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Preparation of a new vanadium-chromium oxide with a two-dimensional structure
Solid State Ionics 161 (2003) 301 – 307 www.elsevier.com/locate/ssi
Preparation of a new vanadium-chromium oxide with a two-dimensional structure O. Pozdnyakova a, E. Kuzmann b, L. Szirtes a,* a
Institute of Isotope and Surface Chemistry, Chemical Research Center for the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary b Department of Nuclear Chemistry, Eo¨tvo¨s University, P.O. Box 32, H-1518 Budapest, Hungary Received 7 May 2003; received in revised form 10 July 2003; accepted 11 July 2003
Abstract New two-dimensional microcrystalline V1xCrxOynH2O phase, where x<0.25 and 6.4
1. Introduction The complex vanadium-chromium oxide systems have been the subject of many investigations primarily with respect to catalytic [1– 9], electronic and ionic [10 – 15] properties of their phases, which find a number of applications in the fields of heterogeneous catalysis, adsorption, sensor, magnetic and ceramic technologies. Mixed vanadium-chromium oxide materials are frequently prepared by high temperature solid state reactions between pure oxide components. Low temperature techniques for preparing these materials have * Corresponding author. Tel.: +36-1-2754346; fax: +36-1-3959075. E-mail address: [email protected] (L. Szirtes). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-2738(03)00296-0
received attention during the last decade. As fare as V5+-related systems are concerned, chromium orthovanadate synthesized via ceramic [16 –19], as well as chimie douce rout [20 – 22] have been reported. A compound Cr2V4O13 has been prepared by solid state reaction between the parent oxides and by co-precipitating chromium (III) nitrate and ammonium metavanadate [23]. The synthesis of the mixed oxide Cr0.11 V2O5.16, has been performed from vanadium oxide gel by ionic exchange from Cr(NO3)3 solution [24]. However, investigations are mainly restricted to anhydrous and usually heat-treated materials. A little attention has been paid to the hydrated V2O5 –CrOx phases and only a few reports on synthesis and some studies of (CrOH)xV12yCryOmnH2O [25], CrVO4nH2O [26], Cr(VO3)3nH2O [27] and Cr4(V2O7)3nH2O [28] compounds can be found in the literature.
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The purpose of the present paper is to report on a low-temperature synthesis of new hydrated vanadiumchromium oxide compounds and characterize their crystallinity.
Works, Budapest) connected to selective water detector [30].
3. Results 2. Experimental Solution of 0.5 M peroxovanadate with molar ratio of H2O2/V=45 was prepared at 5F1 jC by addition of 30% H2O2 (Fluka Chemicals, p.a.) to slurry of deionized water and V2O5 (Aldrich, p.a.). After complete dissolution of V2O5 in hydrogen peroxide, the stoichiometric amount of aqueous 0.001 M CrO3 (Reanal Chemicals, p.a.) solution was added dropwise to achieve V/Cr molar ratio of 25 (sample A), 10 (sample B) and 5 (sample C). The initial pH value of the solutions was in the 1.77 – 1.66 range. The reacting solutions were maintained at 5F1 jC for 1 h. Then, the temperature was slowly increased up to 25 jC while the gelation process took place. When gelation was complete, the aqueous solution initially present was entirely contained within the final solid gel. The dark brown gelatinous substances formed were dried in air at 60 jC giving dark brown solid materials. The reacting mixtures were carefully stirred during the whole synthesis procedure. Temperatures were controlled using a Brinkmann Instruments circulating liquid bath. Vanadium/chromium ratios in the resulting compounds were verified by means of ICP (GBC-Integra XM) analysis. For comparison purpose, sample containing no chromium was also synthesised under the same experimental conditions (sample D). The X-ray powder diffraction study was performed with a Bragg-Brentano geometry, using powder samples with a Philips PW-1050/25 powder diffractometer (at 45 kV and 35 mA) with Cu Ka radiation and graphite monochromator. The powder diffraction patterns were scanned by steps of 0.01j (2u), with fixed accounting times (20 s). After data collection, stability of the X-ray source and of the sample was checked by remeasuring the first few lines of the pattern. The diffractograms were evaluated using laboratory made EXRAY program [29]. Thermoanalytical investigations were carried out for samples of 40 mg weight in argon atmosphere at a heating rate of 5 jC/min by means of a Derivatograph C type instrument (Hungarian Optical
All samples studied showed similar thermal behavior, typical DTA, DTG and TG curves are presented for sample A in Fig. 1. Thermoanalytical investigations revealed that asprepared samples contain about 80% mass of water (calculated from the total mass loss), their dehydration takes place in two steps in 30– 130 jC and 280– 330 jC temperature ranges followed by crystallization and melting of the resulting compounds at ca. 660– 680 jC. It has been observed that upon increasing V/ Cr ratio in the samples, the endothermic process of the structural water loss became more prolonged, taking place in the temperature interval up to 415 jC. The fact that exothermic peak in the differential thermal curves corresponding to crystallization of the samples is single and relatively sharp suggests that our preparations are not a simple mixture of derivatives based on each constituent metal oxide, but a distinct amorphous-like compound. With the increase of chromium content beyond the V/Cr molar ratio of 5, the thermal behaviour of the samples drastically changed and DTA-DTG curves became characteristic of the multiple phase preparations. More detailed DTA-DTG and high temperature XRD characterization of the crystallization process will be reported in other publications [31,58].
