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Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption
Journal Pre-proofs Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption Jinsong Shi, Hongmin Cui, Jianguo Xu, Nanfu Yan, Yuewei Liu PII: DOI: Reference:
S1385-8947(20)30450-2 https://doi.org/10.1016/j.cej.2020.124459 CEJ 124459
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Chemical Engineering Journal
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31 December 2019 3 February 2020 14 February 2020
Please cite this article as: J. Shi, H. Cui, J. Xu, N. Yan, Y. Liu, Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124459
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Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption Jinsong Shi*, Hongmin Cui, Jianguo Xu, Nanfu Yan, Yuewei Liu Institute of Applied Chemistry, Jiangxi Academy of Sciences, 7777# ChangDong Avenue, Nanchang, Jiangxi Province 330096, China *Corresponding author. E-mail: [email protected]. Tel: +86 791 88133295. Abstract Hierarchically porous carbons have triggered lots of research interests due to their unique structure properties. Herein, we presented the synthesis of hierarchically porous carbon frameworks (HPCFs) with the combination of hard template and chemical activation methods. Well-defined cubical macropores were formed by duplication of the hard template’s shape, and developed microporosity in the carbon frames was created by NaOH activation. The HPCFs were also successfully doped with nitrogen using urea. Effects of different synthesis parameters on the carbons’ morphology and textural properties were analyzed and discussed. The carbon yield could be considered as an indicator of the structure’s integrity. Usually the carbon framework collapsed when a very low yield was observed. The carbons’ CO2 adsorption performances were systematically investigated. Effects of nitrogen doping at different pressures were carefully studied. Results from the current and reported works demonstrated that CO2 uptake at 20 bar depended on the carbons’ surface area. The fitting results showed that surface area of 100 m2/g corresponded to CO2 uptake of 0.82 mmol/g. For CO2 adsorption under different conditions, heteroatom doped 1
porous carbons with high porosity and lots of small micropores are the most desirable. Keywords: Hierarchically porous carbon; Nitrogen doping; CO2 adsorption; Hard template; Fe2O3 cube. 1. Introduction The excessive emissions of greenhouse gases, especially CO2, have been considered as the major contributor to global climate change [1]. The burning of fossil fuel is responsible for over half of the greenhouse gas emissions. However, fossil fuel is currently the most affordable and reliable energy form. The development of CO2 capture and storage technologies is quite essential to mitigate this issue. Compared with conventional CO2 capture approach utilizing amine solutions, adsorption in porous materials is an attractive alternative, because the process is clean, reversible and energy efficient. Porous carbons have received lots of attention as CO2 adsorbents, due to the low cost, high porosity and excellent stability. One prominent advantage of porous carbons is their tunable physicochemical properties. For instance, the chemical composition of the carbons can be conveniently modified by doping of heteroatoms such as N [2], S [3], P [4] and F [5]. These heteroatoms doped carbons show great potential in many areas. For CO2 adsorption, it has been reported that nitrogen doped carbons can selectively adsorb CO2 over N2, and enhance the adsorption heat due to the presence of basic nitrogen containing functionalities [6]. On the other hand, the surface area, pore volume and pore size of carbons can be controlled by choosing suitable precursors and synthesis conditions. Carbons with different structures including spheres [7], fibers and tubes [8, 9], sheets [2] and 3D 2
interconnected porous structure [10] can also be synthesized with appropriate methods. Hierarchically porous carbons (HPCs) are especially attractive in many applications, because they combine the advantages of different pores. An ideal CO2 adsorbent should also possess a hierarchically porous structure, in which macropores and mesopores facilitate fast mass transport with minimized diffusive resistance, and micropores especially ultramicropores guarantee the high uptake and selectivity [11]. For the synthesis of HPCs, pores of different sizes are created employing different strategies. Although carbons with high porosity can be prepared from porous hard templates such as porous silica [12] and zeolite [13], but the carbons are either mesoporous or microporous. In addition, the removal of these materials usually involves lengthy reactions with highly toxic HF [14]. A combination of soft/hard templates with other methods is the most commonly used synthetic technique. For instance, carbon material prepared with colloidal silica as hard template was mesoporous, after NH3 activation a hierarchically meso-/microporous structure was constructed [15]. Similarly, HPC was also synthesized by KOH activation of polyurethane sponge templated carbon [16]. The simultaneous use of two templates is also very effective for the synthesis of HPCs. Zhao et al. impregnated polyurethane foam with resol solution and triblock copolymer F127 to prepare HPCs [17]. During the pyrolysis process, polyurethane foam was used as a sacrificial hard template to form the monolithic structure, and F127 was used as a soft template to generate mesopores. Wang and Giannelis et al. dispersed colloidal silica in glucose solution, 3
which was then submerged in liquid nitrogen and freeze-dried, pyrolyzed, and finally physically activated [18]. Colloidal silica and ice acted as hard templates to create mesopores and macropores, respectively, while physical activation introduced microporosity. Herein, we prepared hierarchically porous carbon frameworks (HPCFs) with a novel hard template followed by NaOH activation. The HPCF was featured with well-defined cubical macropores separated by microporous carbon wall. Resol was used as the carbon precursor, and Fe2O3 cubes with uniform size and shape were used as the hard template. The carbon was also successfully doped with nitrogen to prepare the nitrogen doped HPCF (NDHPCF). The carbon exhibited a high specific surface area of 1100 m2/g. CO2 adsorption performances of these carbons were also tested. At 298 K and 20 bar, a comparison of the experiment results revealed that the adsorption correlated with the surface area, and the CO2 uptake was 0.82 (mmol/g)/(100 m2/g). 2. Experimental 2.1. Materials FeCl3·6H2O (99%), urea (99%), NaOH (96%), phenol (99%), formalin (37 wt % formaldehyde), HCl (37 wt %), and ethanol (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the materials were used directly. 2.2. Synthesis of Fe2O3 template Fe2O3 template was prepared based on reported method [19]. FeCl3·6H2O (27.05 g) was dissolved in deionized water (39.5 ml) at 75 °C. The solution was kept stirring at 75 °C for 30 min, then NaOH solution (60.8 g, 17.8 wt %) was added drop by drop 4
in 5 min. The mixture was kept stirring for another 5 min, then transferred into teflon-lined stainless steel autoclave and heated at 100 °C for 4 days. The red products were collected using centrifugation, and washed with deionized water for three times, followed by ethanol washing. The Fe2O3 template was obtained after oven drying at 70 °C for 12 h. 2.3. Synthesis of HPCF Resol prepared according to the literature method [20] was used as the carbon precursor. The synthesis process of the carbons was illustrated in Fig. 1. Typically, Fe2O3 (6.0 g) was added to resol ethanolic solution (6.667 g, 30 wt %). The mixture was kept stirring at 50 °C until a uniform, gel like, and dark red product was formed. After heating at 100 °C for 24 h, the red product turned into a hard solid. The solid was crushed into powders and placed in a tube furnace under nitrogen flow (60 mL/min). The sample was treated at 500 °C for 2 h. The heating rate was set at 5 °C/min. The furnace was then cooled to room temperature, and the obtained carbonization product of resol and Fe2O3 was named as RFC500. RFC500 was thoroughly mixed with NaOH in an agate mortar (mRFC500:mNaOH = 3), and then activated at 700 °C under flowing nitrogen (60 mL/min). The products were hydrothermally treated with HCl (37 wt %) at 120 °C for 3 h to remove the inorganic impurities. The remained carbon was washed with deionized water repeatedly, and finally dried at 100 °C for 3 h. The prepared carbon was labeled as HPCFt, where t (min) was the activation time. 2.4. Synthesis of NDHPCF 5
Urea was used as the nitrogen source for the preparation of NDHPCF. RFC500 was mixed with NaOH and urea in an agate mortar (mRFC500:mNaOH:murea = 6:2:1). The mixture was activated at 700 °C under protection of nitrogen flow (60 mL/min). The heating rate was 5 °C/min. The products were treated with HCl (37 wt %) at 120 °C for 3 h, and then washed with deionized water. The obtained carbon was dried at 100 °C for 3 h. The synthesized material was labeled as NDHPCFt, where t (min) was the activation time. 2.5. Characterization Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on ZEISS-EVO18 and JEM2010, respectively. The nitrogen binding chemistry was characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific 250 Xi). X-ray diffraction (XRD) patterns of the samples were measured on a Bruker D8 Advance. Raman spectroscopy analysis was performed on a Horiba JY LabRAM HR Evolution spectrometer with an excitation laser wavelength of 532 nm. Textural properties of the carbons were analyzed with N2 adsorption-desorption at 77 K (Micromeritics ASAP 2020) after degassing at 250 °C for 3 h. The specific surface area (SBET) was calculated with Brunauer-Emmett-Teller (BET) method using adsorption data in P/P0 range of 0.05-0.20; total pore volume (Vt) was based on the N2 adsorption amount at P/P0 of 0.99; micropore surface area (Smicro) and micropore volume (Vmicro) were obtained with t-plot method; pore size distribution (PSD) curve and ultramicropore volume (Vultra) were obtained with density functional theory (DFT) method assuming slit pores. 6
CO2 adsorption isotherms with pressure up to 20 bar were tested at 273 K and 298 K with volumetric method on a lab-made Sieverts-type equipment. The carbon was degassed at 250 °C for 3 h before the measurements. The volume of the carbon was then calibrated by helium. For CO2 adsorption tests, CO2 was expanded from the reference cell with fixed volume into the sample cell until the pressure reached equilibrium. The isosteric heat of adsorption (Qst) was calculated with Clausius-Clapeyron equation. The CO2/N2 adsorption selectivity was calculated by applying the ideal adsorbed solution theory (IAST). The CO2 and N2 adsorption isotherms at 298 K were first fitted with dual-site Langmuir equations to obtain the related parameters, which were used for the IAST calculations.
