Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped

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Straightforward Synthesis of Hierarchically Porous NitrogenDoped 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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01888 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Sustainable Chemistry & Engineering

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,† Jong-Soo Park† †

Advanced Materials and Devices Laboratory, Korea Institute of Energy Research (KIER), 152 Gajeongro, Yuseong, Daejeon 305-343, Republic of Korea

＊ Correspondence should be addressed to D.-W.L. (e-mail: [email protected]).

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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, 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(6wt%)/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 due to 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 resources such as biomass is considered to be more attractive nitrogen-containing precursors for nitrogen-doped carbon materials due to their sustainability, easy processibility 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 methods to form the mesopores or macropores such as freeze drying,7-9,21,22 hydrothermal carbonization,20,29,33 soft,13,27 or hard14,24,29,30,33 templating. However, in case of 3



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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 nitrogendoped carbon without conventional sophisticated treatments such as freeze drying, hydrothermal carbonization, 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), 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

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), 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 oC and precarbonized for 3 h at 160 oC. The precarbonized chitosan was carbonized for 6 h in the temperature range of 700 oC – 1000 oC with a nitrogen flow (300 mL/min). Afterward, the chitosan-derived carbon was washed with 0.33 M HCl aqueous solution, followed by washing with distilled water. After drying the samples for 2 h at 105 oC, 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 g-C3N4 was prepared by a thermal polymerization method. In a typical synthesis, urea was calcined at 500 oC for 3 h and cooled to room temperature. Preparation

of

Pd/KIE-8

catalysts

for

room-temperature

formic

acid

dehydrogenation. 0.116 g of H2PdCl4 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 aqueous solution was added into the mixture solution, followed by stirring for 1 h. After centrifugation, washing with distilled water, and drying, Pd(6wt%)/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(6wt%)/N-charcoal (Pd/nitrogen-doped activated carbon) catalysts through the same 5



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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 o

C for 2 h with a nitrogen flow (300 mL/min), washing with distilled water, and drying, N-

charcoal (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. 0.055 g of the Pd(6wt%)/KIE8 catalyst was sealed in a 100 mL Teflon-lined 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 oC was measured by the gas burette 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 oC 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 EA-2000 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.

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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 nitrogencontaining 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 well-known activation agent to develop micropores in carbon materials through a reaction of carbon with KOH above 400 oC producing K2CO3 and K, a reaction of carbon with CO2 produced from K2CO3 above 700 oC, and K intercalation among the carbon structure above 800 oC.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 oC 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 oC, the infiltrated amorphous carbon initiates graphitization and is transformed into graphene structure between g-C3N4 layers, followed by g-C3N4 decomposition to nitrogen-containing compound vapor such as NH3, C2N2+ and so on. Such nitrogen-containing vapor dopes nitrogen into as-prepared graphene. 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 chitosan-derived nitrogen-doped carbon materials, and Figure 1 and 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 chitosanderived carbon materials. In case of KIE-8-b 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 7



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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 cm3/g 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 KIE-8-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 graphenelike 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 nitrogendoped carbon prepared from the chitosan/urea/KOH mixture, urea-derived g-C3N4 sheets transform chitosan-derived carbon into graphene-like structure. KOH reacts with such graphene-like carbon sheets, and tears the graphene-like carbon sheets into graphene-like 8