Fig. 1. Thermal analysis of VCr0.035Ox6.4H2O (sample A).
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Based on the analytical data, the compounds synthesized can be considered as the combination V2O5 – 0.070CrO 3 (sample A), V2O5 – 0.198CrO 3 (sample B) and V2O5 –0.451CrO3 (sample C). The general composition of the resulting materials can be expressed as V1xCrxOynH2O, where x<0.25, 6.4< n<8.7 and as VCr0.035Ox6.4H2O, VCr0.099Ox7H2O and VCr0.226Ox8.7H2O for samples A, B and C, respectively. The structural features of vanadium-chromium oxide materials were examined and are discussed in comparison with those exhibited by their parent oxide V2O5. The powder XRD patterns of V1xCrxOynH2O compounds are shown in comparison with that of the orthorhombic V2O5 in Fig. 2. As can be seen, the general profile of the X-ray diffractograms is characteristic of microcrystalline, amorphous-like material. Relatively few lines on XRD patterns were observed for V1xCrxOynH2O compounds comparing with V2O5. The patterns of asprepared samples are peculiar in the broad asymmetrical peak shape, rising very rapidly and then decreasing continuously toward high angle side. The peak intensity decreases and line broadening increases with increasing chromium content in the samples. Every reflection peak in the pattern of samples A and B coincides in the 2u position with the (hk0)
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Table 1 Averaged values of the unit cell parameters for V1xCrxOynH2O compounds and unit cell constants for sol – gel and orthorhombic V2O5 V2O5 [65] V2O51.6H2O [66] Sample A Sample B Sample C Sample D
a (nm)
b (nm)
c (nm)
1.1512 1.1513 1.2593 1.2573 1.2593 1.1512
0.3564 0.3564 0.3593 0.3597 0.3593 0.3613
0.4368 0.4371 0.4518 0.4524 0.4521 0.4368
reflections of orthorhombic V2O5 except for a strong peak at about 8j and a weak one at 23.1j. As has been pointed out [32], these features might suggest a random layer structure which consists of layers (a – b) arranged parallel and equidistant, but random in translation parallel to the layer and rotation about the normal (c) [33]. In such a case, there are (00l) reflections, two-dimensional lattice reflections of (hk) plane and no general reflections (hkl). Thus, the peaks at 8j and 23.1j may correspond to (001) and (003) reflection [34] indicating a basal plane orientation of the material. The (00l) peak shifts towards higher angles upon decreasing V/Cr ratio in the samples. The d-spacing obtained from (001) reflection was found to be 1.1464, 1.0880 and 1.0866 nm for samples A, B and C, respectively and 1.1756 nm for sample prepared under the same experimental conditions but containing no chromium (sample D). Taking into account microcrystalline nature of V1xCrxOynH2O samples, the lattice parameters can be given only to indicate the averaged values tabulated in Table 1. For comparison, the cell parameters of sol –gel and monocrystal V2O5 are also listed. With regard to the cell parameters of monocrystal V2O5, a slight increase of the a- and c-lattice constants was observed for the vanadium-chromium compounds (1.2593 nm cf. 1.1512 nm and 0.4519 A cf. 0.4368 nm, respectively) corresponding to an increase of the unit cell volume.
