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Low temperature biomimetic synthesis of the Li2ZrO3 nanoparticles containing Li6Zr2O7 and high temperature CO2 capture
Materials Letters 64 (2010) 1404–1406
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Low temperature biomimetic synthesis of the Li2ZrO3 nanoparticles containing Li6Zr2O7 and high temperature CO2 capture Shi-Zhao Kang a,⁎, Tan Wu b, Xiangqing Li a, Jin Mu b a b
Department of Chemical Engineering, Laboratory of New Energy Materials, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 19 February 2010 Accepted 19 March 2010 Available online 25 March 2010 Keywords: Lithium zirconate Nanomaterials Low-temperature biomimetic synthesis Gelatin CO2 capture Kinetics
a b s t r a c t Li2ZrO3 nanoparticles containing Li6Zr2O7 were prepared by a biomimetic soft solution route and characterized with X-ray diffraction (XRD), transmission electron microscope (TEM) and nitrogen adsorption. The results show that the tetragonal Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 can be obtained using this simple method. The mean diameter of the nanoparticles is approximately 90 nm and the corresponding specific surface area is 23.7 m2 g− 1. Moreover, the Li2ZrO3 nanoparticles obtained were thermally analyzed under a CO2 flux to evaluate their CO2 capture capacity at high temperature. It was found that the as-prepared Li2ZrO3 nanoparticles would be an effective acceptor for high temperature CO2 capture. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is known that the conventional CO2 absorbents are unfit for the high temperature CO2 capture [1,2]. The materials with high CO2 capture capacity at high temperature are desirable. Bulk Li2ZrO3 seems to be a promising candidate except the CO2 sorption rate is slow [3]. According to the double-shell model [4], Li2ZrO3 nanoparticles would be a proper CO2 absorbent with improved kinetic property due to the large surface and nano-scaled size. In addition, bulk Li6Zr2O7 can capture 4 times more CO2 than bulk Li2ZrO3 and exhibits faster CO2 sorption rate [5]. Therefore, the capture property of Li2ZrO3 nanoparticles may be improved if Li6Zr2O7 would be introduced in the Li2ZrO3 nanoparticles. The solid-state reaction is the most common synthesis route of Li2ZrO3. However, it usually requires high temperature (850 to 1000 °C) and long reaction time [6]. And it is difficult to obtain small particles because of high temperature sintering. Therefore, several new synthetic methods of Li2ZrO3 have been explored [6–8]. However, the calcination at high temperature is still required and the nano-scaled Li2ZrO3 particles cannot be obtained. Inspired by the nature biomineral process, some functional inorganic nanomaterials were obtained using biopolymer as a directing agent [9]. For example, iron oxide nanoparticles can be obtained using a sodium alginate-assisted route [10]. Additionally, the ⁎ Corresponding author. Department of Chemical Engineering, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China. Tel.: +86 21 64941194; fax: +86 21 64252485. E-mail address: [email protected] (S.-Z. Kang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.043
paper reported previously shows that when the metal complex of the polymer is used as a precursor in the solid-state reaction the product can be prepared at a relatively low heat-treatment temperature [11]. Therefore, it is potential to obtain Li2ZrO3 nanoparticles at low temperature if a biopolymer, such as gelatin, is used. Herein we report a biomimetic soft solution route for the formation of the Li2ZrO3 nanoparticles containing Li6Zr2O7 at low temperature. Meanwhile, the CO2 adsorption capacity of the Li2ZrO3 nanoparticles at high temperature is evaluated using thermogravimetric analysis.
