ISSN 09655441, Petroleum Chemistry, 2014, Vol. 54, No. 7, pp. 556–561. © Pleiades Publishing, Ltd., 2014. Original Russian Text © I.A. Prikhno, E.Yu. Safronova, A.B. Yaroslavtsev, W. Wu, 2014, published in Membrany i Membrannye Tekhnologii, 2014, Vol. 4, No. 2, pp. 107–113.
Synthesis and Study of Hybrid Materials Based on Nafion Membranes, Hydrated Silica, Phosphotungstic Acid, and Its Acid Salts I. A. Prikhnoa, E. Yu. Safronovaa*, A. B. Yaroslavtseva, and W. Wub a
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, Russia bZhejiang University, Hangzhou, China *email:
[email protected] Received October 15, 2013
Abstract—The results of research of the properties of hybrid materials based on Nafion membranes, silica, phosphotungstic acid, and its acid salts (potassium, rubidium, and cesium) prepared by the in situ method has been described. Modification leads to an increase in the water content and a significant increase in the conductivity of the hybrid membranes, particularly at a low relative humidity. The materials containing slightly soluble acid salts of phosphotungstic heteropoly acid exhibit a lower conductivity than the material that contains the acid; however, the former are more stable. It has been found that the diffusion permeability of an HCl solution in Nafion + SiO2 + MxH3–xPW12O40 systems is lower than in Nafion + SiO2 and further decreases with increasing ionic radius of the cation. An explanation of the causes of these changes in the prop erties has been proposed. Keywords: hybrid membranes, Nafion, phosphotungstic acid, diffusion permeability, proton conductivity DOI: 10.1134/S0965544114070093
INTRODUCTION Ionexchange membranes are constantly attracting attention of the scientific community [1, 2], since they are extensively used in a variety of modern technolo gies, such as water treatment, separation and concen tration of liquids, and alternative power engineering [3, 4]. One of the most promising ionexchange mem brane materials are Nafion perfluorsulfonic acid cat ionexchange membranes (DuPont), which are used as a solid electrolyte for designing lowtemperature fuel cells (FCs), effective and environmentally friendly power generation devices [1, 2, 5, 6]. The effi cient operation of FCs based on Nafion membranes requires the maintenance of a high humidity because the proton transport rate sharply decreases with decreasing relative humidity (RH) [7]. This feature complicates the design of FCs and, as a consequence, makes their use less economically attractive. The properties of Nafion membranes are improved by using an approach based on their modification with various dopants, in particular hydrated oxides and het eropoly acids (HPAs) [1, 6, 8, 9]. The most highly conductive inorganic compounds include HPA crystal hydrates, in particular phosphotungstic heteropoly acid 29hydrate (PTA) H3PW12O40 ⋅ 29H2O (σ = 0.17 Ω–1 cm–1 at 25°С [10]), which can be regarded as a promising dopant that makes it possible to increase
the proton conductivity of Nafion, particularly at a low humidity. However, since HPAs are highly soluble, they must be stabilized in the membrane matrix. To this end, HPAs are subjected to sorption on the surface of hydrated silica and/or converted to partially soluble and insoluble acid salts [11–13]. After treatment in dilute acid solutions, the surface of these salts contains a large amount of readily dissociating protons; there fore, their conductivity is also fairly high. In addition, the solubility of PTA acid salts of alkali metals decreases with increasing the ionic radius of the cat ion: the potassium salt is partially soluble, while the rubidium and cesium salts are insoluble in water [14]. It has been previously shown that modification of Nafiontype membranes with SiO2 + H3PW12O40 par ticles makes it possible to achieve a significant increase in conductivity at a low RH compared to an unmodi fied membrane (at t = 25°С and RH = 10%, proton conductivity σ = 1.7 × 10–3 and 4 × 10–6 Ω–1 cm–1 for the hybrid and original membranes based on a Nafion analogue, respectively [11]). The conversion of PTA into insoluble acid cesium salt CsxH3 – xPW12O40 leads to a 20–30% decrease in conductivity compared to a membrane containing PTA; however, this procedure prevents the dopant from leaching. Data on the prop erties of hybrid membranes modified with other insol uble salts of PTA with alkali metals have not been reported.
