C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0

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Solid-state dechlorination pathway for thesynthesis of few layered functionalized carbonnanosheets and their greenhouse gas adsorptivityover CO and N2

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.10.081

* Corresponding author: Fax: +91 278 2567562.E-mail addresses: hcbajaj@csmcri.org, hcbajaj13@rediffmail.com (H.C. Bajaj).

Sandesh Y. Sawant a, Rajesh S. Somani a, Sangita S. Sharma b, Hari C. Bajaj a,*

a Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute, Council of Scientific & Industrial

Research (CSIR), G.B. Marg, Bhavnagar 364002, Gujarat, Indiab Department of Chemistry, Hemchandracharya North Gujarat University, Patan, Gujarat, India

A R T I C L E I N F O A B S T R A C T

Article history:

Received 21 July 2013

Accepted 29 October 2013

Available online 7 November 2013

A simple solid-state dechlorination route has been demonstrated to synthesize few layered

functionalized carbon nanosheets (FCNS) utilizing hexachloroethane as carbon source and

copper as reducing agent under the autogenic pressure at 300 �C. The obtained FCNS pos-

sesses the sheet thickness of 6–12 nm as analyzed by transmission electron microscopy.

The particle nature of the FCNS provides the excess porosity having the surface area

836 m2/g. The equilibrium gas adsorption study of FCNS for greenhouse gases (CO2 and

CH4), toxic gas (CO) and light gas (N2) showed the maximum adsorption capacity for CO2

(2.95 mmol/g; at 288 K) with maximum capacity selectivity of 10.1 at 318 K. The very strong

adsorbate–adsorbent interaction was observed in case of CO compare to other gases

resulted in higher heat of adsorption for CO. The gas adsorption and FT-IR study showed

that the interaction of CO with copper present in minute quantities in FCNS improves

the CO adsorption due to p complexation. The FCNS obtained under present methodology

showed the very high equilibrium selectivity for CO over N2 (197) followed by CH4 (61) and

CO2 (7.3) at 288 K.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanosheets (CNS) is a two-dimensional carbon nano-

structure of stacked graphene sheets with few nanometers

thickness [1,2]. CNS is one of the interesting material in the

carbon family due to its potential applicability in various

fields such as gas storage [3], catalyst support [4], nano-elec-

tronics [5], electrode [6] and adsorption [7] etc. The chemical

vapor deposition method has been well elaborated for the

synthesis of CNS [8,9], whereas different synthesis methods

such as close-packing assembly [10], carbonization [11], cata-

lytic thermal pyrolysis [12] and template-like method [13]

have also been reported. Most of the synthesis methods of

CNS included the use of graphite as a carbon source [14]. Very

few methods utilized different carbon sources for the

synthesis of graphene like CNS, but the use of the advanced

techniques and low yield are the major disadvantages

associated with it. Ando et al. [15] have reported the synthesis

of petal-like graphite sheets using the hydrogen arc discharge

method. The dechlorination of chloro-hydrocarbon using

metal catalysts under solvothermal condition is one of the

promising and easy routes for the synthesis of different

carbon nanostructures [16]. Qian et al. [17] and Xiong et al.

[18] synthesized carbon nanotubes with claw-like end and

C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0 211

carbon hollow spheres by the dechlorination of halocarbon

using metal as reducing agent, respectively. Similarly, Kuang

et al. [19] reported the low temperature solvothermal synthe-

sis of crumpled CNS using metallic potassium and methane

tetrachloride at 60 �C.

The drastic increase in the emission of greenhouse gases

mainly CO2, due to industrial exhaust and burning of fossil

fuel creates the environmental hazard like global warming

[20,21]. The direct emission of carbon monoxide into the

atmosphere is restricted because of its high toxicity. Zeolite

and activated carbon are the well known commercial adsor-

bents for the gas storage and separation [22–25]. Clay [26], me-

tal organic framework [27,28] and the porous silica materials

[29] have also been developed and studied as adsorbent for

the CO2 sequestration. Recently, our group reported the horn

shaped carbon nanotubes synthesized using copper catalyzed

dechlorination of tetrachloroethylene as potential adsorbent

for greenhouse gases [30].

