Synergetic Effects of Molybdenum and Magnesium ... - ACS Publications

Sep 28, 2017 - yield increased dramatically with increasing carbonization temperature from 800 to 1000 °C. Beyond 1000 °C, the yield of CNTs was not...
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Synergetic Effects of Molybdenum and Magnesium in Ni−Mo−Mg Catalysts on the One-Step Carbonization of Polystyrene into Carbon Nanotubes Guangdong Li,† Shengnan Tan,‡ Rongjun Song,*,† and Tao Tang*,§ †

Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded Materials, College of Science, Northeast Forestry University, Harbin 150040, PR China ‡ College of Wildlife Resource, Northeast Forestry University, Harbin 150040, PR China § State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China S Supporting Information *

ABSTRACT: The Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg catalysts were systematically explored for the one-step conversion of polystyrene (PS) into carbon nanotubes (CNTs). The catalysts were characterized in the phase structure, morphology, reducibility, and carbonization performance. It was found that the Mo contributed to the small catalyst particle growth by the formation of NiMoO4 phase. The addition of Mg had no effect on catalyst particle sizes but effectively controlled the carbon solubility and led to the growth of CNTs by the generation of NiMgO2. Finally, we demonstrated that the Ni−Mo−Mg catalysts showed the most efficient for the conversion of PS into CNTs owing to the individually synergetic effect of Mo and Mg elements. Besides catalysts, the degradation additives and the carbonization temperature were also discussed for their effects on the carbonization of PS. It is believed that the work provides potential for the conversion of PS waste into CNTs. mesh.9 Acomb et al. used thermal-cracking fraction of PP, PE and PS as carbon source to synthesize CNTs by Ni/Al2O3 catalysts.11 Tang et al. provided a novel route by melt-blending catalysts into plastic matrix, in which a high percentage of plastics were carbonized into CNTs through one-step pyrolyzation.18 Following Tang’s results, varieties of works have been done for the preparation of CNTs from plastic via the one-step plastic carbonization, as this method provides significant feasibility for recycling the plastic waste into highvalued CNTs because of the ultrahigh conversion rate. Jiang et al. utilized Ni oxide as carbonization catalysts and organically modified montmorillonite (OMMT) as a degradation catalyst to achieve a 48% of conversion yield of PP.19 Song et al. found that some of solid acid could effectively substitute the OMMT in this one-step PP carbonization into CNTs.20,21 In Yu’s work, the NiO was also used as catalysts, but the degradation catalyst was replaced by trace amounts of halide compound.22,23 The conversion of PP into CNTs was reached up to 56%. Wen et al. reported a universal degradation additive (nanosized carbon black), which showed high synergetic efficiency for the one-step

1. INTRODUCTION With extreme increase of production and consumption of plastic worldwide, the treatment of waste plastic has everincreasingly aroused attention due to their nonbiodegradable properties.1,2 The conventional treatments, such as landfill and incineration, are far from being accepted owing to the energy waste and serious environmental pollution problems. Chemical recycling can still be considered the most promising for the treatment of culminating plastic waste, among which most of petrochemical component can be recovered and even reutilized as the initial chemical resource.3−5 However, the low quality of recovered products from the pyrolysis of plastic waste often requires long and tedious post-treatment process.6−8 So exploring a new chemical recycling process with relatively pure and high-quality final product is very crucial for the development of plastic waste treatment. Recently, conversion of virgin or waste plastics into pure and high-valued carbon nanomaterials has received worldwide attention owing to its potential and important supplying to the chemical recycling route currently used waste plastic. So far, many attempts have been done to convert virgin or waster plastics including polyethylene (PE), polypropylene (PP), and polystyrene (PS) into carbon nanomaterials.9−17 For example, Zhuo et al. prepared carbon nanotubes (CNTs) by gasification of PE and then catalytic carbonization by a stainless-steel wire © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

July 2, 2017 September 26, 2017 September 28, 2017 September 28, 2017 DOI: 10.1021/acs.iecr.7b02697 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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mechanism of a catalyst in one-step plastic carbonization, it might help to judge its compatibility with a plastic type (such as PS) and the requirement for a degradation additive. In this study, on the basis of Ni (NiO) catalysts, a series of catalyst combinations, denoted as Ni−Mo, Ni−Mg, and Ni− Mo−Mg were systematically studied for the pyrolyzation of PS into CNTs, where the function of Mo and MgO in Ni−Mo− Mg catalyst was clearly addressed. Importantly, the conversion of PS and the quality of CNTs were dramatically enhanced, even without the use of degradation additives in PS matrix. We believe that this work will contribute to a lot for the mass production of CNTs from PS-based plastics via one-step carbonization. Meanwhile, it may provide a significant help to study and clarify the function of promoter and supporter in bimetallic catalysts for some other CNT-synthesized systems.

