Graphene Sheets from Graphitized Anthracite Coal: Preparation

Jul 17, 2012 - Coal has been used as an important resource for the production of chemicals, conventional carbon materials, as well as carbon nanomater...
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Graphene Sheets from Graphitized Anthracite Coal: Preparation, Decoration, and Application Quan Zhou,† Zongbin Zhao,*,† Yating Zhang,‡ Bo Meng,† Anning Zhou,‡ and Jieshan Qiu*,† †

Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ‡ School of Chemistry and Chemical Engineering, Xi'an University of Science and Technology, Xi'an 710054, China ABSTRACT: Coal has been used as an important resource for the production of chemicals, conventional carbon materials, as well as carbon nanomaterials with novel structures, in addition to its main utilization in the energy field. In this work, we present the synthesis of chemically derived graphene and graphene−noble metal composites with coal as the starting material by means of catalytic graphitization, chemical oxidation, and dielectric barrier discharge (DBD) plasma-assisted deoxygenation. It is found that the graphitization degree of the coal-derived carbon remarkably affects the properties of graphene obtained from chemical exfoliation, and high crystallinity of coal-derived carbon is essential for the preparation of high-quality graphene sheets (GS). GS decorated with highly dispersed noble metallic nanoparticles (NP) on their surface (NP/GS) were successfully fabricated via simultaneous reduction of graphite oxide (GO) and noble metal salts by H2 DBD plasma technique. The electrochemical performance of the GS as electrode in supercapacitor and the catalytic activities of NP/GS composites in selective reduction of nitrogen oxides (NOx) were investigated. This work demonstrates an alternative approach for the fabrication of graphene and its composites from coal with promising potential in energy storage and environment preservation. temperature-programmed oxidation method.17 Furthermore, coal possesses abundant polyaromatic structures that are quite similar to sp2 bonding characteristics of graphene; therefore, it is reasonable to expect that coal can be used as starting material for the fabrication of graphene and graphene-based composite materials. Herein, we present the successful synthesis of chemically derived graphene and composites of graphene sheets (GS) decorated with noble metal nanoparticles from coal by means of catalytic graphitization and chemical oxidation combined with dielectric barrier discharge (DBD) plasma technique. The graphitization degree of the coal-derived carbon precursor is controlled by using catalyst, and the correlation between the graphitization degree of the coal-derived graphite-like carbon and the resultant graphene quality has been clarified. The performance of as-prepared chemically derived graphene as electrode in supercapacitor was investigated, and the graphenebased composites of GS decorated with noble metal (Pt, Ru, and PtRu) nanoparticles were used as catalyst in selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia.

1. INTRODUCTION As a valuable and abundant solid fuel in nature, coal provides most of the energy for the economic development and livingused energy in China. Besides the contribution to energy, coal can be exploited as an important resource for the production of chemicals, such as creosote oil, naphthalene, phenol, and benzene, etc. Coal-based carbon materials, such as activated carbon, carbon blocks, and carbon electrodes, have been produced worldwide for a long time.1,2 With the discovery and development of carbon nanomaterials, the preparation of carbon nanomaterials from the cheapest and most abundant natural carbon sourcecoalhas attracted increasing attention in the last few decades.3 Wilson and his co-workers first reported the preparation of C60, C70,4 and CNTs5 from coal by using the arc-discharge method with coal-based carbon as the evaporation anode. Over the past decade, our group has intensively investigated the possibility and feasibility of preparing various carbon nanomaterials from Chinese coal.6 As a novel two-dimensional carbon nanomaterial, graphene has attracted tremendous scientific attention in recent years due to its excellent physical and chemical properties.7−9 To further broaden the application potential of graphene-based materials, graphene nanocomposites are being developed and receiving increased attention, especially the decoration with metal or metal oxide nanoparticles. Various carbon sources, including graphite, 10 hydrocarbons (CH 4 , C 2 H 2 ), 11,12 polymer (PMMA),13 natural biomaterials (food, insects), and even plastic waste14 have been widely used for preparing graphene. However, as the most abundant carbon source in the world, coal has not been investigated for the production of graphene up to now. Graphene layers in the structure of coals have been investigated in molecular level.15,16 Tomita et al. evaluated the size of graphene sheets in several Chinese anthracites by a © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of Coal-Derived Graphite Oxide. Taixi coal (TX), from Shanxi province of China, was used as the starting material for the preparation of GS. The raw coal was ground and sieved to a powder, and the obtained particle size of coal powder is less than 0.5 mm. The coal powders were milled for 1.5 h by ball milling (the ratio of ball and coal is 20:1), and the diameter of more than 90% of them is less than 15 μm after milling. Received: October 28, 2011 Revised: July 17, 2012 Published: July 17, 2012 5186

