Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX pubs.acs.org/EF ‑ Methane Pyrolysis for Carbon Nanotubes and COx Free H2 over Transition-Metal Catalysts † † I-Wen Wang, Deepa Ayillath Kutteri, Bingying Gao, Hanjing Tian, and Jianli Hu* Chemical & Biomedical Engineering Department, West Virginia University, Morgantown, West Virginia 26506, United States *S Supporting Information ABSTRACT: Recently, researchers at West Virginia University reported a promising catalyst innovation for nonoxidative thermochemical conversion of methane to CO2-free hydrogen and solid carbon nanotubes (CNTs). A catalyst system was discovered that promotes “base growth” CNT formation rather than conventional “tip growth”. This enables catalyst regenerability while also generating highly pure and crystalline carbon products. In this study, simultaneous productions of CNTs and CO2-free hydrogen were studied over Fe-based catalysts supported on Al2O3, SiO2, and H-ZSM-5. The experimental results showed that metal−support interaction played a key role in the base growth mechanism. Methane conversion and the property of CNTs depended significantly on metal loading and the type of support. To elucidate the formation mechanism of CNTs, the spent catalysts were characterized by a number of analytical instrumentations including transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and thermogravimetric analysis (TGA). The formation of the active intermediate phase, Fe3C, was discovered. The results of XPS analysis revealed that Fe/Al2O3 had a stronger interaction between metal particles and support than Fe/SiO2 and Fe/ZSM-5. The characterization result is consistent with the performance test results from the methane decomposition reaction. To further explore the fi application of the CNTs, separation and puri cation were carried out using spent Fe/Al2O3 and 9Ni-1Fe/SiO2 catalysts. The fl spent catalysts with CNTs were separated by re uxing in nitric acid. The purity of CNTs recovered from the Fe/Al2O3 catalyst can reach 96% based on TGA analysis. XRD and scanning electron microscopy−energy-dispersive system analyses (SEM- EDS) revealed that most metal particles and supports had been dissolved. In addition, the purified CNTs presented a stable homogeneous dispersion in isopropanol solution, implying the presence of functional groups on CNTs that interacted with the isopropanol solvent. KEYWORDS: methane decomposition, hydrogen, tip and base grown carbon nanotubes, purification 1. INTRODUCTION of high-quality carbon. However, these processes have − ° Because of the increase in population and economic develop- drawbacks like high temperature (1200 3000 C) or require vacuum or low pressure. Hence, it is very challenging to ment, there is a higher demand for energy from the fossil 1 feedstock, which additionally releases greenhouse gases like produce crystalline carbons (CNTs) without a catalyst. For carbon dioxide. Hence, it is a major challenge to find CNT synthesis, catalysts with nanometer-sized metal particles alternative clean energy resources.1,2 Hydrogen, because of are essential, which will also help to reduce the temperature for Downloaded via WEST VIRGINIA UNIV on January 2, 2019 at 22:26:44 (UTC). its availability and efficiency, has been considered as one of the methane decomposition. However, a comprehensive under- most promising forms of energy. Currently, many researchers standing of the whole catalytic chemical vapor deposition have focused on the development of environmental-friendly, (CCVD) process is still largely missing. The catalytic growth of See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. efficient, and economic processes for hydrogen production. CNTs requires fine metal particles, with a certain diameter, Among the various methods available, thermocatalytic which can act as catalysts at high reaction temperatures, decomposition of methane is an attractive way to produce typically 700−900 °C. To prepare such catalysts, support 9−11 clean hydrogen as it generates COx-free hydrogen with low materials with a high surface area have been selected. 