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Phase transformation of zirconia in sulfated zirconia–LiCl system
Materials Letters 61 (2007) 942 – 943 www.elsevier.com/locate/matlet
Phase transformation of zirconia in sulfated zirconia–LiCl system Yubao Zhao a,b,⁎, Xiaohong Yuan a , Chuanxiang Liu b , Chuanjing Huang a , Huilin Wan a,⁎ a
State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China b College of Chemistry and Chemical Engineering, Nanhua University, Hengyang 421001, Hunan province, China Received 2 February 2006; accepted 8 June 2006 Available online 30 June 2006
Abstract The phase transformation from tetragonal to monoclinic of zirconia for a series of sulfated zirconia–LiCl samples was characterized by ultraviolet Raman spectroscopy (UVRS) and X-ray diffraction (XRD). Results show that such phase transformation increases with increasing LiCl content and the monoclinic phase is first detected by XRD than that by UVRS for the sample with Li content of 0.5 wt.%. The comparison between XRD and UVRS characterizations indicates that the phase transition takes place initially at the core. © 2006 Elsevier B.V. All rights reserved. Keywords: Zirconia; Phase transformation; Raman spectroscopy; X-ray diffraction
1. Introduction The amphoteric and other expected properties make zirconia a kind of promising material widely used as ceramic, engineering gemstone, coating, gas sensor, catalyst and catalyst support, etc. Researches have shown that zirconia phases significantly influence the acidic features [1], COx adsorption [2] and the catalytic performance [3–8] of zirconia-based catalysts. For pure zirconia, Raman spectroscopy and XRD characterizations have indicated the phase transition from tetragonal to monoclinic takes place initially at the surface regions [9]. This paper reports the new finding of such phase transformation for sulfated zirconia–LiCl (LSZ) catalysts. Several years ago, it was reported while sulfated zirconia (SZ) shows poor selectivity to ethene, LSZ catalyst possesses high performance for oxidative dehydrogenation of ethane to ethene [10]: 82% yield of ethene can be achieved for the best LSZ catalyst [11]. During the course of our exploring the zirconia phase effect on the catalytic performance of LSZ catalysts, the phase transformation of zirconia from tetragonal to monoclinic
was shown to commence initially from the core, which is the counterexample of the previous report [9]. 2. Experimental 2.1. Sample preparation All reactants including ammonia, ZrOCl2·8H2O, (NH4)2 SO4, LiCl·H2O, were of analytical grade. The preparation of sulfated zirconia was the same as reported previously [6]. Fresh hydrous zirconia precipitate from ZrOCl2·8H2O and ammonia solution at the pH value of 9.3 was refluxed in a roundbottomed Pyrex flask for 50 h. After being washed thoroughly with distilled water till free of chlorine and then with ethanol for three times, separated samples were moderately dried at 353 K for 10 h. Then the dried hydrous zirconia was impregnated with a (NH4)2SO4 solution to give the sulfate content of 6 wt.%, followed by rotary evaporation at 313 K and calcination at 973 K for 3 h in static air. The resultant sulfated zirconia supports were impregnated with LiCl solution at the required concentration by the same method as described above. 2.2. Characterization
⁎ Corresponding authors. Tel.: +86 734 8282735; fax: +86 734 8282375. E-mail addresses: [email protected] (Y. Zhao), [email protected] (H. Wan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.018
XRD measurements were performed by a Rigaku D/Max-C X-ray diffractometer equipped with a Cu target and a Ni filter at
Y. Zhao et al. / Materials Letters 61 (2007) 942–943
943
