) the presence of a considerable amount of narrowed-wall micro
pores in the structure of the DP-CN-1-4 sample, which strengthens the
superposition of the Van der Waals force field related to CO2 and H2S.
This effect causes a limited enhancement of adsorption capacity on N2
and CH4 as well, but not as much as those of CO2 and H2S[75].
3.5. Cyclic gas uptake performance of DP-CN-1-4
To investigate the stability of the DP-CN-1-4 sample, CO2 and H2S
uptake were performed for five consecutive adsorption–desorption cy
cles (Fig. 5d). In detail, a pressure–temperature swing approach was
done after each adsorption test to regenerate the saturated sample. The
used adsorbent was degassed at 160 ◦
C under vacuum for 5 hrs. As ex
pected, the largest drop occurred in the second cycle, and regeneration
efficiency remained in an acceptable range until the fifth cycle. This
reduction in the initial cycles can highlight the partial blockage of DP-
CN-1-4 structure at high operating pressure resulting in a decrease in
Fig. 5. Representative of a) the isosteric heat of adsorption of CO2 and H2S on DP-CN-1-4. IAST predicted selectivity for b) CO2/N2 and CO2/CH4 (15:85, and 50:50),
and c) H2S/N2 and H2S/CH4 (1:99, and 10:90). d) Cyclic adsorption performance of DP-CN-1-4 for CO2 and H2S gases.

9
both CO2 and H2S adsorption capacity. Moreover, the regeneration ef
ficiency of H2S adsorption was slightly lower compared to that of CO2
adsorption, mainly because of the corrosive nature of H2S and its irre
versible chemical reaction with some extra active heteroatoms on the
surface of DP-CN-1-4. The high regeneration efficiency of DP-CN-1-4
(96.5 % and 93.1 % for CO2 and H2S, respectively, at the end of the
fifth cycle) is attributed to the presence of mesopores in the structure of
DP-CN-1-4 that facilitates the desorption process. This demonstrates the
great potential of nanoporous carbon nitride materials for acidic gas
adsorption on large scales.
3.6. DFT results
Dispersion-corrected DFT computations were also performed to
better understand the adsorption behavior of CO2 and H2S molecules on
carbon nitride surfaces. For this purpose, four distinct porous carbon
nitride surfaces with varying pore sizes and carbon/nitrogen atoms were
explored. The relaxed geometries of the carbon nitride nanostructures
employed in the DFT computations, labeled CN-1, CN-2, CN-3, and CN-
4, are shown in Fig. S11. The pore size in these systems increases as CN-
4 > CN-3 > CN-2 > CN-1, while the cell parameter remains almost
constant. Our DFT calculations show that the band gap value of CN-1 is
about 2.40 eV, which is within the range reported for other comparable
structures [83–85]. According to Table 1, the band gap narrows as CN-1
> CN-2 > CN-4 > CN-3, which is mostly related to the stability of the
conduction band minimum in these systems. These findings imply that
the pore size and distribution of carbon/nitrogen atoms of carbon nitride
structures may have a significant effect on their surface reactivity.
Indeed, as shown in Fig. S11, the molecular orbital analysis reveals that
the highest occupied molecular orbital (HOMO) of the carbon nitride
systems is predominantly centered on nitrogen atoms. This shows that
the nitrogen atoms in these systems are the most favorable electron-
donating sites. Furthermore, the molecular electrostatic potential
(MEP) isosurfaces in Fig. S11 show that the pore size and distribution of
carbon/nitrogen atoms of carbon nitride can influence the electronic
structure of these systems. The most negative electrostatic potentials are
connected with nitrogen atoms around the pore site in all circumstances.
This means that nitrogen atoms around pore sites are more likely to
interact with Lewis acids like CO2 and H2S.
Following that, a single CO2 or H2S molecule was placed onto each
carbon nitride surface to examine their adsorption behavior. Fig. 6a–d
display the most stable CO2 and H2S adsorption configurations over
Table 1
The calculated band gap (eV) of different carbon nitride surfaces and adsorption
energy (Eads, kcal/mol) and charge-transfer (qCT, electrons) values of CO2 and
H2S molecules adsorbed on these systems a
.
Surface Band gap Eads (CO2) Eads (H2S) qCT (CO2) qCT (H2S)
CN-1 2.45 − 4.38 − 7.42 − 0.01 − 0.05
CN-2 2.10 − 5.77 − 14.18 − 0.03 − 0.27
CN-3 1.86 − 14.99 − 21.15 − 0.12 − 0.45
CN-4 2.05 − 11.07 − 16.49 − 0.10 − 0.40
a
Negative qCT values reflect the transfer of electrons from the carbon nitride
surface to the adsorbed species.
Fig. 6. The adsorption configurations of CO2 (left) and H2S (right) on different carbon nitride models: (a) CN-1, (b) CN-2, (c) CN-3 and (d) CN-4.

10
different carbon nitride nanostructures, while Table 1 summarizes the
corresponding adsorption energies and charge-transfer values. The CO2
molecule is shown to have an end-on confirmation on all surfaces
(Fig. 6). The calculated CO2 adsorption energies over CN-1, CN-2, CN-3,
and CN-4 are − 4.38, − 5.77, − 14.99, and − 11.07 kcal/mol, suggesting
that CO2 adsorption is thermodynamically favorable in all cases.
Furthermore, these findings indicate that the adsorption strength of CO2
improves as the pore size of the carbon nitride surfaces increases, which
is consistent with the band gap values of these systems. The adsorption
energy of H2S for a given carbon nitride surface, on the other hand, was
found to be more negative than that of CO2, which may be easily
explained by the polarity of H2S (calculated dipole moment = 1.41
Debye). Furthermore, this conclusion, which is consistent with our
experimental data, reveals that the carbon nitride surfaces have a
greater proclivity to adsorb H2S than CO2. According to the Hirshfeld
analysis, both H2S and CO2 molecules act as electron-accepting species
on carbon nitride surfaces. Table 1 shows that the stronger adsorption of
H2S relative to CO2 is correlated with its higher charge-transfer value.
The transferred charge is mainly accumulated on the S atom of H2S and
leads to a decrease in its positive charge. Overall, the DFT calculations
confirmed that porous carbon nitride surfaces are highly active mate
rials for adsorbing and storing both CO2 and H2S gases, and this is
related to the pore size and distribution of C and N atoms in these
systems.
4. Conclusions
In summary, we synthesized a series of M-CN-X and DP-CN-X-Y ad
sorbents using EDA and CTC as precursors, SiO2 as a template, and KOH
as an activator. The resultant M-CN-X and DP-CN-X-Y samples were
characterized and investigated for CO2 and H2S adsorption perfor
mance. It was observed that various engineered carbon nitride adsor
bents with excellent textural properties (surface area, pore volume, pore
diameter) and a wide range of nitrogen contents (10.6 to 22.7 wt%)
were successfully obtained. It was confirmed that the best adsorbent
(DP-CN-1-4) in terms of surface area (2036.9 m2
/g), micropore volume
(1.09 cm3
/g), and appropriate nitrogen content (10.6 wt%) had the
highest CO2 and H2S uptake at 1 bar (8.3 and 13.8 mmol/g, respec
tively), 10 bar (16.9 and 23.1 mmol/g, respectively) and 30 bar (22.9
mmol/g for CO2) at 25 ◦
C making this adsorbent a promising candidate
for CO2 and H2S adsorption.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.cej.2022.140973.
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