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carbon nanoflakes. The agglomeration of such graphene-like carbon nanoflakes leads to three dimensionally porous structure with a large amount of mesopores along with conventional KOH-derived in-plane micropores. Effect of carbonization temperature on physicochemical structure of KIE-8. To observe the effect of carbonization temperature on the pore properties of nitrogen-doped carbon prepared from the chitosan/urea/KOH mixture, we synthesized KIE-8-d, KIE-8-e, KIE-8-f, and KIE-8-g in the carbonization temperature range of 700 oC – 1000 oC. Figure 3 and Table 1 present the nitrogen sorption results of KIE-8-d, KIE-8-e, KIE-8-f, and KIE-8-g. All of the samples gave a type IV isotherm with a H2 hysteresis loop (Figure 3a), which indicates that mesopore structure is formed and originates from interstitial voids among close-packed particles. In addition, all the samples showed a trimodal pore size distribution, consisting of micropores, small mesopores, and large mesopores (Figure 3b). As carbonization temperature increased to more than 800 oC, all of the surface area, total pore volume, and mesopore volume remarkably increased (Table 1), which is attributed to KOH activation, especially carbon gasification with K2CO3-derived CO2 and K intercalation occurring at higher temperature. However, at 1000 oC, the pore properties of KIE-8 slightly decreased due to an increase in graphitization degree. Figure 4 exhibits TEM images of KIE-8-d, KIE-8-f, and KIE-8-g. All of the samples gave three dimensionally porous structure, which could be derived from the agglomeration of torn graphene-like nanoflakes. As shown in Figure S2, KIE-8-f has around 6 graphene layers. Whereas KIE-8-c showed relatively longer graphene layers (Figure S1), graphene layers of KIE-8-f became shorter than KIE-8-c via combination of KOH activation and urea decomposition (Figure S2). Such broken graphene layers of KIE-8-f demonstrate that gC3N4-directed graphene sheets were torn into graphene nanoflakes via KOH activation, and a large amount of mesopores were created through the agglomeration of the graphene nanoflakes. In addition, in-plane micropores in each graphene nanoflake were also formed through KOH activation. Table 2 presents the elemental analysis (EA) of KIE-8-d, KIE-8-e, KIE-8-f, and KIE-8-g with increasing carbonization temperature. As the carbonization temperature increased, the carbon content increased and the hydrogen content decreased due to an increase in carbonization degree. In addition, the nitrogen content decreased with an increase in 9



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carbonization temperature. Especially, when the carbonization temperature increased from 800 oC to 900 oC, the nitrogen content remarkably decreased. The high nitrogen content below 800 oC originated from g-C3N4 produced by urea decomposition. At more than 900 oC, g-C3N4 decomposed into nitrogen-containing vapor, leading to a dramatic decrease in the nitrogen content of KIE-8. Thus, the relatively low nitrogen content at more than 900 oC is estimated to be derived from nitrogen doped into carbon. Figure 5 shows N 1s XPS spectra of KIE-8-d, KIE-8-e, KIE-8-f, and KIE-8-g. All of the samples exhibited two intense peaks at around 398.6 eV and 401.1 eV, being ascribed to pyridinic nitrogen and graphitic nitrogen.9,24,38 The pyridinic-nitrogen-derived peaks at 398.6 eV is the main peak for gC3N4.38 As carbonization temperature increased from 700 oC to 1000 oC, the intensity ratio of graphitic nitrogen to pyridinic nitrogen continuously increased, especially in the carbonization temperature range of 800 oC – 900 oC. This is because g-C3N4 decomposed at temperature between 800 oC and 900 oC while nitrogen-containing vapor was produced. Above 900 oC, the nitrogen-containing vapor doped nitrogen into chitosan-derived graphene, which resulted in the increase in the intensity of a graphitic nitrogen peak. To confirm the presence of g-C3N4, FTIR analysis for KIE-8-e was carried out. As shown in Figure S3, KIE8-e exhibited several peaks in the range of 1360 cm-1 - 1530 cm-1, which is assigned to the stretching modes of C-N heterocycles for g-C3N4.38 However, as for KIE-8-f, the peaks in the range of 1360 cm-1 - 1530 cm-1 disappeared. Based on EA, XPS and FTIR spectra, it was revealed that chitosan-derived carbon was infiltrated into g-C3N4 sheets prepared by urea decomposition below 800 oC, and g-C3N4 decomposed into nitrogen containing vapor above 800 oC. Figure 6 exhibits Raman spectra of KIE-8-d, KIE-8-e, KIE-8-f, and KIE-8-g with increasing carbonization temperature. All of the samples provided two peaks at around 1369 cm-1 (D band) and 1598 cm-1 (G band), being associated with lattice defects and disorder of graphitic carbon atoms, and in-plane stretching vibration of sp2 hybridized carbon atoms, respectively.22,24 The graphitization degree of KIE-8 can be evaluated by the intensity ratio of D band to G band (ID/IG), as the ID/IG ratio decreases with an increase in graphitization degree. When the carbonization temperature increased from 700 oC to 900 oC, the ID/IG ratio continuously decreased from 1.11 to 0.98, indicating that the graphitization degree of KIE-8 increased with an increase in carbonization temperature. However, at 1000 oC, the ID/IG ratio 10