Herein we described the solid-state dechlorination of

hexachloroethane using copper to few layered functionalized

CNS (FCNS) under autogenic pressure and its greenhouse gas

adsorptivity (carbon dioxide and methane) over carbon mon-

oxide and nitrogen at different temperatures.

2. Experimental

2.1. Synthesis of FCNS

All chemicals were procured from S.D. Fine Chem. India and

used as received. Synthesis of FCNS was carried by heating

the SS autoclave (50 ml capacity) containing hexachloroeth-

ane (6.2 g) and copper powder (5 g), at 300 �C for 5 h. After

cooling the autoclave to room temperature, the obtained

black color product was treated with 500 ml of 1:1 HNO3 aque-

ous solution for 24 h under continuous stirring. The product

was recovered by filtration, washed with deionized water till

free from chloride ions (AgNO3 test) and finally dried at

80 �C for 24 h.

Fig. 1 – SEM photograph of agglomerated FCNS. (A colour

version of this figure can be viewed online.)

2.2. Characterization of FCNS

The morphology of the sample was investigated by scanning

electron microscopy (SEM) with Leo 1430 SEM and transmis-

sion electron microscopy (TEM) with JEOL JEM-2100, TEM

using an accelerating voltage of 18 and 200 kV, respectively.

The X-ray diffraction (XRD) pattern of the FCNS was recorded

on PHILIPS X’Pert MPD system equipped with CuKa1 radiation

(k = 1.54056 A), at the scanning rate of 0.05�s�1 in the 2h range

of 2–80�.The surface functionalization of FCNS was investigated

with the Fourier transform infrared (FT-IR) spectroscopy (Per-

kins Elmer Spectrum GX 2500 FT-IR spectrophotometer) on

transmission mode. Thermo gravimetric analysis (TGA) was

done on Mettler Toledo TGA/SDTA 851 equipment under

nitrogen flow (50 ml/min), at a heating rate of 10 �C/min.

The Fourier transform Raman (FT-Raman) spectrum of the

FCNS was recorded using a Renishaw Invia Reflex system at

ambient temperature, employing an argon ion laser at an

excitation wavelength of 514.5 nm.

N2 adsorption–desorption at 77 K and pure gas adsorption

study was carried out using a volumetric gas adsorption

apparatus (ASAP 2020; Micromeritics Inc., USA). The sample

was degassed at 473 K under vacuum (5 · 10�3 mm Hg) for

3 h before the sorption measurements. The adsorption tem-

perature was maintained (±0.1 K) by circulating water from a

constant temperature bath (Julabo F25, Germany). The

elemental composition of FCNS was determined using

CHN/S analysis (Perkin–Elmer CHNS/O analyzer, Series II,

2400) and energy dispersive X-ray (EDX) analysis assembled

with SEM.

3. Results and discussion

3.1. FCNS formation

The structural arrangement or morphology of the obtained

carbon product investigated under SEM depicts an agglom-

erated form of FCNS. Fig. 1 also indicated the particle nature

of FCNS which provides the additional porosity to it. TEM

analysis (Fig. 2a) also reflects the sheets like-arrangement

of the synthesized carbon product with randomly arranged

wrinkled structure. The dark edges appear in the TEM im-

age (Fig. 2a) are the folded region of the FCNS with varying

edge thickness of 6–12 nm. This type of wrinkled arrange-

ment of FCNS was also observed by Kuang et al. [19] using

methane tetrachloride as carbon source. The high resolution

TEM (HR-TEM) analysis of individual FCNS (Fig. 2b) showed

the irregular arrangement of lattice structure. The interlayer

distance measured by multiple layers using the HR-TEM

analysis was �0.344 nm and also in accordance with the

XRD analysis.

The Eq. (1) represents the chemical changes involved in

the synthesis of FCNS from hexachloroethane using copper

as reductant under the autogenic pressure. The yield obtained

in the present transformation is 0.24 g [3.8% on Wt basis

(C2Cl6) and 22.5% on carbon content basis]. Although the both

Fig. 2 – (a) TEM image of agglomerated crumbled FCNS with varying thickness and (b) HR-TEM image of FCNS.

