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Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) Clathrate hydrates of hydrogen and neon Yuri A. Dyadin,* Eduard G. Larionov, Andrei Yu. Manakov, Fridrich V. Zhurko, Evgeny Ya. Aladko, Tamara V. Mikina and Vladislav Yu. Komarov Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russian Federation. Fax: +7 3832 34 4489; e-mail: clat@che.nsk.su At pressures up to 15 kbar, hydrogen and neon were found to form classical clathrate hydrate structures, whereas argon and krypton, already known as polyhedral clathrate formers, can also form hydrates based on the ice II framework at high pressures.Small hydrophobic molecules (including monoatomic molecules) are known to form clathrate hydrates; these molecules can be subdivided into two different groups.The smallest molecules, hydrogen, helium and neon, all of linear dimensions less than 3.5 Å, constitute a group that can be incorporated into the small cavities in the lattices of ices Ih, Ic and II, thus forming solid solutions or clathrates very similar to the original ice frameworks. 1–3 Thus, they are not known to form clathrates based on polyhedral structures.The second group includes substances with larger molecular dimensions of 3.8 to 9.2 Å. These do not dissolve in ice frameworks, but form polyhedral clathrate structures, the best known of which are classical cubic structures I and II (CS-I and CS-II), and a hexagonal structure H (HS-III).4,5 A study of phase diagrams of the (Ar, Kr, Xe)–H2O systems at pressures up to 15 kbar showed6–8 that these gases can form classical clathrate hydrates, which are stable in a wide temperature and pressure range.However, in argon and krypton systems at pressures above 9.6 and 13.4 kbar, respectively, we found new hydrate phases, which are quite dense judging from the fact that their decomposition temperatures rapidly increase with pressure.This high-density packing is impossible in the case of typical clathrate structures where a single guest molecule occupies one cavity in a hydrate framework.† Here, we report the results of studying phase equilibria in the neon–water and hydrogen–water systems. All of the experiments were carried out by differential thermal analysis in an excess of gas. The approximate gas–water molar ratio was 6:1, while the ratio between the components in classical clathrates and ice II structures when every cavity was occupied by one guest molecule was inverse.The temperature was measured with a Chromel– Alumel thermocouple, and the pressure, with a Manganin manometer. The experimental technique was described in detail elsewhere.6–8 The results are shown in Figure 1.In both cases, the initial portions of the curves (lines aIg) represent decomposition of solid solutions of the gases in ice Ih.1 In our case, this manifests itself in a slight increase in the decomposition temperature of the solid solution as compared to the ice melting curve. The final portion of the curve (line bIg) in the hydrogen-containing system corresponds to decomposition of the hydrogen hydrate H2·6H2O with the ice II framework.3,9 Note that in both portions of the P–T curves the equilibrium was established rapidly (in about 20 min), and the results were well reproducible.The most complicated and interesting behaviour is observed in the pressure ranges from 1.0 to 3.6 kbar for hydrogen and from 1.9 to 3.7 kbar for neon systems (lines hlg).In both cases, the equilibria were attained very slowly (in several hours or longer). It is reasonable to suppose that, really, equilibrium states were not achieved in some experiments. The results can be reproduced only with a high excess of gas. We assign these † It is reasonable to consider the above hydrates as belonging to the same group because, provided that all the cavities are occupied by the same guest, their packing coefficients k6,10 are similar.For instance, for the CS-I xenon hydrate, k = 0.584; for the hypothetical xenon hydrates of other structures, they vary from 0.55 to 0.58, i.e., they are appreciably lower than in the case with the densest packing (k = 0.74). Therefore, the effect of pressure on them should be very similar.curves to decomposition of the classical hydrogen and neon clathrates. Figure 2 shows the decomposition curves of hydrate phases in the (Ar, Kr, Xe)–H2O systems6–8 and those of the systems under discussion. It is evident that in the systems with hard noble gases the decomposition curves of classical structure hydrates (solid lines) are top-shaped, and their sizes regularly decrease from xenon to argon hydrate, i.e., with