Kutty, T. R. Narayanan

Murthy, H. S. Gopala Krishna

Rao, M. Subba

English

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Indian Journal of ChemistryVol. 15A, October 1977, pp. 863-867Thermal Decomposition of Ba(II) & Sr(II) Bis(oxalato)-oxovanadates(l V)H. S. GOPALA RRISH> A MURTHY, !Ii. SUBBA RAO & T. R. ARAYANAN RUTTYDepartment of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012Received 15 January 1977; accepted 16 June 1977Preparation, characterization and thermal decomposition of barium and strontium bis-oxalato-oxovanadates, MVO(C20.)2.4H20are reported. Dehydration of the tetrahydrates takesplace in two steps of three and one water molecule. In an oxygen atmosphere, the anhydrousoxalates decompose exotherrnlcally, vanadium(IV) is oxidized to vanadium(V) and the decom-position proceeds through an intermediate carbonate to the pyrovanadate, M2V207• In non-oxidizing atmospheres, the endothermic decomposition of the oxalates results in the forrnattonof the rnetavanadates, MV03 through the intermediate carbonates MV02C03 and M,V205C03• Freecarbon dioxide is found to be trapped in the solid residue during the decomposition of theoxalates.INTEREST in the vanadates of alkaline earthmetals of the type MV03 and their solid solutionswith lanthanide vanadates Ln V03, stems fromtheir electrical properties':". Lack of literatureof the preparation, characterization and thethermal decomposition of alkaline earth bis-oxalato-oxovanadatesfl V), prompted us to undertake thetitle investigation. The titanates of alkaline earthmetals have been prepared by the thermal decomposi-tion of the corresponding titanyl oxalates ; the detailsof these decompositions have been reported by usearlier7-lo. It is of interest to extend a similar workto bis-oxalato-oxovanadatesfl V) wherein it is possiblethat vanadium may exhibit more than one oxi-dation state during thermal decomposition. Thismay bring in considerable variation in the mode ofdecomposition of the oxalate complex.Materials and MethodsAll the reagents used were of either BDH (AR) orE. Merck (pro-analvsi) grade. Vanadium(V) oxidewas prepared by decomposing ammonium metavana-date (Reanal-Budapest) at 4500 to constant weight.Barium and strontium bis-oxalato-oxovanadates-(IV) were prepared by adapting the procedure ofSatyanarayana-! for the preparation of ammoniumbis-oxalato-oxovanadate(I'V). Vanadium (V) oxide(3'638 g, 0·02 mole) was added to aqueous oxalic acid(12·6 g, 0·1 mole). A vigorous reaction took plac~ onwarming; vanadium was reduced and went intosolution. To the resulting deep blue solution,barium chloride dihydrate (9·77 g) or strontiumacetate (8·23 g, 0·04 mole) was added and thesolution concentrated. Excess acetone was addedwhen a deep blue viscous laver separated. Thison vigorous stirring threw out -the bis-oxalato-oxo-vanadatef l V] in the form of an easily settlingpowder. This was filtered, washed with acetoneand air-dried.Barium and strontium in these complexes weredetermined as sulphates after destroying the oxalate.Total oxalate and vanadium(IV) was determinedby permanganimetrv. In order to determine vana-dium, a known weight of the material was heatedat 6000 in air, whereby oxalate was decomposedand vanadiumrl V] was oxidized. The resultingvanadiumj V) was determined bv titration withstandard fer'rous solution using N~ph.enylanthranilicacid as indic':!