Gasification of Sugarcane Bagasse over Supported Ruthenium

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Gasification of Sugarcane Bagasse over Supported Ruthenium Catalysts in Supercritical Water Mitsumasa Osada,† Aritomo Yamaguchi,‡ Norihito Hiyoshi,‡ Osamu Sato,‡ and Masayuki Shirai*,‡ †

Department of Chemical Engineering, Ichinoseki National College of Technology, Takanashi, Hagisho, Ichinoseki, Iwate 021-8511, Japan ‡ Research Center for Compact Chemical System, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1, Nigatake, Miyagino, Sendai 983-8551, Japan ABSTRACT: Gasification of sugarcane bagasse on activated-carbon- and titania-supported ruthenium (Ru/C and Ru/TiO2) catalysts in supercritical water was studied. Sugarcane bagasse was completely gasified to methane, carbon dioxide, and hydrogen over Ru/C and Ru/TiO2 catalysts at 673 K. The carbon yield of the gas products from sugarcane bagasse, cellulose, and lignin over the Ru/TiO2 catalyst at a water density of 0.33 g cm−3 for 15 min was 50.3, 74.4, and 31.1 C %, respectively. In addition, it was observed that an increase in the water density enhanced the initial carbon yield of the gas products for 15 min. The recyclability of the Ru/C catalyst with complete gasification could make it a sustainable process.

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INTRODUCTION Non-food biomass has great potential for use as an energy and chemical resource for the establishment of a sustainable society. The use of cellulose, which is a polymer of glucose, as a chemical resource is desirable because the products are relatively homogeneous.1 On the other hand, lignin is a complex, high- molecular-weight compound with more random structure than that of cellulose, and several kinds of phenolic products are obtained through the decomposition of lignin. However, a large amount of energy and considerable time are needed to separate and refine the various lignin end products. Furthermore, real-world biomass, such as agricultural waste, which is more complex and contains several components, is more desirable for use as an energy resource. Developing an effective gasification process for waste biomass, in particular for use as a high-quality energy source, which can provide energy in the form of electricity using a fuel cell, is important. Steam reformation of biomass in general, however, requires very high temperatures of over 1073 K.2 Moreover, the energy consumed during the pre-drying is an indication that the overall efficiency of the process is not very good. Gasification of biomass using supercritical water (Tc = 647.3 K, and Pc = 22.1 MPa) is a promising green technique because the operating temperatures are relatively low and pre-drying of the feedstock is not required.3,4 The hydrolysis of biomass components occurs rapidly in supercritical water.3−8 In addition, organic reactants and gases are miscible in supercritical water, and it provides a single fluid phase, in which the mass-transfer limitations of the reactant can be minimized.9,10 The rinsing effect of supercritical water for washing out coke precursors from the active catalyst sites can also be expected.11 These characteristics of supercritical water gasification are different from those of the steam reformation process, in which gasification proceeds mainly through high-temperature pyrolysis.1−3 Several research groups have studied the gasification of biomass over supported metal catalysts in supercritical water.12−21 Moreover, we have previously reported that titania© 2012 American Chemical Society

and activated-carbon-supported ruthenium catalysts were highly effective for the gasification of the typical components of biomass, such as cellulose and lignin, in supercritical water at 673 K.22−30 In this work, we report on the supercritical water gasification of sugarcane bagasse, which is the fibrous residue obtained after the extraction of the sugar-containing juice from sugarcane and is sometimes used for pulp production and generating thermal energy through combustion. Further, a large amount of sugarcane bagasse is disposed of as industrial waste.31 In addition, sugarcane bagasse has a high moisture content (>40 wt %), which makes it suitable for supercritical water gasification. In this report, we compare the gasification behaviors of sugarcane bagasse and the typical biomass components cellulose and lignin. Furthermore, we examine the effects of water density and the stability of a supported ruthenium catalyst following repetitive use.