a decrease in the guest molecule radius. The hydrogen and neon systems are adequately described by this scheme. Kuhs et al.11 showed that every two nitrogen molecules can occupy a large cavity in CS-I and CS-II.This is even more true of the equilibrium in the case of hydrogen and neon molecules.12 The presence of more than one guest molecules in a cavity, and slow diffusion of hydrogen and neon molecules in the already formed solid compound can result in rather slow establishment of equilibrium in the case of classical hydrate formation.Figure 2 shows that in all of the systems (except for the xenon system), in addition to the group discussed above, there is a group characterised by similar curves, which go up steeply with increasing pressure (dashed lines).An X-ray study of the compounds formed in the water–hydrogen system showed that this curve branch corresponds to decomposition of a hydrate phase based on the ice II framework. Similar decomposition curves for the systems strongly suggest that, at pressures above 4 kbar in the neon system, 9.6 kbar in the argon system, and 13.4 kbar in the krypton system, hydrates based on the ice II structure are formed.For argon and krypton molecules to be arranged in ice II channel cavities, a slight elongation of hydrogen bonds is required, which is quite realistic. Let us recall that, when the self-clathrate ice VII is formed (two ice Ic frameworks penetrate into each other) at pressures above 20 kbar, the hydrogen bond length increases from 2.76 to 2.96 Å.13 Thus, classical clathrate hydrates are formed in aqueous neon, 80 60 40 20 0 –20 T/°C P/kbar 0 4 8 12 16 metastable continuations of the curves H2–H2O system, this work H2–H2O system, ref. 9 Ne–H2O system, this work alg hlg blg i1hI i3I i5I i6I Figure 1 Decomposition curves of hydrates formed in the water–hydrogen and water–neon systems. Melting curves of ices (inl) are given for comparison: (a) solid solutions of gases in ice Ih, (b) hydrates of the respective gases based on the ice II crystal framework, (h) classical clathrate hydrate phase, (l) liquid phase, (g) fluid phase.Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) hydrogen, argon, krypton and xenon systems. Their stability decreases in the order from xenon to neon. In the first four systems, hydrates are formed on the basis of ice II, and the larger the guest molecules, the higher the pressure required for their formation. A xenon molecule appears to be too bulky to be arranged in an ice II framework cavity, and the CS-I hydrate remains stable at pressures at least up to 15 kbar.We are grateful to Professor J. A. Ripmeester and Professor V.I. Kosjakov for helpful discussions and valuable comments. We also express our gratitude to N. V. Udachina for translating this article into English. This work was supported by the Presidium of the Siberian Branch of the Russian Academy of Sciences (grant no. 97-18) and by the Russian Foundation for Basic Research (grant no. 97-03-33521a). References 1 A.Yu. Namiot and E. B. Bukhalter, Zh. Strukt. Khim., 1965, 6, 911 [J. Struct. Chem. (Engl. Transl.), 1965, 6, 873] (Chem. Abstr., 1966, 64, 10497b). 2 G. P. Arnold, R. G. Wenzel, S. W. Rabideau, N. G. Nereson and A. L. Bowman, J. Chem. Phys., 1971, 55, 589. 3 D. Londono, W. F. Kuhs and J. L. Finney, Nature, 1988, 332, 141. 4 G. A. Jeffrey and R. K. McMillan, Progr. Inorg. Chem., 1967, 8, 43. 5 J. A. Ripmeester, J. S. Tse, C. I. Ratcliffe and B. M. Powell, Nature, 1987, 325, 135. 6 Yu. A. Dyadin, E. G. Larionov, D. S. Mirinski, T. V. Mikina, E. Ya. Aladko and L. I. Starostina, J. Incl. Phenom., 1997, 28, 271. 7 Yu. A. Dyadin, E. G. Larionov, D. S. Mirinski, T. V. Mikina and L. I. Starostina, Mendeleev Commun., 1997, 32. 8 Yu. A. Dyadin, E. G. Larionov, T. V. Mikina and L.I. Starostina, Mendeleev Commun., 1997, 74. 9 W. L. Vos, L. W. Finger, R. J. Hemley and Ho-Kwang Mao, Phys. Rev. Lett., 1993, 71, 3150. 10 A. I. Kitaigorodsky, Molecular Crystals and Molecules, Academic Press, NewYork, part 1. 11 W. F. Kuhs, B. Chazallon, P. Radaelli, F. Pauer and J. Kipfstuhl, Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, 1996, p. 9. 12 K. A. Udachin, J. Lipkowski and M. Tkacz, Supramolecular Chem., 1994, 3, 181. 13 L. Pouling, General Chemistry, Freeman and Co, San Francisco, 1970. ........ 80 60 40 20 0 –20 T/°C P/kbar 0 4 8 12 16 Xe–H2O system Kr–H2O system Ar–H2O system H2–H2O system Ne–H2O system i1hI i3I i5I i6I Figure 2 Decomposition curves of hydrates formed in the (Xe, Kr, Ar, H2, Ne) water systems. Solid lines represent decomposition of polyhedral clathrate hydrates formed in these systems (first portions of decomposition lines in hydrogen and neon systems correspond to decomposition of solid solutions of the gases in ice Ih), dashed lines represent decomposition of hydrates of the gases based on the ice II crystal framework. Dotted lines represent the melting curves of the gases. Melting curves of ices (inl) are given for comparison. .......... Received: 22nd January 1999; Com. 99/1432

