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Preparation and conductivity of ternary undecatungstoferroaluminic heteropoly acid
Materials Science and Engineering B96 (2002) 29 /32 www.elsevier.com/locate/mseb
Preparation and conductivity of ternary undecatungstoferroaluminic heteropoly acid Qingyin Wu *, Xufeng Xie Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Received 12 June 2001; received in revised form 15 October 2001; accepted 25 June 2002
Abstract The ternary undecatungstoferroaluminic heteropoly acid H6[Al(H2O)FeW11O39] ×/14H2O has been prepared by the ion exchangecooling. The optimal proportion of solutions of the component elements and the pH of the synthesis reaction are given. The product is characterized by chemical analysis, potentiometric titration, IR, UV, XRD and TG-DTA. The results of AC impedance measurement show that its proton conductivity is 4.07 /10 4 S cm 1 at room temperature (18 8C). # 2002 Elsevier Science B.V. All rights reserved. Keywords: Heteropoly acid; Infrared spectroscopy; X-ray diffraction; Electrical conductivity
1. Introduction Heteropoly acids (HPA) have attracted increasing interest because of their applications, as catalysts for organic reactions, corrosion resistant coatings, dopants in sol /gel matrixes, membranes in selective electrodes, in gas detection apparatuses, in solid-state electrochromic devices, and in liquid and solid electrolytic cells [1 / 6]. There are two kinds of protons in the HPA crystals. One is the dissociated, hydrated proton that is combined with the HPA anion; the other is the unhydrated proton that is located on the bridging oxygen in the HPA anion. Because the dissociated protons have good mobility, the HPA crystals have some characteristics of a ‘pseudo liquid phase’. They are, in effect, superionic protonic conductors and are promising solid electrolytes. This paper reports on the preparation and characterization, as well as, the protonic conductivity of the ternary undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39]×/14H2O.
* Corresponding author E-mail address: [email protected] (Q. Wu).
2. Experimental 2.1. Instrument and reagents An IR spectrum was recorded on a Perkin-Elmer 1730 FT/IR spectrophotometer using KBr pellets. An UV spectrum was measured on a Hitachi U-3400 UV spectrophotometer. Thermal analysis was carried out on a TAS-100 thermal analyzer with a rate of temperature increase of 10 8C min 1. Impedance measurements were performed on a M378 electrochemical impedance analyzer with copper electrodes over the frequency range from 0.01 Hz to 99.9 kHz. A Mettler DL-21 titrimeter was also used to titration HPA. The purity of Cu is more than 99.8%. All other reagents are analysis grade. 2.2. Preparation of H6[Al(H2O)FeW11O39] ×/14H2O 36.3 g (0.11 mol) of Na2WO4 ×/2H2O were dissolved in 200 ml of water and the pH of the solution was adjusted to 6.3 with acetic acid. The solution was then heated to boiling and a solution of 4.1 g (0.01 mol) of Fe(NO3)3 9H2O in 30 ml of hot water was added dropwise with stirring. After 30 min, a solution of 3.8 g (0.01 mol) Al (NO3)2 ×/9H2O in 50 ml of water was added. The pH was readjusted to 5.0 and stirring was continued for 1.5 h.
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 2 6 - 4
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Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
After cooling, an orange /yellow oil was obtained by adding absolute alcohol. The oily product was extracted by successively dissolving the oil in 80 ml water and passing through an ion exchange column a total of three times. An Amberlite IR-120 cation-exchange column in H -form was used. When the effluent pH B/1, the solid HPA has been separated out by the cooling method. The final yield was about 64%. 2.3. Elemental analysis The water content was determined by thermogravimetry; aluminum was analyzed by EDTA back-titrimetry using xylenol orange as an indicator and Zn2 as a back-titrant; iron by EDTA volumetric titration with sulfo-salicylic acid as an indicator; and tungsten by 8hydroxyquinoline-tannic acid-methyl violet gravimetry. Found: Al, 0.88; Fe, 1.83; W, 67.39; H2O, 8.94. Anal. Calc. for H6[Al(H2O)FeW11O39]×/14 H2O: Al, 0.90; Fe, 1.86; W, 67.30; H2O, 8.99. 2.4. Measurement of protonic conductivity At room temperature (18 8C), H6[Al(H2O)FeW11 O39]×/14H2O was pressed into a tablet of 15-mm diameter and 4.14-mm thickness under a pressure of 20 MPa. Two copper sheets were attached to the two faces and the proton conductivity was measured.