-tor. Water was determined by heatinga known weight of the matenal and absorbing theevolved water in anhydrous magnesium perchlorate.The chemical analysis of the barium compoundis as Iollows : Ba, 31·02; V, 11·2; C204, 38·52; H20,16'31% [Calculated for BaVO(Cz04)z.4H20 (BVO):Ba, 30·36; V" 11·27; C204, 38'91; H20, 15'92%].For the strontium compound, the results of chemicalanalysis are: Sr, 21'82; V, 12'53; C204, 43'97; H20,18·29% [Calculated for SrVO(Cz04)2.4H20 (SVO):Sr, 21·77; V, 12·66; C204, 43·72' H20 17.88°/., , IO.J·Apl?aratus for thermal analyses and other physicaltechniques have been described in an earlier paper".Gas analysis - Gases evolved during the thermaldecomposition were detected by means of a MS 10mass spectrometer and were found to be onlvcarbon monoxide and carbon dioxide. Amountsof carbon monoxide and carbon dioxide evolvedin a flowing atmosphere of oxygen were determinedas follows: A known weight of the material wascompletely decomposed and the evolved carbondioxide was absorbed in ascarite. In a second ex-periment, carbon monoxide was oxidized to carbondioxide by mean~ o.f heated copper(II) oxide andall the carbon dioxide was absorbed in ascarite.Effluent gas analysis (EGA) was carried outUSIng a vacuum system wherein 2 or 3 mg of theanhydrous sample was heated in a quartz reactiontube attached to a vacuum line, at a constant rateof 5° min-I. In one experiment, the total pressuredue to both carbon monoxide and carbon dioxidewas measured at 10.c intervals, using a Pirani gauge.In a second expenment, carbon dioxide was con-densed. using a liquid nitrogen trap and only carbonmonoxide pressure was recorded. Carbon dioxidepressure was found by difference. In each case,863INDIAN J. CHEM., VOL. 15A, OCTOBER 1977the rate of increase in pressure was plotted againsttemperature at 10° interval to obtain the EGA plot.For obtaining residues at different temperatures,known weight of the material was heated under adynamic vacuum of 10-3 mm in a conventionalvacuum line.ResultsThe DTA of BVa in flowing atmospheres of oxygenand carbon dioxide and in a dynamic vacuum are~ _ _ a B ~ ~ ~ ~TEMPERATURE :CFig. 1- DTA of EVO in (a) oxygen, (b) carbon dioxideand (c) a vacuum(a)rv---I(blj(elrl0'/' WI loss!100 200 300 400 500 600 700 800 900TEMPERATURE,oCFig. 2 - DTG of EVO in (a) oxygen, (b) carbon dioxideand (c) a vacuum, (d) TG of EVO in argon864presented in Fig. 1. The corresponding DTG areplotted in Fig. 21 while the EGA results are shownin Fig. 3. Similar results for SVO are presentedin Figs. 4-6. The thermal analyses carried out inflowing atmospheres of nitrogen and argon aresimilar to those in a vacuum. Hence, only the TGtraces in argon for BVa and sva are presented inFigs 2 and 5 respectively for the sake of comparison.From the thermoanalytical curves it is clear thatthe mode of decomposition of both EVa and SVOin oxygen atmosphere is different from that in non-oxidizing atmospheres except for the initial dehy-dration step.The decomposition takes place in three stages:dehydration of the tetrahydrate, decompositionof the oxalate to form a carbonate; and decornposi-,----CO...~a...,1l'----'*n,~, ,~' -±---::::' ~' '~,100 200 300 400 500 600 700 800 900TEMPERATURE,oCFig. 3 - Effluent gas analysis of EVOTEMPERATURE,OCFig. 4 - DTA of SVO in (a) oxygen, (b) carbon dioxide and(c) a vacuum(a)MURTHY et al.: THERMAL DECOMPOSITION OF BlS-OXALATO-OXOVANADATES(b)~(d)I10'/,WUoss100 100 300 400 500 600 700 800 900TEMPERATURE .scFig. 5 - DTG of SVO in (a) oxygen, (b) carbon dioxide and(c) a vacuum, (d) TG of SVO in argonco...:l:!Q.1100 200 300 400 500 600 700 800TEMPERATURE, °CFig. 6 - Effluent gas analysis of SVOtion of the carbonate to form the vanadate. Thereis one to one correspondence between the DTA andDTG curves indicating that every thermal changeis accompanied by a corresponding weight loss.Dehydration - BVO is a dark bluish grey powderand is amorphous to X-rays. It turns dark greyon dehydration. The powder X-ray diffractionpattern of sky-blue SVO indicates its :-rys~allinenature with a low symmetry class. It IS slightlysoluble in water. Dehydration of SVO results ina grey powder, amorphous to X-rays. Magneticmoments of both BVO and SVO correspond to thepresence of one unpaired electron and hence thereis no spin-spin interaction in these as observed inmanv vanadium(IV) compounds'>. The IR spectraof BVO and SVO are almost identical with the IRspectrum of ammonium bis-oxalato-oxovanadate(IV)l1. Thermograms indicate that they dehydrate