Graphene Sheets from Graphitized Anthracite Coal: Preparation ...

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Energy & Fuels Article Table 1. Analysis Data of Taixi Coal and Its Demineralization Coal proximate analysis (wt %) ultimate analysis (daf, wt %) samples M ad A d V daf C H N S O a TX 1.38 15.40 8.56 91.10 3.34 0.89 0.36 4.35 TX-de 1.29 2.52 8.08 93.90 3.47 0.77 0.08 1.81 a 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 Fe 2(SO 4) 3 [TX-de:Fe 2(SO 4) 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 H 2 DBD Plasma. Synthesis of chemically derived graphene was carried out in a DBD reactor with H 2 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 H 2 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 TX- C-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-C- GO and precursors of noble metal (H 2PtCl 6·6H 2O, RuCl 3·3H 2O) 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 H 2 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 NH 3 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 5187 400 ppm NO x (approximately 92% NO, 8% NO 2), 600 ppm NH 3,2% O 2, and N 2 (balance), and a total gas flow rate of 100 mL/min (STP) was used throughout this study. NO x 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 equation 21 La = 1.84 λ/Bacos( φa) Lc = 0.89 λ/Bccos( 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 dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192
Energy & Fuels Article Figure 2. (a, b) Digital photos of samples before and after treatment with H 2 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 H 2 discharge plasma (inset: pore-size distribution). (c) Raman spectra of coal-based graphite, GO, and GS. G = (0.3440 − d )/(0.3440 − 0.3354) 002 where d 002 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-NC- GO 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 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 H 2 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 TX- NC-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 (H 2O and CO 2) 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 dx.doi.org/10.1021/ef300919d | Energy Fuels 2012, 26, 5186−5192
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