212 C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0

of reactants are taken in the (1:3) stoichiometric amount by

considering the 100% reduction of hexachloroethane into

the carbon, but the reactivity of copper is limited due to the

coating/coverage of formed carbon product on the surface

of the copper. Hence the 100% reduction of hexachloroethane

into the carbon product is not attained in the present path-

way thereby parting the chloride functional groups into the

carbon texture.

C2Cl6 þ 3Cu ���!300�C2Cþ 3CuCl2 ð1Þ

The reaction of hexachloroethane with copper at 300 �Cproduced the FCNS and copper (II) chloride as by-product.

The formation of CuCl2 was confirmed by the XRD pattern

of the as-synthesized product (Fig. 3a) showing intense peak

matching with JSPSD file, No. 018–0439 of copper chloride

hydroxide. The diffraction peaks for the CuCl and unreacted

copper has also been observed with a lower intensity. The

XRD pattern (Fig. 3b) of acid treated product showed the less

intense broad diffraction peak for FCNS. The amorphous nat-

ure and irregular arrangement of carbon layers in obtained

FCNS resulted into the broadening of 002 peak at 2h of

Fig. 3 – XRD pattern of FCNS (a) b

20–30� in the XRD pattern (Fig. 3b). The disappearance of peak

observed for 001 plane at 2h of 43.5 also reflects the disorder

in the FCNS.

The agglomeration of FCNS was also confirmed with Ra-

man spectroscopy. The FT-Raman spectra of the FCNS

(Fig. 4) depict three bands namely D-band, G-band and the

mixed band for 2D and D + G bands. The D-band observed

at 1359 cm�1 associated with the defect or disorder in the

FCNS. The comparable intensity of the D- and G-band showed

the defects in the syntheses FCNS. The G-band, representing

the planner configuration of sp2 bonded carbon structure, is

normally observed at �1582 cm�1 for single layer graphene.

In case of obtained FCNS, the position of the G-band was

shifted towards the higher wavenumber (1599 cm�1) due to

the increase in the sheet thickness [14]. The effect of irregular

layer thickness with disordered arrangement and in situ func-

tionalization of nanosheets was also reflected in the shifting

and broadening of 2D-band (second order or overtone of D-

band) and resulted into the mixing with D + G band (normally

observed at 2910 cm�1). The effect of the layer thickness and

functionalization on the 2D-band was also noticed by the

efore and (b) after acid wash.

Fig. 5 – FT-IR spectrum of reaction product (a) before and (b)

after nitric acid treatment.

Fig. 6 – TGA and derivative TGA plot of FCNS under nitrogen

atmosphere.

Fig. 4 – FT-Raman spectrum of FCNS prepared using

hexachloroethane.

C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0 213

Hong et al. [12] and Subrahmanyam et al. [31]. Ferrari and

Robertson [32,33] provides a better description for the disor-

der induced in the carbon network and derived a new relation

for the topological disordered in the graphite/graphene due to

the introduction of 20% sp3 characters. The in-plane crystal-

line size calculated using Ferrari and Robertson formula,

[(ID/IG) = C 0 La2] is 12.4 nm (C 0 � 0.0055 for 514 nm). The FT-IR

analysis of obtained product before and after nitric acid treat-

ment (Fig. 5) clearly showed the introduction of the oxygen

containing functional groups such as ACOOH, AOH, AC@O,

CAOAC into the FCNS during the acid treatment. The band

at 3400 cm�1 was associated with stretching of hydrogen

bonded AOH, whereas C@O group was characterized by the

stretching mode at 1708 cm�1. The sp2 hybridized aromatic

C@C bond was clearly indicated by the strong absorption

band at 1600 cm�1. The CAO and CAH stretching vibration

were also observed at 1250 and 2920 cm�1, respectively [34].