intwo steps losing three and one water molecule respec-tively. The first stage of dehydration takes placearound 1300 for BVO and 2050 for SVO at ambientpressure. In a vacuum, the temperatures arelowered to 115° and 1800 respectively. The seconddehydration step is not well resolved in the thermo-grams and occurs around 3000 for both BVO andSVO. Isothermal experiments also indicate thatthe last water molecule is strongly bound andcan be removed only above 2000 at ambient pressureor at 1500 in a vacuum. Both the dehydrationsteps are irreversible for BVO while SVO can berehydrated. However, it reabsorbs more waterthan required for the tetrahydrate. This rehydratedstuff still retains the grey colour and is amorphousto X-rays. The surface area of the isothermallydehydrated products is around 10 m2g-1.Decomposition of the oxalate - Anhydrous BVOdecomposes exothermically in oxygen. The exo-thermic nature is due to (i) oxidation of carbonmonoxide evolved during the decomposition and(ii) oxidation of vanadium(IV) to vanadiumrV).The residue indicated by the thermo grams around4000 is pale yellow and is amorphous to X-rays. Itschemical analysis and IR spectrum indicate it tocontain unidcntate carbonate and to possess thecomposition BaaV207_..(C03),,(x,,-,0·5). The same-residue can be obtained by isothermal heating of BVOin air at 280°. Both carbon monoxide and carbondioxide are evolved and over 70% of the formeris oxidized to carbon dioxide.For anhydrous SVO decomposition in oxygen,there is no step On the thermogram correspondingto the formation of an intermediate carbonate .However, isothermal heating of SVO at 3000 in airfor 12 hr results in a yellow residue which containscarbonate as indicated by chemical analysis as wellas IR spectrum. It is, however, thermally unstableand continues to lose weight with further heating.This indicates that a transient carbonate is formedduring the decomposition of SVO. The final productof decomposition is Sr2V207•In non-oxidizing atmospheres anhydrous BVOand SVO start decomposing around 300°. TheTG curves show a change in slope after about halfthe decomposition. EGA indicates that both carbonmonoxide and carbon dioxide are simultaneouslyevolved though carbon dioxide is present in largerquantities indicating extensive disproportion at ionof carbon monoxide. The EGA curve for carbonmonoxide is svmmotrical and returns to the baseline after 480o~ On the other hand, the EGA curvefor carbon dioxide is highly asymmetric, broad above4500 and returns to base line only above 750°.This indicates that carbon dioxide continues toevolve after the completion of oxalate decomposition(which is marked by the cessation of carbonmonoxide evolution). This can be explained onthe basis of the formation of unstable intermediatecarbonates. Such intermediate carbonates could,in fa~t, be obtained by careful isothermal heatingexperiments.The results of the isothermal heating experimentsin a vacuum are summarized in Table 1. The865INDIAN J. CHEM., VOL. 15A, OCTOBEI~ 1977TABLE 1- RESULTS OF ISOTHERMAL HEATI!<G OF BVO AND SVO IN A VAClTM AT VARIOUS TEMPERATURESCompound Heating Heating Obs. totaltemp. period wt loss°C (hr) (%)BVO 150-200 12 16·31SVO 150-200 12 18·29BVO 350 24 21·52SVO 350 24 38'5BVO 400 24 41·0SVO 400 24 43·32BVO 500 24 42·11SVO 500 24 48·09BVO 600 24 47·7SVO 600 24 54'4Compo of Calc. wt Surface IR spectralresidue Joss area results(%) (m' g-l)BaVO(C,O,l, 15·92 10 OxalateSrVO(C,O,l, 17·88 11 do33~Oxalate,carbonate,36 carbon dioxideBa.V,05C03 42·89 4 UnidentateSrVO,C03 42·55 56 carbonateBa.V205C03 42·89 2 UnidentatecarbonateSr,V,05C03 48·01 2 Ionic carbonateBaVO. 