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Dec 29, 2012 - State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of. China. ...
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Online Analysis of Volatile Products from Bituminous Coal Pyrolysis with Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry Liangyuan Jia,† Junjie Weng,† Yu Wang,† Shaobo Sun,† Zhouyue Zhou,† and Fei Qi*,†,‡ †

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ‡ State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ABSTRACT: Volatile species from pyrolysis of two kinds of bituminous coal, Huainan (HN) and Yima (YM), were investigated online with tunable synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Mass spectra of products at different photon energies and temperatures were measured during the pyrolysis process. Some isomeric products can be characterized by the SVUV-PIMS based on their different ionization energies. Aromatic compounds are dominant pyrolysis products of these coal samples, while a number of aliphatic products were also observed. In comparison of these two coals, the HN coal probably has more condensed aromatic structures than YM coal. The different structures of the macromolecular network between HN and YM coals probably lead to different pyrolytic products at the same temperature. This work demonstrates the good performance of SVUV-PIMS in online analysis of complex coal pyrolysis.

1. INTRODUCTION Coal counts for approximately 27% of the total world primary energy consumption and makes a large contribution to global air pollution. Therefore, increasing public concerns have been attracted to the development of cleaner and more efficient coal utilization technologies.1 These, of course, require a comprehensive understanding of the fundamental properties of coal. However, coal is such a kind of complex fuel, which consists of tens of thousands of organic and inorganic compounds. It is estimated that more than 130 molecular structures of coal have been proposed over the last 70 years, yet only a few are wellknown.2,3 Many advanced techniques have been applied to characterize the chemical properties and structures of products from coal pyrolysis and coal-derived liquids.4 Among them, mass spectrometry is a useful method for identification of pyrolytic products. For inorganic compounds in coal, inductively coupled-plasma mass spectrometry (ICP-MS) is the most frequently used technique in elemental analysis.5−7 Other methods, such as online molecular-beam quadrupole mass spectrometry (MB-QMS), were also performed well to investigate the release of alkali metal, sulfur, and chlorine species during coal gasification.8,9 For organic compounds, so far the most commonly used technique for analyzing the pyrolytic products of coal is gas chromatography/mass spectrometry (GC/MS) because of its high sensitivity and good performance in separation.10−16 However, GC/MS has some drawbacks, such as long time-consumption and complex sample pretreatment, all suggesting a better method for decent investigation. Detailed information of evolved organic vapors can be given by combination GC-MS with other methods, such as a real-time analytical technique called Fourier transform infrared spectrometry (FTIR)17 and a accurate quantitative method named thermogravimetry (TG).18 Beside these, © 2012 American Chemical Society

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has also been employed to assist the GC/MS, as it is extremely selective for the dihydroxy aromatics presented in coal tar,19 and thus it is very suitable to provide the detailed molecular compositions of acidic species in coal extracts.20 Pyrolysis-GC/MS has also been widely used to examine the structure of coal-derived materials as well.21−24 However, this technique suffers from the limitation that some polar and/or involatile pyrolysis products may be lost and the interpretation of pyrolytic data may lead to errors without knowledge of the chemical nature and thermal behavior of the organic material studied.25 In summary, most of the analytical methods regarding organic compounds involve the pretreatment and isolation processes in the laboratory. These processes probably disturb the original compositions of gas-phase products and result in questionable data. To overcome these difficulties, fast and in situ mass spectrometers combined with soft ionization methods, such as vacuum ultraviolet (VUV) photoionization techniques, have been applied to analyze the coal pyrolysis products,26−28 enabling online analysis of the chemical compositions of aerosols. Gao et al. compared the GC/MS total ion chromatograms with photoionization mass spectra of polycyclic aromatic hydrocarbons contained in coal soot and found out that the VUV photoionization techniques are more sensitive to larger polycyclic aromatic hydrocarbons (>3 rings) than GC/MS.27 This paper reports an experimental study of coal pyrolysis based on SVUV-PIMS, which has been successfully applied to the investigation of biomass pyrolysis very recently.29 The Received: October 16, 2012 Revised: December 22, 2012 Published: December 29, 2012 694

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Table 1. Proximate and Ultimate Analysis of HN and YM Coals proximate analysis (wt %)

a

ultimate analysis (wt %, daf)

coal sample

Ma

VMb

FCc

Ad

N

C

H

S

Oe

HN coal YM coal

1.38 4.79

19.98 32.93

26.17 38.46

52.48 23.81

1.50 1.13

80.68 76.30

5.94 5.35

1.15 0.99

10.93 16.23

M: moisture. bVM: volatile matter. cFC: fixed carbon. dA: ash. eBy difference; daf, on a dry and ash free basis.