Fig. 1. The potentiometric titration curve for H6[Al(H2O)FeW11O39] 14H2O.
3. Results and discussion 3.1. Determination of basicity The number of hydrogen in the HPA can be determined by potentiometric titration experiment. The potentiometric titration shows that its six protons are dissociated in two steps. First, three protons are dissociated; second, the three others (Fig. 1). 3.2. IR and UV spectra There are four kinds of oxygen atoms in M(H2O)XW11O39n , four X /O(a) in which oxygen atom connects with heteroatom (X), twelve W /O(b) / W oxygen-bridges (corner-sharing oxygen-bridge between different W3O13 sets), twelve W/O(c) /W oxygen-bridges (edge-sharing oxygen-bridge within W3O13 sets) and eleven W /O(d) terminal oxygen atoms [7]. In the IR spectrum (Fig. 2), there are six characteristic bands: 962 cm 1, n as[W /O(d)]; 893 cm 1, n as[W / O(b) /W]; 784 and 714 cm 1, n as[W /O(c) /W]; 474 and 440 cm 1, n as[Fe /O(a)] or d (O /Fe /O) [8]. In Keggin structure, intense absorption bands at 200 and 260 nm are caused by charge-transfer of the terminal oxygen and bridge-oxygen to metal atoms respectively. In the
Fig. 2. IR spectrum for H6[Al(H2O)FeW11O39] 14H2O.
UV spectrum (Fig. 3), there are two characteristic bands: 201.8 nm, O(d) 0/W; 258.5 nm, O(b)/O(c) 0/W. 3.3. X-ray powder diffraction X-ray powder diffraction is widely used to study the structural features of HPA and explain their properties [9]. The data of X-ray powder diffraction are listed in Table 1. In each of the four ranges of 2u that are 7 /108, 16/ 228, 25 /308 and 33/388, there is a characteristic peak of
Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
31
Fig. 4. Schematic diagram of the triheteropolyacid structure. The Al(III) ion which in the upper left/ hand corner is depicted as a darker sphere with a terminally coordinated H2O molecule. The Fe(III) heteroatom resides in a central pseudo-tetrahedral hole of the structure; it is the dark central atom. Fig. 3. UV spectrum for H6[Al(H2O)FeW11O39] 14H2O.
HPA anions that have Keggin structure. Combined with IR and UV spectra, it is sure that H6[Al(H2O)FeW11 O39]×/14 ×/H2O possesses Keggin structure (Fig. 4). 3.4. TG-DTA analysis In general, we take the temperature of the exothermic peak of DTA curves as the sign of their thermostability [10]. Fig. 5 is the thermogram of H6[Al(H2O)FeW11 O39]×/14 ×/H2O. The TG curve shows that there are three steps of weight loss. The first is the loss of hydration water, the second is the loss of protonized water and the third is the loss of structural water. In the DTA curve, there is an exothermic peak at 492.8 8C. The thermostability of the HPA is higher than that of H5FeW12O40 (390 8C).
Fig. 5. The thermogram for H6[Al(H2O)FeW11O39]×/14H2O.
14H2O is a new solid high-proton conductor with a proton conductivity of 4.07 /10 4 S cm 1 at 18 8C.
