Fabrication of Carbon Nanotubes Derived from Waste Tire Pyrolytic Carbon and Their Application in the Dehydrogenation of Methylcyclohexane to Produce Hydrogen

: The accumulation of waste tires has resulted in very urgent environmental problems. Pyrolysis has been regarded as a green eco-friendly technology to deal with waste tires, and it is vital to make use of the pyrolysis carbon. Herein, we propose a new way to utilize pyrolysis carbon, to prepare carbon nanotubes with the help of ferrocene. The optimal preparation processes were determined by optimizing the parameters including the solvent, temperature, time, etc. The results of scanning electron microscopy and transmission electron microscopy evidenced the successful formation of carbon nanotubes. Meanwhile, the Brunauer–Emmett–Teller (BET) method and N 2 -adsorption showed that the yielded carbon nanotubes featured a large surface area and abundant pore structure in comparison with the pyrolytic carbon. Finally, the as-prepared carbon nanotubes were applied as the supports for Pt-based catalysts for the dehydrogenation of methylcyclohexane to produce hydrogen. The results showed that the Pt/carbon-nanotubes catalyst exhibited the highest conversion of methylcyclohexane (28.6%), stability, and hydrogen evolution rate (336.9 mmol/g Pt /min) compared to the resulting Pt/commercial-activated-carbon (13.6% and 160.2 mmol/g Pt /min) and Pt/pyrolytic-carbon catalysts (0.19% and 2.23 mmol/g Pt /min).


Introduction
As people's living standards improve and the automobile industry develops, the annual accumulation of discarded tires has become a very serious threat to the environment due to their non-biodegradability [1][2][3].The amount of waste tires reached 200 million tons by 2020 and has further increased by 6 to 8% per year in China [4].This increasingly serious problem requires the technological use of abandoned tires.Compared to direct landfill and incineration, pyrolysis of the discarded tires has been considered the most promising and efficient method to solve the problem due to the advantages of pursuing the reduction in quantity, the high-value utilization of the recycled products, and especially the friendliness to the environment [5,6].During the pyrolysis process, the waste tire rubber is decomposed into fuel gas, pyrolysis oil, and carbon black under the absence of oxygen or hypoxia, and the fuel gas can be burned to supplement the heat required for the pyrolysis process; meanwhile, the pyrolytic oil and carbon black can be used as fuel oil and commercial carbon black, respectively, after simple purification treatment [7].
C 2023, 9 As one of the main pyrolysis products, the utilization of pyrolysis carbon black plays a significant role in dealing with the waste tires.A number of researchers have focused on and discussed the formation of activated carbon derived from pyrolysis carbon black due to the similar composition and structure to industrial carbon black [8,9], which has been successfully applied in adsorption [10,11], electrochemical [12,13], catalysis [14][15][16], and other fields [17].However, few studies have reported the production of carbon nanotubes (CNTs) originating from pyrolysis carbon black.Liquid organic hydrides (boiling points: 80-200 • C) have emerged as a promising hydrogen carrier owing to their easy achievement of hydrogenation and dehydrogenation, large hydrogen storage capacity (6-8%), high security during transportation, etc. [18][19][20][21] Methylcyclohexane (MCH) has been proposed as a potential and satisfactory candidate for liquid organic hydrides for its high reversibility and the low toxicity of the dehydrogenation product [22,23], which accelerates the dehydrogenation reaction process in the presence of Pt-based catalysts over appropriate supports [24].It is vital for the supports to improve the activity and stability of the Pt-based catalyst [25][26][27][28][29][30], and carbon materials have been widely applied to promote the dispersion of Pt atoms, due to their large surface area and abundant oxygen-containing groups [31], including activated carbons [32], carbon filters [33], carbon nanotubes [34], and so on.Kong et al. considered that the features of CNTs rendered them a promising catalyst support, with their excellent thermal/electrical conductivity, high specific surface areas, and favorable adsorption of hydrogen.Wang et al. [35] reported that 0.25% Pt/CNTs exhibited consistent catalytic activity to 1% Pt/Al 2 O 3 .However, there was less reported on CNTs prepared from pyrolysis carbon black derived from waste tires and applied as the dehydrogenation support for MCH for producing hydrogen, which provides a new path to deal with waste tires in an eco-friendly manner.
Herein, we propose a new way to deal with the waste tire pyrolysis carbon, and we developed the preparation of CNTs by pyrolysis carbon black derived from waste tires.The synthesized parameters of the solvent, temperature, and reaction time were investigated to yield the excellent performance of CNTs.The characterization of scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray diffraction (XRD), N 2 -adsorption, Raman spectra, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were utilized to reveal that the as-prepared CNTs had the distinct tubular structure, large surface area, abundant pore structure, high graphitization degree, and major oxygen-containing functional groups in compassion of pyrolysis carbon black.Finally, the satisfactory CNTs were used as the support for Pt-based catalysts and successfully applied to catalyze the dehydrogenation of methylcyclohexane to produce hydrogen.

