Macropore-Stabilized Limestone Sorbents Prepared by the

Mar 16, 2016 - A novel cost-effective method was applied to modified calcium-based sorbents for cyclic high-temperature CO2 capture. The sorbents were...
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Macropore-Stabilized Limestone Sorbents Prepared by the Simultaneous Hydration−Impregnation Method for High-Temperature CO2 Capture Yongqing Xu, Cong Luo,* Ying Zheng, Haoran Ding, and Liqi Zhang* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: A novel cost-effective method was applied to modified calcium-based sorbents for cyclic high-temperature CO2 capture. The sorbents were derived from cheap limestone and sea salt, and the main processes of the preparation involved two simple steps: hydration of CaO and impregnation with sea salt in CaO. Results indicated that the simultaneous hydration− impregnation (SHI) method contributed to the formation of highly improved calcium-based sorbents during cyclic calcination/ carbonation reactions. After 40 cycles, the SHI limestone doped with 3.0 wt % sea salt achieved a CO2 capture capacity of 0.31 g of CO2/g of sorbent, which was 126% higher than that of natural limestone. Moreover, the SHI limestone sorbent contained numerous macropores after several cycles. Further investigation on microstructure changes of the sorbents showed that the macropores were relatively stable during cyclic reactions, which can be attributed to the stable behavior of the sorbent. In contrast, the natural limestone lost its micro- and macropores during initial reactions, thereby rapidly losing its sorption capacity during calcium looping cycles. Furthermore, the SHI limestone sorbents showed slightly better mechanical strength than the natural limestone sorbent demonstrated by attrition tests.

1. INTRODUCTION Carbon capture and storage (CCS) is a series of technologies developed to mitigate climate change by isolating CO2 during fuel combustion.1 Calcium looping is generally accepted as a promising method to remove CO2 from exhaust gases, such as those from a power plant or cement industry. Indeed, CaO readily obtained through the calcination of natural limestone has been proposed as a well-suitable CO2 sorbent that can substantially reduce the costs of the CO2 capture process.1,2 However, the CO2 capture capacity of CaO derived from natural sources generally undergoes a rapid decrease during long-term calcination/carbonation;3,4 this can be ascribed to the sintering of the sorbents.5,6 To develop a sorbent suitable for the calcium looping process, substantial methods were proposed, including flame spray pyrolysis (FSP),7 precipitated calcium precursors (PCCs),8 sol−gel process,9 support with an inert material method,10 calcination of organic calcium precursors,11 preheat treatment,12 self-assembly template synthesis (SATS),13 etc.14−20 However, the cost of these methods is usually expensive. Other inexpensive methods, such as doping with sodium,21,22 were also investigated to prepare calcium-based sorbents. However, their cyclic CO2 capture performance was similar to that of natural sorbents, and such performance failed to achieve an ideal improvement for the sorbents. In this study, a novel simultaneous hydration−impregnation (SHI) process was proposed to prepare calcium-based sorbents for cyclic CO2 capture. This SHI process was developed on the basis of the doping with sodium. However, unlike the previous report, the new SHI limestone doped with sodium showed a much higher CO2 capture behavior than the natural limestone sorbent. The major difference between the previous doping method and the current SHI method was the doped matrix. © XXXX American Chemical Society

The doped matrixes of the previous and current methods were CaCO3 and CaO, respectively. In fact, CaO is a kind of porous medium more suitable for impregnation. Unfortunately, there is little research on CaO-based doping. Therefore, it is significant to explore further research on the CaO-based doping. Hence, this research identified various performances and characteristics by investigating SHI sorbents, traditional impregnation (TI) sorbents, and limestone sorbents during the longterm calcium looping process. Notably, the SHI sorbents were cost-efficient and environmentally friendly for the calcium looping process.

