Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9935-9944
pubs.acs.org/journal/ascecg
Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped Carbon via Pyrolysis of Chitosan/Urea/KOH Mixtures and Its Application as a Support for Formic Acid Dehydrogenation Catalysts Dong-Wook Lee,*,† Min-Ho Jin,† Duckkyu Oh,† Sung-Wook Lee,† and Jong-Soo Park† †
Advanced Materials and Devices Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea S Supporting Information *
ABSTRACT: The development of cheap, simple, and green synthetic methods for hierarchically porous nitrogen-doped carbon, especially derived from renewable biomass, such as chitosan, remains a challenging topic. Here, we first synthesized hierarchically porous nitrogen-doped carbon (KIE-8) having graphene-like structure via simple pyrolysis of a chitosan/urea/KOH mixture without any conventional sophisticated treatments, such as freeze-drying, hydrothermal carbonization, and soft or hard templating. On the basis of various analyses of KIE-8, we demonstrated that effect of urea on mesopore formation was insignificant; however, when KOH is used as an activating agent in the presence of urea, a large amount of mesopores can be created along with conventional KOH-derived micropores. In addition, it was revealed that chitosan-derived carbon nanosheets directed by urea are torn into chitosan-derived carbon nanoflakes via KOH activation, and mesopores originate from interstitial voids in aggregates of the carbon nanoflakes, and micropores are derived from in-plane pores in each nanoflake. KIE-8 was used as a catalyst support for formic acid dehydrogenation at room-temperature. Pd(6 wt %)/KIE-8 catalysts provided excellent catalytic activity (TOF = 280.7 mol H2 mol metal−1 h−1), and we demonstrated that the pore structure and nitrogen structure of KIE-8 are crucial factors to determine the catalytic activity. KEYWORDS: Chitosan, Urea, Hierarchically porous carbon, Nitrogen-doped carbon, Formic acid dehydrogenation
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INTRODUCTION The development of nitrogen-doped carbon materials as catalysts, electrodes, and adsorbents has received considerable attention because of their improved electronic, physicochemical, structural properties, and hydrophilicity.1,2 Especially, hierarchically porous nitrogen-doped carbon has attracted greater attention to achieve easy diffusion of molecules toward and away from active sites, efficient electron and ion transport, and large surface area and pore volume providing more active sites.2−5 The nitrogen doping into carbon materials is generally conducted via in situ synthetic methods or post treatments with nitrogen-containing precursors.2 In case of the post treatment, ammonia,6 urea,7−14 melamine,15 cyanamide,16 dicyandiamide,17 polyaniline, and polypyrrole18 are usually used as a nitrogen-containing precursor and thermal or hydrothermal treatments of as-prepared carbon materials in the presence of such precursors result in nitrogen-doped carbon. In case of the in situ synthetic method, nitrogen-doped carbon can be obtained through pyrolysis of nitrogen-containing complexes, such as biomass,7−9,19−24 organic polymers,10−13,25−28 metal− organic frameworks,14,29,30 or ionic liquids.31−33 In terms of economical feasibility and sustainability issues, renewable © 2017 American Chemical Society
resources, such as biomass is considered to be more attractive nitrogen-containing precursors for nitrogen-doped carbon materials because of their sustainability, easy processability, and low cost.4,5 Especially, chitosan has attracted much attention as a precursor for preparation of nitrogen-doped carbon materials, because it is the second most abundant biomass after cellulose and contains a number of amine groups.21 Meanwhile, regardless of nitrogen doping methods and types of carbon precursors, chemical activation procedures with KOH, ZnCl2, Na2CO3, and so on are inevitable to achieve porous structure with high surface area and pore volume.6,10,11,19,23,25,32 However, such a typical chemical activation method mainly forms micropores, even if mesopore structure or hierarchically porous structure is highly desired for utilization of carbon materials as catalysts, electrodes, and adsorbents. Therefore, additional treatments must be employed to form mesopores or macropores in carbon materials. According to recent publications, there have been several Received: June 12, 2017 Revised: August 25, 2017 Published: September 27, 2017 9935
