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Oxidation-resistant graphitic surface nanostructure of carbon black developed by ethanol thermal decomposition
Diamond & Related Materials 65 (2016) 26–31
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Oxidation-resistant graphitic surface nanostructure of carbon black developed by ethanol thermal decomposition Yuki Kameya a,⁎, Takuhiro Hayashi b, Masahiro Motosuke b,c a b c
Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo, Japan Department of Mechanical Engineering, Tokyo University of Science, Tokyo, Japan Research Institute for Science & Technology, Tokyo University of Science, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 28 November 2015 Received in revised form 14 December 2015 Accepted 4 January 2016 Available online 7 January 2016 Keywords: Carbon black Surface nanostructure Catalyst support Ethanol thermal decomposition Graphitic layers
a b s t r a c t Carbon black surface nanostructure developed by ethanol thermal decomposition was investigated to examine a potential surface treatment process for enhancing oxidation resistance. The product gas composition indicated that carbon deposited from carbon monoxide as well as methane. The development of graphitic structure was revealed by means of Raman spectroscopy, and the increase in oxidation resistance was demonstrated using thermogravimetric analysis. These results indicate that the created nanostructure has higher crystallinity than the original carbon black. Furthermore, carbon black surface morphology and internal crystalline structure were observed using a transmission electron microscope, and the formation of oxidation-resistant graphitic surface nanostructure was confirmed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon black is a carbonaceous nanoparticle material obtained through partial combustion or thermal decomposition of hydrocarbons [1]. Because of its production process, the form of primary particle is nearly spherical and several primary particles coalesce into an aggregate. Depending on the size, aggregate structure, internal crystalline structure, and surface chemistry of particles, carbon black exhibits various physical and chemical characteristics. Carbon black has been used as an important material in various industrial applications. The applications include a reinforcing filler in rubber, black pigment, etc. Carbon black is employed also as a catalyst support in chemical processes. For example, carbon black is used to support platinum catalyst particles on its surface in polymer electrolyte fuel cell (PEFC). A typical PEFC cathode consists of Pt nanoparticles, whose diameter is about 3 nm, dispersed on carbon black support, and the degradation of catalyst occurs under PEFC operation due to the sintering of Pt nanoparticles and oxidation of carbon black support [2]. Therefore, it is required to enhance the durability of carbon black to stably support catalyst nanoparticles on its surface. The durability of carbon black against oxidation is closely related to the surface crystalline structure. The correlation between surface ⁎ Corresponding author at: Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 NE-8, Ookayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail addresses: [email protected], [email protected] (Y. Kameya).
http://dx.doi.org/10.1016/j.diamond.2016.01.002 0925-9635/© 2015 Elsevier B.V. All rights reserved.
structural properties and chemical reactivity has been extensively reported [3–5]. In general, the number of active sites increases as the surface crystalline structure becomes disordered, which results in higher chemical reactivity. Carbon black has highly disordered structure and there are a large number of active sites exposed to the atmosphere. This intrinsic surface structural property is not favorable in terms of oxidation resistance. Hence, it is desirable if we could develop more graphitic surface structure than original carbon black. The authors investigated the kinetics of methane thermal decomposition over carbon black and the surface nanostructure evolving during the reaction [6,7]. It was found that graphitic sheet-like surface structure develops due to carbon deposition by methane decomposition. On the basis of this previous work, it is expected that surface structure development using carbon deposition is used as a surface treatment process to enhance the oxidation resistance of carbon black. In addition, we consider that alcohol as well as hydrocarbons can be used as one of easy-to-handle carbon sources for surface treatment of carbon black. Furthermore, it has been demonstrated that the texture of carbon surface affects the dynamic behavior of supported nanoparticle [8]. Thus, we also expect the modified texture of carbon black positively affects the nanoparticles behavior to inhibit their sintering. In the present work, carbon black surface nanostructure developed by ethanol thermal decomposition was investigated to examine a potential surface treatment process for enhancing oxidation resistance. Carbon black samples were processed by supplying ethanol at elevated temperatures. The product gas analysis during the surface treatment of carbon black was performed to obtain an insight on the mechanism of
Y. Kameya et al. / Diamond & Related Materials 65 (2016) 26–31
carbon deposition. To study the evolution of surface nanostructure during the reaction, the duration time was varied and several samples were prepared for further physicochemical analysis. The developed surface morphology and internal crystalline structure of carbon black are discussed.
27
2. Material and methods
reactor. Then, the temperature of the reactor was raised to the specified reaction temperature while maintaining a constant nitrogen flow. After attaining a thermally steady state, ethanol vapor carried by nitrogen was supplied into the reactor. The duration time of reaction was controlled in order to examine the progress of the surface treatment. The surface treatment of carbon black was performed at 700, 750, 800, 850, and 900°C.
2.1. Carbon samples
2.3. Characterization of carbon black samples
Commercial carbon black SB905 (Asahi Carbon, Japan) was employed because of its superior homogeneity in particulate properties. The mean diameter of the primary particle is 15 nm and the specific surface area is 212 m2/g. The sample was stored in a desiccator after receiving from the manufacturer and used in the experiment without any further pretreatment. Graphite powder Code 072-03845 (Wako Pure Chemical Industries, Japan) was used as a reference material of highly crystallized carbon.
