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Synthesis of carbon nanofibers by catalytic CVD of chlorobenzene over bulk nickel alloy
Applied Surface Science 427 (2018) 505–510
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Synthesis of carbon nanofibers by catalytic CVD of chlorobenzene over bulk nickel alloy Roman M. Kenzhin a , Yuri I. Bauman a , Alexander M. Volodin a , Ilya V. Mishakov a,b , Aleksey A. Vedyagin a,b,∗ a b
Boreskov Institute of Catalysis SB RAS, pr. Ac. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation National Research Tomsk Polytechnic University, Lenin Av., 30, 634050, Tomsk, Russian Federation
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Article history: Received 2 June 2017 Received in revised form 25 August 2017 Accepted 30 August 2017 Available online 6 September 2017 Keywords: Self-disintegration Bulk nickel alloy Chlorobenzene CCVD RAPET conditions Ferromagnetic resonance
a b s t r a c t Catalytic chemical vapor deposition (CCVD) of chlorobenzene over bulk nickel alloy (nichrome) was studied. The bulk Ni-containing samples being exposed to a contact with aggressive reaction medium undergo self-disintegration followed by growth of carbon nanofibers. This process, also known as a metal dusting, requires the simultaneous presence of chlorine and hydrogen sources in the reaction mixture. Molecule of chlorobenzene complies with these requirements. The experiments on CCVD were performed in a flow-through reactor system. The initial stages of nickel disintegration process were investigated in a closed system under Autogenic Pressure at Elevated Temperature (RAPET) conditions. Scanning and transmission electron microscopies and ferromagnetic resonance spectroscopy were applied to examine the samples after their interaction with chlorobenzene. Introduction of additional hydrogen into the flow-through system was shown to affect the morphology of grown carbon nanofibers. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, carbon nanotubes and nanofibers attract a great attention due to the growing number of their application [1–4]. Among the developed methods of their synthesis, chemical vapor deposition (CVD) is of wide use [5–7]. When the dispersed metal particles (Fe, Ni, Co, etc.) participate in the process, so-called catalytic CVD, fabrication of nanostructured carbon becomes more controllable. Different hydrocarbons including substituted ones can be considered as a substrate to be vaporized and decomposed [8]. On the other hand, both production and application of chlorinated organic compounds are known to be accompanied with formation of a huge amount of wastes represented by a difficultto-utilize mixture of toxic compounds having strongly negative impact on the environment. At the same time, halogenated organics were shown to be involved in catalytic CVD process and efficiently used for production of variety of nanostructured carbon materials [9–13]. As we have reported earlier, bulk metal nickel and its alloys can be used as a precursor of the catalyst for halogenated
∗ Corresponding author at: Boreskov Institute of Catalysis SB RAS, pr. Ac. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation. E-mail addresses: [email protected], [email protected] (A.A. Vedyagin). http://dx.doi.org/10.1016/j.apsusc.2017.08.227 0169-4332/© 2017 Elsevier B.V. All rights reserved.
hydrocarbon processing to carbon nanofibers (CNF) [14–16]. It was found that as a result of 1,2-dichloroethane interaction with bulk metal the later undergoes self-disintegration leading to formation of uniform particles of metallic nickel with characteristic size of 200–250 nm. Further, such particles catalyze growth of carbon nanofibers. This approach is based on phenomenon of metal dusting, which is considered in chemical industry as blighting and undesired [17]. Metal dusting results in complete disintegration of bulk metal items being in a contact with aggressive reaction medium. A lot of research works is dedicated to development of effective ways to prevent it [18–20]. On the other hand, this process of self-disintegration can be positively considered as a novel approach for preparation of dispersed metal particles catalyzing the growth of carbon nanofibers. Such catalytic system named as self-organized catalyst (SOC) was recently demonstrated to be very effective in CCVD of chlorinated hydrocarbons (1,2dichloroethane) with formation of nanostructured carbon product [8,14–16]. Observed during the catalytic CVD process disintegration of bulk nickel is stipulated by the consecutive realization of reductive-oxidative processes, which can described by the following equations. Ni + 2HX → NiX2 + H2
(1)
NiX2 + H2 → Ni + 2HX
(2)
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where X is a halogen containing in initial reagents. It should be noted that there are at least two functions of chlorine which have decisive effect on peculiarities of CCVD process. First of all, disintegration of bulk Ni-based alloys was shown to proceed much faster when chlorinated hydrocarbon is used as precursor due to corrosive function of HCl/H2 pair present in reaction atmosphere. Secondly, presence of Cl is believed to have strong influence on the structure of carbon nanofibers produced via CCVD of chlorinated hydrocarbons. Chemisorbed Cl-species may induce the certain perturbations in electronic properties of Ni particles as well as in character of carbon diffusion. It leads to formation of poorly structured, disordered filamentous carbon product. Thereby, reactions (1) and (2) taking place over bulk metal at aggressive reaction conditions result in total disintegration of the metal. In a flow-through regime, gas-phase products pass away from the reaction zone, while nickel particles of 200–300 nm in size are distributed within the structure of growing carbon nanofibers. Formation of nickel halogenides, which are considered as a key intermediate in a scheme depicted by reactions (1) and (2), was just an assumption. No direct evidence of existence of such intermediates was obtained when the solids were retrieved from the reactor. Thereupon, in our recently published work [21] we have carried out the experiments on interaction of bulk nickel-containing items with halogenated organics in a closed system at almost autoclave conditions, when no gases are removed from the reaction volume. In general, this approach on experiments in