Thermal Decomposition of Ammonium Metavanadate

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1 Thermal Decomposition of Ammonium Metavanadate K. C. KHULBE AND R. S. MANN Chemical Engineering Department, Universiw of Ottawa, Ottawa, Canada KIN 6N5 Received February 4, 1975 K. C. KHULBE and R. S. MANN. Can. J. Chem. 53, 2917 (1975). X-Ray, infrared, and electron spin resonance (e.s.r.) studies were made on the samples prepared by the thermal decomposition of ammonium metavanadate at various temperatures between 100 and 700 "C. A new intermediate compound has been identified to be formed between 100 and 150 "C. This new compound is stable up to 200 "C, and is a complex ammonium-vanadyl compound, since while the infrared studies showed the presence of an NH4 group, and V-N bond, the e.s.r. gave the signal of V02+ ions (V = VOZ+). A mechanism of the decomposition of ammonium metavanadate has been given. K. C. KHULBE et R. S. MANN. Can. J. Chem. 53, 2917 (1975). On a ktudik par rayons-x, infra-rouge et resonance paramagnetique Blectronique (r.p.e.) des Bchantillons prkparks par dkomposition thermique, A diverses tempkratures entre 100 et 700 "C, de mktavanadate d'ammonium. On a identifie qu'un nouveau composk intermediaire se forme entre 100 et 150 "C. Ce nouveau composk est stable jusqu'a 200 "C et il s'agit d'un composk complexe de vanadyl-ammonium; cette conclusion dkoule des ktudes i.r. qui montrent la prbence d'un groupe NH4 et d'un lien V-N et des Btudes r.p.e. qui prksentent un signal do aux ions V02+ (V4+ f 02- = VOZ+). On donne un mecanisme de dkcomposition du mktavanadate d'ammonium. [Traduit par le journal] Introduction Ammonium metavanadate is generally the starting material in the preparation of vanadium compounds. The manufacture of vanadium pentoxide from ammonium metavanadate involves a series of chemical reactions (1-5). Although the thermal decomposition of ammonium metavanadate has been studied by several investigators, yet there is no general agreement on the course of the reaction. It has been established that a stable intermediate is formed, but it has not been positively identified and its physical characteristics are not reported. No electron spin resonance (e.s.r.) or infrared studies have been reported on the products formed by the thermal decomposition of ammonia metavanadate at various temperatures. In the present paper, we report the results obtained by e.s.r., X-ray, and infrared techniques on the vanadium compounds prepared by the thermal decomposition of ammonium metavanadate between 100 and 700 "C. Experimental Vanadium pentoxide was prepared by the thermal decomposition of ammonium metavanadate (Sample A). Ammonium metavanadate (Baker Analyzed Reagent) was dissolved in a minimum quantity of warm distilled water, and the solution evaporated over a hot plate with a magnetic stirrer, at low heat. The residue was dried for 12 h at 60 "C. The temperature was increased in stages to 100, 150,200, 250, 300,400, 500,600, and 700 "C and maintained at each of the temperatures for 24 h. The samples were taken from the bulk at these temperatures before the temperature was raised to the next level, Samples B, C, D, E, F, G, H, I, and J refer to the samples obtained by the decomposition of ammonium metavanadate at 100, 150, 200,250, 300,400, 500, 600 and 700 "C respectively. The e.s.r. measurements were carried out using a Bruker 416 spectrometer in which the microwave frequency was in the X-band, g value was calculated by comparison with the g value of the free radical DPPH (2.003). The Phillips defractometer was used for the X-ray analysis of the samples. Measurements were carried out by the Debye-Sherrer method at room temperature using a Cobalt target and an iron filter. Infrared absorption spectra were observed on a Perkin-Elmer model 421 spectrophotometer. The samples were formed into discs with KBr (0.5% sample in KBr). Results and Discussion X-Ray Analysis Table 1 gives the results of the X-ray analysis of the samples prepared by the thermal decomposition of ammonium metavanadate at various temperatures. At 250 "C, all ammonium metavanadate was decomposed and pure V,O, was formed. No other phases (that is other oxides of vanadium) appeared to be present at or above 250 C. The color of the mass changed from light yellow to dark yellow as the temperature