Fig. 1. Schematic illustration of the synthesis process of HPCFs and NDHPCFs. 3. Results and discussion 3.1 Synthesis of Fe2O3 template
7
Fig. 2. SEM images of (a, b) Fe2O3 template, (d, e) HPCF60, and (f, g) HPCF90; TEM images of (c) Fe2O3 template, and (h, i) HPCF60. Fe2O3 cubes are chosen as the hard template, mainly because the uniform cubical shape can lead to the formation of cubical macropores in the carbon. In addition, Fe2O3 can be conveniently removed by HCl after a short reaction time. The SEM and TEM images of Fe2O3 template are shown in Fig. 2a-c. The material after reaction at 100 °C for 4 days has a highly uniform cubical shape, and the average particle size is ~725 nm. XRD analysis result is shown in Fig. S1a, and the diffraction peaks are identified as Fe2O3 (JCPDS no.33-0664). In order to shed some light on the formation mechanism of Fe2O3 cubes, SEM images of Fe2O3 templates treated at 100 °C for different times are demonstrated in Fig. S2. Fig. S2a and b show that without the heating treatment the particles have sphere like shapes and uneven sizes, and each 8
sphere consists of many small particles. After reaction for 1 day, the cubical shape starts to emerge, but the particle size is much smaller (~325 nm). The surface of the particle is rather rough with many small particles attaching on. After 2 days, the particle shape becomes more defined in Fig. S2e and f, and the particle size is approximately 600 nm. Fig. S2g and h show Fe2O3 template after 3 days’ reaction. The particle size is close to that in Fig. 2a. Although small particles can still be found, the surface is getting smooth. Based on the results above, it can be seen that the scattered small particles on the surface are gradually absorbed by the cube at 100 °C, which contributes to the growth of the cube itself. As less and less particles are left, the cube’s surface is also getting smoother. After reaction for 4 days, particles with uniform cubical shape and clean surface are finally formed. 3.2. Synthesis of HPCF In the current work hard template method is combined with NaOH activation for the construction of hierarchically porous structure. XRD pattern of RFC500 is given in Fig. S1b. The reduction of Fe2O3 by carbon gives rise to the formation of Fe3O4 (JCPDS no.65-3107). The diffraction peaks of Fe2O3 are still apparent but much weaker. SEM images of RFC500 after template removal are shown in Fig. S3. The macropores are separated by thick and continuous carbon frames. Most of the pores are isolated from the others. After mixing with NaOH and activation at 700 °C, the products’ XRD test results in Fig. S1c show a mixture of different species. Fe element mainly exists in the forms of metallic Fe (JCPDS no.06-0696) and Fe3C (JCPDS no.03-0400), which should be formed by the reactions between carbon and iron 9
oxides at 700 °C [21]. The detected Na2CO3 (JCPDS no.37-0451) is the product of chemical activation by NaOH [22]. In Fig. 2d and e, well defined cubical pores are observed in the SEM images of HPCF60. The tested particle in Fig. 2d is evenly distributed with these macropores, and the observed pore size is 680-750 nm. The wall thickness of HPCF60 is 40-60 nm. It can be observed in Fig. 2e that the carbon walls consist of closely packed small carbon particles, and connecting windows are formed between some adjacent macropores. This feature is attributed to the consumption of carbon by NaOH and iron oxides, which continuously decreases the thickness of the carbon wall until a window is formed. HPCF90 shows similar morphology in Fig. 2f and g. Cubical pores are distributed across the sample, and the framework is well preserved after activation for 90 min. As shown in Fig. 2g, connecting windows are also formed for some macropores. The TEM image in Fig. 2h gives a direct view of the framework structure of HPCF60. Fig. 2i shows that the carbon wall is filled with abundant micropores created by NaOH activation. In addition, no obvious graphene layers are identified in Fig. 2i, suggesting the amorphous nature of the carbon. Based on the discussion above, the cubical macropores in HPCF60 are separated by microporous carbon walls, forming the hierarchically porous structure. 3.3. Control of HPCF’s morphology Effects of various synthesis parameters on the carbons’ morphology are investigated. Table S1 lists the changes of carbon yield with different synthesis conditions. SEM images of carbon with different mFe2O3/mresol (1.0, 2.0, and 4.0) are 10
shown in Fig. S4. In Fig. S4a and b, macropores with irregular shapes are randomly embedded in the thick carbon matrix. When mFe2O3/mresol is 1.0, excessive resol can completely cover Fe2O3 template. Thick carbon layers are formed outside the template after pyrolysis. The distribution of the macropores is uneven, and large area of carbon with no visible pores can be observed in Fig. S4a. When mFe2O3/mresol is increased to 2.0, the framework structure appears in Fig. S4c and d. However, unlike HPCF60, no obvious connecting windows can be identified. Fig. S4e and f show the existence of many broken carbon chips, suggesting that the high mFe2O3/mresol (4.0) cannot guarantee the formation of a continuous framework after activation. The carbon yield also decreases from 17.7% to 3.7%, due to the reactions between carbon and iron species. Excessive Fe2O3 decreases the thickness of the carbon layer outside Fe2O3 cubes, and the subsequent reductions of iron oxides at high temperature also consume considerable amount of carbon. Both effects can undermine the robustness of the carbon framework. Fig. S5 demonstrates the effects of NaOH’s amount on the morphology. Compared with HPCF60 and HPCF90, the carbons prepared with more NaOH exhibit very disordered structure, and lots of carbon flakes can be seen in Fig. S5. The serious corrosion of carbon by NaOH causes the sharp drop of carbon yield to only 3.5%. Due to the continuous carbon loss, the collapse of the framework structure is inevitable. As an alternative to the proposed synthesis method, a parallel carbon sample is prepared from the direct activation of RFC500 after template removal. The main 11
advantage of this route is that the reactions between carbon and iron oxides at 700 °C can be avoided. 0.144 g of carbon can be obtained from 1.0 g of RFC500, so the actual weight ratio of NaOH to carbon for HPCF60 is calculated to be 2.3. The same weight ratio is also applied in the synthesis of this sample. SEM images of the obtained carbon are shown in Fig. S6. Although macropores are formed by Fe2O3 template, many large and broken carbon particles are also observed. The high carbon yield (17.0%) and thick carbon walls suggest that the structure collapse is not caused by serious carbon loss. The reason should be the lack of the support from Fe2O3 templates during the mixing with NaOH and the activation at high temperature. For NaOH activated carbons, high porosity can be achieved under harsh conditions [23]. The HPCF carbons are also activated with NaOH at higher temperature for longer time. When the activation temperature is 800 °C, no carbon remains after template removal, meaning that the consumption of carbon by NaOH and iron oxides is drastically accelerated at 800 °C. Influences of the activation time are demonstrated in Fig. S7. The carbon yield decreases to 4.1% after activation for 120 min, and is only 1.4% after activation for 240 min. The framework structure starts to collapse after activation for 120 min, as many carbon flakes can be observed in Fig. S7a and b. When the activation time is 240 min, the structure is heavily destroyed and more broken carbon flakes are formed, Fig. S7d even reveals the existence of pulverized carbon particles. The effects of different synthesis parameters show that heavy carbon loss continuously decreases the robustness of the carbon framework, and eventually leads to the collapse of the framework. 12
