Synthesis, structure and adsorption properties of nonstoichiometric carbon nitride in comparison with nitrogen-containing carbons

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JIEC 2737 1–8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6 7 8 9

Synthesis, structure and adsorption properties of nonstoichiometric carbon nitride in comparison with nitrogen-containing carbons D. Shcherban a,*, Svitlana M. Filonenko a, Pavel S. Yaremov a, M. Skoryk b,c, Vladimir G. Ilyin a, Atte Aho d, Dmitry Yu. Murzin d

Q1 Nataliya

a

L.V. Pisarzhevsky Institute of Physical Chemistry, NAS of Ukraine, 31 pr. Nauky, Kyiv 03028, Ukraine

Q2 b NanoMedTeFh LLC, 68 Gorkogo str., Kyiv, Ukraine Q3 c G.V. Kurdyumov Institute for Metal Physics, N.A.S. of Ukraine, 36 Academician Vernadskiy av., 03680 Kyiv, Ukraine d

Johan Gadolin Process Chemistry Centre, A˚bo Akademi University, 20500 Turku/A˚bo, Finland

A R T I C L E I N F O

A B S T R A C T

Article history: Received 14 August 2015 Received in revised form 14 November 2015 Accepted 28 November 2015 Available online xxx

The samples of nonstoichiometric carbon nitride characterized by spatial ordering, large pore volume (up to 0.8 cm3/g) and specific surface area (up to 585 m2/g) were obtained via matrix carbonization of ethylenediamine in the presence of carbon tetrachloride in mesoporous molecular sieves KIT-6 and MCF as exotemplates. In contrast to nitrogen-containing carbons (obtained by modification of carbon samples with nitrogen in the result of compatible thermal treatment of the initial porous carbon with melamine) nonstoichiometric carbon nitride contains much more nitrogen (up to 13.7 wt.% (C/N = 6), compared with preceding 0.6 wt.%) and increased the quantity of basic nitrogen-containing groups (in particular, amino groups)—up to 0.68 mmol/g (vs. 0.46 mmol/g). The increase of adsorption capacity towards hydrogen and carbon dioxide: adsorption potential, differential heat of CO2 adsorption, specific adsorption on the pore surface—from 5.4 to 7.3 mmol H2/m2 and from 2.2 to 5.4 mmol CO2/m2 for carbon and synthesized samples of nonstoichiometric carbon nitride, respectively, due to the incorporation of nitrogen atoms into the carbon framework was noticed. ß 2015 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.

Keywords: Nonstoichiometric carbon nitride Matrix synthesis Specific adsorption Adsorption potential Basic sites

10 11

Introduction

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Materials based on carbon nitride attracted close attention of scientists due to their unique properties—such as high strength, low friction coefficient, chemical inertness, stable autoelectronic emission, transparency in a wide range of optical frequencies and high water resistance. Because of their properties such materials are promising for technological and biological applications, such as biocompatible coatings for medical implants, electrodes in power sources, protective coatings against corrosion, sensors for determination of moisture and composition of gas mixtures [1–6]. Substitution of carbon atoms with nitrogen in the graphite structure preserving its regularity can occur in several ways, leading to existence of a family of related in structure compounds of carbon nitride with different stoichiometry such as C3N4, C3N2, C3N, C5N, C10N3 etc. [7]. Accordingly various strategies were proposed for obtaining such nitrogen-rich carbon materials

* Corresponding author. Tel.: +380 445256771; fax: +380 445256216. E-mail address: [email protected] (N.D. Shcherban).

[8]. Preferably synthesis of carbon nitrides foresees pyrolysis of nitrogen-containing organic precursors at which condensation of C–N bonds occurs. Under their heating a series of intermediate products is formed consistently including melamine with cyclic structure of the molecule, which can also be used as an initial material in the synthesis. Melamine during heating turns into melem–heptazine cycle with a planar triangular structure which due to free aminogroups polymerizes in melon. The last one forms plane nanolayers which under further heating are transformed in carbon nitride [9,10]. Porosity significantly expands the range of applications of carbon nitride based materials due to a possibility of using them for selective adsorption and gases storage at high pressure, in particular hydrogen, adsorption of biomolecules, and base catalysis [11]. In addition, a significant increase in the specific surface area due to porosity enhances their photocatalytic activity [12– 15]. Several new approaches were proposed for synthesis of porous modifications of carbon nitride. Template methods using soft or hard templates are the most important among them. For example, amorphous carbon nitride with an almost stoichiometric composition C3N4 and broad reflexes in diffractograms similar in position