carbonization of PP, PE, and PS into CNTs with the presence of NiO.24 Recently, Shen et al. reported two new carbonization catalysts (i.e., Ni/MgO and Ni/Al2O3) and achieved a very high carbonization rate of ∼80% of PP with the help of OMMT.25,26 In addition, Tang and Gong have done many works on the application of final obtained CNTs and the conversion of PP, PE, and PS into other carbon nanomaterials with one-step carbonization.27−34 Nevertheless, there are still some problems remaining to be resolved in this one-step carbonization. For example, relative to PP and PE, very limited works have been done for the carbonization of PS into CNTs. Furthermore, the PS conversion rate is still very low, and the CNT quality is yet needed to be improved in the morphological uniformity and graphitization degree. Although the addition of degradation additives can change the degraded product composition of plastic to a certain extent, the carbon sources in the one-step plastic carbonization are still composed of complicated and diverse hydrocarbons due to the random chain scission of plastic. As such, the choice of catalysts should be very crucial for overcoming above-mentioned problems. To date, the catalysts employed in one-step plastic carbonization basically belong to monometallic Ni-based catalysts and almost no studies focus on the catalytic mechanism in this special carbonization. In a previous work, Gong has demonstrated that the NiO catalysts with relatively small particle sizes favor the formation of long and uniformed CNT structures in one-step PP carbonization.29 In Shen’s works,25,26 the utilization of Ni/Al2O3 or Ni/MgO has been demonstrated to improve the CNT quality from PP with respect to NiO catalysts. Over the past few years, our group has discovered that further combination of Mo to Ni/ MgO shows high efficiency for the conversion of PP, PE, and PS into CNTs with the presence of carbon black additives.35,36 However, the carbonizing ability of the Ni/Mo/MgO catalysts for the PS carbonization is still very weak; and the addition of additives, such as carbon black, seems to be indispensable. Moreover, similar to the combined catalysts used in one-step plastic carbonization,25,26 the function of Mo and MgO in Ni/ Mo/MgO remains elusive. As such, addressing clearly the role of Mo and MgO in Ni/Mo/MgO should be very crucial for the development of PS carbonization. The MgO is widely employed in chemical vapor deposition (CVD) CNT synthesis, acting as catalyst supporters, which normally accounts for more than 90% of total catalyst weight.37−43 In addition to supporter function, the MgO often functions as a promoter affecting both CNT yield and quality. However, its behavior has been found to be controversial at times, and the interaction between MgO and metal active phase seems to be ambiguous. In contrast to the catalysts in CVD, the Ni/MgO and Ni/Mo/MgO in the onestep plastic carbonization showed high catalytic performance only at the very low mass content of MgO in catalysts, and the supporter function of them obviously became very weak or none.36,44 As a new type of carbonization system, addressing the interaction between metal active phase and MgO in onestep carbonization might reflect the real function of MgO. In the case of Mo, some literature had reported its promoter role in bimetallic catalysts as a so-called shielding effect or an avalanche-like reduction of MgMoO4 to protect the primary metal particles from agglomeration.40,43 In this regard, to further reveal the function of Mo in bimetallic catalysts with the one-step plastic carbonization seems to be requisite. More importantly, through deeply understanding the catalysis