3.5. Conductibility Conductivity is an important parameter. We have recorded the results of the complex impedance measurement of the HPA (the frequency ranges from 0.01 to 9.99 /104 Hz) at room temperature (Fig. 6). We can calculate the conductivity from these results. The calculation shows that ternary heteropolyacid undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39]×/
4. Conclusions A new solid high-proton conductor, the ternary heteropolyacid undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39] ×/14 ×/H2O was prepared and characterized. The results of AC impedance measurement
Table 1 Data of X-ray powder diffraction of H0[Al(H2O)FeW11O39]× 14H2O 2u (8) d (nm) I/I0 2u (8) d (nm) I/I0
8.18 1.080 100 24.82 0.358 26
9.26 0.954 58 27.30 0.327 29
9.62 0.919 40 28.78 0.310 88
10.08 0.877 21 29.96 0.298 22
16.48 0.537 20 33.36 0.268 34
17.36 0.510 26 34.42 0.260 45
18.42 0.481 64 35.86 0.250 42
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Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
Acknowledgements The financial support from the Science Foundation of Zhejiang University and China Postdoctoral Science Foundation for this work is greatly appreciated.
References
Fig. 6. Complex impedance spectrum of H6[Al(H2O)FeW11O39]×/ 14H2O at 18 8C.
show that its proton conductivity is 4.07 /10 4S cm 1 at room temperature (18 8C).
[1] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [2] S.V. Lomakina, T.S. Shatova, L.P. Kazansky, Corros. Sci. 36 (1994) 1645. [3] M. Tasumisago, H. Honjo, Y. Sakai, T. Minami, Solid State Ionics 74 (1994) 105. [4] H.C. Sheu, J.S. Shih, Anal. Chim. Acta 324 (1996) 125. [5] S.J. Babinec, Proc. Electrochem. Soc. 94 (1994) 30. [6] N. Giordano, P. Staiti, A.S. Arico, E. Passalacqua, L. Abate, S. Hocevar, Electrochim. Acta 42 (1997) 1645. [7] Q.Y. Wu, Y.C. Zhai, Chin. J. Nonfer. Met. 8 (1998) 327. [8] Q.Y. Wu, E.B. Wang, Zh. Neorgan. Khim. 38 (1993) 1557. [9] Q.Y. Wu, Y.C. Zhai, Rare Metals 15 (1996) 257. [10] Q.Y. Wu, E.B. Wang, J.F. Liu, Polyhedron 12 (1993) 2563.
Preparation and conductivity of ternary undecatungstoferroaluminic heteropoly acid Qingyin Wu *, Xufeng Xie Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Received 12 June 2001; received in revised form 15 October 2001; accepted 25 June 2002
Abstract The ternary undecatungstoferroaluminic heteropoly acid H6[Al(H2O)FeW11O39] ×/14H2O has been prepared by the ion exchangecooling. The optimal proportion of solutions of the component elements and the pH of the synthesis reaction are given. The product is characterized by chemical analysis, potentiometric titration, IR, UV, XRD and TG-DTA. The results of AC impedance measurement show that its proton conductivity is 4.07 /10 4 S cm 1 at room temperature (18 8C). # 2002 Elsevier Science B.V. All rights reserved. Keywords: Heteropoly acid; Infrared spectroscopy; X-ray diffraction; Electrical conductivity
1. Introduction Heteropoly acids (HPA) have attracted increasing interest because of their applications, as catalysts for organic reactions, corrosion resistant coatings, dopants in sol /gel matrixes, membranes in selective electrodes, in gas detection apparatuses, in solid-state electrochromic devices, and in liquid and solid electrolytic cells [1 / 6]. There are two kinds of protons in the HPA crystals. One is the dissociated, hydrated proton that is combined with the HPA anion; the other is the unhydrated proton that is located on the bridging oxygen in the HPA anion. Because the dissociated protons have good mobility, the HPA crystals have some characteristics of a ‘pseudo liquid phase’. They are, in effect, superionic protonic conductors and are promising solid electrolytes. This paper reports on the preparation and characterization, as well as, the protonic conductivity of the ternary undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39]×/14H2O.
* Corresponding author E-mail address: [email protected] (Q. Wu).