Table 2. Mass Assignment of Main Pyrolytic Products from Two Coals (IE Values Referring to NIST34) MW

name or type

formula

IE (eV)

MW

name or type

formula

IE (eV)

34 64 67 78 80 81 91 92 94 106 106 108 108 110 110 110 117 117 120 122 122 124 124 136 138 142

hydrogen sulfide sulfur dimer pyrrole benzene pyrimidine methylpyrrole benzyl radical toluene phenol benzaldehyde C2 alkyl benzene benzyl alcohol cresols benzenethiol dihydroxybenzens 4-hydroxy benzaldehyde benzyl nitrile indole C3 alkyl benzene benzoic acid C2 alkyl phenol guaiacol dihydroxytoluene 2,4,6-trimethyl phenol 3-ethoxyphenol C1 alkyl naphthalene

H2S S2 C4H5N C6H6 C4H4N2 C5H7N C7H7 C7H8 C6H6O C7H6O C8H10 C7H8O C7H8O C6H6S C6H6O2 C6H6O2 C8H7N C8H7N C9H12 C7H6O2 C8H10O C7H8O2 C7H8O2 C9H12O C7H6O3 C11H10

10.56 9.35 8.21 9.24 8.71 8.01 7.24 8.80 8.51 9.50 8.44−8.77 9.23 8.41 8.49 8.63 9.32 9.34 7.76 8.27−8.42 9.75 8.09−8.26

144 146 156 162 166 168 170 182 184 184 194 196 198 206 210 212 220 228 230 234 234 242 248 252 262 274

2-naphthalenol coumarin C2 alkyl naphthalene benzo[b]thiophene fluorene C1 alkyl biphenyl C3 alkyl naphthalene benzophenone 2,3,6,7-tetramethyl naphthalene dibenzothiophene anthrone xanthone C1 alkyl benzothiophene C2 alkyl phenanthrene sinapyl alcohol, (E) 4,6-dimethyl dibenzothiophene C3 alkyl phenanthrene benz[a]anthracene dimethyl pyrene benzo[b]naphtho[2,1-d]thiophene 3,4,5,6-tetramethyl phenanthrene monomethyl chrysene 6-methyl benzo[b]naphtho[2,3-d]thiophene benzo[e]pyrene dimethyl benzo[b]naphtho[2,3-d]thiophene tricyclopenta[cd]pyrene

C10H8O C9H6O2 C12H12 C10H10S C13H10 C13H12 C13H14 C13H10O C14H16 C12H8S C14H10O C13H8O2 C13H10S C16H14 C11H14O4 C14H12S C17H16 C18H12 C18H14 C16H10S C18H18 C19H14 C17H12S C20H12 C18H14S C22H10

7.89 8.72 7.78−8.30

8.00 8.49 7.91−7.96

7.89 7.80−8.10 9.08 7.60 8.34 8.83 8.42 7.53−8.01 8.77 7.45

7.50 7.44 7.41

The detailed description of the beamline used in this work can be found elsewhere.31,32 Briefly, the VUV light from the undulator beamline covers the photon energy ranging from 8.0 to 17.0 eV and uses a gas filter filled with Ar or Ne to eliminate higher-order harmonic radiation. This beamline utilizes a 1 m Seya-Namioka monochromator with an energy resolving power of 1000 and an average photon flux of 1 × 1013 photons/s. 2.2. Thermogravimetric, Proximate, and Ultimate Analysis. Thermogravimetric analysis experiments were performed in a Q5000 IR thermogravimetric analyzer (America TA). In all experiments, the temperature was raised from room temperature up to 800 °C at the heating rate of 10 °C min−1. The reacting atmosphere was nitrogen, which streamed at a constant volume flow rate of 75 mL min−1. Proximate and ultimate analysis was completed in a TGA-2000 proximate analyzer (Las Navas, Spain) and a Vario EL-II CHONS elemental analyzer (Elementar Analysensysteme Gmbh, Hanau, Germany), respectively. All experiments were run three times for each coal sample, and the average was calculated as the final results. 2.3. Coal Samples and Pyrolysis Experiment. Low volatile HN coal and high volatile YM coal were chosen because they are abundant and widely used in China. The results of proximate and ultimate analysis are presented in Table 1. Both selected HN and YM coals were bituminous; thus the effect of rank variation of coal can be minimized. The coal sample weight for pyrolysis in each measurement was 50 ± 0.1 mg. The mass spectra were collected at different photon energies and temperatures. The background was subtracted by measuring a blank sample at the same condition. The time was recorded after the coal sample was delivered into the center of the