4. Discussion Fig. 2. X-ray diffraction patterns for orthorhombic V2O5 and (A) VCr0.035Ox6.4H2O; (B) VCr0.099Ox7H2O and (C) VCr0.226Ox 8.7H2O samples.
Results of thermoanalytical investigations as well as of X-ray analysis confirm that V1xCrxOynH2O
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material where x<0.25 and 6.4
second set is interleaved between these units in incommensurate positions, similarly to as proposed by Hibino et al. [32] for 2D-V2O5. Taking into account the similarity in XRD patterns of V1xCrxOynH2O compounds with V2O5 xerogels, we examined whether the materials synthesized in the present study might be considered as Cr exchanged or doped V2O5-xerogels. Previous literature [35,36] has indicated that at room temperature, the interlayer distance between planes for V2O51.6H2O gels was 1.160 nm, corresponding to a single interfoliar water layer (0.28 nm thick). Ribbons constituting the xerogel are negatively charged, with ca. 0.2 e charge per vanadium atom. This charge is compensated by 0.3 –0.4 H3O+ ions leading to ionic exchange properties between the H3O+cations and other charged species such as, e.g., Mm+ cations [37]. Exchange reactions are reversible and take place without variation of the ribbon structure. Intercalation can be followed easily by X-ray diffraction via the variation of the basal spacing. It has been demonstrated by Baffier et al. [38] that the variation of the d-spacing between the xerogel ribbons is generally a function of the ionic charge carried by the cation and of the crystallographic radius of the ion. The same authors have also shown that the field strength parameter Z/r is an adequate parameter to encompass the effect of both charge and radius. Indeed, the d-value of 1.440 nm reported by Gregoire et al. [39] for Cr3+ intercalated V2O5 xerogel prepared by ionic exchange is in accordance with the correlation between d-spacing and ionic charge to radius ratio (Z/r) proposed. The d-spacing reported by former group for Al3+and Fe3+ions is 1.42 and 1.41 nm, respectively, comparing with 1.16 nm for non-exchanged xerogel. Increase of the interlayer distance was attributed to two water layers contained in M3+ intercalated compounds. However, the repeating unit distance for the materials produced in our study decreased upon chromium introduction from 1.1576 (sample D) to 1.0866 nm (sample C). The decrease in the d-spacing, as compared with the preparation containing no chromium, is unlikely to be explained by an electrostriction phenomenon [40], when the intensity of the electrical field surrounding the cation leads to a decrease of the water molecular volume, as in the case of M +, M2+exchanged gels. Decrease in the d-spacing might be attributed to the formation of some compounds
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with polyelemental oxide networks incorporating both V and Cr in the same crystallographical site rather than (H3O+-Cr) exchanged V2O5 xerogels with chromium accommodated between the V-O layers. This assumption correlates with the findings of Volkov et al. [41,42] who have recently reported on the synthesis of layered polyvanadium-molybdenum (tungsten) gels of M2V12xTxO31FyH2O composition (T=Mo, W; M=H, Li, Na, K, 0
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vanadium gels. Therefore, the same precursor vanadate species might be present for our synthesis conditions and the same plausible conversion pathway as follows might be applicable: V2 O5 þ 2Hþ þ 2H2 O2 þ 3H2 O ! 2VOðO2 ÞðOH2 Þþ 3 ð1Þ þ 2VOðO2 ÞðOH2 Þþ 3 þ H2 O2 X VOðO2 ÞðOH2 Þ þ 2H
þ 2H2 O 2VOðO2 ÞðOH2 Þ þ 2Hþ ! ½VðO2 Þ2 ðOH2 Þ2 Oo þ H2 O
ð2Þ
ð3Þ
½VðO2 Þ2 ðOH2 Þ2 Oo þ 3H2 O þ 2Hþ ! 2VOðO2 ÞðOH2 Þþ 3 þ O2
ð4Þ
þ 2VOðO2 ÞðOH2 Þþ 3 ! 2VO2 þ O2 þ 6H2 O
ð5Þ
4 þ 10VOþ 2 þ 8H2 O X H2 V10 O28 þ 14H