2. Experimental In a typical procedure, gelatin (1 g) was added into the CH3COOLi solution (0.15 mol dm− 3, 10 mL) at 50 °C under stirring. Subsequently, the ZrO(NO3)2 solution (0.05 mol dm− 3, 10 mL) was dropped into the solution above. After stirred at 50 °C for 30 min, the solution was rapidly cooled to 4 °C. The gel obtained was divided into small pieces and dried in a vacuum desiccator at room temperature. After the xerogel was heat-treated at 350 °C for 1 h in an electric furnace, the product was directly annealed at 600 °C for 8 h to obtain the Li2ZrO3 nanoparticles. X-ray diffraction (XRD) was carried out with a Rigaku D/Max2550 VB/PC X-ray diffractometer (Japan). The morphology of the Li2ZrO3 nanoparticles was examined on an FEI Tecnai G2 F20 S-Twin field emission transmission electron microscope (TEM, USA). The N2 adsorption and desorption isotherms were measured on a Micromeritics ASAP2020 nitrogen adsorption apparatus (USA). The CO2 capture property of
S.-Z. Kang et al. / Materials Letters 64 (2010) 1404–1406
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the Li2ZrO3 nanoparticles was evaluated using thermogravimetric analysis (TGA) (SDT Q600, TA Instruments-Waters LLC, USA) at 500 °C. 3. Results and discussion The XRD pattern of the sample is shown in Fig. 1. It can be observed that there exist several peaks at 22.25, 36.02, 39.95, 42.64, 59.79 and 61.54°, corresponding to the (210), (320), (400), (330), (530) and (521) planes of tetragonal Li2ZrO3, respectively (JCPDS file No. 200647). Meanwhile, the peaks of monoclinic Li6Zr2O7 can also be found in Fig. 1 (JCPDS file No. 34-0312). In addition, the diffraction peaks are fairly strong and sharp. These results indicate that the highly crystalline tetragonal Li2ZrO3 containing monoclinic Li6Zr2O7 can be obtained via the biomimetic soft solution route. Based on the Scherrer's formula [12] and the diffraction peak areas, the mean crystallite size and the fraction of Li6Zr2O7 in the powder are evaluated to be approximately 43 nm and 18%, respectively. Fig. 2 shows the TEM image of the sample. In Fig. 2, the size range of Li2ZrO3 nanoparticles is from 80 nm to 100 nm. This size is larger than that calculated using the Scherrer's formula, suggesting that the as-prepared nanoparticles consist of interconnected nanocrystals. Moreover, although there exists aggregation of nanoparticles due to sintering, the individual nanoparticles and the interparticles pores can be still observed, which may be ascribed to the gel network. The Brunauer–Emmett–Teller (BET) measurement indicates that the specific surface area of the Li2ZrO3 nanoparticles is 23.7 m2 g− 1, which is much larger than that of Li2ZrO3 prepared by solid-state synthesis (b1 m2 g− 1) [13]. In order to understand the formation of the Li2ZrO3 nanoparticles, we propose a possible mechanism. The numerous polar groups present in gelatin make hydrogels form, which can, to some extent, act as nanoreactor to template and stabilize nanocrystals. Additionally, the polar groups (–COO− and –NH2) in the gelatin molecules can provide coordination sites. When ZrO2+ and Li+ are introduced into the aqueous solution, the polar groups are combined with ZrO2+ and Li+, and provide the heterogeneous nucleation sites. Herein, the nanocrystal precursor containing Zr and Li elements is formed. The precursor transforms into the Li2ZrO3 nanoparticles containing Li6Zr2O7 when annealed. Because the surface energy of nanocrystals is very high, the transformation from the precursor to Li2ZrO3 can occur at a relatively low temperature. Thus, gelatin plays an important role in the formation of the Li2ZrO3 nanoparticles containing Li6Zr2O7 at low temperature. Because the CO2 produced in power generation is usually in the temperature range from 430 to 630 °C [14], the CO2 capture curve of the sample was measured at 500 °C, as shown in Fig. 3. The Li2ZrO3
Fig. 1. XRD pattern of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 calcinated at 600 °C.
