Structure and Morphology
Figure 1 (a), (b), and (c) represents the SEM images of samples before carbonization at a magnification of 50x, which reveals that the samples are spheroid in structure with average diameter between 3.0 to 3.2 mm. The cross section of these samples [Fig. 1 (d), (e), (f)] showed an internal layered structure of CB-0, while that of NCB-1 and NCB-3 showed a clogged structure indicating HMT induced more compact internal structures. The diameter of carbonized samples [Fig. 1 (g), (h) and (i)] were between 2.5 and 2.8 mm. The outer surfaces of samples exhibited irregular and heterogeneous porous structure. The higher magnification of carbonized samples showed that the incorporation of HMT caused a decline in the porous structure, which was observed to be the maximum in case of NC-3, followed by NC-1 and C-0 [Fig. 1 (j), (k), (l)]. The decline in the porosity may be ascribed to the hydrogen bonding interaction between nitrogen atoms of HMT and -OH groups of chitosan (Fig. 2). The hydrogen bonding interaction brings the polymer chains closer together resulting the hindrance for the evolution of volatile gases during thermal activation process, thereby progressively reducing the porous structure.
Elemental Analysis
EDS spot scan and mapping were used for the elemental composition survey of the activated carbon samples. The elemental mapping of the samples is shown in Fig. 3 (a), (b) and (c). The elemental analysis by EDS spot scan illustrate the distribution of C, N and O in all the samples that qualitatively confirmed the doping of nitrogen in the increasing order as the composition of HMT was increased [Fig. 3 (d), (e) and (f)]. The percentages of C, N and H in the activated carbon samples were quantitatively obtained using CHN analysis (Table S1). The elemental analysis of C-0, NC-1, and NC-3 obtained from EDS spot scan as well as CHN analysis confirmed an improvement in the percentage of nitrogen as the concentration of HMT was increased. Thus upon carbonization, nitrogen from HMT was effectively doped on to the carbon framework [20]. Therefore, HMT acts as a better nitrogen dopant as compared to other nitrogen dopants such as urea and ammonia reported by Liu et al. [14] and song et al. [24] respectively. The reported percentages of nitrogen obtained for N-doped activated carbon using urea as the nitrogen dopant was 7.5, while using ammonia as the dopant was 9.72 while that of HMT (present study) is 13.06.
Thermal Analysis
The profiles of thermogravimetric analysis (TGA) as well as derivative thermogravimetry (DTG) carried out at 20°C/min. ramping under N2 atmosphereobtained for samples before and after carbonization are shown in Fig. 4 and Fig. 6 respectively. The TGA plot depicted the weight losses as a function of temperature, while the DTG plot portrayed derivative of weight loss as a function of temperature. From Fig. 4 (a&b), it is observed that the removal of moisture happened for CB-0 at 120°C, which is not prominent for the samples, NCB-1 and NCB-3. This may be due to the increase in the intermolecular H-bonding between chitosan and HMT, which restricts the later samples to coordinate water molecules with functional groups of chitosan (Fig. 2). CB-0 exhibited the 1st decomposition stage from 175°C to 330°C with a maximum decomposition temperature at 240°C. This may be due to the degradation of functional groups and side chains, i.e. –NH2, -OH, -CH2OH and -NH(CO)CH3. The decomposition for NCB-1 started around 190°C and ended at 410°C with a maximum decomposition temperature at 275°C. The initial shoulder at 220°C for NCB-1 observed in Fig. 4(b) may be due to the removal of functional groups and side chains of chitosan followed by the decomposition of HMT. The corresponding decomposition for NCB-3 started at 175°C and ended at 415°C with a maximum at 290°C as a single decomposition event, which may be due to the simultaneous decomposition of functional groups, side chains of chitosan and HMT. The enhancement in the thermal decomposition temperatures for NCB-1 and NCB-3 as compared to CB-0 may be due to the existence of H-bonding interaction between chitosan and HMT, due to which a higher thermal energy is required to overcome the bonding interaction and to degrade the structure. The 2nd stage of decomposition for CB-0 started at 330°C and ended at 570°C with a maximum at 410°C, while the corresponding decomposition temperature for NCB-1 and NCB-3 was observed between 430 to 700°C with a maxima at 560°C, which may be attributed to the degradation of polymer backbone structure [25]. The slower degradation profile for NCB-1 and NCB-3 as compared to CB-0 indicates the polymer chains are held by HMT due to stronger physico-chemical interaction. NCB-3 showed the highest thermal stability as compared to CB-0 and NCB-1 both in terms of thermal degradation profile and char yield. Figure 5 (a) & (b) represented the TGA and DTG graphs of activated carbons respectively. The TGA and DTG plot reveals that the decomposition for activated carbon samples occurred around 500°C, which was caused by the elimination of labile functional groups –CO and –OH forming a stable carbon-carbon bond in the framework [26]. The thermal stability of N-doped carbon was found to be better than the undoped activated carbon. While considering the residues around 800°C NC-1, and NC-3 possessed about 61%, whereas C-0 had 57% of residues. A similar result was reported by Hai Long Peng et al. where the doping of activated carbon with pentaethylenehexamine (PEHA) resulted in an improved thermal stability of the carbon [27], which is attributed to the confining effect of nanopores.
