ISSN 00360244, Russian Journal of Physical Chemistry A, 2013, Vol. 87, No. 7, pp. 1200–1202. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.P. Talsi, S.N. Evdokimov, M.V. Trenikhin, O.V. Protasova, V.A. Drozdov, O.B. Bel’skaya, V.A. Likholobov, 2013, published in Zhurnal Fizicheskoi Khimii, 2013, Vol. 87, No. 7, pp. 1224–1226.
PHYSICAL CHEMISTRY OF NANOCLUSTERS AND NANOMATERIALS
On the Possible Discovery of Intercalated Carbonate Anions via the Analysis of 13C NMR Spectra of GraphiteLike Carbon Materials V. P. Talsi, S. N. Evdokimov, M. V. Trenikhin, O. V. Protasova, V. A. Drozdov, O. B. Bel’skaya, and V. A. Likholobov Institute of Hydrocarbon Processing, Siberian Branch, Russian Academy of Sciences, Omsk, 644040 Russia email:
[email protected] Received June 15, 2012
Abstract—Analysis of 13C NMR spectra of graphitelike carbon materials shows that pyrolytic carbon, being a part of carbon–carbon composites, contains carbonate ions that are probably intercalated between the graphitelike layers of composites as in the case of layered double hydroxides. Keywords: 13C NMR, pyrolytic carbon, carbon–carbon composite, intercalated carbonate anions. DOI: 10.1134/S0036024413070315
INTRODUCTION Studies of carbon materials in recent decades have led to impressive results that deserved Nobel Prizes [1, 2]. The whole might of today’s methods of struc tural analysis was applied not only to investigations of fullerenes, nanotubes, and graphene, but also to stud ies of various types of coal and technical carbon [3]. Most of the universally recognized results were obtained by transmission electron microscopy [4, 5], roentgenphase analysis [6], Raman spectroscopy [7], electron spin resonance [8], and, in the case of fullerenes, by mass spectroscopy as well [1]. Solid state nuclear magnetic resonance spectroscopy of car bon13 with magicangle spinning (13C MAS NMR) proved to be effective in studies of ultradispersed dia monds [9] due to the characteristic relatively narrow signal of sp3 hybridized carbon atoms with chemical shifts of ~35 ppm. With different kinds of graphite, coal, technical carbon, nanotubes, and fullerenes, the 13C NMR spectra consist of noninformative broad ened signals with chemical shifts of maxima at ~120 ppm. An additional signal at ~140 ppm was also observed for fullerenes. Glassy carbon absorbs RF radiation in the very wide range of 0 to 150 ppm [10]. In this work, 13C MAS NMR spectroscopy was used to obtain data that validate a new point of view on the morphology of some synthetic graphitelike car bon materials. EXPERIMENTAL Samples of technical carbon P145, pyrolytic car bon, and sibunite, obtained at the pilot factory of the
Institute of Hydrocarbon Processing [11], were used in this work. The sample of hydrotalcite in the carbonate form, Mg6Al2(OH)16[CO3] · 4H2O, was synthesized and identified following the methods in [12]. 13C NMR spectra of carbon materials were recorded in singlepulse experiments on a Bruker Avance400 NMR spectrometer, equipped with a multinuclear SB4 probehead. Spectra were recorded using zirconium oxide rotors (diameters of 4 mm) and magicangle spinning at the magic angle (54.7°) with respect to the magnetic field direction. The spinning rate was 8 × 103 Hz. The Larmor frequency was 100.4 MHz. Pulse repetition time was 20–30 s. Tet ramethylsilane was used as our external standard. The experiment’s duration was 48 h. The conductivity of the samples had to be reduced in order to tune the probehead, and so every sample was mixed with a three times larger amount of talc. Analogous 13C NMR spectra were obtained using other diluents, e.g., TiO2 powder (rutile and anatase). Talc turned out to be preferable for obtaining spectra of a higher quality. Electron microscopy was performed on a JEOL JEM2100 microscope (accelerating voltage, 200 kV; lattice resolution, 0.14 nm for Au). An image of gold (111) single crystals with a typical spacing of 0.235 nm was used as our standard for calibrating the linear sizes of electron microscopy images. Samples were pre pared in the following way: the samples were ground in an agate mortar, ultrasonicated in ethanol at a fre quency of 44 kHz, and then dispersed on a thin poly mer film (~10 nm thick) placed in a copper grid that served as the substrate for the samples. The sample was inserted into the microscope with the aid of a special holder through a preliminary vacuum lock.
