Impacts of Inherent O-Containing Functional Groups on the Surface

Jan 1, 2014 - of the decomposition of O-containing functional groups. ... functional groups and the quantity of surface area without oxygen groups, as...
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Impacts of Inherent O‑Containing Functional Groups on the Surface Properties of Shengli Lignite Yonggang Wang,* Jianlin Zhou, Lei Bai, Yanju Chen, Shu Zhang, and Xiongchao Lin School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, People’s Republic of China ABSTRACT: Shengli (SL) lignite was thermally treated by heating at 200−350 °C in an effort to reduce the number of Ocontaining functional groups and water present. The presence of carboxyl groups, phenolic hydroxyl groups, and methoxy groups was characterized using a chemical titration method. The moisture holding capacity (MHC), wettability, and ζ potential of the SL lignite were measured before and after the low-temperature heat treatment. The results revealed that the main reactions that occurred below 350 °C were decarboxylation and dehydration, corresponding to the decomposition of more than 60% of the carboxyl groups and phenolic hydroxyl groups. SL lignite treated at 350 °C displayed an approximately 50% reduction in its MHC. The O-containing functional groups (the carboxyl groups, in particular) played an important role in water adsorption, indicating that the formation of the hydrogen bond between the O-containing functional groups and water contributed most significantly to the water adsorption process. The contact angle decreased as the number of hydrophilic sites decreased as a result of the decomposition of O-containing functional groups. The ζ potential of the SL lignite decreased significantly as the concentration of O-containing functional groups decreased. MHC was successfully correlated with the presence of O-containing functional groups and the quantity of surface area without oxygen groups, as follows: MHC = 2.655[−COOH] + 2.912[−OH] + 0.209[−OCH3] − 3.321Snon‑O + 1.341, where Snon‑O for the lignite is defined as Snon‑O = Si(1 − Ci/C0), where Si is the surface area of the lignite and Ci and C0 are the total contents of O-containing functional groups in the heat-treatment or as-received lignites.

1. INTRODUCTION The use of abundant lignite deposits has become increasingly important as the minable reserves of high-rank coal in China are rapidly becoming depleted with the development of more advanced emission control technologies.1−3 The high quantities of water and O-containing functional groups present in lignite tend to introduce a variety of problems into the lignite storage and transport processes.4−8 The high moisture content of lignite lowers its energy density. Abundant O-containing functional groups present on the surfaces of lignite coal can facilitate the readsorption of water after drying. Hydrogen may be wasted when it is combined with the O-containing functional groups of lignite during the liquefaction process.9,10 Previous reports11−16 have indicated that the moisture holding capacity (MHC) of coal is important for the rheology and coal concentration in a coal water slurry. The number of O-containing functional groups in lignite is thought to play an important role in the coal−water interactions by providing binding sites for the water molecules.11,17 The ζ potential, which measures the electrical conductivity of a material and may be measured to determine the potential at the boundary of a slipping plane, reflects the polarity of the O-containing functional groups present in a coal sample.15,18 Therefore, it is useful to characterize the factors that affect the MHC and ζ potential in a specific coal sample. Researchers14,19 have found that a high ζ potential reduces the viscosity and improves the dispersion of coal in a water slurry. Schafer17 investigated the water contents of low-rank coals and their relations to the presence of O-containing functional groups. He found that the water concentration was mainly determined by the carboxyl group content. Kaji11 found a fairly good linear relationship between the water concentration and the surface oxygen concentration, multiplied by the specific surface area. The © 2014 American Chemical Society

water adsorption process in a brown coal can be divided into three steps:8 monolayer sorption, multilayer condensation, and capillary condensation. The water in a monolayer is linked to the presence of hydrophilic functional groups, which form hydrogen bonds to the water molecules. Further water sorption occurs through the formation of water clusters at hydrophilic sites on a monolayer, followed by the condensation of water in the capillary structure. Multilayer adsorption and capillary condensation become significant as a coal is soaked in water. The surface of Shengli (SL) lignite, a typical Chinese low-rank coal, is rich in O-containing functional groups. This study was designed to experimentally investigate the process associated with water re-adsorption by SL lignite pretreated with a lowtemperature heat treatment process. The factors that affect the MHC of lignite are discussed. The influences of the O-containing functional groups (carboxyl groups, phenolic hydroxyl groups, and methoxy groups) and their content levels in lignites, both untreated and heat-treated, were investigated quantitatively. The O-containing functional groups of the lignite were correlated with its wettability and ζ potential. Several previous reports have examined the relationship between the presence of O-containing functional groups in coal and the gaseous water adsorption capacity.8,20 The present study is particularly concerned with the adsorption of liquid water.