556
SYNTHESIS AND STUDY OF HYBRID MATERIALS
The aim of this study is to synthesize and examine the transport properties of hybrid materials based on a Nafion 117 membrane, hydrated silica, PTA, and its partially soluble and insoluble acid salts of potassium, rubidium, and cesium. EXPERIMENTAL Materials and Reagents Nafion 117 membranes (Aldrich; thickness, 230– 250 μm; equivalent weight, 1100) were studied. Tetra ethoxysilane Si(OC2H5)4 (≥98%, Fluka), PTA H3PW12O40 ⋅ 44H2O (Acros), potassium carbonate K2CO3 (reagent grade), rubidium nitrate RbNO3 (reagent grade), cesium carbonate Cs2CO3 (special purity grade), solutions of ammonia and hydrochloric acid, and isopropyl alcohol (Khimmed) were used as reagents. Synthesis of Materials To conduct standardization and remove traces of the monomer, the membrane was conditioned as fol lows: it was (a) soaked in a 5% HCl solution at a tem perature of 80°C for 1.5 h and (b) twice exposed to deionized water at a temperature of 80°C for 1.5 h. Hybrid membranes were prepared by the in situ introduction of a dopant into the membrane matrix as described in [11]. At the first stage, hydrated silica nanoparticles were introduced into the membrane. To this end, the Nafion 117 membrane was exposed to a solution of tetraethoxysilane in isopropanol (1 : 1) for 1.5 h and then to a 10% ammonia solution. To intro duce PTA nanoparticles into the matrix of the result ing membrane, the membrane was immersed in a 0.025 M PTA solution under constant stirring for 2 h. To convert PTA into a salt form, Nafion + SiO2 + H3PW12O40 membranes were held in solutions of salts of the respective metals (0.05 M K2CO3, 0.1 М RbNO3, and 0.05 M Cs2CO3) for 3 h. After modifica tion, the membranes were subjected to conditioning, during which the neutral salts were converted to acid salts МxH3 – xPW12O40. Thus, five samples of hybrid membranes were pre pared and studied (Nafion + SiO2, Nafion + SiO2 + H3PW12O40, Nafion + SiO2 + KxH3 – xPW12O40, Nafion + SiO2 + RbxH3 – xPW12O40, Nafion + SiO2 + CsxH3 – xPW12O40). Methods Analysis of microstructure and electron probe microanalysis (EPMA) of the samples were conducted by scanning electron microscopy (SEM) using a JEOL JSM 6380LA microscope combined with a JEOL 2300 analytical station and by transmission electron microscopy (TEM) using a JEOL JEM1011 instru ment at an accelerating voltage of 100 kV. The samples were preliminarily subjected to ultrasonic dispersion. PETROLEUM CHEMISTRY
Vol. 54
No. 7
2014
557
To determine the elemental composition of the introduced dopant and estimate its amount, the mem brane sample was calcined at 600°С to destroy the polymer matrix. A weighed portion of the residue was dissolved in sodium hydroxide. Elemental analysis was conducted using an IRIS Advantage inductively cou pled plasma atomic emission spectrometer (Thermo Jarrell Ash, United States). Thermal analysis of the samples was conducted using a NetzschTG 209 F1 thermobalance in an argon atmosphere in aluminum crucibles in the temperature range of 25–150°С (heating rate of 10°C/min). The water content was determined from the difference in the weight of the membrane before the heat treatment and after holding at 150°С. To conduct standardiza tion, all samples were preequilibrated at RH = 95%. Ionexchange capacity (IEC, mgequiv/g) was determined as follows. A 0.5–1 g portion (mportion) of the membrane held at an RH of 95% was exposed to 50 mL of a 0.5 M NaCl solution (VNaCl, L) under con stant stirring for 24 h. After that, the salt solution was separated from the membrane and titrated with a NaOH solution. The IEC was calculated by the for mula C V IEC = H+ NaCl × 10 −3, (1) m where СH+ is the proton concentration (mol/L) in the NaCl solution after holding the membrane in it and m is the weight of the membrane sample on a dry cation exchanger basis (m = mportionWH2O, g). Ionic conductivity was determined at different temperatures in the range of 20–100°C (measure ments were conducted in contact with deionized water) and at different RH values (measurements were conducted at 25°С). To prepare samples with different water contents, the membranes were held in a dessica tor at an RH provided by saturated solutions of the fol lowing salts (the RH values are shown in parentheses): СаCl2 (32%), NaBr (58%), NaCl (75%), and Na2HPO4 (95%). The measurements were made using a 2V1 alternatingcurrent bridge (a frequency range of 10 Hz to 6 MHz) on symmetrical carbon/mem brane/carbon cells with an active area of 0.25 cm2. Conductivity (Ω–1 cm–1) was found via extrapolating the impedance spectrum to the active resistance axis. The diffusion permeability and H+/Na+ interdiffu sion of all samples, except of Nafion + SiO2 + H3PW12O40, were measured as described in [15]. The samples were placed in a cell composed of two cham bers separated by a membrane. The volume of each chamber was 32 cm3. One of the chambers was filled with a 0.1 M HCl solution; the other, with deionized water (in the case of measuring the diffusion perme ability) or a 0.1 M NaCl solution (in the case of mea suring the H+/Na+ interdiffusion). In the experiment, the change in the specific pH value of the solution in the chamber with deionized water or the NaCl solu tion (in the case of interdiffusion) was measured using
558
PRIKHNO et al.
