In this paper the latest results of our continuing investigation of heteropoly acids and their salts are reported. Specially attention was paid to the influence of cations on the dynamic equilibrium of protonic species, as well as on the structure of the host lattice itself, i.e., the Keggin anions. The investigations were done by IR and Raman spectroscopy within the range of 1200-40 cm-1.U ovom radu saopšteni su poslednji rezultati naših sistematskih ispitivanja heteropolikiselina i njihovih soli. Posebna pažnja posvećena je uticaju katjona na dinamičku ravnotežu protonskih vrsta, kao i na strukturu kristalne rešetke, odnosno Keginovih anjona. Ispitivanja su vršena metodama infracrvene i ramanske spektroskopije u oblasti 1200-40 cm-1. Dobijeni rezultati su važni za objašnjenje mehanizma provodljivosti, kao i za rešavanje problema praktične primene ovih jedinjenja kao čvrstih elektrolita. PR Projekat Ministarstva nauke Republike Srbije

Bošnjaković, Nada S.

Mioč, Ubavka B.

Nedić, Zoran P.

Todorović, Marija R.

Uskoković-Marković, Snežana

Journal of the Serbian Chemical Society

FarFar - Repository of the Faculty of Pharmacy, University of Belgrade

J. Serb. Chem. Soc. 65(5-6)399-406(2000) UDC 661.878/54-381/:543.42JSCS-2758 Original scientific paperA spectroscopic investigation of 12-tungstophosphoric acidalkali salts*UBAVKAB. MIO^,1**MARIJAR. TODOROVI],2** SNE@ANAM. USKOKOVI]--MARKOVI],3** ZORAN P. NEDI]1** and NADAS BO[NJAKOVI]1**1Faculty of Physical Chemistry, University of Belgrade, P. O. Box 137, YU-11001 Belgrade,2Fac-ulty of Chemistry, University of Belgrade, P. O. Box 158, YU-11001 Belgrade and3Faculty of Phar-macy, University of Belgrade, P. O. Box 146, YU-11001 Belgrade, Yugoslavia(Received 27 October 1999)In this paper the latest results of our continuing investigation of heteropoly acidsand their salts are reported. Specially attention was paid to the influence of cations onthe dynamic equilibrium of protonic species, as well as on the structure of the host latticeitself, i.e., the Keggin anions. The investigations were done by IR and Raman spectros-copy within the range of 120040 cm-1.Keywords: IR spectra, Raman spectra, 12-tungstophosphoric acid, alkali salts of 12-tungstophosphoric acid, Keggin anion.INTRODUCTIONAlthough heteropoly compounds (HPC) have been the subject of detailedinvestigation formore than 30years, there are stillmanyobscurities in their structureand characteristics. Especially the HPC of the Keggin structure has drawn specialattention, because of the property of superionic protonic conductivity at roomtemperature (s = (1001)· 103 S/cm).1,2 Besides the host lattice made of Kegginanions, these compounds are characterized by the presence of a great number ofwater molecules. This causes the existence of a great number of protonic species3,4that are in permanent dynamic equilibrium, which explains their high protonicconductivity.5,6 On the other hand, the great number of water moleculs in thestructure also cause a dependence of their stability on external conditions, such astemperature and relative humidity of the surrounding. In this way, there are changesin the protonic properties, which aggruates their application as solid electrolytes infuel cells H2/O2, etc. A moderate increase of the WPA stability, without a bigreduction of the conductivity values, was noticed after the substitution of a certainnumber of protons, i.e., oxoniumH3O+ ions,with alkaline and earth alkaline cations.399* Dedicated to Professor Slobodan Ribnikar on the occasion of his 70thbirthday** Serbian Chemical Society active memberAlmost all the