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Fig. 1. 13C NMR spectra of (1) sibunite, (2) pyrolytic carbon, (3) technical carbon P145, and (4) synthetic hydrotalcite in the carbonate form. Asterisks (*) denote spinning sidebands.
RESULTS AND DISCUSSION After decades of investigations and exploitation, carbon–carbon composite materials have proved themselves as efficient sorbents and unique carrier materials for heterogeneous catalysts [13]. The pro cess of obtaining one type of such composites— sibunite—includes compression of the surface of pri mary technical carbon globules resulting from keeping pyrolytic carbon in hightemperature contact with hydrocarbon gases. The second step after the applica tion of pyrolytic carbon is hightemperature water vapor activation, which leads to the preferential removal of the inner parts of carbon globules and the formation of a mesopore system in the obtained mate rial [11]. Figure 1 shows 13C NMR spectra of the samples of carbon materials, sibunite and pyrolytic carbon, which is actually similar to sibunite that was not sub ject to vapor activation. The spectrum of technical carbon P145 is also shown in Fig. 1 for the sake of comparison. The 13C NMR spectra of sibunite and pyrolytic carbon are almost the same. Broad signals similar to those observed in Fig. 1 were decomposed in [5] into three Gaussian components with maxima at 170, 115, and 20 ppm. The main component with chemical shift of the maximum at ~115 ppm was attributed to nanosized graphene clusters; the two oth ers, to carbon atoms in the sp3 hybridization state (20 ppm) and to highly defective graphene clusters (170 ppm). The graphitelike arrangement of structural ele ments in the subsurface layers of pyrolytic carbon and sibunite, unlike that in technical carbon, is shown in the electron microscopy images in Fig. 2. This well known morphological difference [13] could be RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
responsible for the shift of the maximum of the main signal by 5–10 ppm to higher frequencies that occurs in the spectra of pyrolytic carbon and sibunite, in con trast to the spectrum of technical carbon (Fig. 1). An additional weak signal at ~167 ppm, which is not observed in the spectrum of technical carbon P145, is found in the spectra of sibunite and pyrolytic carbon (Fig. 1). The signal in the region 165–170 ppm was observed earlier in spectra of technical or amor phous carbon and was usually attributed to absorption of carboxyl in benzoate groups. The same weak and relatively narrow signal (as compared to the signal at ~115 ppm) as in Fig. 1 was observed recently in the 13C NMR spectrum of rice straw carbonizate [14]. It was attributed to bicarbonate ions in the alkaline ash admixture or, as an alternative, to carboxyl groups in benzoates. Following this alternative, we attributed the signal near 167 ppm (Fig. 1) to carbonate anions. The 13C NMR spectrum of a synthetic sample of hydrotalcite
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Fig. 2. Electron microscopy images of the graphitelike structure of a pyrolytic carbon globule (left) and technical carbon P145 carbon (right). Vol. 87
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in carbonate form is shown in Fig. 1 for comparison. This mineral is a layered aluminum–magnesium hydroxide consisting of positively charged brucitelike layers, water molecules, and carbonate ions embedded in the interlayer space. The signals from hydrotalcite, pyrolytic carbon, and sibunite with a chemical shift of 167 ppm (Fig. 1) are identical. In our opinion, if the considered signal was produced by the carboxyl car bons in benzoates, it would be difficult to explain why it is several times narrower than the main absorption band with the maximum at 115 ppm. In addition, the continued existence of carboxyl groups at the temper ature of the formation of a pyrolytic carbon layer (700°С) seems quite improbable.
ACKNOWLEDGMENTS We are grateful to E.A. Rayskaya and G.V. Plaksin for kindly providing the samples of carbon materials and their helpful discussions. REFERENCES 1. 2. 3. 4. 5. 6.
CONCLUSIONS If carbonate ions are present in the considered car bon materials, it is logical to suggest that they are local ized between their graphitelike layers. Electrostatic interaction between positively charged (due to interac tion with protons of carbonic acid (H2O + CO2 = 2H+ + CO32− ) graphitelike layers and interlayer car bonate anions promotes the formation of surface areas of pyrolytic carbon, granules of which exhibit ten times higher abrasion resistance and crushing strength than granules of technical carbon [13]. The difference between the physical and mechanical properties of talc and mica is explained by an analogous electrostatic interaction [15]. Carbonate anions could therefore serve as an adhesive cementing graphene layers during the formation of pyrolytic carbon. Such intercalation apparently takes place in the formation of a layer pyro lytic carbon, since the gas environment used in the carbonization of the surface of technical carbon con tains carbon dioxide in addition to carbon.
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Translated by S. Efimov
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