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Table 1. Analysis Data of Taixi Coal and Its Demineralization Coal proximate analysis (wt %)

a

ultimate analysis (daf, wt %)

samples

Mad

Ad

Vdaf

C

H

N

S

Oa

TX TX-de

1.38 1.29

15.40 2.52

8.56 8.08

91.10 93.90

3.34 3.47

0.89 0.77

0.36 0.08

4.35 1.81

By difference.

The raw coal was treated with hydrofluoric acid and concentrated hydrochloride in plastic beakers at 50 °C for 4 h to make deashed coal (TX-de), and then it was washed with distilled water until no Cl− and F− were detected in the filtrate. The transformation of coal into graphite-like carbon was conducted by heat treatment in the presence of Fe. First, the TX-de and Fe2(SO4)3 [TX-de:Fe2(SO4)3 = 16:12.6] was well-mixed by ball milling for 2 min, and then the mixture was subjected to catalytic graphitization (C-G) at 2400 °C for 2 h under argon, and the product obtained was named TX-C-G. For comparison, TX was noncatalytically graphitized (NC-G, without the additional catalyst) under similar condition to give rise to the product defined as TX-NC-G. The proximate analysis and ultimate analysis of TX and TX-de are listed in Table 1. Both TX-C-G and TX-NC-G described above were oxidized by Hummers method to get their corresponding graphite-like carbon oxides,18 TX-C-GO and TX-NC-GO, respectively. 2.2. Preparation of Coal-Derived Graphene by H2 DBD Plasma. Synthesis of chemically derived graphene was carried out in a DBD reactor with H2 as working gas at ambient atmosphere pressure.19 A weighted amount of TX-C-GO or TX-NC-GO powder was put into a vertical quartz tube (10 mm in diameter, about 100 mm in length of discharge) with porous plate in the middle. Prior to the discharge, the reactor was purged with H2 to exhaust the air in the quartz tube. Afterward, the DBD plasma was initiated under 50 V × 1.2 A input ac power at room temperature and atmospheric pressure for 5 min; significant volume expansion can be observed in a flash. The product GS derived from TX-C-GO and TX-NC-GO was named TXC-GS and TX-NC-GS, respectively. 2.3. Preparation of GS Decorated with Noble Metallic Nanoparticles. GS decorated with noble metal nanoparticles (NP/ GS) were prepared as follows.20 The TX-C-GO made as in section 2.1 with a high oxidation degree was used as the starting material for the support of metal nanoparticles. As a typical procedure to synthesize the NP/GS composite nanostructures, a certain proportion of TX-CGO and precursors of noble metal (H2PtCl6·6H2O, RuCl3·3H2O) were dispersed in ethanol with ultrasonication for 1 h. Subsequently, the mixed solutions were vacuum-dried at 100 °C for 10 h. The assynthesized solid powder mixtures were exposed in H2 plasma with 50 V × 1.2 A ac input power for 10 min to simultaneously reduce the GO and precursors of noble metal ions to produce NP/GS composites. 2.4. Electrochemical Measurement. For the electrochemical measurement, the fabrication of working electrodes was carried out as follows. Typically, the electroactive materials and polytetrafluoroethyene (PTFE) binder (without any other carbon additive) were mixed in a mass ratio of 9:1 and dispersed in ethanol, and the resulting mixture was dried at 60 °C. After drying, it was pressed into a wafer of 8 mm diameter and assembled into an electrode with two pieces of nickel foam substrate (10 mm in diameter). The electrochemical properties of chemically derived graphene were measured in a standard three-electrode system with a Pt sheet as counter electrode and Hg/HgO electrode as reference electrode in a 6 M KOH aqueous electrolyte. CV curves (scanning rates varying from 2 to 100 mV/s) were measured with an electrochemical workstation (CHI 660D). Galvanostatic charge/discharge measurements (current density ranged from 50 to 1000 mA/g) were conducted with a charge−discharge tester (Arbin BT2000), and the electrochemical capacitances were obtained from charge−discharge curves. 2.5. SCR Catalytic Activity Measurement. SCR of NO with NH3 over the catalysts was carried out in a continuous-flow fixed-bed quartz reactor under atmospheric pressure. Typically, 50 mg catalyst was packed inside the reaction zone, the initial gas composition was