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年代:1999

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Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) A Mössbauer study of pentavalent iron in a vanadium(V) oxide matrix Sergey K. Dedushenko,*a Yurii D. Perfiliev,b Dmitrii E. Tcheboukov,b Denis A. Pankratovb and Yurii M. Kiselevb a N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 117907 Moscow, Russian Federation. Fax: +7 095 954 1279 b Department of Chemistry, M.V. Lomonosov Moscow State University, 119899 Moscow, Russian Federation. Fax: +7 095 932 8846 The formation of iron in the 5+ oxidation state was observed in iron-doped vanadium(V) oxide; this state is characterised by a singlet with the isomer shift d = –0.56±0.01 mm s–1 relative to a-iron in the Mössbauer spectrum at room temperature. The preparation and study of the elements in unusual oxidation states are an important branch of present-day inorganic chemistry.These species are of considerable interest because, on the one hand, they are of practical importance creating compounds and materials with new and unusual properties. On the other hand, the data on the physico-chemical properties of these systems expand the experimental basis of empirical laws that remain still quantitatively unsubstantiated.In this context, a study of iron in high oxidation states is of paramount importance. Indeed, the wine-coloured FeO4 2– ion has been known since the 19th century. However, the compounds of iron in the oxidation states higher than 4+ are known as only a few of examples to the present day, and the data on their chemical properties are limited.The compounds of iron in high oxidation states are of particular interest in Mössbauer spectroscopy. Iron seems to be the most convenient element for Mössbauer studies. The correlation between the Mössbauer spectral characteristics and the oxidation state of iron allows us to reveal the potentialities of this technique for studying the electronic and geometric structures of substances.A study of various compounds doped by different elements is used in modern chemistry and related sciences for determining the positions and properties of dopants. In particular, both simple and sophisticated oxides doped by iron have long been examined by Mössbauer spectroscopy.1 This approach is promising for the preparation of derivatives of the element in uncommon oxidation states.2 We decided on a vanadium(V) oxide matrix for stabilising iron in the unusual 5+ oxidation state for the reasons given below.First, the ionic radii of vanadium are close to the radii of isovalent iron ions.3–7 Thus, it is reasonable in terms of crystallography to expect the formation of the Fe5+ ion by isovalent isomorphous substitution of iron for vanadium in V2O5.Second, V2O5 is the highest oxide of vanadium. Consequently, it cannot be a reducing agent for the highly oxidised iron ions. Finally, V2O5 melts at a moderate temperature (674 °C) and remains stable even above the melting point.8 Thus, the problem of uniform iron distribution in the bulk of V2O5 can be solved, for example, by dissolving Fe2O3 in a V2O5 melt.Here we report the data on the 5+ oxidation state of iron in a vanadium(V) oxide matrix. Iron was introduced into V2O5 by dissolving 57Fe2O3 in molten vanadium(V) oxide in a platinum crucible at about 700 °C in an oxygen atmosphere. Next, the melt was cooled rapidly by immersing the crucible in metallic mercury. The iron content of the product was approximately 1 mol% on a vanadium basis.Absorption Mössbauer spectra were measured on a Perseus spectrometer (a modified instrument from the Research Institute of Instrument-making, Moscow)9 operating in a constant velocity regime; the control and correction (stabilisation) of the spectrometer vibrator velocity were performed with a laser interferometer. Standard g-radiation sources of 57Co in chromium metal matrices with the activities up to 35 mCi (Cyclotron Co., Ltd., Obninsk, Russia) were used.Isomer shifts are given with respect to a-iron.10 The Mössbauer nomenclature is given according to the recommendations submitted to IUPAC by Intrnetional Board on the Application of the Mössbauer