properties of these compounds are influenced by the nature of thecation. The nature and the strength of the hydrogen bonds between the entities ascharge carriers are especially affected by the nature of the cation (ionic size,polarizability, solvatation number, etc.).This paper reports the results of the influence of the nature of the cation onthe host lattice of alkali WPA salts, a studied by IR and Raman spectroscopy.EXPERIMENTALThe examined alkaliWPAsaltswere synthesized according to a previously reported procedure.7The IR spectra of the samples, as KBr pellets and Nujol mulls, were recorded at roomtemperature with a Perkin Elmer 983G spectrophotometer. A variable temperature cell, model VLT-2,RIIC, London, was used for recording spectra at different temperatures. The spectra at high tempera-tures were recorded in situ, with a recording step of 10 ºC.The Raman spectra of the polycrystalline samples, sealed in glass tubes, were recorded on aDilor RII instrument, using the 457.94 nm excitation line from a Spectra Physics laser, with a constantpower of 3 mW.RESULTS ANDDISCUSSIONThe observed effects of the influence of the cation on the characteristics ofthe investigated salts have been classified as the influence of big-strong andsmall-weak cations.7,8Thus, all theWPAsalts are divided into twogroups: the groupof the small-weak cations (Li+, Na+, Mg2+, Ca2+) and the group of the big-strongcations (K+, Rb+, Cs+, Sr2+, Ba2+). TheWPA salts of the first group are transparentcubic crystals, slightly soluble in water, which crystallize with a large number ofwater molecules. During the process of calcination, these salts undergo two-stepdehydrations. The second group of WPA salts are small, white crystals, slightlysoluble in water.The results of the IR andRaman spectroscopywill be discussed from the pointof view of the nature of the cation and the influence of the protonic species on thehost lattice. The IR spectra in the region of the stretching and bending vibration ofwater and proton entities were partialy analyzed in a previous paper.7Data obtained fromXRPD show that the structure of the investigated alkali saltsis influenced by the degree of hydration. The salts of the group of small cations, with alarge number of water molecules, have the structure of WPA-29 and the salts whichbelong to the group of big cations have a structure very similar to that of WPA-6.The Raman spectra of Na3WPAand Cs3WPA, as examples of salts with smalland big cations, respectively, and that of WPA are shown in Fig. 1. The assignmentof the characteristic band frequencies is given in Table I. The Raman spectraconfirmed the difference between the two groups of alkali WPA salts in agreementwith the above mentioned division. The Raman spectra of WPA salts with smallcations are simillar to the spectrum of WPA. From Fig. 1 it is evident that thestrongest band, characteristic for the n1(PO4) vibration at about 1010 cm1 appears400 MIO^ et al.as a doublet. TheWPAsalts of weak cations, besides this band, show another strongband at 989 cm1. For the salts of the strong cations this second band appears at992995 cm1. In spectra of the second groupWPAsalts, a band ofmedium intensityat 978 cm1 appears, which is not registered in the Raman spectra of small cationsalts. There is also a difference in the band intensities of the n2(PO4) vibrations forsalts with big cations and those with small cations.The strong cations cause a dynamic dissociation of water molecules and thenumber of proton species (H+,OH and hydrated proton entities) increase relativelyto WPA and the small cation salts. These protonic species may interact with theFig. 