400 ppm NOx (approximately 92% NO, 8% NO2), 600 ppm NH3, 2% O2, and N2 (balance), and a total gas flow rate of 100 mL/min (STP) was used throughout this study. NOx concentrations were analyzed using a flue gas analyzer (KM9106 Quintox, Kane International Limited) online.

3. RESULTS AND DISCUSSION The proximate analysis and ultimate analysis results of TX coal and its demineralization coal TX-de are shown in Table 1. The high carbon content (91.10%) and low content of volatile matter (8.56%) suggest that the coal is a typical anthracite coal with high metamorphic degree. After the treatment with hydrofluoric acid and concentrated hydrochloride, the ash content was decreased to 2.52 wt % from an initial value of 15.40 wt %, indicating that the mineral matter in coal has been efficiently removed. XRD patterns of samples, including coal-based graphite-like carbons (TX-NC-G, TX-C-G), their corresponding oxides (TX-NC-GO, TX-C-GO), and the final product graphene sheets (TX-NC-GS, TX-C-GS) derived from the coal-based graphite oxides after DBD plasma treatment, are shown in Figure 1. There are significant differences in crystallite size

Figure 1. XRD patterns of coal-based graphite (TX-NC-G, TX-C-G), coal-based graphite oxides (TX-NC-GO, TX-C-GO), and graphene sheets (TX-NC-GS, TX-C-GS) from DBD plasma.

(La), stacking height (Lc), and degree of graphitization between TX-NC-G and TX-C-G. The La and Lc of the crystallite can be determined using the Scherrer equation21 La = 1.84λ /Ba cos(φa) Lc = 0.89λ /Bc cos(φc)

where λ is the wavelength of the radiation used, Ba and Bc are the width of the (100) and (002) peaks, respectively, at 50% height, and φa and φc are the corresponding scattering angles or peak positions. The La of TX-NC-G and TX-C-G is 0.23 and 0.92 nm, and the Lc of TX-NC-G and TX-C-G is 0.27 and 0.48 nm, respectively. The graphitization degree (G) was calculated by using the following equation 5187

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Figure 2. (a, b) Digital photos of samples before and after treatment with H2 discharge plasma. SEM and TEM images of TX-NC-GS (c, d, e) and TX-C-GS (f, g, h) with different magnification.

Figure 3. (a) FTIR spectra of coal-based graphite oxides and their chemically derived GS. (b) Nitrogen adsorption and desorption isotherms at 77 K of GS from H2 discharge plasma (inset: pore-size distribution). (c) Raman spectra of coal-based graphite, GO, and GS.

Figure 2a,b is the optical photos of coal-based graphite oxides (TX-NC-GO, TX-C-GO) and their corresponding resultant products (TX-NC-GS, TX-C-GS) after H2 discharge plasma under the same experimental conditions. Obviously, the volume of TX-C-GS increased remarkably compared with its precursor TX-C-GO, while the volume change from TX-NC-GO to TXNC-GS increased only slightly after the DBD plasma treatment. It can also be observed that TX-C-GS is bulky and fluffy in appearance while TX-NC-GS is relatively compacted. Taking into consideration the higher graphitization degree of TX-C-G compared with TX-NC-G, it can be concluded that the expansion level of graphite oxide has a close relationship with the graphitization degree of its precursor. In other words, the high graphitization degree is of great benefit to the intercalation of graphite by the oxygen-containing groups during oxidation. The oxygen intercalated into the interlayer spacing of graphite will be removed to form gases (H2O and CO2) during the discharge process, and the yielding pressure overcomes the van der Waals force between the layers, so that the oxides can be exfoliated immediately during DBD plasma process. For the low level graphitization carbon, the interaction hardly takes