The oxygen content of FCNS (35.3 Wt%) analyzed by EDX

analysis also support the above observations. The carbon,

chlorine and copper content of the FCNS (by EDX analysis)

were 62.8, 1.4 and 0.5 Wt%, respectively. The elemental

analysis of FCNS was also found in good agreement with

the above results and resulted as 60.2% (C) and 1.1% (H).

The thermal stability of the prepared FCNS was evaluated

with the TGA analysis under nitrogen flow. Fig. 6 showed the

percentage decrease in weight of the FCNS with respect to

temperature. The major weight loss (16.5%) in the FCNS was

observed in the range of the 400–600 �C due to the degrada-

tion of the carbonyl containing functional groups such as

ACOOH, C@O, lactone, etc. Whereas the second major weight

loss (130–320 �C; 8.8%) was due to the decomposition of hy-

droxyl groups and the chemisorbed water molecules. The

moisture or physisorbed water (4.1%) was removed at

<130 �C. The total weight loss of the obtained FCNS was

�29.4% up to 700 �C in the nitrogen atmosphere. The surface

functionality (mainly oxygen containing functional group)

present on the FCNS makes it thermally less stable compared

to pure carbon materials.

The surface area or the texture properties of the carbon

materials plays an important role in their prospective applica-

tions such as adsorbent material, catalyst support, sensors,

etc. Graphene possesses the very high theoretical surface area

of 2650 m2/g, however, this is yet to be achieved due to

agglomerated nature of the synthesized graphene sheets.

The surface area of the FCNS prepared using present protocol

(Table 1) was determined with nitrogen sorption at 77 K. The

nitrogen sorption on FCNS exhibits (Fig. 7a) the Type IV phys-

isorption isotherm, classified by International Union of Pure

and Applied Chemistry (IUPAC), characteristics of the meso-

porous materials. The hysteresis loop of Type-H4 exhibits

the filling and emptying of the mesopores by capillary con-

densation. The derivative (dV/dD) pore size distribution of

the FCNS (Fig. 7b) determined using Barrett–Joyner–Halenda

(BJH) method with desorption data showed the FCNS contains

the maximum number of pores in the range of 3–4 nm (mes-

opores). The adsorption average pore width determined using

4 V/A by Brunauer–Emmer–Teller method (BET) is 4.2 nm. The

Fig. 7 – (a) Nitrogen sorption isotherm of FCNS at 77 K and (b) cumulative and derivative pore size distribution of FCNS

determined by applying BJH method to desorption data.

Table 1 – Specific surface areas, pore volumes, and average pore size of FCNS.

Specific surface area (m2/g) Pore volume (cm3/g) Pore size (nm)

SBETa SLang

b Smicroc Sext

d VTe Vmicro

f DBETg DBJH

h

836.3 1079.3 312.8 523.5 0.883 0.138 4.2 8.5a BET surface area.b Langmuir surface area.c Micropore surface area, calculated using t-plot method.d External surface area, calculated using t-plot method.e Single point adsorption total pore volume, obtained at P/P0 = 0.9732.f t-Plot micro pore volume.g Adsorption average pore diameter, obtained from 4 V/A by BET.h BJH desorption average pore diameter.

214 C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0

cumulative plot of pore volume versus pore size (up to

100 nm) by BJH method (Fig. 7b) reflects the presence of few

large pores in the range of meso and macro-meters compared

to the average pore size (3–4 nm); and their contribution in the

pore volume is comparable with the average sized (3–4 nm)

pores. The presence of the macro porosity is attributed to

the particle nature of the FCNS.

3.2. Greenhouse gas (CO2 and CH4) adsorptivity of FCNSover CO and N2

3.2.1. Adsorption equilibriumThe obtained FCNS was utilized as the adsorbent for the

adsorption of carbon dioxide, methane, carbon monoxide

and nitrogen. The adsorption isotherms of carbon dioxide,

methane, carbon monoxide and nitrogen on FCNS at 288,

303, 318 K and gas pressure up to 113 kPa are given in Fig. 8.

