Journal of Magnetism and Magnetic Materials 360 (2014) 67–72
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NiFe2O4/activated carbon nanocomposite as magnetic material from petcoke Sarah Briceño a,n, W. Brämer-Escamilla a, P. Silva a, J. García b, H. Del Castillo b, M. Villarroel b, J.P. Rodriguez c, M.A. Ramos d, R. Morales d, Y. Diaz e a Laboratorio de Física de la Materia Condensada, Centro de Física, Instituto Venezolano de Investigaciones Científicas IVIC, Apartado 20632, Caracas 1020-A, Venezuela b Laboratorio de Cinética y Catálisis, Departamento de Química, Facultad de Ciencias, Universidad de Los Andes ULA, Mérida 5101-A, Venezuela c Laboratorio de Microscopia Electrónica. Instituto de Estudios Científicos y Tecnológicos IDECYT. Apartado 47925 - Caracas 1041-A, Venezuela d Instituto Zuliano de Investigaciones Tecnológicas INZIT. Apdo. Postal 331. La Cañada-Maracaibo, Venezuela e Centro de Química, Instituto Venezolano de Investigaciones Científicas IVIC, Apartado 20632, Caracas 1020-A, Venezuela
art ic l e i nf o
a b s t r a c t
Article history: Received 9 April 2013 Received in revised form 13 January 2014 Available online 7 February 2014
Nickel ferrite (NiFe2O4) was supported on activated carbon (AC) from petroleum coke (petcoke). Potassium hydroxide (KOH) was employed with petcoke to produce activated carbon. NiFe2O4 were synthesized using PEG-Oleic acid assisted hydrothermal method. The structural and magnetic properties were determined using thermogravimetric and differential thermal analysis (TGA–DTA), X-ray diffraction (XRD), Fourier Transform Infrared (IR-FT), surface area (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). XRD analysis revealed the cubic spinel structure and ferrite phase with high crystallinity. IR-FT studies showed that chemical modification promoted the formation of surface oxygen functionalities. Morphological investigation by SEM showed conglomerates of spherical nanoparticles with an average particle size of 72 nm and TEM showed the formation of NiFe2O4/carbon nanofibers. Chemical modification and activation temperature of 800 1C prior to activation dramatically increased the BET surface area of the resulting activated carbon to 842.4 m2/g while the sulfur content was reduced from 6 to 1%. Magnetic properties of nanoparticles show strong dependence on the particle size. & 2014 Elsevier B.V. All rights reserved.
Keywords: Petcoke Activated carbon Ferrite Nanoparticle Hydrothermal synthesis Magnetic property
1. Introduction Petroleum coke (petcoke) including delayed coke, fluid coke, needle coke, shot coke, and flexi coke is a by-product of the upgrading process for oil sand derived bitumen. Petcoke is the most abundant byproduct of oil refining in Venezuela accounting for up to 30% of all products. The global production of petroleum coke was 80.815 MMTPA in 2000, and it was increased to 123.058 MMTPA in 2010. In 2015, the production is expected to grow to 161.271 MMTPA [1]. Currently Venezuela produces 20,000 tons per day of petcoke [2]. Effort has been directed towards petcoke utilization in the fields of production of carbon nanotubes, water purification, catalyst support, combustion and gasification to generate electric power or produce syngas. The applications for petcoke, however, have been limited mainly due to the high
n
Corresponding author. Tel.: þ 58 2125041547. E-mail addresses:
[email protected] (S. Briceño),
[email protected] (W. Brämer-Escamilla). http://dx.doi.org/10.1016/j.jmmm.2014.01.073 0304-8853 & 2014 Elsevier B.V. All rights reserved.
sulphur content (6%) and low surface area (5 m2/g) [3]. Due to its large carbon content (80%) and relatively low ash content, this petroleum coke can be converted to a value added product such as activated carbon, which may have many catalytic and adsorptive applications [1]. NiFe2O4 is an inverse spinel in which half of the ferric ions occupy the tetrahedral sites (A-sites) and the rest occupy the octahedral sites (B sites). Thus, the compound can be represented by ðFe31:0þ Þ½Ni21:0þ Fe31:0þ O24 , where the round and the square brackets represent A and B sites, respectively. NiFe2O4 is used in applications including high density magnetic recording media, magnetic refrigeration, magnetic liquids, microwave absorber, and catalysts [4]. The interest in the preparation of magnetic nanocomposites has grown tremendously in recent years due to the novel properties presented by these materials, such as limited agglomeration and narrow grain size distribution under space confinement effect [5]. Activated carbon supported ferrite has been used for absorbing hydrogen sulfide at a low temperature [6] and in lithium ion battery anodes [7]. We report the synthesis, structural and magnetic properties of NiFe2O4/AC nanocomposites from petcoke.