To evaluate the degree of crystallization of produced carbon, Raman spectra were measured using a NRS-3200 laser Raman spectroscopy system (JASCO, Japan). The wavelength of the excitation beam was 532 nm. The objective with 20 × magnification was used. The spectra of samples were obtained in the wavenumber range 900–2000 cm−1 (Stokes shift) to evaluate the first-order Raman spectra. Measurement was performed for ten different locations of each sample pasted on a glass plate. The spectra were processed via Spectra Manager Version 2 software (JASCO, Japan). Thermogravimetric analysis (TGA) was conducted under the atmospheric pressure using a TG-DTA 2010SA (Bruker). An alumina pan was used to contain samples. The internal diameter of the pan is 9 mm and the depth is 1.8 mm. To induce sample oxidation, dry air was introduced into the furnace at a constant flow rate of 300 sccm. To observe the surface texture and internal structure of carbon samples, a JEM-2000EX transmission electron microscope (JEOL, Japan), was used. Images representing the overall trend of each sample were selected for the evaluation.
2.2. Surface treatment using ethanol A schematic diagram of the experimental setup for the carbon black surface treatment using ethanol is shown in Fig. 1. The reaction test was carried out in a vertical quartz tube reactor heated by an ARF-30KC electric tube furnace (Asahi Rika, Japan). The quartz tube has a height of 700 mm and inner diameter of 7 mm. The electric tube furnace having a height of 300 mm covered the central region of the tube reactor. The carbon black sample was packed on the quartz wool bed supported by a quartz tube thinner than the tube reactor. A type-K sheathed thermocouple was set on the outer surface of the quartz tube at the height of the carbon black-packed bed location. The input power to the furnace was controlled by an AMF-S PID controller (Asahi Rika, Japan). In addition, the furnace has enough heat-supplying capacity to keep the specified temperature against heat removal by the endothermic reaction. The reactant was supplied with a carrier gas into the reactor and the gases flowed downward in the reactor. The pressure was measured in the upstream piping of the reactor. Gas sampling was conducted using a gastight syringe at the downstream piping of the reactor where the gas temperature decreased to approximately the room temperature. The product gas was analyzed using a GC-8AIT gas chromatograph with a thermal conductivity detector (Shimadzu, Japan). To analyze H2, CH4, and CO, argon gas was used as a carrier gas for gas chromatograph and gas separation was conducted using a MS-5 A packed column (Shinwa Chemical Industries, Japan). When detecting CO2, the carrier gas was changed to helium and a silica-gel packed column (Shinwa Chemical Industries, Japan) was used. After packing a weighed sample in the reactor, nitrogen gas was introduced in order to purge other gas species from the inside of the
3. Results and discussion 3.1. Carbon deposition mechanism examined by product gas analysis The product gas analysis during the surface treatment of carbon black provides an insight on the mechanism of carbon deposition. The gas-phase chemical reaction in ethanol thermal decomposition has been extensively studied [9,10]. Although the case where carbon black exists in the reaction filed of ethanol decomposition has not been reported, it would be helpful to review the known gas-phase reactions in order to discuss the ethanol decomposition over carbon black. Ethanol vapor is dehydrogenated to acetaldehyde: C2 H5 OH → CH3 CHO þ H2 ; ΔH ¼ þ285 kJ mol
−1
ð1Þ
where ΔH denotes the standard reaction enthalpy. Then, the decarbonylation of acetaldehyde takes place, and methane and carbon monoxide are produced: −1
CH3 CHO → CH4 þ CO; ΔH ¼ −80 kJ mol
:
ð2Þ
Solid carbon would be obtained by carbon deposition from these gaseous species. The first mechanism of solid carbon production is the deposition from carbon monoxide [10]: 2CO → CðsÞ þ CO2 ; ΔH ¼ −172 kJ mol
−1
:
ð3Þ
The thermodynamic stability of CO rises with increasing temperature and decreasing pressure. The second mechanism of solid carbon production is the deposition from methane: −1
CH4 → CðsÞ þ 2H2 ; ΔH ¼ þ75 kJ mol
Fig. 1. Schematic diagram of the experimental setup.
:
ð4Þ
The thermal decomposition of methane was intensively investigated by the authors regarding the kinetics [6] and product carbons [5,7]. On the basis of these known reaction pathways, we consider the mechanism of carbon deposition on carbon black surface. In the gas analysis,
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H2, CH4, CO, and CO2 were detected. Fig. 2 shows a representative chromatogram of gases produced at 900°C, which was obtained in the GC operating condition for H2 detection and hence CO2 did not appear here. Since argon was used as a carrier gas in this GC condition, H2 was detected with high sensitivity (high signal-to-noise ratio) while the sensitivity for CO was relatively low. The amount of each gas species was calculated by integrating the peak in chromatogram and processing the value based on the relative sensitivity. The surface treatment of carbon black was performed at 700, 750, 800, 850, and 900°C. Since the flow rate of nitrogen was constant, the amount of product gas was evaluated by the mole ratio of each species to nitrogen. The result is shown in Fig. 3. At 700°C, we did not detect CH4 and CO. There is no remarkable time-variation in the amount of each product gas during each run. Concerning the amounts of H2, CH4, and CO, we can observe the relation H2 N CH4 N CO. This relation can be interpreted by considering Eqs. (1)–(4). Hydrogen is produced at the same amount as acetaldehyde through dehydrogenation (Eq. (1)). Then, a partial amount of acetaldehyde decomposes into CH4 and CO by decarbonylation (Eq. (2)), and therefore, the amount of H2 is larger than those of CH4 and CO. The decomposition of CH4 also produces H2, which means the amount of H2 is always greater than the others. Although CH4 and CO are produced at the same amount (Eq. (2)), CO is consumed twice as many as CH4 when the same amount of C(s) is produced via two pathways (Eqs. (3,4)). Furthermore, we detected CO2, which indicates the progress of carbon deposition from CO as described in Eq. (3). Next, we investigated the temperature dependence of the product gas composition. We calculated the time-averaged amount of product gas at each temperature. On the basis of the discussion above, H2 is the largest amount species in the product gas. Hence, we used the mole ratio of each species to H2 to examine the relative amount of gas species. The obtained result is shown in Fig. 4(a). The mole ratio CH4/ H2 showed no remarkable temperature dependence. On the other hand, the mole ratio CO/H2 increased with temperature. The thermodynamic equilibrium composition of CO and CO2 is shown in Fig. 4(b) (Boudouard equilibrium diagram [11]). The thermodynamic stability of CO rises with increasing temperature, so that the mole fraction of CO increases with temperature. This behavior corresponds to the temperature dependence of the amount of CO shown in Fig. 4(a). In addition to the detection of CO2, the observed temperature dependence of the CO amount is an evidence for the progress of carbon deposition from CO (Eq. (3)).