G = (0.3440 − d002)/(0.3440 − 0.3354)

where d002 is the interlayer spacing of (002) calculated from XRD patterns. From the XRD patterns, it can be calculated that the graphitizing degree of TX-C-G (91.98%) is much higher than that of TX-NC-G (66.28%). It is well-known that the diffraction peaks of (100), (004), and (110) will appear when high graphitization degree was achieved, and these peaks can be observed in the XRD pattern of TX-C-G rather than TX-NC-G. Therefore, it can be concluded that the applied catalyst substantially promoted graphitization of the coal. After oxidation of the coal-based graphite, their sharp (002) peaks around 26.2° basically disappeared, while the as-prepared TX-NC-GO and TX-C-GO gave rise to (001) peaks located at 11°, corresponding to an increasing interlayer spacing from 0.335 to 0.749 nm, indicating that the TX-NC-G and TX-C-G were efficiently oxidized and oxygen was bonded to their planar surface. After DBD plasma treatment, (001) peaks of TX-NCGO and TX-C-GO disappeared almost completely to give rise to TX-NC-GS and TX-C-GS, respectively, without any distinct peaks in their XRD profiles. 5188

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Figure 4. XPS spectra C1s peaks of coal-based graphite oxides and graphene sheets: TX-C-GO (a), TX-C-GS (b), TX-NC-GO (c), and TX-NC-GS (d).

stronger than that of TX-C-GS, which may indicate that the former has more oxygen-containing groups. The coal-based graphene sheets were further characterized via N2 adsorption and desorption at 77 K for deeply understanding their pore structure. According to the results, the BET surface area values of TX-C-GS and TX-NC-GS are 306 and 135 m2/g, respectively. As shown in Figure 3b, the isotherms have the characteristics of type IV with a type H3 hysteresis loop at relatively high pressure, revealing that these materials are composed of aggregated sheets (loose assemblages) possessing typical mesopore structures, corresponding with distributions of the most probable pore sizes of 3.69 nm (TX-NC-GS) and 4.02 nm (TX-C-GS), as shown in the inset of Figure 3b and the microscopy observations mentioned above. The significant structural changes occurring during the transformation from coal-based graphite to the GS are reflected in their Raman spectra (Figure 3c). As known, the G band corresponds to the first-order scattering of the E2g mode observed for sp2 carbon domains, and the pronounced D band is the disordered band associated with structural defects, amorphous carbon, or edges that can break the symmetry and selection rule.23 The Raman spectra of TX-NC-G and TX-C-G display a prominent G peak as the feature at 1581 cm−1. The strong 2D peak at 2663 cm−1 indicates that the Taixi coal has been transformed into graphite-like carbon after graphitization treatment. As expected, the ID/IG ratio of TX-C-G is smaller than that of TX-NC-G, which indicates that TX-C-G possesses higher graphitization degree than TX-NC-G, consistent with the results obtained from XRD. From the Raman spectra of coal-based graphite oxides (TX-NC-GO and TX-C-GO), it can be seen that the G bands seem to be broadened compared with

place during the oxidation process; therefore, the driving force resulting from gases (H2O and CO2) formed during DBD is not strong enough to overcome interlayer van der Waals force for complete expansion. Typical morphologies of TX-NC-GS and TX-C-GS are shown in Figure 2. It is revealed that layer structures were formed after expansion via DBD plasma (Figure 2c,f). Highmagnification SEM images (Figure 2d,g) of GS show a wrinkled, thin, sheetlike structure similar to that of graphene prepared from natural flake graphite by thermal expansion22 and sonication-assisted chemical reduction.9 TEM images (Figure 2e,h) show that these sheet structures have plenty of ripples and folded regions in their basal planes and seem to be transparent, possibly owing to their being ultrathin or even single sheets. It is obvious that the layer of TX-NC-GS appears to be thicker than that of TX-C-GS due to the incomplete exfoliation of the former. This is consistent with the volume change observation during plasma-assisted exfoliation, as shown in Figure 2a,b. There are a variety of oxygen-containing groups on the surface of TX-NC-GO and TX-C-GO (FTIR spectra; see Figure 3a), which mainly include the CO, C−O, and O−H. After H2 plasma reduction, the peaks for oxygen functional groups were significantly reduced with fewer CO groups left in TX-NC-GS and TX-C-GS compared with their corresponding precursors. Generally, the wide peak at 1100−1420 cm−1 could mainly be assigned to the C−H stretch, and the peak at 1583 cm−1 could be assigned to the aromatic CC stretch. According to the FTIR spectra of coal-based graphite oxides, the CC bond almost cannot be detected, while there is a visible CC peak located at 1583 cm−1 in their GS. The typical peak of CO located at 1731 cm−1 of TX-NC-GS is 5189

