Mixed Solvents Assisted Flame Spray Pyrolysis Synthesis of TiO2

Furthermore, the dye-sensitized solar cells (DSSCs) performance of TiO2 HPHSs as light ... Renewable and Sustainable Energy Reviews 2016 64, 543-568 ...
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Mixed Solvents Assisted Flame Spray Pyrolysis Synthesis of TiO2 Hierarchically Porous Hollow Spheres for Dye-Sensitized Solar Cells Junchao Huo, Yanjie Hu,* Hao Jiang, Wenjuan Huang, Yunfeng Li, Wei Shao, and Chunzhong Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: A novel one-step and template-free preparation process had been developed to synthesize TiO2 hierarchically porous hollow spheres (HPHSs) by mixed solvents assisted flame spray pyrolysis (FSP). The as-obtained TiO2 HPHSs had hierarchically porous hollow structure such as central cavities, macropores on shells, and mesopores accumulated by TiO2 nanocrystallites. The unique hierarchically porous structure endowed the TiO2 spheres with high specific surface area and excellent light scattering property. A mechanism of the formation of TiO2 HPHSs depending on the competition between chemical reaction rate and diffusion rate of the components of the precursor was proposed, in which mixed solvents and short flame residence time were of importance. Furthermore, the dye-sensitized solar cells (DSSCs) performance of TiO2 HPHSs as light scattering layer was investigated. The photoelectric conversion efficiency (η) was improved by 38.2% (from 5.00% to 6.91%), comparing to that of single layer P25 films.

1. INTRODUCTION Due to the serious problem of traditional fossil energy sources exhaustion, study of renewable energy sources has became a worldwide hot topic over the recent years. As a typical renewable energy source, solar energy is considered to be green and inexhaustible. Dye-sensitized solar cells (DSSCs), one of the photovoltaic devices, are becoming a significant focus, because of their abundant raw material, low cost, facile fabrication process, efficient photovoltaic performance, and stability.1,2 In 1991, a breakthrough in DSSCs was achieved, a photoelectric conversion efficiency as high as 7% was obtained by O’Regan and Grätzel3 who innovatively employed TiO2 nanoparticle as the transporting medium of photoinduced electrons while using a ruthenium complex (N719) as the sensitizer to absorb the solar light. This value has been improved to over 12% nowadays by optimizing the corresponding components of the devices.4 In highly efficient DSSCs, the photoanode is normally formed by two layers, 5 which are usually made of anatase TiO2 nanoparticles with a diameter of about 20 nm and anatase TiO2 spheres of about 400 nm in diameter, respectively. The first layer of small TiO2 nanoparticles is to ensure a large surface area for loading large amount of dye molecules. Because such a film is usually well transparent, long wavelength (red) part of the incident light can transmit it without exciting dye molecules. The second layer of TiO2 anatase spheres are used to scatter back the transmitted light, in order to enhance light harvesting performance.6,7 However, the second layer often has low surface area, inevitably resulting in insufficient dye adsorption and reduced the energy conversion efficiency. Therefore, an ideal scattering material should fulfill two foremost requirements, a high specific surface area to obtain more dye loading capacity8 and a large particle size to enhance the harvest of red and near-infrared light.9 To satisfy these requirements, several examples using hollow microspheres or © 2013 American Chemical Society

hierarchical hollow spheres with a high surface area as the scattering layer to enhance light harvest,10 such as nanoembossed hollow spherical TiO2 and doubleshell multilayered SnO2@TiO2 hollow spheres, have been previously reported.11,12 Submicrometer-sized hollow TiO2 spheres were prepared by directly self-assembled of TiO2 nanoparticles and showed good matching to visible light, which significantly improved photoelectric conversion efficiency.13 Hierarchical hollow spherical TiO2 with higher surface area compared to the usual hollow structured TiO2 has received more and more attention nowadays.14−17 The multiple-reflection effect occurring inside the interior cavities of hierarchical hollow spherical TiO2 could trap the incident light for a longer time, which brought forth more opportunities for light absorption.18−20 Many different approaches, such as ultrasonic spray pyrolysis, Ostwald Ripening, sol−gel, template method and hydrothermal method, have been developed to prepare metal oxide semiconductor hollow spheres.21−26 Conventional methods for the preparation of metal oxide semiconductor hollow spheres usually require removable or sacrificial templates to guide the formation of inorganic nanoparticles on their surfaces. However, the use of templates usually suffers from disadvantages of high cost and tedious synthetic procedures. Flame spray pyrolysis (FSP) approach with many advantages, such as being scalable and continuous, requiring no posttreatment, and usable in large quantities, has become an established way to produce metal oxide semiconductor nanoparticles. In recent years, FSP has been adopted to prepare hollow structured metal oxide semiconductor materials, Pratsinis SE27 prepared various oxide powders, such as Al2O3, TiO2, ZrO2 and Y2O3 by the Received: Revised: Accepted: Published: 11029