2 2918 CAN. J. CHEM. VOL. 53, 1975 TABLE 1. X-Ray analysis of the products obtained by thermal decomposition of ammonium metavanadate at different temperatures and g value of different samples Samples Temperature ("C) Identified phases g A (ammonium metavanadate) Room tempel -ature B 100 C 150 D 200 E 250 F 300 G 400 H 500 I 600 J 700 was raised.the products obtained by the decomposition of ammonium metavanadate at 150 and 200 C could not be identified specifically by X-ray analysis, though they appeared to be the same as indicated by their similar X-ray diffraction pattern. Table 2 shows the d-values and intensity of lines of the unidentifiable product. Thus X-ray analysis showed beyond doubt that a new compound really formed between 100 and 200 C, though it could not be specifically identified. Lamure and Colin (5) studied the pyrolysis of ammonium metavanadate and reported the formation of bivanadate (2V205.0(NH4),) and trivanadate (3V (NH4),) preceding the formation of V205. Taniguchi and Ingraham (1) studied the thermal decomposition of ammonium metavanadate at 225 "C and reported the compound's formula as (NH4)2.0.V205. But the X-ray analysis data obtained for (NH4), 0 -V205 was not the same as that obtained in the present investigation. It TABLE 2. Powder diffraction data for samples C and D 'Interplanar spacing. trelative peak intensity Plus other faint lines Ammonium metavanadate Ammonium metavanadate Unidentified (X) Unidentified (same as C) Vanadium pentoxide is possible that the compound obtained by the decomposition of ammonium metavanadate in the temperature range of 150 to 200 "C was not the same as identified by Lamure and Colin (5) and Taniguchi and Ingraham (1). Infrared Absorption Measurements Figure 1 a and b show the infrared absorption spectra obtained for various samples. The spectra for sample A were very similar to that observed for sample B, which indicated that there was no structural change, when the temperature of pure ammonium metavanadate was raised to 100 "C and maintained at that temperature for 24 h. However when the temperature of ammonium metavanadate was raised to 150 Cand its (sample C) spectra observed, a definite change in its structure was noticed. The spectra for sample C was different from that observed for sample B. The spectra for samples C and D were identical, indicating that when the decomposition temperature was raised from 150 to 200 "C no noticeable structural change took place. When the decomposition temperature was raised from 200 to 250 C a change in the structure and formation of a new compound was indicated by the spectra for sample E, which was different from the one observed for sample D. The spectra of all samples prepared by the decomposition of ammonium metavanadate at temperatures greater than 250 C were identical to the one observed for sample E. This indicated that no structural change took place when the decomposition temperature was increased from 250 to 700 "C. The infrared spectra for samples B, C, and D show that as the temperature of decomposition was raised from 100 to 200, two new bands appeared. This band is entirely different from the one observed for ammonium

3 KHULBE AND MANN: AMMONIUM METAVANADATE L I I OOO Frequency (cm-') I I Frequency (cm-'1 FIG. 1. (a) Infrared absorption spectra for samples A (-), B (-.-.-), and C (---). (b) Infrared absorption spectra for samples D (-) and E (---). 02V-0-V02 + NH3 [31 6 *V02+ + V03'- t NH3 -IyOV-O+ + V02- OV-ONH, I I + I I OH NH2 OH VOz- metavanadate in the region 600 to 1000 cm-i. The peak observed at about 10IUcm-I for samples B, C, and D is possibly due to V=O (6). This peak shifts towards higher frequency on raising the decompositi~n temperature on the formation of V205 (characteristic peak of V=O in V205 is 1020 cm-l) (6-9) which indicates that the band strength of V=O strengthens with the increase of the decomposition temperatures. The characteristic peak of ammonia (NH,), 1397 to 1400 cm-i (lo), disappeared totally at 250 "C. A comparison of the intensity and shape of the peak at cm-' for the samples B, C, D, and E indicates that the product obtained by the decomposition of ammonium metavanadate in the temperature range of "C cannot be a mixture of the compound and V205. The peak observed for samples C and D at about 980 cm-i may be attributed to V-0 band with slightly weaker bonding strength (characteristic peak of Vf :O is 990 cm-l) (6). The observed 825 cm-i band in Figs. 1 and 2 is apparently due to the vibrations of the V-0-V bond (8, 9). The band in the region of absorption at cm-i is assigned to lattice vibrations. The band observed at around 750 cm-i in samples C and D may be due to the V-N bond (11-13) but it is not possible to postulate at present as to what exactly is bonded to N, i.e. V-NO, V-NH,, etc. (10, 11). Usually covalent nitrite gives a peak in this region (R-&NO) (14). Taniguchi and Ingraham (1) observed that the