3.4. Synthesis of NDHPCF
Fig. 3. N 1s XPS spectra of (a) NDHPCF60, and (b) NDHPCF90. Urea is used as the nitrogen source because it is cheap, easy to handle, and environmentally friendly. Previous works have proved that the pyrolysis of urea and nitrogen free precursors can produce nitrogen doped carbons [24-26]. The success of nitrogen doping into the carbon framework is confirmed by XPS tests. NDHPCF60 and NDHPCF90 exhibit very similar nitrogen contents of 2.2 and 2.3 wt %, respectively. As shown in Fig. S8, when doubled and tripled amounts of urea are used, the nitrogen contents are 3.0 and 3.5 wt %, respectively. These values are only slightly higher than that of NDHPCF90. It is speculated that most of the nitrogen containing groups are destroyed at 700 °C, regardless of the urea amount. The carbon yield drops drastically to 0.7% when tripled amount of urea is used. The carbon walls suffer from serious corrosion effects from both of NaOH and urea at high temperature. The N 1s core level spectra in Fig. 3 are deconvolved into three components: N-5 (pyrrolic/pyridonic nitrogen, ~400.2 eV), N-6 (pyridinic nitrogen, 398.5 eV), and N-Q (quaternary nitrogen, 401.5 eV) [25]. The detailed peak positions and the contents of 13
the nitrogen species are listed in Table S2. N-5 is the dominant nitrogen form, and the content of N-Q increases obviously for the carbons activated for 90 min. It has been reported that N-Q represents the most stable nitrogen functionality, and N-5 can be transformed into N-Q under harsh conditions [27, 28]. The simultaneous decrease of N-5 contents in Table S1 implies that such transformation is possible when the activation time is extended to 90 min.
Fig. 4. SEM images of (a, b) NDHPCF60, (c, d) NDHPCF90; (e-g)TEM images, and (h) the C/N/O elemental mapping of NDHPCF60. As shown in Fig. 4a and b, the framework structure is retained in NDHPCF60 after nitrogen doping, and many connecting windows between the adjacent macropores are observed. In Fig. 4b, unlike HPCF60, the carbon walls consist of many nanosized carbon particles, and obvious voids are formed among these particles. These voids indicate that more carbon is consumed after the addition of urea. Compared with HPCF60, NDHPCF60 has a more fragile structure. The SEM image of NDHPCF90 in Fig. 4c shows the existence of broken carbon pieces, suggesting that the framework structure has already been partially destroyed. The 14
voids among the carbon particles in Fig. 4d become larger. The morphology suggests a more serious carbon loss, as the carbon yield decreases from 13.1% to 8.0%. The serious carbon loss leads to the destruction of the carbon wall, and eventually the partial collapse of the framework structure. TEM images of NDHPCF60 are demonstrated in Fig. 4e-g. The carbon walls are uniformly filled with lots of micropores, so the hierarchically porous structure in HPCF60 is also formed in NDHPCF60. Also no graphene layer can be found in Fig. 4f and g. The C/N/O elemental mapping results are shown in Fig. 4h. Nitrogen element is homogeneously distributed across the tested carbon frames, and it overlaps with the signal of carbon element. The above results also confirm the success of uniform nitrogen doping by urea.
Fig. 5. (a) XRD test results, (b) Raman spectra, (c) N2 adsorption-desorption 15
isotherms at 77 K, and (d) PSD characteristics of the carbons.
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Table 1. Nitrogen contents, textural properties, and adsorption test results of the carbons. sample
nitrogen
SBET
Smicro
Vt
Vmicro
Vultra
CO2 uptake at CO2 uptake at N2
CO2/N2
content
(m2/g)
(m2/g)
(cm3/g)
(cm3/g)
(cm3/g)
298 K (mmol/g)
273 K (mmol/g)
uptake
selectivity
1 bar
20 bar
1 bar
20 bar
(298 K)
(298 K, 1 bar)
(wt %) HPCF60
/
866
476
0.54
0.22
0.16
2.48
7.94
3.81
9.94
0.34
20
HPCF90
/
1088
442
0.78
0.20
0.16
2.44
10.04
3.80
13.47
0.33
16
NDHPCF60 2.2
972
563
0.56
0.26
0.15
2.76
9.22
4.08
11.13
0.37
19
NDHPCF90 2.3
1100
489
0.73
0.22
0.16
2.62
10.00
4.00
13.51
0.36
19
17
3.5. Physicochemical properties of the carbons XRD test results of the carbons are presented in Fig. 5a. No sharp peaks are detected, confirming that all the inorganic impurities have been completely removed. The carbons have an amorphous structure, as shown by the two broad peaks at approximately 21.5 and 44 °. The D band (~1330 cm-1) and G band (~1581 cm-1) of the Raman spectra in Fig. 5b correspond to disordered and graphitic carbon structures, respectively [29, 30]. The relatively high ID/IG (1.0) also demonstrates the low degree of graphitization in the carbons, in agreement with XRD and TEM results. The carbons’ textural properties are characterized with N2 adsorption-desorption measurements, and the detailed results are listed in Table 1. Fig. 5c show typical characteristics of type I and IV isotherms, i.e., the fast increase of adsorption amount in the low pressure region and the presence of hysteresis loops. The hysteresis loops with parallel and almost horizontal branches are characterized as Type H4, arising from the presence of mesopores surrounded by abundant micropores [31]. The loop’s size grows larger for carbons activated for 90 min, corresponding to the development of mesoporosity in the carbons. The results in Table 1 also demonstrate the significant improvements of SBET and Vt. NDHPCF90 has the highest SBET of 1100 m2/g. However, the Smicro and Vmicro of HPCF90 and NDHPCF90 are lower than the carbons activated for 60 min. For instance, the Smicro of NDHPCF60 is 563 m2/g, and it decreases to 489 m2/g for NDHPCF90, which is a direct evidence of the destruction of micropores. The increase of SBET with prolonged activation time is mainly contributed by mesopores. It is also noticed that the SBET of NDHPCF is slightly higher than that 18