2. EXPERIMENTAL SECTION 2.1. Materials and Sorbent Preparation. The natural sea salt was from Binzhou, China, and the natural limestone was from Wuhan, China. The chemical component of the sea salt and limestone were analyzed by a X-ray fluorescence (XRF) spectrometer, as shown in Table 1. The major ingredients of the limestone and sea salt are CaCO3 and NaCl, respectively. The limestone was ground and sieved to the particles with the size range of 0.2−0.3 mm before experiments. To remove the crystal water, the crude sea salt was ground into powders and then dried at 110 °C overnight. The SHI method was prepared as follows: 10 g of natural limestone particle was calcined at 850 °C, then it was placed in 50 mL of seawater that contained a certain mass of sea salt (the mass ratio of sea salt/limestone precursor was 0−4 wt %). The mixture was stirred for 45 min at 80 °C and then dried at 100 °C overnight. Finally, the sample was ground and sieved to particles with the size range of 0.2−0.3 mm. After SHI preparation, the main contents of the sample were Ca(OH)2 and CaCO3, as shown in Figure 1. Received: November 4, 2015 Revised: March 4, 2016

A

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Energy & Fuels Table 1. Chemical Element (wt %) Analysis of the Samples element (wt %)

Na

Mg

Al

Si

S

Cl

K

Ca

Fe

others

limestone sea salt

1.60 32.51

1.59 1.65

0.19 0.66

0.20 0.13

0.17 0.75

0.99 63.99

0.27 0.14

93.93 0.14

0.17 0.02

0.89 0.11

Figure 3. Schematic of the friability tester. dried at 100 °C overnight. Finally, the sample was sieved to particles with the size range of 0.2−0.3 mm. The prepared samples are listed in Table 2. The major difference between the SHI and TI methods was that the precursor of the sorbent derived from the TI method was CaCO3, whereas the precursor of the SHI sorbent was CaO. In the SHI method, CaO was porous, and when it was hydrated with seawater, sea salt can be impregnated uniformly among the sorbent. 2.2. Sorbent Testing. The cyclic calcination/carbonation performances of calcium-based sorbents were investigated in a fixedbed reactor system (Figure 2). Carbonation and calcination proceeded in pure CO2 and N2, respectively, with a flow rate of 1 L/min at atmospheric pressure. The duration of calcination and carbonation was 15 min each. The calcination/carbonation reactions proceeded at a constant high temperature of 850 °C, which was in accordance with previous literature.23−25 The variation in the mass of the sample during the calcination/ carbonation cycles was measured by a delicate electronic balance, and the CO2 capture capacity of the sorbent was calculated as follows:

Figure 1. XRD patterns of the SHI sorbent after hydration.

Table 2. Definition of the Samples Prepared by SHI and TI Methods definition

calcium material

TI (0.125) TI (0.25) TI (0.5) TI (1.0) SHI (0.0) SHI (0.125) SHI (0.25) SHI (0.5) SHI (1.0) SHI (2.0) SHI (3.0) SHI (4.0)

limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone limestone

doping material sea sea sea sea

salt salt salt salt

sea sea sea sea sea sea sea

salt salt salt salt salt salt salt

preparation method

sea salt ratio (wt %)

TI TI TI TI SHI SHI SHI SHI SHI SHI SHI SHI

0.125 0.25 0.5 1.0 0.0 0.125 0.25 0.5 1.0 2.0 3.0 4.0

CN = (mN − m0)/m0

(1)

where CN is the CO2 sorption volume of the sorbent during the Nth cycle, mN is the mass of the sorbent after the Nth carbonation, and m0 is the mass of the sorbent after initial calcination. The crystal structure parameters for calcined samples were determined by an X-ray diffraction (XRD, PANalytical B.V.) analyzer with Cu Kα radiation with the 2θ range of 20−70° at a scanning speed of 0.05°/s. The microstructures of the different calcined sorbents were analyzed with field-emission scanning electron microscopy

To compare the different characteristics of the new method and traditional impregnation (TI) method, limestone was also modified by the TI method as follows: 10 g of limestone (ground and sieved to 0.2−0.3 mm) was placed in 50 mL of seawater that contained a certain mass of sea salt (the mass ratio of sea salt/limestone precursor was 0−1 wt %). The mixture was stirred for 90 min at 80 °C and then

Figure 2. Schematic of the fixed-bed reactor system. B

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Energy & Fuels (SEM, Nova NanoSEM 450) with 10 kV of accelerating voltage. The surface area and pore size distribution of the calcined samples were analyzed with the ASAP 2020 accelerated surface area and porosimetry system. The attrition resistance analysis of the sorbents was examined by a friability tester, as shown in Figure 3. The mass percentage of sorbent particles reduced to less than a certain size in a specified time within a specified test apparatus may be used as an index representing the attrition propensity of the sorbents.26−29 Here, 6.5 g of sorbent sieved within 0.2−0.3 mm was rotated in a rotatable drum (inner diameter of about 286 mm) of the friability tester at 60 revolutions/min for 1800 revolutions. The sample was then sieved again into the particle sizes of 0.2−0.3 mm, and its weight was measured by a precise balance. The mass loss ratios were calculated as follows:

R = (ma − mb)/ma

(2)

where R is the mass loss ratio, ma is the mass of the initial sorbent (6.5 g), and mb is the mass of sorbent within the particle sizes of 0.2−0.3 mm, which remained after 1800 revolutions.

3. RESULTS AND DISCUSSION 3.1. Cyclic CO2 Capture Behavior of Modified Limestone Sorbents. The cyclic CO2 capture behavior of natural limestone sorbents and the sorbents derived from the TI method was shown in Figure 4a. The CO2 capture capacities of natural, TI (0.125), and TI (0.25) limestone showed similar trends, and they all underwent a rapid decrease during cyclic calcination/carbonation reactions. The CO2 capture capacities of TI (0.5) and TI (1.0) were similar, and they showed relatively stable CO2 capture behavior. However, their CO2 capture capacities were relatively low in the first few cycles. The results showed that the sea salt doping process can improve the cyclic CO2 capture behavior of the limestone sorbent. However, this improvement was quite limited. This finding was in agreement with the previous literature.21,22 Actually, the microstructure of limestone is cubic, and its pore structure is fairly limited in comparison to its calcined lime. Thus, the impregnated sodium failed to be well-dispersed in the sorbent and showed little improvement for the sorbent. To achieve further improvement by salt doping, a SHI method was developed to dope sodium salt in porous lime, instead of limestone. The CO2 capture behavior of SHI-made sorbents during 20 cycles is shown in Figure 4b. Both the SHI (0.0) and natural limestone sorbents underwent a rapid decay during the repeated calcination/carbonation process. Notably, the SHI (0.0) sorbent experienced only hydration during preparation, and no salt doping existed in the sorbent; thus, the enhancement was quite limited. However, when the sorbents underwent the SHI process with some sea salt doped, they showed much higher CO2 capture capacity and more stable reactivation during calcium looping cycles. The capture capacity and cyclic stability of the SHI-made sorbents increased with the increase of the doped sea salt ratio from 0 to 2 wt %. In particular, the capture capacity of SHI (2.0) sorbent showed almost no decrease within 3−20 cycles, and its CO2 capture capacity achieved as high as 0.368 g of CO2/g of sorbent after 20 cycles, which was 119% higher than that of the natural limestone sorbent. However, when the doped ratio of sea salt was over 2 wt %, an interesting phenomenon occurred. The CO2 capture capacities of SHI (3.0) and SHI (4.0) sorbents suffered a rapid decrease during the initial several cycles and then gradually rose to a relatively high value. The possible reason is that the main chemical component of sea salt is NaCl, which

Figure 4. Cyclic CO2 capture behavior of different sorbents during 20 cycles: (a) TI sorbents, (b) SHI sorbents, and (c) comparison of the sorbents in the 20th cycle.

can accelerate sintering during the gas−solid reaction.22,30 Thus, the sorbents rapidly lost their capacity during the first few cycles. However, the sorbents can form a stable microstructure after the initial severe sintering, which showed numerous stable macropores in the sorbents (as discussed later). The stable microstructure resulted in the stable CO2 capture performance of the sorbents. This phenomenon was similar to the CO2 capture behavior of the thermally pretreated sorbents, which suffered from severe sintering during the initial extremely hightemperature calcination.31 Finally, SHI (3.0), SHI (4.0), and thermal pretreated sorbents showed an increment in CO2 capture capacity after the initial few cycles. To compare the CO2 capture performance of TI- and SHImade sorbents, Figure 4c shows the CO2 capture capacities of those sorbents doped with different sea salt ratios after 20 cyclic reactions. In summary, the SHI-made sorbents gained much C