Instrumentations
SEM and TEM images were performed on a JEOL JSM-7500F (JEOL, Tokyo, Japan) and a FEI Tecnai G2 F20 (FEI, Eindhoven, The Netherlands), respectively.A microspores instrument ASAP 2020 (Micromeritics, Atlanta, GA, USA) and a Bruker D8 FOCUS diffractometer (Bruker, Karlsruhe, Germany) were used to analyze the pore structure and XRD patterns of the materials.The FTIR spectra and XPS profiles were measured by a Vector C 2023, 9, 121 3 of 11 22 Fourier-transform infrared spectrophotometer (Bruker, Fällanden, Switzerland) and a Kratos AXIS-ULTRA DLD multifunctional photoelectron spectrometer (Shimadzu, Kyoto, Japan), respectively.All the XPS peaks were calibrated with the C1s peak binding energy of 284.5 eV.The inductively coupled plasma atomic emission spectrometry (ICP-AES, ICP-9000 (N + M), Shimadzu, Kyoto, Japan) was utilized to calculate the load capacity of Pt.The TG and DTG curves were measure by a Thermo Plus EVO thermogravimetric differential thermal analyzer (TG-DTA, Rigaku, Tokyo, Japan) at a heating rate of 10 • C/min under nitrogen gas.All measurements were taken at room temperature.

Preparation of CNTs
The pyrolysis carbon derived from waste tires was extracted in toluene to remove the organic impurities and washed in dilute sulfuric acid to eliminate the inorganic ash (e.g., ZnS), which was marked as C. CNTs were synthesized by using the purified C as the carbon source and ferrocene as the catalyst with a mass ratio of 1:10 in dimethylsulfoxide solvent in a muffle furnace at 700~850 • C for 4~10 h.

Preparation of Pt/CNTs Catalyst
The Pt/CNTs catalyst was prepared as follows.First, 2.0 g of CNTs were uniformly dispersed into 20 mL of deionized water, and then a certain volume of chloroplatinic acid solution was slowly dropped into the suspension with a final Pt load amount of 0.2% and continuously stirred.After 30 min, 20 mL of ethylene glycol solution was sequentially added into the reaction system.The precursor reacted for 4 h, and the solid products (i.e., Pt/CNTs) were filtered, washed, and dried in a vacuum oven for 12 h at 55 • C.Meanwhile, C was used to synthesize the Pt/C catalyst as a contrast using the same operation.The Pt/CNTs and Pt/C catalysts were reduced by H 2 at 400 • C for 2 h and further applied to catalyze the dehydrogenation reaction of MCH.

The Dispersion of the Active Component Pt
The CO pulse technique was used to test the dispersion of the active component Pt.About 0.100 g of carbon materials was reduced by H 2 at 400 • C for 40 min, cooled to 25 • C under helium protection; then, the adsorption capacity of CO was tested.

Catalytic Reaction
The dehydrogenation reaction of methylcyclohexane to produce hydrogen was performed in a fixed-bed flow reactor (10 mm inner diameter (ID) × 250 mm) with the Pt-based catalyst weight of 0.30 g and the MCH flow rate of 0.03 mL/min.A Rock GC 7800 gas chromatograph and an SE-30 column were used to determine the conversion of MCH.