SVUV light has many advantages for the online analysis of complex chemical systems, such as a broad tunable photon energy range, good energy resolution, and high photon flux.30−32 Fragment-free and/or fragment-controllable mass spectra can be obtained by SVUV-PIMS, and part of the isotopes can be characterized by tunable SVUV light due to their different ionization energies.

2. EXPERIMENTAL METHODS 2.1. SVUV-PIMS Pyrolysis Apparatus. Mass spectrometric analysis was performed at the National Synchrotron Radiation Laboratory in Hefei, China. The detailed description of the SVUVPIMS pyrolysis apparatus has been reported elsewhere.29 Briefly, the experimental setup consists of three parts: a pyrolysis chamber with a Shimadzu furnace inside, a photoionization chamber designed for introducing the SVUV light, and a modified quadrupole time-of-flight (QTOF) mass spectrometer (AB Sciex, Toronto, Canada). Nitrogen as the carrier gas was used to bring gas-phase pyrolysis products to the photoionization region at a constant flow rate of 5 mL min−1, which was controlled by a mass flow controller. In our experiment, a quartz sample pole (o.d. 6 mm) with a boat was used to introduce the coal sample into the middle of furnace. The gas-phase products entered into the photoionization region from the pyrolysis chamber, and ions produced by SVUV light were guided into the QTOF-MS for analysis by a repeller plate and a skimmer. The temperature of the furnace was controlled by a temperature controller. 695

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furnace, and the complete heating process for each measurement lasted from the beginning to the end of the pyrolysis process.

aromatics, which is in agreement with the reported high aromaticity of coal soot in a previous study.3 Figure 2a−c shows the mass spectra of volatile products obtained at the fixed photon energy of 10.5 eV and at different temperatures, including 450, 550, and 650 °C. We can see that temperature obviously affects the relative abundance of pyrolysis products. As shown in Figure 2, it is important to note that peaks in the high m/z region (240−350), corresponding to aromatics with 4−7 rings, can be apparently observed (see Figure 2a), while a significant fall in the signal intensity of these aromatics occurs at higher temperatures (see Figure 2b,c). Figure 2b shows the mass spectrum of volatile products from HN coal at 550 °C, which suggests a decrease in the amount of aromatics with two and three rings. However, this decrease is caused by the lower total ion signal for all evolved products at 550 °C comparing to the one at 450 °C, not the fragmentation of these aromatics. Therefore, there is no significant change for the two and three rings with temperature increase. However, the relative intensity of the phenol series (1-ring) rises steadily when the heating temperature increases to 550 and 650 °C (see Figure 2b,c), which indicates a better vaporization efficiency of the phenol series at higher temperatures. Besides, O-containing ions (m/z 110, 124, 138, and 146) and methyl-naphthols (m/z 158 and 172) continuously increase as the temperature rises. When the temperature reaches to 650 °C, the amount of larger PAHs over m/z 210 is further reduced, while the content of benzene increases obviously. In addition, it is worth mentioning that a new ion at m/z 64 appears at 550 °C, which probably can be identified as SO2 and/or S2. Since the IE of SO2 (12.3 eV) is above 10.5 eV, m/z 64 in Figure 2b is ascribed to S2. Furthermore, a small signal at m/z 66 from 34S (see Figure 2b) verifies this conclusion. Previous thermogravimetric analysis also displayed a weight loss around 560 °C in coal pyrolysis process and demonstrated its correspondence to the thermal decomposition of pyritic sulfur in coal.35 Figure 2d,a,e presents the mass spectra of evolved products at a fixed temperature of 450 °C and several photon energies, including 9.0, 10.5, and 12.0 eV. The total ion signal is observed to increase with elevated photon energy due to the increasing photoionization efficiencies for most products. Beside the total ion signal, no significant difference in mass spectra can be observed between the evolved products at 9.0 and 10.5 eV, which implies negligible fragmentation at these photon energies. The fragmentation becomes significant at 12.0 eV as a lot of odd-mass fragment peaks appear. This reveals the ability of tunable SVUV-PIMS to achieve both fragment-free and fragment-controllable photoionization. It is specially mentioned that the peak at m/z 91 in Figure 2d (9.0 eV) is assigned to the benzyl radical (IE = 7.2 eV)34 as a product from coal pyrolysis instead of fragment from large aromatic products since fragmentation of aromatics is inhibited at such a low photon energy. Furthermore, comparing panels d and e in Figure 2, a much stronger peak at m/z 91 can be detected with a high photon energy of 12.0 eV. The new increasing abundance of m/z 91 is probably originated from the fragmentation of alkylbenzenes, such as toluene. In addition, the peak at m/z 34, corresponding to H2S, can be detected at 12.0 eV (see the inset in Figure 2e). The weak signal of H2S is most likely caused by its low concentration and low ionization cross section at 12 eV. Figure 2f,c shows the mass spectra of products emitted from HN coal pyrolysis at 650 °C and photon energies of 9.0 and 10.5 eV. Compared to the similar mass spectra at 450 °C (see Figure 2d,a), it seems surprising that peaks, such as m/z 106,