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emulsion combustion method. Takao Tani28 investigate the dynamics of hollow and solid alumina particle formation in spray flames: hollow particles maintained their shapes in the flame using air as the dispersion/oxidant gas, whereas hollowto-solid restructuring of the particles took place in the flame using oxygen. Our group has made some processes by using this interesting method for the synthesis of hollow Al2O3,29 ball-in-shell structured TiO2,30 and so on. In this work, a one-step and template-free new route has been developed to synthesized TiO2 hierarchically porous hollow spheres (HPHSs) by flame spray pyrolysis (FSP) approach using mixed solution of ethanol and ethylene glycol as solvents. Based on the results, a possible mechanism of the formation of TiO2 HPHSs was proposed. On account of the hierarchically porous structure such as central cavity, macropores on the shells and mesopores accumulated by TiO2 nanocrystallites, the TiO2 HPHSs had high absorption property of light and large surface area. In addition, the photoelectric property of the as-prepared TiO2 HPHS as scattering layer of DSSCs was investigated.

The precursor solution was beforehand prepared by dissolving appropriate amounts of titanium tetrabutoxide (Ti(OC4H9-n)4, 98%, Lingfeng Chemical Reagent Co., Ltd., China) in mixed solvents of anhydrous ethanol (C2H5OH, 99.7%, Sinopharm Chemical Reagent Co. Ltd., China), and ethylene glycol (C2H6O, 99%, Lingfeng Chemical Reagent Co., Ltd., China) with a volume ratio of 4:1. Then, the precursor was stirred for 5 min to obtain a solution with a concentration of 0.6 M. The precursor solution was delivered by a syringe pump with a speed of 3 mL/min to the capillary tube. In addition, dense TiO2 (Figure S2, Supporting Information) nanospheres were prepared using the same precursor with a low atomizing pressure (1.5 bar). 2.2. DSSCs Fabrication. To prepare a screen-printable paste, 1.00 g prepared TiO2 HPHSs were mixed with 0.025 g of ethyl cellulose (EC; M70, Sinopharm) and ground for 20 min. Then, 1.00 mL of terpineol (Sigma-Aldrich) and 0.40 mL of Triton X-100 (Sinopharm, cp) were added into the above mixture and ground for 30 min. Subsequently, 4.00 mL of terpineol was introduced and the mixture was ground for another 30 min. Ethyl cellulose was selected as a binder to improve the connectivity between the TiO2 hollow spheres, and the amount of the terpineol was adjusted to control the viscosity of the paste and the thickness of the electrodes. The paste of dense TiO2 and P25 was prepared by the same method. The resulting screen-printable paste was printed on fluorine-doped tin oxide (FTO with sheet resistance of 15 Ω/ square) glass precoated with a 50 mM TiCl4 solution with an active area of 0.25 cm2, by using the screen-printing technique. The films were annealed at 450 °C for 30 min in air. To obtain the bilayer films, the TiO2 HPHSs layer was printed on the P25 film and then annealed at 450 °C for 30 min in air. After annealing, the films were treated with 50 mM TiCl4 aqueous solution at 70 °C for 30 min and washed with distilled water and ethanol. Finally, the films were sintered at 450 °C for 30 min. The TiO2 films were immersed in an anhydrous ethanol solution containing 0.5 mM Ru dye ((Bu4N)2[Ru(Hdcbpy)2(NCS)2], known as N719, Solaronix) for 24 h at room temperature to ensure complete uptake of the sensitizer. A Pt counter electrode was prepared on the transparent conducting glass using 0.50 mM H2PtCl6 solution, which was subsequently annealed at 380 °C for 20 min in air. The dye-sensitized TiO2 electrode and the Pt-counter electrode were assembled into a sandwich-type cell and sealed with a thermal adhesive film. The redox electrolyte consisted of 0.60 M BMII (1-butyl-3methylimidazolium iodide), 0.03 M I2, 0.10 M guanidinium thiocyanate, and 0.50 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (volume ratio, 85:15). 2.3. Characterization and Measurements. The general crystallinity and morphology of the samples were examined by X-ray diffraction (XRD; Rigaku D/max 2550), scanning electron microscopy (SEM; Hitachi S-4800), high-resolution transmission electron microscopy (TEM; JEM-2010). Diffusereflectance spectra were measured on the same film samples with a spectrophotometer (UV−vis, Cary-500 spectrometer, 200−800 nm wavenumber, Varian Ltd.). Photocurrent voltage I−V measurements were performed using a Newport I−V tester (Oriel Class A 91160A) at 1 sun condition (100 mW/ cm2 at AM1.5). Dye loading measurements were conducted by immersing the films in 0.05 M NaOH solution and monitoring the concentration of desorbed dye by UV/vis spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Particle Synthesis. TiO2 HPHSs were prepared by a FSP reactor. Figure 1 showed the experimental setup of the