Preparation and Characterizations of CNTs
The main chemical components of waste tire pyrolysis carbon black were carbon, oxygen, copper, zinc, etc., and its ash and fine powder content were relatively high, which could cause carbon deposition and lead to catalyst deactivation.Hence, it was significant to remove the ash before preparing carbon nanotubes.Meanwhile, pickling could not only effectively remove the ash on the surface of the pyrolytic carbon black but also increase the specific surface area and surface functional groups [16].The preparation parameters were investigated to obtain the excellent properties, including the solvent, synthetic temperature, time, etc.In the SEM photos, the products without any solvent (Figure 1A) and those with the solvent of ultrapure water (Figures 1B and S1) were composed of angular and different size blocks, and a large number of amorphous spherical particles with a uniform size appeared; yet, no CNTs were observed.However, as displayed in Figure 1C, a large number of CNTs were discovered with the addition of DMSO, which might mean that the presence of DMSO was conducive to the sufficient and effective dispersal of the reactant, and CNTs were formed under the catalytic action of ferrocene.Meanwhile, as shown in Figure 2, a small number of CNTs were formed at 700 • C, which was consistent with the report [36], the number continuously grew with the temperature increase to 750 • C and 800 • C, producing more energy and accelerating the catalytic reaction.However, some of the small carbon particles disappeared and became larger ones, which could possibly influence the surface area and pore structure; this was possibly caused by the excessive temperature.As depicted in Figure 3, the production and shape of CNTs at 6 h were better (such as in yield, length, shape, etc.) than those of CNTs at the other times with the field of view of 1 µm or 100 nm.Therefore, the CNTs were prepared at 750 • C or 800 • C for 6 h in DMSO in this work.TEM was utilized to study the morphology of the as-prepared CNTs.As displayed in Figure 4, the structures of the tube walls and inner cavity were clear and complete.The thickness of the tube wall was in the range of 2~8 nm; moreover, the internal diameter and the external diameter were between 6 and 11 nm and 11 and 28 nm, respectively, which was beneficial to the dispersion of Pt catalyst, the complete contact between the reactant and catalyst, and the rapid overflow of the hydrogen product.
uniform size appeared; yet, no CNTs were observed.However, as displayed in Figure 1C, a large number of CNTs were discovered with the addition of DMSO, which might mean that the presence of DMSO was conducive to the sufficient and effective dispersal of the reactant, and CNTs were formed under the catalytic action of ferrocene.Meanwhile, as shown in Figure 2, a small number of CNTs were formed at 700 °C, which was consistent with the report [36], the number continuously grew with the temperature increase to 750 °C and 800 °C, producing more energy and accelerating the catalytic reaction.However, some of the small carbon particles disappeared and became larger ones, which could possibly influence the surface area and pore structure; this was possibly caused by the excessive temperature.As depicted in Figure 3, the production and shape of CNTs at 6 h were better (such as in yield, length, shape, etc.) than those of CNTs at the other times with the field of view of 1 µm or 100 nm.Therefore, the CNTs were prepared at 750 °C or 800 °C for 6 h in DMSO in this work.TEM was utilized to study the morphology of the as-prepared CNTs.As displayed in Figure 4, the structures of the tube walls and inner cavity were clear and complete.The thickness of the tube wall was in the range of 2~8 nm; moreover, the internal diameter and the external diameter were between 6 and 11 nm and 11 and 28 nm, respectively, which was beneficial to the dispersion of Pt catalyst, the complete contact between the reactant and catalyst, and the rapid overflow of the hydrogen product.The specific surface area and pore structure were significant factors for the evaluation of the properties of carbon materials.Compared to C, the structure of the CNTs was significantly changed.As listed in Table 1, the specific surface area of C was high at 64.6 m 2 /g, which increased to 138 m 2 /g for the CNTs.The same downtrends were observed in the micropore area and pore volume; however, the average pore width decreased from C to CNTs, which may be caused by the formation of tubular structures.The large specific surface area and pore volume could promote the dispersion of Pt atoms to improve the catalytic performance.The specific surface area and pore structure were significant factors for the evaluation of the properties of carbon materials.Compared to C, the structure of the CNTs was significantly changed.As listed in Table 1, the specific surface area of C was high at 64.6 m 2 /g, which increased to 138 m 2 /g for the