4 CAN. J. CHEM. VOL. 53, 1975 FIG. 2. Electron spin resonance spectra for samples C (I) and D (11). amount of ammonia fell short of the theoretical value, i.e. some part of ammonia changed into nitrogen through either its oxidation or decomposition (15). The loss in weight of ammonium metavanadate to vanadium pentoxide was very small in the temperature range between 100 and 200 "C. Thus it can be concluded from infrared measurements that the compound formed during the thermal decomposition of ammonium metavanadate at temperatures greater than 100 "C and less than 200 "C should be a new compound of ammonia-vanadium-nitrogen-oxygen or a mixture of ammonia-vanadium-oxygen and vanadium-oxygen-nitrite. Electron Spin Resonance Measurements All samples gave e.s.r. signals except samples A and B. The thermal decomposition of ammonium metavanadate started at temperatures higher than 100 "C. Representative spectra for the samples C, D, H, and J are shown in Figs. 2 and 3. In studying the sequence of the spectra of the samples prepared by the thermal decomposition of the ammonium metavanadate at temperatures between 100 and 700 "C, it was observed that the two lines (a-a') observed for samples D broadened to form a large broad line near the center of the spectral pattern (Fig. 2). The spectra observed for samples E, F, G, and H contained some broad lines (h.f.s.) (Fig. 3). These also disappeared as the samples were heated over 500 "C (samples H, I, and J). The spectra obtained for samples 1 and J were essentially similar. Vanadium contains 99.8x of an isotope with a spin of 712 and should either give a spectrum consisting of eight components or one broad line of about 80 G observed in the absence of exchange interaction. The e.s.r. spectra in V20, is due to V02+ ion in the unit structure V '- (16). The V02+ ion (3d' electron configuration) is formed when a V4+ ion is bound to an oxygen ion by a strong, double covalent bond. The nature of this bond has been described in terms of molecular orbitals of the vanadyl pentahydrate complex (V02+.5H20) by Ballhausen and Gray (17). Table 2 gives the g values calculated (16) for all the samples. It appears that in all cases the spectra is due to V02+ ions (V4+ + 0'- = V02+). The two lines observed at low temperature (i.e. 200 "C) are due to + V02 ions. The sharp line (a) corresponds to perpendicular orientation of the internal magnetic field (H) and the electric field (E) of the V02+ ions, whereas the broad, (a'), line corresponds to parallel orientation of (H) and (E). The lines are due to V4+ ions in the compound surrounded by six distorted 0 atoms. It seems from the i.r. and e.s.r. study that the new con~pound should be an ammonium vanadyl compound. The line width increased for the samples C, D, E, and F; for samples G, H, I, and J, the line width remained constant. There are various factors affecting the line width. The magnetic dipole interaction is assumed to be the major factor (16). In other words the local fields produced by the nearest V4+ ions contribute mainly to the line width. The concentration of V4+ ions increases with the increase of temperature up to 300 "C and then remains constant. Conclusion During the thermal decomposition of ammonium metavanadate between 100 and 200 "C, a new intermediate compound has been identified FIG. 3. Electron spin resonance spectra for samples H (111) and J (IV).

5 KHULBE AND MANN: AMMONIUM METAVANADATE 2921 from e.s.r., infrared, and X-ray studies. This new compound is stable up to 200 "C. While the infrared studies showed the presence of an NH, group and N-V bond, the e.s.r. gave the signal of V02+ ions. It can thus be concluded that the new intermediate identified in the present study is a complex of vanadyl compound. The following mechanism is possible for the decomposition of ammonium metavanadate. The authors are grateful to the National Research Council of Canada for financial assistance for the project (A-1125). The authors wish to express their gratitude to Dr. A. Manoogian, Physics Department and Dr. J. L. Argo, Chemical Engineering Department, Ottawa University for useful discussions. The authors are aiso indebted to Mr. E. J. Murray of the Mineral Science Division of Mines Branch, Department of Energy, Mines and Resources for the X-ray analysis. 1. M. TANIGUCHI and T. R. INGRAHAM. Can. J. Chem. 42,2467 (1964). 2. J. TRAU. ROCZ. Chem. 36, 1365 (1962). 3. K. TARAMA and S. TAERANISHI. J. Chem. Soc. Jpn. 55,68 (1952). 4. P. Du~ors and P. BRETON. C. R. 206, 1969 (1938). 5. J. LAMURE and COLIN. C. R. 258,6433 (1964). 6. S. YOSHIDA. Shokubai 10,90 (1968). 7. B. E. ZAITSEU, Z. I. EZHKOVA, and I. I. IOFFE. Kinet. Katal. 7,755 (1966). 8. K. YOSHIYA and K. HIROTA. J. Phys. Chem. 73,3973 (1969). 9. Y. KERA, S. TERATANI, and K. HIROTA. Bull. Chem. Soc. Jpn. 40,2458 (1967). 10. H. W. MORGAN, P. A. STAATS, and J. H. GOLDSTIEN. J. Chem. Phys. 27, 1212 (1957). 11. C ADDISON, D. W. ANOS, D. SUTTON, and W. H. H. HOYLE. J. Chem. Soc. A, 808(1967). 12. S. D. Ross. Inorganic infrared and Raman spectra. McGraw Hill BookCo. Ltd.,New York p Table 613b. 13. R.J.H.CLARK~II~C.S.WILLIAMS.J.C~~~.S 1425 (1966). 14. N. B. COLTHUP. J. Opt. SOC. Am. 40,397 (1950). 15. M.ONCHI and E. MA. J. Phys. Chem.67,2241(1963). 16. K. C. KHULBE, R. S. MANN, and A. MANOOGIAN. J. Chem. Phys. 66,4810 (1974). 17. G. J. BALLHAUSEN and H. B. GRAY. Inorg. Chem. 1, 111 (1962).

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