2. Experimental 2.1. Instrument and reagents An IR spectrum was recorded on a Perkin-Elmer 1730 FT/IR spectrophotometer using KBr pellets. An UV spectrum was measured on a Hitachi U-3400 UV spectrophotometer. Thermal analysis was carried out on a TAS-100 thermal analyzer with a rate of temperature increase of 10 8C min 1. Impedance measurements were performed on a M378 electrochemical impedance analyzer with copper electrodes over the frequency range from 0.01 Hz to 99.9 kHz. A Mettler DL-21 titrimeter was also used to titration HPA. The purity of Cu is more than 99.8%. All other reagents are analysis grade. 2.2. Preparation of H6[Al(H2O)FeW11O39] ×/14H2O 36.3 g (0.11 mol) of Na2WO4 ×/2H2O were dissolved in 200 ml of water and the pH of the solution was adjusted to 6.3 with acetic acid. The solution was then heated to boiling and a solution of 4.1 g (0.01 mol) of Fe(NO3)3 9H2O in 30 ml of hot water was added dropwise with stirring. After 30 min, a solution of 3.8 g (0.01 mol) Al (NO3)2 ×/9H2O in 50 ml of water was added. The pH was readjusted to 5.0 and stirring was continued for 1.5 h.
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 3 2 6 - 4
30
Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
After cooling, an orange /yellow oil was obtained by adding absolute alcohol. The oily product was extracted by successively dissolving the oil in 80 ml water and passing through an ion exchange column a total of three times. An Amberlite IR-120 cation-exchange column in H -form was used. When the effluent pH B/1, the solid HPA has been separated out by the cooling method. The final yield was about 64%. 2.3. Elemental analysis The water content was determined by thermogravimetry; aluminum was analyzed by EDTA back-titrimetry using xylenol orange as an indicator and Zn2 as a back-titrant; iron by EDTA volumetric titration with sulfo-salicylic acid as an indicator; and tungsten by 8hydroxyquinoline-tannic acid-methyl violet gravimetry. Found: Al, 0.88; Fe, 1.83; W, 67.39; H2O, 8.94. Anal. Calc. for H6[Al(H2O)FeW11O39]×/14 H2O: Al, 0.90; Fe, 1.86; W, 67.30; H2O, 8.99. 2.4. Measurement of protonic conductivity At room temperature (18 8C), H6[Al(H2O)FeW11 O39]×/14H2O was pressed into a tablet of 15-mm diameter and 4.14-mm thickness under a pressure of 20 MPa. Two copper sheets were attached to the two faces and the proton conductivity was measured.
Fig. 1. The potentiometric titration curve for H6[Al(H2O)FeW11O39] 14H2O.
3. Results and discussion 3.1. Determination of basicity The number of hydrogen in the HPA can be determined by potentiometric titration experiment. The potentiometric titration shows that its six protons are dissociated in two steps. First, three protons are dissociated; second, the three others (Fig. 1). 3.2. IR and UV spectra There are four kinds of oxygen atoms in M(H2O)XW11O39n , four X /O(a) in which oxygen atom connects with heteroatom (X), twelve W /O(b) / W oxygen-bridges (corner-sharing oxygen-bridge between different W3O13 sets), twelve W/O(c) /W oxygen-bridges (edge-sharing oxygen-bridge within W3O13 sets) and eleven W /O(d) terminal oxygen atoms [7]. In the IR spectrum (Fig. 2), there are six characteristic bands: 962 cm 1, n as[W /O(d)]; 893 cm 1, n as[W / O(b) /W]; 784 and 714 cm 1, n as[W /O(c) /W]; 474 and 440 cm 1, n as[Fe /O(a)] or d (O /Fe /O) [8]. In Keggin structure, intense absorption bands at 200 and 260 nm are caused by charge-transfer of the terminal oxygen and bridge-oxygen to metal atoms respectively. In the
Fig. 2. IR spectrum for H6[Al(H2O)FeW11O39] 14H2O.