a closed volume was successfully applied in numerous works of Prof. A. Gedanken [22–27] for synthesis of various structured materials. This method based on thermolysis of organic precursors in a closed volume was called as Reactions under Autogenic Pressure at Elevated Temperature (RAPET). It has allowed the inventors to prepare uniform in size and shape structured materials including composite ones (carboncontaining materials, sulphides, borides, selenides of metals, etc.), which particle sizes lie in a submicron interval. As we have reported recently [21], interaction of metallic nickel with halogen-substituted hydrocarbons (C6 F6 , C6 Cl6 ) at RAPET conditions results in a total suppression of catalytic formation of nanostructured carbon product. At the same time, appearance of a new type of highly ordered materials with unique structure and properties – presumably microcrystals of nickel halogenides, were observed. It was found that such microcrystals are metastable: they are formed and exist in the narrow ranges of temperature and composition of gas phase. On the other hand, as it was already mentioned, process of bulk metal disintegration requires presence in the reaction mixture not only halogen but hydrogen as well. On this reason, hexamethylbenzene was added to hexachlorine(fluorine)benzene as a hydrogen source in RAPET experiments. It should be emphasized that disintegration process of bulk metal accompanied with growth of halogenide microcrystals and formation of nanostructured carbon takes place in a simultaneous presence of halogen and hydrogen sources only. On the other hand, chlorobenzene is known as constituent in composition of multi-component organochlorine waste, along with various aliphatic chlorine-substituted hydrocarbons such as 1,2-dichloroethane. The features of catalytic CVD process of 1,2-dichloroethane in flow-through reactor over supported Nicatalysts [28] and self-organized Ni-based systems [14–16] has been rather well established. Among the chlorinated hydrocarbons, chlorobenzene is one of the attractive precursors for the synthesis of multi-walled carbon nanotubes with high purity as well as for synthesis of disordered carbon nanofibers [28–32]. At the same time, there is no information in literature concerning the possible usage of chlorinated benzenes as carbon precursors for preparation of self-organizing catalyst from the bulk Ni-based alloys, neither in flow-through regime, nor in a closed system at autogenic pressure.
In general, the growth mechanism of carbon nanomaterials from chlorinated hydrocarbons is fairly similar to that described for many examples of CCVD with respect to unsubstituted hydrocarbons. However, the presence of Cl in the reaction system exerts the significant influence on regularities of CNF growth which is illustrated by structural and morphologic peculiarities of the produced carbon nanomaterial. Additionally, using the bulk Ni-based alloys as precursors of catalyst for CVD process can be considered as a principle thing which makes this approach different from many other CCVD-based processes. Hereby, present work was aimed to study the catalytic CVD of chlorobenzene over bulk nickel-containing items with formation of carbon nanostructures. Chlorobenzene was chosen as a precursor containing both halogen and hydrogen. The effect of hydrogen addition in reaction mixture upon the observed rate of CNF growth was investigated using flowthrough reactor equipped with McBain balances. Ferromagnetic resonance spectrometry was applied to follow the appearance of dispersed nickel particles as a result of bulk metal disintegration at RAPET conditions.
2. Experimental A commercial Ni-Cr alloy known as nichrome (Soyuznichrome, Russia; wire of 0.1 mm in diameter; 80 wt.% Ni, 20 wt.% Cr) was used as bulk starting materials. Each sample (2 mg) prior to the experiments was subjected to short-time (2–3 min) etching in a mixture of hydrochloric and nitric acids (in a ratio of 3:1). Kinetic studies on the disintegration of bulk Ni samples with formation of nanostructured carbon product during the interaction with chlorobenzene vapors were performed in a quartz flow reactor equipped with McBain balances. The vapors of chlorobenzene were fed into reactor by passing the inert purge gas (argon) through the saturator with liquid substrate at 20 ◦ C. Hydrogen of varied concentration was added into the reaction mixture when required. The temperature in reaction zone was varied in a range of 500–750 ◦ C. The weight change was registered in a real-time regime every 2 min with accuracy of 0.1 mg. The enlarged amount of carbon nanomaterials was obtained using scaled-up flow reactor with evaporator. Prior the experiments the samples were kept in a hydrogen flow for 15 min. Liquid halogenated hydrocarbon was fed into evaporator by infusion pump. After the evaporation, the vapors interact with bulk Ni-containing sample inside the reactor. Finally, the sample was cooled down to the room temperature in an argon flow. Studies on nickel-containing samples interaction with chlorobenzene in a RAPET regime were performed as follows. The quartz ampoules (d = 4–5 mm, V ∼ 0.2 ml) were used as a reactor allowing one to collect the spectra of ferromagnetic resonance for the materials inside the ampoule [15]. A piece of nichrome wire of about 0.2–0.3 mg was placed in an ampoule together with 2–3 mg of chlorobenzene. The ampoule with reaction mixture was sealed and brought to thermal treatment at certain temperature for 2 h. The accuracy of temperature measurements was ±2 ◦ C. Both electron paramagnetic resonance (EPR) and FMR spectra were collected for the samples in sealed ampoules. The FMR spectra of samples were registered at room temperature using an experimental setup based on an ERS-221 ESR spectrometer described in [33]. The processing of the spectra obtained was performed with the use of ESR CAD software developed earlier. After the RAPET procedure, the ampoule was carefully opened for further investigation. Note, that amount of solid-state product was insufficiently small to study it with a wide range of characterization techniques. The intermediates after RAPET experiments and carbon nanomaterials obtained in flow reactor were studied by scanning
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Fig. 1. Kinetic curves for carbon deposition during interaction of nichrome wire with chlorobenzene at 600 ◦ C.