2. EXPERIMENTAL SECTION 2.1. Materials. Polystyrene (PG-33) particle was purchased from Zhenjiang Chimei Chemical Co., Ltd. Ni(NO3)2·6H2O(AR), (NH4)6Mo7O24·4H2O(AR), and Mg(NO3)2·6H2O(AR) were from Tianjin Damao Chemical Reagent Factory, poly(ethylene glycol)200(PEG-200) with a molecular weight of about 200 was kindly provided by Tianjin Zhiyuan Chemical Reagent Co., Ltd. 2.2. Fabrication Procedure. The catalyst, including Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg, were synthesized by a combustion method according to our previous report.22 Briefly speaking, the metal salts are weighted and mixed at a certain molar ratio, and then a proper amount of PEG-200 is added to facilitate further fusion between them. After that, it was combusted for 1 h in a muffle furnace at 650 °C. The catalysts were finally obtained by grinding these cooling products into the powder with a diameter about 5 μm. The PS blends were made by blending prepared catalyst into PS via Brabender mixer (Hapro-200C) at 50 rpm and 190 °C for 10 min, and then the mixture was shaped into the required sizes with a mass of 5−6 g. A piece of the above PS blend was placed into a quartz tube which was put in a resistance furnace. The carbonization process was performed at 800 °C ∼ 1050 °C for 10 min under the protection of nitrogen. After cooling to room temperature, the carbonaceous residue was collected as the as-prepared product. 2.3. Characterization. The yield (γ) of CNTs was calculated by the following formula: γ(100%) =

M r − Mc − Ma × 100% MPS

(1)

where Mr, Mc, Ma, and MPS represented the mass of residue, catalyst additives, and PS in the sample, respectively. Temperature-programmed reduction (TPR) was carried out using a Micromeritics Chemisorb 2705 instrument to measure the interaction of Mo, Mg, and Ni active phase. About 20 mg of catalyst sample was heated from 50 to 700 °C at 10 °C min−1 in a reduction atmosphere (10 vol % H2/90 vol % Ar). The hydrochloric acid (HCl) was used to remove most of the metallic catalysts in the product. The morphology of CNTs was observed by means of scanning electron microscope (SEM; FEI Quanta200, United States), transmission electron microscope (TEM; JEM-1011, operated at 100 kV voltage), and high-resolution TEM (HRTEM; JEM-3010, operated at 300 kV voltage). The purity and phase structure of the catalysts and the B

DOI: 10.1021/acs.iecr.7b02697 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. XRD diffraction patterns of the Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg catalysts. (a) Ni catalyst, (b) Ni−Mo (5:0.05), (c) Ni−Mo (5:0.1), (d) Ni−Mo (5:0.15), (e) Ni−Mg (5:1), and (f) Ni−Mo−Mg (5:0.1:1).

3. RESULTS AND DISCUSSION 3.1. Effects of Mo and Mg on the Morphology and Phase Structure of Final Ni-Based Catalysts. With the same standard process mentioned in the experimental part, all catalysts were put into the same perspective. Their phase structures were first examined by XRD measurements. Figure1a displayed the diffraction peaks at 37.4°, 43.2°, 62.7°, and 75.4°,

obtained CNTs were analyzed by X-ray powder diffraction (XRD) that was operated at 40 kV and 200 mA) via X-ray diffractometer with Cu Kα radiation. The thermal stability of CNTs was measured by PerkinElmer TGA 7 thermal analyzer. The vibrational properties of CNTs were analyzed by the Raman spectrum with an excitation beam wavelength at 532 nm. C