ð6Þ
VOþ 2 ! solid
ð7Þ
Depending on the H2O2/V molar ratio and total concentration of vanadium, diperoxovanadate anion or monoperoxovanadate cation can be predominant species in solution. Duration of their stability also depends on peroxide concentration. In accordance with Ref. [57], high H2O2/V molar ratio used in our study would initially favour the formation of more monomer peroxo species than dimer peroxo species. Once the excess peroxide had been consumed, reaction 2 ceased, and reaction 5 proceeded. The peroxovanadates present at lower H2O2/V ratios and lower vanadium concentrations decompose before gelation began, leaving a solution containing the diprotonated decavanadate ion and the dioxovanadate cation. In accordance with Ref. [48], at H2O2/ V ratios and vanadium concentration used in our study gelation might begin and proceed in the presence of several peroxovanadates. In the present study, chromic acid solution was added to the fine V2O5 –H2O2 solution before gelation
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began. It has been observed that upon addition of H2CrO4 a deep blue colour rapidly appeared but did not persist long and mixed solution became dark brown. This could indicate the rapid formation of blue perchromic acid, CrO5H2O that in acidic solution quickly undergoes decomposition to Cr(III) with the evolution of dioxygen [59 – 62]. (The formation of several Cr(V) peroxo complexes, however, is also probable [63]). In acidic solution, Cr3+ exists as octahedral hexaquo ion, Cr(H2O)63+ that might be hydrolyzed with increasing the temperature [64], resulting in the formation of polynuclear complexes containing OH bridges. Under the present experimental conditions, the subsequent interactions between different vanadium and chromium species in solution may result in formation of polyelemental oxide network incorporating both V and Cr through possible (i) condensation of molecular precursors with the involvement of both vanadium hydroxo and chromium aquo/hydroxo and/or hydroxo complexes via olation/oxolation reactions; or (ii) formation of polynuclear molecular precursors as a result of interaction between coexisting vanadium and chromium peroxo complexes. Species in the solid phase, however, might be somewhat different from those expected in solution. Decrease in the crystallinity of the samples with decreasing V/Cr ratio could be attributed to the more random orientation of smaller two-dimensional crystallites formed upon drying of the gelatinous materials. Distortion of the long-range order in the solid phase may result from inhibition of oxolation reaction due to the presence of more aquo ligand in the coordination sphere of the metals upon increasing chromium content, and therefore, more random orientation of molecular precursor units upon condensation in solution.
5. Conclusion New two-dimensional microcrystalline V1xCrx OynH2O phase, where x<0.25 and 6.4
structural periodicity corresponding to the basal spacing of ca. 1.088 –1.146 nm, which might be associated with the formation of polyelement oxide network incorporating both V and Cr in the same crystallographical site. The degree of crystallinity decreases and the thermal stability of the materials increases upon decreasing V/Cr ratio. Acknowledgements The support from the Hungarian Science Foundation (OTKA project no. T-043687) is greatly appreciated. References [1] C.L. Thomas, Catalytic Processes and Proven Catalysts, Academic Press, New York, 1970. [2] C. Pradier, F. Rodrigues, P. Marcus, M. Landau, M. Kaliya, A. Gutman, M. Herskowitz, Appl. Catal., B Environ. 