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EXPERIMENTAL SECTION

The composition of the sugarcane bagasse used in this experiment was 35.0% cellulose, 35.8% hemicellulose, 16.1% lignin, and 3.5% water content,32 and its elementary composition, as determined by an ultimate CHNS analyzer (Perkin-Elmer, model 2400), is listed in Table 1. The sugarcane bagasse powder was almost insoluble in both tetrahydrofuran (THF) and water at ambient conditions. THF (+97%) was purchased from Wako Pure Chemicals Industries, Ltd. All chemicals were used without further purification. Distilled water was obtained from a water distillation apparatus (Yamato Co., model WG-220). Catalysts used in this study were Ru/TiO2 [2 wt % ruthenium on TiO2; metal dispersion, 27%; Brunauer−Emmett− Teller (BET) surface area, 24 m2/g] and Ru/C (5 wt % ruthenium on activated carbon; metal dispersion, 51%; BET surface area, 768 m2/g). Moles of surface ruthenium atoms of the fresh and used catalysts were determined by a carbon monoxide desorption method at 323 K (Bel Japan, Inc., BEL-CAT). It was assumed that a carbon monoxide Received: March 9, 2012 Revised: May 12, 2012 Published: May 14, 2012 3179

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Table 1. Elemental Analysis elemental analysis (wt %)

raw sugarcane bagasse THF-insoluble products from sugarcane bagassea celluloseb hemicellulose (xylane)b ligninb

C

H

N

S

O (difference)

44.3 70.2

6.0 4.0

0.4 0.1

0.1 0.1

49.2 25.6

43.9 45.4 67.8

7.3 6.1 6.2

0.0 0.0 0.0

0.0 0.0 0.0

48.8 48.5 26.0

product yield based on carbon (C %) moles of carbon in product = × 100 moles of carbon in sugarcane bagasse loaded

(1)

millimoles of gas product mass of sugarcane bagasse loaded

(2)

gas yield (mmol/g) =

gas composition (%) =

moles of gas product × 100 sum of moles of gas product

(3)

Some experiments were repeated 3 times to quantify run to run variability. The averages of these trials are included in the figures and tables along with the standard deviations.

a

After reaction without catalyst at 673 K, 0.5 g/cm3 water density, and 15 min of reaction time. bFrom refs 5 and 22.

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RESULTS AND DISCUSSION Effect of the Ruthenium Catalyst. Figure 1a shows the gasification profiles of sugarcane bagasse in supercritical water at 673 K. The carbon yield of the gas products was only 10 C % at 15 min and did not increase with increasing time. The carbon yields of the water- and THF-soluble products were 51 and 33 C %, respectively, at 15 min. The confirmed water-soluble products were 5-(hydroxymethyl)-furaldehyde, dihydroxyacetone, glycolaldehyde, and acetic acid; however, the yields of each product accounted for sugarcane bagasse > lignin. THF-insoluble products were not observed in the gasification of each reactant over the Ru/TiO2 catalyst. For

Figure 3. (a) Carbon yield, (b) gas yield, and (c) gas composition for sugarcane bagasse gasification over Ru/TiO2 in supercritical water of 0.5 g cm−3 of water density at 673 K. The amounts of Ru/TiO2 and sugarcane bagasse were 0.375 and 0.10 g, respectively. Carbon yield: (○) gas, (△) water soluble, (◇) THF soluble, and (□) THF insoluble. Gas yield and gas composition: (●) H2, (▲) CH4, (■) CO2, (◆) CO, and (×) C2−C4 gases.

yields of products (gas, water-soluble, THF-soluble, and THFinsoluble products) of sugarcane bagasse are similar to those of cellulose in the absence of a catalyst. The carbon yields of the gas and water-soluble products of sugarcane bagasse and cellulose were higher than those of lignin. Furthermore, the THF-insoluble, namely, char, yield of sugarcane bagasse was lower than that of lignin. The gas yield and gas composition of sugarcane bagasse were similar to those of cellulose, and the main gas products were CO2 and CO. The gas yield of lignin was lower than those of sugarcane bagasse and cellulose, and the main gas products from lignin were CO2 and CH4. It has been reported that cellulose is hydrolyzed to watersoluble oligomers and monosaccharides in supercritical water within a few seconds of reaction time3,5 and that lignin is hydrolyzed to water- and/or THF-soluble alkylphenols and formaldehyde in supercritical water within a few minutes of 3182