of HPCF. Nitrogen containing functionalities originated from urea are reported to preferably locate at the edges of graphene layers, which may block small pores and cause the decrease of SBET [32]. However, as discussed above, carbon is also corroded by urea at high temperature, promoting the formation of pores and enhancing the porosity. The increase of SBET after nitrogen doping is thus attributed to the reactions between carbon and urea. The PSD curves derived from DFT method are shown in Fig. 5d. HPCF60 exhibits two sharp peaks at 0.5 and 0.7 nm, while for NDHPCF60 the peak corresponding to the smallest pores shifts to 0.6 nm, and the peak at 0.7 nm weakens and broadens obviously. For NDHPCF90 the peak at 0.5 nm is preserved, but its intensity is lower than that of HPCF90. It can be concluded that the reactions between carbon and urea result in the loss of the smallest pores in the carbons. The influences of activation time on ultramicropores are a little complicated. Compared with HPCF60, HPCF90 loses the sharp peak at 0.7 nm, although a new peak at 0.6 nm with moderate intensity emerges. On the other hand, a sharp peak at 0.5 nm is found in the PSD curve of NDHPCF90, which is absent in the PSD curve of NDHPCF60. Although the PSD characteristics of the ultramicropores vary significantly for different carbons, the Vultra stays in a narrow range of 0.15-0.16 cm3/g. 3.6. CO2 adsorption performances
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Fig. 6. CO2 adsorption isotherms at (a) 273 K and (b) 298 K; (c) isosteric heat of adsorption (Qst); (d) N2 adsorption isotherms at 298 K; (e) CO2/N2 adsorption selectivity calculated with IAST method (CO2:N2 = 15:85); (f) Linear fitting of the CO2 uptakes (298 K, 20 bar) against the SBET with fixed intercept at 0. The combination of nitrogen doping and high porosity makes HPCFs/NDHPCFs excellent candidates for CO2 adsorption. The CO2 adsorption performances are tested at 273 and 298 K with pressure up to 20 bar. The adsorption isotherms are presented in Fig. 6a and b, and the uptakes are also listed in Table 1. At 298 K and 1 bar, NDHPCF60 exhibits the highest CO2 uptake of 2.76 mmol/g. This value is lower than some of the reported results (for instance, WSC-500-1 (4.50 mmol/g) [33], NGC-650-4 (3.92 mmol/g) [34], GN-650-1 (4.26 mmol/g) [35], C500-K (4.17 mmol/g) [36]). It has been emphasized that the uptake at 1 bar largely depends on ultramicropores [37, 38]. The relatively low uptake of NDHPCF60 is due to the high portion of large micropores and mesopores in the carbon. The Vultra values of HPCF60 and NDHPCF60 are 0.16 and 0.15 cm3/g, respectively. Even so, HPCF60 has a lower 20
uptake of 2.48 mmol/g. This result shows that nitrogen doping can enhance the uptake to a certain degree. The Qst in Fig. 6c are calculated to investigate the interactions between CO2 and the carbon surface. The carbons show Qst of 23.4-27.6 kJ/mol at low CO2 loading. Unexpectedly, the Qst values of NDHPCF60 (23.4 kJ/mol) and NDHPCF90 (24.2 kJ/mol) are lower than HPCF60 (27.6 kJ/mol) and HPCF90 (26.5 kJ/mol). The Qst at low loading results from the interactions between CO2 and the most favorable adsorption sites, for instance ultrafine pores and basic nitrogen functionalities [11]. The loss of the smallest micropores in the NDHPCFs causes the decrease of Qst at low CO2 uptake. In order to study the CO2/N2 adsorption selectivity, the N2 adsorption isotherms at 298 K are shown in Fig. 6d for comparison, and the N2 uptake is 0.33-0.37 mmol/g. The selectivity plots calculated with IAST (CO2:N2 = 15:85) are displayed in Fig. 6e. At 1 bar, the selectivity varies in a narrow range of 16-20, and the highest value is achieved by the nitrogen free HPCF60, which also has the highest Qst. The above results highlight the importance of small micropores to CO2 adsorption at low pressure, while the positive effects of nitrogen doping could be completely disguised by the influences of pore enlargement. On the other hand, CO2 adsorption at high pressure benefits from the contribution of both micropores and mesopores [39, 40]. At 20 bar, the CO2 uptakes of HPCF90 are 13.47 mmol/g (273 K) and 10.04 mmol/g (298 K), which are very close to the uptakes of NDHPCF90. To evaluate the adsorption stability and recyclability, we select HPCF90 and test five consecutive CO2 adsorption cycles at 298 K. As shown in Fig. S9, no obvious degradation of the uptake is observed, 21
showing the promising stability of HPCF90. With the decrease of porosity, the adsorption performances deteriorate apparently. NDHPCF60 shows the highest CO2 uptake at 1 bar, but the uptake at 20 bar and 298 K is only 9.22 mmol/g. The results in Table 1 suggest that nitrogen doping shows no obvious effects at 20 bar, and the uptake correlates more with the SBET. To support this point, CO2 uptake (298 K, 20 bar) is plotted against SBET in Fig. 6f with data collected from this work and reported results (the specific data are given in Table S3) [41-50]. Since the uptake heavily depends on the carbon’s porosity, it will be reasonable to assume zero uptake in nonporous adsorbents. The intercept of the fitted line in Fig. 6f is thus set at zero. Regardless of the nitrogen content of the carbons, a correlation coefficient of 0.854 is generated, and the slope of the fitted line shows that SBET of 100 m2/g corresponds to CO2 uptake of 0.82 mmol/g at 298 K and 20 bar. To sum up, highly porous carbons with heteroatom doping and high portion of small micropores are the most desirable for excellent adsorption at different pressures. 4. Conclusions In conclusion, HPCFs were prepared with Fe2O3 as the hard template followed by chemical activation. Cubical macropores were formed in the carbon by Fe2O3 cubes, and the carbon walls of the frameworks were filled with micropores due to NaOH activation. The HPCFs were also successfully doped with nitrogen with the assistance of urea during NaOH activation. The carbons exhibited high SBET up to 1100 m2/g. The phenomenon of pore enlargement was observed after nitrogen doping. The structure integrity of the prepared carbon frameworks were under influences from 22
different synthesis parameters, including the amount of Fe2O3 template, NaOH and urea. Uncontrolled consumption of carbon by reactions with these chemicals could result in very low carbon yield, and hence the destruction of the framework structure. At 298 K, CO2 uptakes of 2.76 and 10.04 mmol/g were achieved at 1 and 10 bar, respectively. At 1 bar, the uptake was enhanced by nitrogen doping to a certain degree. However, no significant effects on Qst and adsorption selectivity were observed. For the adsorption at 20 bar, the experiment results from the current and reported works revealed that the uptake was mainly decided by SBET with a ratio of 0.82 (mmol/g)/(100 m2/g). Acknowledgements The research was supported financially by the National Natural Science Foundation of China (No. 21805120 and No. 51804143), Jiangxi Academy of Sciences (2018-YZD2-19), and the Key R & D Program of Jiangxi Province (20181BBF68011).
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Hierarchically porous carbon frameworks (HPCFs) are prepared using hard templates. The HPCFs have well-defined cubical macropores and abundant micropores. The HPCFs are successfully doped with nitrogen. SBET of 100 m2/g corresponds to CO2 uptake of 0.82 mmol/g at 298 K and 20 bar.