electron microscopy (SEM) using a JSM-6460 (Jeol, Japan) electron microscope with a resolution of 4 nm in the range of magnifications from 5× to 300,000× . Transmission electron microscopy (TEM) examinations were performed on a JEM-1400 (Jeol, Japan) instrument (accelerating voltage 80 kV). The local energy-dispersion X-ray (EDX) microanalysis was carried out using EDAX spectrometer with energy resolution 127 eV.
3. Results and discussion 3.1. Interaction of bulk Ni-containing samples with chlorobenzene in a flow-through regime According to our previous studies [14–16], decomposition of chlorinated hydrocarbons over bulk Ni-containing alloys is characterized with a presence of prolonged induction period (IP). Duration of IP depends on different factors (temperature, presence of odd hydrogen in reaction feed, etc.) and can be shortened by preliminary activation of the surface of studied solids. Here, we have used the most efficient treatment procedure of acidic etching. After the procedure, the samples were subjected to a contact with chlorobenzene vapors. As it was shown recently for catalytic CVD of 1,2-dichloroethane [14–16], metal dusting of bulk nickel alloy requires excess hydrogen in a reaction mixture. It is connected with the alloy surface blockage by adsorbed HCl, which is released at decomposition of 1,2-dichloroethane. The formation of nickel chloride phase on the surface is occurred in accordance with reaction (1). Addition of excess hydrogen into reaction mixture leads to alternating of surface chlorination/dechlorination processes, thus facilitating the loosening and activation of bulk metal surface, and removing adsorbed chlorine as well. Molecule of chlorobenzene contains much higher amount of hydrogen atoms then halogen atoms if compare with 1,2-dichloroethane. Thereby, it was of special interest to study the effect of additional hydrogen on peculiarities of catalytic CVD of chlorobenzene over bulk nickel accompanied with metal dusting of the later. Fig. 1 illustrates the effect of hydrogen addition in reaction mixture upon the observed rate of CNF growth at 600 ◦ C accompanied with continuous disintegration of bulk Ni-Cr alloy. It is seen from the figure that intensive disintegration of nichrome wire in the presence of additional hydrogen starts after less than 30 min of its interaction with chlorobenzene vapors. If no hydrogen was added into reaction mixture, the process begins after 1 h and goes not so intensively (see the insertion in Fig. 1).
Fig. 2. SEM images of carbon materials obtained via catalytic CVD of chlorobenzene over bulk nichrome at 600 ◦ C: a – without hydrogen; b,c – with additional hydrogen (35 vol%).
Figs. 2 and 3 show the SEM and TEM images of carbon product obtained by catalytic CVD of chlorobenzene over nichrome wire. The methods reveal the impact of additional hydrogen on structure and morphology of carbon being deposited. A presence of odd hydrogen favors the formation of smaller nickel particles resulted from disintegration of bulk nichrome alloy, which, in its turn, affects the diameter of CNF. As seen from Fig. 2a, the carbon product is represented by dense agglomerates of filaments composed from chaotic arranged graphene fragments. At the same time, addition of hydrogen leads to formation of noticeably friable layer of thin carbon fibers as demonstrated in Fig. 2b and c. TEM images of the samples exposed to interaction with chlorobenzene are shown in Fig. 3. The main feature of the nanofibers obtained in absence of hydrogen is their globular structure (Fig. 3a and b). Each fiber consists of separated fragments of hollow sphere-like shape. The thickness of their walls is varied in a range of 15–30 basal planes
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Fig. 3. TEM images of carbon materials obtained via catalytic CVD of chlorobenzene over bulk nichrome at 600 ◦ C: a, b – without hydrogen; c, d – with additional hydrogen (35 vol%).
of graphite. The similar spherical structure of carbon deposits was reported by Nieto-Marquez et al. [29] and by Shaikjee and Coville [34] for decomposition of various chlorinated hydrocarbons over supported nickel catalysts. Effect of hydrogen is also evident from Fig. 3. Addition of hydrogen into reaction mixture results in narrowing of the carbon fibers (from 50–70 nm to 20–40 nm) and fluffing of their edges, which is in good agreement with data from scanning electron microscopy. It was also found that addition of hydrogen into reaction mixture at temperatures below 550 ◦ C results in cardinal change of the process route from carbide cycle mechanism to mechanism of chlorobenzene hydrodechlorination. At such conditions, no carbon deposition on nickel surface takes place, while the main product of interaction becomes benzene. Nevertheless, in the absence of hydrogen, process of catalytic CVD of chlorobenzene realizes effi-
ciently with formation of carbon nanostructures at relatively low temperatures. According to averaged EDX analysis data, the obtained carbon product contains about 1.5 at.% of chlorine which is in good agreement with data reported earlier for different carbon nanomaterials produced by decomposition of various chlorinated hydrocarbons over Ni-containing catalysts, where the chlorine concentration was in a range of 0.07–2.1% [34–36]. Chlorine was found to be mainly chemisorbed on surface of active metallic particles resulted from disintegration of parent Ni-Cr alloy. Thereby, it can be concluded that catalytic chemical vapor deposition of chlorobenzene over bulk nickel alloy with no excess hydrogen leads to formation of carbon product represented by ensemble of independent globular structures composed into integrated fibrous system. The reasons for appearance of such unusual structures are not evident, and require further detailed investigation. 3.2. Interaction of bulk Ni-containing samples with chlorobenzene in a RAPET regime
Fig. 4. FMR spectra for the bulk Ni-Cr alloy exposed to interaction with chlorobenzene under RAPET conditions at different temperatures.