Fig. 3. Product gas composition at (a) 700°C, (b) 750°C, (c) 800°C, (d) 850°C, and (e) 900°C.
black is characterized by two strong peaks: the G (“graphite”) band around 1580 cm−1 and the D1 (“disorder”) band around 1350 cm−1. The G band is attributed to an ideal graphitic lattice vibration mode with E2g symmetry [12]. When analyzing Raman spectra of highly disordered carbons, curve fitting can be achieved better by introducing a broad band around 1500 cm−1 [13]. This band is denoted as the D3 (or A) band and associated with amorphous carbon content. In addition to the aforementioned bands, other bands are often used: the D2 band
3.2. Graphitic surface structure and its oxidation resistance After performing surface treatment of carbon black at 900°C, the crystalline structure was evaluated by means of Raman spectroscopy. Fig. 5(a) shows an exemplary Raman spectrum and its deconvolution by 5-band curve fitting. The first-order Raman spectrum of carbon
Fig. 2. Exemplary chromatogram of product gas obtained at 900°C. The inset shows the magnified part for CH4 and CO.
Fig. 4. (a) Mole ratios of CH4 and CO to H2. (b) Thermal equilibrium mole fractions of CO and CO2.
Y. Kameya et al. / Diamond & Related Materials 65 (2016) 26–31
29
decrease in both R2 and FWHM(D1) is attributed to the development of graphitic surface structure due to carbon deposition through ethanol decomposition. In addition, it should be noted that the contribution of amorphous carbon to Raman spectra remained even after surface treatment at 900°C for 120 min. It is considered that the surface treatment does not affect the internal structure of pristine carbon black: more graphitic carbon than internal amorphous part develops on the surface of carbon black during ethanol decomposition. Since more graphitic structure than the original carbon black develops during ethanol surface treatment, it is expected that the oxidation resistance increases after the surface treatment. To demonstrate this aspect, we performed TGA of carbon oxidation. Dry air was used to induce sample oxidation. The TG curve of carbon black which was processed with ethanol at 900°C for 120 min is shown in Fig. 6 as a representative of surface-treated carbon black. For comparison, the TG curves of fresh sample and graphite powder are plotted also in Fig. 6. The onset of oxidation was observed for fresh carbon black at around 500°C. Concerning graphite powder, oxidation initiated at higher temperature and the oxidation rate (the slope of curve after the initial transient state) was much slower than fresh carbon black. The surfacetreated carbon black exhibited the onset of oxidation at around 600°C. The oxidation rate was faster than graphite powder, which indicates the difference in the internal crystalline structure between carbon black and graphite powder. The temperature where the weight loss reaches 3% of the initial weight was 527°C for fresh carbon black and 607°C for surface-treated carbon black. Therefore, it is demonstrated that the development of graphitic structure by carbon deposition on fresh carbon black surface makes the surface more oxidation resistant than the original one. 3.3. Surface nanostructure developed by carbon deposition
Fig. 5. Raman spectrum analysis. (a) Exemplary Raman spectrum of carbon black and the 5-band curve fitting result. Time variation of spectral parameters of Raman spectra: (b) FWHM(D1), and (c) R2 ratio.
around 1620 cm−1 and the D4 band around 1180 cm−1. The 5-band curve fitting by a combination of four Lorentzian curves for the G and D2–4 bands and a Gaussian curve for the D1 band allows us to determine Raman spectral parameters [14]. In this study, two spectral parameters of Raman spectrum were evaluated because of their high sensitivities in the structural evaluation of highly disordered carbon: R2 ratio, which is defined as I(D1)/(I(G) + I(D1) + I(D2)) where I is the intensity of each band, and the full-width at half maximum (FWHM) of the D1 band. Fig. 5(b,c) shows the time variation of R2 ratio and the FWHM of the D1 band, respectively. The decrease in both R2 ratio and FWHM(D1) generally indicates a decrease in the structural disorder of highly disordered carbons, or in other words, an increase in graphitic crystalline structure. To discriminate the effect of heat on the structural characteristics of carbon black from that of developed carbon structure, Raman spectra were obtained also for samples treated by heat at 900°C without supplying ethanol. The results are plotted in Fig. 5(b,c). Considering the standard error associated with Raman spectral parameters, we do not recognize the effect of heat treatment on structural characteristics. This observation corresponds to the experimental result showing that an effect of heat treatment on the internal structure of carbon particle becomes remarkable only above ~ 1900°C [16]. Consequently, the
Because the deposited carbon from ethanol decomposition on carbon black surface showed more graphitic characteristics than the original carbon black as described in the previous section, we were motivated to directly observe the surface and internal structure of ethanol-treated samples. TEM observation is used to characterize the internal crystalline structure and the degree of graphitization can be evaluated from the obtained images [15]. TEM images of pristine carbon black (at low and high magnifications) and samples treated at 900°C for 20, 30, 40, and 120 min are shown in Fig. 7(a–f), respectively, and the featured parts described in the text are emphasized by arrows in the figures. Fresh carbon black had smooth surface and the lattice fringe at shell part was parallel to the outline of primary particles as can be seen in TEM images (Fig. 7(a,b)) [6]. As the surface treatment proceeds, carbon produced from methanol deposits on carbon black surface, and the initially smooth surface morphology changes. Small structure (b~3 nm) protruding from the initially smooth surface is observed for 20-min treated sample (Fig. 7(c)). Then, the number of protrusions appearing on the surface increases as well as the size of each structure
Fig. 6. TG curves of carbon black samples and graphite powder.