All adsorption isotherms were found to be similar with the

Type-I classified by IUPAC, showing the stronger adsorbate–

adsorbent interaction than adsorbate–adsorbate interaction

in the bulk state. FCNS showed the highest adsorption capac-

ity for carbon dioxide (2.95 mmol/g at 288 K; 2.24 mmol/g at

303 K and 1.64 mmol/g at 318 K) followed by carbon monoxide

(1.34 mmol/g at 288 K; 1.06 mmol/g at 303 K and 0.73 mmol/g

at 318 K), methane (1.12 mmol/g at 288 K; 0.81 mmol/g at

303 K and 0.55 mmol/g at 318 K) and nitrogen (0.39 mmol/g

at 288 K; 0.27 cm3/g at 303 K and 0.16 mmol/g at 318 K) at all

studied temperatures. The carbon monoxide showed the

higher affinity towards the FCNS at lower pressure as com-

pared to the other gases although the carbon dioxide pos-

sesses the high adsorption capacity at 113 kPa. In the case

of carbon adsorbents, non-specific interactions such as dis-

persion and close-range repulsion energy plays an important

role during adsorption due to the absence/less amount of

charges on the surface [35]. Therefore the polarizability of

gases governs their adsorption on carbon materials. Carbon

dioxide, possessing the higher polarizability showed the high-

er adsorption than the other studied gases at final pressure.

The high adsorption affinity between the FCNS and carbon

monoxide at lower pressure can be attributed to the copper

adventitiously present in minute quantities in FCNS. The Cu

salt incorporated activated carbon has been used commer-

cially for the carbon monoxide separation [35]. The enhance-

ment in the carbon monoxide adsorption in FCNS may be due

to the formation of p complexation between copper and car-

bon monoxide. The FT-IR spectrum of the FCNS (Fig. 9) in

the carbon monoxide atmosphere clearly showed the two

additional adsorption bands at 2117 and 2174 cm�1 attributed

for Cu–CO interaction [36,37]. The presence of the element

Fig. 9 – FT-IR spectra of FCNS before and after carbon

monoxide adsorption. (A colour version of this figure can be

viewed online.)

Fig. 8 – Carbon dioxide, carbon monoxide, methane and nitrogen gas adsorption isotherms on FCNS at (a) 288, (b) 303 and (c)

318 K. (A colour version of this figure can be viewed online.)

C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0 215

with high polarizability also improves the adsorption affinity

for adsorbate due to the enhancement in the dispersion po-

tential of adsorbent [35]. Chlorine (2.18) possesses the high

polarizability than the carbon (1.76), hence the presence of

chlorine atom in the texture of the FCNS also have

advantageous for adsorption. The low adsorption capacity

and linear shape of the nitrogen isotherm showed the weaker

interaction of nitrogen with FCNS due to the lower

polarizability.

From the different proposed models for evaluation of the

experimental gas adsorption data, Langmuir and Freundlich

models were used to correlate the experimental adsorption

data using non-linear regression method [38,39]. The data

was fitted with help of Origin Pro 9.0 software.

The Langmuir isotherm [40] is written as:

q ¼ qmBP1þ BP

ð2Þ

The Freundlich isotherm [41] is written as:

q ¼ kP1=t ð3Þ

216 C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0

Where q is the amount of gas adsorbed (mmol/g), P is the

equilibrium pressure (kPa), qm is the monolayer adsorption

capacity (mmol/g), B is the Langmuir constant (kPa�1), k

(mmol/g) and n are the Freundlich parameters related with

the adsorption capacity and adsorption intensity, respectively.

The regression coefficient (R2) and average relative error

(ARE) methods were used to estimate the degree of fitness

of each isotherm.

Calculation of ARE was carried by following formula and

given in Table 2.

%ARE ¼ ðq� qcalÞq

��������100=N ð4Þ

Where q is the experimental adsorption amount, qcal is calcu-

lated adsorption amount using models and N is the number

of adsorption points.