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the tendencies of the gasification of cellulose and hemicellulose were almost the same.36 Thereafter, we used GC (74.4 C %), GH (74.4 C %), and GL (31.1 C %) from the results in Table 3. We obtained an estimated GSB = 57.7 C %, which is slightly higher than the experimental value of 50.3 C % (Table 3), indicating that there is an interaction between biomass components. It has been reported that the estimated carbon yield of the gas products of real biomass (sawdust and rice straw) based on weight fractions was higher than their experimental data and claimed that this is due to the interactions between the cellulose and lignin components.36,37 One possible explanation of the interactions is that the aldehydes from cellulose and phenolic compounds from lignin underwent a polymerization reaction and produced a condensation product, which is difficult to gasify, namely, char. Therefore, the carbon yield of the gas products of real biomass would be lower than that estimated from its model components. On the other hand, Azadi et al. reported that the gasification yield of hardwood bark (containing 40−50% lignin) was comparable to that of cellulose and positively deviates from the rule of mixtures like eq 4 but the gas yield of extracted lignin is much lower than that of native lignin naturally within real biomass.38 It is probable that the gasification behavior depends upon the biomass species and/or its pretreatment process. Then, a further study of the biomass structure effect on the gasification should be performed in the future. Effect of the Water Density. Figure 4a shows the dependence of the carbon yields of each product upon the water density for sugarcane bagasse gasification for 15 min in the absence of a catalyst. The carbon yields of the gas products were very small, less than 10%, regardless of the water density. The carbon yields of the water-soluble products from sugarcane bagasse increased, while those of THF-insoluble products decreased, with increasing water density. We have reported that water-soluble products increased with water density in lignin gasification in supercritical water at 673 K because the hydrolysis rate of lignin to low-molecular-weight compounds was enhanced with an increase in the water density.23 As mentioned above, hydrolysis of cellulose and hemicellulose proceeds within a few seconds of the reaction time and is significantly faster than that of lignin, which requires a few minutes of reaction time.3 Therefore, the result in Figure 4a for 15 min of reaction time, which showed an increase of the watersoluble yield with an increasing water density, would be due to an enhancement of the hydrolysis of lignin. The gas yield and gas composition in panels b and c of Figure 4 show that H2 slightly increased and CO slightly decreased with increasing water density. We have also reported that an increase of H2 and a decrease of CO with increasing water density occurs in the gasification of lignin and formaldehyde.23,39 In supercritical water, the water−gas shift reaction does not proceed efficiently.10,40 We have reported that the formaldehyde converted into formic acid and the dominant reaction pathway of formic acid decomposition changed dehydration (HCOOH → CO + H 2 O) to decarboxylation (HCOOH → CO2 + H2) with increasing water density.39 One possible explanation for the formation of CO and H2 is that formaldehyde and formic acid would form during the decomposition of the sugarcane bagasse, although they were not detected in the recovered solutions in this study because of their high reactivity rates, and decarboxylation would be dominant with increasing water density.

Table 3. Product Yields and Composition of Gas Products from Gasification in the Presence of the Ru/TiO2 Catalyst Condition in Supercritical Watera sugarcane bagasse

celluloseb

carbon yield gas (C %) 50.3 ± 2.3 74.4 water soluble (C %) 17.3 ± 3.4 9.5 THF insoluble (C %) 0.0 ± 0.0 16.1c THF solubled (C %) 32.5 ± 5.7 TONe 119 172 gas yield H2 (mmol/g) 3.22 ± 0.07 2.80 CH4 (mmol/g) 8.53 ± 0.64 13.61 CO2 (mmol/g) 9.67 ± 1.64 14.25 CO (mmol/g) 0.21 ± 0.09 0.02 C2−C4 gases (mmol/g) 0.21 ± 0.19 0.33 gas composition H2 (%) 14.4 ± 0.3 9.0 CH4 (%) 39.4 ± 4.7 43.9 CO2 (%) 44.2 ± 5.6 45.9 CO (%) 1.0 ± 0.4 0.1 C2−C4 gases (%) 1.0 ± 0.8 1.1 equilibrium gas composition calculated by CHEMKIN III35 H2 (%) 20.8 21.0 CH4 (%) 34.2 34.3 CO2 (%) 45.0 44.7 CO (%) 0.0 0.0 C2−C4 gases (%) 0.0 0.0