10 nm Fig. 1. TEM micrograph of the Nafion + SiO2 membrane.
an EconixExpert Expert001 pH meter. The pH meter was calibrated using standard buffer solutions. The end of the experiment was determined from the stabilization of the pH values of the solution. The dif fusion permeability of the membranes was determined by the relationship P = (dc/dt)Vl/SΔc, (2) 3 where V is the solution volume, cm ; l is the membrane thickness, cm; Δc is the concentration, mol/cm3; and t is the time, s (the error in the determination of P was less than 1%). RESULTS The efficiency and uniformity of modification of the membranes was proved by EPMA. All hybrid membrane samples contain silicon atoms; the samples containing PWA and its salts contain phosphorus and tungsten atoms in a ratio close to 1 : 12 and atoms of the alkali metals corresponding to the salts, i.e., potas sium, rubidium and cesium. Elemental analysis has shown that the Nafion 117 + SiO2 membrane contains the dopant in an amount of 1 wt % of the dry basis, whereas the Nafion 117 + SiO2 + CsxH3 – xPW12O40 membrane has the dopant content of 2.9 wt %. In addition, the silicon content is almost the same for all the samples. The efficiency of modification was also confirmed by the TEM data. The hybrid membranes contain nanoparticles with a size of 3–5 nm (Fig. 1). The IEC of the hybrid membranes is slightly higher than that of the original Nafion membrane; further more, there is a tendency toward an increase in the IEC of the samples containing silica and a PTA salt with increasing the ionic radius of the alkali cation. The water content of all the hybrid membranes exceeds that of the original Nafion 117 sample, which is 19.5% (Table 1). The water content of the samples containing PTA and its acid salts is slightly lower than that of the Nafion + SiO2 membrane.
Figure 2 shows the temperature dependence of conductivity measured in contact with water for the different samples. For the Nafion + SiO2 + H3PW12O40 membranes, the measurements were not conducted because of the risk of leaching of PTA from the membrane matrix during the experiment. The pro ton conductivity of the membranes containing SiO2 and acid salts of PTA is slightly lower than the conductivity of the Nafion + SiO2 sample. The Nafion + SiO2 + RbxH3 – xPW12O40 membrane exhib its the lowest conductivity, and the highest conductiv ity for the ternary systems (0.119 Ω–1 cm–1 at a temperature of 90°C) was obtained for the membrane containing the cesium salt of PTA. During the modification of the membranes, the activation energy of conductivity remains close to that of the original Nafion membrane (18.1 ± 0.5 kJ/mol for Nafion + SiO2 + RbxH3 – xPW12O40 and Nafion + SiO2 + CsxH3 – xPW12O40) or slightly increases (to 22.9 ± 0.5 kJ/mol in the case of Nafion + SiO2 and Nafion + SiO2 + KxH3 – xPW12O40). Figure 3 shows the dependence of conductivity on RH for all the membrane samples examined. The modification of the Nafion membrane makes it possi ble to achieve a significant increase in conductivity under lowRH conditions; the lower the humidity, the more pronounced the observed hybrid effect. The highest conductivity at RH = 32% was found for