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Chen, T., Luo, L., Li, Z., Zhang, Z., Zheng, S., Zhu, Z., He, J., & Zhao, W. (2018). "Preparation and characterization of nitrogen and oxygen heteroatom codoped activated biocarbons from edamame shell," BioRes. 13(2), 3932-3948.

Abstract

A simple procedure was evaluated to prepare cost-effective heteroatom self-doped porous carbons from edamame shell using a two-step carbonization and activation process. The morphological, structural, textural properties and N2/CO2 adsorption–desorption were investigated. The results showed that edamame shell, which is abundant in nitrogen and oxygen, is an ideal precursor for preparing heteroatom self-doped porous carbons. The N and O contents of the prepared activated carbons (ACs) ranged from 1.20 wt% to 1.81 wt% and 5.13 wt% to 9.98 wt%, respectively. Furthermore, the specific surface area of 1835 m2/g of the N and O doped ACs resulted in mainly microporosity, which suggested that it has promising potential for wide applications in the fields of catalysis, energy conversion, energy storage devices, and adsorption.


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Preparation and Characterization of Nitrogen and Oxygen Heteroatom Codoped Activated Biocarbons from Edamame Shell

Tingting Chen,a Lu Luo,a Zeliang Li,a Zhicheng Zhang,a Shouwen Zheng,a Zhicheng Zhu,a Ju He,b,* and Weigang Zhao a,*

A simple procedure was evaluated to prepare cost-effective heteroatom self-doped porous carbons from edamame shell using a two-step carbonization and activation process. The morphological, structural, textural properties and N2/CO2 adsorption–desorption were investigated. The results showed that edamame shell, which is abundant in nitrogen and oxygen, is an ideal precursor for preparing heteroatom self-doped porous carbons. The N and O contents of the prepared activated carbons (ACs) ranged from 1.20 wt% to 1.81 wt% and 5.13 wt% to 9.98 wt%, respectively. Furthermore, the specific surface area of 1835 m2/g of the N and O doped ACs resulted in mainly microporosity, which suggested that it has promising potential for wide applications in the fields of catalysis, energy conversion, energy storage devices, and adsorption.

Keywords: Edamame shell; Activated carbon; Heteroatom self-codoping; Pore texture

Contact information: a: College of Material Engineering, Fujian Agriculture and Forestry University, 63 Xiyuangong Road, Fuzhou 350002, P.R. China; b: College of the Arts and School of Landscape Architecture, Fujian Agriculture and Forestry University, 15 Shangxiadian Road, Fuzhou 350002, P.R. China; *Corresponding authors: weigang-zhao@hotmail.com; heju@foxmail.com

INTRODUCTION

Carbon-based materials have attracted considerable interest for many energy- and environment-related applications, such as supercapacitors (Hwang et al. 2017), batteries (Tai et al. 2017), hydrogen storage (Luo et al. 2017), fuel cells (Zhang et al. 2011), CO2 capture (Zhao et al. 2017a), and water pollution treatment (Liu et al. 2014). This high level of interest can be attributed to their abundance, chemical and thermal stability, processability, and textural and structural tunability to fulfill the requirements of specific applications. In particular, activated carbon (AC) is an allotrope of disordered carbon materials that is distinguished for its high specific surface area and microstructure porous system, and stability for large-scale production (Karnan et al. 2016; Zhao et al. 2017b).

Activated carbons can be produced from any inexpensive material that has a high C content and moderate inorganic content, such as degraded or fossil biomass (e.g., peat, lignite, and all ranks of coal) and fresh biomass (e.g., wood, coconut shells, etc.). They can be produced using physical activation with different oxidizing gases, such as air, O2, CO2, steam, and their mixtures, or through chemical activation with KOH, NaOH, H3PO4, ZnCl2, and other chemical compounds (Şahin and Saka 2013; Ahmad et al. 2014). It is also well known that, although the surface of ACs is primarily non-polar, some surface functional groups do exist that play a very important role and influence their performance in the final application (Yamashita et al. 2006; Dang et al. 2017; Jiang et al. 2017).