Fig. 10 shows the fitting of Langmuir and Freundlich mod-

els to experimental adsorption isotherms for nitrogen, meth-

ane, carbon monoxide and carbon dioxide on FCNS at 288, 303

and 318 K. In case of nitrogen and methane, the both Lang-

muir and Freundlich models (Fig. 10a and b) were well fitted

with the experimental data having the regression coefficient

of >0.99; due to linear nature of the isotherm and the lack

of saturation in the adsorption capacity at studied pressure.

The same observation was also noted by Khalili et al. [38]

for nitrogen and carbon dioxide adsorption on multi-walled

carbon nanotubes. The Freundlich adsorption model best ex-

plained the adsorption of carbon monoxide on FCNS (Fig. 10c,

Table 2). The best fitting of Freundlich model for carbon mon-

oxide adsorption also supports the multilayer adsorption of

carbon monoxide on FCNS involving p complexation.

Although the carbon dioxide adsorption on FCNS was more

suitable with Freundlich models (Fig. 10d), the regression

coefficient values obtained with Langmuir model were also

found satisfactory (>0.99, Table 2). In case of carbon dioxide,

the lower adsorption temperature favors the adsorbate–

adsorbate interaction and formation of multilayer resulted

into the best fitting with Freundlich model. The %ARE (Table

2) calculated from the experimental and simulated gas

adsorption data also supported the above findings.

Table 2 – Langmuir and Freundlich isotherm constant for adsordioxide on FCNS.

Adsorbate Temperature (K) Langmuir

qm B R2

Nitrogen 288 2.05 0.0021 0.9303 2.18 0.0012 0.9318 1.25 0.0013 0.9

Methane 288 2.75 0.0059 0.9303 2.59 0.0038 0.9318 1.69 0.0042 0.9

Carbon monoxide 288 1.50 0.0348 0.9303 1.26 0.0281 0.9318 0.80 0.0373 0.9

Carbon dioxide 288 4.31 0.0162 0.9303 3.38 0.0150 0.9318 3.09 0.0095 0.9

3.2.2. Adsorption selectivityThe high adsorption capacity and selectivity is the basic crite-

ria for the adsorbent to be utilized in the gas separation appli-

cations. The adsorbent possessing high adsorption capacity

but low selectivity is not viable for the gas separation. Flue

gas exhausts from the power stations mainly contains carbon

dioxide (10–14%) and nitrogen, along with carbon monoxide.

The separation and purification of the carbon dioxide and car-

bon monoxide from the flue gas requires the adsorbent with

high selectivity for these gases over nitrogen. The capacity

selectivity (Scap) of the FCNS for different gases at particular

pressure and temperature was calculated using Eq. (5) [26].

Scap ¼qA

qB

� �P;T

ð5Þ

where qA and qB are the adsorbed quantity of gas A and B,

respectively at given pressure (P) and temperature (T).

The adsorption data was fitted into the Virial equation (Eq.

(6)), to determine the value of first Virial coefficient (A) re-

quired further for the calculation of Henry’s constant using

Eq. (7).

lnp

q¼ Aþ Bqþ Cq2 þ � � � ð6Þ

K ¼ Expð�AÞ ð7Þ

where A, B and C are the first, second and third virial coeffi-

cients, respectively, K is the Henry’s constant.

The equilibrium selectivity (Seq) at particular temperature

was determined using the Henry’s constant as follows [30]:

Seq ¼KA

KB

� �T

ð8Þ

where KA and KB are the Henry’s constant at temperature (T).

Fig. 11 showed the plot of capacity selectivity and equilib-

rium selectivity of FCNS for carbon dioxide, carbon monoxide,

methane and nitrogen over each other at different tempera-

ture and equilibrium pressure. The FCNS showed the higher

capacity selectivity for carbon dioxide over nitrogen (8.4) fol-

lowed by methane (2.8) and carbon monoxide (2.1) at 303 K.

