Synthesis and Conductivity of Solid High-Proton Conductor

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Materials Research Bulletin 40 (2005) 405–410 www.elsevier.com/locate/matresbu

Synthesis and conductivity of solid high-proton conductor H5GeW10MoVO4021H2O Qingyin Wu*, Xiaoguang Sang Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China

Received 13 October 2003; received in revised form 8 March 2004; accepted 6 June 2004

Abstract

A new solid high-proton conductor decatungstomolybdovanadogermanic heteropoly acid (HPA) H5GeW10- MoVO4021H2O has been synthesized for the ﬁrst time by stepwise acidiﬁcation and stepwise addition of solutions of the component elements. The product was characterized by chemical analysis, potentiometric titration, IR, UV, XRD and TG-DTA. The IR, UV and XRD indicate that H5GeW10MoVO4021H2O possesses the Keggin structure. The TG-DTA curve shows the sequence of water loss in the acid, the amount of the loss, as well as the thermostability. The results of AC impedance measurement show that its proton conductivity is 3.58 104 Scm1 at 18 8C and the activation energy for proton conduction is 31.82 kJ/mol. # 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; B. Chemical synthesis; C. Infrared spectroscopy; D. Ionic conductivity

1. Introduction

Heteropoly acids (HPAs), are a fascinating class of inorganic metal-oxygen cluster compounds [1]. Their chemistry, dictated by remarkable structural and electronic properties, is the subject of an intense interdisciplinary research activity in some major scientiﬁc areas, namely catalysis (e.g., photochemical dehydrogenations of organic substrates), materials science (e.g., secondary batteries and charge-transfer materials) and medicine (e.g., anti-viral and anti-tumor drugs) [2–8]. An aspect of the research in materials science has been the conductivity of HPAs because HPAs and its salts have good electric

* Corresponding author. Tel.: +86 571 8795 3258; fax: +86 571 8795 1895. E-mail address: [email protected] (Q. Wu).

406 Q. Wu, X. Sang / Materials Research Bulletin 40 (2005) 405–410 conductivity either in solution or in the solid state [9]. Solid HPAs have also been shown to be good proton conductors. In recent years, a number of solid high-proton conductor HPAs have been reported, which show that this ﬁeld continues to have a prominent place at the forefront of HPAs chemistry. For example, the conductivity of binary 12-tungstophosphoric heteropoly acid was reported [10,11] and the conductivity of a series of trinary heteropoly acids vanadotungstogermanic acid [12], molybdovanadophosphoric acid [13], undecatungstochromoferrous acid [14], and tungstovanadophosphoric acid [15] were also reported. To investigate the effect of component elements in HPA on conductivity, we report the synthesis of tetranary decatungstomolybdovanadogermanic heteropoly acid H5GeW10MoVO4021H2O by the stepwise acidiﬁcation and stepwise addition of solutions of the component elements. We also report the results on the conductivity.

2. Experimental

2.1. Instrument and reagents

IR spectrum was recorded on a P-E 1730 FT/IR spectrometer in the range 400–4000 cm1 using KBr pellets. UV spectrum was measured on a Hitachi U-3400 UV spectrophotometer in the range 190– 400 nm. X-ray powder diffraction analysis was obtained on a D/Max-III A, X-ray diffractometer using a Cu tube operated at 40 kV and 30 mA in the range of 2u =5–408 at a rate of 0.58 min1. TG-DTA was carried out on a TAS-100 thermal analyzer with a rate of temperature increase of 10 8C min1. Impedance measurements were performed on M378 electrochemical impedance analyzer with copper electrodes over the frequency range from 0.01 Hz to 99.9 kHz. A Mettler DL-21titrimeter and an 8410 ICP spectrometer were also used. The purity of Cu is more than 99.8%. All other reagents are analysis grade.

2.2. Preparation of HPA

Germanium dioxide (0.8 g) was dissolved in 20 ml of 5% sodium hydrate solution with stirring. An aqueous solution (20 ml) of sodium vanadate (1.2 g) was dropped into the above solution. After heating and boiling for 30 min with vigorous stirring, sodium molybdate (1.8 g) was added to reaction solution, pH was adjusted to 5.0 with sulfuric acid. After 70 min boiling an aqueous solution (100 ml) of sodium tungstate (25 g) was added, pH was adjusted to 1.5 and the stirred solution was continuously boiled for 5 h. The cooled solution was extracted with ether in a sulfuric acid medium. The etherate was dissolved with a little water and kept in a desiccator with sulfuric acid. The yield was above 80%.

2.3. Elemental analysis

Tungsten was analyzed by 8-hydroxy-quinoline-tannin acid-methylviolet gravimetry; germanium, vanadium and molybdenum were analyzed by ICP; the amount of water was analyzed by thermo- gravimetry. Found: Ge, 2.30; W, 59.53; Mo, 3.04; V, 1.61; H2O, 12.28. Calc. for H5GeW10Mo- VO4021H2O: Ge, 2.36; W, 59.67; Mo, 3.11; V, 1.65; H2O, 12.27. 中国科技论文在线 http://www.paper.edu.cn

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2.4. Measurement of conductivity

At room temperature (18 8C), H5GeW10MoVO4021H2O was pressed into a tablet 15 mm in diameter and 3.88 mm in thickness under a pressure of 20 MPa. Two copper sheets were attached to two sides of the tablet. The proton conductivity was measured using a cell: copperjsamplejcopper.