As we have reported earlier [15], ferromagnetic resonance spectrometry is a quite sensitive technique, which allows one to follow the formation of dispersed nickel particles from very beginning due their ferromagnetism. Evolution of FMR spectrum during the interaction of nichrome wire with chlorobenzene at RAPET conditions and different temperatures is shown in Fig. 4. In the case of chlorobenzene decomposition, process of self-disintegration of bulk nichrome with appearance of ferromagnetic nickel particles starts at 550 ◦ C, that is slightly higher if compare with decomposition of hexachlorinebenzene and hexamethylbenzene mixture (480 ◦ C) [21]. A wide signal with gav ∼ 2.3, which is typical for FMR spectra of dispersed nickel particles, was found to appear at 560 ◦ C. Along with it, intensive narrow singlet (g = 2.003) characteristic for various carbon materials was observed. When the ampoule was heated up to 610 ◦ C, the intensity of FMR signal increases significantly, thus facilitating towards higher degree of corrosion
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Fig. 5. SEM image of Ni-Cr samples after being exposed to interaction with chlorobenzene at 610 ◦ C (a, b) and 730 ◦ C (c, d) under RAPET conditions.
of the bulk metal alloy with formation of larger amount of dispersed nickel particles. It should be noted here, that registration of FMR spectra at higher temperatures is complicated due to strong microwave absorption by the sample. Fig. 5 shows microscopic images for the nichrome samples after interaction with chlorobenzene at 610 and 730 ◦ C. As seen in Fig. 5(a, b), the surface of alloy is covered with thick carbon fibers of about 3–5 in diameter. Similarly to the flow-through regime, the observed carbon fibers contain trace amount of chlorine. When the temperature was 730 ◦ C, two types of carbon fibers were formed: thick fibers similar to the previous ones and carbon spheres of 2 in diameter. The larger size of nickel particles and grown carbon fibers can be explained by few reasons. First of all, the realization of the process in a closed reactor system does not allow chlorinecontaining intermediates to be removed from the surface of the sample. Secondly, the only hydrogen present in reaction volume is generated via decomposition of chlorobenzene. These assumptions correlate well with previously discussed effect of additional hydrogen on regularities of metal dusting, including the size of nickel particles being formed. Unfortunately, no structured solid intermediates (like nickel halogenides found in [21]) were detected in the case of chlorobenzene. Most probably, it is connected with that just one halogen atom is contained in molecule of chlorobenzene. Such low amount of chlorine seems to be enough to initialize the process of selfdisintegration of bulk metal, but insufficient for formation of intermediates (nickel halogenides) due to hydrogen excess, which leads to their reduction in accordance with reaction (2). It is important to emphasize that no self-disintegration of bulk nichrome alloy or formation of carbon nanofibers was observed under RAPET conditions when benzene or hexacholobenzene was used as an organic substrate instead of chlorobenzene.
4. Conclusions In present study, catalytic chemical vapor deposition of chlorobenzene over bulk nichrome alloy was performed. The
results obtained testify towards previously made assumption [21] that the process of self-disintegration of bulk metal precursor requires presence of both hydrogen and halogen sources in a reaction medium. Herewith, hydrogen and halogen might be contained in one compound (like in chlorobenzene). In a flow reactor, such processes of Ni-based alloys self-disintegration in halogensubstituted organic medium occur in a presence of additional hydrogen only [8]. In the case of C6 H5 Cl, deposition of nanostructured carbon was observed even without hydrogen excess. Carbon nanofibers formed are characterized with globular structure. Addition of excess hydrogen into reaction mixture results in formation of more narrow filaments. Interaction of chlorobenzene with bulk nichrome takes place under RAPET conditions as well. Unfortunately, no intermediate solid-phase halogenides were detected in these experiments. It might be stipulated by few reasons including insufficient amount of halogen. At the same time, carbon fibers formed under RAPET regime have just remote resemblance with nanostructures obtained in a flow regime. There are not thin but thickened and dense fibers. A good correlation between the starting temperatures of carbon nanofibers formation and FMR data on disperse Ni particles appearance (initial point of bulk nichrome self-dispersion process) was observed. In a whole, results of flow reactor correlate well with that for RAPET regime. All it indicates the importance of simultaneous presence of hydrogen and halogen sources during the catalytic CVD process.
Acknowledgments Financial support by Russian Foundation for Basic Research (project 16-33-60034) is acknowledged with gratitude.