Figure 1. Schematic setup for mixed solvents assisted FSP synthesis of TiO2 HPHSs.

FSP reactor. Schematically, it contained a control unit, a reaction unit, and a particle collection unit. The reaction unit was a flame-spray apparatus consists of an external mixing gasassisted nozzle, which was made of a capillary tube with an outer diameter of 1 mm (inner diameter 0.6 mm) and an opening of 1.4 mm in diameter. The capillary tube lied in the opening, creating an annular gap of 0.75 mm2 in area, which was surrounded by a flamelet openings ring. The flamelet openings were uniformly distributed at a radius of 8 mm from the center of the nozzle. Oxygen was used as oxidant and dispersion gas, atomizing the liquid precursor supplied by a syringe pump into a fine spray. The spray was evaporated and ignited by the eight supporting flamelets. The flow rate of the dispersion gas was controlled by a flow controller, which maintained a constant pressure (3 bar) at the nozzle tip. Hydrogen gas flowed through the eight supporting flamelets at a flow rate of 1.67 L/min to obtain the supporting flamelets. Additional air (33.3 L/min) was provided through the outermost sintered metal ring as sheath for the supported flame. The product was collected on a glass fiber with the assistance of a vacuum pump. 11030

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3. RESLULTS AND DISCUSSION 3.1. Structure and Morphology of the TiO2 HPHSs. Figure 2 showed the XRD pattern of TiO2 HPHSs, suggesting

Figure 2. XRD pattern of TiO2 HPHSs.

Figure 3. (a) SEM image, (b) TEM image, (c) SEM image of a macropore, (d) TEM of one TiO2 HPHSs, (e) corresponding SAED pattern.

that the samples have good crystalline nature. All diffraction peaks can be indexed to the anatase TiO2 (JCPDS No. 211272) and rutile TiO2 (JCPDS No. 21-1276), respecting the TiO2 HPHSs were mixture of annatase TiO2 and rutile TiO2, in which the anatase fraction was estimated to be 67.8% while the rutile fraction was 32.2%. As determined from the anatase (101) and rutile (110) peaks using the Scherrer eqution,31 the crystallite sizes of TiO2 HPHSs were 4.6 nm and 6.0 nm, respectively. This indicated that the synthesized TiO2 HPHSs were composed of small nanocrystallites. In addition, the XRD patterns of dense TiO2 and P25 were provided (Figure S2, Supporting Information), the structure of P25, dense TiO2, and TiO2 HPHSs are summarized in Table 1. Dense TiO2 and P25