the Nafion + SiO2 + H3PW12O40 membrane: σ = 9.45 × 10–3 Ω–1 cm–1. This value is more than an order of magnitude higher than the conductivity of the original Nafion membrane under these conditions (σ = 6.38 × 10–4 Ω–1 cm–1). The conversion of PTA (the Nafion + SiO2 + H3PW12O40 membrane) to insoluble salt forms leads to a decrease in conductivity by 20– 25% in the case of potassium and rubidium (Nafion + SiO2 + КxH3 – xPW12O40 and Nafion + SiO2 + RbxH3 – xPW12O40) or 55% in the case of the cesium salt (at RH = 32%). Comparison of the diffusion permeability and interdiffusion values gives the possibility to evaluate the rate of diffusion of the anions and cations across the membrane. The H+/Na+ interdiffusion coeffi cients are 1–1.5 orders of magnitude higher than the diffusion permeability of the HCl solution; however, they vary symbatically as a function of composition (Table 2). The appearance of PTS salts in the Nafion + SiO2 system leads to a decrease in the diffusion perme Table 1. Water content of different membrane samples Sample
Water content, %
Nafion + SiO2 Nafion + SiO2 + H3PW12O40 Nafion + SiO2 + KxH3 – xPW12O40 Nafion + SiO2 + RbxH3 – xPW12O40 Nafion + SiO2 + CsxH3 – xPW12O40
22.6 20.2 21.4 22.4 21.1
PETROLEUM CHEMISTRY
Vol. 54
No. 7
2014
SYNTHESIS AND STUDY OF HYBRID MATERIALS
559
Nafion + SiO2 Nafion + SiO2 + KxH3 – xPW12O40 Nafion + SiO2 + RbxH3 – xPW12O40 Nafion + SiO2 + CsxH3 – xPW12O40
logσ [Ω–1 cm–1] –0.7 –0.8 –0.9 –1.0 –1.1 –1.2 –1.3 –1.4 2.7
2.8
2.9
3.0
3.1
3.2
3.3 3.4 1000/T, K–1
Fig. 2. Arrenius dependence of proton conductivity on the temperature as measured in contact with water.
logσ [Ω–1 cm–1] –1.0 –1.2 –1.4 –1.6 –1.8 –2.0 –2.2 –2.4 –2.6 –2.8 –3.0 –3.2
Nafion Nafion + SiO2 Nafion + SiO2 + H3PW12O40 Nafion + SiO2 + KxH3 – xPW12O40 Nafion + SiO2 + RbxH3 – xPW12O40 Nafion + SiO2 + CsxH3 – xPW12O40
30
40
50
60
70
80
90
100
RH, %
Fig. 3. Dependence of the proton conductivity of the membranes on humidity.
ability; in addition, with an increase in the ionic radius upon switching from potassium to cesium, the diffu sion permeability decreases by a factor of 1.5 from 4.81 × 10–7 to 3.2 × 10–7 cm2/s. Upon passing from the potassium to the cesium salt, the H+/Na+ interdiffu sion coefficient decreases more significantly—by a factor of 3.5. DISCUSSION The effects observed during modification can be explained in terms of the structure of perfluorinated ionexchange membranes and the model of limited PETROLEUM CHEMISTRY
Vol. 54
No. 7
2014
elasticity of membrane pore walls described in [2, 16, 17]. During the in situ modification, dopant parti cles are formed in the membrane pores. Since the water uptake of the hybrid membranes is greater than that of the original Nafion sample, in accordance with the model of limited elasticity of pore walls [17], the modification is accompanied by an increase in the pore size and, as a consequence, in the channels that connect them. This is the main cause of increase in the ionic conductivity of the hybrid membranes at 100% RH compared to the original Nafion membrane (Fig. 2). A decrease in the conductivity of the ternary systems based on the membrane, SiO2, and acid salts
560
PRIKHNO et al.