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Figure 5. Electrochemical performance of coal-derived graphene sheets. (a) Cyclic voltammetry curves of GS with the scanning rates of 5 mV/s. (b) Specific capacitance against different discharge current density with 6 M KOH solution as an electrolyte. (c) Galvanostatic charge/discharge curves with the current density of 100 mA/g. (d) The cycling performance at a charge/discharge current of 100 mA/g.

their precursors, meanwhile, the D bands at 1337 cm−1 become much more prominent, indicating the reduction in size of the in-plane sp2 domains, possibly due to the extensive oxidation.10 The Raman spectra of the TX-NC-GS and TX-C-GS also contain both G and D bands (at 1590 and 1350 cm−1, respectively), however, with a decreased ID/IG intensity ratio compared to that in TX-NC-GO and TX-C-GO, which suggests an increase in the average size of the sp2 domains and restoration of sp2 network during reduction of GO in H2 discharge plasma process.24,25 It should be noted that no obvious 2D bands can be observed in the Raman spectra of TXNC-GS and TX-C-GS due to the disordered structure (defects, vacancies, or distortions of the sp2 domains) caused by the use of strong acid (H2SO4) and strong oxidant (KMnO4) during the oxidation stage. We can reasonably believe that it is very difficult for the graphene sheets to retrieve the graphitic structures through DBD plasma treatment. Compared with TXNC-GS, the D band of TX-C-GS is weaker and wider, which suggests fewer structural defects in the latter than the former. As mentioned above, it can be concluded that the crystallinity of coal-derived carbon remarkably affects the structures and the properties of the graphene obtained from chemical exfoliation.26 The oxygen-containing functional groups of TX-NC-GO, TX-NC-GS, as well as TX-C-GO and TX-C-GS were characterized by X-ray photoelectron spectroscopy (XPS). The high-resolution C1s spectrum of the TX-C-GO (Figure 4a) and TX-NC-GO (Figure 4c) reveal that there are three main components arising from CC (aromatic rings), C−O (alkoxy and epoxy), and CO (carboxyl) groups. After DBD plasma treatment, carbon with different chemical valences remains for TX-NC-GS and TX-C-GS in XPS spectra; however, the peak intensities of them are much smaller than those in TX-

NC-GO and TX-C-GO. As shown in Figure 4b,d, there are no O−CO groups left in TX-NC-GS and TX-C-GS and the CC bonds become dominant, while the peak intensities of CO and C−O are obviously lower than that of TX-NC-GO and TX-C-GO, which indicates that most of the oxygencontaining groups in TX-NC-GO and TX-C-GO have been removed and the majority of the conjugated graphene networks are restored27 by DBD plasma treatment for only 5 min. The residual oxygen-containing groups on the graphene sheets will benefit the wettability of the electrode in supercapacitor and metal or metal oxide catalyst grafting for heterogeneous catalysis. However, compared with TX-C-GS, it is worth noting that there are more significant CO and C−O peaks left in the XPS spectrum of TX-NC-GS, which is consistent with the FTIR results. The electrochemical properties of these coal-derived graphene were measured in a 6 M KOH aqueous electrolyte with a three-electrode supercapacitor cell at room temperature by a CHI 660D electrochemical workstation. Figure 5a shows the CV measurement results of TX-NC-GS and TX-C-GS at the scanning rate of 5 mV/s. The CV curve of TX-C-GS exhibits a more typical rectangular shape than that of TX-NCGS, indicating that the TX-C-GS has a better capacitive behavior and charge propagation at the surface of electrode following the electric double layer charging mechanism compared with TX-NC-GS. The specific capacitances of TXNC-GS and TX-C-GS against the discharge current density are shown in Figure 5b, which was calculated from the galvanostatic charge and discharge curves at different current density, using the following equation28 C = I Δt /mΔV 5190

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Figure 6. TEM images of Pt/GS (a), Ru/GS (b), and PtRu/GS (c). (d) XRD patterns of NP/GS composites. (e) NO conversion as a function of temperature over chemically derived graphene and noble NP/GS.