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Further investigation on the long-term cyclic CO2 capture performance of SHI-made sorbents is shown in Figure 5. The SHI (1.0), SHI (2.0), SHI (3.0), and SHI (4.0) sorbents all showed stable CO2 capture performance within 20−40 cycles. In particular, the SHI (3.0) and SHI (4.0) sorbents displayed nearly horizontal lines after 30 cycles. The results indicated that the more sea salt doped, the more stability of the sorbent was achieved. However, an excessive amount of sea salts doped in limestone sorbent resulted in a low capacity in the initial cycles. As shown in Figure 4b, the CO2 capture capacity of SHI (4.0) sorbent was even lower than that of the natural sorbent within 1−10 cycles. Considering both initial capture capacity and cyclic stability, the most reasonable doped ratio of sea salt into limestone should be 2−3 wt % according to the SHI method. 3.2. Characteristics of the Sorbents during Long-Term Cyclic Reactions. The XRD patterns of the SHI-made sorbent and natural limestone sorbent after calcination of different cycles are shown in Figure 6. Evidently, the XRD patterns of the sorbents showed similar characteristic peaks of CaO. The highest peak intensities of the SHI (3.0) sorbent were detected after the initial calcination, suggesting the high degree of crystallinity of the SHI-CaO sorbent. However, as the number of cycles increased, the peak intensity of the sorbents gradually decreased and the location of the major peak suffered from slight deviation. This finding may be attributed to the sintering of a certain lattice face. The average CaO grain sizes were calculated by the Scherrer equation as follows:

better CO2 capture behavior than the TI-made sorbents. Moreover, the optimal mass ratios of doped sea salt were 2.0 and 0.25 wt % for SHI- and TI-made sorbents, respectively. The reason should be ascribed to the different doped matrices of SHI and TI methods. The doped matrix of the TI-made sorbents was CaCO3, which contained few pores, whereas the doped matrix of the SHI-made sorbents was porous CaO. Sodium cations can disperse well in the porous surface of the SHI sorbents.

Figure 5. Cyclic CO2 capture behavior of SHI-made sorbents during 40 cycles.

L = kλ /(β cos θ )

(3)

Figure 6. XRD patterns of calcined natural and SHI-made sorbents.

Table 3. Crystallite Size and Lattice Distortion of the Sorbents by XRD Analysis Dhkl (nm) sample

200

220

111

D̅ (nm)

lattice distortion (%)

limestone-CaO, 0 cycles limestone-CaO, 20 cycles limestone-CaO, 40 cycles SHI (3 wt %)-CaO, 0 cycles SHI (3 wt %)-CaO, 20 cycles SHI (3 wt %)-CaO, 40 cycles

49.3 49.3 78.4 49.3 78.4 49.3

38.1 83.2 52.4 83.3 52.4 83.3

48.6 77.3 77.3 77.3 77.3 77.3

45.3 69.9 69.4 70.0 69.4 70.0

0.311 0.239 0.229 0.238 0.229 0.237

D

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Energy & Fuels where L is the grain size of CaO after different cycles, β is the main peak breadth in XRD spectra, λ is the wavelength of the X-ray, θ is the Bragg angle, and k is the Scherrer constant. The results are displayed in Table 3. The average grain size of the limestone sorbent significantly increased from 0 to 20 cycles and then remained unchanged from 20 to 40 cycles. The large increase in grain size usually indicates that the sorbent experienced severe sintering of the grains in the sorbent. In contrast, the average grain size of the SHI sorbent remained constant within 0−40 cycles. The results indicated that the hydration−impregnation process stabilized and protected the

grain morphology in the sorbent, thereby potentially increasing the cyclic reaction stability of the sorbent. However, the grain size changes cannot explain why the SHI sorbent showed a much higher CO2 capture capacity than the natural sorbent. In particular, the grain sizes of SHI and natural sorbents were equal after 40 cycles, but the CO2 capture capacity of the SHI sorbent was over twice as high as that of the natural sorbent. The XRD analysis failed to show the pore structure characteristics of the sorbent. The CaO−CO2 gas−solid reaction usually contained two stages: chemical-reaction-controlled stage and product-diffusion-controlled stage. The pore microstructure of CaO was quite significant for both stages. A further investigation was conducted on the comparison of the pore changes of the natural and SHI limestone sorbents during the cyclic reactions via N2 adsorption−desorption and SEM analysis. The Brunauer−Emmett−Teller (BET) surface area changes of SHI (3.0) and natural sorbents are listed in Table 4. Interestingly, the surface area of the two sorbents exhibited opposite changes from 0 to 40 cycles. The BET surface area of natural limestone after initial calcination was the largest among all of the samples. However, as the number of cycles increased,