owing to the accumulated of TiO2 nanocrystallites. The SAED pattern is given in Figure 3e, indicating that TiO2 HPHSs spheres were polycrystalline. Furthermore, the dense TiO2 nanoparticles prepared at low atomizing pressure were composed of little TiO2 nanoparticles and bigger dense TiO2 particles with an unequal diameter of 100−500 nm (Figure S3, Supporting Information). Figure 4 showed the nitrogen adsorption−desorption isotherms of the TiO2 HPHSs together with the BJH analysis of desorption isotherms in the inset. The isotherm was of type IV (BDDT classification) with a hysteresis loop, indicating the presence of mesopores. The BET surface area for the sample was 80.0 m2/g, and the total pore volume was 0.218 m3/g. Moreover, the pore size distribution measurement suggested that the sample had macropores and mesopores with a total average pore diameter of 10.9 nm. The BJH analysis of desorption isotherms showed a bimodal pore size distribution. The first peak (9 nm) suggested the existing of mesopores, while the second peak (35 nm) showed the existing of both mesopores (20−50 nm) and macropores (>50 nm). The mesopores were related to the accumulating of TiO2 nanocrystallites, while the macropores were associated with the inner cavities and macroholes on the shells. The hierarchically porous structure of TiO2 HPHSs could facilitate electrolyte diffusion and dye loading and enhance light harvesting efficiency when used as scattering centers in DSSCs. In addition, the nitrogen adsorption−desorption isotherms and corresponding pore size distribution of P25 and dense TiO2 are provided in the Supporting Information Figures S4 and S5, respectively. The BET specific surface area and total pore volume (dense TiO2 and P25) are also summarized in Table 1, suggested the specific surface area and total pore volume of TiO2 HPHSs was the highest of the three examples. 3.2. Formation Mechanism of TiO2 HPHSs. The formation mechanism of the TiO2 HPHSs (Figure 5) can be proposed by considering the competition between chemical

Table 1. Structure of P25, Dense TiO2, and TiO2 HPHSs phase fractions (%)

crystallite sizes (nm)

samples

anatase

rutile

anatase

rutile

total pore vol (cm3/g)

BET (m2/g)

P25 dense TiO2 TiO2 HPHSs

79.4 56.3

20.6 43.7

23.0 11.3

26.2 14.6

0.17 0.15

49.1 40.1

67.8

32.2

4.6

6.0

0.22

80.0

were both mixtures of annatase TiO2 and rutile TiO2; the anatase fraction of dense TiO2 was 56.3% while that of P25 was 79.4%. The different phase fractions of the three samples were derived from the different synthesize temperatures. As shown in Figure 3a and b, TiO2 hollow spheres with a diameter of about 200− 400 nm were observed, and there were one or more macropores (>50 nm) on the shells of the spheres. For detailed information, Figure 3c and d showed the SEM and TEM of one TiO2 HPHSs, suggesting the hollow spheres had relative rough shells with an average shell thickness of 30 nm. In addition, the as-obtained TiO2 HPHSs spheres were composed of dense TiO2 nanocrystallites of 5−10 nm in diameter. Additionally, it was clear that mesopores existed, 11031

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Figure 4. (a) Nitrogen adsorption−desorption isotherms and (b) corresponding pore size distribution of TiO2 HPHSs.

Figure 5. Illustration of the formation process of TiO2 HPHSs.

3.3. DSSCs Performance. In order to investigate the DSSCs performance of TiO2 HPHSs, a bilayer film was constructed by printing a layer (∼ 5 μm) of TiO2 HPHSs which was employed as a scattering layer on the top of a layer (∼ 7.5 μm) of P25. For comparison, single layer films consisting of P25 nanoparticles with the same thickness of the bilayer film (Figure S6, Supporting Information) and a bilayer film using dense TiO2 as the scattering layer were also fabricated. The photocurrent (I)−voltage (V) over an active area of 0.25 cm2 using simulated sunlight was examined, and the I−V curves and photovoltaic parameters are shown in Figure 6 and Table 2, respectively. As a result, after TiO2 HPHSs were employed as scattering layer, the cell showed the highest efficiency of 6.91%, corresponding to a 38.2% and 16.7% increment compared to that of P25 (5.00%) and dense TiO2 (5.92%) as scattering layer. It should be noted that, compared to the film of P25, the short circuit current density (Jsc) of the TiO2 HPHSs film and the dense TiO2 film was improved by 53.1% (11.27 to 17.26 mA/cm2) and 22.4% (11.27 to 13.80 mA/cm2), respectively. The improvement was mostly owing to the scattering effect of the introduced scattering layer. The short circuit current density of TiO2 HPHSs film was higher than dense TiO2 film, indicating that TiO2 HPHSs showed a better scattering effect than dense TiO2 nanoparticles. The amount of dye (N719) adsorbed was investigated and is also summarized in Table1. The dye loading amount of the bilayer film of TiO2 HPHSs was 1.17 times