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JIEC 2737 1–8 2

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to the graphite reflections was obtained during decomposition of melamine derivative at temperatures of 400–500 8C [16]. The method of obtaining carbon nitride via solvothermal condensation of melamine with cyanuric chloride, which can be used for largescale production was also described [17]. This work was restored by the authors [18] who synthesized carbon nitride by condensation of cyanuric chloride with calcium cyanamide and established temperature limits (500–600 8C) for the formation of the ordered product with the composition close to stoichiometric. The authors [19] used 2-amino-4,6-dichlorotriazine as the precursor and obtained at high temperature and pressure graphite-like carbon nitride with high degree of crystallinity as a product. Crystalline carbon nitride was also obtained via solvothermal synthesis from cyanuric chloride and sodium amide in benzene under heating above 200 8C for 8–12 h [20]. Carbon nitride in particular nonstoichiometric one (with a lower nitrogen content than in C3N4) attracts attention as base catalyst in a number of acid–base processes (Knoevenagel condensation, transesterification of ethylene carbonate and ethyl acetoacetate to dimethyl carbonate, Friedel–Crafts acylation, oxidation of cyclic olefins to epoxides, dehydrogenation) [1,21– 25]. Nonstoichiometric carbon nitride due to a large concentration of basic nitrogen-containing groups and developed porosity is also a promising carbon dioxide adsorbent [8,26], component of the enzyme-based biosensors [27], sensor for acidic/basic molecules [28]. Nonstoichiometric porous carbon nitride causes therefore a large interest in particular for base catalysis and adsorption of carbon dioxide due to the presence of basic groups. Comparison of such materials with nitrogen-containing carbon nanostructures obtained as a result of porous carbons modifications in the process of their heat treatment with nitrogen-containing compounds can be also interesting. The aim of this paper is to determine the conditions of nonstoichiometric porous carbon nitride synthesis via matrix carbonization of ethylenediamine in the presence of carbon tetrachloride as well as clarification of the features of its structure, composition and sorption properties towards hydrogen and carbon dioxide, in comparison with nitrogen-containing carbons. In the current work a comparison is provided for the composition and sorption properties of different N-doped carbons prepared using various initial organic precursors thus resulting in carbons with different nitrogen content. According to our knowledge such comparative studies are absent in the literature.

92

Experimental

93

Preparation

94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110

Carbon nitride samples with different porous structure were obtained via matrix synthesis using silica mesoporous molecular sieves (MMS) such as KIT-6 and MCF as exotemplates [8]. For this purpose a weighted amount of the initial KIT-6 0.5 g (MCF weight was 0.3 g) was added to a mixture of 2 ml of ethylenediamine and 1.5 ml of carbon tetrachloride and heated to 90 8C for 6 h under reflux. The resulting dark brown mass was dried at 60 8C for 12 h and mixed with a mixture of 2 ml of ethylenediamine and 1.5 ml of carbon tetrachloride. The mixture was re-heated to 90 8C for 6 h under reflux and dried. The obtained light brown powder was heated in an inert atmosphere to 600 8C with a rate 3 8C/min and kept at this temperature for 5 h. Silica matrix was removed by treatment in HF solution (HF:H2O = 1:3), the obtained products were washed with distilled water and ethanol several times, then dried at 100 8C. Carbon nitride samples obtained in MMS KIT-6 were marked as CN–KIT-6, samples obtained in MMS MCF were marked as CN–MCF.

A carbon replica of the matrix KIT-6 (CMK-8) was prepared via carbonization of sucrose according to the method [29]. For synthesis of mesoporous N-containing carbon in the KIT-6 matrix (N–CMK-8) carbon–silica composites prepared via matrix carbonization of sucrose in silica MMS KIT-6 were used. Functionalization of these composites with nitrogen was done in accordance with [30] using melamine.