27 (2000) 73. [3] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [4] R. Mariscal, M. Galan-Fereres, J. Anderson, L. Alemany, J. Palacios, J. Fierro, in: G. Centi, et al. (Eds.), Environmental Catalysis, SCI, Rome, 1995, p. 223. [5] S. De Rossi, G. Ferraris, S. Fermiotti, A. Cimini, V. Indovina, J. Appl. Catal. A: General 81 (1992) 113. [6] P. Wittgen, C. Groeneveld, P. Zwaans, H. Morgenstern, A. Van Heughten, C. Van Heumen, G. Schuit, J. Catal. 77 (1982) 360. [7] Y. Iwasawa, S. Ogasawa, Chem. Lett. 8 (1980) 127. [8] Y. Iwasawa, J. Mol. Catal. 17 (1982) 93. [9] S. De Rossi, G. Ferraris, S. Fremiotti, A. Cimino, V. Indovina, Appl. Catal. 81 (1992) 113. [10] P. Soudan, J.P. Pereira-Ramos, J. Farcy, G. Gregoire, N. Baffier, Solid State Ionics 135 (2000) 291. [11] S.M. Tsheshnickii, A.A. Fotiev, L.L. Surat, Zh. Neorg. Khim. 8 (1983) 699. [12] W. Bencsh, E. Worner, M. Muhler, Mater. Res. Bull. 29 (1994) 155. [13] C. Sanchez, F. Babonneau, R. Morineau, J. Livage, Phila. Mag. 47B (1983) 279. [14] J. Bullot, O. Gallais, M. Gauthier, J. Livage, J. Appl. Phys. Lett. 36 (1980) 986. [15] J. Bullot, P. Cordier, O. Gallais, M. Gauthier, J. Livage, Phys. Status Solidi 68 (1981) 357. [16] E.C. Completosn, M.Y.C. Simons, B. Barham, Trans. J. Br. Ceram. Soc. 76 (1977) 68. [17] J. Amiel, D. Colaitis, D. Oliverm, C. R. Acad. Sci. Paris 263 (1966) 224. [18] R.C. Kerby, J.R. Wilson, Can. J. Chem. 51 (1973) 1032. [19] S.M. Tsheshnitski, A.A. Fotiev, L.L. Surat, Zh. Neorg. Khim. 28 (1983) 2699.
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Preparation of a new vanadium-chromium oxide with a two-dimensional structure O. Pozdnyakova a, E. Kuzmann b, L. Szirtes a,* a
Institute of Isotope and Surface Chemistry, Chemical Research Center for the Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary b Department of Nuclear Chemistry, Eo¨tvo¨s University, P.O. Box 32, H-1518 Budapest, Hungary Received 7 May 2003; received in revised form 10 July 2003; accepted 11 July 2003
Abstract New two-dimensional microcrystalline V1xCrxOynH2O phase, where x<0.25 and 6.4
1. Introduction The complex vanadium-chromium oxide systems have been the subject of many investigations primarily with respect to catalytic [1– 9], electronic and ionic [10 – 15] properties of their phases, which find a number of applications in the fields of heterogeneous catalysis, adsorption, sensor, magnetic and ceramic technologies. Mixed vanadium-chromium oxide materials are frequently prepared by high temperature solid state reactions between pure oxide components. Low temperature techniques for preparing these materials have * Corresponding author. Tel.: +36-1-2754346; fax: +36-1-3959075. E-mail address: [email protected] (L. Szirtes). 0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0167-2738(03)00296-0
received attention during the last decade. As fare as V5+-related systems are concerned, chromium orthovanadate synthesized via ceramic [16 –19], as well as chimie douce rout [20 – 22] have been reported. A compound Cr2V4O13 has been prepared by solid state reaction between the parent oxides and by co-precipitating chromium (III) nitrate and ammonium metavanadate [23]. The synthesis of the mixed oxide Cr0.11 V2O5.16, has been performed from vanadium oxide gel by ionic exchange from Cr(NO3)3 solution [24]. However, investigations are mainly restricted to anhydrous and usually heat-treated materials. A little attention has been paid to the hydrated V2O5 –CrOx phases and only a few reports on synthesis and some studies of (CrOH)xV12yCryOmnH2O [25], CrVO4nH2O [26], Cr(VO3)3nH2O [27] and Cr4(V2O7)3nH2O [28] compounds can be found in the literature.
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The purpose of the present paper is to report on a low-temperature synthesis of new hydrated vanadiumchromium oxide compounds and characterize their crystallinity.
Works, Budapest) connected to selective water detector [30].