ligninb 31.1 13.9 0.0 55.0 108 2.45 6.95 7.46 0.00 0.17 14.4 40.8 43.8 0.0 1.0 15.3 44.3 39.9 0.0 0.0

a Gasification conditions: reactant, 0.1 g; catalyst, 0.30 g; reaction temperature, 673 K; water density, 0.33 g cm−3; and reaction time, 15 min. bFrom ref 22. cWater-insoluble products of cellulose gasification: water-insoluble yield (C %) = 100 − (gas yield (C %) + water-soluble yield (C %)). dTHF-soluble yield (C %) = 100 − (gas yield (C %) + water-soluble yield (C %) + THF-insoluble yield (C %)). eTurnover number (TON) = moles of carbon atoms in gas products/moles of surface ruthenium atoms.

the gas yield and gas composition of all reactants, the main gas products were CO2, CH4, and H2 gases. The gas yields of CO2 and CH4 were in the order of cellulose > sugarcane bagasse > lignin. There is a possibility that sugarcane bagasse contains inorganic ashes; in this case, the gas yield would be underestimated in comparison to pure cellulose and lignin. The experimental gas compositions of all reactants show high CO2 and CH4 selectivity and correspond within 5%, whereas the gas compositions of the calculation depended slightly upon the reactants. Next, we will discuss the interactions between cellulose and lignin because sugarcane bagasse is a complex mixture of these components. First, we assumed that there is no interaction between the cellulose and lignin components and estimated the carbon yield of the gas products of sugarcane bagasse by eq 4 GSB = GCXC + G HXH + G LXL

(4)

where GSB is defined as the carbon yield of the gas products of sugarcane bagasse and estimated from the fractions of its components, GC, GH, and GL are the carbon yields of the gas products of cellulose, hemicellulose, and lignin, respectively, and XC, XH, and XL are the weight fractions of cellulose (35.0%), hemicellulose (35.8%), and lignin (16.1%) in the sugarcane bagasse, respectively. In eq 4, we assumed that the carbon yield of the gas products of hemicellulose (GH) was the same as that of cellulose (GC) because it has been reported that 3183

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Figure 4. Effect of the water density on (a) carbon yield, (b) gas yield, and (c) gas composition for sugarcane bagasse gasification in the absence of the catalyst condition in supercritical water at 673 K. The amount of sugarcane bagasse was 0.10 g. Carbon yield: (○) gas, (△) water soluble, (◇) THF soluble , and (□) THF insoluble. Gas yield and gas composition: (●) H2, (▲) CH4, (■) CO2, (◆) CO, and (×) C2−C4 gases.

Figure 5. Effect of the water density on (a) carbon yield, (b) gas yield, and (c) gas composition for sugarcane bagasse gasification over Ru/C in supercritical water at 673 K. The amounts of Ru/C and sugarcane bagasse were 0.150 and 0.10 g, respectively. Carbon yield: (○) gas, (△) water soluble, (◇) THF soluble, and (□) THF insoluble. Gas yield and gas composition: (●) H2, (▲) CH4, (■) CO2, (◆) CO, and (×) C2−C4 gases.