Table 4. BET Surface Area of Calcined Natural Limestone and SHI (3.0) after Various Cycles sample

BET surface area (m2/g)

limestone after 0 cycles limestone after 20 cycles limestone after 40 cycles SHI (3.0) after 0 cycles SHI (3.0) after 20 cycles SHI (3.0) after 40 cycles

22.2 4.9 2.2 3.1 5.8 8.3

Figure 7. BJH analysis of pore size distributions of calcined sorbents: (a) natural limestone and (b) SHI (3.0) sorbent.

Figure 8. Isotherm adsorption/desorption behavior of natural and SHI (3.0) sorbents. E

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Figure 9. SEM images of calcined sorbents after different cycles: (a and b) limestone after 0 cycle, (c) limestone after 20 cycles, (d) limestone after 40 cycles, (e) SHI (3.0) after 0 cycle, (f) SHI (3.0) after 2 cycles, (g) SHI (3.0) after 20 cycles, and (h) SHI (3.0) after 40 cycles.

the BET surface area declined rapidly and the surface area of natural limestone was about one-tenth of its initial status. On the contrary, the surface area of the SHI (3.0) sorbent increased as the cycle numbers increased. Figure 7 presents the pore size distribution of natural and SHI (3.0) sorbents from 0 to 40 cycles. The Barrett−Joyner− Halenda (BJH) analysis is based on mesopores, and the pores from 30 to 40 nm are the major pores for the natural and

SHI-made sorbents. As the cycles increased, the peak of limestone-CaO rapidly declined, as shown in Figure 7a; in contrast, SHI-CaO attained the highest peak after 40 cycles, as shown in Figure 7b. Figure 8 shows the nitrogen adsorption− desorption isotherms profile of natural and SHI (3.0) sorbent from different cycles. The nitrogen adsorption performance of SHI (3.0) sorbent increased with the increase of the cycle number. The nitrogen adsorption performance of the SHI (3.0) F

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Energy & Fuels sorbent was a little lower than that of limestone from 0 to 20 cycles, but the nitrogen adsorption performance of the SHI (3.0) sorbent was a little larger than that of limestone after 40 cycles. It should be noted that the BJH analysis is only studying pores in the mesopore range.21 Further investigation via SEM analysis showed visual changes of macropores in the sorbents. The morphology changes of the calcined natural limestone and SHI (3.0) sorbent after different cycles are shown in Figure 9. The calcined limestone after initial calcination was cubic, and the surface appeared compact and smooth, as shown in Figure 9a; however, when the magnification times (100000×) are enlarged, as shown in Figure 9b, the surface showed numerous linked pores and contained

amounts of well-developed micropores, which supported high CO2 capture capacity during the initial cycles. The surface became much smoother when the number of cycles increased, as shown in panels c and d of Figure 9, and severe sintering may occur. The CO2 molecule became much more difficult to react with the core CaO inside the grain. The surface of the SHI-CaO after 0 cycle is porous and fluffy, similar to “popcorn”, as shown in Figure 9e, however, after 2 cycles of reactions, the macropores on the surface of the SHI sorbent were evident in a well-interlinked macropore frame structure, which was gradually formed, as shown in Figure 9f. As the number of cycles increased, the size of the macropores in the SHI-CaO became larger, similar to the SHI-CaO after 20 cycles, shown in Figure 9g. However, the macropores seemed to be comparatively stable in the later further cycles, as shown in Figure 9h, and the pore structure of the SHI-CaO demonstrated almost no changes within 20−40 cycles. The surface of the SHI (3.0) sorbent after 40 cycles almost showed the same porous structures, which indicated the cyclic stability of the CO2 capture behavior during high-temperature reactions. The well-interlinked macropores should be good scaffolds that inhibited agglomeration of the CaO sorbents. Therefore, this finding can well explain the reason why the SHI limestone sorbent maintained significantly higher CO2 capture capacity during the multiple cycles. 3.3. Attrition Resistance Analysis of the Different Sorbents. The comparison of friability of the natural and SHI-made sorbents is shown in Figure 10. The mass loss ratio of the SHI-made sorbents is slightly less than that of the natural limestone after the same abrasion tests. Moreover, with the increase of sea salt ratios, the abrasion of the SHI-made sorbents became alleviative. This finding was in accordance with the previous report31 that the sodium salt (Na2CO3) as a binder can increase the strength of pelletized particles. The current results showed that the SHI process with sea salt doping was beneficial to the attrition performance of the limestone sorbent. 3.4. Economic Analysis of the SHI Method. To develop a sorbent suitable for the calcium looping process, various modification methods were proposed. A preliminary economic analysis of different methods was investigated by comparing the CO2 capture performance, raw material cost, and preparation process of the sorbents. The cyclic CO2 capture capacities of SHI-made and other modified calcium-based sorbents in the literature are shown in Figure 11. All of these sorbents achieved higher CO2 capture behavior than limestone. CO2 capture capacity of the SHI-made sorbent was a little higher than that of the precipitated calcium carbonated (PCC)-made sorbent, but it was a little lower than that of the wet-chemistry-, sol−gel-, and flame-spray-made sorbents. The raw material costs and CO2 capture capacities (after 20 cycles) of each modified calcium-based sorbents are listed in Table 5. The raw material cost of the SHI-made sorbent is