1. Raman spectra of WPA, Na3WPA and Cs3WPA (region characteristic for host lattice vibra-tions).12-TUNGSTOPHOSPHORIC ACID SALT 401terminal oxygen atoms in the Keggin anions and cause a change of the distancesbetween the oxygen atom inWO6octahedra and the PO4tetrahedra. This is reflectedon the bands at about 1000 and 530 cm1, corresponding to the n1and n2(PO4)vibrations, which is also a doublet. The components of this band in the Ramanspectra of small cations salts (at 536 and 518 cm1) are of approximately the sameintensities. In the case of the big cation salts, the components have differentintensities; the band at 540 cm1 ismore intense that that at 520 cm1,which appearsas a shoulder. The frequency of the band at about 540 cm1 increaseswith increasingcation radius. In the Raman spectra of the investigated compounds other doubletbands are also noticed; for group of small cations these are at 235220, 157148and 10389 cm1, and for the big cations at 238218, 159148 and 10789 cm1.The IR spectra, at room temperature, of WPA, Na3WPA and Cs3WPA, in theregion below 1200 cm1, with the bands characteristic for the host lattice-Kegginanions, are shown in Fig. 2. The assignment of the bands is given in Table I. FromFigs. 1 and 2 it may be noticed that the most intensive bands in the IR spectra areFig. 2. IR spectra of WPA, Na3WPA and Cs3WPA at room temperature (KBr technique).402 MIO^ et al.veryweak in theRaman spectra. FromFig. 2 andTable I it is evident that the n1(PO4)band (983 cm1) is split. As the spiting could be caused by the presence of differentproton entities and/or different cations, it was of interest to investigate inmore detailTABLE I. Characteristic frequencies (cm-1) of the host lattice for WPA alkali saltsLi3WPARamanLi3WPAIRNa3WPARamanNa3WPAIRK3WPARamanK3WPAIRCs3WPARamanCs3WPAIRAssignement1079 vs 1078 s 1079 s 1078 sn 3(PO4)1010 vs 1010.9 vs 1009.8 vs 1010.1 vsn 1(PO4)988.7 s 994 sh 988.7 s 999 sh 992.1 s 995.5 s 994 shn (W=O)982 vs 979 vs 978 m 982 978.6 m 984 s934 m 932 m 919 mn (O-W-O)908 s 906 sh 907 s 907 m892.4 m 892 m 891.4 m 894 892.3 w 890 mn 2(WO6)814.1 w 812.3 w 814 w 836.1 vw 813 w 831.5 vw797 s 799 b s580.6 w 595 vw 578 w 597 vw 597.7 vw 596 vw 589.6 vw 595 wn 4(PO4)536.8 m 537.6 m 542.1 s 540.3 sn 2(PO4)518.8 m 524 w 524.1 m 510 w 518.7 w 526 w 525 w 523 w477.2 w 484 sh 477.2 w 471.7 w 474.4 w 482 sh428 vw 445.2 w 428 vw 429 vw417.2 vw 416.3 vw 413.6 w 415.4 w373.4 w 378 m 367.9 vw 384 m 387.1 w 384 s 382.5 w 381 sn (W-PO4)336.3 vw 336 w 335.5 vw 340.7 w 340 m 339.9 w 337 mn (W-PO4)311.5 vw 313.5 w 315.1 w 317 w267.4 sh 263.4 w 256 vwd (O-W-O)240.3 s 247.8 sh233.9 s 236 vs 235.8 vs218.1 vs 222.1 vs 219 vs 217.5 vsn (W-O-W)205 sh R(PO4)197.7 sh 188.6 m 198.6 shlatticemodes156.7 sh 157.8 sh 159.4 m 159.5 m146.4 s 149.4 vs 148.1 m 148.2103.2 s 103.4 vs 106.9 vs 107 vs88.2 s 89.3 vs 88.1 m 91 s49.5 w26.8 vs 26.1 vs12-TUNGSTOPHOSPHORIC ACID SALT 403the influence of the cation. Hence, this band was deconvoluted, Fig. 3. From Fig. 3it is obvious that deconvolution of this band for the small cation salts gives threebands: at 999, 979 and 960 for Na3WPA, as in the case ofWPA, and only two bands:at 999 and 979 cm1, for the group of big cations. The difference in the intensitiesof the common bands in these two group of sailts is also very prononced, Fig. 3.The above mentioned results show that the nature of the cation influences thenumber and quantity of proton species, as well the host lattice and because of thisit has to have an influence on the conductivity of the salt too. The conductivity valuesFig. 