CNTs.The same downtrends were observed in the micropore area and pore volume; however, the average pore width decreased from C to CNTs, which may be caused by the formation of tubular structures.The large specific surface area and pore volume could promote the dispersion of Pt atoms to improve the catalytic performance.Figure 5 shows the structural characterizations of C and the as-prepared CNTs.As shown in Figure 5A, the diffraction peaks at 24° and 44° in the XRD patterns were derived from the (110) crystal face and (002) crystal face of graphite microcrystalline (PDF# 041-1487), respectively, which evidenced the presence of the crystal structure in the CNTs and C. Compared to C, the two signals of the CNTs were strengthened, which indicated the formation of CNTs.As observed in Figure 5B, the adsorption isotherms of the CNTs and C attached to IV type hysteresis loops, which implied the presence of mesopores.The pore diameter distributions curves suggested that the pore structures of the supports mainly consisted of the mesopores, and the most available apertures were 3~4 nm (Figure 5C), while the pore volume increased from C to the CNTs, manifesting the pore structure was further improved in comparison to that of C. The FTIR spectra (Figure 5D) confirmed that  Figure 5 shows the structural characterizations of C and the as-prepared CNTs.As shown in Figure 5A, the diffraction peaks at 24 • and 44 • in the XRD patterns were derived from the (110) crystal face and (002) crystal face of graphite microcrystalline (PDF# 041-1487), respectively, which evidenced the presence of the crystal structure in the CNTs and C. Compared to C, the two signals of the CNTs were strengthened, which indicated the formation of CNTs.As observed in Figure 5B, the adsorption isotherms of the CNTs and C attached to IV type hysteresis loops, which implied the presence of mesopores.The pore diameter distributions curves suggested that the pore structures of the supports mainly consisted of the mesopores, and the most available apertures were 3~4 nm (Figure 5C), while the pore volume increased from C to the CNTs, manifesting the pore structure was further improved in comparison to that of C. The FTIR spectra (Figure 5D) confirmed that the surface of the CNTs and C possessed abundant and identical oxygen-containing functional groups.The absorption peak at 1630 cm −1 was assigned to the stretching vibration of the C=O bond [37].The region of 3200~3600 cm −1 was due to the stretching vibration of the O-H bond [38].The peak at 1118 cm −1 was attributed to the asymmetric stretching vibration of the -O-C=O bond [39].Raman spectra revealed the graphitization C 2023, 9, 121 6 of 11 degree of the carbon materials.As shown in Figure 5E, the peaks at 1353 cm −1 (D band) and 1584 cm −1 (G band) in the Raman spectrum of the CNTs were attributed to the amorphous carbon (sp 3 ) and the crystal carbon (sp 2 ), respectively [40], which were also found in the Raman spectrum of C (i.e., 1341 cm −1 and 1591 cm −1 ).Moreover, the ratio value of I D /I G was 0.817 for the CNTs and a little less than that of C (0.943), which indicated that the graphitization degree was improved after C was produced into CNTs [41].As displayed in Figure 5F, showing the CNTs' TG and DTG profiles, the peak at 106 • C resulted from the loss of moisture, and the peaks at 250 • C and 420~630 • C were ascribed to the decomposition of the carboxyl group and carboxyl anhydrides, respectively [42-44], which was in agreement with the results of the FTIR.
C 2023, 9, x FOR PEER REVIEW 8 of 14 degree was improved after C was produced into CNTs [41].As displayed in Figure 5F, showing the CNTs' TG and DTG profiles, the peak at 106 °C resulted from the loss of moisture, and the peaks at 250 °C and 420~630 °C were ascribed to the decomposition of the carboxyl group and carboxyl anhydrides, respectively [42-44], which was in agreement with the results of the FTIR.The XPS spectra further verified the surface functional groups of the as-prepared CNTs.The survey spectrum indicated that the surface of CNTs was comprised of C and O elements (Figure 6A).And the C 1s spectrum (Figure 6B) was constituted by the peaks of 284.6, 285.2, and 288.1 eV, which were assigned to the C-C, C-O, and C=O bonds, respectively [45,46].Meanwhile, the bonds of O-C (531.8 eV) and O=C (533.0 eV) [47] were observed in the O 1 s spectrum of the CNTs (Figure 6C), which was agreement with the results of the FTIR.The XPS spectra further verified the surface functional groups of the as-prepared CNTs.The survey spectrum indicated that the surface of CNTs was comprised of C and O elements (Figure 6A).And the C 1s spectrum (Figure 6B) was constituted by the peaks of 284.6, 285.2, and 288.1 eV, which were assigned to the C-C, C-O, and C=O bonds, respectively [45,46].Meanwhile, the bonds of O-C (531.8 eV) and O=C (533.0 eV) [47] were observed in the O 1 s spectrum of the CNTs (Figure 6C), which was agreement with the results of the FTIR.