3. RESULTS AND DISCUSSION Compounds evolved from HN and YM coals were identified by measurement of mass spectra at different photon energies and temperatures. The most important products are listed in Table 2. The effects of temperature and photon energy would be illustrated in this experiment. Mass spectra obtained with different temperatures were compared at the fixed photon energy of 10.5 eV, a moderate photon energy that both ionizes most organic compounds and minimizes fragmentation. Previous TG results showed that no significant gasification starts below 450 °C for different kinds of coal.26,33 Our TG results basically confirmed this conclusion. As shown in the Figure 1, the majority of weight loss occurs between 400 and

Figure 1. Thermogravimetric weight loss curves during the pyrolysis of HN and YM coals.

550 °C during the HN and YM coal pyrolysis. A higher temperature may lead to serious thermal fragmentation, which causes the difficulty for discriminating the thermal and ionized fragments. Therefore, the fixed temperature of 450 °C was chosen to compare the mass spectra obtained with different photon energies. Amounts of volatile products with a wide mass range up to m/z 800 were observed, but we only focus on the volatile products smaller than m/z 350 here due to the lower detection efficiency for larger volatile matters. Most peaks are assigned according to the measured molecular weights and previous literature for coal pyrolysis. In addition, main volatile products from both HN and YM coals are listed in Table 2 along with their ionization energies (IEs) taken from the NIST database.34 The structures of some isomers are difficult to be identified by SVUV MS because their IE values are very close to each other. Therefore, IE values of these unidentified species in Table 2 are shown in an energy range referring to all possible isomers. 3.1. Pyrolysis of HN Coal. SVUV photoionization mass spectra of volatile products from HN coal pyrolysis with different photon energies and temperatures are presented in Figure 2. The dominant pyrolytic compounds of HN coal are 696

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Figure 2. Mass spectra of volatile products from HN coal pyrolysis at (a) 450 °C and 10.5 eV, (b) 550 °C and 10.5 eV, (c) 650 °C and 10.5 eV, (d) 450 °C and 9.0 eV, (e) 450 °C and 12.0 eV, and (f) 650 °C and 9.0 eV. Peaks marked with a star indicate that they increase obviously as the temperature rises. The inset in (e) suggests that the compound of H2S can be detected. The partial enlargement (the bottom panel) of (a) shows some detected aromatics and nitrogen-containing components with their molecular structures.