2. EXPERIMENTAL SECTION 2.1. Material. SL lignite was obtained from the No. 2 field in Xilinhot, Inner Mongolia, China. The sample was pulverized and passed Received: October 5, 2013 Revised: December 31, 2013 Published: January 1, 2014 862

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Table 1. Proximate and Ultimate Analyses of the Samples (wt %)a proximate analysisd SL I II III IV V VI VII VIII

ultimate analysisdaf

V

ash

FC

C

H

N

S

Ob

O/C

42.56 42.56 42.80 42.40 42.56 41.92 40.63 39.19 38.63

13.95 13.88 13.05 12.91 12.72 13.28 13.75 14.01 14.23

43.50 43.58 44.15 44.70 44.72 44.81 45.62 46.81 47.15

73.24 72.96 73.09 72.71 72.98 73.07 72.77 73.75 74.06

4.50 4.62 4.67 4.56 4.58 4.50 4.48 4.37 4.33

1.12 1.09 1.07 1.06 1.07 1.06 1.07 1.10 1.08

0.80 0.80 0.77 0.79 0.81 0.78 0.77 0.74 0.78

20.35 20.53 20.39 20.90 20.55 20.60 20.92 20.05 19.76

0.21 0.21 0.21 0.22 0.21 0.21 0.22 0.20 0.20

d, dry basis; daf, dry and ash-free basis; SL, Shengli raw coal; I, 200 °C (treating temperature) for 30 min (holding time); II, 200 °C for 60 min; III, 250 °C for 30 min; IV, 250 °C for 60 min; V, 300 °C for 30 min; VI, 300 °C for 60 min; VII, 350 °C for 30 min; and VIII, 350 °C for 60 min. bBy difference.

a

through a sieve to obtain particle sizes between 106 and 147 μm. The sample was then dried under vacuum at 40 °C for 12 h and stored in brown glass containers in a freezer. The proximate and ultimate analyses of the sample are summarized in Table 1. 2.2. Experimental Section. Heat treatment was applied in a horizontally placed tube reactor (inner diameter, 550 mm; length, 1200 mm). The effective volume of the reactor was approximately 2.85 L. The central part (approximately 1000 mm) of the reactor was heated electrically. The temperatures were measured using a thermocouple and were maintained at the prescribed value using a proportional−integral− derivative (PID) controller. A schematic diagram of the heat treatment system is shown in Figure 1. Around 9 g of SL coal was placed in a silica

hydroxide solution to determine the amount of acid consumed during the conversion process. The number of phenolic hydroxyl groups present in the lignite was then calculated using the following equation: phenolic hydroxyl groups = total acidic groups − carboxyl groups. The mean value was obtained in triplicate tests, and the error was found to be within 1%. 2.3.2. Methoxy Groups. CH3I, produced by the reaction between the methoxy groups in USP29-NF24 and the added hydroiodic acid, was absorbed in a Br2− acetic acid solution, and IO3− was quantified. Nitrogen gas was bubbled through the solution at a rate of 2 bubbles per second. The solution was heated in an oil bath to 150 °C, and the reaction was continued for 40 min to determine the methoxy group content. The mean value was obtained in triplicate tests, and the error was found to be within 2%. 2.4. Properties of Lignite and Its Heat-Treated Products. 2.4. 1. Physical Structures. The Brunauer−Emmett−Teller (BET) surface areas in the various lignite samples were calculated from the N2 adsorption isotherms, which had been measured at 77 K using a sorptometer (Builder SSA-4300). Prior to conducting this measurement, the samples were dried overnight in an oven heated at 130 °C. The total pore volume (Vp) was calculated from the adsorption data using the Kelvin equation.22,23 2.4.2. MHC Values. About 20 g of lignite particles were fully wetted by soaking the particles in distilled water for 3 h at 30 °C. The particles were filtered and patted dry with filter paper to completely remove water from the external surfaces of the particles. The moisture adsorption equilibrium was reached under a 96% relative humidity in a nitrogen atmosphere with heating at 30 °C over 48 h. The MHC was found to be the weight loss obtained by drying at 110 °C for 3 h under a N2 flow of about 350 mL/min. The experimental results are reported in detail for the GB/T4632-2008 sample. The MHC of each sample was measured 3 times. The mean value is reported in the study. 2.4.3. ζ Potentials. The ζ potentials were measured using a Nano ZS90 ζ potentiometer with heating at 25 °C. The pH was varied over the range of 2−7. The mean values were obtained from triplicate tests, and the error was found to be within 5%. 2.4.4. Contact Angles. The contact angles were measured using a Germany OCA contact angle measurement system with heating at 30 °C. Coal particles were passed through a 100 mesh and pressed at 10 MPa for 10 min to form a pellet, 10 mm in radius and 2 mm in thickness.