The decrease in the capacity selectivity was observed with in-

crease in the pressure. The decrease in the capacity selectivity

ption of nitrogen, methane, carbon monoxide and carbon

Freundlich

ARE% K t R2 ARE%

999 0.8 0.0058 1.12 0.9996 3.9999 1.3 0.0032 1.07 0.9997 3.2998 1.0 0.0020 1.08 0.9997 1.9990 3.9 0.0310 1.31 0.9992 7.7987 3.8 0.0162 1.21 0.9991 6.8998 2.6 0.0118 1.23 0.9993 8.1484 30.4 0.1635 2.31 0.9945 6.5691 26.9 0.1062 2.10 0.9976 3.0638 25.6 0.0907 2.33 0.9976 2.8925 11.9 0.1828 1.70 0.9987 5.6953 8.4 0.1281 1.65 0.9979 9.4984 5.2 0.0647 1.46 0.9985 11.0

Fig. 10 – Experimental and stimulated (Langmuir and Freundlich) adsorption isotherms of (a) nitrogen, (b) methane, (c) carbon

monoxide and (d) carbon dioxide on FCNS at different temperatures. (A colour version of this figure can be viewed online.)

C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0 217

over pressure was attributed to the decrease in the adsorption

capacity with pressure of other adsorbate in comparison with

linear adsorption of nitrogen on FCNS. The exception was ob-

served with above findings in case of selectivity of carbon

dioxide over carbon monoxide due to the initially high

adsorption affinity of carbon monoxide to FCNS. The capacity

selectivity trend for the gases studied was CO2/N2 > CO/

N2 > CH4/N2 > CO2/CH4 > CO2/CO > CO/CH4 at all studied tem-

peratures because of the high adsorption capacity of FCNS for

CO2 and CO. Both the pore size distribution and surface func-

tionality played an important role in the adsorption capacity

by effecting adsorbate–adsorbent interaction. The micropore

volume with uniform pore size distribution is more favorable

for the gas adsorption on carbon material [42,43]. Such type of

uniform microporosity can be achieved in the carbon material

synthesized using zeolite as template. Boisgontier et al. [44]

synthesized the zeolite templated carbon replicas having high

micropore volume with uniform pore size distribution. The

prepared carbon replicas showed the maximum equilibrium

selectivity of 2.9 and 8.4 for carbon dioxide over methane

and nitrogen, respectively. Here the adsorbate properties

and pore structure of carbon replicas were responsible for

the adsorption and separation efficiency. The hydrophobic

nature (i.e., less surface oxygen functionality) of the carbon

material is favorable for methane adsorption [45,46]. Contre-

ras et al. [43] also supported the above findings and observed

the significant enhancement in the methane adsorption

capacity of the commercial activated carbon thorough the

elimination of oxygen functionality by heat treatment. The

simulation and experimental studies showed that the oxygen

functionality such as hydroxyl, carbonyl, carboxylic, etc. sig-

nificantly influenced the electrostatic adsorbate–adsorbent

interactions and favors the carbon dioxide adsorption on car-

bon materials [47,48]. The obtained FCNS possesses the larger

average pore diameters (3–4 nm) in comparison with the ki-

netic diameters of the adsorbate gas molecules (<3.9 A) and

hence have negligible effect on the selectivity. The high oxy-

gen surface functionality of FCNS supports the high adsorp-

tion as well as selectivity for the carbon dioxide as

compared to the other adsorbents due to the electrostatic

interaction of carbon dioxide with oxygen-containing func-

tional groups. Furmaniak et al. [49] observed the effect of

Fig. 11 – Capacity selectivity of FCNS at (a) 288, (b) 303, (c) 318 K and (d) equilibrium selectivity at different temperatures. (A

colour version of this figure can be viewed online.)

218 C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0

carbon pore size and oxygen functionality on the carbon diox-

ide and methane selectivity. The introduction of the oxygen

functional groups enhances the carbon dioxide equilibrium

selectivity over methane (�2–3 times) than the parent carbon

material, and same phenomena is also expected for the

adsorbate possessing the weaker electrostatic and dispersion

interactions [49]. The equilibrium selectivity of the materials

decides the theoretical separation efficiency of the adsorbent

at particular temperature. The zeolites are widely used as

commercial adsorbent for carbon dioxide separations from

flue gas due to their low cost and high equilibrium selectivity

for carbon dioxide over nitrogen. The FCNS obtained in the

present study showed the high equilibrium selectivity of

Table 3 – Heat of adsorption, Henry’s constant and Virial coefficion FCNS.