Fig. 2. TEM image of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7.
nanoparticles containing monoclinic Li6Zr2O7 can hold CO2 equivalent to 12 wt.% sample weight, and the saturation is reached about 50 min. According to the results reported by Pannocchia et al. [2], the sorption rate of bulk Li2ZrO3 at 550 °C is very low with a weight increase of 0.1% after 40 min. Veliz-Enriquez et al. [15] also reported that approximately 300 min was required to reach the maximum sorption capacity when the CO2 caption on bulk Li2ZrO3 was performed at 550 °C. And the weight change was only 8.13%. Pfeiffer et al. [5] reported that bulk Li2ZrO3 absorbed only 6.3 wt.% CO2 at 500 °C after 180 min and the saturation was not reached. Therefore, the sorption rate of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 is much faster than that of bulk Li2ZrO3. In addition, the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 show much higher capture capacity than that of bulk Li2ZrO3. This good capture property may be ascribed to three causes: (1) nanosize and large specific surface area of particles; (2) introduction of Li6Zr2O7; (3) effect of crystalline structure. Most of the previous studies for the development of Li2ZrO3 as a CO2 absorbent were performed on the pure or modified monoclinic Li2ZrO3. Recently, Nair et al. [16] reported that tetragonal Li2ZrO3 shows a faster sorption rate in comparison with that of monoclinic Li2ZrO3. Therefore, it can be expected that the tetragonal Li2ZrO3 nanoparticles possess a fast capture rate. Among these causes,
Fig. 3. CO2 capture on the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 at 500 °C.
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S.-Z. Kang et al. / Materials Letters 64 (2010) 1404–1406
we suggest that the nanoscaled size and tetragonal phase are critical for the enhancement of the CO2 capture. 4. Conclusions In conclusion, we have prepared the highly crystalline tetragonal Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 via a gelatin assisted biomimetic soft solution method. Compared with bulk Li2ZrO3, the Li2ZrO3 nanoparticles containing Li6Zr2O7 show significantly improved CO2 capture property, which may be applicable in the field of high temperature CO2 capture. Acknowledgements This work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 20933007) and the Key Discipline Development Program of Shanghai Municipal Education Commission (No. J51503).
References [1] Mosqueda HA, Vazquez C, Bosch P, Pfeiffer H. Chem Mater 2006;18:2307–10. [2] Pannocchia G, Puccini M, Seggiani M, Vitolo S. Ind Eng Chem Res 2007;46: 6696–706. [3] Ochoa-Fernandez E, Ronning M, Grande T, Chen D. Chem Mater 2006;18:1383–5. [4] Ida JI, Lin YS. Environ Sci Technol 2003;37:1999–2004. [5] Pfeiffer H, Bosch P. Chem Mater 2005;17:1704–10. [6] Yi KB, Eriksen DO. Sep Sci Technol 2006;41:283–96. [7] Pfeiffer H, Bosch P, Bulbulian S. Mater Chem Phys 2002;78:558–61. [8] Ochoa-Fernandez E, Ronning M, Yu X, Grande T, Chen D. Ind Eng Chem Res 2008;47:434–42. [9] Tseng YH, Lin HY, Liu MH, Chen YF, Mou CY. J Phys Chem C 2009;113:18053–61. [10] Gao S, Shi Y, Zhang S, Jiang K, Yang S, Li Z, Takayama-Muromachi E. J Phys Chem C 2008;112:10398–401. [11] Kim HG, Hwang DW, Bae SW, Jung JH, Lee JS. Catal Lett 2003;91:193–8. [12] Nutz T, Felde U, Haase M. J Chem Phys 1999;110:12142–50. [13] Xiong R, Ida J, Lin YS. Chem Eng Sci 2003;58:4377–85. [14] Ochoa-Fernandez E, Ronning M, Grande T, Chen D. Chem Mater 2006;18:6037–46. [15] Veliz-Enriquez MY, Gonzalez G, Pfeiffer H. J Solid State Chem 2007;180:2485–92. [16] Nair BN, Yamaguchi T, Kawamura H, Nakao SI. J Am Ceram Soc 2004;87:68–74.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Low temperature biomimetic synthesis of the Li2ZrO3 nanoparticles containing Li6Zr2O7 and high temperature CO2 capture Shi-Zhao Kang a,⁎, Tan Wu b, Xiangqing Li a, Jin Mu b a b