DOI: 10.1021/acssuschemeng.7b01888 ACS Sustainable Chem. Eng. 2017, 5, 9935−9944
Research Article
ACS Sustainable Chemistry & Engineering
aqueous solution was added into the mixture solution, followed by stirring for 1 h. After centrifugation, washing with distilled water, and drying, Pd(6 wt %)/KIE-8 catalysts were prepared. Preparation of Pd/N-Charcoal Catalysts. To compare catalytic activity of Pd/KIE-8 catalysts with that of catalysts supported on conventional activated carbon, we also prepared Pd(6 wt %)/Ncharcoal (Pd/nitrogen-doped activated carbon) catalysts through the same method as Pd/KIE-8. Before deposition of Pd nanoparticles on the activated charcoal, nitrogen was doped on the activated charcoal support to improve the catalytic activity for formic acid dehydrogenation. In a typical synthesis, 1 g of activated charcoal (Aldrich) was mixed with 2 g of urea (99%, SAMCHUN) and 2 g of melamine (99%, SAMCHUN) as a nitrogen source, and the mixture was finely ground. After carbonization of the mixture at 900 °C for 2 h with a nitrogen flow (300 mL/min), washing with distilled water, and drying, Ncharcoal (nitrogen-doped activated charcoal) was prepared. The Pd nanoparticle deposition on N-charcoal was conducted through the same method as Pd/KIE-8. Formic Acid Dehydrogenation Tests at Room Temperature. Pd(6 wt %)/KIE-8 catalyst (0.055 g) was sealed in a 100 mL Teflonlined stainless steel reactor, followed by a nitrogen purge for 30 min. Afterward, a mixture of 10 mL of distilled water, 0.19 mL of formic acid (95%, Aldrich), and 0.34 g of sodium formate (99%, Aldrich) was injected into the reactor. The volume of gas produced at 25 °C was measured by the gas buret system. Characterization. The pore properties of KIE-8 samples were taken by nitrogen sorption tests with a Micromeritics ASAP 2420 instrument. Degassing of samples was conducted at 200 °C for 5 h. Transmission electron microscopy (TEM) analyses were conducted by using a FEI/TECNAI G2 instrument. For investigation into the nitrogen content, structure, and graphitization degree of KIE-8, we conducted elemental analysis (EA), Raman, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) analyses by using a Thermo Scientific FLASH EA2000 Organic Elemental Analyzer, BRUKER SENTERRA, Kratos 165XP spectrometer, a Rigaku D/MAX-2200 V instrument, and Thermo Nicolet 5700 instrument, respectively. To measure the Pd nanoparticle dispersion of Pd/KIE-8 catalysts, H2 chemisorption was conducted by using an AutoChem II 2920 instrument.
methods to form the mesopores or macropores, such as freezedrying,7−9,21,22 hydrothermal carbonization,20,29,33 soft,13,27 or hard14,24,29,30,33 templating. However, in case of freeze-drying and hydrothermal carbonization, expensive equipment is required. As for soft templating, we need surfactant templates, which are expensive and not eco-friendly. The hard templating is a complicated process including a preparation procedure of hard templates and an etching procedure for elimination of the hard templates. Thus, the development of cheap, simple, and green synthetic methods for hierarchically porous nitrogen-doped carbon, especially derived from renewable biomass, such as chitosan, remains a challenging topic. However, there have been few reports to provide a straightforward strategy for preparation of hierarchically porous chitosan-derived nitrogen-doped carbon without conventional sophisticated treatments, such as freezedrying, hydrothermal carbonization, and soft or hard templating. Here we first synthesized hierarchically porous nitrogen-doped carbon (KIE-8, Korea Institute of Energy Research-8) having graphene-like structure via simple pyrolysis of a chitosan/urea/KOH mixture without any conventional sophisticated treatments. Compared