Figure 1. Schematic diagram of the heat treatment system: (1) N2 source, (2) electrically heated furnace, (3) quartz tube, (4) quartz boat and coal sample, (5) thermocouple, (6) PID controller, (7) pressure meter, (8) vacuum pump, (9) cooling trap, (10) absorption bottle, (11) gas bag, and (12) valve. boat in the tube. The reactor was then firmly sealed and flushed with a continuous stream of nitrogen (99.999%, 1.5 L/min, 30 min) through the tube. The nitrogen flow rate was set at 200 mL/min, and the reactor was heated at an average heating rate of 10 °C/min. The temperature was maintained for 30 or 60 min after the desired temperature (200, 250, 300, or 350 °C) had been reached. The reactor was then cooled to room temperature using an electric fan. The coal tar produced during the heat treatment process was condensed in a cool trap at a temperature of −10 °C to separate the liquid from the gas products. The properties of these heat-treated coal samples are provided in Table 1. 2.3. Quantification of the O-Containing Functional Groups. 2.3.1. Carboxyl and Phenolic Hydroxyl Groups.21 The carboxyl group content in the coal was determined by exchanging the ions with barium chloride in a triethanolamine−hydrochloric acid buffer (pH 8.3). The total acidity was obtained using barium chloride in barium hydroxide as a buffer. The coal/buffer mixture was filtered and rinsed, and the Ba-form coal was converted into the acid form by adding a given quantity of hydrochloric acid to the solution and then back-titrating using a sodium

3. RESULTS 3.1. Effects of Heat Treatment on the O-Containing Functional Groups and Surface Structures of the Lignites. The O-containing functional group contents were plotted as a function of the heat treatment temperature in panels a and b of Figure 2. The values obtained were 1.65 mmol/g carboxyl groups, 1.12 mmol/g phenolic hydroxyl groups, and 0.20 mmol/ g methoxy groups in the raw coal after drying at 40 °C. The oxygen weight percentages of carboxyl groups, phenolic hydroxyl 863

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3.2. MHC Values of the Original and Heat-Treated Lignites. Figure 3 plots the MHC values obtained from the original lignite samples and the samples heated at different temperatures for 30 or 60 min.

Figure 3. Effects of the temperature on the MHC value for heating times of 30 or 60 min.

The MHC values were observed to decrease as the temperature or heating time increased. The MHC of the original lignite was 16.37%, and this value decreased to 9.25 or 8.64% after heating at 350 °C for 30 or 60 min, respectively. 3.3. Contact Angles and ζ Potentials of the Original and Heat-Treated Lignites. The contact angle of the original lignite and its heat-treated counterpart are plotted as a function of the heat treatment temperature in Figure 4. The contact angle

Figure 2. Effect of the low-temperature heat treatment on the contents of O-containing functional groups of SL lignite with a holding time of (a) 30 min or (b) 60 min.

groups, and methoxy groups in the SL lignite were calculated to be 40.54, 10.40, and 3.39%, respectively. As the temperature was increased, the respective O-containing functional group contents decreased. This trend resembled the trends obtained by holding the samples at each temperature for 30 or 60 min. The carboxyl, phenolic hydroxyl, and methoxy group contents decreased to 0.73, 0.50, and 0.02 mmol/g, respectively, after heating at 350 °C for 30 min and decreased to 0.61, 0.43, and 0.01 mmol/g after heating at 350 °C for 60 min. A distinct decrease in the carboxyl group content was observed upon heating at 200 °C for 30 or 60 min. The rate of loss of phenolic hydroxyl groups was higher than the rate of loss of other O-containing functional groups above 300 °C. Heating the samples at 350 °C for 60 min removed 63.00% of carboxyl groups, 61.60% of phenolic hydroxyl groups, and 95.00% of methoxy groups. The specific surface areas and pore volumes of the raw and heat-treated lignites are summarized in Table 2. As is shown in Table 2, the specific surface areas and pore volumes gradually increased with temperature. It is interesting to note that the specific surface area increased slightly from 0.85 to 17.50 and 17.90 m2/g after heating at 350 °C for 30 or 60 min, respectively.