Department of Chemical Engineering, Laboratory of New Energy Materials, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e
i n f o
Article history: Received 19 February 2010 Accepted 19 March 2010 Available online 25 March 2010 Keywords: Lithium zirconate Nanomaterials Low-temperature biomimetic synthesis Gelatin CO2 capture Kinetics
a b s t r a c t Li2ZrO3 nanoparticles containing Li6Zr2O7 were prepared by a biomimetic soft solution route and characterized with X-ray diffraction (XRD), transmission electron microscope (TEM) and nitrogen adsorption. The results show that the tetragonal Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 can be obtained using this simple method. The mean diameter of the nanoparticles is approximately 90 nm and the corresponding specific surface area is 23.7 m2 g− 1. Moreover, the Li2ZrO3 nanoparticles obtained were thermally analyzed under a CO2 flux to evaluate their CO2 capture capacity at high temperature. It was found that the as-prepared Li2ZrO3 nanoparticles would be an effective acceptor for high temperature CO2 capture. © 2010 Elsevier B.V. All rights reserved.
1. Introduction It is known that the conventional CO2 absorbents are unfit for the high temperature CO2 capture [1,2]. The materials with high CO2 capture capacity at high temperature are desirable. Bulk Li2ZrO3 seems to be a promising candidate except the CO2 sorption rate is slow [3]. According to the double-shell model [4], Li2ZrO3 nanoparticles would be a proper CO2 absorbent with improved kinetic property due to the large surface and nano-scaled size. In addition, bulk Li6Zr2O7 can capture 4 times more CO2 than bulk Li2ZrO3 and exhibits faster CO2 sorption rate [5]. Therefore, the capture property of Li2ZrO3 nanoparticles may be improved if Li6Zr2O7 would be introduced in the Li2ZrO3 nanoparticles. The solid-state reaction is the most common synthesis route of Li2ZrO3. However, it usually requires high temperature (850 to 1000 °C) and long reaction time [6]. And it is difficult to obtain small particles because of high temperature sintering. Therefore, several new synthetic methods of Li2ZrO3 have been explored [6–8]. However, the calcination at high temperature is still required and the nano-scaled Li2ZrO3 particles cannot be obtained. Inspired by the nature biomineral process, some functional inorganic nanomaterials were obtained using biopolymer as a directing agent [9]. For example, iron oxide nanoparticles can be obtained using a sodium alginate-assisted route [10]. Additionally, the ⁎ Corresponding author. Department of Chemical Engineering, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China. Tel.: +86 21 64941194; fax: +86 21 64252485. E-mail address: [email protected] (S.-Z. Kang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.03.043
paper reported previously shows that when the metal complex of the polymer is used as a precursor in the solid-state reaction the product can be prepared at a relatively low heat-treatment temperature [11]. Therefore, it is potential to obtain Li2ZrO3 nanoparticles at low temperature if a biopolymer, such as gelatin, is used. Herein we report a biomimetic soft solution route for the formation of the Li2ZrO3 nanoparticles containing Li6Zr2O7 at low temperature. Meanwhile, the CO2 adsorption capacity of the Li2ZrO3 nanoparticles at high temperature is evaluated using thermogravimetric analysis.