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slightly increased from 0.98 to 0.99 in comparison with 900 oC, which might be attributed to the increase in graphitic nitrogen defects as shown in Figure 5. Pore formation mechanism of hierarchically porous nitrogen-doped carbon KIE-8. On the basis of results from Figure 1 to Figure 6, we can deduce the pore formation mechanism for hierarchically porous nitrogen-doped carbon KIE-8 prepared from the chitosan/urea/KOH mixture. Figure 7 shows the pore formation mechanism of KIE-8. Above 500 oC of carbonization temperature, urea is transformed into g-C3N4 sheets,7,9 into which chitosan-derived carbon infiltrates and is transformed into carbon nanosheets. When carbonization temperature increases to above 700 oC, in-plane micropore formation is boosted by the reaction of the carbon nanosheets with K2CO3-derived CO2 and K intercalation, while the carbon nanosheets are torn into carbon nanoflakes, followed by the agglomeration of the nanoflakes. Above 900 oC, g-C3N4 decomposes with production of nitrogen-containing compounds such as NH3, C2N2+ and so on, resulting in nitrogen-doping in the aggregates of carbon nanoflakes. The in-plane pores in each nanoflake are mainly composed of micropores. In addition, small and large mesopores originate from interstitial voids among the carbon nanoflakes. For further investigation into pore formation mechanism, we conducted XRD analyses for g-C3N4 (urea calcined at 500 oC), KIE-8-d, and KIE-8-f. As shown in Figure 8, urea calcined at 500 oC gave diffraction peaks at 27.1o, attributed to (002) peak of g-C3N4, indicating that g-C3N4 sheets can be easily formed by calcination of urea above 500 oC. KIE-8-d (700 oC) had only (002) peak of g-C3N4, which demonstrates that chitosan-derived carbon is formed between g-C3N4 sheets, and it has graphene-like structure with interlayer spacing of 0.329 nm. In addition, KIE-8-f (900 oC) gave a broad peak at 24.4o, which indicates that urea-derived g-C3N4 decomposes and chitosan-derived carbon sheets are torn into graphene-like carbon nanoflakes with larger interlayer distance (0.364 nm). The transformation of chitosan-derived carbon morphology from carbon nanosheets to carbon nanoflakes gives rise to a decrease in crystal size, which makes the diffraction peak broader. Moreover, the shift of diffraction peak from 27.1o to 24.4o may be attributed to aggregation of the graphene-like carbon nanoflakes. Catalytic activity of KIE-8 for room-temperature formic acid dehydrogenation. To verify the performance of KIE-8 as a catalyst support, we prepared Pd(6wt%)/KIE-8 catalysts for formic acid dehydrogenation at room-temperature. KIE-8-d, KIE-8-e, KIE-8-f, and KIE11