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Y. Kameya et al. / Diamond & Related Materials 65 (2016) 26–31
Fig. 7. TEM images of pristine carbon black (a) at low magnification and (b) at high magnification, and samples treated at 900°C for (c) 20 min, (d) 30 min, (e) 40 min, and (f) 120 min.
(Fig. 7(d)). Further carbon deposition makes the lattice fringe of graphitic layers clearly observable inside the protruding structure (Fig. 7(e)). This aspect becomes more prominent after the surface treatment for 120 min (Fig. 7(f)). Large structure (N~5 nm) having several graphitic layers is remarkable at this stage. Since the amount of deposited carbon is large, the outline of original primary particles is unclear. It appears that the original particles are almost covered by highlygraphitized deposited carbon, which can be inferred also from the Raman spectral result (Fig. 5) and TGA (Fig. 6). It should be noted that some structural features are different from those observed for carbon black treated by methane thermal decomposition [7]. In the case of methane, dissociative adsorption of methane molecule on the surface active sites of carbon black progresses, which
results in the evolution of broad sheet-like graphitic structure on carbon black surface. In the present case, carbon deposition takes place via carbon monoxide as well as methane generated by ethanol decomposition. It appears that the observed difference in surface nanostructure is due to carbon deposition mechanism. This result suggests that the surface nanostructure developed through surface treatment using hydrocarbon or alcohol could be controlled by properly choosing the source of carbon as well as the conditions of chemical reaction. Fig. 8 summarizes the surface treatment process of carbon black using ethanol thermal decomposition in which the oxidation-resistant graphitic surface nanostructure is created. Ethanol is in the liquid state at room temperature and easy to handle, which makes ethanol one of promising sources of carbon. The proposed process have the potential
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the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References
Fig. 8. Carbon black surface treatment process using ethanol as a carbon source to produce oxidation-resistant graphitic surface nanostructure.
to be applied to an after-treatment of commercial carbon black so that the unique characteristics of surface nanostructure is utilized. 4. Conclusions We investigated the surface treatment process of carbon black using ethanol thermal decomposition. We have demonstrated that the oxidation-resistant graphitic surface nanostructure develops on carbon black surface. The product gas analysis during ethanol decomposition have revealed that carbon monoxide is used as a carbon source to create unique surface nanostructure. Since we observed the difference in surface structural characteristics depending on carbon source, the surface nanostructure could be controlled by choosing the source of carbon as well as the conditions of chemical reaction. In future work, it is expected to examine the performance of unique nanostructure on carbon black surface in a potential application such as stable catalyst support. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 26889056 (Grant-in-Aid for Research Activity Start-up). A part of this work was supported by “Nanotechnology Platform” (project No. 12024046) of
[1] J.B. Donnet, C. Bansal, M.J. Wang, Carbon Black, second ed. Marcel Dekker, New York, 1993. [2] M. Hara, M. Lee, C.H. Liu, B.H. Chen, Y. Yamashita, M. Uchida, H. Uchida, M. Watanabe, Electrochemical and Raman spectroscopic evaluation of Pt/graphitized carbon black catalyst durability for the start/stop operating condition of polymer electrolyte fuel cells, Electrochim. Acta 70 (2012) 171–181. [3] J.-O. Müller, D.S. Su, R.E. Jentoft, J. Kröhnert, F.C. Jentoft, R. Schlögl, Morphologycontrolled reactivity of carbonaceous materials towards oxidation, Catal. Today 102-103 (2005) 259–265. [4] M. Knauer, M.E. Schuster, D. Su, R. Schlögl, R. Niessner, N.P. Ivleva, Soot structure and reactivity analysis by Raman microspectroscopy, temperature-programmed oxidation, and high-resolution transmission electron microscopy, J. Phys. Chem. A. 113 (2009) 13871–13880. [5] Y. Kameya, K. Hanamura, Variation in catalytic activity of carbon black during methane decomposition: active site estimations from surface structural characteristics, Catal. Lett. 142 (2012) 460–463. [6] Y. Kameya, K. Hanamura, Kinetic and Raman spectroscopic study on catalytic characteristics of carbon blacks in methane decomposition, Chem. Eng. J. 173 (2011) 627–635. [7] Y. Kameya, K. Hanamura, Carbon black texture evolution during catalytic methane decomposition, Carbon 50 (2012) 3503–3512. [8] M.S. Moldovan, H. Bulou, Y.J. Dappe, I. Janowska, D. Bégin, C. Pham-Huu, O. Ersen, On the evolution of Pt nanoparticles on few-layer graphene supports in the hightemperature range, J. Phys. Chem. C 116 (2012) 9274–9282. [9] D.A. Morgenstern, J.P. Fornango, Low-temperature reforming of ethanol over copper-plated raney nickel: a new route to sustainable hydrogen for transportation, Energy Fuel 19 (2005) 1708–1716. [10] D.M. Sedlovets, A.N. Redkin, V.I. Korepanov, O.V. Trofimov, Electrical conductivity and optical properties of thin carbon films grown from ethanol vapor, Inorg. Mater. 48 (2012) 34–39. [11] R. Kneer, D. Toporov, M. Förster, D. Christ, C. Broeckmann, E. Pfaff, M. Zwick, S. Engels, M. Modigell, OXYCOAL-AC: towards an integrated coal-fired power plant process with ion transport membrane-based oxygen supply, Energy Environ. Sci. 3 (2010) 198–207. [12] S. Reich, C. Thomsen, Raman spectroscopy of graphite, Philos. Trans. R. Soc. Lond. A. 362 (2004) 2271–2288. [13] T. Jawhari, A. Roid, J. Casado, Raman spectroscopic characterization of some commercially available carbon black materials, Carbon 33 (1995) 1561–1565. [14] A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner, U. Pöschl, Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information, Carbon 43 (2005) 1731–1742. [15] R.L. Vander Wal, A.J. Tomasek, K. Street, D.R. Hull, W.K. Thompson, Carbon nanostructure examined by lattice fringe analysis of high-resolution transmission electron microscopy images, Appl. Spectrosc. 58 (2004) 230–237.