31
S1385-8947(20)30450-2 https://doi.org/10.1016/j.cej.2020.124459 CEJ 124459
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Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
31 December 2019 3 February 2020 14 February 2020
Please cite this article as: J. Shi, H. Cui, J. Xu, N. Yan, Y. Liu, Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124459
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Design and fabrication of hierarchically porous carbon frameworks with Fe2O3 cubes as hard template for CO2 adsorption Jinsong Shi*, Hongmin Cui, Jianguo Xu, Nanfu Yan, Yuewei Liu Institute of Applied Chemistry, Jiangxi Academy of Sciences, 7777# ChangDong Avenue, Nanchang, Jiangxi Province 330096, China *Corresponding author. E-mail: [email protected]. Tel: +86 791 88133295. Abstract Hierarchically porous carbons have triggered lots of research interests due to their unique structure properties. Herein, we presented the synthesis of hierarchically porous carbon frameworks (HPCFs) with the combination of hard template and chemical activation methods. Well-defined cubical macropores were formed by duplication of the hard template’s shape, and developed microporosity in the carbon frames was created by NaOH activation. The HPCFs were also successfully doped with nitrogen using urea. Effects of different synthesis parameters on the carbons’ morphology and textural properties were analyzed and discussed. The carbon yield could be considered as an indicator of the structure’s integrity. Usually the carbon framework collapsed when a very low yield was observed. The carbons’ CO2 adsorption performances were systematically investigated. Effects of nitrogen doping at different pressures were carefully studied. Results from the current and reported works demonstrated that CO2 uptake at 20 bar depended on the carbons’ surface area. The fitting results showed that surface area of 100 m2/g corresponded to CO2 uptake of 0.82 mmol/g. For CO2 adsorption under different conditions, heteroatom doped 1
porous carbons with high porosity and lots of small micropores are the most desirable. Keywords: Hierarchically porous carbon; Nitrogen doping; CO2 adsorption; Hard template; Fe2O3 cube. 1. Introduction The excessive emissions of greenhouse gases, especially CO2, have been considered as the major contributor to global climate change [1]. The burning of fossil fuel is responsible for over half of the greenhouse gas emissions. However, fossil fuel is currently the most affordable and reliable energy form. The development of CO2 capture and storage technologies is quite essential to mitigate this issue. Compared with conventional CO2 capture approach utilizing amine solutions, adsorption in porous materials is an attractive alternative, because the process is clean, reversible and energy efficient. Porous carbons have received lots of attention as CO2 adsorbents, due to the low cost, high porosity and excellent stability. One prominent advantage of porous carbons is their tunable physicochemical properties. For instance, the chemical composition of the carbons can be conveniently modified by doping of heteroatoms such as N [2], S [3], P [4] and F [5]. These heteroatoms doped carbons show great potential in many areas. For CO2 adsorption, it has been reported that nitrogen doped carbons can selectively adsorb CO2 over N2, and enhance the adsorption heat due to the presence of basic nitrogen containing functionalities [6]. On the other hand, the surface area, pore volume and pore size of carbons can be controlled by choosing suitable precursors and synthesis conditions. Carbons with different structures including spheres [7], fibers and tubes [8, 9], sheets [2] and 3D 2
interconnected porous structure [10] can also be synthesized with appropriate methods. Hierarchically porous carbons (HPCs) are especially attractive in many applications, because they combine the advantages of different pores. An ideal CO2 adsorbent should also possess a hierarchically porous structure, in which macropores and mesopores facilitate fast mass transport with minimized diffusive resistance, and micropores especially ultramicropores guarantee the high uptake and selectivity [11]. For the synthesis of HPCs, pores of different sizes are created employing different strategies. Although carbons with high porosity can be prepared from porous hard templates such as porous silica [12] and zeolite [13], but the carbons are either mesoporous or microporous. In addition, the removal of these materials usually involves lengthy reactions with highly toxic HF [14]. A combination of soft/hard templates with other methods is the most commonly used synthetic technique. For instance, carbon material prepared with colloidal silica as hard template was mesoporous, after NH3 activation a hierarchically meso-/microporous structure was constructed [15]. Similarly, HPC was also synthesized by KOH activation of polyurethane sponge templated carbon [16]. The simultaneous use of two templates is also very effective for the synthesis of HPCs. Zhao et al. impregnated polyurethane foam with resol solution and triblock copolymer F127 to prepare HPCs [17]. During the pyrolysis process, polyurethane foam was used as a sacrificial hard template to form the monolithic structure, and F127 was used as a soft template to generate mesopores. Wang and Giannelis et al. dispersed colloidal silica in glucose solution, 3
which was then submerged in liquid nitrogen and freeze-dried, pyrolyzed, and finally physically activated [18]. Colloidal silica and ice acted as hard templates to create mesopores and macropores, respectively, while physical activation introduced microporosity. Herein, we prepared hierarchically porous carbon frameworks (HPCFs) with a novel hard template followed by NaOH activation. The HPCF was featured with well-defined cubical macropores separated by microporous carbon wall. Resol was used as the carbon precursor, and Fe2O3 cubes with uniform size and shape were used as the hard template. The carbon was also successfully doped with nitrogen to prepare the nitrogen doped HPCF (NDHPCF). The carbon exhibited a high specific surface area of 1100 m2/g. CO2 adsorption performances of these carbons were also tested. At 298 K and 20 bar, a comparison of the experiment results revealed that the adsorption correlated with the surface area, and the CO2 uptake was 0.82 (mmol/g)/(100 m2/g). 2. Experimental 2.1. Materials FeCl3·6H2O (99%), urea (99%), NaOH (96%), phenol (99%), formalin (37 wt % formaldehyde), HCl (37 wt %), and ethanol (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All the materials were used directly. 2.2. Synthesis of Fe2O3 template Fe2O3 template was prepared based on reported method [19]. FeCl3·6H2O (27.05 g) was dissolved in deionized water (39.5 ml) at 75 °C. The solution was kept stirring at 75 °C for 30 min, then NaOH solution (60.8 g, 17.8 wt %) was added drop by drop 4
in 5 min. The mixture was kept stirring for another 5 min, then transferred into teflon-lined stainless steel autoclave and heated at 100 °C for 4 days. The red products were collected using centrifugation, and washed with deionized water for three times, followed by ethanol washing. The Fe2O3 template was obtained after oven drying at 70 °C for 12 h. 2.3. Synthesis of HPCF Resol prepared according to the literature method [20] was used as the carbon precursor. The synthesis process of the carbons was illustrated in Fig. 1. Typically, Fe2O3 (6.0 g) was added to resol ethanolic solution (6.667 g, 30 wt %). The mixture was kept stirring at 50 °C until a uniform, gel like, and dark red product was formed. After heating at 100 °C for 24 h, the red product turned into a hard solid. The solid was crushed into powders and placed in a tube furnace under nitrogen flow (60 mL/min). The sample was treated at 500 °C for 2 h. The heating rate was set at 5 °C/min. The furnace was then cooled to room temperature, and the obtained carbonization product of resol and Fe2O3 was named as RFC500. RFC500 was thoroughly mixed with NaOH in an agate mortar (mRFC500:mNaOH = 3), and then activated at 700 °C under flowing nitrogen (60 mL/min). The products were hydrothermally treated with HCl (37 wt %) at 120 °C for 3 h to remove the inorganic impurities. The remained carbon was washed with deionized water repeatedly, and finally dried at 100 °C for 3 h. The prepared carbon was labeled as HPCFt, where t (min) was the activation time. 2.4. Synthesis of NDHPCF 5
Urea was used as the nitrogen source for the preparation of NDHPCF. RFC500 was mixed with NaOH and urea in an agate mortar (mRFC500:mNaOH:murea = 6:2:1). The mixture was activated at 700 °C under protection of nitrogen flow (60 mL/min). The heating rate was 5 °C/min. The products were treated with HCl (37 wt %) at 120 °C for 3 h, and then washed with deionized water. The obtained carbon was dried at 100 °C for 3 h. The synthesized material was labeled as NDHPCFt, where t (min) was the activation time. 2.5. Characterization Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were obtained on ZEISS-EVO18 and JEM2010, respectively. The nitrogen binding chemistry was characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific 250 Xi). X-ray diffraction (XRD) patterns of the samples were measured on a Bruker D8 Advance. Raman spectroscopy analysis was performed on a Horiba JY LabRAM HR Evolution spectrometer with an excitation laser wavelength of 532 nm. Textural properties of the carbons were analyzed with N2 adsorption-desorption at 77 K (Micromeritics ASAP 2020) after degassing at 250 °C for 3 h. The specific surface area (SBET) was calculated with Brunauer-Emmett-Teller (BET) method using adsorption data in P/P0 range of 0.05-0.20; total pore volume (Vt) was based on the N2 adsorption amount at P/P0 of 0.99; micropore surface area (Smicro) and micropore volume (Vmicro) were obtained with t-plot method; pore size distribution (PSD) curve and ultramicropore volume (Vultra) were obtained with density functional theory (DFT) method assuming slit pores. 6
CO2 adsorption isotherms with pressure up to 20 bar were tested at 273 K and 298 K with volumetric method on a lab-made Sieverts-type equipment. The carbon was degassed at 250 °C for 3 h before the measurements. The volume of the carbon was then calibrated by helium. For CO2 adsorption tests, CO2 was expanded from the reference cell with fixed volume into the sample cell until the pressure reached equilibrium. The isosteric heat of adsorption (Qst) was calculated with Clausius-Clapeyron equation. The CO2/N2 adsorption selectivity was calculated by applying the ideal adsorbed solution theory (IAST). The CO2 and N2 adsorption isotherms at 298 K were first fitted with dual-site Langmuir equations to obtain the related parameters, which were used for the IAST calculations.