111 112 113 114 115 116 117

Characterization

118

The phase composition of the samples was analyzed using Xray diffractometer Bruker D8 Advance with monochromated CuKa-radiation. Scanning electron images (SEM) were recorded using the MIRA3 TESCAN microscope at an accelerating voltage of 5–20 kV. IR-specters were recorded using Perkin Elmer Spectrum One spectrometer in 400–4000 cm1 frequency range. Samples were pelleted by pressing 2 mg of highly dispersed powder with 30 mg of KBr. A Perkin-Elmer PHI 5400 spectrometer with a Mg Ka X-ray source operated at 14 kV and 200 W was used in the XPS-analysis of the samples. The pass energy of the analyzer was 17.9 eV and the energy step 0.025 eV. Peak fitting was performed with the program XPS Peak 4.1. The background was corrected with the Shirley function. The sensitivity factors used in the quantitative analysis for C 1s, N 1s, and O 1s were 0.296, 0.477 and 0.711, respectively. Elemental composition of the obtained materials was determined using CHN-analyzer Carlo Erba 1106. The analysis method is based on the complete and instantaneous oxidation of the sample by ‘‘flash combustion’’ on the catalyst in an oxygen atmosphere. The resulting combustion gases passed through a reduction furnace, swept on the chromatographic column and thereafter detected by the thermal conductivity detector. Adsorption isotherms of nitrogen and hydrogen with a purity of 99.999% were measured by a volumetric method at 77 K up to ambient pressure (up to 1 atm) on Sorptomatic 1990. Micropore size was calculated by Horvath–Kavazoe equation [31]. For micromesoporous materials (S-like isotherms) adsorption parameters were determined by a comparative t-plot method [32] using standard isotherms for non-porous compounds (micropore and mesopore volume Vmicro, Vmeso, mesopore surface area Smeso). External surface area Sext was calculated according to [33]. The total specific surface area SBET was estimated by the BET equation [32]. Mesopore size was determined by the BJH method [34]. For isotherms in which ad(de)sorption hysteresis ends at relative pressure 0.45–0.50 and corresponds to the ultimate strength of the liquid meniscus to break, the mesopore size distribution was determined from the adsorption branches of isotherms. Additionally, the initial adsorption potential jDm0j was determined. It was calculated by the method of excess surface work for determination of the specific surface area [35]. The CO2-adsorption isotherms were measured using a Sorptomatic 1990 at 253 and 273 K. Boehm titration. Samples of obtained N-containing carbons with the mass of about 0.05 g were placed in 20 ml of 0.05 N hydrochloric acid solution. To establish equilibrium, suspensions were stirred for 24 h at room temperature. After this, 5 ml of each filtrate was pipetted and the excess of acid was titrated with NaOH. Concentration of basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon sample (namely, total concentration of basic sites was obtained). Total concentration of basic sites was calculated for 1 g of N-containing carbon and expressed in mmol/g. Thermogravimetric studies were performed using serial derivatograph Q-1000 in the range of the temperatures from ambient (20 8C) to 1000 8C. The rate of heating of the sample and the

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JIEC 2737 1–8 N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

Ethylenediamine is already known as the initial precursor for synthesis of carbon nitride. This organic substance possesses a high content of both C and N and its pyrolysis results in the formation of nonstoichiometric C3N4. Prolonged heating of a mixture of ethylenediamine and carbon tetrachloride in the presence of a silica matrix at 90 8C leads to the formation of a copolymer with a branched structure in the pores of MMS. Nitrogen-containing carbon is formed as a result of carbonization of this copolymer. Polymerization reaction can be represented schematically as follows:

CN-MCF

KIT-6

CMK-8

N-CMK-8

1.0

1.5

2.0

2.5

2θ, deg., Cu Kα Considering that carbon in (CH2NH2)2 and CCl4 is in the state of sp3 hybridization, it is assumed that after polymerization and during the subsequent heat treatment formation of nitrogensubstituted aromatic rings and interplanar binding of graphite-like CN-planes is also occurred involving sp3 hybridized carbon. Diffraction patterns of carbon nitrides are characterized by broad reflexes at 2u = 25.28 (Fig. 1) corresponding to the diffraction from the plane 0 0 2 with the interplanar distance of 3.53 A˚, which is close to the interplanar distance in nonporous carbon nitride. It points to the turbostratic ordering of carbon and nitrogen atoms in CN graphene planes—according to [36]. However, the lack of clear intense reflexes in diffraction patterns indicates a small degree of crystallinity of the samples. The obtained materials have spatially ordered mesostructure which corresponds to the initial MMS. XRD pattern (Fig. 2a) of the initial matrix KIT-6 is characterized by well divided h k l reflexes which determine a highly ordered cubic structure with Ia3d symmetry. There is a shift of reflexes maxima in the XRD pattern towards larger angles compared to the initial MMS because of diminishing interplanar distances in carbon nitride obtained in the KIT-6 matrix. Lower intensity of the peaks in the XRD pattern of CN–KIT-6 testifies thickening of the walls of the product compared to the KIT-6 matrix—similar to the explanation given in [37] for the hexagonal MCM-41 structure. For carbon nitride sample prepared in the MCF matrix the spatial ordering of mesopores is also stored. However, there is a shift of the second peak in the diffraction pattern towards larger angles and significant redistribution of intensities of the reflexes (Fig. 2b). Probably such changes in the structure are caused by the incorporation of nitrogen into the framework due to a minor change of interatomic distances and its 70 60

Intensity, a.u.

50 40 30

CN-KIT-6

20 10

CN-MCF

0 10

20

30

40

50

60

2θ, deg., Cu Kα Fig. 1. XRD patterns of the samples of nonstoichiometric carbon nitride CN–KIT-6 and CN–MCF.

MCF

CN-KIT-6

0.5

190 189 188 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219

Intensity, a.u.

178 179 180 181 182 183 184 185 186 187

b

420 332 422

Results and discussion

211

177

a 220

standard was 108/min. The DTA, TG, DTG curves were registered simultaneously.

Intensity, a.u.

175 176

3

3.0

0.5

1.0

1.5

2.0

2.5

3.0

2θ, deg., Cu Kα

Fig. 2. XRD patterns of carbon nitride samples CN–KIT-6, carbon and nitrogencontaining carbon (a) and CN–MCF (b) in small-angle region.

electron-donor properties. Thus the obtained samples after removal of the matrix are replicas of the corresponding MMS. According to XRD data (Fig. 2a) synthesized nitrogen-containing carbon N–CMK-8 has a high degree of spatial ordering— position, intensity and dividing of low-angle reflexes for Ncontaining sample is close to those of a mesoporous carbon which does not contain nitrogen (E;K-8), and corresponds to hexagonal ordered mesophases. The incorporation of nitrogen into mesostructure E;K-8 leads probably to some its distortion, as evidenced by the redistribution of the intensity of low-angle reflexes— reducing the intensity of the first small-angle reflex and increasing the intensity of the 2nd and 3rd diffraction peaks compared to the reflexes for pure carbon E;K-8 (Fig. 2a). SEM-images (Fig. 3) demonstrate formation of carbon nitride with spherical morphology in the case of MCF as the initial hard template (CN–MCF sample). CN–KIT-6 possesses a more compact structure with the particle size of few hundreds nm. Nitrogendoped carbon N–CMK-8 and carbon CMK-8 are characterized by a similar spherical morphology with the particle size of ca. 0.4– 1.5 mm). IR spectrum of nonstoichiometric carbon nitride CN–KIT-6 possesses absorption bands at 1260 cm1, 1490 cm1 and 1630 cm1, which are characteristic for stretching vibrations of nitrogen in an aromatic cycle, vibration of 5 5C5 5N– bond and –NH2 group, respectively. The broad absorption band with the center at 3432 cm1 corresponds to the stretching vibration of N–H bond in the aromatic cycle (Fig. 4). Such spectra were also observed for non-porous carbon nitrides [38]. The presence of nitrogen is confirmed by CHN-analysis (Table 1). The obtained data indicate the incorporation of nitrogen into carbon structure, namely the formation of carbon nitride. IR spectrum of CMK-8 (Fig. 4) contains an absorption band at 1590–1650 cm1 that characterizes unsaturated carbon structures (vibrations of C5 5C bond). Absorption bands at 2800–3000 cm1 correspond to the stretching vibrations of a CH bond in aliphatic compounds. Nitrogen atoms are incorporated into the carbon framework of N–CMK-8 as a part of pyridine and pyrrole cycles, quaternary nitrogen (absorption band at 2320 cm1) and pyridine N-oxide (absorption band at 1380–1385 cm1) as was shown in our previous manuscript [30]. The surface binding states were additionally clarified using X-ray photoelectron spectroscopy (XPS). The deconvolution of N 1s spectrum of nitrogen-containing carbon N–CMK-8 (Fig. 5a) gives four peaks: A—398.7 eV, pyridinic nitrogen, B—400.1 eV,