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held at 500 1C for 2 h. The sample was then allowed to cool down to room temperature under nitrogen flow.
2. Materials and methods 2.1. Synthesis of NiFe2O4 ferrite
2.4. Characterization methods For hydrothermal reactions, all the reagents were of analytical grade and used as received. A mixture of 1 mmol FeðNO3 Þ3 9H2 O, 0.5 mmol NiðCl2 Þ3 :6H2 O, 2.0 g of PEG 6000 and 3 ml of oleic acid was mixed in distilled water, followed by addition of 6 M KOH solution at pH 12. The mixture was then transferred into a Teflon lined stainless steel autoclave of 100 ml capacity. The sealed tank was heated and stirred at 200 1C for 10 h. The resulting black precipitates were collected by filtration and washed with deionized water, 10 M HCl and ethanol for several times, and dried in an oven for 24 h at 60 1C. To obtain the sample NiFe2O4 500, prepared NiFe2O4 were heat-treated in a vertical furnace in a nitrogen stream from room temperature to 500 1C at a heating rate of 10 1C/min, and held at 500 1C for 2 h. The sample was then allowed to cool down to room temperature under nitrogen flow.
Thermogravimetry and differential thermal analyses were carried out by STD Q600 in air 100 ml/min at 20 1C/min heating rate. Phase identification and average size of the samples were examined at room temperature using a polycrystalline sample brand X-ray diffractometer model Bruker D8 Focus using Cu K radiation (λ ¼1.5406 Å) and resolution of 0.021 in 2θ. Infrared spectra were recorded in the range of 400–4000 cm 1 with a Bruker vector 22 FT-IR spectrometer from samples in KBr pellets. The size and the morphology of particles were examined by scanning electron microscope Hitachi S-4500 and a transmission electron microscope Hitachi H-7100, respectively. The surface area of samples was measure by N2 physisorption, using the BET method on a Micromeritics ASAP 2010. Magnetic characterization was made in a homemade vibrating sample magnetometer.
2.2. Preparation of activated carbon (AC) 3. Results and discussion Venezuelan petcoke from INTEVEP–PDVSA Company was used as raw material. The petcoke was first added into H2 O2 15% solution. To mix completely, more solution was used (1 g of petcoke per 10 ml of solution), then it was stirred for 3 h at 120 1C. Finally, modified petcoke was thoroughly washed with distilled water. After rinsing under suction, the obtained materials were dried at 60 1C in an oven during 12 h. Modified sample was physically mixed with KOH at a mass ratio of 1:3. These mixture were heat-treated in a vertical furnace in a nitrogen stream from room temperature to 400 1C at a heating rate of 10 1C/min and held at 400 1C for 1 h, then a heating rate of 10 1C/min was applied up to the final activation temperature 800 1C, which was maintained for 1 h. The sample was then allowed to cool down to room temperature under nitrogen flow. After that, the activated carbon was washed first with a 1 M HCl aqueous solution, then with distilled water until the pH of the rinse remains constant and close to 6. Such washings were required for eliminating metallic potassium, excess of hydroxide and soluble impurities. The acidic washing would dissolve part of mineral matters in the raw petroleum coke [10]. After rinsing under suction, the activated carbon was dried in an oven for 24 h.