Figure 10. Mass loss ratio of the natural and SHI-made sorbents during the attrition test.

Figure 11. CO2 capture behavior during multiple cycles of the sorbents derived from different preparation methods.

Table 5. Raw Material Costs of Different Modified CaO-Based Sorbents method

raw material

number of cycles

CO2 capture capacity (g of CO2/g of sorbent)

cost (USD/ton of CaO sorbent)

wet chemistry10 sol−gel32 PCC33 flame spray7 SHI limestone

CaO, Al(NO3)3, and 2-propanol Ca(NO3)3 and citric acid Ca(OH)2 and Al(NO3) 3 Ca−naphthenate and xylene lime and sea salt limestone (powder)

13 20 30 20 40 20

0.45 0.37 0.23 0.46 0.31 0.09

∼4600 ∼2800 ∼400 ∼12000 ∼80 ∼62

G

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about one-fiftieth of that of the sol−gel- and wet-chemistrymade sorbents and about one-hundredth of that of the flamespray-made sorbent. The costs were calculated on the basis of the raw material price from Alibaba (which is the largest online trading platform in China), as shown in Table 6.

price (USD/ton)

Al(NO3)3·9H2O Ca(NO3)3·4H2O waste acetic acid citric acid monohydrate 2-propanol Ca−naphthenate xylene Ca(OH)2 lime powder limestone powder sea salt

∼490 ∼250 ∼300 ∼470 ∼780 ∼1500 ∼780 ∼90 ∼80 ∼35 ∼15

The main preparation of the SHI method is similar to that of the wet chemistry method, mainly including two simple processes of dissolution and calcination. In comparison, PCC, sol−gel, and flame spray methods are more complex than the SHI method, because the PCC method contains an additional filtration process and the sol−gel method needs a long time to form dry gel. The flame spray method applies the combustion of CH4, O2, and organic calcium. Thus, the manufacture cost of the SHI method is still competitive compared to the other methods. In summary, SHI is a promising method for large-scale sorbent preparation, which is the most cost-effective candidate among all of the modified sorbents.

4. CONCLUSION The results showed that the SHI process is an excellent method for the manufacture of calcium-based sorbents. The SHI-made sorbent showed high and stable reactivity during long-term CO2 capture cycles and good attrition resistance properties. The optimal doped ratio of sea salt in the SHI-made limestone was 2−3 wt %. The SHI limestone sorbent contained numerous macropores after several reactions at high temperatures. The macropores were relatively stable during cyclic reactions, which can be attributed to the stable behavior of the sorbent. Furthermore, the SHI-made sorbent is an environmentally friendly and cost-effective sorbent.

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Table 6. Price of Various Raw Materials sample (industrial grade)

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-27-87542417-409. Fax: +86-27-87545526. E-mail: [email protected]. *Telephone: +86-27-87542417-312. Fax: +86-27-87545526. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (51276078) and the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for XRD and SEM measurements. H

DOI: 10.1021/acs.energyfuels.5b02603 Energy Fuels XXXX, XXX, XXX−XXX