30 kV/40 mA at room temperature. UVRS characterizations were carried out by a Renishaw UV–Vis 1000 confocal microprobe Raman system equipped with a laser at 325-nm line. Its lower cutoff frequency is about 200 cm− 1 and the laser spot size is about 1–2 μm. 3. Results and discussion Fig. 1 shows the XRD patterns of LSZ samples with Li content varied from 0 to 5 wt.%. There are no diffraction lines other than those of zirconia. Letters of “t” and “m” in the figure denote the tetragonal and monoclinic phases, respectively. After being calcined at 973 K, only the diffraction peaks corresponding to tetragonal phase (JCPDS No. 17-923) appear in the XRD pattern of SZ [6]. This indicates SZ support itself is purely tetragonal. Introducing 0.5 wt.% Li to SZ gives birth to the peaks belonging to monoclinic phase (JCPDS No. 371484). Furthermore, monoclinic fraction increases with increasing Li loading in LSZ samples. Quantitatively phase analysis, based on the – intensities of (111) plane for t-ZrO2, (111) and ( 111) planes for m-ZrO2 [12], shows that the volume fractions of phase transformation from tetragonal to the monoclinic are 22% (about 1/5 in volume) for 0.5 wt. % Li loading and 64% for 5 wt.% Li loading respectively. It has been shown that strong electronic absorption of ZrO2 in the UV region makes UVRS more sensitive at the surface region than XRD [9]. Fig. 2 shows the Raman spectra of samples with different LiCl contents. It can be seen in the Raman spectrum of SZ that only the bands for t-ZrO2 at 648, 465, 318, and 276 cm− 1 (ca 5 cm− 1 higher than that reported in Refs. [9] and [13] for pure zirconia) appear, which means that the SZ support itself is purely tetragonal (the band at 149 cm− 1 characteristic of t-ZrO2, lower than the cutoff frequency of the preset Raman spectrometer, cannot be seen here). It is just the same result as what is obtained from XRD. With the Li content larger than 1 wt.%, Raman bands at 339, 353, 380, 476,508, 544, 560, 622, and 643 cm− 1 appear and increase significantly with further increasing Li loading. In the Raman spectra of LSZ containing 0.5–1.0 wt.% Li, the fact that there hardly exists bands at 380 and 560 cm− 1 due to m-ZrO2 [13] reveals the existence of few monoclinic zirconia in their surface regions. From the comparison between Figs. 1 and 2, it can be seen that there are distinct differences in the results from UVRS and XRD. For low LiCl-containing samples, while the outcome of XRD shows that no less than 1/5 in volume of t-ZrO2 transformed to m-ZrO2, the result of UVRS indicates that such m-ZrO2 is almost invisible. Considering the
Fig. 2. UV Raman spectra of LiCl–SO2− 4 ZrO2 samples with various Li contents in x wt.%.
fact that UVRS is more sensitive to the surface than XRD, it is suggested that the LiCl loading makes the phase transformation from tetragonal to monoclinic for zirconia develops initially from the center. Thus the m-ZrO2 in the core of LSZ with a few LiCl transformed from t-ZrO2 could be unobservable by the bulk-insensitive UVRS characterization.
4. Conclusion Tetragonal zirconia begins to change into monoclinic zirconia with the loading of LiCl to sulfated zirconia (SZ). The results of UVRS and XRD suggest that the phase transition of ZrO2 from tetragonal to monoclinic takes place initially at the core of LiCl/SZ. Acknowledgements The authors gratefully acknowledged the support of the Ministry of Science and Technology of China (2005C B221408), “Key Scientific Project of Fujian Province, China” (2005HZ01–3), and of the National Natural Science Foundation of China (20433030, 20021002 and 20423002). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Fig. 1. XRD patterns of LiCl–SO2− 4 ZrO2 samples with various Li contents in x wt.%.
[13]
Y. Zhao, W. Li, M. Zhang, K. Tao, Catal. Commun. 3 (2002) 239. K. Pokrovski, K.T. Jung, A.T. Bell, Langmuir 17 (2001) 4297. W. Li, Y. Yin, R. Gao, R. Hou, J. Mol. Catal. (China) 13 (1997) 186. J.C. Yori, J.M. Parera, Catal. Lett. 65 (2000) 205. W. Stichert, F. Schüth, S. Kuba, H. Knözinger, J. Catal. 198 (2001) 277. Y. Zhao, Y. Zeng, K. Tao, Chin. J. Catal. 23 (2002) 168. K.T. Jung, A.T. Bell, Catal. Lett. 80 (2002) 63. Y. Zhao, W. Li, M. Zhang, K. Tao, Acta Pet. Sin., Pet. Process. Sect. 18 (2002) 21. M. Li, Z. Feng, G. Xiong, P. Ying, Q. Xin, C. Li, J. Phys. Chem., B 105 (2001) 8107. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa, K. Suzuki, Chem. Commun. (1999) 103. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa, K. Suzuki, Catal. Lett. 62 (1999) 191. P.D.L. Mercera, J.G. van Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. L. Shi, K. Tin, N. Wong, J. Mater. Sci. 34 (1999) 3367.