to chitosan/urea and chitosan/KOH mixtures, the chitosan/urea/KOH mixture gave a remarkable improvement in mesopore surface area and pore volume, which indicates that KOH facilitated the mesopore formation in the presence of urea. We investigated the pore formation mechanism of KIE-8 on the basis of nitrogen sorption, transmission electron microscopy (TEM), elemental analysis (EA), Raman, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) analyses. In addition, we employed KIE-8 as a catalyst support for formic acid dehydrogenation at room temperature, which is one of the significant topics in the field of energy and catalysis.34,35
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EXPERIMENTAL SECTION
RESULTS AND DISCUSSION Effect of a Combination of Urea and KOH on Mesopore Formation. We prepared various chitosan-derived porous carbon materials and investigated the pore formation mechanism to establish more industry-friendly synthetic strategies for chitosan-derived nitrogen-doped carbon materials with hierarchically porous structure. Chitosan was used as a nitrogen-containing carbon precursor, and KOH as an activating agent for micropore formation. In addition, urea was employed as a nitrogen source and structure directing agent for the formation of graphene structure. KOH is a wellknown activation agent to develop micropores in carbon materials through a reaction of carbon with KOH above 400 °C producing K2CO3 and K, a reaction of carbon with CO2 produced from K2CO3 above 700 °C, and K intercalation among the carbon structure above 800 °C.6,36 According to previous publications,7,37 urea is recognized as a structure directing agent for the preparation of graphene nanosheets. The decomposition of urea around 500 °C results in the formation of graphitic carbon nitride (g-C3N4) nanosheets, between which carbon precursors can infiltrate and be carbonized. When increasing temperature to above 700 °C, the infiltrated amorphous carbon initiates graphitization and is transformed into graphene structure between g-C3N4 layers, followed by gC3N4 decomposition to nitrogen-containing compound vapor such as NH3, C2N2+, and so on. Such nitrogen-containing vapor dopes nitrogen into as-prepared graphene.
Preparation of Hierarchically Porous Nitrogen-Doped Carbon KIE-8. Hierarchically porous nitrogen-doped carbon KIE-8 was synthesized via simple pyrolysis of a mixture of chitosan (Aldrich), KOH (95%, SAMCHUN), and urea (99%, SAMCHUN). In a typical synthesis, 0.3 g of chitosan and 1.8 g of urea were dissolved in 10 mL of 5.2 wt % acetic acid aqueous solution. Subsequently, 10 mL of 3.8 wt % KOH aqueous solution was added to the chitosan/urea mixture solution. The chitosan/urea/KOH mixture solution was dried for 12 h at 100 °C and precarbonized for 3 h at 160 °C. The precarbonized chitosan was carbonized for 6 h in the temperature range of 700−1000 °C with a nitrogen flow (300 mL/min). Afterward, the chitosanderived carbon was washed with 0.33 M HCl aqueous solution, followed by washing with distilled water. After the samples were dried for 2 h at 105 °C, hierarchically porous nitrogen-doped carbon KIE-8 was successfully synthesized. To investigate the effect of urea and KOH on pore formation, KIE-8 samples were also prepared without addition of urea or 3.8 wt % KOH aqueous solution. The preparation conditions of KIE-8 samples are listed in Table S1. Preparation of Graphitic Carbon Nitride (g-C3N4). Bulk gC3N4 was prepared by a thermal polymerization method. In a typical synthesis, urea was calcined at 500 °C for 3 h and cooled to room temperature. Preparation of Pd/KIE-8 Catalysts for Room-Temperature Formic Acid Dehydrogenation. H2PdCl4 (0.116 g) aqueous solution (10 wt % Pd basis, PM RESEARCH) was added into 10 mL of distilled water, followed by addition of 0.18 g of KIE-8 samples into the H2PdCl4 solution. After the mixture solution was stirred for 3 h, pH of the solution was adjusted to 9.5 by using 0.1 M NaOH solution. Subsequently, the mixture solution was additionally stirred for 21 h at room temperature. Afterward, 2 mL of 0.85 M NaBH4 9936