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JIEC 2737 1–8 N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

4

Fig. 3. SEM-images of carbon nitride samples CN–KIT-6 (a) and CN–MCF (b), nitrogen-containing carbon N–CMK-8 (c) and carbon CMK-8 (d).

pyrrolic/pyridone–N, C—401.1 eV, quaternary nitrogen, D— 402.2 eV, pyridine-N-oxide [39–41]. The amounts of pyridinic and pyrrolic/pyridone N are the highest being equal to 35.0 and 34.9 at.%, respectively. The relative amounts of quaternary N and

СN-KIT-6

Transmittance, a.u.

264 265 266 267

N-CMK-8

CMK-8 0

4000

3500

3000

2500

pyridine-N-oxide are lower (17.7 and 12.4 at.%, respectively). The total nitrogen mass content according to XPS data is 2.8%. The deconvolution of N 1s spectrum of nonstoichiometric carbon nitride CN–KIT-6 (Fig. 5b) gives two peaks: 398.5 eV (N bonded with the sp2 carbon in the aromatic ring) and 400.8 eV (N trigonally bonded to all the sp2 carbons) [11]. A high nitrogen content and the presence of amino groups in the composition of nonstoichiometric carbon nitride lead to an increased concentration of the basic sites (Boehm titration) in such samples compared to nitrogen-containing carbons obtained as a result of carbon MMS functionalization with nitrogen via their heat treatment with melamine. Thus, the total concentration of basic sites for samples of nonstoichiometric carbon nitride CN– KIT-6 and CN–MCF is 0.68 mmol/g, while for N–CMK-8 it was 0.46 mmol/g.

Table 1 CHN-analysis data for samples of carbon nitride, carbon and nitrogen-containing carbon.

2000

1500

Wavenumber, cm

1000

500

-1

Fig. 4. IR spectra of nonstoichiometric carbon nitride CN–KIT-6, carbon (CMK-8) and nitrogen-containing carbon (N–CMK-8).

Sample

N

C

H

CN–KIT-6 CN–MCF N–CMK-8 CMK-8

13.58 13.69 2.56 –

71.51 72.56 86.01 90.83

1.44 1.41 0.36 2.17

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JIEC 2737 1–8 N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

5

250

N 1s B

N 1s A

240 0.8

230

CPS

0.7

Intensity (a.u.)

N 1s C N 1s D

220

210

0.6 0.5 0.4

200

0.3 405

404

403

402 401 400 399 Binding Energy (eV)

398

397

a)

404

b)

402

400

398

396

B.E.(eV)

Fig. 5. XPS spectra of (a) nitrogen-containing carbon N–CMK-8 and (b) nonstoichiometric carbon nitride CN–KIT-6.

Thermogravimetric analysis was done to investigate the thermal stability of nonstoichiometric carbon nitride in atmospheric conditions (Fig. 6). The weight loss up to 90 8C corresponds to water removal which content in the sample reaches to 25%. The second bend of the TG curve corresponds to the substance combustion and almost complete weight loss in the temperature range 490–580 8C. Thus the obtained samples of nonstoichiometric carbon nitride exhibit thermal stability in air up to ca. 500 8C. Analysis of nitrogen ad(de)sorption isotherms (Fig. 7, Table 2) shows that synthesized nonstoichiometric carbon nitride samples are characterized by a predominantly mesoporous structure (Vmeso = 0.54–0.58 cm3/g, Vmicro = 0.06–0.08 cm3/g), comparatively high specific surface area (SBET—up to 585 m2/g), total pore volume (VS—up to 0.81 cm3/g), but low mesopore size uniformity due to the peculiarities of the replication process of the porous structure of the initial silica matrices KIT-6 and MCF. A comparative analysis of mesopore size distribution curves of carbon nitride and carbon replica of MMC KIT-6 (E;K-8, precursor–sucrose) shows the presence of three maxima in their structure (Table 2, Fig. 7c and d), which is due to similar features of replication of the initial matrix by precursors of different nature. Position of the first maximum of the mesopore size distribution (3.1–3.8 nm) practically coincides for all replicas based on MMS KIT-6 and MCF, when position of other depends on the type of the matrix and the precursor nature. A homogeneous mesoporous 0

30

10 40

DTA

60

DTG

0 -10 80

-20

TG 100

-30 0

200

400

600

800

Temperature, °С Fig. 6. Derivatogram of the sample CN–KIT-6.