2.3. Preparation of NiFe2O4/AC NiFe2O4 were mixed with AC at a mass ratio of 1:1. These mixture were heat-treated in a vertical furnace in a nitrogen stream from room temperature to 500 1C at a heating rate of 10 1C/min, and
3.1. Structural properties Fig. 1(a) shows the thermogravimetric analysis (TGA) trace of NiFe2O4 sample. The decomposition process consists of three regions. They are 50–150 1C, 150–480 1C and 480–650 1C. Owing to the initial breakdown of the complex and spontaneous combustion, the first weight loss region from 50 to 150 1C indicates the evaporation of absorbed water with the liberation of H2 O, CO2 and NOx and the nitrate ions providing an oxidizing environment for the combustion of the organic components, the spontaneous combustion is caused from the interactions of PEG, oleic and nitrate ions. The second weight loss region observed between 150 1C and 480 1C is ascribed to dehydration of OH group in the spinel structure of some constituents such as NO3 and PEG that lead to two degradation systems involving both inter and intramolecular transfer reactions, the oxidation of complexes and formation of semi-organic carbon metal/metal oxide. The third weight loss region in the temperature range of 480–650 1C is believed to be due to the formation of corresponding metal oxide and the spinel phase. Above 640 1C there is no weight loss [11]. TGA of NiFe2O4/AC (Fig. 1(b)) exhibited a slight weight loss from 30 to 100 1C and an about 40% weight loss starting from about 250 to 800 1C. The slight weight loss was attributed to the release of the moisture and the later loss could be due to the decompositions of volatiles. The DTA profile of NiFe2O4/AC sample further revealed that the volatiles decomposition started at about 300 1C and lasted a wide temperature range with a broad peak
Fig. 1. TGA and DTA of (a) NiFe2O4 ferrite and (b) NiFe2O4/AC.
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Fig. 2. XRD of the activated carbon and petcoke.
Table 1 Average particle size, specific surface area and elemental analysis of petcoke, activated carbon, NiFe2O4, NiFe2O4/AC and NiFe2O4 500. Sample
Size (nm)
SBET (m2/g)
C
S (wt%)
O
Petcoke AC NiFe2O4 NiFe2O4 500 NiFe2O4/AC
1.05 0.49 72.83 161.70 100.79
31.61 842.40 9.55 71.33 515.00
79.96 65.10 15.69 15.69 48.40
6.84 1.17 — — 0.62
9.85 20.75 17.14 15.69 27.63
temperature at about 482 1C. The wide temperature range may be due to the overlap of thermal decomposition of multi unstable species on the surface of the sample, implying the presence of various structures in the NiFe2O4/AC sample [9].
3.1.1. XRD characterization The XRD analysis was carried out on powder samples to investigate the structural changes with the activation and thermal treatment. The typical XRD patterns of the activated carbon and petcoke are shown in Fig. 2. It can be clearly seen that there are diffraction peaks around 2θ ¼ 251 in each spectrum, corresponding to the diffraction of (002). The (002) peak of AC has no obvious difference, compared to that of petcoke. But the (002) peak of AC is broader than the (002) peak of petcoke. Table 1 lists the structural parameters of the activated carbon and the petcoke. The XRD pattern of the nanocomposite NiFe2O4/AC and the sample NiFe2O4 thermally treated at 500 1C is shown in Fig. 3. These samples present many strong and sharp crystalline peaks attributed to the face-centered cubic NiFe2O4 phase (PDF #100,325). The XRD patterns of sample exhibited the reflection plan (220), (311), (222), (400), (422), (511) and (440) that indicate the spinel cubic structure. Besides in Fig. 3(a) and (b), the two peaks between (100) and (220), (311) and (222) came from the existence of an important amount of αFe2 O3 phase (PDF # 840,309) and the (002) correspond to the AC support. Fig. 3 also shows an obvious transition from amorphous to crystal phase with the thermal treatment. The intensity (I ) of the peaks increases with the calcination process. This can be attributed to the increase of the rate of crystal growth as a result of expansion of volume and reduction of supersaturation of the system at elevated temperature.
Fig. 3. XRD pattern of (a) NiFe2O4, (b) NiFe2O4 500 and (c) NiFe2O4/AC.