Phase transformation of zirconia in sulfated zirconia–LiCl system Yubao Zhao a,b,⁎, Xiaohong Yuan a , Chuanxiang Liu b , Chuanjing Huang a , Huilin Wan a,⁎ a
State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry and Institute of Physical Chemistry, Xiamen University, Xiamen 361005, China b College of Chemistry and Chemical Engineering, Nanhua University, Hengyang 421001, Hunan province, China Received 2 February 2006; accepted 8 June 2006 Available online 30 June 2006
Abstract The phase transformation from tetragonal to monoclinic of zirconia for a series of sulfated zirconia–LiCl samples was characterized by ultraviolet Raman spectroscopy (UVRS) and X-ray diffraction (XRD). Results show that such phase transformation increases with increasing LiCl content and the monoclinic phase is first detected by XRD than that by UVRS for the sample with Li content of 0.5 wt.%. The comparison between XRD and UVRS characterizations indicates that the phase transition takes place initially at the core. © 2006 Elsevier B.V. All rights reserved. Keywords: Zirconia; Phase transformation; Raman spectroscopy; X-ray diffraction
1. Introduction The amphoteric and other expected properties make zirconia a kind of promising material widely used as ceramic, engineering gemstone, coating, gas sensor, catalyst and catalyst support, etc. Researches have shown that zirconia phases significantly influence the acidic features [1], COx adsorption [2] and the catalytic performance [3–8] of zirconia-based catalysts. For pure zirconia, Raman spectroscopy and XRD characterizations have indicated the phase transition from tetragonal to monoclinic takes place initially at the surface regions [9]. This paper reports the new finding of such phase transformation for sulfated zirconia–LiCl (LSZ) catalysts. Several years ago, it was reported while sulfated zirconia (SZ) shows poor selectivity to ethene, LSZ catalyst possesses high performance for oxidative dehydrogenation of ethane to ethene [10]: 82% yield of ethene can be achieved for the best LSZ catalyst [11]. During the course of our exploring the zirconia phase effect on the catalytic performance of LSZ catalysts, the phase transformation of zirconia from tetragonal to monoclinic
was shown to commence initially from the core, which is the counterexample of the previous report [9]. 2. Experimental 2.1. Sample preparation All reactants including ammonia, ZrOCl2·8H2O, (NH4)2 SO4, LiCl·H2O, were of analytical grade. The preparation of sulfated zirconia was the same as reported previously [6]. Fresh hydrous zirconia precipitate from ZrOCl2·8H2O and ammonia solution at the pH value of 9.3 was refluxed in a roundbottomed Pyrex flask for 50 h. After being washed thoroughly with distilled water till free of chlorine and then with ethanol for three times, separated samples were moderately dried at 353 K for 10 h. Then the dried hydrous zirconia was impregnated with a (NH4)2SO4 solution to give the sulfate content of 6 wt.%, followed by rotary evaporation at 313 K and calcination at 973 K for 3 h in static air. The resultant sulfated zirconia supports were impregnated with LiCl solution at the required concentration by the same method as described above. 2.2. Characterization
⁎ Corresponding authors. Tel.: +86 734 8282735; fax: +86 734 8282375. E-mail addresses: [email protected] (Y. Zhao), [email protected] (H. Wan). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.018
XRD measurements were performed by a Rigaku D/Max-C X-ray diffractometer equipped with a Cu target and a Ni filter at
Y. Zhao et al. / Materials Letters 61 (2007) 942–943
943