ϕ, mV

20

20

Weight loss, %

283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307

structure (Dmeso = 3.8  0.9 nm, other maxima are absent) is formed using melamine as a precursor (sample N–CMK-8). Decrease of the total adsorption volume is observed with increase of the nitrogen content in the products of initial matrices replication. It can be due to a complex formation of a monolithic strong framework in the mesopores of MMC especially in the case of carbon nitride samples with a dense framework. The presence of micropores in the samples at the level of 0.06–0.08 cm3/g is mainly due to the peculiarities of matrix carbonization of the precursors of different nature. In the case of melamine pyrolysis the micropore content in the framework (N– CMK-8), according to the t-plot method, significantly increased (Vmicro = 0.20 cm3/g). The analysis of the calculated values of the adsorption potential jDm0j for the matrix synthesis products shows a dependence of nitrogen adsorption energy in standard conditions on the pores parameters of the samples, nature and functionalization of their surfaces. In particular, there was an increase of jDm0j (from 21 to 49 kJ/mol) with reduction of the micropore size (from 0.55 to 0.51 nm) and am increase of the micropore volume (from 0.07 to 0.20 cm3/g) and in the presence of N-containing surface groups (for matrix KIT-6 jDm0j was ca. 13 kJ/mol). Analysis of adsorption properties of the synthesized samples of nonstoichiometric carbon nitride and nitrogen-containing carbon, and the corresponding carbon MMS towards hydrogen was performed considering the characteristics of their porous structure obtained from adsorption isotherms of standard adsorbate nitrogen in identical adsorption conditions for nitrogen and hydrogen (T = 77 K, p  760 Torr). Adsorption capacity towards hydrogen (at p = 760 Torr, Fig. 8, Table 3) of carbon nitride samples is in the range of 0.8–0.9 wt.%. This value is lower than the available ones from the literature (ca. 2–3 wt.% at 1 bar and 6–7 wt.% at 20–25 bar) [42,43] being rather high considering the corresponding parameters of the porous structure, namely micropore volume and size and the total specific surface area (SBET = 530–585 m2/g). Moreover, the surface of the obtained samples is characterized by the adsorption specificity towards hydrogen, in particular due to the presence of nitrogencontaining centers of hydrogen polarization. A significant increase of the total specific surface area of the N–CMK-8 and CMK-8 samples (SBET—up to 1190 and 1570 m2/g, respectively) is accompanied by a corresponding increase in the adsorption capacity to hydrogen (ca. 1.3 and 1.7 wt.%, respectively). To characterize the adsorption specificity of the surface of synthesized samples towards hydrogen the values of specific adsorption on

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JIEC 2737 1–8 N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

1000 800

b

-1 -2 -3 -4

0.6

600 400

0.8 0.6

0.4

0.4 0.2 0.2

200

0.0

0 0.0

0.2

0.4

0.6

0.8

0.5

1.0

1.0

-1 -2 -3 -4

1.5

1.0

0.3 0.2

0.5

0.1

0.0 2

4

6

d

0.4

0.0 10

8

-1 -2 -3 -4

3

VPore / cm3g-1

c

dV/dD / cm3nm-1g-1

2.0

0.0 2.0

1.5

D / nm

p/p0

VPore / cm3g-1

1.0

dV/dD / cm3nm-1g-1

-1 -2 -3 -4

0.15

2

0.10

1

0.05

0 0

10

20

30 40 D / nm

D / nm

50

dV/dD/ cm3nm-1g-1

Vads / cm3g-1, STP

0.8

a

1200

VPore / cm3g-1

6

0.00 60

Fig. 7. Nitrogen adsorption (77 K) for samples CN–KIT-6 (1), CN–MCF (2), N–CMK-8 (3) and CMK-8 (4): (a) ad(de)sorption isotherms; (b) micropore size distribution (Horvath–Kawazoe method); (c) mesopore size distribution (adsorption branch of isotherm, BJH method); (d) mesopore size distribution (desorption branch of isotherm, BJH method). Table 2 Structural and sorption properties of the samples carbon nitride, carbon, nitrogen-containing carbon and initial silica matrices (N2, 77 K). Vmicro (cm3/g)