2. Experimental In a typical procedure, gelatin (1 g) was added into the CH3COOLi solution (0.15 mol dm− 3, 10 mL) at 50 °C under stirring. Subsequently, the ZrO(NO3)2 solution (0.05 mol dm− 3, 10 mL) was dropped into the solution above. After stirred at 50 °C for 30 min, the solution was rapidly cooled to 4 °C. The gel obtained was divided into small pieces and dried in a vacuum desiccator at room temperature. After the xerogel was heat-treated at 350 °C for 1 h in an electric furnace, the product was directly annealed at 600 °C for 8 h to obtain the Li2ZrO3 nanoparticles. X-ray diffraction (XRD) was carried out with a Rigaku D/Max2550 VB/PC X-ray diffractometer (Japan). The morphology of the Li2ZrO3 nanoparticles was examined on an FEI Tecnai G2 F20 S-Twin field emission transmission electron microscope (TEM, USA). The N2 adsorption and desorption isotherms were measured on a Micromeritics ASAP2020 nitrogen adsorption apparatus (USA). The CO2 capture property of
S.-Z. Kang et al. / Materials Letters 64 (2010) 1404–1406
1405
the Li2ZrO3 nanoparticles was evaluated using thermogravimetric analysis (TGA) (SDT Q600, TA Instruments-Waters LLC, USA) at 500 °C. 3. Results and discussion The XRD pattern of the sample is shown in Fig. 1. It can be observed that there exist several peaks at 22.25, 36.02, 39.95, 42.64, 59.79 and 61.54°, corresponding to the (210), (320), (400), (330), (530) and (521) planes of tetragonal Li2ZrO3, respectively (JCPDS file No. 200647). Meanwhile, the peaks of monoclinic Li6Zr2O7 can also be found in Fig. 1 (JCPDS file No. 34-0312). In addition, the diffraction peaks are fairly strong and sharp. These results indicate that the highly crystalline tetragonal Li2ZrO3 containing monoclinic Li6Zr2O7 can be obtained via the biomimetic soft solution route. Based on the Scherrer's formula [12] and the diffraction peak areas, the mean crystallite size and the fraction of Li6Zr2O7 in the powder are evaluated to be approximately 43 nm and 18%, respectively. Fig. 2 shows the TEM image of the sample. In Fig. 2, the size range of Li2ZrO3 nanoparticles is from 80 nm to 100 nm. This size is larger than that calculated using the Scherrer's formula, suggesting that the as-prepared nanoparticles consist of interconnected nanocrystals. Moreover, although there exists aggregation of nanoparticles due to sintering, the individual nanoparticles and the interparticles pores can be still observed, which may be ascribed to the gel network. The Brunauer–Emmett–Teller (BET) measurement indicates that the specific surface area of the Li2ZrO3 nanoparticles is 23.7 m2 g− 1, which is much larger than that of Li2ZrO3 prepared by solid-state synthesis (b1 m2 g− 1) [13]. In order to understand the formation of the Li2ZrO3 nanoparticles, we propose a possible mechanism. The numerous polar groups present in gelatin make hydrogels form, which can, to some extent, act as nanoreactor to template and stabilize nanocrystals. Additionally, the polar groups (–COO− and –NH2) in the gelatin molecules can provide coordination sites. When ZrO2+ and Li+ are introduced into the aqueous solution, the polar groups are combined with ZrO2+ and Li+, and provide the heterogeneous nucleation sites. Herein, the nanocrystal precursor containing Zr and Li elements is formed. The precursor transforms into the Li2ZrO3 nanoparticles containing Li6Zr2O7 when annealed. Because the surface energy of nanocrystals is very high, the transformation from the precursor to Li2ZrO3 can occur at a relatively low temperature. Thus, gelatin plays an important role in the formation of the Li2ZrO3 nanoparticles containing Li6Zr2O7 at low temperature. Because the CO2 produced in power generation is usually in the temperature range from 430 to 630 °C [14], the CO2 capture curve of the sample was measured at 500 °C, as shown in Fig. 3. The Li2ZrO3
Fig. 1. XRD pattern of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 calcinated at 600 °C.
