Morphology and nanostructrue of PCMCA-X.
To illustrate the influence of the carbonization temperature on the structure of the PCM derived carbon materials, an SEM examination of the morphology was carried out first, and the results are shown in Fig. 1b-d. The obtained PCM derived carbon materials showed similar morphology. These carbon materials were the aggregate of some nanosheets and irregular hollow boxes. The formation of these structures is related to the template effect from K2CO3 [40]. With the thermal decomposition of PCM, K2CO3 and carbon can be produced, and the K2CO3 particles are usually coated by carbon. After the removal of K2CO3 during the following water washing procedure, the irregular hollow box-like carbon can be obtained. In PCMCA-700 and PCMCA-800, since the carbonization temperature is relatively low, some irregular hollow box-like carbon can be found. While in PCMCA-900, due to the thermal decomposition of the irregular hollow box-like carbon at high temperature, the aggregate of nanosheets can mainly be observed. These irregular nanosheets can also be found from the TEM images (Fig. 1e-g), and with the increase of carbonization temperature, it is more conducive to the formation of micropores.
Characterization of PCMCA-X.
Figure 2a shows the Raman spectrum of PCMCA-X, from which the D peak at 1340 cm− 1 and the G peak at 1592 cm− 1 can be seen [41]. The size of ID /IG reflects the defect degree of the carbon material. The higher the ratio, the more defect structures in the carbon material. The largest ID/IG of PCMCA-900 indicates that it has more defect structures. These defects are caused by a large number of micropores generated by the increase in carbonization temperature. The XRD patterns of PCMCA-X are recorded in Fig. 2b. There are two abroad diffraction peaks at 2θ = 26 and 43°, which correspond to the (002) and (101) crystal planes of amorphous carbon, respectively [42].
The textural properties of PCM derived carbon materials were determined by the N2 adsorption/desorption method. As shown in Fig. 2c, PCMCA-700 shows a type-IV isotherm with an H4 type hysteresis loop, which means that PCMCA-700 is a mesoporous carbon material [43]. The isotherms of PCMCA-800 and PCMCA-900 exhibited a typical type I isotherm. Compared with PCMCA-800, the isotherm curve of PCMCA-900 presents relatively high nitrogen adsorption in the low pressure range (P/P0 < 0.05), which indicates the formation of a more microporous structure in PCMCA-900 [44]. Meanwhile, a wide adsorption knee can be observed in the isotherm curve of PCMCA-900, demonstrating that the mesopores are existing in PCMCA-900 [45–47]. Pore size distribution diagrams of PCM derived carbon materials indicate that most of the pores of PCMCA-700 are distributed in the pore diameter range from 2 to10 nm, while PCMCA-800 and PCMCA-900 process somewhat broader pore size distribution with the most of their pores distributed in the pore diameter range from 0.5 to10 nm (Fig. 2d).
To illustrate the difference in textural properties of the three PCM derived carbon materials, detailed textural parameters of the three adsorbents are listed in Table 1. The BET surface areas of PCMCA-700, PCMCA-800, and PCMCA-900 are calculated to be 506, 1032, and 1476 m2 g− 1, respectively. PCMCA-900 shows the highest micropore volume (Vmicro) of 0.497 cm3 g− 1 and total pore volume (Vtotal) of 0.951 cm3 g− 1 among the three adsorbents. During the investigation of CO2 adsorption, it has been reported that the volume of narrow micropores (the pores distributed between 0.7 to 0.9 nm, P0.7−0.9) can significantly affect the adsorption result. Usually, the relatively strong interaction between CO2 and the surface of narrow micropores can greatly improve the CO2 adsorption capacity [48, 49]. It can be observed from Table 1 that the P0.7−0.9 in PCMCA-900 is 19.21%, which is the highest among the three adsorbents. Thus, the high surface areas and suitable pore volume distribution of PCMCA-900 can provide more active surface sites and diffusion channels for adsorption, which can enhance the CO2 and CR adsorption performance of PCMCA-900.