Vmeso (cm3/g)

Dmeso (nm)

CN–KIT-6

0.06

0.52  0.08

0.58

0.08

0.48  0.04

0.54

N–CMK-8 E;K-8

0.20 0.07

0.51  0.13 0.55  0.19

0.96 1.86

KyG-6 MCF

0.10 0.04

0.57  0.10 0.51

1.23 1.70

3.2  0.8 13.7  3.9 26.3  5.7 4.1  1.5 18.4  4.0 48.5  4.3 3.8  0.9 3.1  0.8 10.9  0.9 17.0  3.5 7.4  0.6 14.5  1.5

CN–MCF

Dmicro (nm)

200

SBET (m2/g)

VS (cm3/g)

jDm0j (kJ/mol)

440

585

0.70

23.5

330

530

0.81

34.9

710 1300

1190 1570

1.16 1.93

49.0 20.7

590 360

830 470

1.33 1.74

12.5 13.5

9

a

- 1 ads. - 1 des. - 2 -"- 2 -"- 3 -"- 3 -"- 4 -"- 4 -"- 5 -"- 5 -"-

160

120

3

Vads., cm /g, STP

Smeso (m2/g)

80

b

-1 -2 -3 -4 -5

8

ln(|Δμ|)

Sample

7

40

6

0 0

200

400

600

p, tor

800

1000

0

50

100

150

Vads / cm3 g-1, STP

Fig. 8. Hydrogen adsorption (77 K) for samples CN–KIT-6 (1), CN–MCF (2), N–CMK-8 (3), CMK-8 (4), KIT-6 (5): (a) adsorption isotherms; (b) graph for calculation of the initial potential of hydrogen adsorption jDm0 jH2 (linear form of the equation: nads = nmono ln jDm/Dm0j [35]).

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G Model

JIEC 2737 1–8 N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

7

Table 3 Adsorption characteristics of carbon nitride and nitrogen-containing carbons based on matrices KIT-6 and MCF towards hydrogen (77 K). Sample

3

CN–KIT-6 CN–MCF N–CMK-8 E;K-8 KyG-6

Potential of hydrogen adsorption jDm0j (kJ/mol)

Hydrogen adsorption (p = 760 Torr) 2

Vads (STP) (cm /g)

a (wt.%)

Specific adsorption on the pore surface (SBET) rH2 (mmol/m )

95.2 84.0 145.6 190.4 56.2

0.85 0.75 1.30 1.70 0.50

7.3 7.1 5.5 5.4 3.0

5.6 5.5 5.1 4.2 2.8

160

9.5

a - 1; - 3; - 5;

b

-1 -2 -3 -4 -5 -6

9.0

ln(|Dµ|)

Vads., cm3/g, STP

120

-2 -4 -6

80

40

8.5 8.0 7.5 7.0

0 0

200

400

600

800

0

20

40

60

80

Vads / cm3g-1, STP

p, tor

Fig. 9. Adsorption of carbon dioxide at 253 K (1, 3, 5) and 273 K (2, 4, 6) for CN–KIT-6 (1, 2), N–CMK-8 (3, 4), CMK-8 (5, 6): (a) adsorption isotherms; (b) graph for calculation of the initial potential of carbon dioxide adsorption Dm0 CO (linear form of the equation: nads = nmono ln jDm/Dm0j [35]). 2

Table 4 Adsorption characteristics towards carbon dioxide (273 K) of carbon nitride, nitrogen-containing carbon and carbon based on KIT-6 matrix. Sample

CN–KIT-6 N–CMK-8 E;K-8

352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376

Adsorption potential of carbon dioxide jDm0j (kJ/mol)

Adsorption of carbon dioxide (p = 760 Torr) 2)

a (mmol/g)