Contents lists available at ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Oxidation-resistant graphitic surface nanostructure of carbon black developed by ethanol thermal decomposition Yuki Kameya a,⁎, Takuhiro Hayashi b, Masahiro Motosuke b,c a b c
Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo, Japan Department of Mechanical Engineering, Tokyo University of Science, Tokyo, Japan Research Institute for Science & Technology, Tokyo University of Science, Tokyo, Japan
a r t i c l e
i n f o
Article history: Received 28 November 2015 Received in revised form 14 December 2015 Accepted 4 January 2016 Available online 7 January 2016 Keywords: Carbon black Surface nanostructure Catalyst support Ethanol thermal decomposition Graphitic layers
a b s t r a c t Carbon black surface nanostructure developed by ethanol thermal decomposition was investigated to examine a potential surface treatment process for enhancing oxidation resistance. The product gas composition indicated that carbon deposited from carbon monoxide as well as methane. The development of graphitic structure was revealed by means of Raman spectroscopy, and the increase in oxidation resistance was demonstrated using thermogravimetric analysis. These results indicate that the created nanostructure has higher crystallinity than the original carbon black. Furthermore, carbon black surface morphology and internal crystalline structure were observed using a transmission electron microscope, and the formation of oxidation-resistant graphitic surface nanostructure was confirmed. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Carbon black is a carbonaceous nanoparticle material obtained through partial combustion or thermal decomposition of hydrocarbons [1]. Because of its production process, the form of primary particle is nearly spherical and several primary particles coalesce into an aggregate. Depending on the size, aggregate structure, internal crystalline structure, and surface chemistry of particles, carbon black exhibits various physical and chemical characteristics. Carbon black has been used as an important material in various industrial applications. The applications include a reinforcing filler in rubber, black pigment, etc. Carbon black is employed also as a catalyst support in chemical processes. For example, carbon black is used to support platinum catalyst particles on its surface in polymer electrolyte fuel cell (PEFC). A typical PEFC cathode consists of Pt nanoparticles, whose diameter is about 3 nm, dispersed on carbon black support, and the degradation of catalyst occurs under PEFC operation due to the sintering of Pt nanoparticles and oxidation of carbon black support [2]. Therefore, it is required to enhance the durability of carbon black to stably support catalyst nanoparticles on its surface. The durability of carbon black against oxidation is closely related to the surface crystalline structure. The correlation between surface ⁎ Corresponding author at: Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 NE-8, Ookayama, Meguro-ku, Tokyo 152-8552, Japan. E-mail addresses: [email protected], [email protected] (Y. Kameya).
http://dx.doi.org/10.1016/j.diamond.2016.01.002 0925-9635/© 2015 Elsevier B.V. All rights reserved.
structural properties and chemical reactivity has been extensively reported [3–5]. In general, the number of active sites increases as the surface crystalline structure becomes disordered, which results in higher chemical reactivity. Carbon black has highly disordered structure and there are a large number of active sites exposed to the atmosphere. This intrinsic surface structural property is not favorable in terms of oxidation resistance. Hence, it is desirable if we could develop more graphitic surface structure than original carbon black. The authors investigated the kinetics of methane thermal decomposition over carbon black and the surface nanostructure evolving during the reaction [6,7]. It was found that graphitic sheet-like surface structure develops due to carbon deposition by methane decomposition. On the basis of this previous work, it is expected that surface structure development using carbon deposition is used as a surface treatment process to enhance the oxidation resistance of carbon black. In addition, we consider that alcohol as well as hydrocarbons can be used as one of easy-to-handle carbon sources for surface treatment of carbon black. Furthermore, it has been demonstrated that the texture of carbon surface affects the dynamic behavior of supported nanoparticle [8]. Thus, we also expect the modified texture of carbon black positively affects the nanoparticles behavior to inhibit their sintering. In the present work, carbon black surface nanostructure developed by ethanol thermal decomposition was investigated to examine a potential surface treatment process for enhancing oxidation resistance. Carbon black samples were processed by supplying ethanol at elevated temperatures. The product gas analysis during the surface treatment of carbon black was performed to obtain an insight on the mechanism of
Y. Kameya et al. / Diamond & Related Materials 65 (2016) 26–31
carbon deposition. To study the evolution of surface nanostructure during the reaction, the duration time was varied and several samples were prepared for further physicochemical analysis. The developed surface morphology and internal crystalline structure of carbon black are discussed.