where I is charge or discharge current, Δt is the time for a full charge or discharge, m indicates the mass of the active material, and ΔV represents the voltage change after a full charge or discharge. As shown in the Figure 5c, there are almost no voltage drops observed; this indicates that the electrodes can carry out fast charge/discharge under the experimental current density and show good rate capability.29,30 Moreover, lifecycle is one of the most important performance indexes for supercapacitor. The TX-C-GS and TX-NC-GS present the feature of good cycling performance (as shown in Figure 5d) tested using the galvanostatic charge−discharge technique. The capacitance of them basically remains invariant during the 1000 cycles under the operation current density of 100 mA/g after the unstable cycles at the beginning. It can be found that the specific capacitance of TX-C-GS is higher than that of TX-NC-GS at any specified discharge current density used in the present experiment. This may be attributed to the high-quality of graphene sheets from TX-CGS, which has more open structures and higher BET specific surface area (306 vs 135 m2/g) as well as higher conductivity compared with the TX-NC-GS. These features can significantly improve the electrical double layer capacitance; thus, TX-G-GS presented a higher specific capacitance and better electrochemical performance. Recently, graphene nanocomposites are receiving tremendously increasing attention because they possess much more novel properties compared with pure graphene and therefore enable more promising potential applications. It is easy to imagine that the GS can be used as catalyst support due to its unique two-dimensional basal network plane structure and high surface area, as well as the presence of surface functional groups in chemically derived graphene. Noble metal nanoparticles (Pt, Ru, and PtRu) decorated coal-based graphene sheet (TX-CGS) catalysts were successfully fabricated via impregnation and DBD plasma technique. The loading amounts of Pt, Ru, and

PtRu were kept constant (∼3.5 wt %), and the atomic ratio of Pt:Ru in PtRu/GS was 3:1. From the TEM images (Figure 6a−c), it can be clearly seen that the exfoliated GS was decorated by uniform nanoparticles less than 2 nm in size. It has been demonstrated that metal nanoparticles can be deposited on both sides of GS due to their large surface area and unique basal 2D planar structure, and furthermore, the attachment of nanoparticles onto the GS surface can significantly prevent their aggregation and restacking.31−33 The highly dispersed metal nanoparticles on supports with large surface area are favorable to promote their interfacial contact with the other reactant molecules; therefore, they have potential advantages in catalytic activity and sensor sensitivity.34 The XRD patterns of the NP/GS composites are shown in Figure 6d. The typical diffraction peaks of the facecentered cubic (fcc) Pt lattice (111) and (200) in the assynthesized Pt/GS composites can be clearly observed; however, the peaks of Ru (Ru/GS) and PtRu alloy (PtRu/ GS) cannot be recognizable in XRD diffraction due to the small size and low loading.35,36 Selective catalytic reduction (4NO + 4NH3 + O2 = 4N2 + 6H2O) has been wildly used for the removal of NO in the flue gas from coal combustion.37 The NH3−SCR reaction activities of GS, Pt/GS, Ru/GS, and PtRu/GS are shown in Figure 6e. It can be seen that GS itself does not show any catalytic activity in SCR, while high catalytic activities appeared after the decoration of the graphene sheets with Pt and PtRu nanoparticles. Although Ru/GS shows negligible catalytic activity under the present reaction condition, PtRu/GS bimetallic alloy catalyst shows the highest catalytic activity at low temperature among the obtained graphene-based catalysts, which is similar to the performance of the electrocatalytic reaction in the fuel cell.38 The light-off temperature (the temperature at which the conversion of NO reaches 50%) for this reaction is as low as 150 °C under present experiment conditions, and more than 90% NO conversion can be achieved 5191

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at 165 °C. Moreover, the reaction temperature window of the alloy catalyst is relatively wide, ranging from 150 to 230 °C with 80% NOx conversion. The excellent activity for PtRu/GS catalyst is possibly due to the synergistic effect between Pt and Ru caused by the Ru that entered into the lattice of Pt in the PtRu alloy.