Table 1
Detailed textural parameters of PCM derived carbon materials
Sample
|
SBET (m2 g− 1)
|
Smicro (m2 g− 1)
|
Vtotal (cm3 g− 1)
|
Vmicro (cm3 g− 1)
|
P0.7−0.9 a (%)
|
PCMCA-700
|
506
|
93
|
0.539
|
0.044
|
0
|
PCMCA-800
|
1032
|
909
|
0.556
|
0.358
|
14.47
|
PCMCA-900
|
1476
|
1140
|
0.951
|
0.497
|
19.21
|
a Percentage of the pores distributed between 0.7 to 0.9 nm |
The surface element state of the PCM derived carbon materials were identified by XPS. As shown in Fig. 3a, two peaks appearing at 285 eV and 533 eV, which suggests the main elements in PCM derived carbon materials are C and O [50]. In Fig. 3b, the high resolution spectra of C 1s can be further fitted into four separate peaks at 284.3 eV, 285.3 eV, 286.5 eV, and 288.6 eV, which was attributed to C-C (C1), C-OH (C2), C = O (C3), and COOH (C4), respectively [51, 52]. The percentage of each component is listed in Table 2. The percentage of the O-containing functional groups (C2, C3, and C4) of the PCMCA-900 sample is highest among the three examined samples, indicating that more O-containing functional groups can be introduced at the relatively high carbonization temperature [53]. In Fig. 3c, The high resolution spectra of O 1s can be divided into four separate peaks at 531.5 eV, 533.5 eV, 533.6 eV, and 535.0 eV, which can be assigned to carbonyl, ketone or lactone groups (O1), ether and alcohol groups (O2), carboxyl group (O3) and oxygen in water (O4), respectively [54, 55]. The detailed percentage of these components is presented in Table 3. The total percentage of O1 and O2 in the PCMCA-900 sample is 73.09%, which is the highest among the three samples. Since O1 and O2 are responsible for alkalinity, and O3 is responsible for acidity, the high percentage of O1 and O2 in the PCMCA-900 sample indicates that PCMCA-900 shows more alkalinity properties than the other two carbon materials. The presence of these O-containing functional groups can improve the adsorption properties of PCM derived carbon materials [51].
Table 2
XPS data of C 1s composition of PCM derived carbon adsorbents
Sample
|
C1 (%)
|
C2 (%)
|
C3 (%)
|
C4 (%)
|
PCMCA-700
|
47.69
|
28.98
|
11.47
|
11.86
|
PCMCA-800
|
48.81
|
23.55
|
16.88
|
10.76
|
PCMCA-900
|
45.59
|
31.06
|
13.85
|
9.5
|
Table 3
XPS data of O 1s composition of PCM derived carbon adsorbents
Sample
|
O1 (%)
|
O2 (%)
|
O3 (%)
|
O4 (%)
|
PCMCA-700
|
6.60
|
45.12
|
33.57
|
14.71
|
PCMCA-800
|
10.73
|
44.36
|
42.82
|
2.09
|
PCMCA-900
|
15.28
|
57.81
|
23.14
|
3.77
|
CO2 adsorption analysis
To compare the CO2 adsorption performance of the PCM derived carbon adsorbents, the CO2 isotherms of PCMCA-X were examined at 0°C, and the corresponding results are presented in Fig. 4a. The static CO2 adsorption capacities of the three adsorbents at 0°C and 1 bar are 4.12 mmol g− 1, 6.52 mmol g− 1, and 7.67 mmol g− 1, respectively. The difference in the static CO2 adsorption capacity of the three adsorbents can be related to their different textural properties. PCMCA-900 has the highest surface area and suitable pore volume distribution, which can not only provide more CO2 adsorption sites but also provide more CO2 diffusion channels, leading to the best CO2 adsorption performance of PCMCA-900 among the three adsorbents. Moreover, as shown in Fig. 4b, the SBET of the PCMCA-X materials has a strong influence on the CO2 storage with a high R2 = 0.954, indicating the higher specific surface is favor for improvement of CO2 stroage at 0°C.