Effect (IBAME).11 X-ray measurements were carried out on a DRON-3M diffractometer (Russia) using CuKa radiation and a Ni filter.Hardening of the iron-doped vanadium(V) oxide melt leads to the formation of a black glassy product, which becomes dark brown in a fine powder. An X-ray powder diffraction study of this product revealed no phases other than V2O5. The X-ray diffraction patterns are similar to those of pure V2O5 (both hardened under the above conditions and non-hardened) and correlate with the published data.12 Figure 1(a) shows a typical room-temperature Mössbauer spectrum of the substance. To a first approximation, the spectrum contains two resolved lines with different intensities and widths on the background of extended absorption.The spectrum can be formally described by a model containing three components, a singlet, a doublet and an unresolved constituent of extended absorption.The envelope of the extended absorption can be formally described, e.g., by a broad singlet.† However, it is evident that this component results from magnetic interactions in the substance, as confirmed by low-temperature measurements. Indeed, the lines of the hyperfine magnetic structure are resolved in the spectrum [Figure 1(b)] as the temperature is decreased up to –196 °C.However, the resolution is inadequate, and it is most likely that the lines are related to two sextets. Moreover, the central portion of the spectrum becomes more complicated. Thus, it is impossible to suggest an unambiguous model for the description of the spectrum. However, an analysis of the spectrum shows that the presence of a divalent iron derivative among magnetic species is quite possible.† In the spectrum, the parameters of this line are d = 0.45±0.06 mm s–1 and Gexp = 8.8±0.6 mm s–1. a b Absorption (%) v/mm s–1 0 6 12 0 3 6 9 –12 –6 0 6 12 Figure 1 Mössbauer absorption spectra of iron-doped V2O5 at (a) 291 and (b) 77 K.Mendeleev Communications Electronic Version, Issue 5, 1999 (pp. 171–212) An increase in the sample temperature up to 400 °C allowed us to exclude the presence of unresolved magnetic structures in the spectrum, and the shape of the central portion of the spectrum remained unchanged. However, the spectrum changed at this temperature because of the annealing of the substance. It is safe to say that the right component of the central portion of the spectrum [Figure 1(a)] is due to a trivalent iron doublet.The fact that the doublet line widths are considerable‡ suggests the presence of several different trivalent iron species, which are similar in nature and geometrical arrangement. Indeed, the doublet parameters, especially the quadrupole splitting and line widths, changed with a little variation of the synthesis conditions and the iron content of the system.This part of the spectrum can correspond to trivalent iron ions occupying vacancies that are available in the vanadium(V) oxide structure13 and can also be formed under deformation of the V2O5 crystal lattice in the course of doping of this oxide by iron.14 Of course, an attempt to describe the substance on the basis of the vanadium(V) oxide crystal structure is provisional, because the substance can contain an amorphous phase in considerable amounts.15 The singlet with the parameters d = –0.56±0.01 mm s–1 and Gexp = 0.41±0.02 mm s–1 is of primary interest [Figure 1(a)].The relationship between the isomer shift and the oxidation state of iron in an oxygen environment is well known.16 Isomer shifts for hexavalent iron derivatives were measured in a study of alkali and alkaline earth metal ferrates, in which the Fe6+ ion is in the tetrahedral arrangement of oxygen ions.The roomtemperature isomer shift values for these compounds are in the range from –0.98 to –0.88 mm s–1.16,17 In addition, the isomer shift of sodium ferrate(VI) was found to be equal to –0.80 mm s–1 at room temperature.18 The isomer shift for pentavalent iron is known only for the compound La2LiFeO6 with the octahedral coordination of the Fe5+ ion, and it is equal to –0.41 mm s–1.5,6 According to the well-known fact that the isomer shift of iron decreases with decreasing coordination number,16,19 we can conclude that the isomer shift d = –0.56 mm s–1 corresponds to the 5+ oxidation state of iron with a coordination number lower than six.The position of the line with d = –0.56 mm s–1 remained unchanged with varying the synthesis conditions and the iron content. A change in the temperature of measuring the spectrum leads to a systematic drift of the line; this drift is characterised by the value of dd/dT = (5.0±0.2)×10–4 mm s–1 