Among ACs, N- and/or O-doped ACs have gained increasing interest over the last decade, mainly because doping carbonaceous materials with the heteroatoms is an effective way to modify their surface chemistry and improve their application performance (Hulicova-Jurcakova et al. 2009; Meng et al. 2014; Alabadi et al. 2016; Li et al. 2017; Ma et al. 2017; Ye et al. 2017). The N and O are predominantly introduced by one of two ways to fabricate N/O-doped ACs, which are carbonization/activation of N/O-enriched precursors and post-treatment with a N/O source, such as urea, melamine, ozone, plasma oxidation, etc. (Boudou 2003; Hulicova-Jurcakova et al. 2009; Xia et al. 2011; Zhao et al. 2013; Zhai et al. 2016). However, a negative impact of the post-treatment has been demonstrated by several authors because of the decreasing of the specific surface area by the pore blocking. Therefore, reasonable efforts have been devoted to developing heteroatom self-doped carbon materials from biowaste because of the low cost and simple procedure (Gao et al. 2015; Xu et al. 2015; Gao et al. 2016).

The symbiotic nitrogen fixation of legumes with Azotobacter has been a hot topic of scientific research that attracted the attention of the authors because of the much higher N content in their stems and shells compared with other biomass materials (Wani et al. 1995; Ci and Gao 2005). Legumes and especially edamame, which originates from China, have a high global annual production (Ren et al. 2011). Most edamame shells are a large-quantity by-product; however it is usually abandoned or sintered, which is harmful to the environment. The most important, but often ignored, characteristic of edamame shell is that it is rich in carbon, oxygen, and nitrogen.

This present work used edamame shell as a raw material to prepare heteroatom self-doped activated carbons and KOH as the activation agent. This material would not only generate value-added products from agriculture waste but also offer a potentially cost-effective alternative to ACs precursors. The aim of this experiment was to investigate the impact of the weight ratio of KOH/ edamame shell charcoal and to characterize the N and O dual doped ACs from edamame shell.

EXPERIMENTAL

Materials

Edamame shell (vegetable soybean shell) was used as a carbon precursor, and it was obtained from a commercial plantation area in Fuzhou, Fujian, China. It was repeatedly cleaned with tap water to remove impurities, placed in the sun for three days to remove most of the water, and then placed in an oven at 105 °C for 24 h to remove the residual moisture. All of the reagents, including KOH and HCl, were of analytical grade and used without further purification or treatment.

Activated carbon synthesis

The edamame shell was put into ceramic pots and then placed in a muffle furnace under a pure N2 environment. The furnace was heated from room temperature to 450 °C at a heating rate of 5 °C/min. The target temperature was maintained for 1 h. The N2 flow rate was 300 mL/min. The furnace was cooled down to room temperature under a N2 atmosphere, and the edamame shell charcoal (BS) was finally obtained.

The BS was ground and sieved to generate a uniform powder with particle sizes that ranged from 120 μm to 200 μm. The BS powder and KOH pellets were then mixed at KOH/BS weight ratios (W) of 2, 3, 4, 5, and 6. They were placed in a muffle furnace under a pure N2atmosphere, the temperature was increased from room temperature to 800 °C, and the target temperature was maintained for 2 h. The whole process was conducted under a stream of N2 with a flow rate of 300 mL/min. The crucible was then allowed to cool down to room temperature. The activated product was first washed with 1 M HCl, then with distilled water, and finally with a Soxhlet extractor with a low-temperature cooling circulating pump for three days. Pure activated carbons (BC) were obtained after drying in an oven for 24 h. The schematic procedure of the complete synthesis route of N/O-doped porous carbon from edamame shell is shown in Fig. 1.

All of the prepared BCs were as labeled BCxx. In this nomenclature, the xx at the end refers to the W. For example, BC13 means that the BS/KOH W was 1:3.