The average particle sizes D were calculated according to Debye–Scherrer equation [8] considering the most intense peak (311): D¼
0:9λ β cos θ
ð1Þ
where λ is the wavelength of the radiation, θ is the diffraction angle, and β is the (FWHM) of the diffraction peak. The average particle sizes obtained by Scherrer are listed in Table 1, which reveals that the activated carbon matrix could be inhibited the growth of the nanoparticles in the sample NiFe2O4/AC with the thermal treatment when compare to the sample NiFe2O4 500 treated without AC. 3.1.2. Infrared analysis FT-IR spectroscopy was employed to explore changes in functional groups induced by modification of petcoke. The FTIR spectra of petcoke (Fig. 4(a)) present bands at 1370 cm 1 and 2914 cm 1 which can be assigned respectively to asymmetric and symmetric –C–H stretching vibrations in aliphatic such as –CH3, CH3, and –CH2 CH3 . The bands around 1588 cm 1 and 1438 cm 1 are assigned to the –C–C– aromatic bonds and exalted by the presence of phenolic groups, respectively. The FTIR spectra of activated carbon from petcoke (Fig. 4(b)) show the presence of the bending vibration of –C–H of aromatic rings which is supported by the weak absorption peak at 3151 cm 1. The broad band at about 3427 cm 1, appearing as intense and broad peak, was associated with the –O–H stretching vibration mode of hydroxyl functional groups [10]. From the FTIR studies, it could be concluded that the surface functional groups such as C–O–H, C–O–O–H and some alkyl groups of petroleum cokes play a important role in the chemical activation process [13]. Fig. 4(c) shows the FT-IR absorption spectra of NiFe2O4 samples, which were recorded in the range of 400–4000 cm 1. On the bases of literature data, in the range of
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3.1.4. BET The nitrogen adsorption isotherms of activated carbon and petcoke samples are shown in Fig. 6. The N2 adsorption isotherms are all type I, indicative of microporous carbons. Similar isotherms have been observed by some previous worker for different activated carbons [10,12]. The adsorption isotherms of AC are typical for highly microporous carbon, with a very steep initial uptake and a relative absence of meso and macroporosity. The results show that chemical modification prior to activation dramatically increased the BET surface area from 31.6 m2/g to 842.4 m2/g of the resulting activated carbon. These results suggest that activated carbon prepared from petcoke, using chemical modification followed by chemical activation, can be a way to produce activated carbons with high surface area. According to the element analysis from Table 1, the chemical composition of the activated carbons changed with treatment temperature. It can be seen that after KOH activation the majority of the sulphur was removed with treatment at 800 1C but the percent of sulphur continued to decrease with the incorporation of NiFe2O4. The percent of carbon initially decreased from 79% in petcoke to 65% in AC. 3.2. Magnetic properties NiFe2O4 is a spinel ferrite with a bulk volume magnetization of 275 kA/m and a crystal anisotropy constant K a ¼ 4:36 103 J=m3 [20]. The diameter below which becomes monodomain (D0) is 200 nm [21], and the diameter below which the nanoparticle is superparamagnetic (Dp) is 33 nm, and is obtained using sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 180kB T Dp ¼ ; ð2Þ πK a
Fig. 4. IR-FT spectra of (a) petcoke, (b) AC and (c) NiFe2O4.
1000–100 cm 1, the FT-IR bands of solids are usually assigned to vibration of ions in the crystal lattice. In all spinels and particularly in ferrites, two main broad metal oxygen bands are seen in the FT-IR spectra. Therefore the highest one, observed at 672 cm 1, corresponds to intrinsic stretching vibrations of the metal at the tetrahedral site, (Mt-O), whereas the lowest band that is observed at 476 cm 1 is assigned to octahedral metal stretching vibration (Mo–O) [14]. The band at 3412 cm 1 could be attributed to the O–H stretching vibration of H2 O absorbed by the sample.
3.1.3. SEM and TEM SEM was used in order to investigate the morphology of the synthesized NiFe2O4 (Fig. 5(a)) and thermally treated NiFe2O4 500 (Fig. 5(b)) the activated carbon from petcoke (Fig. 5(c)) and nanocomposite NiFe2O4/AC (Fig. 5(d)). We observe in Fig. 5 (a) that the prepared nanoparticles of NiFe2O4 have a conglomerates of 1 μm of individual particles about 72 nm according to Eq. (1), with spherical morphology. The cohesion of particles is due to the magnetic attraction and the presence of the surfactant PEG 6000 and the oleic acid. Fig. 5(b) shows a broad size distribution of the nanoparticles of NiFe2O4 thermally treated at 500 1C with octahedral morphology. The NiFe2O4 ferrite is clearly embedded in the activated carbon as further confirmed by TEM (Fig. 5(d)). This figure shows the incorporation of the nanoparticles to the activated carbon support and the formation of carbon nanofibers, this result has been reported by other authors using ferrite nanoparticles as catalyst [15,16].