3,4 energy consumption. In addition to COx-free hydrogen, Support facilitates the uniform distribution of metal nano- methane decomposition generates carbon in the form of particles and also controls the agglomeration of nanoparticles nanotubes and nanofibers (R1). at high temperatures. In the case of supported metal catalysts, there are two factors which have to be taken into account: (i) ↔+ CH4(CNT)2 C 2H (R1) the presence and the size of nanopores in the support and (ii) Carbon nanotubes (CNTs) or nanofibers are extensively the interaction between the metal particles and the support employed as adsorbents, catalyst supports, conductive material. Some background knowledge is essential before polymers, and so forth because of their unique physical, starting to investigate the peculiarities of the growth − chemical, mechanical, electrical, and optical properties.5 8 It mechanism of CNTs. The catalytic decomposition of carbon- has been reported that the structure and morphology of carbon (amorphous or crystalline) depend on the process conditions. Received: October 8, 2018 Various methods such as laser ablation and arc discharge Revised: December 13, 2018 chemical vapor deposition are well-known for the production Published: December 18, 2018 © XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.8b03502 Energy Fuels XXXX, XXX, XXX−XXX Energy & Fuels Article containing molecules by means of utilizing methane, acetylene, These studies have also shown that the support material ethylene, propylene, benzene, toluene, hexane, alcohol, and significantly influences the catalytic activity of metal catalysts, acetone12,13 as carbon feedstock has been studied in establish- suggesting the importance of the metal−support, in this case, ing a continuous process that drives the interest in high- metal−substrate, interaction. Recently, a thorough study was volume and low-cost CNT production. The use of methane published on the role of the metal−support interaction in the gas in the catalytic decomposition process is reported to be the formation of CNTs.48 Three metals Cu, Fe, and Ni of low, most suitable over the aforementioned carbon-containing moderate, and high catalytic activity were chosen for this molecules because of the fact that methane is a cheaper comparison on substrates with varying ease of reduction (CaO, 14 fi carbon source which produces little or practically no Al2O3, SiO2, and TiO2). Nickel provided the highest nano ber amorphous carbon and that it has a high stability characteristic yield among the 12 systems tested when supported on TiO2. 14,15 at elevated temperatures in preventing self-pyrolysis. Ni supported on SiO2 and Al2O3 was far less active and was Transition-metal (Ni, Fe, and Co)-based supported metal virtually unreactive when supported on CaO. Cu was most catalysts are widely used for methane decomposition to active on CaO and SiO2 and less so when supported on TiO2 produce hydrogen and CNTs at moderate temperatures or Al2O3. − (500−800 °C).16 18 It is accepted that metal nanoparticles Technological applications of CNTs such as storage devices, dispersed over oxide supports can enhance the catalytic the catalysts in drug delivery, and so forth require purity. The performance as the stronger metal−support interaction as-synthesized CNTs obtained from the methane CVD process changes the particle size or tuning of the electronic state of contain impurities such as amorphous carbon as well as metal metal species.19,20 In our previous work, we demonstrated the catalyst. These impurities can hinder the analytical study of the catalytic activity, selectivity, and stability of mono- and CNT properties and limit its applications.33,34 As supported bimetallic Ni/Fe/Co-based catalysts for the methane decom- metal catalysts have been used in the large-scale synthesis of position study.21 Also, we have explored and selectively CNTs, aggressive treatments have to be employed during the “ ” fi synthesized base-grown CNTs over Fe/SiO2 catalysts, CNT puri cation process. Among the various methods used which can be easily harvested without sacrificing the catalyst for CNT purification, chemical oxidation is the one widely sites. Previous studies have shown that majority of reports on used. The principle behind the process is the selective the catalytic decomposition of methane forms CNTs by “tip oxidative etching of carbon and metal particles because of growth”.22,23 A major disadvantage of tip-grown CNTs is that their higher oxidation rate than CNTs that leaves behind during CNT extraction from the catalyst, acid or base CNTs. During this process, the tip or end of the nanotube gets treatment has to be applied, which dissolves the catalyst opened up, creates defects in the sidewalls, and may also metal nanoparticles. As a result, during each CNT extraction introduce certain functional groups on to the CNTs. Currently, after methane decomposition, the catalyst is being sacrificed. mineral acids, such as sulfuric acid, nitric acid, or hydrochloric 35−37 This problem can be overcome if a catalyst system is developed acid, are widely used for purification. Even though there in such a way to produce “base-grown CNT’s”.As“base- are many reports which discuss either methane decomposition grown” CNTs can be easily harvested, the catalyst regeneration or purification of CNTs, there are very few studies