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Industrial & Engineering Chemistry Research indicative of the formation of NiO crystalline phase in the monometallic Ni catalyst. Then the Mo element was introduced with the Ni/Mo molar ratio of 5/0.05, 5/0.1, and 5/0.15, respectively. In their XRD patterns (Figure 1, panels b, c, and d), except for NiO reflections, some new diffraction peaks representing Ni/Mo alloy (NiMoO4) appeared. With the increase of the Mo content in Ni−Mo catalysts, the diffraction peak intensity ratio of NiMoO4 to NiO became larger. It is wellknown that MoO3 crystal can be easily formed under this condition in the absence of other metal elements.43 Thus, it implies that all the added Mo elements have reacted with NiO by the formation of NiMoO4 phase during calcinations. For the Mg element, it was incorporated into NiO catalysts with the Ni/Mg molar ratio of 5/0.5, 5/1, and 5/1.5. However, the XRD patterns of them exhibited exactly the same diffraction peaks with NiO catalysts, as shown in Figure 1e. Two possible reasons can explain this phenomenon: one is the formation of MgNiO2 alloy that has the same lattice with that of crystalline NiO; another is the formation of amorphous MgO. Nevertheless, it is totally different from some reported MgO supporters that exhibited perfect crystalline MgO structure.35,36,44 By the combination of Mo and Mg with Ni, a kind of Ni−Mo−Mg catalysts with molar ratio of 5:0.1:1 was synthesized. Its XRD pattern in Figure 1f showed the characteristic reflections of NiO and NiMoO4. Compared with the Ni−Mo (5:0.1) catalyst, the introduction of Mg obviously decreased the intensity ratio of NiMoO4 to NiO in Ni−Mo−Mg pattern. It can be reasonably explained by the formation of MgNiO2 crystals, which has the same diffraction peaks with the NiO crystalline phase. TEM was employed to directly observe the morphology and phase structure of the above Ni, Ni−Mo (5:0.1), Ni−Mg (5:1), and Ni−Mo−Mg (5:0.1:1) catalysts (Figure 2). The TEM image of Ni catalysts displays some very dense and grainlike structures with irregular shape and diameters in the range of 90−110 nm (Figure 2a). The D-spacing values of 0.24 nm corresponding to the (111) plane of NiO confirmed the formation of crystalline NiO structure (Figure 2b). A typical TEM image of Ni−Mo catalyst was shown in Figure 2c. It was quite intriguing to find that the catalyst particle became very small (10−30 nm). We attributed it to the generation of NiMoO4, which can effectively enhance the thermal stability of NiO and results in the formation of very smaller catalyst particles. HRTEM image in Figure 2d demonstrated the coexisting of crystalline NiMoO4 and NiO in one catalyst particle. In previous reports associated with the MgOsupported and Mo-containing bimetallic catalysts, the NiMoO4 phase was difficult to detect owing to the presence of large amounts of MgO crystals.41 For the Ni−Mg catalyst, its TEM images were displayed in Figure 2 (panels e and f). Different from Ni−Mo catalyst, the Ni−Mg had similar particle sizes with that of Ni catalyst. The HRTEM image in Figure 2f further revealed that the amorphous MgO was present and located at the surface of NiO instead of the mutual fusing form like Ni−Mo particles. Meanwhile, it indicates that the formed MgNiO2 phase should be localized at the contact region of NiO and MgO. As for the Ni−Mo−Mg catalyst, some combined features of both Ni−Mo and Ni−Mg catalysts were observed in Figure 2 (panels g and h), namely having very small-size catalyst particles that consist of NiMoO4, NiO, and amorphous MgO coatings. On the basis of the above results, it was found that the added Mo contributed mainly to the generation of small-sized catalyst particles, where the crystalline NiMoO4 was

Figure 2. TEM images of the Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg catalysts. (a and b) Ni catalyst, (c and d) Ni−Mo catalyst, (e and f) Ni−Mg catalyst, and (g and h) Ni−Mo−Mg catalyst.

clearly detected and as primary evidence for the small-size catalyst formation. The MgO present on the surface of NiO in amorphous state had no influence on the catalyst particle sizes. Meanwhile, it revealed that the detected MgNiO2 phase should be localized between NiO and MgO phases. TPR was used to further study the action of Mo and Mg on the Ni active phase (Figure 3). The Ni catalyst showed a pronounced reduction peak at about 300 °C, which was attributed to the reduction of NiO crystals. The Ni−Mo (5:0.1) also had a pronounced reduction peak in the TPR profile and appeared at a relatively higher temperature of 437 °C. It further indicated that there was a homogeneous oxide structure in Ni− Mo catalysts and a strong interaction between Mo and Ni with the formation of NiMoO4. For the Ni−Mg (5:1), we observed a very wide H2 consumption domain. Consistent with some previous reports,45,46 the reduction of Ni2+ normally took place at a large range according to the location of NiO in the outer or inner layer of MgO particles. The results implied the presence D

DOI: 10.1021/acs.iecr.7b02697 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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carbon spheres reveals that the carbon solubility of Ni catalysts is too high to catalyze the formation of tubular graphite structure from PS. In the case of Ni−Mo(5:0.1), a slight decrease of carbon yield (19%) was found. Figure 5 displayed

Figure 3. TPR profiles of Ni, Ni−Mo (5:0.1), Ni−Mg (5:1), and Ni− Mo−Mg (5:0.1:1).