Table 2. Diffusion permeability and H+/Na+ interdiffusion coefficients (P, cm2/s) for the test membranes Contacting solution Sample Nafion + SiO2 Nafion + SiO2 + KxH3 – xPW12O40 Nafion + SiO2 + RbxH3 – xPW12O40 Nafion + SiO2 + CsxH3 – xPW12O40
0.1 M HCl/H2O
0.1 M HCl/0.1 M NaCl
5.60 × 10–7 4.81 × 10–7 4.38 × 10–7 3.20 × 10–7
1.43 × 10–5 1.04 × 10–5 1.05 × 10–5 3.98 × 10–6
of PTA relative to the binary Nafion + SiO2 system at 100 % RH can be attributed to two factors. First, since the particles of PTA salts are localized on the SiO2 sur face, the particle size of the dopant is higher in the ter nary than in the binary systems, which can lead to the formation of new regions hindering the ionic conduc tivity between the nanoparticle surface and the mem brane pore wall because the elasticity of the latter is extremely limited. It is for this reason that the conduc tivity is lower than in the Nafion + SiO2 membrane. Second, the simultaneous presence of a proton and an alkali metal cation in the membrane matrix can result in a slowdown in the ion transport rate (the socalled mixed alkali effect). In addition, the size of alkali metal cations is significantly greater than the size of protons, and they can further hinder the transport. This assumption is also confirmed by a decrease in the H+/Na+ interdiffusion coefficient upon passing from the potassium to the cesium salt (Table 2). The slightly lower conductivity values of the Nafion + SiO2 + RbxH3 – xPW12O40 membrane com pared with Nafion + SiO2 + CsxH3 – xPW12O40 at a 100% RH appeared surprising. In [14], it was shown that the solubility of MxPW12O40 ⋅ nH2O samples decreases with an increase in the ionic radius of M+. A lower solubility of salts with larger cations provides for higher supersaturation. In addition, the nucleation rate increases faster than the nucleus growth rate [18] and the particle size of the salt decreases. Since acidic protons exchange occurs only on the surface of the particles, which increases with a decrease in their size, the cesium salt is characterized by a higher degree of substitution. Owing to this, the ionic conductivity of the salts increases in the order K < Rb < Cs. During the synthesis of a salt from PTA in a mem brane pore, the particle size is determined by the pore size and the amount of HPA preliminarily introduced into the pores. Therefore, it should not significantly depend on the cation type. At the same time, the potassium salt can partially dissolve and leach from the membrane matrix; owing to this, the size of the result ing dopant particle can be less than in the case of the rubidium and cesium salts. The low ionic conductivity of the membrane containing the rubidium salt is apparently due to the fact that, unlike the other two samples, it was prepared from the nitrate, not the car bonate used for other systems. In addition, lower pH values are maintained during the synthesis, which can
lead to lower MxH3 – xPW12O40 ⋅ nH2O concentrations in the membrane. As the relative humidity decreases, the membrane undergoes dehydration, which alters the transport mechanism: the distance between the contiguous oxy gen atoms becomes larger, the activation energy of proton hopping increases, and the hopping frequency decreases. As a result, the conductivity of the mem brane sharply decreases [19]. In the case of hybrid membranes, the oxygen atoms of the dopant are involved in the proton transport at a low humidity [20], thereby enhancing the conductivity (Fig. 3). In addition, the presence of the particles in the mem brane pore prevents it from decreasing and hinders its “collapse” during dehydration. At a low relative humidity, conductivity can be further increased by introducing PTA or its salts. The effect is due to an increase in the concentration of acidic protons involved in the ion transport. A certain decrease in conductivity upon passing from the potassium to the cesium salt at a low humidity can be attributed to steric hindrances arising from the presence of larger cations. The study of diffusion permeability has revealed that ions of both types are simultaneously transported across the membrane and the ratedetermining step of the entire process is the rate of diffusion of the anions [21]. The cations move along the walls of the mem brane pores