here T is the temperature of the sample and kB is the Boltzman constant. The coercivity Hc and the remanence Mr as a function of the nanoparticle diameter, in a first approximation, can be calculated using [22,23] 3=2 ! Dp Hc ¼ H0 1 ð3Þ D and
"
M r ¼ M 0 exp tf 0 exp
K a D3p π 6kB T
!# ;
ð4Þ
were M0 is the maximum value of the remanence, t is the time elapses after turning off an external field and f0 is of the order of the Larmor precession period of the magnetic moments ð⋍0:7 s 1 Þ. Another magnetic parameter of great importance is the remanence ratio (R ¼ M r =M s ) that is used to identify the existence and type of interaction between particles. Systems with values of R above 0.5 are believed to be governed by exchange interactions while values below are attributed to dipolar interactions, the value of R¼ 0.5 is attributed to systems with non-interacting nanoparticles [25]. The magnetic measurements were performed by using a homemade VSM. The hysteresis curves are shown in Fig. 7. The three curves are typical for soft magnetic materials. The variation of the saturation magnetization (M s ), remanent magnetization (M r ) and coercive force (H c ) were shown in Table 2. This type of behavior is entirely consistent with a model of particle growth in the system in such a way that the differences in the magnetic parameters are associated with changes in the particle size [17]. As shown in Fig. 5(d) nanoparticles of NiFe2O4 grown on the activated carbon support, increasing the particle size of 72–100 nm with the heat treatment. Our results show that the magnetic properties of the samples increase with increasing the particle sizes. These properties originate from the large surface area, reduced size and modification in
S. Briceño et al. / Journal of Magnetism and Magnetic Materials 360 (2014) 67–72
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Fig. 5. SEM of (a) NiFe2O4, (b) NiFe2O4 500, (c) activated carbon from petcoke and (d) TEM of NiFe2O4/AC.
Fig. 6. N2 adsorption isotherm of activated carbon and petcoke.
interparticle interactions. Due to the proportionality of the magnetic particle energy in the external field to the particle sizes via the number of molecules in a single magnetic domain, the increase of the M s values with the increase of particle sizes can be attributed to the surface effects which are the result of the finite size scaling of nanoparticles [19]. Table 2 shows both experimental and theoretical results for M s , M r and R. Theoretical values for Hc and Mr were calculated from Eqs. (3) and (4) after including a statistical weight with respect to particle size and integrated over the entire range of sizes. For all the samples of Table 2 R indicates the existence of magnetostatic
Fig. 7. Magnetization curve of NiFe2O4, NiFe2O4/AC and NiFe2O4 500.
interactions, resulting in a decrease in the values of M r and H c . The drastic decline of Mr and Ms in sample NiFe2O4 turn is due to the low crystallinity and to the surface distortion due to the
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Table 2 Average particle size and magnetic properties of NiFe2O4,NiFe2O4 500 and NiFe2O4/AC. Sample
Size (nm) Hc (Oe)
72.83 NiFe2O4 NiFe2O4 500 161.70 NiFe2O4/AC 100.79
Ms (emu/g) Mr (emu/g)
38.0 (121.5a) 1.97 165.0 (160.0a) 61.80 144.0 (143.2a) 66.10
R
0.11 (11.57a) 0.056 12.30 (referenceb) 0.20 10.35 (12.92a) 0.16
the as-prepared nanocomposite. It was found that the modification of petcoke with KOH contributes to the formation of activated carbon with higher specific surface area and the percent of sulphur is reduced dramatically with the chemical and thermal treatment.