Structure Characterizations of the Pt-Based Catalysts
As shown in Figure S2, the diffraction peaks of Pt were not discovered in the XRD patterns of the Pt/CNTs and Pt/C catalysts, which might result from the low loading amount of Pt or the effective dispersion of the Pt particles on the CNTs and C supports.Figure 7 depicts the TEM images of the Pt/C and Pt/CNTs, which exhibited that the Pt particles appeared well dispersed in the catalysts.The average sizes of the Pt/C and Pt/CNTs catalysts were calculated at about 8.71 nm and 7.01 nm, respectively.The dispersion degrees of the Pt-based catalysts were investigated by the CO pulse adsorption technique, which was listed in Table 2.The dispersion degrees and sizes of the active component Pt were 17.8% and 6.12 nm in the Pt/CNTs catalyst and 12.6% and 8.83 nm in the Pt/C catalyst, which was identical to the result of the TEM images, further indicating the successful preparation of the Pt-based catalysts.

Structure Characterizations of the Pt-Based Catalysts
As shown in Figure S2, the diffraction peaks of Pt were not discovered in the XRD patterns of the Pt/CNTs and Pt/C catalysts, which might result from the low loading amount of Pt or the effective dispersion of the Pt particles on the CNTs and C supports.Figure 7 depicts the TEM images of the Pt/C and Pt/CNTs, which exhibited that the Pt particles appeared well dispersed in the catalysts.The average sizes of the Pt/C and Pt/CNTs catalysts were calculated at about 8.71 nm and 7.01 nm, respectively.The dispersion degrees of the Pt-based catalysts were investigated by the CO pulse adsorption technique, which was listed in Table 2.The dispersion degrees and sizes of the active component Pt were 17.8% and 6.12 nm in the Pt/CNTs catalyst and 12.6% and 8.83 nm in the Pt/C catalyst, which was identical to the result of the TEM images, further indicating the successful preparation of the Pt-based catalysts.Figure 8 shows the conversion of MCH and the hydrogen evolution using Pt/CNTs, Pt/AC, and Pt/C as catalysts at the reaction temperature of 300 • C and with the space velocity of 5 h −1 .As displayed in Figure 8A, the conversion of MCH taking Pt/C as the catalyst was almost zero, which indicated that the carbon black derived from waste tires was not suitable to catalyze the dehydrogenation of MCH.Meanwhile, the conversion of MCH utilizing Pt/AC as the catalyst was 21.2% at 2 h and gradually decreased below 10% at 12 h.Meanwhile, the conversion of MCH applying Pt/CNTs as the catalyst was around 29.6% at 2 h and only slightly decreased to 27.6% at 12 h, which indicated that the Pt/CNTs exhibited much higher and more stable than those of Pt/C and Pt/AC, probably benefiting from the high BET surface and tubular structure to promote the diffusion of MCH, the dispersion of the Pt atoms, and the spilling of hydrogen, as well as the adsorption capacity of hydrogen of CNTs.According to the average conversions of MCH adopting Pt/CNTs (28.6%),Pt/AC (13.6%), and Pt/C (0.19%) as the catalysts, the hydrogen evolution rate of the Pt/CNTs was 336.9 mmol/g Pt /min, much higher than the 160.2 mmol/g Pt /min of the Pt/AC and the 2.23 mmol/g Pt /min of the Pt/C (Figure 8B), further indicating the good performance of the Pt/CNTs in the catalytic dehydrogenation of MCH.Table S1 [16,[48][49][50] lists the comparisons of the hydrogen production reported by the previous literature, which demonstrated the high performance of the resultant CNTs on the hydrogen production at a low Pt load.