3. Results 2. Experimental Solution of 0.5 M peroxovanadate with molar ratio of H2O2/V=45 was prepared at 5F1 jC by addition of 30% H2O2 (Fluka Chemicals, p.a.) to slurry of deionized water and V2O5 (Aldrich, p.a.). After complete dissolution of V2O5 in hydrogen peroxide, the stoichiometric amount of aqueous 0.001 M CrO3 (Reanal Chemicals, p.a.) solution was added dropwise to achieve V/Cr molar ratio of 25 (sample A), 10 (sample B) and 5 (sample C). The initial pH value of the solutions was in the 1.77 – 1.66 range. The reacting solutions were maintained at 5F1 jC for 1 h. Then, the temperature was slowly increased up to 25 jC while the gelation process took place. When gelation was complete, the aqueous solution initially present was entirely contained within the final solid gel. The dark brown gelatinous substances formed were dried in air at 60 jC giving dark brown solid materials. The reacting mixtures were carefully stirred during the whole synthesis procedure. Temperatures were controlled using a Brinkmann Instruments circulating liquid bath. Vanadium/chromium ratios in the resulting compounds were verified by means of ICP (GBC-Integra XM) analysis. For comparison purpose, sample containing no chromium was also synthesised under the same experimental conditions (sample D). The X-ray powder diffraction study was performed with a Bragg-Brentano geometry, using powder samples with a Philips PW-1050/25 powder diffractometer (at 45 kV and 35 mA) with Cu Ka radiation and graphite monochromator. The powder diffraction patterns were scanned by steps of 0.01j (2u), with fixed accounting times (20 s). After data collection, stability of the X-ray source and of the sample was checked by remeasuring the first few lines of the pattern. The diffractograms were evaluated using laboratory made EXRAY program [29]. Thermoanalytical investigations were carried out for samples of 40 mg weight in argon atmosphere at a heating rate of 5 jC/min by means of a Derivatograph C type instrument (Hungarian Optical
All samples studied showed similar thermal behavior, typical DTA, DTG and TG curves are presented for sample A in Fig. 1. Thermoanalytical investigations revealed that asprepared samples contain about 80% mass of water (calculated from the total mass loss), their dehydration takes place in two steps in 30– 130 jC and 280– 330 jC temperature ranges followed by crystallization and melting of the resulting compounds at ca. 660– 680 jC. It has been observed that upon increasing V/ Cr ratio in the samples, the endothermic process of the structural water loss became more prolonged, taking place in the temperature interval up to 415 jC. The fact that exothermic peak in the differential thermal curves corresponding to crystallization of the samples is single and relatively sharp suggests that our preparations are not a simple mixture of derivatives based on each constituent metal oxide, but a distinct amorphous-like compound. With the increase of chromium content beyond the V/Cr molar ratio of 5, the thermal behaviour of the samples drastically changed and DTA-DTG curves became characteristic of the multiple phase preparations. More detailed DTA-DTG and high temperature XRD characterization of the crystallization process will be reported in other publications [31,58].
Fig. 1. Thermal analysis of VCr0.035Ox6.4H2O (sample A).
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Based on the analytical data, the compounds synthesized can be considered as the combination V2O5 – 0.070CrO 3 (sample A), V2O5 – 0.198CrO 3 (sample B) and V2O5 –0.451CrO3 (sample C). The general composition of the resulting materials can be expressed as V1xCrxOynH2O, where x<0.25, 6.4< n<8.7 and as VCr0.035Ox6.4H2O, VCr0.099Ox7H2O and VCr0.226Ox8.7H2O for samples A, B and C, respectively. The structural features of vanadium-chromium oxide materials were examined and are discussed in comparison with those exhibited by their parent oxide V2O5. The powder XRD patterns of V1xCrxOynH2O compounds are shown in comparison with that of the orthorhombic V2O5 in Fig. 2. As can be seen, the general profile of the X-ray diffractograms is characteristic of microcrystalline, amorphous-like material. Relatively few lines on XRD patterns were observed for V1xCrxOynH2O compounds comparing with V2O5. The patterns of asprepared samples are peculiar in the broad asymmetrical peak shape, rising very rapidly and then decreasing continuously toward high angle side. The peak intensity decreases and line broadening increases with increasing chromium content in the samples. Every reflection peak in the pattern of samples A and B coincides in the 2u position with the (hk0)