Acknowledgments
a
Theoretical values. b M0 ¼13.89 emu/g was calculated using the size distribution and the M r value of the sample (b).
interaction of transition metal ions in the spinel lattice with the oxygen atoms, which can reduce the net magnetic moment in the particle [24]. The lower M s value related to the NiFe2O4 nanoparticles with the smaller size could be attributed to the surface distortion due to the interaction of transition metal ions in the spinel lattice with the oxygen atoms, which can reduce the net magnetic moment in the particle [24]. This effect is particularly prominent for the small particles due to their large surface to volume ratio. In addition, the magnetocrystalline anisotropy of the particles is dependent on the degree of the crystallinity of the nanoparticles. Large proportion of crystal defects and dislocations can occur within the lattice of most samples prepared at lower temperatures which causes a significant reduction of the magnetic moment within the particles as a result of the magnetocrystalline anisotropy distortion [26]. The M s of the sample NiFe2O4 500 was 66.10 emu/g, close to the bulk value of NiFe2O4. It is well known that for magnetic particles the size has significant influence on their magnetic properties. For relatively larger particles, magnetic domains are formed to reduce the static magnetic energy. The number of domains increases with increasing particle size. The particles turn into single domain ones with their size under a critical radius (for NiFe2O4, this parameter is about 100 nm), resulting in the increasing coercive force due to vanishing of the magnetization caused by the movement of domain walls [21]. 4. Conclusions NiFe2O4 supported on activated carbon from petcoke (NiFe2O4/AC) was successfully synthesized. The TGA, XRD, IR-FT, SEM, TEM, BET surface area and magnetic measurements were used to characterize
We want to acknowledge to FONACIT for funding the project PEII 2012000216. The Unit and Structure Characterization of Materials UCEM–INZIT for the diffraction data collection in the X-ray diffractometer polycrystalline sample brand model Bruker D8 Focus, which was purchased with funds allocated to the research Project no. G-20050000433 and Lic. Jorge Rivas for the digitalization of SEM images. References [1] N. Rambabu, et al., Fuel Process. Technol. 106 (2013) 501–510. [2] Fundación Venezolana de Investigaciones Sismológicas. 〈http://www.funvisis. gob.ve/noticia.php?id=742〉. [3] J. Choi, G. Zaine Barnard, et al., Can. J. Chem. Eng. 90 (3) (2012) 631–636. [4] M.A. Gabal, J. Phys. Chem. Solids 64 (2003) 1375–1385. [5] S. Pattanaik, et al., J. Hazard. Mater. 178 (2010) 804–813. [6] N. Ikenaga, N. Chiyoda, et al., Fuel 81 (11–12) (2002) 1569–1576. [7] Y. Jin, S. Deok Seo, et al., Nanotechnology 23 (2012) 125402. [8] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, third ed., Prentice-Hall, Englewood Cliffs, NJ, 2001. [9] L. Chunlan, et al., Carbon 43 (2005) 2295–2301. [10] B. Jiang, et al., Fuel 87 (2008) 1844–1848. [11] P.P. Hankare, et al., J. Alloy. Compd. 553 (2013) 383–388. [12] M. Wu, et al., Fuel 84 (2005) 1992–1997. [13] R. Xiao, et al., J. Anal. Appl. Pyrolysis 96 (2012) 120–125. [14] K. Nejati, R. Zabihi, Chem. Cent. J. 6 (2012) 23–29. [15] R. Hosseini Akbarnejad, V. Daadmehr, et al., J. Supercond. Nov. Magn. 26 (2013) 429–435. [16] H. Zhang, J. Lin, et al., J. Phys. Conf. Ser. 188 (2009) 012041. [17] M. Gharagozlou, J. Alloy. Compd. 495 (2010) 217–223. [19] H. Nathani, R.D.K. Misra, Mater. Sci. Eng. B 113 (2004) 228–235. [20] R.M. Bozorth, Elizabeth F. Tilden, Albert J. Williams, Phys. Rev. 99 (1955) 1788. [21] Yao Cheng, Yuanhui Zheng, Yuansheg Wang, Feng Bao, Yong Qin, J. Solid State Chem. 178 (2005) 2394. [22] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [23] E.F. Kneller, Ferromagnetismus, Springer-Verlag, Berlin, Göttingen, Heidelberg, 1962 (Chapter 27). [24] M. Rajendran, R.C. Pullar, et al., J. Magn. Magn. Mater. 232 (2001) 71. [25] Michael J. Bonder, Yunhe Hunge, George C. Hadjipanayis, in: R. Skomski, D.J. Sellmyer (Eds.), Advanced Magnetic Nanostructures, Springer, 2006, p. 184 (Chapter 7). [26] S.P. Gubin, Yu.A. Koksharov, Russ. Chem. Rev. 74 (6) (2005) 489–520.
