Selective synthesis of carbon nanotubes by catalytic decomposition of methane using Co-Cu/cellulose derived carbon catalysts: A comprehensive kinetic study
Graphical abstract
Introduction
During the last decades, production of carbon nanotubes (CNTs) with different wall structures (single-walled (SWCNTs) [1], multiwalled (MWCNTs) [2]) and tubular morphologies (straight [3], bamboo-like [4], cup-stacked [5], helical [6]) has received a great industrial and scientific interest due to their extraordinary physical and chemical properties. Their high aspect-ratio, thermal conductivity, tensile strength, chemical surface and electronic properties give rise to new opportunities in a large variety of disciplines, including catalysis [7], [8], sensors [9], [10], energy storage [11], [12], environmental remediation [13] and biomedicine [14], [15]. Despite extensive progress in CNTs synthesis, obtaining CNTs with a well-defined structure at the industrial level is still a difficult process to achieve by a cost-effective technique. Therefore, enormous efforts have been devoted worldwide to find the most appropriate operating conditions and catalyst compositions that provide both high carbon yield, quality and selectivity towards the desired carbon structure [16], [17], [18]. Among the currently available CNTs production technologies, catalytic chemical vapor deposition (CCVD) stands for the most attractive method to meet this demand, due to its low-cost and easy scalability for mass production in large-scale manufacturing scenarios [19], [20].
In this technique, transition metal nanoparticles are exposed to a gaseous carbon source, commonly a light hydrocarbon (e.g., CH4, C2H2, C2H4), which is decomposed into carbon atoms, molecular hydrogen and, in some cases, cracking by-products at high reaction temperatures (600–1200 °C). When CH4 is used as a carbon feedstock, the co-production of COx-free hydrogen and the absence of cracking by-products constitute an additional advantage [21]. CNTs growth starts with the hydrocarbon dissociation by the metal nanoparticles into carbon atoms that diffuse either through the bulk metal or along its surface. After the metal reaches its carbon solubility limit, the diffusing carbon atoms precipitate on the opposite side, forming the carbonaceous nanomaterial. Depending on the metal nanoparticle–catalyst support interaction, the CNTs can grow via a tip-growth or a base-growth mechanism, whether the metal is lifted up or remains attached to the substrate, respectively [16]. Carbon growth continues as long as the metal nanoparticles have active sites available for the carbon source dissociation, and there is a favorable carbon concentration gradient between the surface of the nanoparticles exposed to the gas and the exit points where the precipitation of carbon atoms occurs [22].
Thus far, the most commonly used transition metals are Fe, Co, Ni because of their high carbon solubility and diffusion rate at elevated temperatures [16]. Furthermore, the high melting point and the low equilibrium vapor pressure of these metals offer a wide range of operating conditions in the CVD process. Other non-iron group metals such as Cu [23], Mn [24], Cr [25], Mo [26], Pt [27], Pd [28], and Au [29] have been incorporated as promoters since they allow modulating the carbon diffusion and the final structure of the carbonaceous nanomaterial obtained. In this regard, our group is investigating the effect of the reaction operating conditions and catalyst compositions on the yield and morphology of the carbon nanomaterial grown [30], [31], [32], [33], [34], [35], [36], [37], [38]. In the case of Co-based catalysts, we have recently found that the promotion with Cu or Mn has a strong effect on the type of carbon nanomaterials synthesized [39]. Thus, the addition of Cu promotes the formation of CNTs, while Mn intensifies the selectivity towards graphene-related materials (GRMs).
Cu-based catalysts have usually been quoted as not efficient for CVD-grown CNTs [40]. This fact has been explained considering the filled electronic d-shell of Cu which causes a barrier for the dissociative adsorption of hydrocarbons and does not form stable carbides [41]. However, its electron density can be enhanced by doping with alkaline elements [42] or by accepting the electron density of Lewis-base supports [43], attaining a stable [Ar] 4s2 3d10 configuration. More recent reports have shown notable improvements in the catalytic activity of Cu for growing both random networks and aligned arrays of CNTs [44], [45], [46]. Besides, Cu has some attractive properties that make the understanding of its behavior interesting for co-catalyzed CVD processes. Compared with Co, Ni or Fe, carbon atoms could precipitate on the outer surface of Cu more easily because its carbon solubility is much lower. Additionally, Cu presents a lower hydrocarbon dissociation rate, which slows down the carbon amount released into the reactive gas atmosphere. This could be an advantage to reduce the formation of amorphous carbon deposits. On the other hand, although the low melting point of Cu promotes the sintering of the metal nanoparticles, CNTs growth could proceed at lower temperatures. In that view, these features encourage the exploration of CVD operating conditions such as reaction temperature, carbon feedstock and flow rates over new catalyst compositions for CNTs growth.