Fig. 2. TEM image of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7.
nanoparticles containing monoclinic Li6Zr2O7 can hold CO2 equivalent to 12 wt.% sample weight, and the saturation is reached about 50 min. According to the results reported by Pannocchia et al. [2], the sorption rate of bulk Li2ZrO3 at 550 °C is very low with a weight increase of 0.1% after 40 min. Veliz-Enriquez et al. [15] also reported that approximately 300 min was required to reach the maximum sorption capacity when the CO2 caption on bulk Li2ZrO3 was performed at 550 °C. And the weight change was only 8.13%. Pfeiffer et al. [5] reported that bulk Li2ZrO3 absorbed only 6.3 wt.% CO2 at 500 °C after 180 min and the saturation was not reached. Therefore, the sorption rate of the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 is much faster than that of bulk Li2ZrO3. In addition, the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 show much higher capture capacity than that of bulk Li2ZrO3. This good capture property may be ascribed to three causes: (1) nanosize and large specific surface area of particles; (2) introduction of Li6Zr2O7; (3) effect of crystalline structure. Most of the previous studies for the development of Li2ZrO3 as a CO2 absorbent were performed on the pure or modified monoclinic Li2ZrO3. Recently, Nair et al. [16] reported that tetragonal Li2ZrO3 shows a faster sorption rate in comparison with that of monoclinic Li2ZrO3. Therefore, it can be expected that the tetragonal Li2ZrO3 nanoparticles possess a fast capture rate. Among these causes,
Fig. 3. CO2 capture on the Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 at 500 °C.
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S.-Z. Kang et al. / Materials Letters 64 (2010) 1404–1406
we suggest that the nanoscaled size and tetragonal phase are critical for the enhancement of the CO2 capture. 4. Conclusions In conclusion, we have prepared the highly crystalline tetragonal Li2ZrO3 nanoparticles containing monoclinic Li6Zr2O7 via a gelatin assisted biomimetic soft solution method. Compared with bulk Li2ZrO3, the Li2ZrO3 nanoparticles containing Li6Zr2O7 show significantly improved CO2 capture property, which may be applicable in the field of high temperature CO2 capture. Acknowledgements This work was financially supported by the Key Project of the National Natural Science Foundation of China (No. 20933007) and the Key Discipline Development Program of Shanghai Municipal Education Commission (No. J51503).
References [1] Mosqueda HA, Vazquez C, Bosch P, Pfeiffer H. Chem Mater 2006;18:2307–10. [2] Pannocchia G, Puccini M, Seggiani M, Vitolo S. Ind Eng Chem Res 2007;46: 6696–706. [3] Ochoa-Fernandez E, Ronning M, Grande T, Chen D. Chem Mater 2006;18:1383–5. [4] Ida JI, Lin YS. Environ Sci Technol 2003;37:1999–2004. [5] Pfeiffer H, Bosch P. Chem Mater 2005;17:1704–10. [6] Yi KB, Eriksen DO. Sep Sci Technol 2006;41:283–96. [7] Pfeiffer H, Bosch P, Bulbulian S. Mater Chem Phys 2002;78:558–61. [8] Ochoa-Fernandez E, Ronning M, Yu X, Grande T, Chen D. Ind Eng Chem Res 2008;47:434–42. [9] Tseng YH, Lin HY, Liu MH, Chen YF, Mou CY. J Phys Chem C 2009;113:18053–61. [10] Gao S, Shi Y, Zhang S, Jiang K, Yang S, Li Z, Takayama-Muromachi E. J Phys Chem C 2008;112:10398–401. [11] Kim HG, Hwang DW, Bae SW, Jung JH, Lee JS. Catal Lett 2003;91:193–8. [12] Nutz T, Felde U, Haase M. J Chem Phys 1999;110:12142–50. [13] Xiong R, Ida J, Lin YS. Chem Eng Sci 2003;58:4377–85. [14] Ochoa-Fernandez E, Ronning M, Grande T, Chen D. Chem Mater 2006;18:6037–46. [15] Veliz-Enriquez MY, Gonzalez G, Pfeiffer H. J Solid State Chem 2007;180:2485–92. [16] Nair BN, Yamaguchi T, Kawamura H, Nakao SI. J Am Ceram Soc 2004;87:68–74.