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Table 1 Averaged values of the unit cell parameters for V1xCrxOynH2O compounds and unit cell constants for sol – gel and orthorhombic V2O5 V2O5 [65] V2O51.6H2O [66] Sample A Sample B Sample C Sample D
a (nm)
b (nm)
c (nm)
1.1512 1.1513 1.2593 1.2573 1.2593 1.1512
0.3564 0.3564 0.3593 0.3597 0.3593 0.3613
0.4368 0.4371 0.4518 0.4524 0.4521 0.4368
reflections of orthorhombic V2O5 except for a strong peak at about 8j and a weak one at 23.1j. As has been pointed out [32], these features might suggest a random layer structure which consists of layers (a – b) arranged parallel and equidistant, but random in translation parallel to the layer and rotation about the normal (c) [33]. In such a case, there are (00l) reflections, two-dimensional lattice reflections of (hk) plane and no general reflections (hkl). Thus, the peaks at 8j and 23.1j may correspond to (001) and (003) reflection [34] indicating a basal plane orientation of the material. The (00l) peak shifts towards higher angles upon decreasing V/Cr ratio in the samples. The d-spacing obtained from (001) reflection was found to be 1.1464, 1.0880 and 1.0866 nm for samples A, B and C, respectively and 1.1756 nm for sample prepared under the same experimental conditions but containing no chromium (sample D). Taking into account microcrystalline nature of V1xCrxOynH2O samples, the lattice parameters can be given only to indicate the averaged values tabulated in Table 1. For comparison, the cell parameters of sol –gel and monocrystal V2O5 are also listed. With regard to the cell parameters of monocrystal V2O5, a slight increase of the a- and c-lattice constants was observed for the vanadium-chromium compounds (1.2593 nm cf. 1.1512 nm and 0.4519 A cf. 0.4368 nm, respectively) corresponding to an increase of the unit cell volume.
4. Discussion Fig. 2. X-ray diffraction patterns for orthorhombic V2O5 and (A) VCr0.035Ox6.4H2O; (B) VCr0.099Ox7H2O and (C) VCr0.226Ox 8.7H2O samples.
Results of thermoanalytical investigations as well as of X-ray analysis confirm that V1xCrxOynH2O
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material where x<0.25 and 6.4
second set is interleaved between these units in incommensurate positions, similarly to as proposed by Hibino et al. [32] for 2D-V2O5. Taking into account the similarity in XRD patterns of V1xCrxOynH2O compounds with V2O5 xerogels, we examined whether the materials synthesized in the present study might be considered as Cr exchanged or doped V2O5-xerogels. Previous literature [35,36] has indicated that at room temperature, the interlayer distance between planes for V2O51.6H2O gels was 1.160 nm, corresponding to a single interfoliar water layer (0.28 nm thick). Ribbons constituting the xerogel are negatively charged, with ca. 0.2 e charge per vanadium atom. This charge is compensated by 0.3 –0.4 H3O+ ions leading to ionic exchange properties between the H3O+cations and other charged species such as, e.g., Mm+ cations [37]. Exchange reactions are reversible and take place without variation of the ribbon structure. Intercalation can be followed easily by X-ray diffraction via the variation of the basal spacing. It has been demonstrated by Baffier et al. [38] that the variation of the d-spacing between the xerogel ribbons is generally a function of the ionic charge carried by the cation and of the crystallographic radius of the ion. The same authors have also shown that the field strength parameter Z/r is an adequate parameter to encompass the effect of both charge and radius. Indeed, the d-value of 1.440 nm reported by Gregoire et al. [39] for Cr3+ intercalated V2O5 xerogel prepared by ionic exchange is in accordance with the correlation between d-spacing and ionic charge to radius ratio (Z/r) proposed. The d-spacing reported by former group for Al3+and Fe3+ions is 1.42 and 1.41 nm, respectively, comparing with 1.16 nm for non-exchanged xerogel. Increase of the interlayer distance was attributed to two water layers contained in M3+ intercalated compounds. However, the repeating unit distance for the materials produced in our study decreased upon chromium introduction from 1.1576 (sample D) to 1.0866 nm (sample C). The decrease in the d-spacing, as compared with the preparation containing no chromium, is unlikely to be explained by an electrostriction phenomenon [40], when the intensity of the electrical field surrounding the cation leads to a decrease of the water molecular volume, as in the case of M +, M2+exchanged gels. Decrease in the d-spacing might be attributed to the formation of some compounds