Fig. 1. Schematic illustration of the synthesis process of HPCFs and NDHPCFs. 3. Results and discussion 3.1 Synthesis of Fe2O3 template
7
Fig. 2. SEM images of (a, b) Fe2O3 template, (d, e) HPCF60, and (f, g) HPCF90; TEM images of (c) Fe2O3 template, and (h, i) HPCF60. Fe2O3 cubes are chosen as the hard template, mainly because the uniform cubical shape can lead to the formation of cubical macropores in the carbon. In addition, Fe2O3 can be conveniently removed by HCl after a short reaction time. The SEM and TEM images of Fe2O3 template are shown in Fig. 2a-c. The material after reaction at 100 °C for 4 days has a highly uniform cubical shape, and the average particle size is ~725 nm. XRD analysis result is shown in Fig. S1a, and the diffraction peaks are identified as Fe2O3 (JCPDS no.33-0664). In order to shed some light on the formation mechanism of Fe2O3 cubes, SEM images of Fe2O3 templates treated at 100 °C for different times are demonstrated in Fig. S2. Fig. S2a and b show that without the heating treatment the particles have sphere like shapes and uneven sizes, and each 8
sphere consists of many small particles. After reaction for 1 day, the cubical shape starts to emerge, but the particle size is much smaller (~325 nm). The surface of the particle is rather rough with many small particles attaching on. After 2 days, the particle shape becomes more defined in Fig. S2e and f, and the particle size is approximately 600 nm. Fig. S2g and h show Fe2O3 template after 3 days’ reaction. The particle size is close to that in Fig. 2a. Although small particles can still be found, the surface is getting smooth. Based on the results above, it can be seen that the scattered small particles on the surface are gradually absorbed by the cube at 100 °C, which contributes to the growth of the cube itself. As less and less particles are left, the cube’s surface is also getting smoother. After reaction for 4 days, particles with uniform cubical shape and clean surface are finally formed. 3.2. Synthesis of HPCF In the current work hard template method is combined with NaOH activation for the construction of hierarchically porous structure. XRD pattern of RFC500 is given in Fig. S1b. The reduction of Fe2O3 by carbon gives rise to the formation of Fe3O4 (JCPDS no.65-3107). The diffraction peaks of Fe2O3 are still apparent but much weaker. SEM images of RFC500 after template removal are shown in Fig. S3. The macropores are separated by thick and continuous carbon frames. Most of the pores are isolated from the others. After mixing with NaOH and activation at 700 °C, the products’ XRD test results in Fig. S1c show a mixture of different species. Fe element mainly exists in the forms of metallic Fe (JCPDS no.06-0696) and Fe3C (JCPDS no.03-0400), which should be formed by the reactions between carbon and iron 9
oxides at 700 °C [21]. The detected Na2CO3 (JCPDS no.37-0451) is the product of chemical activation by NaOH [22]. In Fig. 2d and e, well defined cubical pores are observed in the SEM images of HPCF60. The tested particle in Fig. 2d is evenly distributed with these macropores, and the observed pore size is 680-750 nm. The wall thickness of HPCF60 is 40-60 nm. It can be observed in Fig. 2e that the carbon walls consist of closely packed small carbon particles, and connecting windows are formed between some adjacent macropores. This feature is attributed to the consumption of carbon by NaOH and iron oxides, which continuously decreases the thickness of the carbon wall until a window is formed. HPCF90 shows similar morphology in Fig. 2f and g. Cubical pores are distributed across the sample, and the framework is well preserved after activation for 90 min. As shown in Fig. 2g, connecting windows are also formed for some macropores. The TEM image in Fig. 2h gives a direct view of the framework structure of HPCF60. Fig. 2i shows that the carbon wall is filled with abundant micropores created by NaOH activation. In addition, no obvious graphene layers are identified in Fig. 2i, suggesting the amorphous nature of the carbon. Based on the discussion above, the cubical macropores in HPCF60 are separated by microporous carbon walls, forming the hierarchically porous structure. 3.3. Control of HPCF’s morphology Effects of various synthesis parameters on the carbons’ morphology are investigated. Table S1 lists the changes of carbon yield with different synthesis conditions. SEM images of carbon with different mFe2O3/mresol (1.0, 2.0, and 4.0) are 10
shown in Fig. S4. In Fig. S4a and b, macropores with irregular shapes are randomly embedded in the thick carbon matrix. When mFe2O3/mresol is 1.0, excessive resol can completely cover Fe2O3 template. Thick carbon layers are formed outside the template after pyrolysis. The distribution of the macropores is uneven, and large area of carbon with no visible pores can be observed in Fig. S4a. When mFe2O3/mresol is increased to 2.0, the framework structure appears in Fig. S4c and d. However, unlike HPCF60, no obvious connecting windows can be identified. Fig. S4e and f show the existence of many broken carbon chips, suggesting that the high mFe2O3/mresol (4.0) cannot guarantee the formation of a continuous framework after activation. The carbon yield also decreases from 17.7% to 3.7%, due to the reactions between carbon and iron species. Excessive Fe2O3 decreases the thickness of the carbon layer outside Fe2O3 cubes, and the subsequent reductions of iron oxides at high temperature also consume considerable amount of carbon. Both effects can undermine the robustness of the carbon framework. Fig. S5 demonstrates the effects of NaOH’s amount on the morphology. Compared with HPCF60 and HPCF90, the carbons prepared with more NaOH exhibit very disordered structure, and lots of carbon flakes can be seen in Fig. S5. The serious corrosion of carbon by NaOH causes the sharp drop of carbon yield to only 3.5%. Due to the continuous carbon loss, the collapse of the framework structure is inevitable. As an alternative to the proposed synthesis method, a parallel carbon sample is prepared from the direct activation of RFC500 after template removal. The main 11