Figure 4. Effect of the heat treatment temperature on the contact angle measurements for the original and treated lignites.

increased linearly with the heat treatment temperature from 55.6° for the original lignite to 128° for the lignite heated at 350 °C for 60 min. The ζ potential is the difference between the electric potentials of the solution and a stationary fluid layer present on the surface of a particle in the solution. The ζ potential measurements as a function of pH are shown in Figure 5. The absolute value of the ζ potential increased rapidly with the pH over the range of 2.6−5.4 and then increased moderately over the

Table 2. Specific Surface Areas and Pore Volumes of the Lignites before and after Heat Treatmentd samplea

SL

I

II

III

IV

V

VI

VII

VIII

specific surface area (m2/g) pore volume (1000×, cm3/g)

0.85 17.71

2.75 18.62

2.76 18.17

2.63 17.93

2.82 18.39

3.30 19.56

3.00 19.96

17.50 21.63

17.90 21.37

d, dry basis; SL, Shengli raw coal; I, 200 °C (treating temperature) for 30 min (holding time); II, 200 °C for 60 min; III, 250 °C for 30 min; IV, 250 °C for 60 min; V, 300 °C for 30 min; VI, 300 °C for 60 min; VII, 350 °C for 30 min; and VIII, 350 °C for 60 min.

a

864

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Figure 5. ζ potential of the heat-treated lignite as a function of pH, after heating at 350 °C for 30 min.

Figure 7. Effects of the O-containing functional groups on the contact angles of the original and treated lignites.

range of 5.4−7.4. Other heat-treated samples displayed the same trend. The ζ potentials of the original and heat-treated lignites as a function of pH at various heat treatment temperatures, applied for 30 or 60 min, are shown in Figure 6. The absolute values of the ζ potential also increased with the pH.

calculated from the linear fits of the O-containing functional group contents of the heat-treated lignites versus the contact angle. The R2 values were found to be 0.92, 0.98, 0.78, and 0.80 for the total number of acidic O-containing functional groups, carboxyl groups, phenolic hydroxyl groups, and methoxy groups, respectively. During the heat treatment process, the surface properties of the SL lignite changed from hydrophilic to hydrophobic as a result of the decomposition of hydrophilic O-containing functional groups from the coal surface. Figure 7 shows that the contact angles of the heat-treated samples increased linearly as the O-containing functional group content decreased. This trend was mainly attributed to the reduction in the number of acidic O-containing functional groups. The carboxyl group content (about 1.7 mmol/g) in the original coal sample deviated from the trendline because the original coal sample contained approximately 10% water, which formed hydrogen bonds with the surface O-containing functional groups, thus affecting the contact angle. The dominant factor affecting the contact angles of the heat-treated samples was the O-containing functional group content, which decreased as the moisture content decreased. The relationship between the phenolic hydroxyl group content and the contact angle was quite similar to that between the carboxyl group content and the contact angle. The O-containing functional groups on the surfaces of the coal particles were negatively charged and attracted cations present in the solution, leading to the formation of an electric double layer in the fluid.18 The number of O-containing functional groups on the coal particle surfaces may potentially affect the ζ potential value. The absolute value of the ζ potential increased with the pH for a fixed O-containing functional group content. This trend was similar to the results obtained by Yu and co-workers.15 The concentration of H+ ions present on the surfaces of the coal particles (the Stern plane) was determined by the number of Ocontaining functional groups present on the coal particle surface; hence, the ζ potential corresponded to the difference between the concentrations of H+ ions in the slipping plane and the solution. The absolute value of the ζ potential increased as the H+ concentration decreased or the pH increased. At low pH values, the ionization of the polar O-containing functional groups was suppressed and the absolute value of the ζ potential decreased as the pH value decreased. At a given solution concentration of H+, i.e., at a fixed pH, the absolute value of the ζ potential increased as the O-containing functional group content increased. A larger number of O-containing functional groups adsorbed a higher number of H+ ions on the coal particle surfaces (in the Stern plane). The absolute values of the ζ potentials were, therefore,