In order to illustrate the binding affinity between the PCMCA-900 adsorbent and CO2, the isosteric adsorption heat was calculated according to the method provided in the literature [52]. The isosteric adsorption heat value of the prepared PCMCA-900 adsorbent is in the range of 22.37 to 27.47 kJ mol− 1 in Fig. 4c, which indicates the adsorption of CO2 on PCMCA-900 adsorbent is a typical physical adsorption process [56]. The adsorption mechanism diagram was shown in Fig. 4d. Due to the van der Waals force between molecules, carbon dioxide molecules are adsorbed on the surface of porous carbon.
Temperature is an important factor affecting the CO2 adsorption performance of adsorbents [57]. Thus, the influence of adsorption temperature on the CO2 adsorption performance of PCMCA-900 was explored as well. It can be observed from Fig. 4e that the CO2 adsorption capacity of PCMCA-900 decreased from 7.67 mmol g− 1 to 2.11 mmol g− 1 when the adsorption temperature increased from 0°C to 50°C [58]. The adsorption capacity of CO2 decreases with the increase of adsorption temperature. This phenomenon is due to the fact that, when the temperature increases, the surface adsorption energy and molecular diffusion rate of CO2 increase, resulting in the instability between CO2 and adsorbent and the decrease of CO2 adsorption capacity [59, 60].
To investigate the recyclability of PCMCA-900, the successive adsorption-desorption experiment was applied. At each cycle, PCMCA-900 was degassed at 250°C for 1 h under vacuum. Then, PCMCA-900 was cooled to 0°C to start the next cycle. 10 cycles were carried out to evaluate the recyclability of PCMCA-900. It can be seen from Fig. 4f that, after 10 cycles of CO2 adsorption and desorption, the CO2 adsorption capacity decreased by less than 5% (7.67 mmol g− 1 to 7.29 mmol g− 1), demonstrating the high recyclability of PCMCA-900.
Congo red adsorption analysis
Since PCMCA-900 showed good adsorption performance in the CO2 adsorption, PCMCA-900 was further used in the study of CR adsorption. The adsorption kinetics of the PCMCA-900 for the removal of CR was examined firstly, and the results are shown in Fig. 5a. The CR adsorption ability of the PCMCA-900 is greatly affected by the CR initial concentration. When the CR initial concentration was increased from 50 mg L− 1 to 200 mg L− 1, the CR adsorption capacity of the PCMCA-900 increased from 297.7 mg g− 1 to 408.5 mg g− 1. Generally, adsorption can be divided into physical adsorption and chemical adsorption. To demonstrate the adsorption mechanism of PCMCA-900, the pseudo-first-order and pseudo-second-order kinetic models were used to illustrate the adsorption mechanism [61]. The pseudo-first-order and pseudo-second-order kinetic models can be described in the following equations:
$$\text{ln}\left({Q}_{e}-{Q}_{t}\right)=\text{ln}{Q}_{e}-{K}_{1}t$$
2
\(\frac{t}{{Q}_{t}}=\frac{1}{{K}_{2}{Q}_{e}^{2}}+\frac{1}{{Q}_{e}}\) t (3)
where Qe (mg g− 1), Qt (mg g− 1), K1 (min− 1), and K2 (g (mg·min)−1) are corresponding to the adsorption capacity at adsorption equilibrium, the adsorption capacity at time t (min), the adsorption rate constant, and the rate constant of the pseudo-second-order adsorption, respectively [62]. The simulated results are shown in Table 4. When the isothermal adsorption kinetic data of CR was simulated by the pseudo-second-order model, the obtained correlation coefficient (R2) was greater than that of the pseudo-first-order model. Meanwhile, the calculated adsorption capacities at adsorption equilibrium (Qe,cal) based on the pseudo-second-order model are much closer to the experimental data than that based on the pseudo-first-order model. These results indicate that the pseudo-second-order model is more suitable for describing the CR adsorption behavior of the PCMCA-900. Therefore, the kinetic study results demonstrate that the overall adsorption kinetics of CR by PCMCA-900 is mainly a chemisorption-controlled process and governed by the diffusion time of adsorbate into the narrow pores [63–65].