advantage of this route is that the reactions between carbon and iron oxides at 700 °C can be avoided. 0.144 g of carbon can be obtained from 1.0 g of RFC500, so the actual weight ratio of NaOH to carbon for HPCF60 is calculated to be 2.3. The same weight ratio is also applied in the synthesis of this sample. SEM images of the obtained carbon are shown in Fig. S6. Although macropores are formed by Fe2O3 template, many large and broken carbon particles are also observed. The high carbon yield (17.0%) and thick carbon walls suggest that the structure collapse is not caused by serious carbon loss. The reason should be the lack of the support from Fe2O3 templates during the mixing with NaOH and the activation at high temperature. For NaOH activated carbons, high porosity can be achieved under harsh conditions [23]. The HPCF carbons are also activated with NaOH at higher temperature for longer time. When the activation temperature is 800 °C, no carbon remains after template removal, meaning that the consumption of carbon by NaOH and iron oxides is drastically accelerated at 800 °C. Influences of the activation time are demonstrated in Fig. S7. The carbon yield decreases to 4.1% after activation for 120 min, and is only 1.4% after activation for 240 min. The framework structure starts to collapse after activation for 120 min, as many carbon flakes can be observed in Fig. S7a and b. When the activation time is 240 min, the structure is heavily destroyed and more broken carbon flakes are formed, Fig. S7d even reveals the existence of pulverized carbon particles. The effects of different synthesis parameters show that heavy carbon loss continuously decreases the robustness of the carbon framework, and eventually leads to the collapse of the framework. 12
3.4. Synthesis of NDHPCF
Fig. 3. N 1s XPS spectra of (a) NDHPCF60, and (b) NDHPCF90. Urea is used as the nitrogen source because it is cheap, easy to handle, and environmentally friendly. Previous works have proved that the pyrolysis of urea and nitrogen free precursors can produce nitrogen doped carbons [24-26]. The success of nitrogen doping into the carbon framework is confirmed by XPS tests. NDHPCF60 and NDHPCF90 exhibit very similar nitrogen contents of 2.2 and 2.3 wt %, respectively. As shown in Fig. S8, when doubled and tripled amounts of urea are used, the nitrogen contents are 3.0 and 3.5 wt %, respectively. These values are only slightly higher than that of NDHPCF90. It is speculated that most of the nitrogen containing groups are destroyed at 700 °C, regardless of the urea amount. The carbon yield drops drastically to 0.7% when tripled amount of urea is used. The carbon walls suffer from serious corrosion effects from both of NaOH and urea at high temperature. The N 1s core level spectra in Fig. 3 are deconvolved into three components: N-5 (pyrrolic/pyridonic nitrogen, ~400.2 eV), N-6 (pyridinic nitrogen, 398.5 eV), and N-Q (quaternary nitrogen, 401.5 eV) [25]. The detailed peak positions and the contents of 13
the nitrogen species are listed in Table S2. N-5 is the dominant nitrogen form, and the content of N-Q increases obviously for the carbons activated for 90 min. It has been reported that N-Q represents the most stable nitrogen functionality, and N-5 can be transformed into N-Q under harsh conditions [27, 28]. The simultaneous decrease of N-5 contents in Table S1 implies that such transformation is possible when the activation time is extended to 90 min.
Fig. 4. SEM images of (a, b) NDHPCF60, (c, d) NDHPCF90; (e-g)TEM images, and (h) the C/N/O elemental mapping of NDHPCF60. As shown in Fig. 4a and b, the framework structure is retained in NDHPCF60 after nitrogen doping, and many connecting windows between the adjacent macropores are observed. In Fig. 4b, unlike HPCF60, the carbon walls consist of many nanosized carbon particles, and obvious voids are formed among these particles. These voids indicate that more carbon is consumed after the addition of urea. Compared with HPCF60, NDHPCF60 has a more fragile structure. The SEM image of NDHPCF90 in Fig. 4c shows the existence of broken carbon pieces, suggesting that the framework structure has already been partially destroyed. The 14
voids among the carbon particles in Fig. 4d become larger. The morphology suggests a more serious carbon loss, as the carbon yield decreases from 13.1% to 8.0%. The serious carbon loss leads to the destruction of the carbon wall, and eventually the partial collapse of the framework structure. TEM images of NDHPCF60 are demonstrated in Fig. 4e-g. The carbon walls are uniformly filled with lots of micropores, so the hierarchically porous structure in HPCF60 is also formed in NDHPCF60. Also no graphene layer can be found in Fig. 4f and g. The C/N/O elemental mapping results are shown in Fig. 4h. Nitrogen element is homogeneously distributed across the tested carbon frames, and it overlaps with the signal of carbon element. The above results also confirm the success of uniform nitrogen doping by urea.
Fig. 5. (a) XRD test results, (b) Raman spectra, (c) N2 adsorption-desorption 15
isotherms at 77 K, and (d) PSD characteristics of the carbons.
16
Table 1. Nitrogen contents, textural properties, and adsorption test results of the carbons. sample
nitrogen
SBET
Smicro
Vt
Vmicro
Vultra
CO2 uptake at CO2 uptake at N2
CO2/N2
content
(m2/g)
(m2/g)
(cm3/g)
(cm3/g)
(cm3/g)
298 K (mmol/g)
273 K (mmol/g)
uptake
selectivity
1 bar
20 bar
1 bar
20 bar
(298 K)
(298 K, 1 bar)
(wt %) HPCF60
/
866
476
0.54
0.22
0.16
2.48
7.94
3.81
9.94
0.34
20
HPCF90
/
1088
442
0.78
0.20
0.16
2.44
10.04
3.80
13.47
0.33
16
NDHPCF60 2.2
972
563
0.56
0.26
0.15
2.76
9.22
4.08
11.13
0.37
19
NDHPCF90 2.3
1100
489
0.73
0.22
0.16
2.62
10.00
4.00
13.51
0.36
19
17
3.5. Physicochemical properties of the carbons XRD test results of the carbons are presented in Fig. 5a. No sharp peaks are detected, confirming that all the inorganic impurities have been completely removed. The carbons have an amorphous structure, as shown by the two broad peaks at approximately 21.5 and 44 °. The D band (~1330 cm-1) and G band (~1581 cm-1) of the Raman spectra in Fig. 5b correspond to disordered and graphitic carbon structures, respectively [29, 30]. The relatively high ID/IG (1.0) also demonstrates the low degree of graphitization in the carbons, in agreement with XRD and TEM results. The carbons’ textural properties are characterized with N2 adsorption-desorption measurements, and the detailed results are listed in Table 1. Fig. 5c show typical characteristics of type I and IV isotherms, i.e., the fast increase of adsorption amount in the low pressure region and the presence of hysteresis loops. The hysteresis loops with parallel and almost horizontal branches are characterized as Type H4, arising from the presence of mesopores surrounded by abundant micropores [31]. The loop’s size grows larger for carbons activated for 90 min, corresponding to the development of mesoporosity in the carbons. The results in Table 1 also demonstrate the significant improvements of SBET and Vt. NDHPCF90 has the highest SBET of 1100 m2/g. However, the Smicro and Vmicro of HPCF90 and NDHPCF90 are lower than the carbons activated for 60 min. For instance, the Smicro of NDHPCF60 is 563 m2/g, and it decreases to 489 m2/g for NDHPCF90, which is a direct evidence of the destruction of micropores. The increase of SBET with prolonged activation time is mainly contributed by mesopores. It is also noticed that the SBET of NDHPCF is slightly higher than that 18