3. Results and discussion

3.1. Determination of basicity

The number of hydrogen in the HPA and the states of ionization can be determined by potentiometer titration [16]. The potentiometric titration curve (Fig. 1) shows that the ﬁve protons of H5GeW10Mo- VO4021H2O are equivalent and they are ionized in the same step.

3.2. IR and UV spectra

The IR spectrum of HPA shows the jump between two vibration energy levels of the electron basic state. The vibrations of the oxygen bond reﬂect the change of mechanical and electronic properties, every change has its own characteristic frequency. The absorptive band of the HPA UV spectrum shows the charge-transfer between oxygen and a coordinate metal atom [17]. n The [GeM12O40] structure (Keggin structure) consists of one GeO4 tetrahedron surrounded by four M3O13 sets formed by three edge-sharing octahedra. The M3O13 sets are linked together through oxygen n atoms. Thus, there are four kinds of oxygen atoms in [GeM12O40] , four Ge–Oa in which one oxygen atom connects with heteroatom (Ge), 12 M–Ob–M oxygen-bridges (corner-sharing oxygen-bridge between different M3O13 sets), 12 M–Oc–M oxygen-bridges (edge-sharing oxygen-bridge within

Fig. 1. The potentiometric titration curve of H5GeW10MoVO4021H2O. 中国科技论文在线 http://www.paper.edu.cn

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Fig. 2. IR spectrum of H5GeW10MoVO4021H2O.

M3O13 sets) and 12 M–Od terminal oxygen atoms. In general, the symmetric and asymmetric stretching of the different kinds of M–O bonds are observed in the following spectral regions: M–Od bonds (1000– 1 1 1 900 cm ), M–Ob–M bridges (800–900 cm ), M–Oc–M bridges (700–800 cm ) [18]. In the IR spectrum of H5GeW10MoVO4021H2O(Fig. 2), there are ﬁve characteristic bands: 1 1 1 1 978 cm , vas (M–Od); 877 cm , vas (M–Ob–M); 767 cm , vas (M–Oc–M); 816 cm , vas (Ge– 1 Oa); 458 cm , d(O–Ge–O), all of which correspond to the spectrum of the heteropoly complex of Keggin structure previously reported [19]. In the 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 UV spectrum, there are two characteristic bands: 197 nm, Od ! M; 265 nm Ob/Oc ! M [20].

3.3. X-ray powder diffraction

X-ray powder diffraction is widely used to study the structural features of HPA and explain their properties [21]. 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 are characteristic peaks of HPA anions that have Keggin structure. Combined with IR and UV spectra, we are sure that H5GeW10MoVO4021H2O possesses Keggin structure.

Table 1 Data of X-ray powder diffraction of H5GeW10MoVO4021H2O 2u(8) 7.80 8.59 9.61 16.10 18.05 19.90 23.80 d(nm) 1.133 1.030 0.920 0.551 0.491 0.446 0.374 I 100.0 47.2 11.0 12.2 11.6 6.8 8.6 2u(8) 27.41 28.34 29.85 30.63 31.90 33.17 38.05 d(nm) 0.325 0.315 0.299 0.292 0.281 0.270 0.236 I 22.4 20.8 12.4 6.8 10.0 17.8 8.0 中国科技论文在线 http://www.paper.edu.cn

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Fig. 3. Thermogram of H5GeW10MoVO4021H2O. 3.4. Thermal analysis

HPA consists of protons, HPA anions and hydration water. Fig. 3 is the thermogram of H5GeW10- MoVO4021H2O. The TG curve shows that the total percent of weight loss is 12.30%, which indicates that each HPA molecule has 21 molecules of water and there are three steps of weight loss. The ﬁrst is the loss of 11 molecules of hydration water, the second is the loss of 10 molecules of protonized water and the third is the loss of 2.5 molecule of structural water. Thus, the accurate molecular formula of the product is + (H5O2 )5 GeW10MoVO4011H2O [22]. In general, we take the temperature of the exothermic peak of DTA curves as the sign of their thermostability [23]. In the DTA curve, there is an exothermic peak at 476.5 8C. The thermostability of the HPA is between that of H4GeMo12O40 and H4GeW12O40. This shows that when W is replaced by Mo, or V, the thermostability decreases.

3.5. Conductivity

Conductivity is an important parameter. We 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 and can calculate the conductivity from these results. The calculation shows that at room temperature 4 1 (18 8C), the conductivity of H5GeW10MoVO4021H2O is 3.58 10 Scm . This compound is a new solid high-proton conductor HPA. Fig. 4 shows the Arrhenius plot. From the slope, we can calculate the activation energy of proton conduction, which is 31.82 kJ/mol. In the range of 18–48 8C, the conductivity of H5GeW10Mo- VO4021H2O is increased with an increase in temperature. 中国科技论文在线 http://www.paper.edu.cn

410 Q. Wu, X. Sang / Materials Research Bulletin 40 (2005) 405–410

Fig. 4. Arrhenius plot for H5GeW10MoVO4021H2O.

Acknowledgements

We greatly appreciate the ﬁnancial support received from the National Natural Science Foundation of China under Grant No. 20271045 and the Foundation of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University.

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