Table 4
CR kinetic adsorption fitting parameters.
C0
|
Qe,exp
|
Pseudo-first-order
|
Pseudo-second-order
|
Qe,cal
|
K1
|
R2
|
Qe,cal
|
K2
|
R2
|
50
|
297.7
|
293.526
|
0.091
|
0.905
|
297.546
|
0.006
|
0.987
|
100
|
349.1
|
335.954
|
0.157
|
0.972
|
348.129
|
0.008
|
0.996
|
200
|
408.2
|
395.135
|
0.282
|
0.983
|
404.918
|
0.001
|
0.998
|
To illustrate the equilibrium state of CR molecules in the liquid and solid phase at a constant temperature, Langmuir and Freundlich isotherm models were used, which can be respectively represented by following equations: [66].
$$\frac{1}{{Q}_{\text{e}}}=\frac{1}{{Q}_{\text{m}}{K}_{\text{L}}}\frac{1}{{c}_{\text{e}}}+\frac{1}{{Q}_{\text{m}}}$$
4
$$\text{log}{Q}_{\text{e}}=\text{log}{K}_{\text{F}}+\frac{1}{n}\text{log}{c}_{\text{e}}$$
5
where Qe (mg g− 1), Qm (mg g− 1), Ce (mg L− 1), KL (L mg− 1), KF ((mg g− 1) (L mg− 1) 1/n), and 1/n refer to the adsorption capacity of the adsorbent for the adsorbate at adsorption equilibrium, the maximum adsorption capacity of the adsorbent for the adsorbate, the concentration of adsorbate in solution at adsorption equilibrium, the equilibrium constant in the Langmuir isotherm model, the equilibrium constant in the Freundlich isotherm model, and the adsorption intensity, respectively [67]. It can be seen from Fig. 5b that the Langmuir model is more suitable for simulating the experimental data due to its high R2 value of 0.999. Since the Langmuir model is related to the monolayer adsorption, the isothermal adsorption results illustrated that CR molecules are adsorbed on the PCMCA-900 surface in the monolayer form at the equilibrium state.
Since the pH value can affect the chemical properties of adsorbent and adsorbate, the effect of pH value on the CR adsorption performance of PCMCA-900 was studied, and the results are presented in Fig. 5c. The CR adsorption capacity of PCMCA-900 is strongly affected by the pH value. With the increase of the pH value of the CR solution, the CR adsorption capacity of PCMCA-900 decreased clearly. The CR adsorption capacity of PCMCA-900 at the pH value of 4 was 652.3 mg g− 1, which was about 2 times higher than that at the pH value of 10. This phenomenon can be explained by the different concentrations of H+ and OH− ions in CR solution at different pH values. When the pH value of CR solution is lower than 7, with the decrease of solution pH value, the H+ ion concentration increases, and the protonation of the PCMCA-900 surface results in the increase of the CR adsorption [63]. When the pH value of the CR solution is higher than 7, the concentration of OH− ion in the CR solution increases, which can compete with the CR molecule to adsorb on the PCMCA-900, leading to the decrease of the CR adsorption capacity of PCMCA-900 [68].
As shown in Fig. 5d, the adsorption of CR is due to electrostatic attraction and ion exchange. In addition, the benzene ring in CR molecule can interact with porous carbon through π-π stacking, thus facilitating the adsorption of CR. In addition, there is -NH2 group in CR molecule, and -OH group in PCMCA-900 can form hydrogen bond with it, which is helpful for CR adsorption.