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Full Length Article
Synthesis of carbon nanofibers by catalytic CVD of chlorobenzene over bulk nickel alloy Roman M. Kenzhin a , Yuri I. Bauman a , Alexander M. Volodin a , Ilya V. Mishakov a,b , Aleksey A. Vedyagin a,b,∗ a b
Boreskov Institute of Catalysis SB RAS, pr. Ac. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation National Research Tomsk Polytechnic University, Lenin Av., 30, 634050, Tomsk, Russian Federation
a r t i c l e
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Article history: Received 2 June 2017 Received in revised form 25 August 2017 Accepted 30 August 2017 Available online 6 September 2017 Keywords: Self-disintegration Bulk nickel alloy Chlorobenzene CCVD RAPET conditions Ferromagnetic resonance
a b s t r a c t Catalytic chemical vapor deposition (CCVD) of chlorobenzene over bulk nickel alloy (nichrome) was studied. The bulk Ni-containing samples being exposed to a contact with aggressive reaction medium undergo self-disintegration followed by growth of carbon nanofibers. This process, also known as a metal dusting, requires the simultaneous presence of chlorine and hydrogen sources in the reaction mixture. Molecule of chlorobenzene complies with these requirements. The experiments on CCVD were performed in a flow-through reactor system. The initial stages of nickel disintegration process were investigated in a closed system under Autogenic Pressure at Elevated Temperature (RAPET) conditions. Scanning and transmission electron microscopies and ferromagnetic resonance spectroscopy were applied to examine the samples after their interaction with chlorobenzene. Introduction of additional hydrogen into the flow-through system was shown to affect the morphology of grown carbon nanofibers. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, carbon nanotubes and nanofibers attract a great attention due to the growing number of their application [1–4]. Among the developed methods of their synthesis, chemical vapor deposition (CVD) is of wide use [5–7]. When the dispersed metal particles (Fe, Ni, Co, etc.) participate in the process, so-called catalytic CVD, fabrication of nanostructured carbon becomes more controllable. Different hydrocarbons including substituted ones can be considered as a substrate to be vaporized and decomposed [8]. On the other hand, both production and application of chlorinated organic compounds are known to be accompanied with formation of a huge amount of wastes represented by a difficultto-utilize mixture of toxic compounds having strongly negative impact on the environment. At the same time, halogenated organics were shown to be involved in catalytic CVD process and efficiently used for production of variety of nanostructured carbon materials [9–13]. As we have reported earlier, bulk metal nickel and its alloys can be used as a precursor of the catalyst for halogenated
∗ Corresponding author at: Boreskov Institute of Catalysis SB RAS, pr. Ac. Lavrentieva, 5, 630090, Novosibirsk, Russian Federation. E-mail addresses: [email protected], [email protected] (A.A. Vedyagin). http://dx.doi.org/10.1016/j.apsusc.2017.08.227 0169-4332/© 2017 Elsevier B.V. All rights reserved.
hydrocarbon processing to carbon nanofibers (CNF) [14–16]. It was found that as a result of 1,2-dichloroethane interaction with bulk metal the later undergoes self-disintegration leading to formation of uniform particles of metallic nickel with characteristic size of 200–250 nm. Further, such particles catalyze growth of carbon nanofibers. This approach is based on phenomenon of metal dusting, which is considered in chemical industry as blighting and undesired [17]. Metal dusting results in complete disintegration of bulk metal items being in a contact with aggressive reaction medium. A lot of research works is dedicated to development of effective ways to prevent it [18–20]. On the other hand, this process of self-disintegration can be positively considered as a novel approach for preparation of dispersed metal particles catalyzing the growth of carbon nanofibers. Such catalytic system named as self-organized catalyst (SOC) was recently demonstrated to be very effective in CCVD of chlorinated hydrocarbons (1,2dichloroethane) with formation of nanostructured carbon product [8,14–16]. Observed during the catalytic CVD process disintegration of bulk nickel is stipulated by the consecutive realization of reductive-oxidative processes, which can described by the following equations. Ni + 2HX → NiX2 + H2
(1)
NiX2 + H2 → Ni + 2HX
(2)
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where X is a halogen containing in initial reagents. It should be noted that there are at least two functions of chlorine which have decisive effect on peculiarities of CCVD process. First of all, disintegration of bulk Ni-based alloys was shown to proceed much faster when chlorinated hydrocarbon is used as precursor due to corrosive function of HCl/H2 pair present in reaction atmosphere. Secondly, presence of Cl is believed to have strong influence on the structure of carbon nanofibers produced via CCVD of chlorinated hydrocarbons. Chemisorbed Cl-species may induce the certain perturbations in electronic properties of Ni particles as well as in character of carbon diffusion. It leads to formation of poorly structured, disordered filamentous carbon product. Thereby, reactions (1) and (2) taking place over bulk metal at aggressive reaction conditions result in total disintegration of the metal. In a flow-through regime, gas-phase products pass away from the reaction zone, while nickel particles of 200–300 nm in size are distributed within the structure of growing carbon nanofibers. Formation of nickel halogenides, which are considered as a key intermediate in a scheme depicted by reactions (1) and (2), was just an assumption. No direct evidence of existence of such intermediates was obtained when the solids were retrieved from the reactor. Thereupon, in our recently published work [21] we have carried out the experiments on interaction of bulk nickel-containing items with halogenated organics in a closed system at almost autoclave conditions, when no gases are removed from the reaction volume. In general, this approach on experiments in a closed volume was successfully applied in numerous works of Prof. A. Gedanken [22–27] for synthesis of various structured materials. This method based on thermolysis of organic precursors in a closed volume was called as Reactions under Autogenic Pressure at Elevated Temperature (RAPET). It has allowed the inventors to prepare uniform in size and shape structured materials including composite ones (carboncontaining materials, sulphides, borides, selenides of metals, etc.), which particle sizes lie in a submicron interval. As we have reported recently [21], interaction of metallic nickel with halogen-substituted hydrocarbons (C6 F6 , C6 Cl6 ) at RAPET conditions results in a total suppression of catalytic formation of nanostructured carbon product. At the same time, appearance of a new type of highly ordered materials with unique structure and properties – presumably microcrystals of nickel halogenides, were observed. It was found that such microcrystals are metastable: they are formed and exist in the narrow ranges of temperature and composition of gas phase. On the other hand, as it was already mentioned, process of bulk metal disintegration requires presence in the reaction mixture not only halogen but hydrogen as well. On this reason, hexamethylbenzene was added to hexachlorine(fluorine)benzene as a hydrogen source in RAPET experiments. It should be emphasized that disintegration process of bulk metal accompanied with growth of halogenide microcrystals and formation of nanostructured carbon takes place in a simultaneous presence of halogen and hydrogen sources only. On the other hand, chlorobenzene is known as constituent in composition of multi-component organochlorine waste, along with various aliphatic chlorine-substituted hydrocarbons such as 1,2-dichloroethane. The features of catalytic CVD process of 1,2-dichloroethane in flow-through reactor over supported Nicatalysts [28] and self-organized Ni-based systems [14–16] has been rather well established. Among the chlorinated hydrocarbons, chlorobenzene is one of the attractive precursors for the synthesis of multi-walled carbon nanotubes with high purity as well as for synthesis of disordered carbon nanofibers [28–32]. At the same time, there is no information in literature concerning the possible usage of chlorinated benzenes as carbon precursors for preparation of self-organizing catalyst from the bulk Ni-based alloys, neither in flow-through regime, nor in a closed system at autogenic pressure.
In general, the growth mechanism of carbon nanomaterials from chlorinated hydrocarbons is fairly similar to that described for many examples of CCVD with respect to unsubstituted hydrocarbons. However, the presence of Cl in the reaction system exerts the significant influence on regularities of CNF growth which is illustrated by structural and morphologic peculiarities of the produced carbon nanomaterial. Additionally, using the bulk Ni-based alloys as precursors of catalyst for CVD process can be considered as a principle thing which makes this approach different from many other CCVD-based processes. Hereby, present work was aimed to study the catalytic CVD of chlorobenzene over bulk nickel-containing items with formation of carbon nanostructures. Chlorobenzene was chosen as a precursor containing both halogen and hydrogen. The effect of hydrogen addition in reaction mixture upon the observed rate of CNF growth was investigated using flowthrough reactor equipped with McBain balances. Ferromagnetic resonance spectrometry was applied to follow the appearance of dispersed nickel particles as a result of bulk metal disintegration at RAPET conditions.
2. Experimental A commercial Ni-Cr alloy known as nichrome (Soyuznichrome, Russia; wire of 0.1 mm in diameter; 80 wt.% Ni, 20 wt.% Cr) was used as bulk starting materials. Each sample (2 mg) prior to the experiments was subjected to short-time (2–3 min) etching in a mixture of hydrochloric and nitric acids (in a ratio of 3:1). Kinetic studies on the disintegration of bulk Ni samples with formation of nanostructured carbon product during the interaction with chlorobenzene vapors were performed in a quartz flow reactor equipped with McBain balances. The vapors of chlorobenzene were fed into reactor by passing the inert purge gas (argon) through the saturator with liquid substrate at 20 ◦ C. Hydrogen of varied concentration was added into the reaction mixture when required. The temperature in reaction zone was varied in a range of 500–750 ◦ C. The weight change was registered in a real-time regime every 2 min with accuracy of 0.1 mg. The enlarged amount of carbon nanomaterials was obtained using scaled-up flow reactor with evaporator. Prior the experiments the samples were kept in a hydrogen flow for 15 min. Liquid halogenated hydrocarbon was fed into evaporator by infusion pump. After the evaporation, the vapors interact with bulk Ni-containing sample inside the reactor. Finally, the sample was cooled down to the room temperature in an argon flow. Studies on nickel-containing samples interaction with chlorobenzene in a RAPET regime were performed as follows. The quartz ampoules (d = 4–5 mm, V ∼ 0.2 ml) were used as a reactor allowing one to collect the spectra of ferromagnetic resonance for the materials inside the ampoule [15]. A piece of nichrome wire of about 0.2–0.3 mg was placed in an ampoule together with 2–3 mg of chlorobenzene. The ampoule with reaction mixture was sealed and brought to thermal treatment at certain temperature for 2 h. The accuracy of temperature measurements was ±2 ◦ C. Both electron paramagnetic resonance (EPR) and FMR spectra were collected for the samples in sealed ampoules. The FMR spectra of samples were registered at room temperature using an experimental setup based on an ERS-221 ESR spectrometer described in [33]. The processing of the spectra obtained was performed with the use of ESR CAD software developed earlier. After the RAPET procedure, the ampoule was carefully opened for further investigation. Note, that amount of solid-state product was insufficiently small to study it with a wide range of characterization techniques. The intermediates after RAPET experiments and carbon nanomaterials obtained in flow reactor were studied by scanning
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Fig. 1. Kinetic curves for carbon deposition during interaction of nichrome wire with chlorobenzene at 600 ◦ C.