Figure 6. ζ potential of original and heat-treated lignites as a function of pH at variable heat treatment temperatures in 30 or 60 min [SL, Shengli raw coal; I, 200 °C (treating temperature) for 30 min (residence time); II, 200 °C for 60 min; VII, 350 °C for 30 min; and VIII, 350 °C for 60 min].

4. DISCUSSION Heat treatment changed the amounts of O-containing functional groups in the lignites and the surface structures of the lignite. As a result, heat treatment significantly affected the MHC value, the contact angle, and the ζ potential of a lignite sample. The number of O-containing functional groups present decreased as the temperature increased, but rates at which this reduction took place differed depending upon whether the heat treatment occurred above or below 300 °C. This result was probably because of the fact that the phenolic hydroxyl groups were more stable than the carboxyl groups or the methoxy groups below 300 °C, and the carboxyl groups were more likely to decompose at low temperatures. These results agree with those reported by Schafer.24 The decomposition of O-containing functional groups mainly produced CO2 or H2O during the low-temperature treatment process.25,26 A reduction in the number of polar groups present decreased the hydrophilicity of the heat-treated coal particle surface. On the other hand, the specific surface areas of the heat-treated samples changed slightly below 300 °C. Figure 7 shows a linear plot of the contact angle as a function of the O-containing functional groups. The coefficients of determination (R2) were 865

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determined by the number of polar groups present, including carboxyl groups, phenolic hydroxyl groups, and methoxy groups. After heat treatment, the absolute values of the ζ potential of the coal samples decreased, corresponding to the decomposition of O-containing functional groups. During soaking in water, the lignite pores filled and surfaces became coated with water. The surface area and pore volume, therefore, may potentially play important roles in determining the MHC value. The relationship between the MHC and the pore volume is shown in Figure 8a, and the relationship between

Figure 9. MHC as a function of the O-containing functional group content.

functional groups) have a high affinity to water because of the formation of various hydrogen-bonding interactions. Water associates with O-containing functional groups, which form hydrophilic sites. Carboxyl groups and phenolic hydroxyl groups can interact with water molecules in two ways: the oxygen atoms in these groups can form a hydrogen bond to a water molecule, and the carboxyl or phenolic hydroxyl groups can form a hydrogen bond to an oxygen atom in a water molecule. On the other hand, none of the methoxy group hydrogen atoms is capable of forming a hydrogen bond. Therefore, methoxy can only interact with water by forming a hydrogen bond between the oxygen atom in the methoxy group and the hydrogen atom in the water molecule. Hence, the carboxyl and phenolic hydroxyl groups had two hydrophilic sites, whereas the methoxy group had only one hydrophilic site. A multiple regression analysis of the O-containing functional group content versus the MHC values was performed as follows: Figure 8. Effects of the surface area and pore size on the MHC (a, surface area; b, pore size).

MHC = 3.127[−COOH] + 1.308[−OH] + 0.234[−OCH3] + 3.437

the MHC and the specific surface area is presented in Figure 8b. The R2 values obtained from the two plots were 0.14 and 0.35. The low-temperature heat treatment process did not significantly alter the surface area and pore volume. These results indicate that the physicochemical properties of the coal may play an important role in determining the MHC. The presence of O-containing functional groups on the surface of the lignite must contribute significantly to the MHC value. The O-containing functional groups on the lignite surfaces played a dominant role in the sorption of water vapor, i.e., monolayer sorption, as reported previously.8 Soaking the lignite in water resulted in multilayer condensation and capillary condensation of water. Figure 9 shows the plots of the MHC as a function of the quantity of each type of O-containing functional group, as determined using a linear fitting method. The total O-containing functional group content decreased, yielding a proportional decrease in the equilibrium moisture content of the SL lignite. These results agreed with the hypothesis that a reduction in the number of polar groups should decrease the MHC of a coal sample.11,15 Several reports have described the correlation between the number of O-containing functional groups and the adsorption of stream11,27 to the lignite surface. In those studies, less than 20 wt %/weight of water was adsorbed by the sample at a relative humidity in excess of 96%.27 The MHC is expected to be related to the number of polar groups present on the coal surface because the polar groups (O-containing