Figure 5a shows the dependence of product yields on water density for sugarcane bagasse gasification over the Ru/C catalyst. The carbon yields of the gas products increased with an increasing water density, and it reached about 100% at 0.5 g cm−3 of water density, whereas the carbon yields of the gas products were about 60−70% at water densities of 0.2−0.4 g cm−3. Further, the carbon yield of the gas products at 15 min was only 30% in the absence of water. The carbon yields of the THF-soluble products decreased with an increasing water density. THF-insoluble products were not formed in water above 0.33 g cm−3 of water density. Below 0.2 g cm−3 of water density, THF-insoluble products formed. The carbon yields of the water-soluble products were less than 3%. These were the main products in the absence of a catalyst, as shown in Figure 4a, indicating that the water-soluble products were gasified immediately over the Ru/C catalyst. For the case of water

density conditions of less than 0.2 g cm−3, the hydrolysis of sugarcane bagasse components did not proceed efficiently and gasification did not proceed well. The gas yield of CO2 and CH4 increased up to 24 and 14 mmol/g, respectively, with increasing water density (Figure 5b). On the other hand, the gas composition, which is 50% CO2, 40% CH4, and 10% H2, was independent of the water density level (Figure 5c). We have reported that the gas composition of lignin gasification over the supported ruthenium catalyst is also independent of the water density.23 As shown in panels b and c of Figure 2, the gas composition depends upon the gas yield at a shorter reaction time of 5 min but reached an equilibrium condition within 10 min. The reaction time of the results in Figure 5 is 15 min, and the gas products would have already reached a state of equilibrium, both with and without water conditions. 3184

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Table 4. Stability of the Ru/C Catalyst on Gasification of Sugarcane Bagasse in Supercritical Watera carbon yield gas (C %) water soluble (C %) THF insoluble (C %) THF solubleb (C %) gas yield H2 (mmol/g) CH4 (mmol/g) CO2 (mmol/g) CO (mmol/g) C2−C4 gases (mmol/g) gas composition H2 (%) CH4 (%) CO2 (%) CO (%) C2−C4 gases (%)

first

second

third

fourth

fifth

± ± ± ±

87.6 3.4 0.0 9.0

101.4 4.7 0.0 0.0

87.5 10.3 0.0 2.4

71.5 11.4 0.0 17.1

100.4 0.4 0.0 0.0

1.92 15.55 22.33 0.00 0.09 9.1 37.1 53.3 0.0 0.5

9.6 0.2 0.0 0.1

± ± ± ± ±

± ± ± ± ±

0.01 1.20 1.09 0.00 0.09

1.2 5.8 6.7 0.0 0.2

1.89 14.08 17.94 0.02 0.74

2.49 15.11 21.26 0.00 0.90

2.49 11.12 19.63 0.00 0.84

2.70 9.17 15.05 0.03 1.11

10.1 37.5 47.7 0.0 4.7

11.5 34.8 49.0 0.0 4.6

13.2 29.6 52.2 0.0 5.1

16.7 28.8 46.5 0.1 8.3

Reaction conditions: sugarcane bagasse, 0.10 g; Ru/C catalyst, 0.150 g; water density, 0.50 g cm−3; reaction temperature, 673 K; and reaction time, 15 min. bTHF-soluble yield (C %) was calculated as 100 − gas yield (C %) − water-soluble yield (C %) − THF-insoluble yield (C %).

a

Stability of the Catalyst. We studied the stability of the Ru/C catalysts by repetitive use of the same catalyst for five cycles at 673 K and 0.5 g cm−3 of water density for 15 min for each run. Table 4 lists the carbon yields of each product, the gas yield, and the gas compositions of the repetitive experiments. The gas yield after five repetitions remained 71.5 C %. The yields of water- and THF-soluble products increased for repetitive uses, and THF-insoluble products were not formed. Although we washed the catalyst with water and THF, there is a possibility that the residue remained on the catalyst surface. Some active sites might be covered by the residue, which caused deactivation. For repetitive use of catalysts, the gas yields of CO2 and CH4 decreased slightly and the gas yields of H2 and C2−C4 gas increased. In the gas composition, CH4 selectivity decreased and H2 and C2−C4 gas selectivities increased with repetitive use of catalysts, indicating deterioration of the methanation sites. The characterization of the Ru/ C catalysts before and after sugarcane bagasse gasification was conducted using an XPS technique (Table 5). From the XPS