of heterogeneous phase (NiO, NiMgO2, and MgO) in Ni−Mg catalysts, while revealing that the MgO had a strong interaction with the NiO phase. The Ni−Mo−Mg showed the combined character in the TPR profile, having a much higher reduction peak at round 510 °C with a very wide reduction domain. 3.2. Morphology and Microstructure of Carbonaceous Product Obtained from One-Step Conversion of Virgin PS by Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg Catalysts. Among polyolefin plastics, the effective conversion of PS into CNTs was considered to be the most difficult owing to the formation of large amounts of aromatic hydrocarbons with the property of high thermal stability. Up to now, very limited works related to PS carbonization have been reported.23 Here the virgin PS was selected as model plastic to investigate the catalytic performance of the above catalysts by one-step pyrolyzation of PS at 1000 °C with the addition of 5 wt % catalysts. For the Ni catalysts, 21 wt % of carbon yield from PS was obtained and the microstructure of products was examined by SEM and TEM. In Figure4 (panels a and b), a kind of carbon spheres with the diameter of 500−1000 nm were found to be the main constituent of the final product. In addition, many of carbon spheres had merged together and formed a chainlike structure in Figure 4 (panels c and d). The absence of any CNT structures indicates that the Ni catalyst is not matched with the PS degraded products. The formation of

Figure 5. SEM and TEM images of carbon products obtained by the Ni−Mo (5/0.1) catalysts. (a and b) SEM images and (c and d) TEM images.

the microstructure of carbon products. It was observed that the product mainly consisted of the carbon spheres with diameter of 300−500 nm, in which a low amount of irregular carbon fibers was dispersed (Figure 5, panels a and b). Figure 5 (panels c and d) confirmed the formation of carbon spheres and the CNTs with very narrow inner diameters. It implies that the Ni−Mo catalysts are still not matched with PS carbon source, where most of Ni−Mo particles induced the generation of carbon spheres, and a few of the Ni−Mo particles containing high concentration of NiMoO4 phase led to the growth of thick CNTs. The Mo/MgO catalysts have been found to produce thick CNTs in a previous report.35 For the Ni−Mg catalysts, an obviously increased carbon yield of 35% was achieved. The morphology and microstructure of carbon products were showed in Figure 6. Interestingly, numerous fiberlike carbon structures appeared with the diameter at about 40−50 nm in Figure 6 (panels a and b). Typical TEM images (Figure 6, panels c and d) further confirmed the formation of CNT structures. It demonstrates that the presence of small amounts of amorphous MgO can impose proper carbon solubility in Ni−Mg catalysts for the growth of CNTs from PS. In Shen’s work,26 it was demonstrated that the MgO could effectively enhance the thermal stability and catalytic lifetime of Ni active phase. In this work, we further revealed that the presence of interaction between NiO and MgO, namely the formation of MgNiO2, played an important role for the control of carbon solubility. In the one-step plastic carbonizations, the particle size of catalysts basically has a striking influence on the formation of CNTs according to previous reports.29 The catalysts with relatively small particle diameters are considered to be the most suitable for the growth of long and straight CNTs. Then on the basis of Ni−Mo and Ni−Mg catalysts, the Ni−Mo−Mg catalyst with molar ratio of 5:0.1:1 was studied. As expected, the Ni− Mo−Mg catalyst got the highest product yield of 42 wt % at 5 wt % loadings. The carbon product mainly consisted of

Figure 4. SEM and TEM images of carbon products from PS catalyzed by the Ni catalysts. (a and b) SEM images and (c and d) TEM images. E