Accordingly, the design of new formulations of both metal nanoparticles and supports is being increasingly investigated [17], [47], [48]. It is well known that the catalyst support plays an important role in the dispersion and catalytic activity of metals, which affects some properties of the CNTs grown such as diameter distribution and growth arrangement [49], [50]. Renewable lignocellulosic raw materials for catalyst supports emerge as an attractive alternative to the commonly expensive and laborious-synthesized metal oxides, due to their high availability, low-cost and unique textural properties [51]. Natural lignocellulosic materials present a rich hierarchical structure that serves as a high surface area template to provide a fine dispersion of metal nanoparticles [52]. An additional advantage is that the catalysts can be prepared in a single step by a biomorphic mineralization technique, which involves the reductive thermal decomposition of the lignocellulosic material previously impregnated with the metallic precursors [53]. The use of cellulose and vine shoots wastes as raw catalyst supports has been previously studied in our group for growing several carbonaceous nanomaterials (carbon nanotubes, carbon nanofibers, few-layer graphene) via Ni [30] or Ni-Cu [31] catalyzed CVD reactions. The developed catalysts exhibited high catalytic activity and good dispersion of metal nanoparticles, which resulted in a high number of nucleation sites for carbon precipitation and a high performance in the growth of carbonaceous nanomaterials.
Owing to the crucial role of the catalyst efficiency in the mass production of CNTs with a well-defined structure, the investigation of its activation, deactivation and growth mechanism is certainly essential. Unfortunately, there are still several unanswered questions today about what are the critical steps that allow better structural control in large-scale CNTs manufacturing processes. Hence, this work aims at contributing to the CNTs research field through a fundamental understanding of the influence of the operating conditions —reaction temperature and feed gas composition— on the quality and yield of CVD-grown CNTs. For this purpose, we have developed a kinetic model involving the main stages of the CNTs growth mechanism, which has been continuously adapted to different synthesis conditions and catalyst compositions [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], this time a novel Co-Cu/cellulose derived carbon (CDC) catalyst.
Section snippets
Catalysts preparation
Co-Cu/CDC catalyst was prepared by thermal decomposition of a commercial cellulose (Sigma Aldrich, ref: C6288) under reductive atmosphere, as described elsewhere [53]. First, the dried cellulose was impregnated by incipient wetness with the appropriate amount of an aqueous solution containing the precursor salts (Co(NO3)2·6H2O and Cu(NO3)2·3H2O, Sigma-Aldrich), in order to obtain a nominal weight composition of 5%Co–1.35%Cu (atomic ratio Co/Cu = 4) regarding the initial amount of cellulose.
Fresh catalyst characterization
Morphology and particle size distribution of the bulk Co-Cu/CDC catalyst and the impregnated metal nanoparticles were addressed by electron microscopy images shown in Fig. 1. SEM-EDS elemental mapping depicted in Fig. 1(A–C) indicates that Co and Cu transition metals were well distributed on the CDC support owing to its characteristic biomorphic surface texture. TEM images were used to estimate the particle size of the incorporated metals by measuring at least 500 particles from different
Conclusions
In this work, a 23.9%Co-6.4%Cu catalyst supported on a carbon derived from cellulose (CDC) was proved to be active for the selective production of CNTs via catalytic decomposition of methane under moderated reaction temperatures (700–800 °C). A maximum carbon productivity of 0.33 gC/gcat∙h was achieved at 800 °C under 28.6% CH4:14.3% H2. The increment of the reaction temperature above 800 °C caused a transition in the CNTs growth towards the formation of two-dimensional graphite-like
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge financial support from the Ministry of Science and Innovation (Madrid, Spain), Projects ENE2017-82451-C3-1-R and ENE2017-82451-C3-2-R. W. Henao acknowledges the financial support from Fundación Carolina (Madrid, Spain) and the University of Zaragoza.
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