catalyst of this work in the absence of sugarcane bagasse, in which the dispersion of ruthenium metal particles was almost constant and the surface area values of the carbon support did not change by just treating with supercritical water.24 However, the amount of CO molecules adsorbed on the ruthenium metal particles after five repetitions became only 9.1% of that of the fresh Ru/C. In addition, we have also reported that the surface area values of the carbon support decreased in the presence of biomass.24 Possible explanations for the decrease in the amount of CO adsorption and surface area values after gasification are (1) the pore structure of the carbon support changed and/or (2) the pores were blocked by small amounts of carbonaceous products during gasification. These results indicate that the ruthenium metal in the pores would not work for gasification after repetitive use; hence, gasification effectiveness decreased gradually. To maintain high activity of the Ru/C catalysts, it would be important to gasify sugarcane bagasse completely to keep the catalyst surface fresh. Ru/C catalysts are effective for sugarcane bagasse gasification in supercritical water in a batch system; however, more active and durable catalysts should be developed for commercialization because the ratio of catalyst/sugarcane bagasse is high and the activity level decreased after several uses.

Table 5. Summary of Results from XPS of the Catalyst Surfacea

■

Ru (3d) 0

Ru

4+

Ru

Ru6+ or Ru8+

total

0.8 0.5

2.8 2.1

CONCLUSION Gasification of sugarcane bagasse was conducted in supercritical water in the presence of supported ruthenium catalysts at 673 K. The following results were obtained: (1) Sugarcane bagasse was completely gasified over the Ru/C and Ru/TiO2 catalysts in supercritical water at 673 K. (2) The initial carbon yields of the gas products over Ru/TiO2 were in the order of cellulose > sugarcane bagasse > lignin. (3) The gas yield of sugarcane bagasse increased with an increase in the water density. (4) The gasification activity in supercritical water continued after five repetitions.

Ru/C fresh usedb

1.2 1.0

0.8 0.6

a

The unit is atomic percent. bConditions: sugarcane bagasse, 0.10 g; reaction temperature, 673 K; water density, 0.50 g cm−3; and reaction times, 15 min × 5.

analysis, for the case of the fresh Ru/C catalyst, the ruthenium forms are 40% metallic ruthenium and 60% ruthenium cations, namely, ruthenium oxides, such as RuO2. The amounts of ruthenium species detected by XPS decreased slightly after repetitive use of the catalysts (Table 5). Ruthenium in the recovered aqueous solution after gasification was not detected by ICP analysis. We have reported the effect of supercritical water treatment on the same Ru/C

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-22-237-5219. Fax: +81-22-237-5224. E-mail: [email protected]. 3185

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Notes

(31) Cardona, C. A.; Quintero, J. A.; Paz, I. C. Bioresour. Technol. 2010, 101, 4754−4766. (32) Sasaki, M.; Adschiri, T.; Arai, K. Bioresour. Technol. 2003, 86, 301−304. (33) Arai, K.; Adschiri, T.; Watanabe, M. Ind. Eng. Chem. Res. 2000, 39, 4697−4701. (34) Wagner, W.; Purb, A. J. Phys. Chem. Ref. Data 2002, 31, 387− 535. (35) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixson-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; Adigun, O. CHEMKIN Collection, Release 3.6; Reaction Design, Inc.: San Diego, CA, 2001. (36) Yoshida, T.; Matsumura, Y. Ind. Eng. Chem. Res. 2001, 40, 5469−5474. (37) Yoshida, T.; Oshima, Y.; Matsumura, Y. Biomass Bioenergy 2004, 26, 71−78. (38) Azadi, P.; Khan, S.; Strobel, F.; Azadi, F.; Farnood, R. Appl. Catal., B 2012, 117−118, 330−338. (39) Osada, M.; Watanabe, M.; Sue, K.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 2004, 28, 219−224. (40) Sato, T.; Kurosawa, S.; Smith, R. L., Jr.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 2004, 29, 113−119.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Special Coordination Funds for Promoting Science and Technology “Development of Sustainable Catalytic Reaction System Using Carbon Dioxide and Water”.

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REFERENCES

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dx.doi.org/10.1021/ef300460c | Energy Fuels 2012, 26, 3179−3186