DOI: 10.1021/acs.iecr.7b02697 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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positive effect of catalyst addition is attributed to the one-step carbonized character, in which the higher concentration of catalysts in plastic can deposit more amounts of degraded fractions. A similar trend was found for the Ni−Mo catalysts (Figure 8b). Among three Ni−Mo catalysts, the molar ratio of 5:0.1 had relatively better catalytic performance, where the 5 wt % of catalysts gave about 19% of carbon yield. However, the highest yield value by Ni and Ni−Mo catalysts are still very low and appear at relatively low catalyst content, meaning that the too high carbon solubility in both catalysts made the catalysts with short catalytic lifetime owing to the coating of amorphous carbon. Comparatively, the Ni−Mg catalysts had different yield curves in Figure 8c. It always has a much higher product yield than that of Ni or Ni−Mo catalysts at every catalyst loading point. Meanwhile, the highest yield is about 36%, corresponding to the 7 wt % of Ni−Mg (5:1). The Ni−Mo−Mg with the molar ratio of 5:0.1:1 was investigated in Figure 8d. It had a similar trend with Ni−Mg catalysts; however, the product yield was further improved. The carbon yield as high as 46% was achieved at 7 wt % loading of catalyst. In addition, with the 7 wt % catalyst addition, the effect of Mo and Mg content in Ni− Mo−Mg catalysts on the carbon yield was further investigated (Figure S1). Similar to the Ni−Mo and Ni−Mg catalysts, both the Mo addition at the molar ratio of Ni/Mo of 5/0.1 and the Mg introduction at the molar of Ni/Mg of 5/1 showed the best catalytic performance (46%), when the molar ratio of the other two elements in Ni−Mo−Mg catalysts is fixed. The results further revealed that both Mo and Mg elements played important and different roles, in which the Mo effectively reduced the particle size of the Ni active phase and the Mg modified the Ni carbon solubility and increased its catalyst lifetime. Thus, both appropriate additions imposed the best catalytic performance of Ni−Mo−Mg catalysts. 3.4. XRD Examination of As-Prepared Products. The as-prepared carbon products without purification were examined by XRD to analyze the phase change of catalysts and thus further address the effect of Mo and Mg in the Ni− Mo−Mg catalysts. In Figure 9a, it was clearly observed that the NiO in Ni catalyst had been completely transformed into the metal Ni after carbonization. For the products of Ni−Mo catalysts (Figure 9b), all NiO phase disappeared. Differently, the NiMoO4 characteristic peaks were still detectable. The above observations indicate that the Ni catalysts have very high carbon solubility; the addition of Mo cannot change the carbon solubility but make the Ni−Mo catalysts exist in small size particles with the formation of the NiMoO4 phase. Subsequently, we examined the carbon product of Ni−Mg catalysts. Interestingly, the diffraction peaks of NiO are still detectable in Figure 9c. It has been demonstrated that the NiO is very easily reduced into metal Ni, thus these diffraction peaks should be assigned to the MgNiO2 phase. It confirmed our assumption that the MgNiO2 was formed indeed and functioned as a joint between amorphous MgO and NiO. It reveals that the formation of MgNiO2 are very crucial for the carbon solubility in catalysts and the final growth of CNT structures from PS. In some previous studies about the MgOsupported metal catalysts,41,45 the promoted action of MgO was just demonstrated by some indirect evidence. The results here provide a direct evidence for the interaction between MgO and the active metal phase. Finally, the products based on the Ni−Mo−Mg catalyst were recorded by XRD. Besides metal Ni diffraction peaks, both NiMoO4 and MgNiO2 diffraction peaks were all observed. The results further demonstrated the effects

Figure 6. SEM and TEM images of carbon products obtained by the Ni−Mg (5:1) catalysts. (a and b) SEM images and (c and d) TEM images.

uniform, long and smooth CNTs, as shown by the SEM and TEM images in Figure 7. Compared to the CNTs obtained by

Figure 7. SEM and TEM images of carbon products obtained from PS by the Ni−Mo−Mg catalysts. (a and b) SEM images and (c and d) TEM images.

Ni−Mg catalysts, these CNTs showed much smaller outer diameters (15−20 nm) and bigger inner hollow. From these results, it was concluded that the Ni−Mo−Mg catalyst possessed simultaneously the features of small particle size and proper carbon solubility, which resulted in the synthesis of CNTs with the highest yield and the highest quality in one-step PS carbonization. 3.3. Effect of the Content of Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg Catalysts on the Product Yield in the OneStep Conversion of PS into CNTs. Figure 8 shows the influence of the content of catalysts (Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg) on the carbon yield in one-step PS carbonization. For the Ni catalyst (Figure 8a), increasing catalyst content led to a synchronous yield enhancement until the 5 wt % of addition with the carbon yield of 21%, after which the yield stopped increasing and even showed a slight reduction. It was consistent with the results in some previous works.28,36 The F

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Figure 8. Effect of the amount of Ni, Ni−Mo, Ni−Mg, and Ni−Mo−Mg catalysts on the yield of carbon products from PS. (a) Ni, (b) Ni−Mo, (c) Ni−Mg, and (d) Ni−Mo−Mg.