of 10.5 eV would lead to the increase of total ion signal due to the higher ionization efficiency, but there is no overall change for the relative content of each product compared to other ions. However, when the temperature is up to 650 °C, the relative content of these ions has a significant growth as the photon energy rises from 9.0 to 10.5 eV, which indicates that new compounds with the same m/z appear at 650 °C. As shown in Table 2, besides the compounds mentioned above, m/z 106, 108, 110, 117, and 122 at 650 °C probably also include benzaldehyde, benzyl alcohol, 4-hydroxy-benzaldehyde, benzyl

108, 110, 117, and 122, are obviously affected by the photon energy, which indicates that each increasing peak probably corresponds to different components at the temperature of 650 °C. Previous works have characterized these ions by GC/MS, and a consistency was shown, as they presented different components for the same ion as well.22,36 In Figure 2d,a (450 °C), m/z 106, 108, 110, 117, and 122 are identified as pure C2 alkyl benzene, C1 alkyl phenol, benzenethiol (or dihydroxybenzene), indole, and C2 alkyl phenol, respectively. The IEs of all of these ions are lower than 9.0 eV. A higher photon energy 697

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Figure 3. Mass spectra of volatile products from YM coal pyrolysis at (a) 450 °C and 10.5 eV, (b) 550 °C and 10.5 eV, (c) 650 °C and 10.5 eV, (d) 450 °C and 9.0 eV, (e) 450 °C and 13.5 eV, and (f) 650 °C and 9.0 eV. The partial enlargement (the bottom panel) of (a) shows some detected oxygen- and sulfur-containing components with their molecular structures.

generally has similar compound components, with aromatic compounds and their derivatives as the dominant pyrolytic products. The main differences include that the peak at m/z 64 corresponding to S2 is very weak in Figure 3, whereas the peak at m/z 234 is strong. Previous literature assign this peak (m/z 234) to two compounds named benzonaphthothiophene37 and 3,4,5,6-tetramethyl phenanthrene.27,28 However, there is no significant change in the shapes of mass spectra of the pyrolytic products when the temperature increases from 450 to 650 °C. This temperature-insensitive phenomenon illustrates that chemical compositions of those particles are similar. No new

nitrile, and benzoic acid, respectively, which were also observed in some previous GC/MS studies.22 Since the IEs of these new compounds at 650 °C are all between 9.0 and 10.5 eV, they can contribute to the abrupt changes of mass peaks at m/z 106, 108, 110, 117, and 122 as the photon energy rises from 9.0 to 10.5 eV. 3.2. Pyrolysis of YM Coal. Figure 3a−c presents the SVUV photoionization mass spectra of volatile products from YM coal pyrolysis with a fixed photon energy of 10.5 eV and different temperatures of 450, 550, and 650 °C. Compared with the mass spectra of HN coal in Figure 2a−c, the YM coal sample 698

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Figure 4. Relative contents of three aromatics evolved from HN and YM coal pyrolysis with a fixed photon energy of 10.5 eV and different temperatures of 450, 550, and 650 °C.

compounds with the same m/z are produced in the pyrolysis process when the temperature rises. Previous studies showed that only higher temperatures (up to 800 °C) can lead to the decomposition of unstable compounds, such as substituted PAHs, to their skeletal structures.24 Figure 3d,a,e shows the mass spectra of volatile products from YM coal pyrolysis with different photon energies, including 9.0, 10.5, and 13.5 eV and the fixed temperature of 450 °C. No significant difference in mass spectra can be observed between the volatile products at 9.0 and 10.5 eV, even when the temperature rises to 650 °C (see Figure 3f,c). When the photon energy rises to 13.5 eV, many ionization fragments with odd mass appear. Hydrogen elimination from aromatics is the most common fragmentation pathway in the high photon energy of 13.5 eV. For example, m/z 155 and 169 are originated from m/z 156 and 170, respectively. However, a sharper rising peak at m/z 219 in Figure 3e is probably originated from m/z 234 losing methyl radical as the peak at m/ z 234 decreases obviously. Therefore, m/z 234 most likely corresponds to 3,4,5,6-tetramethyl phenanthrene with methyl side chains, not the benzo[b]naphthothiophene. 3.3. Comparison between HN and YM Coal. The partial enlargements (bottom panels) of Figures 2a and 3a show the comparison of mass spectra of volatile products from HN and YM coal pyrolysis at 10.5 eV and 450 °C. The main products