27
2. Material and methods
reactor. Then, the temperature of the reactor was raised to the specified reaction temperature while maintaining a constant nitrogen flow. After attaining a thermally steady state, ethanol vapor carried by nitrogen was supplied into the reactor. The duration time of reaction was controlled in order to examine the progress of the surface treatment. The surface treatment of carbon black was performed at 700, 750, 800, 850, and 900°C.
2.1. Carbon samples
2.3. Characterization of carbon black samples
Commercial carbon black SB905 (Asahi Carbon, Japan) was employed because of its superior homogeneity in particulate properties. The mean diameter of the primary particle is 15 nm and the specific surface area is 212 m2/g. The sample was stored in a desiccator after receiving from the manufacturer and used in the experiment without any further pretreatment. Graphite powder Code 072-03845 (Wako Pure Chemical Industries, Japan) was used as a reference material of highly crystallized carbon.
To evaluate the degree of crystallization of produced carbon, Raman spectra were measured using a NRS-3200 laser Raman spectroscopy system (JASCO, Japan). The wavelength of the excitation beam was 532 nm. The objective with 20 × magnification was used. The spectra of samples were obtained in the wavenumber range 900–2000 cm−1 (Stokes shift) to evaluate the first-order Raman spectra. Measurement was performed for ten different locations of each sample pasted on a glass plate. The spectra were processed via Spectra Manager Version 2 software (JASCO, Japan). Thermogravimetric analysis (TGA) was conducted under the atmospheric pressure using a TG-DTA 2010SA (Bruker). An alumina pan was used to contain samples. The internal diameter of the pan is 9 mm and the depth is 1.8 mm. To induce sample oxidation, dry air was introduced into the furnace at a constant flow rate of 300 sccm. To observe the surface texture and internal structure of carbon samples, a JEM-2000EX transmission electron microscope (JEOL, Japan), was used. Images representing the overall trend of each sample were selected for the evaluation.
2.2. Surface treatment using ethanol A schematic diagram of the experimental setup for the carbon black surface treatment using ethanol is shown in Fig. 1. The reaction test was carried out in a vertical quartz tube reactor heated by an ARF-30KC electric tube furnace (Asahi Rika, Japan). The quartz tube has a height of 700 mm and inner diameter of 7 mm. The electric tube furnace having a height of 300 mm covered the central region of the tube reactor. The carbon black sample was packed on the quartz wool bed supported by a quartz tube thinner than the tube reactor. A type-K sheathed thermocouple was set on the outer surface of the quartz tube at the height of the carbon black-packed bed location. The input power to the furnace was controlled by an AMF-S PID controller (Asahi Rika, Japan). In addition, the furnace has enough heat-supplying capacity to keep the specified temperature against heat removal by the endothermic reaction. The reactant was supplied with a carrier gas into the reactor and the gases flowed downward in the reactor. The pressure was measured in the upstream piping of the reactor. Gas sampling was conducted using a gastight syringe at the downstream piping of the reactor where the gas temperature decreased to approximately the room temperature. The product gas was analyzed using a GC-8AIT gas chromatograph with a thermal conductivity detector (Shimadzu, Japan). To analyze H2, CH4, and CO, argon gas was used as a carrier gas for gas chromatograph and gas separation was conducted using a MS-5 A packed column (Shinwa Chemical Industries, Japan). When detecting CO2, the carrier gas was changed to helium and a silica-gel packed column (Shinwa Chemical Industries, Japan) was used. After packing a weighed sample in the reactor, nitrogen gas was introduced in order to purge other gas species from the inside of the
3. Results and discussion 3.1. Carbon deposition mechanism examined by product gas analysis The product gas analysis during the surface treatment of carbon black provides an insight on the mechanism of carbon deposition. The gas-phase chemical reaction in ethanol thermal decomposition has been extensively studied [9,10]. Although the case where carbon black exists in the reaction filed of ethanol decomposition has not been reported, it would be helpful to review the known gas-phase reactions in order to discuss the ethanol decomposition over carbon black. Ethanol vapor is dehydrogenated to acetaldehyde: C2 H5 OH → CH3 CHO þ H2 ; ΔH ¼ þ285 kJ mol
−1
ð1Þ
where ΔH denotes the standard reaction enthalpy. Then, the decarbonylation of acetaldehyde takes place, and methane and carbon monoxide are produced: −1
CH3 CHO → CH4 þ CO; ΔH ¼ −80 kJ mol
:
ð2Þ
Solid carbon would be obtained by carbon deposition from these gaseous species. The first mechanism of solid carbon production is the deposition from carbon monoxide [10]: 2CO → CðsÞ þ CO2 ; ΔH ¼ −172 kJ mol
−1
:
ð3Þ
The thermodynamic stability of CO rises with increasing temperature and decreasing pressure. The second mechanism of solid carbon production is the deposition from methane: −1
CH4 → CðsÞ þ 2H2 ; ΔH ¼ þ75 kJ mol
Fig. 1. Schematic diagram of the experimental setup.