12), thus it would have more wide applications, such as electrochemical devices, hydrogen storage, and field emission devices. 3.6. Discussions. Over the past few decades, considerable effort has been devoted to study the one-step carbonization of PP or PE for the mass production of CNTs. It has been found that the PP and PE can be highly converted into CNTs by monometallic Ni catalysts in the presence of a proper synergistic additive. Much work has been reported to address the function of these additives during the carbonization of plastic by Ni catalysts.28,29 A general accepted explanation is that these additives can influence the degradation process of plastic and make the decomposed products more suitable to be transformed into CNTs. For example, in the PP/Ni2O3/solid acid system, Tang and his colleagues discovered that the added solid acids render PP degrade in a catalytic way and form greater amounts of light hydrocarbons and hydrogen.29 Similar phenomena were revealed in the cases containing the halidecontaining species and carbon black additives.23,32 By examining these additives in the one-step PS carbonization with Ni−Mo−Mg catalysts, it was found that these additives did not show positive effect on the yield of CNTs from PS (Table 1). It revealed that the function of all these additives just alter the degradation component composition of plastic to have proper carbon solubility in catalysts without the effect on the catalytic performance of catalysts. Apparently, these additives had no positive effect on the carbon solubility of PS-degraded products in Ni−Mo−Mg catalyst in this work. Another question was whether the carbonization temperatures and the Ni−Mo−Mg amounts in PS had important effects on the yield and thermal stability of prepared CNTs in the one-step PS carbonization. Similar to the effect of catalyst

of Mo and Mg in Ni−Mo−Mg catalysts on the one-step conversion of PS into CNTs. 3.5. HRTEM, Raman, and TG Characterization of the Pure CNTs Obtained by Ni−Mo−Mg Catalysts. For effective evaluation of the quality of obtained CNTs by Ni− Mo−Mg catalysts, HRTEM, Raman, and TG techniques were applied. The HRTEM images in Figure 10 displayed clear and continuous graphitic layers in the wall of CNTs and 0.34 nm of interlayer spacing. It directly demonstrated that the CNTs had a high-level of quality in terms of lattice integrity. The Raman spectrum was recorded for further examining the microstructure of the prepared CNTs. In a typical CNT Raman spectrum, D-band at 1350 cm−1 and G-band 1580 cm−1 represent stretching vibration for the defect of C atom lattice and the C atom sp2 hybrid in-plane, respectively. In Figure 11, the intensity ratio of G-band to D-band is approximately 1.3, meaning that the CNTs obtained by Ni−Mo−Mg are predominantly made up of fine graphite carbon structures. The TG testing was performed to measure the thermal stability of CNTs in air. In Figure 12, it was found that the CNTs only lost negligible weight at the temperature below 550 °C. It indicates that the prepared CNTs are free from unstable carbon structure. We attribute it to the high carbonization temperature (1000 °C) and the fine graphite structures. Clearly, the CNTs degraded dramatically at the temperature 600 °C and lost most of the weight at 600−650 °C. It implies that the purified products are made up of relatively pure and uniform CNT structures. The CNT materials from pure and waste plastic has been used as reinforced materials,47 adsorbents,48 catalyst supports,49 and energy storage materials.24 In this work, the prepared CNTs from PS showed very high graphitization degree and thermal stability (Figure 10, Figure 11, and Figure G

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Figure 9. XRD patterns of carbon products without purification. (a) Ni, (b) Ni−Mo (5:0.1), (c) Ni−Mg (5:1), and (d) Ni−Mo−Mg (5:0.1:1).

Figure 10. HRTEM images of CNTs obtained by Ni−Mo−Mg catalysts. (a) CNT tip and (b) CNT body.

Figure 12. TGA curves of the CNTs obtained from PS by the Ni− Mo−Mg catalysts.

content in Figure 8d, the carbonization temperature also had a remarkable influence on the yield of CNTs. In Figure 13, the yield increased dramatically with increasing carbonization temperature from 800 to 1000 °C. Beyond 1000 °C, the yield of CNTs was not further increased. Table S1 provided the temperatures of maximum degradation rate of CNTs obtained at different carbonization temperatures and different Ni−Mo− Mg catalyst loadings. It showed that the higher CNT thermal stability was achieved at the 1000 °C of carbonization and the catalyst loadings at 3 wt % 5 and 7 wt % of Ni−Mo−Mg. From these observations, it demonstrated that the 1000 °C and the 7

Figure 11. Raman spectrum of the CNTs obtained from PS by the Ni−Mo−Mg catalysts.