can be roughly classified in four groups: polycyclic aromatic hydrocarbons (including aryl-PAHs and cyclopenta-fused PAHs), nitrogen-containing compounds (e.g., pyrrole, pyrimidine, and methyl-pyrrole), sulfur-containing compounds (e.g., sulfur-containing aromatics, S2 and H2S), and oxygencontaining compounds. The dominant evolved components from the pyrolysis of these two coals are generally similar, but the total ion signals for all volatile matters of YM coal are higher than those of HN coal (see Figures 2 and 3), which is consistent with the proximate analysis (see Table 1). This phenomenon suggests that most of the volatile products from HN and YM coal pyrolysis can be detected by our photoionization MS with the given range of mass (m/z 20−400) and photon energy (9.0−13.5 eV). Besides, among the aromatic compounds identified, the HN coal has a relative lower content of 1-ring aromatics (m/z 108, 110, 122, 124, et al.) and a relative higher content of aromatics with more than 3 rings (m/z 252, 260, 274, et al.) than the YM coal at 450 °C, which points out that HN coal probably has a more condensed aromatic structure than YM coal. In addition, the amount of oxygen-containing aromatics (m/z 94, 110, 124, 138, 146) in the YM coal is higher than that in HN coal (see the enlargements of Figures 2a and 3a); the results suggest that aromatic structures of YM coal are more likely linked by a small quantity of aliphatic chains and some oxygen-containing 699

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Chinese Academy of Sciences. We thank Prof. Minghou Xu of the State Key Laboratory of Coal Combustion in China for his kind help in the part of sample testing.

groups. This difference has a significant effect on the role and fate of different forms of coal in the environment, because the formation of oxygen-containing radicals can lead to a faster decomposition of organics. Sulfur-containing compounds (m/z 162, 198, 212, 248) were also detected in both HN coal and YM coal, and their relative contents of contained sulfur are similar. The aromatic and hydroaromatic units can be connected by thioether bonds (−S−, −S−S−, etc.).12 Figure 4 presents the relative contents of three typical aromatics (alkyl-phenol, alkyl-naphthalene, and alkyl-phenanthrene) evolved from HN and YM coal pyrolysis at different temperatures of 450, 550, and 650 °C. Each mass spectrum is normalized with the signal intensity of the most intense peak. As shown in Figure 4a, the signal ratios of 1-/3-ring and 2-/3ring aromatics continuously increase when the temperature rises. However, the relative intensities of aromatics from YM coal were hardly affected by the temperature, as shown in Figure 4b. Previous literature has confirmed that the majority of coal emissions observed in the temperature range of 450−650 °C is due to thermal cleavage of bonds within the coal structure, but not thermal desorption of compounds from coal.35 Therefore, the fact that relative intensities of aromatics in Figure 4a,b changed differently with increasing temperature is probably caused by the different structures of the macromolecular network between HN and YM coals. The macromolecular structure of HN coal seems to be ruptured more readily than that of YM coal.

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4. CONCLUSIONS This paper reports the online analysis of the volatile matters evolved from HN and YM coal pyrolysis with synchrotron vacuum ultraviolet photoionization mass spectrometry. The mass spectra recorded as functions of photon energy and temperature exhibit that both coals were characterized as highly aromatic. Among the aromatic compounds identified, the HN coal has a relative lower content of 1-ring aromatics and a higher content of over or equal to 4-ring aromatics than the YM coal, further indicating that HN coal probably has a more condensed aromatic structure than YM coal. In addition, the relative contents of pyrolytic products from HN coal are more sensitive than the ones from YM coal at the same temperature, which is most likely caused by different structures of the macromolecular network between HN and YM coals. Mass spectra obtained with different photon energies suggest that some of the peaks include different isomers, and part of these isomers can be online characterized by SVUV-PIMS due to their different ionization energies. However, serious fragmentation derived from the high photon energy suggests that only the soft ionization method is adaptable to analyze the volatile matter from solid pyrolysis.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-551-65141078. Tel: +86551-63602125. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (2013CB834602), the Natural Science Foundation of China (50925623), and the 700

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dx.doi.org/10.1021/ef301670y | Energy Fuels 2013, 27, 694−701