Conclusions
Herein, we proposed a novel utilization of the waste tire pyrolytic carbon for prep ing the carbon nanotubes and investigated their application as the support for Pt-bas catalysts in the dehydrogenation of methylcyclohexane (MCH) to produce hydrog SEM and TEM confirmed the tubular structure and proved the successful production CNTs.The FTIR, N2-adsorption, TG profiles, and XPS showed that the yielded CNTs hibited a large surface area, abundant pore structure, and oxygen-containing groups.the dehydrogenation reaction of MCH, the Pt/CNTs catalyst displayed the highest co

Conclusions
Herein, we proposed a novel utilization of the waste tire pyrolytic carbon for preparing the carbon nanotubes and investigated their application as the support for Pt-based catalysts in the dehydrogenation of methylcyclohexane (MCH) to produce hydrogen.SEM and TEM confirmed the tubular structure and proved the successful production of CNTs.The FTIR, N 2 -adsorption, TG profiles, and XPS showed that the yielded CNTs exhibited a large surface C 2023, 9, 121 9 of 11 area, abundant pore structure, and oxygen-containing groups.In the dehydrogenation reaction of MCH, the Pt/CNTs catalyst displayed the highest conversion of MCH (28.6%) and the highest hydrogen evolution rate (336.9mmol/g Pt /min) over those of the Pt/AC (13.6%, 160.2 mmol/g Pt /min) and Pt/C (0.19%, 2.23 mmol/g Pt /min), which was attributed to the tubular structure promoting sufficient contact between the reactants and the catalyst and the rapid leakage of hydrogen.This work provided a new path to deal with waste tires with an eco-friendly and efficient hydrogen storage.Next, we will devote how to improve the yields of the CNTs and further evaluate their application in other fields such as drug delivery, sensors, and electronics.

Figure 1 .
Figure 1.Effect of the solvent on the preparation of CNTs at 800 °C: (A) without any solvent, (B) H2O, (C) DMSO.

Figure 1 .
Figure 1.Effect of the solvent on the preparation of CNTs at 800 • C: (A) without any solvent, (B) H 2 O, (C) DMSO.C 2023, 9, x FOR PEER REVIEW 5 of 14

Figure 4 .
Figure 4. TEM images (A, B, C, and D) of the as-prepared CNTs at 800 °C for 6 h in DMSO.

Figure 4 .
Figure 4. TEM images (A-D) of the as-prepared CNTs at 800 • C for 6 h in DMSO.

Figure 6 .
Figure 6.XPS spectra of the as-prepared CNTs: (A) survey spectrum, (B) C 1 s spectrum, and (C) O 1 s spectrum.

23, 9 ,Figure 8 .
Figure 8. (A) The conversion of methylcyclohexane dehydrogenated into toluene and (B) the hyd gen evolution based on Pt catalysts.

Figure 8 .
Figure 8. (A) The conversion of methylcyclohexane dehydrogenated into toluene and (B) the hydrogen evolution based on Pt catalysts.

Table 1 .
Surface structure information of carbon supports.

Table 1 .
Surface structure information of carbon supports.

Table 2 .
The CO pulse adsorption data of the Pt-based catalysts.
t-Based atalystsAdsorptionCapacity of CO (µL CO/g Catalyst) Surface Area of Metal (m 2 /g Sample) Dispersion of Active Component (%)

Table 2 .
The CO pulse adsorption data of the Pt-based catalysts.