FT-IR Analysis
Fig S2 (a) shows the FT-IR spectra of samples before carbonization. The bands obtained around 3600 − 3400 cm− 1 for the three samples correspond to the O-H stretching vibration. The band at 1593 cm− 1 for the samples corresponds to the primary amine N-H bending while the bands around 1050 cm-1 attributed to the C-N stretching of amine groups [28]. The sharp peak at 1380 cm− 1 could be assigned to the –CH3 symmetrical deformation mode. Even though there arises a H-bonding due to the HMT addition in case of NCB-1 and NCB-3, it is not prominently observed due to the broadened FT-IR peaks. Fig. S2 (b) represents the FT-IR spectra of samples after carbonization. The bands around 3600 − 3400 cm− 1 correspond to the O-H stretching vibrations, which was similar in the case of samples before carbonization. The bands around 1740 cm-1 was attributed to the –C = O stretching vibrations. The appearance of a band at 1452 cm− 1 was attributed to the C-H stretching vibration [29].
XRD Analysis
The diffraction patterns of C-0, NC-1, and NC-3 are depicted in Fig S3. No significant difference in the diffraction patterns was observed among the three samples. All the samples possessed patterns of a non-ordered material i.e. an amorphous structure with a very low degree of graphitization [30]. The broad and weak peaks obtained at 2ϴ = 25° was ascribed to the (002) plane of polycyclic aromatic C- sheets of amorphous carbon in irregular orientation and reflection of graphene, whereas 2ϴ = 42° represented the (100) plane of 2D in-plane diffraction of graphene sheets. A similar result wasobserved by Limin et al. in the case of activated carbon prepared from urea modified coconut shell [31]. The percentage of crystallinity from the XRD peaks were calculated to be 52.3%, 34.7% and 31.1% respectively for C-0, NC-1 and NC-3 which indicated that the amorphous nature of the carbons increased with the incorporation of HMT. The decrease in the crystallinity can be attributed to the fact that during nitrogen doping, elemental nitrogen enters into the carbon ring by replacing carbon, thereby destroying its graphitic lamellar structure [32].
Raman Spectral Analysis
Figure 6 represents the Raman spectra of C-0, NC-1 and NC-3. All the samples showed two peaks at 1350 cm− 1 and 1580 cm− 1 corresponding to the D and G bands of graphitic carbon respectively. D band corresponds to the A1g mode similar to the in-plane breathing vibration, which is Raman inactive in large crystals that becomes active due to the presence of disorder parts like grain edges. The G band is attributed to the E2g in-plane vibrational mode of sp2 carbon framework in the planar hexagonal crystal lattice [33]. The degree of disorder in the graphitic structure is determined by the intensity ratio, ID/IG. In the case of C-0, NC-1 and NC-3 the ID/IG values were calculated from the peak heights and found to be 1.21, 1.23 and 1.25 respectively. The increase in the values of ID/IG indicated the increase of disorder in the graphitic structure, which may be caused by the doping of nitrogen. As the defect in the N-doped activated carbons increases due to the incorporation of nitrogen into the graphitic framework, the intensity of D band increased and as a result the ID/IG value also increased. A similar result was reported by Shuai Zhang et al., where the undoped activated carbon possessed an ID/IG of 1.15 and the doping of activated carbon using polyethylenimine (PEI) in different ratios resulted in the increase in ID/IG to 1.32, 1.35 and 1.45 due to the irregularity in carbon atom configuration by the incorporation of nitrogen [34]. The increase in the ID/IG value was consistent with the XRD data and this reveals doping of nitrogen in case of NC-1 and NC-3.
N2 adsorption desorption isotherm
The N2 adsorption desorption isotherms represented in Fig. 7 (a) showed characteristics of mesoporous materials having a typical Type 4 isotherm with a hysteresis loop caused by the capillary condensation [35]. The N2 adsorption decreased as the ratio of HMT impregnation was increased. Subsequently, the surface areas for NC-1 and NC-3 were lesser than that of C-0 as shown in Table S2. Hai Long Peng et al. reported a comparable results with PEHA loadings on the activated carbon [27]. Considering the BJH pore size distribution in Fig. 7 (b), the distribution around 2 nm showed the presence of micropores and the distribution around 2–50 nm showed presence of mesopores. The mean pore diameters 2.211 nm, 2.105 nm and 2.125 nm for C-0, NC-1, and NC-3 respectively calculated from the BJH pore size distribution indicates that all the three samples possessed mesoporous characteristics [36].