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8-g prepared at different carbonization temperature were used as a catalyst support to investigate the effect of physicochemical properties of KIE-8 on the catalytic activity. Figure 9 shows formic acid dehydrogenation results for Pd(6wt%)/KIE-8 catalysts. As carbonization temperature of catalyst supports increased, the volume of produced hydrogen and carbon dioxide mixture gas increased (Figure 9a) and the corresponding conversion at 80 min and turnover frequency (TOF) at initial 10 min and 25 oC increased from 23.3 % and 21.6 mol H2 mol metal-1 h-1 to 96.7 % and 280.7 mol H2 mol metal-1 h-1 (Figure 9b)(Comparison with previously reported catalysts was shown in Table S2)39-47. Especially, when carbonization temperature increased from 800 oC to 900 oC, there was a noticeable improvement in the catalytic activity. The conversion at 80 min and initial TOF significantly increased from 28.4 % and 43.2 mol H2 mol metal-1 h-1 to 94.1 % and 259.1 mol H2 mol metal-1 h-1. Interestingly, such a significant range of carbonization temperature (800 oC – 900 oC) is consistent with that showing a remarkable change in the nitrogen content and intensity ratio of graphitic nitrogen to pyridinic nitrogen through the g-C3N4 decomposition and initiation of nitrogen doping (Figure 5, S3, and Table 2). However, comparing pore properties of KIE-8-e prepared at 800 oC with those of KIE-8-f prepared at 900 oC, there was little difference in surface area and pore volume (Table 1). In addition, at 1000 oC, even though surface area and pore volume decreased, TOF and conversion slightly increased. This is because the increase in the intensity ratio of graphitic nitrogen to pyridinic nitrogen improved the catalytic activity. On the basis of such results, we can deduce that the activity of Pd/KIE-8 catalysts for formic acid dehydrogenation is significantly dependent on the graphitic nitrogen content in nitrogendoped porous carbon supports. To investigate the effect of graphitic nitrogen on Pd nanoparticle formation, we conducted XPS and H2 chemisorption analyses for Pd/KIE-8 catalysts. Figure 10 exhibits Pd 3d XPS spectra of Pd/KIE-8-d, Pd/KIE-8-e, Pd/KIE-8-f, and Pd/KIE-8-g. All of the samples gave four peaks at 336.5 eV (Pd 3d5/2), 338.2 eV (Pd2+ 3d5/2), 341.6 eV (Pd 3d3/2), and 343.3 eV (Pd2+ 3d3/2). Pd/KIE-8-d, Pd/KIE-8-e, and Pd/KIE-8-f prepared below 900 oC have main peaks at 338.2 eV and 343.3 eV, whereas Pd/KIE-8-g prepared at 1000 oC has main peaks at 336.5 eV and 341.6 eV. The peaks at 338.2 eV and 343.3 eV is ascribed to the interaction between Pd and the pyridinic nitrogen via a covalent chemical bonding.48,49 As the intensity ratio of graphitic nitrogen to pyridinic nitrogen increased with increasing temperature up to 1000 oC, 12


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the intensity of peaks at 338.2 eV and 343.3 eV decreased and the intensity of peaks at 336.5 eV and 341.6 eV increased. Table 3 shows H2 chemisorption results of Pd/KIE-8-d, Pd/KIE8-e, Pd/KIE-8-f, and Pd/KIE-8-g. As temperature increased from 800 oC to 900 oC, Pd dispersion and Pd nanoparticle size significantly increased and decreased, respectively. At 1000 oC, Pd dispersion and Pd nanoparticle size slightly decreased and increased, which might be attributed to the decrease in the surface area and pore volume of KIE-8 (Table 1). Considering the results in Figure 10 and Table 3 along with Figure 5, and Table 1, it was revealed that the electronic structure of Pd nanoparticles depends on the intensity ratio of graphitic nitrogen to pyridinic nitrogen, and Pd nanoparticle size and dispersion are dependent on a combination of pore structure and the intensity ratio of graphitic nitrogen to pyridinic nitrogen. In summary, the pore structure and nitrogen structure of nitrogen-doped carbon KIE-8 are determined by changing the carbonization temperature. The combination of those structures of KIE-8 influences the electronic structure, particle size and dispersion of Pd nanoparticles deposited on KIE-8. The catalytic activity of Pd/KIE-8 is significantly dependent on such properties of Pd nanoparticles. Ultimately, it can be concluded that the pore structure and nitrogen structure of nitrogen-doped carbon KIE-8 are crucial factors to determine the catalytic activity of Pd/KIE-8, and higher surface area, pore volume and graphitic nitrogen content give rise to better catalytic activity. Moreover, in order to compare catalytic activity of Pd/KIE-8 catalysts with that of catalysts supported on conventional activated carbon, we also prepared Pd(6wt%)/N-charcoal (Pd/nitrogen-doped activated carbon) catalysts. We used activated charcoal having 2.5 nm of BJH desorption average pore diameter, 1129 m2/g of BET surface area and 0.58 cm3/g of total pore volume, and doped nitrogen on the activated charcoal to improve the catalytic activity for formic acid dehydrogenation. Figure S4 shows formic acid dehydrogenation results for Pd(6wt%)/N-charcoal, Pd(6wt%)/KIE-8-f, and Pd(6wt%)/KIE-8-g catalysts. The conversion at 80 min and TOF at initial 10 min and 25 oC for Pd(6wt%)/N-charcoal catalysts were 81.2 % and 246.1 mol H2 mol metal-1 h-1. Pd(6wt%)/KIE-8-f and Pd(6wt%)/KIE-8-g catalysts provided 5-16 % higher catalytic activity in comparison with Pd(6wt%)/N-charcoal, and lower catalytic activity of Pd(6wt%)/N-charcoal is attributed to microporous structure of activated charcoal. 13