47·75 SmallSrVO; 53·47 doresidue at 350° contains both oxalate and carbonate.It is not stable and the oxalate group decomposeson further heating. The major features of the IRspectra of the intermediates are given in Table 1.The residue at 350° shows, in addition to the bandsdue to oxalate and carbonate, the 2350 crrr- banddue to gaseous carbon dioxide. The IR spectraof the carbonate residues have been interpreted onthe basis of whether the V3band is split or not,and, if split, on the magnitude of separation-".All the intermediate residues are dark grey andare amorphous to X-rays.Decomposition of carbonate - In oxygen, the car-bonate Ba2V207-,,(C03)% decomposes in the tempe-rature range 360°-600° to form Ba2V207 which isalmost colourless. SVO forms directly Sr2V207•In non-oxidizing atmospheres, the final step inthe thermograms in the temperature range 550°-800° corresponds to the decomposition of thecarbonates M2V20SC03 to form MV03. DTA doesnot clearly show this peak probably due to lowenthalpy change involved and also due to the largetemperature range. EGA shows the evolution ofonly carbon dioxide as expected.The final products of decomposition, MY03' ~redistinctly crystalline. The X-ray powder rl~ffractlOnpattern of BaV03 is in good agreement WIth thosereported by Feltz and Schrnalfuss" as :vel! as byPalanisamy et al.". The powder diffraction patternof SrV03 is in agreement with those reported byChamberland and Danielson'. Further, the preli-minarv electrical conductivity measurements onpelletized samples indicate that BaV03 is a semi-conductor and SrV03 is metallic, in agreementwith available reports-A The vanadat~s M2V207obtained by decomposing the oxalates In oxygencan be reduced to MV03 by means of hydrogenaround 800° as reported by Feltz and Schmalfussas also by Palanisamy ei al.DiscussionThe position of peaks on thennograms dependon experimental conditions. Yet in related com-pounds, the V indicate the strength of the bond thatIS rupturing. Hence, structural deta~ls of thematerials under investigation assume Importance.The structure of both BVO and SVO are notknown. However, the structure of ammonium866bis-oxalato-oxovanadate(IV) has been recentlysolvedH,15 and the close resemblance between thisand BVO and SVO in spectral and magneticproperties indicates that at least, the coordinationsphere around vanadium atom in these compoundsis the same. Accordingly, vanadium in both BVOand SVO can be six-coordinated; the two oxalategroups, cis to each other, occupy four coordinationpositions; the vanadyl (V= 0) oxygen is in the fifthposition and the sixth position is occupied by awater molecule. The remaining three water mole-cules are structural waters. On dehydration, thesethree water molecules of crystallization are removedfirst bringing about a collapse in the crystal structure.The higher temperature required for this processin S·"'O compared to BVO is because of its crystal-linity. The fourth water molecule, coordinatedto vanadium, is removed around 300° as indicatedon the therrnograms,The scheme of dehydration may, therefore, bewritten as shown in Eqs. (1) and (2):<200·MVO(C204)2.4H20 ~ MVO(C204)Z·H20+3H20...(1)~OO-300·MVO(C204hH20 ---+ MVO(C204)2+H20... (2)Decomposition of the oxalate takes place in asingle step, though, to start with, the two oxalat esare not equivalent. When the coordinated watermolecule is removed, the coordination sphere aroundvanadium is rearranged [most probably to asquare pyramidal configuration which is preferredby oxovanadium(IV) in many of its complexes-v,to bring about equivalence of the two oxalate groups.In oxygen atmosphere, the steps (3 and 4) areinvolved in the decomposition of the anhydrousoxalates.2MVO(C204)2 + 1!202--+M2 "20;--z(C03) .