DOI: 10.1021/acssuschemeng.7b01888 ACS Sustainable Chem. Eng. 2017, 5, 9935−9944
Research Article
ACS Sustainable Chemistry & Engineering
prepared with only KOH, high surface area and pore volume were obtained, however such high pore properties originated from mainly micropores (Table 1). KIE-8-b provided a typical type I isotherm with no hysteresis loops and its peak pore diameter was below 2 nm, indicating that KIE-8-b has microporous structure being typical pore structure for activated carbon (Figure 1). In contrast, KIE-8-c synthesized with only urea gave a type IV isotherm with a H2 hysteresis loop in the relative pressure range of 0.9−1.0 (Figure 1a), demonstrating that mesoporous structure was established. However, its surface area and pore volume were too low to use it as a catalyst, adsorbent or electrode (Table 1). As previous publications reported,4,7,37 urea treatments combined with the freeze-drying contribute to formation of crumpled graphene structure and corresponding mesopore formation. However, in our case without the freeze-drying process, it was revealed that the effect of urea on the mesopore formation was insignificant. However, in case of KIE-8-e prepared by using both of KOH and urea, both of total pore volume and mesopore volume remarkably increased to 1.44 and 0.76 cm3/g in comparison with KIE-8-b and KIE-8-c (Table 1). In addition, KIE-8-e gave a type IV isotherm with two obvious hysteresis loops at 0.5−0.7 and 0.9−1.0 of relative pressure (Figure 1a), and the corresponding pore size distribution showed a trimodal distribution consisting of micropores smaller than 2 nm, small mesopores centered at 3.9 nm, and large mesopores centered at 27.2 nm (Figure 1b). On the basis of these results, it was revealed that when KOH is used as an activating agent in the presence of urea, a large amount of mesopores can be created along with conventional KOH-derived micropores, even if sophisticated treatments such as freeze-drying and so on are not used. Figure 2 presents TEM images of KIE-8-b, KIE-8-c, and KIE8-e. KIE-8-b prepared by only KOH activation has sheet-like morphology with rough surface originating from a large amount of micropores (Figures 2a and 2b). In case of KIE-8-c prepared by using urea as a structure directing agent for the synthesis of graphene, it showed highly crumpled graphene-like morphology with smooth surface, meaning that it mainly has large mesopores, derived from crumpled morphology, along with very low in-plane micropores (Figures 2c and 2d). As shown in Figure S1, KIE-8-c showed several layers of graphene, which is directed by g-C3N4 sheets produced through urea decomposition.7,37 However, as for KIE-8-e prepared by using both of KOH and urea, it has the agglomeration morphology among carbon nanoflakes, to which g-C3N4-directed graphene sheets seem to be torn. In summary, in case of nitrogen-doped carbon prepared from the chitosan/urea/KOH mixture, urea-derived
To investigate the effect of each agent on the pore structure of chitosan-derived carbon materials, we synthesized various chitosan-derived carbon materials without any agents, by using only KOH, by using only urea, or by using both of KOH and urea. Table S1 shows synthesis conditions of various chitosanderived nitrogen-doped carbon materials, and Figure 1 and
Figure 1. (a) Nitrogen sorption isotherms and (b) pore size distributions of KIE-8-b, KIE-8-c, and KIE-8-e.
Table 1 present nitrogen sorption results of KIE-8-a, KIE-8-b, KIE-8-c, and KIE-8-e. As for KIE-8-a prepared by pyrolysis of chitosan without any agents, no pores were detected (Table 1), which indicates that activating agents are necessary to create pores in chitosan-derived carbon materials. In case of KIE-8-b
Table 1. Pore Properties for KIE-8 Samples Obtained from Nitrogen Sorption Tests sample code KIE-8-a KIE-8-b KIE-8-c KIE-8-d KIE-8-e KIE-8-f KIE-8-g
SABET [m2/g]a ND
h
SAmicro [m2/g]b ND
2564 43 604 1648 1633 1165
h
Vtot [cm3/g]c ND
2522 25 520 1472 1362 993
h
Vmicro [cm3/g]d ND
1.23 0.16 0.57 1.44 1.43 0.85
h
Vmeso [nm]e h
ND 1.12 0.01 0.22 0.68 0.66 0.47
DBJH [nm]f ND
0.11 0.15 0.35 0.76 0.77 0.38
h
2.9 27.6 9.3 6.5 5.4 4.5
Dpeak [nm]g NDh