UV spectrum (Fig. 3), there are two characteristic bands: 201.8 nm, O(d) 0/W; 258.5 nm, O(b)/O(c) 0/W. 3.3. X-ray powder diffraction X-ray powder diffraction is widely used to study the structural features of HPA and explain their properties [9]. The data of X-ray powder diffraction are listed in Table 1. In each of the four ranges of 2u that are 7 /108, 16/ 228, 25 /308 and 33/388, there is a characteristic peak of
Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
31
Fig. 4. Schematic diagram of the triheteropolyacid structure. The Al(III) ion which in the upper left/ hand corner is depicted as a darker sphere with a terminally coordinated H2O molecule. The Fe(III) heteroatom resides in a central pseudo-tetrahedral hole of the structure; it is the dark central atom. Fig. 3. UV spectrum for H6[Al(H2O)FeW11O39] 14H2O.
HPA anions that have Keggin structure. Combined with IR and UV spectra, it is sure that H6[Al(H2O)FeW11 O39]×/14 ×/H2O possesses Keggin structure (Fig. 4). 3.4. TG-DTA analysis In general, we take the temperature of the exothermic peak of DTA curves as the sign of their thermostability [10]. Fig. 5 is the thermogram of H6[Al(H2O)FeW11 O39]×/14 ×/H2O. The TG curve shows that there are three steps of weight loss. The first is the loss of hydration water, the second is the loss of protonized water and the third is the loss of structural water. In the DTA curve, there is an exothermic peak at 492.8 8C. The thermostability of the HPA is higher than that of H5FeW12O40 (390 8C).
Fig. 5. The thermogram for H6[Al(H2O)FeW11O39]×/14H2O.
14H2O is a new solid high-proton conductor with a proton conductivity of 4.07 /10 4 S cm 1 at 18 8C.
3.5. Conductibility Conductivity is an important parameter. We have recorded the results of the complex impedance measurement of the HPA (the frequency ranges from 0.01 to 9.99 /104 Hz) at room temperature (Fig. 6). We can calculate the conductivity from these results. The calculation shows that ternary heteropolyacid undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39]×/
4. Conclusions A new solid high-proton conductor, the ternary heteropolyacid undecatungstoferroaluminic acid H6[Al(H2O)FeW11O39] ×/14 ×/H2O was prepared and characterized. The results of AC impedance measurement
Table 1 Data of X-ray powder diffraction of H0[Al(H2O)FeW11O39]× 14H2O 2u (8) d (nm) I/I0 2u (8) d (nm) I/I0
8.18 1.080 100 24.82 0.358 26
9.26 0.954 58 27.30 0.327 29
9.62 0.919 40 28.78 0.310 88
10.08 0.877 21 29.96 0.298 22
16.48 0.537 20 33.36 0.268 34
17.36 0.510 26 34.42 0.260 45
18.42 0.481 64 35.86 0.250 42
32
Q. Wu, X. Xie / Materials Science and Engineering B96 (2002) 29 /32
Acknowledgements The financial support from the Science Foundation of Zhejiang University and China Postdoctoral Science Foundation for this work is greatly appreciated.
References
Fig. 6. Complex impedance spectrum of H6[Al(H2O)FeW11O39]×/ 14H2O at 18 8C.
show that its proton conductivity is 4.07 /10 4S cm 1 at room temperature (18 8C).
[1] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113. [2] S.V. Lomakina, T.S. Shatova, L.P. Kazansky, Corros. Sci. 36 (1994) 1645. [3] M. Tasumisago, H. Honjo, Y. Sakai, T. Minami, Solid State Ionics 74 (1994) 105. [4] H.C. Sheu, J.S. Shih, Anal. Chim. Acta 324 (1996) 125. [5] S.J. Babinec, Proc. Electrochem. Soc. 94 (1994) 30. [6] N. Giordano, P. Staiti, A.S. Arico, E. Passalacqua, L. Abate, S. Hocevar, Electrochim. Acta 42 (1997) 1645. [7] Q.Y. Wu, Y.C. Zhai, Chin. J. Nonfer. Met. 8 (1998) 327. [8] Q.Y. Wu, E.B. Wang, Zh. Neorgan. Khim. 38 (1993) 1557. [9] Q.Y. Wu, Y.C. Zhai, Rare Metals 15 (1996) 257. [10] Q.Y. Wu, E.B. Wang, J.F. Liu, Polyhedron 12 (1993) 2563.