•+4CO+(4-x)C02 ••. (3)M2V207-z(C03)X-.,..M2V207+XC02 ... (4)In non-oxidizing atmospheres, the decompositionfollows steps (5-7) (temperatures are for isothermalheating).400·MVO(C204)2 ~ MV02C03+2CO+C02 •.. (5)MURTHY et al.: THERMAL DECOMPOSITION OF BIS-OXALATO-OXOVANADATES500'2MV02C03 --»- M2V205C03+C02 ... (6)600'M2V205C03 --»- 2MV03+C03 ••• (7)Though for BVO the intermediate BaV02C03cannot be isolated, EG A clearly indicates itstransient formation.The scheme of decomposition given above forBVO and SVO in non-oxidizing atmospheres isclosely similar to those proposed by us for thethermal decomposition of titanyloxalatesv t''. Thetemperature range of decomposition for BVO andSVO are higher than those for the titanyloxalates,indicating that the oxalato complexes of oxovana-dium(IV) are stabler than those of oxotitanium(IV).Further, the surface area increases considerablvduring the decomposition of BVO and SVO wherea"sit is almost constant in the decomposition of thetitanyloxalates. Here again, we observe that freecarbon dioxide is trapped in the solid residue duringthe initial stages of the decomposition of theoxalate. The trapped gas is given out as the oxalatedecomposition is completed. Since the surfacearea and hence the porosity also increases withoxalate decomposition, the trapped carbon dioxidemay find easy channels for diffusing out withincreasing extent of decomposition of oxalate. Sucha situation is not present in the case of the residuesobtained by decomposing tit anyloxalates whichremain non-porous throughout the decompositionand hence escape of trapped carbon dioxide ishindered in them. This is in accordance withGafner's-? treatment of the problem of occlusionof gases in a solid as a result of slow diffusion fromhomogeneously decomposing particles.Though it is claimed-" that the IR band at 2350cm! often occurs in the spectra of carbonates andcarbonatocornplexes, the reported IR spectra13•19do not bear this out. Indeed, it is significant toobserve that this band appears in only thosematerials that are obtained by thermal decompositionas was noted bv van de Velde et al. in the thermaldecomposition vof ammoniumw and lead titanyl-oxalatesl" and in our own studies on titanyloxalates.AcknowledgementThe authors are grateful to Prof. A. R. VasudevaMurthy for his encouragement. One of the authors(H.S.G.) is grateful to the CSIR, New Delhi, for afellowship.References1. CHAMBERLAND, B. L. & DAXIELSON, P. S., J. Solid SlateChem., 3 (1971), 243.2. FELTZ, A. & SCHMALF1:SS, S., Z. anorg . allg, Chem., 417(1975), 130, 137.3. PALANISAMY, T., GOPALAKRISHNAN, J. & SASTRI, M. V. c..Z. anorg, allg. cu«, 415 (1975), 275.4. SHIN-IKE, T., SAKAI, T., ADACHI, G. & SHIOKAWA, J.,Mal. Res. Bull., 11 (1976), 249.5. DOUGIER, P. & HAGENMULLER, H., J. solid state Che111.,15 (1975), 158.6. DOUGIER, P., FAN, J. c. C. & GOODENOUGH, J. B., J.solid slate cis«, 14 (1975), 247.7. GOPALA KRISHNA MURTHY. H. S., SUBBA RAO, M. &NARAYANAN KUTTY, T. R., J. inorg, nucl. Chem., 37(1975), 391.8. GOPALA KRISHNA MURTHY, H. S., SUBBA RAO, J\L &KARAYAXAN KUTTY, T. R., J. inorg, nucl, ci-«, 38(1976), 417.9. GOPALA KRISHNA MURTHY, H. S., SUBBA RAO, M. &NARAYANAN KUTTY, T. R., Thermochim. Acta, 13 (1975),183.10. GOPALA KRISHNA MURTHY, H. S., SUBBA RAO, M. &NARAYANAN KUTTY, T. R., .J. inorg. nucl, ci-«, 38·(1976), 596.11. SATYANARAYANA, D. N., Ph.D. Thesis, Indian Instituteof Science, Bangalore, 1965.12. SYAMAL, A., Coord. chem, Rev., 16 (1975), 309.13. NAKAMOTO, K., IR spectra of inorganic and coordinationcompounds (John Wiley, New York), 1969, 169.14. OUGHTRED, R. E., RAPER, E. S. & SHEARER, H. M. M.,Acta Crystallogr., 32B (1976), 82.15. FORM, G. E., RAPER, E. S., OUGHTRED, R. E. & SHEARER,H. M. M., cu»: Commun., (1972), 945.16. CLARK, R. J. H., in Comprehensive inorganic chemistry,Vol. 3, edited by J. C. Bailar, H. J. Emelius & R.Nyholm (Pergamon Press, London), 1973, 54.17. GAFNER, Trans. Faraday Soc., 55 (1959), 981.18. VAN DE VELDE, G. M. H. & ORAN]E, P., Thermochim.Acta, 14 (1976), 269.19. NYQUIST, R. A. & KAGEL, R. 0., IR spectra of inorganiccompounds (John Wiley, New York), 1971.20. VAN DE VELDE, G. M. H. & ORAN]E, P. J. D., Thermal'analysis, Vol. I, 1974, 851.867

Thermal Decomposition of Ba(II) & Sr(II) Bis(oxalato)-oxovanadates(IV)

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Thermal Decomposition of Ba(II) & Sr(II) Bis(oxalato)-oxovanadates(IV)

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