reaction rate and diffusion rate of the components of the precursor. A mixed solvents system, that is, ethanol and ethylene glycol, was adopted for the synthesis of TiO2 HPHSs. At the beginning, the solution was injected into the capillary tube and dispersed into small droplets by high pressured oxygen, which was flowing through the annular gap surround the capillary tube. Owing to the low boiling point, the volatilization and combustion of ethanol occurred and resulted in the decrease of the droplet size and the increase of TBT concentration.32 Then, the TBT molecules near the surface of droplets reacted with water molecules produced by the combustion of ethanol vapor and hydrogen, forming a dense layer of TiO2 nanocrystallites on the surface of the droplets. As the temperature became higher, ethylene glycol with higher boiling point evaporated and produced the central cavities and macropores on the shells. On the other hand, the as-obtained hierarchically porous hollow structure could be successfully preserved in short flame residence time. However, when a flame with higher temperature or long length was employed, dense TiO2 spheres instead of hollow structured TiO2 spheres were obtained due to the further burning (Figure S3, Supporting Information).28 Therefore, mixed solvents and short flame residence time were the two most important requirements for the preparation of TiO2 HPHSs. Moreover, short flame residence time (Figure S1, Supporting Information) was realized by using high atomizing pressure (3 bar). 11032

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Figure 7. Diffuse-reflectance spectra of DSSCs based on films of P25, dense TiO2, and TiO2 HPHSs.

Figure 6. I−V curves of DSSCs based on films of P25, dense TiO2, and TiO2 HPHSs.

Table 2. Performance Results of DSSCs Based on Films of P25, Dense TiO2, and TiO2 HPHSsa samples P25 dense TiO2 TiO2 HPHSs

Jsc (mA/cm2)

Voc (V)

FF

η (%)

adsorbed dye (10−7 mol/cm2)

11.27 13.80 17.26

0.74 0.76 0.76

0.60 0.57 0.53

5.00 5.92 6.91

1.55 1.43 1.81

scattering effect. In order to more clearly investigate the scattering effect of TiO2 HPHSs, normalized IPCE spectra was showed in Figure 8b. The normalized IPCE values of TiO2 HPHSs film were much higher than those of the P25 film, and a little higher than those of the film of dense TiO2 in all spectra, which were in good agreement with the diffuse-reflectance spectra. Therefore, it could be deduced that the TiO2 HPHSs had a stronger light-scattering effect, eventually leading to a better photovoltatic performance. There were two key factors contributing to the higher photoelectric conversion efficiency of the photoanode made of TiO2 HPHSs as scattering layer. According to the Mie theory,33 the effective scattering particles were those with a size comparable to the wavelength of light, and the light scattering led to higher light trapping in the device. Therefore the asprepared TiO2 spheres were beneficial to the multiple reflectance of the light, because of the suitable size (200−400 nm) and unique hierarchically porous hollow structure. The light scattering and the optical path length of light in TiO2 HPHSs film could be effectively improved. As discussed above, the excellent light scattering of the TiO2 HPHSs enhanced the light trapping and utilization performance in the photoanode, which greatly improved the short circuit current density (Jsc) of the cells. The overall pore volume of the TiO2 HPHSs film was larger than that of the P25 film, owing to the higher specific surface area of 80 m2/g. As shown in Table 1, the amount of the absorbed dye of the TiO2 PHPSs film was higher than that of the P25 film. Therefore, the film employed TiO2 HPHSs as scattering layer could absorb more light. As considered the two factors mentioned above, although the impedance at the oxide/ dye/electrolyte interface of the TiO2 HPHSs film was a little larger than that of P25 film (Figure S7, Supporting Information), the cell of TiO2 HPHSs as scattering layer still obtained the highest photoelectric conversion efficiency.