Specific adsorption on the pore surface (SBET) rCO2 (mmol/m

3.14 3.36 3.40

5.4 2.8 2.2

their surface (SBET) rH2 (at 760 Torr) were calculated. A corresponding comparative analysis of the specific hydrogen adsorption on the pore surfaces of the replicas of the initial matrices shows a significant growth of rH2 values with an increase of nitrogen content in the structure of carbon nitride samples (from 5.4 to 7.3 mmol/m2). At the same time specificity of the surface of the initial silica matrix KIT-6 towards hydrogen is much lower  rH2 ¼ 3:0 mmol=m2 . Energetic parameters of hydrogen adsorption (jDm0j) more correctly demonstrate surface affinity of the synthesized samples towards hydrogen because they do not depend on the degree of filling of the surface with hydrogen (the values rH2 at p < 760 Torr are significantly higher for samples with high jDm0j). The corresponding analysis of adsorption potential confirms surface affinity increase towards hydrogen with an increase nitrogencontaining sites concentrations which can be seen from the change of the values jDm0j from 4.2 for CMK-8 to 5.1–5.6 kJ/mol for nitrogen-containing samples (Table 3, the maximum values of jDm0 jH2 are observed for samples CN–KIT-6 and CN–MCF being 5.6 and 5.5 kJ/mol, respectively). It should be noted that the difference in the values of hydrogen adsorption potential for nitrogen-containing samples N–CMK-8 and CN–KIT-6 is caused not only by the total nitrogen concentration on their surface (0.46 and 0.68 mmol/g of basic sites, respectively), but also by specificity of surface functionalization, namely the presence of

12.3 11.0 9.9

surface groups –NH2 and nitrogen-substituted aromatic rings confirmed by IR spectroscopy. Existence of nitrogen-containing basic sites (up to 3.4 mmol/g) on the surface of the obtained samples is confirmed by carbon dioxide adsorption (T = 253 and 273 K, p  850 Torr, Fig. 9a and b, Table 4). Inparticular an increase of the differential heat of E?2  adsorption Q CO2 for replicas of the KIT-6 matrix with increasing of their nitrogen content was obtained. For CMK-8, N–CMK-8 and CN–KIT-6 the values of Q CO2 are 26, 30 and 33 kJ/mol, respectively, (at filling 0.15 mmol/g), the values of adsorption potential Dm 0 CO2 and specific adsorption rCO2 at 273 K are 9.8, 11.0 and 12.3 kJ/mol and 2.2, 2.8 and 5.4 mmol/m2, respectively. The obtained values of E?2 adsorption are comparable with those reported in the literature [8,43].

377 378 379 380 381 382 383 384 385 386 387 388 389 390

Conclusions

391

Nonstoichiometric carbon nitride characterized by significant porosity (pore volume—up to 0.8 cm3/g, surface area—up to 585 m2/g) and spatial ordering was obtained via polymerization of ethylenediamine in the presence of carbon tetrachloride in silica mesoporous matrices MCF and KIT-6, followed by pyrolysis of CNpolymer in the pores of MMS and subsequent removal of silica components. It is shown that the obtained samples are characterized by a high content of nitrogen (up to 13.7 wt.% (C/N = 6)), which

392 393 394 395 396 397 398 399

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N.D. Shcherban et al. / Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416

is a part of the nitrogen-substituted aromatic rings and amino groups, and as a result by high concentration of basic sites (0.68 mmol/g). Corresponding characteristics for nitrogen-containing carbons prepared by the functionalization of the mesoporous carbons using melamine are lower—ca. 0.6 wt.% of nitrogen and 0.46 mmol/g of basic sites. An increase of surface affinity of the samples towards hydrogen was found with an increase of nitrogen-containing sites concentrations, resulting in the values of jDm0j = 4.2 for CMK-8 and 5.1–5.6 kJ/mol for nitrogen-containing carbons. Maximal values of Dm0 H were observed for 2 nonstoichiometric carbon nitride–5.6 kJ/mol. An increase of the differential heat of CO2 adsorption for nonstoichiometric carbon nitride in comparison with carbon and nitrogen-containing carbon materials (33, 26 and 30 kJ/mol, respectively) was obtained, the values of adsorption potential and specific adsorption at 273 K were also highest for carbon nitride (12.3 kJ/mol and 5.4 mmol/m2, respectively).

417

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