:
ð4Þ
The thermal decomposition of methane was intensively investigated by the authors regarding the kinetics [6] and product carbons [5,7]. On the basis of these known reaction pathways, we consider the mechanism of carbon deposition on carbon black surface. In the gas analysis,
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H2, CH4, CO, and CO2 were detected. Fig. 2 shows a representative chromatogram of gases produced at 900°C, which was obtained in the GC operating condition for H2 detection and hence CO2 did not appear here. Since argon was used as a carrier gas in this GC condition, H2 was detected with high sensitivity (high signal-to-noise ratio) while the sensitivity for CO was relatively low. The amount of each gas species was calculated by integrating the peak in chromatogram and processing the value based on the relative sensitivity. The surface treatment of carbon black was performed at 700, 750, 800, 850, and 900°C. Since the flow rate of nitrogen was constant, the amount of product gas was evaluated by the mole ratio of each species to nitrogen. The result is shown in Fig. 3. At 700°C, we did not detect CH4 and CO. There is no remarkable time-variation in the amount of each product gas during each run. Concerning the amounts of H2, CH4, and CO, we can observe the relation H2 N CH4 N CO. This relation can be interpreted by considering Eqs. (1)–(4). Hydrogen is produced at the same amount as acetaldehyde through dehydrogenation (Eq. (1)). Then, a partial amount of acetaldehyde decomposes into CH4 and CO by decarbonylation (Eq. (2)), and therefore, the amount of H2 is larger than those of CH4 and CO. The decomposition of CH4 also produces H2, which means the amount of H2 is always greater than the others. Although CH4 and CO are produced at the same amount (Eq. (2)), CO is consumed twice as many as CH4 when the same amount of C(s) is produced via two pathways (Eqs. (3,4)). Furthermore, we detected CO2, which indicates the progress of carbon deposition from CO as described in Eq. (3). Next, we investigated the temperature dependence of the product gas composition. We calculated the time-averaged amount of product gas at each temperature. On the basis of the discussion above, H2 is the largest amount species in the product gas. Hence, we used the mole ratio of each species to H2 to examine the relative amount of gas species. The obtained result is shown in Fig. 4(a). The mole ratio CH4/ H2 showed no remarkable temperature dependence. On the other hand, the mole ratio CO/H2 increased with temperature. The thermodynamic equilibrium composition of CO and CO2 is shown in Fig. 4(b) (Boudouard equilibrium diagram [11]). The thermodynamic stability of CO rises with increasing temperature, so that the mole fraction of CO increases with temperature. This behavior corresponds to the temperature dependence of the amount of CO shown in Fig. 4(a). In addition to the detection of CO2, the observed temperature dependence of the CO amount is an evidence for the progress of carbon deposition from CO (Eq. (3)).
Fig. 3. Product gas composition at (a) 700°C, (b) 750°C, (c) 800°C, (d) 850°C, and (e) 900°C.
black is characterized by two strong peaks: the G (“graphite”) band around 1580 cm−1 and the D1 (“disorder”) band around 1350 cm−1. The G band is attributed to an ideal graphitic lattice vibration mode with E2g symmetry [12]. When analyzing Raman spectra of highly disordered carbons, curve fitting can be achieved better by introducing a broad band around 1500 cm−1 [13]. This band is denoted as the D3 (or A) band and associated with amorphous carbon content. In addition to the aforementioned bands, other bands are often used: the D2 band
3.2. Graphitic surface structure and its oxidation resistance After performing surface treatment of carbon black at 900°C, the crystalline structure was evaluated by means of Raman spectroscopy. Fig. 5(a) shows an exemplary Raman spectrum and its deconvolution by 5-band curve fitting. The first-order Raman spectrum of carbon
Fig. 2. Exemplary chromatogram of product gas obtained at 900°C. The inset shows the magnified part for CH4 and CO.
Fig. 4. (a) Mole ratios of CH4 and CO to H2. (b) Thermal equilibrium mole fractions of CO and CO2.
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decrease in both R2 and FWHM(D1) is attributed to the development of graphitic surface structure due to carbon deposition through ethanol decomposition. In addition, it should be noted that the contribution of amorphous carbon to Raman spectra remained even after surface treatment at 900°C for 120 min. It is considered that the surface treatment does not affect the internal structure of pristine carbon black: more graphitic carbon than internal amorphous part develops on the surface of carbon black during ethanol decomposition. Since more graphitic structure than the original carbon black develops during ethanol surface treatment, it is expected that the oxidation resistance increases after the surface treatment. To demonstrate this aspect, we performed TGA of carbon oxidation. Dry air was used to induce sample oxidation. The TG curve of carbon black which was processed with ethanol at 900°C for 120 min is shown in Fig. 6 as a representative of surface-treated carbon black. For comparison, the TG curves of fresh sample and graphite powder are plotted also in Fig. 6. The onset of oxidation was observed for fresh carbon black at around 500°C. Concerning graphite powder, oxidation initiated at higher temperature and the oxidation rate (the slope of curve after the initial transient state) was much slower than fresh carbon black. The surfacetreated carbon black exhibited the onset of oxidation at around 600°C. The oxidation rate was faster than graphite powder, which indicates the difference in the internal crystalline structure between carbon black and graphite powder. The temperature where the weight loss reaches 3% of the initial weight was 527°C for fresh carbon black and 607°C for surface-treated carbon black. Therefore, it is demonstrated that the development of graphitic structure by carbon deposition on fresh carbon black surface makes the surface more oxidation resistant than the original one. 3.3. Surface nanostructure developed by carbon deposition
Fig. 5. Raman spectrum analysis. (a) Exemplary Raman spectrum of carbon black and the 5-band curve fitting result. Time variation of spectral parameters of Raman spectra: (b) FWHM(D1), and (c) R2 ratio.