Fig. 1. Schematic procedure of the complete synthesis route of the N/O-doped porous carbons from edamame shell

Analysis Method

Physicochemical characterization of the materials

The TG curves representing the carbonization and activation processes at a W value of 3 were obtained by means of thermogravimetric analysis (TGA; SDT-Q600, NETZSCH-Gerätebau GmbH, Wittelsbacherstraße, Germany). The samples were heated from room temperature to a final temperature of 1273 K at a rate of 10 K/min under a N2 atmosphere with a flow rate of 20 mL/min.

The surface morphology of the samples was determined using scanning electron microscopy (SEM; FEG SEM Hitachi S 3400, Chiyoda-ku, Tokyo, Japan) and high-resolution transmission electronic microscopy (HRTEM; JEM-2100, JEOL, Tokyo, Japan) at an accelerating voltage of 200 kV.

The elemental analysis was performed with a Vario EL elemental analyzer (Elementar, Langenselbold, Germany) and X-ray photoelectron spectrometer (XPS; SPECS XPS System, Surface Nano Analysis Inc., Berlin, Germany) using 150 W Al Kα radiation.

N2 physisorption at 77 K

The N2 adsorption–desorption isotherms were obtained at 77 K using an ASAP 2020 automatic apparatus (Micromeritics Instrument Co., Norcross, GA, USA). The samples were degassed for 48 h under vacuum at 523 K prior to the adsorption experiments. Carbon dioxide adsorption was not used to characterize the materials prepared in this study because of the full accessibility of their pores to N2 at 77 K. Indeed, the porosity was well developed, and no N2 diffusion limitation was observed. The N2 adsorption data were obtained and treated as described elsewhere (Luo et al. 2017; Zhao et al. 2017a,b) to determine the following properties: (i) surface area by the Brunauer-Emmett-Teller (BET) calculation method (SBET); (ii) micropore volume according to the Dubinin-Radushkevich (DR) method (VDR,N2); and (iii) total pore volume defined as the volume of liquid nitrogen corresponding to the amount adsorbed at a relative pressure (P/P0) of 0.99 (V0.99). The average micropore diameter (L0) and pore size distribution (PSD) were also calculated with the density functional theory (DFT).

CO2 adsorption at 273 K

An ASAP 2020 automatic apparatus (Micromeritics Instrument Co., Norcross, GA, USA) was used for the CO2 adsorption analysis. The CO2 adsorption isotherms were determined by dosing CO2 volumes of 0.2 STP (standard temperature and pressure) cm3/g up to a P/P0 of 10-4, and a P/P0 table was used to construct the CO2 adsorption isotherm up to a final P/P0 of 1. An equilibration time of 50 s was used. The CO2 data was interpreted based on the DR model with correlation coefficients equal to or higher than 0.999. The micropore volume determined by the CO2 adsorption data (VDR,CO2) provides information about micropores narrower than 0.7 nm. Finally, the PSDs were calculated by applying the DFT model using a Micromeritics ASAP 2020 HD 88 system (Xia et al. 2017).

RESULTS AND DISCUSSION

TGA

The carbonization and activation processes of the edamame shell in a N2 atmosphere were studied by TGA, and the results are given in Fig. 2. The carbonization process curve of the BS had three distinct stages of weight loss. The temperature range of less than 403 K corresponded to the weight loss of physisorbed water. The second stage of remarkable weight loss occurred from 473 K to 673 K and was primarily because of the evaporation of H2O, and the escape of H and O atoms. Hence, the yield of edamame shell carbonization was 28.5% with an inferred favorable temperature of 723 K, which was similar to that of other biomass charcoals prepared in previous studies (Xu et al. 2015; Zhao et al. 2017b).