(1)

where [−COOH], [−OH], and [−OCH3] represent the number of hydrophilic sites present in the carboxyl groups, phenolic hydroxyl groups, and methoxy groups, respectively. The coefficient and eq 1 constant were calculated using a multiple regression analysis with a R2 of 0.965. The value of eq 1 indicated that the amount of water associated with the carboxyl groups was 1.5 times the amount of water associated with the phenolic hydroxyl groups as reported elsewhere.17 The carboxyl groups played the most important role in determining the MHC value. The soaking process resulted in strong interactions between the water molecules and the O-containing functional groups. A total of 60% of the carboxyl and phenolic groups decomposed upon heat treatment at 350 °C, and the methoxy groups were almost completely lost. The resulting MHC was 50% of the value measured for the as-received lignite. Several factors may contribute to this effect. The loss of O-containing functional groups generated a nonpolar hydrophobic carbon surface that inhibited water adsorption. The surface area of the lignite sample devoid of O-containing functional groups could be described according to Snon‐O = Si(1 − Ci /C0)

(2)

where i represents I, II, ..., VIII [I, 200 °C (treating temperature) for 30 min (holding time); II, 200 °C for 60 min; III, 250 °C for 30 min; IV, 250 °C for 60 min; V, 300 °C for 30 min; VI, 300 °C 866

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for 60 min; VII, 350 °C for 30 min; and VIII, 350 °C for 60 min], Snon‑O and Si are the surface areas devoid of oxygen-containing functional groups, either after or before heat treatment, respectively, and Ci and C0 are the O-containing functional group contents of the heat-treated and as-received lignite samples, respectively. The surface area devoid of O-containing functional groups was approximately zero in the as-received lignite. An increase in the surface area and a decrease in the number of O-containing functional groups increased the nonoxygen-containing surface area of the sample. The relationship between the MHC, O-containing functional group content, and Snon‑O was fit using multiple regression methods and was found to be

ACKNOWLEDGMENTS The authors gratefully acknowledge funding support from the Natural Science Foundation of China (21076222). The authors are also grateful to Prof. Isao Mochida of Kyushu University for his invaluable comments and suggestions.

■

(3)

The contribution of the surface lacking oxygen-containing functional groups to the MHC is expressed in eq 3. The calculated R2 of 0.977 indicated that eq 3 provided a better fit than eq 1. Taken together, eqs 2 and 3 indicated that the presence of O-containing functional groups reduced the MHC value. Snon‑O significantly affected the MHC of a material. The MHC decreased as Snon‑O increased and as the O-containing functional group content increased. Equation 3 indicated that a reduction in the O-containing functional group content reduced the number of hydrogen bonds between H2O molecules and the hydrophilic sites. As a result, Snon‑O inhibited water adsorption. The significant contributions of the carboxyl groups, phenolic hydroxyl groups, methoxy groups, and surface area lacking oxygen-containing functional groups, as described in eq 3, accurately modeled the MHC effects. It should be noted that the contributions of the pore size distribution or condensation in micropores were not analyzed. The contribution of micropore condensation will be analyzed in future work.

5. CONCLUSION The percentages of carboxyl groups, phenolic hydroxyl groups, and methoxy groups removed upon heating at 350 °C for 60 min were 63.00, 61.60, and 95.00% of the corresponding amounts present in the as-received lignite sample, respectively. The water content decreased as the concentration of O-containing functional groups decreased. The correlation coefficients between the water content and each O-containing functional group were 0.94 for the total content of acidic O-containing functional groups, 0.93 for the carboxyl group content, 0.67 for the phenolic hydroxyl group content, and 0.90 for the methoxy group content. The contact angles of samples exceeded 90° after heat treatment at 300 °C. The contact angle increased as the Ocontaining functional group content decreased, whereas the ζ potential followed the opposite trend.

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MHC = 2.655[−COOH] + 2.912[−OH] + 0.209[−OCH3] − 3.321Snon‐O + 1.341

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The authors declare no competing financial interest. 867

dx.doi.org/10.1021/ef402004j | Energy Fuels 2014, 28, 862−867