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CONCLUSIONS

We prepared hierarchically porous nitrogen-doped carbon KIE-8 via simple pyrolysis of a chitosan/urea/KOH mixture without any conventional sophisticated treatments such as freeze drying, hydrothermal carbonization, soft or hard templating. On the basis of various analyses of KIE-8, we could draw conclusions as follows. (1) It is well-known that urea treatments combined with the freeze drying contribute to the formation of crumpled graphene structure and corresponding mesopore formation. However, in our case without the freeze drying process, the effect of urea on the mesopore formation was insignificant. In contrast, 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. (2) We deduced the pore formation mechanism of KIE-8. During pyrolysis of a chitosan/urea/KOH mixture, g-C3N4 sheets can be easily formed by calcination of urea above 500 oC, and chitosan-derived carbon is formed between g-C3N4 sheets. Chitosan-derived carbon is torn into graphene-like carbon nanoflakes through KOH activation above 700 oC, and g-C3N4 decomposes and nitrogen is doped into carbon nanoflakes above 900 oC. Small and large mesopores originate from interstitial voids in the aggregates of carbon nanoflakes. (3) KIE-8 was used as a catalyst support for formic acid dehydrogenation. When carbonization temperature of KIE-8 increased from 800 oC to 900 oC, there was a noticeable improvement in the catalytic activity. Such an improvement is attributed to higher graphitic nitrogen content. Thus, it is demonstrated that the graphitic nitrogen content as well as the pore structure of nitrogen-doped carbon KIE-8 are crucial factors to determine the catalytic activity.

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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free for charge on the ACS Publications website at DOI: 10.1021/acssuschemeng. Chemical reactions during activation with KOH, calculation method of conversion, synthesis conditions of KIE-8 samples, catalytic activity comparison of various heterogeneous catalysts for formic acid dehydrogenation, high magnification TEM images of KIE-8-c and KIE-8-f, FTIR spectra of KIE-8-e and KIE-8-f, comparison of formic acid dehydrogenation activity of Pd/KIE-8 with Pd/N-charcoal.

ACKNOWLEDGMENT

This work was supported by a research program (B7-2461-03) of the Korea Institute of Energy Research (KIER).

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Table 1. Pore properties for KIE-8 samples obtained from nitrogen sorption tests. Sample code KIE-8-a

SABET [m2/g]a NDh

SAmicro [m2/g]b NDh

Vtot [cm3/g]c NDh

Vmicro [cm3/g]d NDh

Vmeso [nm]e NDh

DBJH [nm]f NDh

Dpeak [nm]g NDh

KIE-8-b

2564

2522

1.23

1.12

0.11

2.9

Straightforward Synthesis of Hierarchically Porous Nitrogen-Doped

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