The activation process of the sample with a W of 3 (BC13) is shown in Fig. 2, and it had three different stages of mass loss. The first stage of mass loss at less than 393 K correlated to the dehydration of the KOH activation agent. The second stage was nearly a plateau from 393 K to 973 K, which indicated that there may have been no activation reaction. The third stage was observed at high temperatures (973 K to 1193 K) and corresponded to a sharp mass loss that was caused by the reaction of C-KOH. Pores are produced by the removal of C. In general, the chemical reaction between KOH and a carbon material can be written as follows (Rodríguez-Reinoso 2002; Azargohar 2009; Viswanathan et al. 2009):

4 KOH + C K2CO3 + K2O + 2 H2 (1)

The activation temperature is an important parameter for preparing ACs. Increasing the activation temperature results in noticeable changes to the porosity of ACs (Yoon et al. 2004; Kim et al. 2007; Cardoso et al. 2008). It was speculated from the TG curve that the appropriate activation temperature could be 1073K, which corresponded to a yield of 64.8%.

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Fig. 2. TG curves of the BS (solid line) and BC13 (dotted line) under a N2 atmosphere

Elemental Composition of all of the Materials

The elemental analyses of all of the materials prepared at different stages are shown in Table 1. The carbon content of the natural edamame shell used in this work was 42.1%. The carbon content increased remarkably after the carbonization and activation processes, and yielded 62.4% after carbonization and 70.8% to 78.2% after further activation. The N, O, and H contents decreased from 10.0%, 38.4%, and 5.0% to 6.5%, 25.6%, and 4.2% after carbonization, respectively. For the BC samples, the N and O contents were in the range of 1.20 wt% to 1.81 wt% and 5.13 wt% to 9.98 wt%, respectively.

Table 1. Elemental Analysis Results for All of the Samples

As was expected, pyrolysis caused a major decrease in the N, O, and H contents, but the N and O atoms were successfully incorporated into the C framework. The challenge for synthesizing N/O-doped carbon materials was to prepare materials that had both a high porosity and high N content because N is a volatile element that changes more easily with an increase in the heat-treatment temperature and/or the porosity of the material in which it is contained (Boudou 2003; Hulicova-Jurcakova et al. 2009; Xia et al. 2011; Zhao et al. 2013; Luo et al. 2017).

XPS Analysis

The XPS analysis was intensively utilized to investigate the elemental composition and chemical states. Figure 3A shows that the full survey spectra of the BS and BC13 all included obvious peaks ascribed to the C 1s (284.56 eV), N 1s (400.15 eV), and O 1s (532.23 eV) orbitals, and no other impurity peaks were observed, which validated the compositional analysis of the C, O, and N elements. The C, N, and O contents of the edamame shell were 62.2%, 10.4%, and 27.3%, respectively. The C content increased obviously up to 80.4%, while N and O contents decreased to 7.2% and 12.4% after carbonization. At the end, after activation, the C, N, and O contents for BC13 were 95.0%, 3.7%, and 1.3%, respectively. During carbonization, the C content increased considerably, and the O and N contents decreased due to the evaporation of H, N, and O in the gas phase (Braghiroli et al. 2012). Then, during thermal activation at high temperatures, the KOH activation involved the reaction of C and KOH via Eq. 1. Additionally, the abundant O functional groups in the biochar can react with the KOH during heating to generate H2O, CO2, and NOx gases because of dehydration, denitrogenation, and decarboxylation (Rodríguez-Reinoso 2002). This demonstrated that the carbonization and activation processes caused more obvious denitrogenation and deoxygenation of BC13 accompanied by the pore creation effect. The XPS results confirmed that the N and O atoms were successfully incorporated into the C framework, which was also in agreement with the elemental analysis results.