30 kV/40 mA at room temperature. UVRS characterizations were carried out by a Renishaw UV–Vis 1000 confocal microprobe Raman system equipped with a laser at 325-nm line. Its lower cutoff frequency is about 200 cm− 1 and the laser spot size is about 1–2 μm. 3. Results and discussion Fig. 1 shows the XRD patterns of LSZ samples with Li content varied from 0 to 5 wt.%. There are no diffraction lines other than those of zirconia. Letters of “t” and “m” in the figure denote the tetragonal and monoclinic phases, respectively. After being calcined at 973 K, only the diffraction peaks corresponding to tetragonal phase (JCPDS No. 17-923) appear in the XRD pattern of SZ [6]. This indicates SZ support itself is purely tetragonal. Introducing 0.5 wt.% Li to SZ gives birth to the peaks belonging to monoclinic phase (JCPDS No. 371484). Furthermore, monoclinic fraction increases with increasing Li loading in LSZ samples. Quantitatively phase analysis, based on the – intensities of (111) plane for t-ZrO2, (111) and ( 111) planes for m-ZrO2 [12], shows that the volume fractions of phase transformation from tetragonal to the monoclinic are 22% (about 1/5 in volume) for 0.5 wt. % Li loading and 64% for 5 wt.% Li loading respectively. It has been shown that strong electronic absorption of ZrO2 in the UV region makes UVRS more sensitive at the surface region than XRD [9]. Fig. 2 shows the Raman spectra of samples with different LiCl contents. It can be seen in the Raman spectrum of SZ that only the bands for t-ZrO2 at 648, 465, 318, and 276 cm− 1 (ca 5 cm− 1 higher than that reported in Refs. [9] and [13] for pure zirconia) appear, which means that the SZ support itself is purely tetragonal (the band at 149 cm− 1 characteristic of t-ZrO2, lower than the cutoff frequency of the preset Raman spectrometer, cannot be seen here). It is just the same result as what is obtained from XRD. With the Li content larger than 1 wt.%, Raman bands at 339, 353, 380, 476,508, 544, 560, 622, and 643 cm− 1 appear and increase significantly with further increasing Li loading. In the Raman spectra of LSZ containing 0.5–1.0 wt.% Li, the fact that there hardly exists bands at 380 and 560 cm− 1 due to m-ZrO2 [13] reveals the existence of few monoclinic zirconia in their surface regions. From the comparison between Figs. 1 and 2, it can be seen that there are distinct differences in the results from UVRS and XRD. For low LiCl-containing samples, while the outcome of XRD shows that no less than 1/5 in volume of t-ZrO2 transformed to m-ZrO2, the result of UVRS indicates that such m-ZrO2 is almost invisible. Considering the
Fig. 2. UV Raman spectra of LiCl–SO2− 4 ZrO2 samples with various Li contents in x wt.%.
fact that UVRS is more sensitive to the surface than XRD, it is suggested that the LiCl loading makes the phase transformation from tetragonal to monoclinic for zirconia develops initially from the center. Thus the m-ZrO2 in the core of LSZ with a few LiCl transformed from t-ZrO2 could be unobservable by the bulk-insensitive UVRS characterization.
4. Conclusion Tetragonal zirconia begins to change into monoclinic zirconia with the loading of LiCl to sulfated zirconia (SZ). The results of UVRS and XRD suggest that the phase transition of ZrO2 from tetragonal to monoclinic takes place initially at the core of LiCl/SZ. Acknowledgements The authors gratefully acknowledged the support of the Ministry of Science and Technology of China (2005C B221408), “Key Scientific Project of Fujian Province, China” (2005HZ01–3), and of the National Natural Science Foundation of China (20433030, 20021002 and 20423002). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Fig. 1. XRD patterns of LiCl–SO2− 4 ZrO2 samples with various Li contents in x wt.%.
[13]
Y. Zhao, W. Li, M. Zhang, K. Tao, Catal. Commun. 3 (2002) 239. K. Pokrovski, K.T. Jung, A.T. Bell, Langmuir 17 (2001) 4297. W. Li, Y. Yin, R. Gao, R. Hou, J. Mol. Catal. (China) 13 (1997) 186. J.C. Yori, J.M. Parera, Catal. Lett. 65 (2000) 205. W. Stichert, F. Schüth, S. Kuba, H. Knözinger, J. Catal. 198 (2001) 277. Y. Zhao, Y. Zeng, K. Tao, Chin. J. Catal. 23 (2002) 168. K.T. Jung, A.T. Bell, Catal. Lett. 80 (2002) 63. Y. Zhao, W. Li, M. Zhang, K. Tao, Acta Pet. Sin., Pet. Process. Sect. 18 (2002) 21. M. Li, Z. Feng, G. Xiong, P. Ying, Q. Xin, C. Li, J. Phys. Chem., B 105 (2001) 8107. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa, K. Suzuki, Chem. Commun. (1999) 103. S. Wang, K. Murata, T. Hayakawa, S. Hamakawa, K. Suzuki, Catal. Lett. 62 (1999) 191. P.D.L. Mercera, J.G. van Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. L. Shi, K. Tin, N. Wong, J. Mater. Sci. 34 (1999) 3367.