Adsorbate Heat ofadsorption

(kJmol�1)

Henry’s constant (10�3 mmol

288 K 303 K

CO2 28.8 80.8 50.1CO 39.2 591.0 385.6CH4 25.1 10.0 8.8N2 24.0 3.0 2.0

197, 197 and 193 for carbon monoxide over nitrogen at 288,

303 and 318 K, respectively. Although, the FCNS possesses

the high CO2 adsorption capacity than CO, the equilibrium

selectivity of carbon dioxide was less (27 at 288 K; 26 at

303 K and 18 at 318 K) than that of CO over nitrogen. The high

loading of carbon monoxide on FCNS at lower pressure due to

the Cu–CO interaction attributed for the high value of Henry’s

constant (Table 3) than that of carbon dioxide and resulted

into the high equilibrium selectivity. Besides, FCNS also

showed good equilibrium selectivity for carbon monoxide

over carbon dioxide and methane at all studied temperature

(Fig. 11). The equilibrium selectivity of FCNS for carbon diox-

ide over methane (5.7) and methane over nitrogen (4.5) at

ent at 288, 303 and 318 K for CO2, CH4, CO and N2 adsorption

g�1 kPa�1) Virial coefficient A

318 K 288 K 303 K 318 K

26.2 2.516 2.994 3.640288.8 0.526 0.953 1.242

7.6 4.637 4.728 4.8731.5 5.808 6.234 6.502

C A R B O N 6 8 ( 2 0 1 4 ) 2 1 0 – 2 2 0 219

303 K is also promising. Hence the FCNS can be a potential

adsorbent material for carbon dioxide/carbon monoxide sep-

aration due to its high equilibrium selectivity and adsorption

capacity for CO2 and CO. Isosteric heat of adsorption (Table 3)

was calculated with the help of software provided with ASAP

2020 by fitting the adsorption data obtained at 288, 303 and

318 K into Clausius–Clapeyron equation given below.

�DH ¼ R½d lnP=dð1=TÞ�h ð9Þ

The positive sign of heat of adsorption indicated the

adsorption of studied gases on FCNS was exothermic process.

The carbon monoxide adsorption on FCNS associated with

the higher heat of adsorption followed by carbon dioxide,

methane and nitrogen showing the greater interaction of

the carbon monoxide with FCNS. The present study showed

that the synthesized FCNS is the possible potential candidate

for the gas separation, but the work on the actual separation

of gas mixture is under progress and will be discuss later.

4. Conclusion

Few layered FCNS was successfully synthesized using hexa-

chloroethane as carbon source and copper powder as reduc-

ing agent under autogenic pressurized condition. The

obtained CNS showed the varying layer thickness of

6–12 nm (as characterized by TEM analysis) and possesses

the surface and textural oxygen; and chlorine functionality.

The equilibrium gas adsorption study of greenhouse gases

(CO2 and CH4), toxic gas (CO) and light gas (N2) showed that

the CNS possesses the high adsorption capacity for CO2 over

CO, CH4 and N2. The carbon monoxide showed greater bind-

ing tendency towards FCNS at lower pressure due to the cop-

per adventitiously present in minute quantities in FCNS and

resulted into highest equilibrium selectivity for CO over other

gases at different temperatures. The p complexation of CO

with copper on FCNS was also supported by the FT-IR analy-

sis. The adsorption of N2 and CH4 on FCNS was fitted with

the both Langmuir and Freundlich model whereas the CO

adsorption exhibits better fit for Freundlich model. In the case

of carbon dioxide, the lower adsorption temperature favors

the adsorbate–adsorbate interaction and formation of multi-

layer resulted into the best fitting with Freundlich model.

Acknowledgments

The authors are grateful to Council of Scientific and Industrial

Research (CSIR), New Delhi, India for financial support under

Network project CSC-102. Sandesh Sawant acknowelegdes

CSIR, New Delhi for the award of Senior Research Fellowship.

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