The results of the C, O, and N analysis were studied by peak-differentiating and imitating to determine the detailed bonding configurations. According to previous literature (Boudou 2003; Hulicova-Jurcakova et al. 2009; Xia et al. 2011; Zhao et al. 2013; Gao et al. 2015; Xu et al. 2015; Zhai et al. 2016; Luo et al. 2017), the regions corresponding to N 1s binding energies in carbon materials are 398.5 eV ± 0.4 eV for pyridinic-N (N-6), 400.5 eV ± 0.3 eV for pyrrolic-N (N-5), 400.5 eV for pyridinic-N (N-6(O)), 399.6 eV ± 0.2 eV for pyrrolidinic-N (N-5(O)), 401.1 eV ± 0.3 eV for quaternary-N (N-Q), and 402.5 eV to 403.7 eV for pyridinic-N-oxide (N-X). Figures 3F and 3G show the N 1s peak of BC13 that can be divided into three components, which corresponded to pyridinic-N (N-6, approximately 398.3 eV), pyrrolic-N (N-5, approximately 399.8 eV), and quaternary-N (N-Q, approximately 401.5 eV). The C 1s spectrum displayed an asymmetric shape in Figs. 3B and 3C, with the four components of the C species corresponding to C-C/C=C at 284.6 eV, C-OH/C-O-C at 285.3 eV, C=O/C-N at 286.2 eV, and C(O)OH/C(O)-O-C at 288.5 eV in the BS and BC13. Among them, three of the C peak areas (C-N/C-O, C=N/C=O, and –O-C=O) decreased and the C=C peak area increased during KOH activation, which agreed with the conclusion that the N and O contents were reduced during activation. Figures 3D and 3E show the O 1s spectrum with three O components, which were C=O at 531.7 eV, C-OH at 532.8 eV, and –COOH at 533.4 eV. It was obvious that for the O 1s spectrum, the C=O and C-OH peak intensities increased and decreased after activation, respectively.

Fig. 3A-F. XPS survey of the BS and BC13 (A); C 1s (B), O 1s (D), and N 1s (F) core-level spectra of the BS; and C 1s (C), O 1s (E), and N 1s (G) core-level spectra of BC13

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Fig. 3G. XPS survey of the BS and BC13 (A); C 1s (B), O 1s (D), and N 1s (F) core-level spectra of the BS; and C 1s (C), O 1s (E), and N 1s (G) core-level spectra of BC13

Morphology Analysis

Figure 4 shows the material morphology of the edamame shell, BS, and BC13 determined by SEM. The representative SEM image in Fig. 4a shows that the surface of the edamame shell exhibited a plant fiber structure that was rough and heterogeneous with a rugged morphology. The morphology of the BS in Fig. 4b was entirely different from the edamame shell; it contained many cavities with a heterogeneous porous structure, which was attributed to the evaporation during the carbonization process. Figures 4c through 4e show SEM images of the resultant BC13 at different magnifications. Serious structural changes were induced, which indicated that the prepared porous carbon sample possessed a continuous branched porous framework that consisted of C nanosheets and fully interconnected pores that ranged from several nanometers to hundreds of nanometers in size. The porous sheet-like structure was further studied by HRTEM (Fig. 4f), which illustrated and proved the amorphous and microporous structures of the AC sample. These results were in good agreement with the results from the N2adsorption/desorption analysis that showed that the SBET obviously increased after carbonization and activation.

Porous Texture

Figure 5a shows the N2 adsorption–desorption isotherms of the ACs from various W values. All the N2 isotherms were type I according to the International Union of Pure and Applied Chemistry (IUPAC) classification system (Rodríguez-Reinoso 2002). The type I isotherm is common to microporous materials with a narrow range of pore distributions. The key features of type I isotherms for these microporous materials are a sharp increase in the N2 adsorption capacity at a very low P/P0 (lower than 0.05), almost a plateau at a higher P/P0, and no adsorption–desorption hysteresis cycle. As the Wincreased from 2 to 6, the knee of the isotherm widened, which indicated a broadening of the micropore width. The CO2 adsorption isotherms of all ACs are shown in Fig. 5b. It was obvious that the total adsorbed CO2 volume first increased and then decreased, which indicated a decrease in the number of the narrowest micropores with a higher W.