(13) Sun, Z. Z.; Yan, Z.; Yao, J.; Beitler, E.; Zhu, Y.; Tour, J. M. Nature 2010, 468 (7323), 549. (14) Ruan, G. D.; Sun, Z. Z.; Peng, Z. W.; Tour, J. M. Acs Nano 2011, 5 (9), 7601. (15) Wertz, D. L.; Bissell, M. Energy Fuels 1994, 8 (3), 613. (16) Saikia, B. K.; Boruah, R. K.; Gogoi, P. K. J. Chem. Sci. 2009, 121 (1), 103. (17) Aso, H.; Matsuoka, K.; Sharma, A.; Tomita, A. Energy Fuels 2004, 18 (5), 1309. (18) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80 (6), 1339. (19) Zhou, Q.; Zhao, Z. B.; Chen, Y. S.; Hu, H.; Qiu, J. S. J. Mater. Chem. 2012, 22 (13), 6061. (20) Xu, W. Y.; Wang, X. Z.; Zhou, Q.; Meng, B.; Zhao, J. T.; Qiu, J. S.; Gogotsi, Y. J. Mater. Chem. 2012, 22 (29), 14364. (21) Sonibare, O. O.; Haeger, T.; Foley, S. F. Energy 2010, 35 (12), 5347. (22) Schniepp, H. C.; Li, J. L.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110 (17), 8535. (23) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Phys. Rev. Lett. 2006, 97 (18), 187401. (24) Ramaprabhu, S.; Eswaraiah, V.; Aravind, S. S. J. J. Mater. Chem. 2011, 21 (19), 6800. (25) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970, 53 (3), 1126. (26) Wu, Z. S.; Ren, W. C.; Gao, L. B.; Liu, B. L.; Jiang, C. B.; Cheng, H. M. Carbon 2009, 47 (2), 493. (27) Liu, Y. Z.; Li, Y. F.; Zhong, M.; Yang, Y. G.; Wen, Y. F.; Wang, M. Z. J. Mater. Chem. 2011, 21 (39), 15449. (28) Wang, H. L.; Casalongue, H. S.; Liang, Y. Y.; Dai, H. J. J. Am. Chem. Soc. 2010, 132 (21), 7472. (29) Li, Y. G.; Tan, B.; Wu, Y. Y. Nano Lett. 2008, 8 (1), 265. (30) Lin, R.; Taberna, P. L.; Chmiola, J.; Guay, D.; Gogotsi, Y.; Simon, P. J. Electrochem. Soc. 2009, 156 (1), A7. (31) Wang, X.; Xu, C.; Zhu, J. W. J. Phys. Chem. C 2008, 112 (50), 19841. (32) Samulski, E. T.; Si, Y. C. Chem. Mater. 2008, 20 (21), 6792. (33) Yuge, R.; Zhang, M. F.; Tomonari, M.; Yoshitake, T.; Iijima, S.; Yudasaka, M. ACS Nano 2008, 2 (9), 1865. (34) Xing, Y. C. J. Phys. Chem. B 2004, 108 (50), 19255. (35) Abu Bakar, N. H. H.; Bettahar, M. M.; Abu Bakar, M.; Monteverdi, S.; Ismail, J.; Alnot, M. J. Catal. 2009, 265 (1), 63. (36) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. R. J. Am. Chem. Soc. 2004, 126 (25), 8028. (37) Galvez, M. E.; Lazaro, M. J.; Moliner, R. Catal. Today 2005, 102, 142. (38) Liu, Z. L.; Ling, X. Y.; Su, X. D.; Lee, J. Y. J. Phys. Chem. B 2004, 108 (24), 8234.

4. CONCLUSION We demonstrate that coal-derived graphene can be successfully prepared from graphitized coal with the graphite oxide pathway combined with H2 DBD plasma technique. Fe can serve as an effective catalyst to promote the graphitization of coal, which is critical to the structures, quality, and properties of the final product of coal-derived graphene. Noble metallic nanoparticles as small as 2 nm decorated on the surface of GS were successfully fabricated with high uniformity and dispersity via the impregnation and H2 DBD plasma technique. The electrochemical performance of the produced graphene from coal presents a mild capacitance and excellent electrochemical stability in KOH electrolyte. The noble metal/graphene composites can be used as catalyst, of which PtRu/GS show high catalytic activity and wide operating temperature window, as evidenced in SCR of NOx with ammonia under atmospheric pressure. Our synthesis strategy may lead to an alternative approach to the preparation of graphene and graphene-based composites with novel structures and various applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.Z.), [email protected] (J.Q.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the NSFC (Nos. 51072028, 20876026, 20836002, 20725619) and the Fundamental Research Funds for the Central Universities (no. DUT 11ZD120).

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dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192