electron microscopy (SEM) using a JSM-6460 (Jeol, Japan) electron microscope with a resolution of 4 nm in the range of magnifications from 5× to 300,000× . Transmission electron microscopy (TEM) examinations were performed on a JEM-1400 (Jeol, Japan) instrument (accelerating voltage 80 kV). The local energy-dispersion X-ray (EDX) microanalysis was carried out using EDAX spectrometer with energy resolution 127 eV.
3. Results and discussion 3.1. Interaction of bulk Ni-containing samples with chlorobenzene in a flow-through regime According to our previous studies [14–16], decomposition of chlorinated hydrocarbons over bulk Ni-containing alloys is characterized with a presence of prolonged induction period (IP). Duration of IP depends on different factors (temperature, presence of odd hydrogen in reaction feed, etc.) and can be shortened by preliminary activation of the surface of studied solids. Here, we have used the most efficient treatment procedure of acidic etching. After the procedure, the samples were subjected to a contact with chlorobenzene vapors. As it was shown recently for catalytic CVD of 1,2-dichloroethane [14–16], metal dusting of bulk nickel alloy requires excess hydrogen in a reaction mixture. It is connected with the alloy surface blockage by adsorbed HCl, which is released at decomposition of 1,2-dichloroethane. The formation of nickel chloride phase on the surface is occurred in accordance with reaction (1). Addition of excess hydrogen into reaction mixture leads to alternating of surface chlorination/dechlorination processes, thus facilitating the loosening and activation of bulk metal surface, and removing adsorbed chlorine as well. Molecule of chlorobenzene contains much higher amount of hydrogen atoms then halogen atoms if compare with 1,2-dichloroethane. Thereby, it was of special interest to study the effect of additional hydrogen on peculiarities of catalytic CVD of chlorobenzene over bulk nickel accompanied with metal dusting of the later. Fig. 1 illustrates the effect of hydrogen addition in reaction mixture upon the observed rate of CNF growth at 600 ◦ C accompanied with continuous disintegration of bulk Ni-Cr alloy. It is seen from the figure that intensive disintegration of nichrome wire in the presence of additional hydrogen starts after less than 30 min of its interaction with chlorobenzene vapors. If no hydrogen was added into reaction mixture, the process begins after 1 h and goes not so intensively (see the insertion in Fig. 1).
Fig. 2. SEM images of carbon materials obtained via catalytic CVD of chlorobenzene over bulk nichrome at 600 ◦ C: a – without hydrogen; b,c – with additional hydrogen (35 vol%).
Figs. 2 and 3 show the SEM and TEM images of carbon product obtained by catalytic CVD of chlorobenzene over nichrome wire. The methods reveal the impact of additional hydrogen on structure and morphology of carbon being deposited. A presence of odd hydrogen favors the formation of smaller nickel particles resulted from disintegration of bulk nichrome alloy, which, in its turn, affects the diameter of CNF. As seen from Fig. 2a, the carbon product is represented by dense agglomerates of filaments composed from chaotic arranged graphene fragments. At the same time, addition of hydrogen leads to formation of noticeably friable layer of thin carbon fibers as demonstrated in Fig. 2b and c. TEM images of the samples exposed to interaction with chlorobenzene are shown in Fig. 3. The main feature of the nanofibers obtained in absence of hydrogen is their globular structure (Fig. 3a and b). Each fiber consists of separated fragments of hollow sphere-like shape. The thickness of their walls is varied in a range of 15–30 basal planes
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Fig. 3. TEM images of carbon materials obtained via catalytic CVD of chlorobenzene over bulk nichrome at 600 ◦ C: a, b – without hydrogen; c, d – with additional hydrogen (35 vol%).
of graphite. The similar spherical structure of carbon deposits was reported by Nieto-Marquez et al. [29] and by Shaikjee and Coville [34] for decomposition of various chlorinated hydrocarbons over supported nickel catalysts. Effect of hydrogen is also evident from Fig. 3. Addition of hydrogen into reaction mixture results in narrowing of the carbon fibers (from 50–70 nm to 20–40 nm) and fluffing of their edges, which is in good agreement with data from scanning electron microscopy. It was also found that addition of hydrogen into reaction mixture at temperatures below 550 ◦ C results in cardinal change of the process route from carbide cycle mechanism to mechanism of chlorobenzene hydrodechlorination. At such conditions, no carbon deposition on nickel surface takes place, while the main product of interaction becomes benzene. Nevertheless, in the absence of hydrogen, process of catalytic CVD of chlorobenzene realizes effi-
ciently with formation of carbon nanostructures at relatively low temperatures. According to averaged EDX analysis data, the obtained carbon product contains about 1.5 at.% of chlorine which is in good agreement with data reported earlier for different carbon nanomaterials produced by decomposition of various chlorinated hydrocarbons over Ni-containing catalysts, where the chlorine concentration was in a range of 0.07–2.1% [34–36]. Chlorine was found to be mainly chemisorbed on surface of active metallic particles resulted from disintegration of parent Ni-Cr alloy. Thereby, it can be concluded that catalytic chemical vapor deposition of chlorobenzene over bulk nickel alloy with no excess hydrogen leads to formation of carbon product represented by ensemble of independent globular structures composed into integrated fibrous system. The reasons for appearance of such unusual structures are not evident, and require further detailed investigation. 3.2. Interaction of bulk Ni-containing samples with chlorobenzene in a RAPET regime
Fig. 4. FMR spectra for the bulk Ni-Cr alloy exposed to interaction with chlorobenzene under RAPET conditions at different temperatures.