K–1 in the temperature range examined. The fact that noticeable changes in the shape of the spectrum lines in the central portion, namely, the ratios of intensities and widths, were not observed in the temperature range from –196 to 400 °C did not allow us to consider the asymmetry of the central portion of the spectrum as a result of the Gol’danskii–Karyagin effect20 or the Blume mechanism.21 Because of this, the suggested interpretation of the asymmetry observed as a result of the appearance of the singlet with d = –0.56 mm s–1 seems to be reasonable.The possible existence of divalent iron in the substance is not surprising; moreover, in our opinion, this fact can be an additional argument in support of the formation of iron in a higher oxidation state. In fact, the presence of divalent iron can correspond either to the reduction of Fe3+ placed in a V2O5 matrix to Fe2+ and the release of oxygen or to the disproportionation of trivalent iron 3Fe3+ ® Fe5+ + 2Fe2+, which was initially introduced into the system as Fe2O3.As mentioned above, vanadium(V) oxide cannot be a reducing agent. However, note that the reduction of trivalent iron to a divalent state was not detected under similar experimental conditions.14,15 ‡ The parameters of this symmetric doublet in the spectrum are d = = 0.34±0.01 mm s–1, D = 0.34±0.01 mm s–1 and Gexp = 1.02±0.04 mm s–1.If the system is considered on a basis of the crystal structure of vanadium(V) oxide, we can suggest that the formation of pentavalent iron takes place by isovalent isomorphous replacement of vanadium with iron in the V2O5 structure. The coordination polyhedron of vanadium in V2O5 can be considered as a strongly distorted octahedron.22 This distortion is significant, so that the coordination polyhedron is, in fact, an irregular trigonal bipyramid,14,22 which corresponds to the coordination number 5.Further distortion, which appears with the introduction of iron into the system as a result of the difference in the ionic radii of vanadium and iron and of the filling of vacancies in the vanadium oxide structure by Fe2+ and Fe3+ ions, can lead to a decrease in the coordination number.14 Thus, we found for the first time that pentavalent iron can be formed in the course of doping vanadium(V) oxide by iron.References 1 Z. Homonnay, S. Music, T. Nishida, N. S. Kopelev and A.Vertes, Mössbauer Spectroscopy of Sophisticated Oxides, Akademiai Kiado, Budapest, 1997. 2 Yu. D. Perfil’ev, Ross. Khim. Zh., 1998, 42 (3), 47 (in Russian). 3 R. D. Shannon, Acta Crysallogr., Sect. A: Fundam. Crystallogr., 1976, 32, 751. 4 B. Buffat, G. Demazeau, M. Pouchard, L. Fournes, J.-M. Dance, P. Fabritchnyi and P. Hagenmuller, C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Astron., 1981, 292, 509. 5 B. Byuffa, G. Demazeau, M. Pouchard, L. Fournes, J. M. Dance, P. Fabrichnyi and P. Hagenmuller, Fiz. Tverd. Tela, 1981, 23, 2262 (Sov. Phys. Solid State, 1981, 23, 1324). 6 J. L. Soubeyroux, B. Buffat, N. Chevreau and G. Demazeau, Physica B (Amsterdam), 1983, 120, 227. 7 J. H. Choy, G. Demazeau and S. H. Byeon, Solid State Commun., 1991, 77, 647. 8 Handbuch der Präparativen Anorganischen Chemie, ed.G. Brauer, Ferdinand Enke Verlag, Stuttgart, 1975, vol. 3. 9 V. I. Gol’danskii, A. V. Dolenko, B. G. Egizarov, V. P. Romashko and A. I. Shamov, Prib. Tekh. Eksp., 1970, 101 (Instruments and Experimental Techniques, 1970, 1071). 10 J. G. Stevens, Hyperfine Interact., 1983, 13, 221. 11 Mössbauer Effect Reference and Data Journal, 1998, 21 (10). 12 D. Schulz, F. Larson and G. McCarthy, in JCPDC–ICDD Database, Card no. 41-1426. 13 R. Enjalbert, P. Lecante and J. Galy, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1986, 42, 1465. 14 A. A. Abdullaev, L. M. Beliaev, T. V. Dmitrieva, G. F. Dobrjanskii, V. V. Il’uhin and I. S. L’ubutin, Kristallografiya, 1969, 14, 473 (Crystallography USSR, 1969, 14, 389). 15 S. Mandal, S. Hazra, D. Das and A. Ghosh, J. Non-Cryst. Solids, 1995, 183, 315. 16 F. Menil, J. Phys. Chem. Solids, 1985, 46, 763. 17 R. H. Herber and D. Johnson, Inorg. Chem., 1979, 18, 2786. 18 N. S. Kopelev, Yu. D. Perfiliev and Yu. M. Kiselev, J. Radioanal. Nucl. Chem., 1992, 162, 239. 19 S. K. Dedushenko, L. A. Kulikov and Yu. D. Perfil’ev, Radiokhimiya, 1998, 40, 403 (Radiochemistry,1998, 40, 416). 20 S. V. Karyagin, Dokl. Akad. Nauk SSSR, 1963, 148, 1102 [Dokl. Phys. Chem. (Engl. Transl.), 1963, 148, 110]. 21 M. Blume, Phys. Rev. Lett., 1965, 14, 96. 22 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 1984, p. 568. Received: 23rd April 1999; Com. 99/1483

ISSN:0959-9436     

出版商:RSC    

年代:1999

数据来源: RSC