3. Deconvolution of Na3WPA a) and Cs3WPA b) IR spectra (region characteristic for then 1(PO4) vibration).404 MIO^ et al.for some hydrates of WPA and its alkali salts are given in Table II. The higherhydrates of WPA have high conductivity caused by the large number of protonicspecies, as charge carriers, and the well developed network of hydrogen bonds,which enables fast proton transport. The salts with big cations also have very highconductivities because of influence of these ions on the dynamic dissociation ofwater molecules, resulting in an increased number of protons and oxonium ions.From the obtained results and the results published in a previous paper,7 it is evidentthat the influence of the nature of the cation (ionc size, polarization, hydrationnumber etc.) is more pronounced in the H2OH2O than in the H2OH3O+ interac-tion. With increasing ionic radius, the strength of the hydrogen bond also increases,which may be followed by the ROOdistances and by the shift of the bandcharacteristic for this interaction. The Dn value of about 40 cm1 and the decreasein the ROOdistance from 0.282 to 0.271 nm follow the order from Li+ to Rb+. Thechanges of the ROOdistances for the H2OH3O+ hydrogen bonds are smaller anddecrease from 0.269 to 0.266 nm in the same order.TABLE II: Conductivity values of WPA and its alkali saltsCompound conductivity (S/cm)H3PW12O40.29H2O 8 ·· 10-2H3PW12O40.14H2O 3.3 ·· 10-4Li3PW12O40.27H2O 1.25 ·· 10-2Na2HPW12O40.15H2O 1.7 ·· 10-4Na3PW12O40.16H2O 6.6 ·· 10-5K3PW12O40.10H2O 1.7 ·· 10-2Rb3PW12O40.6H2O 1.7 ·· 10-2Cs3PW12O40.8H2O 2.5 ·· 10-2Acknowledgement: This paper was supported by the Ministry of Science and Technology ofthe Republic of Serbia.I Z V O DSPEKTRALNOISPITIVAWE SOLIALKALNIH ELEMENATA 12-VOLFRAMFOS-FORNE KISELINEUBAVKA B. MIO^,*MARIJA R. TODOROVI],**SNE@ANAM. USKOKOVI]-MARKOVI],***ZORANP. NEDI]*i NADA S. BO[WAKOVI]**Fakultet za fizi~ku hemiju, Univerzitet u Beogradu, p. pr. 137, 11001 Beograd,**Hemijski fakultet, Uni-verzitet u Beogradu, p. pr. 158, 11001 Beograd i***Farmaceutski fakultet, Univerzitet u Beogradu, p. pr.146, 11001 BeogradU ovom radu saop{teni su posledwi rezultati na{ih sistematskih ispitivawaheteropolikiselina i wihovih soli. Posebna pawa posve}ena je uticaju katjona nadinami~kuravnoteuprotonskih vrsta, kaoina strukturukristalnere{etke, odnosno12-TUNGSTOPHOSPHORIC ACID SALT 405Keginovih anjona.Ispitivawa su vr{enametodamainfracrveneiramanske spektrosk-opije u oblasti 1200–40 cm–1. Dobijeni rezultati su vani za obja{wewe mehanizmaprovodqivosti, kao i za re{avawe problema prakti~ne primene ovih jediwewa kao~vrstih elektrolita.(Primqeno 27. oktobra 1999)REFERENCES1. O. Nakamura, T. Kodama J. Ogino, Y. Miyake, Chem. Lett. (1979) 172. O. Nakamura, I. Ogino, T. Kodoma, Solid State Ionics 3/4 (1981) 3473. U. Mio~, Ph. Colomban, A. Novak, J. Mol. Struct. 218 (1990) 1234. U. Mio~, M. Davidovi}, N. Tjapkin, Ph. Colomban, A. Novak, Solid State Ionics 46 (1991) 1035. R. C. T. Slade, H. A. Pressman, E. Skou, Solid State Ionics 38 (1990) 2076. R. C. T. Slade, I. M. Thomson, R. C.Ward, C. Poinsignon, J. Chem. Soc., Chem. Commun. (1987)7267. U. B.Mio~,M. Davidovi}, B. R. Stanisavljev, M. R. Todorovi}, Z. P. Nedi}, S. Uskokovi}, J. Serb.Chem. Soc. 61 (1996) 7598. E. A. Uksche, L. S. Leonova, A. I. Korosteleva, Solid State Ionics 36 (1989) 219.406 MIO^ et al.

Spektralno ispitivanje soli alkalnih elemenata 12-volframfosforne kiseline

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Spektralno ispitivanje soli alkalnih elemenata 12-volframfosforne kiseline

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