of HPCF. Nitrogen containing functionalities originated from urea are reported to preferably locate at the edges of graphene layers, which may block small pores and cause the decrease of SBET [32]. However, as discussed above, carbon is also corroded by urea at high temperature, promoting the formation of pores and enhancing the porosity. The increase of SBET after nitrogen doping is thus attributed to the reactions between carbon and urea. The PSD curves derived from DFT method are shown in Fig. 5d. HPCF60 exhibits two sharp peaks at 0.5 and 0.7 nm, while for NDHPCF60 the peak corresponding to the smallest pores shifts to 0.6 nm, and the peak at 0.7 nm weakens and broadens obviously. For NDHPCF90 the peak at 0.5 nm is preserved, but its intensity is lower than that of HPCF90. It can be concluded that the reactions between carbon and urea result in the loss of the smallest pores in the carbons. The influences of activation time on ultramicropores are a little complicated. Compared with HPCF60, HPCF90 loses the sharp peak at 0.7 nm, although a new peak at 0.6 nm with moderate intensity emerges. On the other hand, a sharp peak at 0.5 nm is found in the PSD curve of NDHPCF90, which is absent in the PSD curve of NDHPCF60. Although the PSD characteristics of the ultramicropores vary significantly for different carbons, the Vultra stays in a narrow range of 0.15-0.16 cm3/g. 3.6. CO2 adsorption performances
19
Fig. 6. CO2 adsorption isotherms at (a) 273 K and (b) 298 K; (c) isosteric heat of adsorption (Qst); (d) N2 adsorption isotherms at 298 K; (e) CO2/N2 adsorption selectivity calculated with IAST method (CO2:N2 = 15:85); (f) Linear fitting of the CO2 uptakes (298 K, 20 bar) against the SBET with fixed intercept at 0. The combination of nitrogen doping and high porosity makes HPCFs/NDHPCFs excellent candidates for CO2 adsorption. The CO2 adsorption performances are tested at 273 and 298 K with pressure up to 20 bar. The adsorption isotherms are presented in Fig. 6a and b, and the uptakes are also listed in Table 1. At 298 K and 1 bar, NDHPCF60 exhibits the highest CO2 uptake of 2.76 mmol/g. This value is lower than some of the reported results (for instance, WSC-500-1 (4.50 mmol/g) [33], NGC-650-4 (3.92 mmol/g) [34], GN-650-1 (4.26 mmol/g) [35], C500-K (4.17 mmol/g) [36]). It has been emphasized that the uptake at 1 bar largely depends on ultramicropores [37, 38]. The relatively low uptake of NDHPCF60 is due to the high portion of large micropores and mesopores in the carbon. The Vultra values of HPCF60 and NDHPCF60 are 0.16 and 0.15 cm3/g, respectively. Even so, HPCF60 has a lower 20
uptake of 2.48 mmol/g. This result shows that nitrogen doping can enhance the uptake to a certain degree. The Qst in Fig. 6c are calculated to investigate the interactions between CO2 and the carbon surface. The carbons show Qst of 23.4-27.6 kJ/mol at low CO2 loading. Unexpectedly, the Qst values of NDHPCF60 (23.4 kJ/mol) and NDHPCF90 (24.2 kJ/mol) are lower than HPCF60 (27.6 kJ/mol) and HPCF90 (26.5 kJ/mol). The Qst at low loading results from the interactions between CO2 and the most favorable adsorption sites, for instance ultrafine pores and basic nitrogen functionalities [11]. The loss of the smallest micropores in the NDHPCFs causes the decrease of Qst at low CO2 uptake. In order to study the CO2/N2 adsorption selectivity, the N2 adsorption isotherms at 298 K are shown in Fig. 6d for comparison, and the N2 uptake is 0.33-0.37 mmol/g. The selectivity plots calculated with IAST (CO2:N2 = 15:85) are displayed in Fig. 6e. At 1 bar, the selectivity varies in a narrow range of 16-20, and the highest value is achieved by the nitrogen free HPCF60, which also has the highest Qst. The above results highlight the importance of small micropores to CO2 adsorption at low pressure, while the positive effects of nitrogen doping could be completely disguised by the influences of pore enlargement. On the other hand, CO2 adsorption at high pressure benefits from the contribution of both micropores and mesopores [39, 40]. At 20 bar, the CO2 uptakes of HPCF90 are 13.47 mmol/g (273 K) and 10.04 mmol/g (298 K), which are very close to the uptakes of NDHPCF90. To evaluate the adsorption stability and recyclability, we select HPCF90 and test five consecutive CO2 adsorption cycles at 298 K. As shown in Fig. S9, no obvious degradation of the uptake is observed, 21
showing the promising stability of HPCF90. With the decrease of porosity, the adsorption performances deteriorate apparently. NDHPCF60 shows the highest CO2 uptake at 1 bar, but the uptake at 20 bar and 298 K is only 9.22 mmol/g. The results in Table 1 suggest that nitrogen doping shows no obvious effects at 20 bar, and the uptake correlates more with the SBET. To support this point, CO2 uptake (298 K, 20 bar) is plotted against SBET in Fig. 6f with data collected from this work and reported results (the specific data are given in Table S3) [41-50]. Since the uptake heavily depends on the carbon’s porosity, it will be reasonable to assume zero uptake in nonporous adsorbents. The intercept of the fitted line in Fig. 6f is thus set at zero. Regardless of the nitrogen content of the carbons, a correlation coefficient of 0.854 is generated, and the slope of the fitted line shows that SBET of 100 m2/g corresponds to CO2 uptake of 0.82 mmol/g at 298 K and 20 bar. To sum up, highly porous carbons with heteroatom doping and high portion of small micropores are the most desirable for excellent adsorption at different pressures. 4. Conclusions In conclusion, HPCFs were prepared with Fe2O3 as the hard template followed by chemical activation. Cubical macropores were formed in the carbon by Fe2O3 cubes, and the carbon walls of the frameworks were filled with micropores due to NaOH activation. The HPCFs were also successfully doped with nitrogen with the assistance of urea during NaOH activation. The carbons exhibited high SBET up to 1100 m2/g. The phenomenon of pore enlargement was observed after nitrogen doping. The structure integrity of the prepared carbon frameworks were under influences from 22
different synthesis parameters, including the amount of Fe2O3 template, NaOH and urea. Uncontrolled consumption of carbon by reactions with these chemicals could result in very low carbon yield, and hence the destruction of the framework structure. At 298 K, CO2 uptakes of 2.76 and 10.04 mmol/g were achieved at 1 and 10 bar, respectively. At 1 bar, the uptake was enhanced by nitrogen doping to a certain degree. However, no significant effects on Qst and adsorption selectivity were observed. For the adsorption at 20 bar, the experiment results from the current and reported works revealed that the uptake was mainly decided by SBET with a ratio of 0.82 (mmol/g)/(100 m2/g). Acknowledgements The research was supported financially by the National Natural Science Foundation of China (No. 21805120 and No. 51804143), Jiangxi Academy of Sciences (2018-YZD2-19), and the Key R & D Program of Jiangxi Province (20181BBF68011).
23
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Hierarchically porous carbon frameworks (HPCFs) are prepared using hard templates. The HPCFs have well-defined cubical macropores and abundant micropores. The HPCFs are successfully doped with nitrogen. SBET of 100 m2/g corresponds to CO2 uptake of 0.82 mmol/g at 298 K and 20 bar.
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