As we have reported earlier [15], ferromagnetic resonance spectrometry is a quite sensitive technique, which allows one to follow the formation of dispersed nickel particles from very beginning due their ferromagnetism. Evolution of FMR spectrum during the interaction of nichrome wire with chlorobenzene at RAPET conditions and different temperatures is shown in Fig. 4. In the case of chlorobenzene decomposition, process of self-disintegration of bulk nichrome with appearance of ferromagnetic nickel particles starts at 550 ◦ C, that is slightly higher if compare with decomposition of hexachlorinebenzene and hexamethylbenzene mixture (480 ◦ C) [21]. A wide signal with gav ∼ 2.3, which is typical for FMR spectra of dispersed nickel particles, was found to appear at 560 ◦ C. Along with it, intensive narrow singlet (g = 2.003) characteristic for various carbon materials was observed. When the ampoule was heated up to 610 ◦ C, the intensity of FMR signal increases significantly, thus facilitating towards higher degree of corrosion
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Fig. 5. SEM image of Ni-Cr samples after being exposed to interaction with chlorobenzene at 610 ◦ C (a, b) and 730 ◦ C (c, d) under RAPET conditions.
of the bulk metal alloy with formation of larger amount of dispersed nickel particles. It should be noted here, that registration of FMR spectra at higher temperatures is complicated due to strong microwave absorption by the sample. Fig. 5 shows microscopic images for the nichrome samples after interaction with chlorobenzene at 610 and 730 ◦ C. As seen in Fig. 5(a, b), the surface of alloy is covered with thick carbon fibers of about 3–5 in diameter. Similarly to the flow-through regime, the observed carbon fibers contain trace amount of chlorine. When the temperature was 730 ◦ C, two types of carbon fibers were formed: thick fibers similar to the previous ones and carbon spheres of 2 in diameter. The larger size of nickel particles and grown carbon fibers can be explained by few reasons. First of all, the realization of the process in a closed reactor system does not allow chlorinecontaining intermediates to be removed from the surface of the sample. Secondly, the only hydrogen present in reaction volume is generated via decomposition of chlorobenzene. These assumptions correlate well with previously discussed effect of additional hydrogen on regularities of metal dusting, including the size of nickel particles being formed. Unfortunately, no structured solid intermediates (like nickel halogenides found in [21]) were detected in the case of chlorobenzene. Most probably, it is connected with that just one halogen atom is contained in molecule of chlorobenzene. Such low amount of chlorine seems to be enough to initialize the process of selfdisintegration of bulk metal, but insufficient for formation of intermediates (nickel halogenides) due to hydrogen excess, which leads to their reduction in accordance with reaction (2). It is important to emphasize that no self-disintegration of bulk nichrome alloy or formation of carbon nanofibers was observed under RAPET conditions when benzene or hexacholobenzene was used as an organic substrate instead of chlorobenzene.
4. Conclusions In present study, catalytic chemical vapor deposition of chlorobenzene over bulk nichrome alloy was performed. The
results obtained testify towards previously made assumption [21] that the process of self-disintegration of bulk metal precursor requires presence of both hydrogen and halogen sources in a reaction medium. Herewith, hydrogen and halogen might be contained in one compound (like in chlorobenzene). In a flow reactor, such processes of Ni-based alloys self-disintegration in halogensubstituted organic medium occur in a presence of additional hydrogen only [8]. In the case of C6 H5 Cl, deposition of nanostructured carbon was observed even without hydrogen excess. Carbon nanofibers formed are characterized with globular structure. Addition of excess hydrogen into reaction mixture results in formation of more narrow filaments. Interaction of chlorobenzene with bulk nichrome takes place under RAPET conditions as well. Unfortunately, no intermediate solid-phase halogenides were detected in these experiments. It might be stipulated by few reasons including insufficient amount of halogen. At the same time, carbon fibers formed under RAPET regime have just remote resemblance with nanostructures obtained in a flow regime. There are not thin but thickened and dense fibers. A good correlation between the starting temperatures of carbon nanofibers formation and FMR data on disperse Ni particles appearance (initial point of bulk nichrome self-dispersion process) was observed. In a whole, results of flow reactor correlate well with that for RAPET regime. All it indicates the importance of simultaneous presence of hydrogen and halogen sources during the catalytic CVD process.
Acknowledgments Financial support by Russian Foundation for Basic Research (project 16-33-60034) is acknowledged with gratitude.
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