a The active areas of the films were about 0.25 cm2, and the thickness of the films was about 4 μm. Each entry represents the average data of three cells.

higher than that of single layer film of P25. This was attributed to the relative large pore size and porosity of TiO2 HPHSs, which was in accordance with the above discussion. On account of the highest short circuit current density (Jsc), TiO2 HPHSs film exhibited the best DSSCs performance of the three films, although the open-circuit voltage (Voc) was similar and fill factor (FF) was a little smaller than the other two films. Besides, because of lower surface area, the dye loading amount of the bilayer film of dense TiO2 was a little lower than that of single layer film of P25. This was also a factor attributing to the smaller short circuit current density (Jsc) improvement. The difference in light-scattering capacity was revealed by the diffuse-reflectance spectra of the three films (Figure 7). In general, the reflectance of the bilayer films was much higher than that of the single layer P25 film in the visible range 400− 800 nm. Furthermore, the film of TiO2 HPHSs as scattering layer showed higher reflectance value than that of dense TiO2 as scattering layer, indicating that the TiO2 HPHSs film had higher light-scattering ability and could trap more light. The scattered light obviously increased the amount of electrons generated in the photoanode, which greatly improved the short circuit current density (Jsc). Therefore, the cell of TiO2 HPHSs showed highest photoelectric conversion efficiency. Figure 8a showed incident photo to current efficiency (IPCE) spectra of the cells, indicating that film of TiO2 HPHSs exhibited the highest IPCE values in all spectra. The increase in the short wavelength region (400−600 nm) should be attributed to the high dye loading and the enhanced lightscattering effect, while that in the long wavelength region (600−800 nm) probably arose only from the enhanced light-

4. CONCLUSION In summary, a one-step and template-free new route had been developed to synthesize TiO2 hierarchically porous hollow spheres (HPHSs) by FSP process using ethanol and ethylene glycol mixed solution as solvents. On account of the hierarchically porous structure such as central cavity, macro11033

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Figure 8. (a) IPCE spectra and (b) normalized IPCE spectra of DSSCs based on films of P25, dense TiO2, and TiO2 HPHSs.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21236003, 21106038), the Special Projects for Nanotechnology of Shanghai (11 nm0500200), the Basic Research Program of Shanghai (11JC1403000), Program for New Century Excellent Talents in University (NCET-11-0641), the Fundamental Research Funds for the Central Universities.

pores on the shells, and mesopores accumulated by the TiO2 nanocrystallites, the as-obtained TiO2 HPHSs had larger surface area for higher dye loading amount and excellent light scattering property for better trapping of incident light. A mechanism of the formation of TiO2 HPHSs depends on the competition between chemical reaction rate and diffusion rate of the components of the precursor was proposed. Owing to low boiling point, volatilization and combustion of ethanol first occurred, resulting in decrease of the droplet size and increase of TBT concentration. Then, the TBT molecules began hydrolysis and formed a layer of TiO2 dense sphere on the surface of the droplets. Finally, the evaporation of ethylene glycol, which was the high-boiling-point component in the solvents, occurred and produced the central cavities and macropores on the shells. Mixed solvents and short flame residence time were the most important requirements for the synthesis of TiO2 HPHSs. Furthermore, the DSSCs performance of TiO2 HPHSs as light scattering layer was investigated. The photoelectric conversion efficiency (η) of TiO2 HPHSs film was improved by 38.2% (from 5.00% to 6.91%) and 16.7% (from 5.92% to 6.91%), compared to that of P25 film and dense TiO2 film. The unique hierarchically porous hollow structure and high specific surface area were the two factors contributing to the increment of photoelectric conversion efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

Photographs of spray flame at differernt atomizing pressure, Figure S1; TEM of mixed solvents assisted FSP synthesized TiO2 spheres at different dispersion pressure, Figure S2; SEM top and cross section of films printed on FTO, Figure S3; Nyquist plots of the cells, Figure S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 21 64250624. Tel: 86 21 6425 0949. E-mail: czli@ ecust.edu.cn (C. Z. Li); [email protected] (Y. J. Hu). Notes

The authors declare no competing financial interest. 11034

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dx.doi.org/10.1021/ie4006222 | Ind. Eng. Chem. Res. 2013, 52, 11029−11035