The iodine values for the three samples obtained using ASTM D4607-94 [37] were 1262 mg/g, 1196 mg/g and 1080 mg/g respectively, which also indicated that HMT doping results in the decrease of porosity. The total pore volume of C-0, NC-1 and NC-3 were 0.138 cc/g, 0.095 cc/g and 0.075 cc/g respectively. Among the three samples C-0 possessed the maximum surface area and pore volume. Therefore, it was observed that by the doping of activated carbon with HMT, there occurred a deterioration in the structural characteristics, which corroborate with XRD analysis. The surface characteristics, nitrogen content as well as the CO2 adsorption capacity of these samples were compared with other activated carbons recorded in the literatures and is listed in Table 1. On comparison, it was observed that the activated carbon adsorbents in this studyexhibited better adsorption capacity but using simple synthesis method than that of many other activated carbons. In the case of carbon sorbents, which showed higher surface area as well as adsorption capacity in the previous literatures,the method of preparations were complex and further, the raw materials used in the processes were costlier.
Table 1
Comparison of N content, BET surface area and CO2 adsorption capacity with other works
Sample
|
Carbon precursor, nitrogen dopant
|
N content (%)
|
BET surface area (m2/g)
|
CO2 adsorption capacity
(mg/g) at 25°C (method)
|
Ref
|
C-0
|
Chitosan
|
7.563
|
391.502
|
97.98 (U-Tube method)
|
This work
|
NC-1
|
Chitosan, HMT
|
9.458
|
259.017
|
72.95 (U-Tube method)
|
This work
|
NC-3
|
Chitosan,HMT
|
13.06
|
111.717
|
55.11 (U-Tube method)
|
This work
|
Biochar
|
Cotton stalk, NH3
|
0.71
|
435
|
79.20 (Thermogravimetric analysis)
|
[38]
|
AC-NH-400
|
Eucalyptus wood, NH3
|
3.14
|
1637
|
48.40 (Volumetric method)
|
[39]
|
ANF-0.5T
|
PAN, TEPA
|
9.51
|
59.66
|
18.48 (Monosorb)
|
[15]
|
SU-25-1-650
|
Sucrose, urea
|
7.7
|
1745
|
189.20 (Thermogravimetric analysis)
|
[40]
|
PTSA-β-CD-2.5
|
β-cyclodextrin
|
Not reported
|
620
|
112.64 (Thermogravimetric analysis)
|
[41]
|
CSc-2-700
|
Carboxymethyl cellulose
|
Not reported
|
1705
|
182.00 (ASAP 2020)
|
[42]
|
KQA-1/1-500
|
Furfural, urea
|
7.5
|
1013
|
202.4 (ASAP 2020)
|
[14]
|
Carbon cryogel
|
Phenol-Urea-Formaldehyde resin
|
2.08
|
1710
|
242.44 (ASAP 2020)
|
[43]
|
CO2 adsorption studies
The CO2 adsorption uptake of C-0 was calculated to be 97.98 mg/g,whereas for NC-1 and NC-3 were 72.95 and 55.11 mg/g respectively (Fig. 8) with an average uncertainty of 2%. TGA and DTG studies were performed on the CO2adsorbed carbon samples (Fig. S4) to evaluate the optimal regeneration temperature. The initial weight loss started at 30°C and ended at 150°C with a maximum weight loss at about 60°C indicated the removal of CO2from the carbon samples [Fig. S4 (b)]. The amounts of residue after the initial weight loss was found to be 78%, 81% and 86% for C-0, NC-1 and NC-3 respectively [Fig. S4 (a)], which also confirms that the maximum CO2 adsorption capacity is for C-0. The TGA plot also revealed that the complete desorption of CO2 from samples occurred around 150°C. Therefore regeneration studies were performed on the samples at 150°C (Fig. S5). There observed an excellent regeneration capacity for C-0, NC-1 and NC-3. Among the three samples C-0 showed that maximum adsorption capacity with adsorption values of 97.98, 97.55, 96.34, 97.45, 95.34 mg/g respectively. It was expected an increase in the CO2 adsorption for N-doped activated carbons as reported in previous studies [44, 45]. The basic nitrogen functionalities react with the Lewis acidic CO2 to increase the chemisorption to a greater extent. However, in the present study, the decline in the adsorption capacity of activated carbon due to the N-doping with HMT was due to deterioration of structural characteristics (i.e. surface area and pore volume). The plausible reason may be due to (i) the hydrogen bonding interaction between HMT and chitosan polymer chain, which reduces the free volume between the polymer networks as discussed in previous paragraph (Fig. 2) and (ii) increase in pH of chitosan precursor solution with the addition of HMT. The addition of HMT to chitosan solution marked a significant increase in the pH. The pH of chitosan solution was measured to be 2.56, while that of chitosan solution mixed with 1:1 HMT and 1:3 HMT were 3.28 and 4.19 respectively. According to the course grind (CG) model reported by Hongcheng Xu et al. the chitosan polymer chains self-aggregate with gradual increase in pH of solution [46]. As depicted in Fig. 9, the self-assembly of chitosan polymer chains starts to form a physically cross-linked network as the pH of the solution is increased from 2.56 to 3.28 and then to 4.19 with respect to the increase in HMT concentration. The resulting physical cross-linking of polymer chains resulted in the deterioration in the surface characteristics such as surface area and pore volume. As a result, the adsorption capacity of N-doped activated carbon prepared from chitosan/HMT precursor system reduced as compared to that of the undoped activated carbon obtained from chitosan alone.