H

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still mainly result in the generation of carbon sphere structures with small amounts of thick CNTs. Differently, the introduced Mg just locates on the surface of Ni and shows no effect on the size of catalysts, but it can lead to the formation of CNTs. A well-known fact in CVD CNT synthesis is that the MgO supports can highly influence the catalytic action of metal active phase.50,51 Some explanations were given, such as the reduction of diameters of metal active phase, the increase of the surface area of catalysts and the improvement of catalyst stability, due to the metal−support-interaction. In the present work, the MgO had almost lost the support function owing to the very small addition, and the catalyst particle sizes of Ni−Mg were not reduced. The formation of CNTs should be attributed to the significant interaction with the NiO active phase by the generation of NiMgO2. Small amounts of NiMgO2 can effectively control the carbon solubility of Ni catalysts and finally leads to the formation of CNTs, as shown in Figure 14. When both Mo and Mg are incorporated, the Ni−Mo−Mg catalysts possess the features of Ni−Mo and Ni−Mg (i.e., small particle sizes and proper carbon solubility). Thus, the smallsized and high-quality CNTs are successfully achieved in large amount.

Table 1. Effect of Some Additives on the Yield of CNTs from PS Catalyzed by Ni−Mo−Mg Catalysts

a

exp.

PS (wt %)

Ni−Mo−Mg (wt %)

additivesa

γ (%)

1 2 3 4 5 6 7

88 88 88 88 88 88 93

7 7 7 7 7 7 7

OMMT ZSM-5 NH4Cl CuCl2 active carbon carbon black none

36 38.5 29.4 26.3 39 39.6 46

The content of additives: 5 wt %.

4. CONCLUSIONS In summary, the Ni−Mo−Mg catalysts have been demonstrated for the efficient conversion of PS into high-quality CNTs by one-step carbonization without the need of any degradation additives. The function of Mo and Mg in Ni−Mo− Mg catalysts was clearly addressed: the Mo addition helped to form very small-sized catalyst particles with the formation of NiMoO4, while the formation of carbon spheres from Ni−Mo catalysts revealed its weak effect on the carbon solubility of Ni catalysts. The introduction of Mg was discovered having no influence on the catalyst particle size, but it could effectively decline the carbon solubility and led to the growth of CNTs from PS by the formation to NiMgO2. As such, the combined addition of Mo and Mg imposed the Ni active phase with small particle sizes and proper carbon solubility in the one-step PS carbonization, and consequently, 46% of CNT yield was successfully achieved. In addition, with the experimental demonstration and theoretical analyses, it revealed that the one-step PS carbonization did not need any degradation additives, and the 1000 °C was the optimal carbonization temperature. The work was believed to have an important contribution to the further development of one-step plastic carbonization and make it achieve the conversion of PS-based waste into CNTs. In addition, owing to the distinct catalytic carbonization features, the obtained results should be helpful to address the function of Mo promoter and MgO supporter in other CNT synthesis systems.

Figure 13. Influence of the carbonization temperature on the yield of CNTs from PS with 7 wt % of Ni−Mo−Mg catalysts.

wt % of Ni−Mo−Mg were the optimal carbonization conditions for the CNT preparation from PS by Ni−Mo−Mg catalysts. Nevertheless, the introduction of Mo and Mg imposed the best catalytic performance of Ni catalysts. The influence of Mo and Mg on the catalytic performance of Ni active phase was further described in Figure 14. The Ni catalysts formed under

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Figure 14. Schematic illustrations for the Mo and Mg effects on the Ni active phase in catalyst preparation and one-step carbonization of PS.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02697. Effects of the Mo and Mg content in Ni−Mo−Mg on carbon yield with fixed Ni/Mo and Ni/Mg molar ratio at 7:0.1 and 7:1 (Figure S1) and effects of carbonization temperatures and Ni−Mo−Mg content on the thermal stability of CNTs (Table S1) (PDF)

this condition have relatively large particle sizes, which only enable the formation of large carbon spheres from PS owing too high carbon solubility. With the presence of Mo, a kind of Ni/Mo fused structure with very small particle sizes can be achieved. However, owing to the limited influence on the carbon solubility of PS degraded product, the Ni−Mo catalysts I

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 (0) 45182190679. Fax: +86 (0) 451 82191753. Email: [email protected]. *E-mail: [email protected]. ORCID

Rongjun Song: 0000-0001-8184-2004 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work has been supported by the Natural Science Foundation of Heilongjiang Province of China (B201401), the Postdoctoral Scientific Development fund of Heilongjiang Province (LBH-Q15008), and a General Financial Grant from the China Postdoctoral Science Fundation (2016M600240).

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