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with polyelemental oxide networks incorporating both V and Cr in the same crystallographical site rather than (H3O+-Cr) exchanged V2O5 xerogels with chromium accommodated between the V-O layers. This assumption correlates with the findings of Volkov et al. [41,42] who have recently reported on the synthesis of layered polyvanadium-molybdenum (tungsten) gels of M2V12xTxO31FyH2O composition (T=Mo, W; M=H, Li, Na, K, 0
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vanadium gels. Therefore, the same precursor vanadate species might be present for our synthesis conditions and the same plausible conversion pathway as follows might be applicable: V2 O5 þ 2Hþ þ 2H2 O2 þ 3H2 O ! 2VOðO2 ÞðOH2 Þþ 3 ð1Þ þ 2VOðO2 ÞðOH2 Þþ 3 þ H2 O2 X VOðO2 ÞðOH2 Þ þ 2H
þ 2H2 O 2VOðO2 ÞðOH2 Þ þ 2Hþ ! ½VðO2 Þ2 ðOH2 Þ2 Oo þ H2 O
ð2Þ
ð3Þ
½VðO2 Þ2 ðOH2 Þ2 Oo þ 3H2 O þ 2Hþ ! 2VOðO2 ÞðOH2 Þþ 3 þ O2
ð4Þ
þ 2VOðO2 ÞðOH2 Þþ 3 ! 2VO2 þ O2 þ 6H2 O
ð5Þ
4 þ 10VOþ 2 þ 8H2 O X H2 V10 O28 þ 14H
ð6Þ
VOþ 2 ! solid
ð7Þ
Depending on the H2O2/V molar ratio and total concentration of vanadium, diperoxovanadate anion or monoperoxovanadate cation can be predominant species in solution. Duration of their stability also depends on peroxide concentration. In accordance with Ref. [57], high H2O2/V molar ratio used in our study would initially favour the formation of more monomer peroxo species than dimer peroxo species. Once the excess peroxide had been consumed, reaction 2 ceased, and reaction 5 proceeded. The peroxovanadates present at lower H2O2/V ratios and lower vanadium concentrations decompose before gelation began, leaving a solution containing the diprotonated decavanadate ion and the dioxovanadate cation. In accordance with Ref. [48], at H2O2/ V ratios and vanadium concentration used in our study gelation might begin and proceed in the presence of several peroxovanadates. In the present study, chromic acid solution was added to the fine V2O5 –H2O2 solution before gelation
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began. It has been observed that upon addition of H2CrO4 a deep blue colour rapidly appeared but did not persist long and mixed solution became dark brown. This could indicate the rapid formation of blue perchromic acid, CrO5H2O that in acidic solution quickly undergoes decomposition to Cr(III) with the evolution of dioxygen [59 – 62]. (The formation of several Cr(V) peroxo complexes, however, is also probable [63]). In acidic solution, Cr3+ exists as octahedral hexaquo ion, Cr(H2O)63+ that might be hydrolyzed with increasing the temperature [64], resulting in the formation of polynuclear complexes containing OH bridges. Under the present experimental conditions, the subsequent interactions between different vanadium and chromium species in solution may result in formation of polyelemental oxide network incorporating both V and Cr through possible (i) condensation of molecular precursors with the involvement of both vanadium hydroxo and chromium aquo/hydroxo and/or hydroxo complexes via olation/oxolation reactions; or (ii) formation of polynuclear molecular precursors as a result of interaction between coexisting vanadium and chromium peroxo complexes. Species in the solid phase, however, might be somewhat different from those expected in solution. Decrease in the crystallinity of the samples with decreasing V/Cr ratio could be attributed to the more random orientation of smaller two-dimensional crystallites formed upon drying of the gelatinous materials. Distortion of the long-range order in the solid phase may result from inhibition of oxolation reaction due to the presence of more aquo ligand in the coordination sphere of the metals upon increasing chromium content, and therefore, more random orientation of molecular precursor units upon condensation in solution.
5. Conclusion New two-dimensional microcrystalline V1xCrx OynH2O phase, where x<0.25 and 6.4
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