around 1620 cm−1 and the D4 band around 1180 cm−1. The 5-band curve fitting by a combination of four Lorentzian curves for the G and D2–4 bands and a Gaussian curve for the D1 band allows us to determine Raman spectral parameters [14]. In this study, two spectral parameters of Raman spectrum were evaluated because of their high sensitivities in the structural evaluation of highly disordered carbon: R2 ratio, which is defined as I(D1)/(I(G) + I(D1) + I(D2)) where I is the intensity of each band, and the full-width at half maximum (FWHM) of the D1 band. Fig. 5(b,c) shows the time variation of R2 ratio and the FWHM of the D1 band, respectively. The decrease in both R2 ratio and FWHM(D1) generally indicates a decrease in the structural disorder of highly disordered carbons, or in other words, an increase in graphitic crystalline structure. To discriminate the effect of heat on the structural characteristics of carbon black from that of developed carbon structure, Raman spectra were obtained also for samples treated by heat at 900°C without supplying ethanol. The results are plotted in Fig. 5(b,c). Considering the standard error associated with Raman spectral parameters, we do not recognize the effect of heat treatment on structural characteristics. This observation corresponds to the experimental result showing that an effect of heat treatment on the internal structure of carbon particle becomes remarkable only above ~ 1900°C [16]. Consequently, the
Because the deposited carbon from ethanol decomposition on carbon black surface showed more graphitic characteristics than the original carbon black as described in the previous section, we were motivated to directly observe the surface and internal structure of ethanol-treated samples. TEM observation is used to characterize the internal crystalline structure and the degree of graphitization can be evaluated from the obtained images [15]. TEM images of pristine carbon black (at low and high magnifications) and samples treated at 900°C for 20, 30, 40, and 120 min are shown in Fig. 7(a–f), respectively, and the featured parts described in the text are emphasized by arrows in the figures. Fresh carbon black had smooth surface and the lattice fringe at shell part was parallel to the outline of primary particles as can be seen in TEM images (Fig. 7(a,b)) [6]. As the surface treatment proceeds, carbon produced from methanol deposits on carbon black surface, and the initially smooth surface morphology changes. Small structure (b~3 nm) protruding from the initially smooth surface is observed for 20-min treated sample (Fig. 7(c)). Then, the number of protrusions appearing on the surface increases as well as the size of each structure
Fig. 6. TG curves of carbon black samples and graphite powder.
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Fig. 7. TEM images of pristine carbon black (a) at low magnification and (b) at high magnification, and samples treated at 900°C for (c) 20 min, (d) 30 min, (e) 40 min, and (f) 120 min.
(Fig. 7(d)). Further carbon deposition makes the lattice fringe of graphitic layers clearly observable inside the protruding structure (Fig. 7(e)). This aspect becomes more prominent after the surface treatment for 120 min (Fig. 7(f)). Large structure (N~5 nm) having several graphitic layers is remarkable at this stage. Since the amount of deposited carbon is large, the outline of original primary particles is unclear. It appears that the original particles are almost covered by highlygraphitized deposited carbon, which can be inferred also from the Raman spectral result (Fig. 5) and TGA (Fig. 6). It should be noted that some structural features are different from those observed for carbon black treated by methane thermal decomposition [7]. In the case of methane, dissociative adsorption of methane molecule on the surface active sites of carbon black progresses, which
results in the evolution of broad sheet-like graphitic structure on carbon black surface. In the present case, carbon deposition takes place via carbon monoxide as well as methane generated by ethanol decomposition. It appears that the observed difference in surface nanostructure is due to carbon deposition mechanism. This result suggests that the surface nanostructure developed through surface treatment using hydrocarbon or alcohol could be controlled by properly choosing the source of carbon as well as the conditions of chemical reaction. Fig. 8 summarizes the surface treatment process of carbon black using ethanol thermal decomposition in which the oxidation-resistant graphitic surface nanostructure is created. Ethanol is in the liquid state at room temperature and easy to handle, which makes ethanol one of promising sources of carbon. The proposed process have the potential
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the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References
Fig. 8. Carbon black surface treatment process using ethanol as a carbon source to produce oxidation-resistant graphitic surface nanostructure.
to be applied to an after-treatment of commercial carbon black so that the unique characteristics of surface nanostructure is utilized. 4. Conclusions We investigated the surface treatment process of carbon black using ethanol thermal decomposition. We have demonstrated that the oxidation-resistant graphitic surface nanostructure develops on carbon black surface. The product gas analysis during ethanol decomposition have revealed that carbon monoxide is used as a carbon source to create unique surface nanostructure. Since we observed the difference in surface structural characteristics depending on carbon source, the surface nanostructure could be controlled by choosing the source of carbon as well as the conditions of chemical reaction. In future work, it is expected to examine the performance of unique nanostructure on carbon black surface in a potential application such as stable catalyst support. Acknowledgments This work was supported by JSPS KAKENHI Grant Number 26889056 (Grant-in-Aid for Research Activity Start-up). A part of this work was supported by “Nanotechnology Platform” (project No. 12024046) of
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