and channels within the thin Debye layer formed by the dissociated functional sulfo groups of the membrane and the cations. The anions are excluded from this region, and their transport occurs through an almost electrically neutral solution, which is localized in the middle of the pore and has a compo sition close to that of the contacting solution [22]. During modification of the Nafion membrane, the dopant particles are incorporated into the center of the pore and have hardly any effect on the thin Debye layer along the walls. Incorporation of a neutral dopant— SiO2 —leads to an increase in the free space within the pore (as evidenced by an increase in the water content of the membrane) in which the anions can diffuse. The appearance of PTA salts on the surface of SiO2 results in the formation of another electric double layer near the dopant surface, although it is weaker than that located near the pore wall. The space in which the anions can diffuse decreases, thereby leading to a decrease in the diffusion permeability (Table 2). A cer tain decrease in the diffusion permeability in the PETROLEUM CHEMISTRY
Vol. 54
No. 7
2014
SYNTHESIS AND STUDY OF HYBRID MATERIALS
Nafion + SiO2 + MxH3 – xPW12O40 ternary systems with increasing ionic radius of the alkali cation is apparently due to a further decrease in the free space for anion diffusion owing to steric hindrances. CONCLUSIONS Modification of the Nafion membrane with nano particles of hydrated silica, PTA, and its acid salts leads to an increase in the water retention capacity and ionic conductivity. The highest increase in conductiv ity is observed under lowhumidity conditions; at RH = 32%, the maximum value achieves 9.45 × 10 ⎯3 Ω–1 cm–1. It has been shown that the anion trans port rate in the membranes that contain both silica and HPA salts is lower than in the membrane containing only silica and decreases with increasing ionic radius of the cation contained in the salt. The causes of changes in the properties of Nafion membranes by incorporation of different HPA acid salts have been proposed. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project nos. 1208 91161_GFEN_a and 130812103_ofi_m. REFERENCES 1. H. Ahmad, S. K. Kamarudin, U. A. Hasran, and W. R. W. Daud, Int. J. Hydrogen Energy 35, 2160 (2010). 2. A. B. Yaroslavtsev and V. V. Nikonenko, Ross. Nan otekhnol. 4 (3), 33 (2009). 3. D. J. Jones and J. Roziere, Handbook of Fuel Cells: Fun damentals, Technology and Applications, vol. 3: Fuel Cells Technology and Applications, Ed. by W. Vielstich, H. A. Gasteiger, and A. Lamm (Wiley, Chichester, 2003), p. 219.
PETROLEUM CHEMISTRY
Vol. 54
No. 7
2014
561
4. Membrane and membrane Technologies, Ed. by A. B. Yaroslavtsev (Nauchnyi Mir, Moscow, 2013) [in Russian]. 5. F. Lufrano, V. Baglio, P. Staiti, et al., J. Power Sources 243, 519 (2013). 6. A. B. Yaroslavtsev, Yu. A. Dobrovol’skii, N. S. Shag laeva, et al., Usp. Khim. 81, 191 (2012). 7. D. R. Morris and X. Sun, J. Appl. Polym. Sci. 50, 1445 (1993). 8. M. P. Rodgers, Z. Shi, and S. Holdcroft, J. Membr. Sci. 325, 346–356 (2008). 9. P. Staiti, A. S. Arico, V. Baglio, et al., Solid State Ionics 145, 101 (2001). 10. A. B. Yaroslavtsev, Usp. Khim. 63, 449 (1994). 11. E. Yu. Safronova, I. A. Stenina, and A. B. Yaroslavtsev, Russ. J. Inorg. Chem. 55, 13 (2010). 12. M. Amirinejad, S. S. Madaeni, M. A. Navarra, et al., J. Power Sources 196, 988 (2011). 13. A. Mahreni, A. B. Mohamad, A. A. H. Kadhum, et al., J. Membr. Sci. 327, 32 (2009). 14. A. B. Yaroslavtsev, E. M. Yaroslavtseva, and V. F. Chu vaev, Zh. Neorg. Khim. 39, 948 (1994). 15. A. G. Mikheev, E. Yu. Safronova, and A. B. Yaroslavt sev, Pet. Chem. 53, 504 (2013). 16. W. Y. Hsu and T. J. Gierke, J. Membr. Sci. 13, 307 (1983). 17. S. A. Novikova, E. Yu. Safronova, A. A. Lysova, and A. B. Yaroslavtsev, Mendeleev Commun. 20, 156 (2010). 18. A. B. Yaroslavtsev, Solid State Chemistry, (Nauchnyi Mir, Moscow, 2009) [in Russian]. 19. A. B. Yaroslavtsev, Polym. Sci., Ser. A 55, 674 (2013). 20. E. Yu. Safronova, A. A. Lysova, S. A. Novikova, and A. B. Yaroslavtsev, Izv. Akad. Nauk, Ser. Khim., No. 1, 21 (2011). 21. A. B. Yaroslavtsev, Ross. Nanotekhnol. 7 (9/10), 8 (2012). 22. A. B. Yaroslavtsev, V. V. Nikonenko, and V. I. Zabo lotskii, Usp. Khim. 72, 438 (2003). Translated by M. Timoshinina
