Carbonaceous

Carbon is one of the most versatile types of elements found in Earth. Due to its allotropic nature, carbon forms two completely different compounds by rearranging the adjacent carbon atoms. Since the last few decades, carbon compounds in nanodimensions are being synthesized and being used for many advanced and innovative applications. Carbon-based nanomaterials with different forms, e.g. tube, spherical, ellipsoids, or sheets, are frequently being synthesized. In this subsection, each type of carbonaceous nanomaterials along with their application potential will be briefly discussed.

Carbon Nanotube (CNT)

Carbon nanotube (CNT) is a cylindrical carbon nanostructure that can be compared with the rolled graphene sheets. CNT has been considered as one of the most studied carbon nanostructures in the last few decades. This nanomaterial has a diameter of only a few nanometres with a length of very higher magnitude compared to the diameter that makes them nearly one dimensional in its structure. CNT has very unique mechanical and electrical properties when compared with other carbon allotrophs such as graphite and diamond. In 1991, Sumio Lijima first reported a pathway of CNT synthesis. Thereafter, numerous applications of CNTs make them to be studied rigorously. CNTs have many outstanding properties such as high reflex strength and elasticity; excellent structural, chemical, and thermal stability; and the high conductivity (Shenderova, Zhirnov, and Brenner 2002; Katz and Willner 2004; Gohardani, Elola, and Elizetxea 2014). Single-walled carbon nanotube (SWNT) and multiwalled carbon nanotube (MWNT) are the two main structural configurations. SWNT is a simple cylindrical structure, whereas MWNT contains several concentric SWNTs. Thus, the properties of these two forms of CNTs are different from one another. All the properties of CNTs mainly depend on diameter, length, and the morphology of the nanotubes. Till date, many methods have been proposed for CNT synthesis. Among them, three different procedures, namely pulsed laser, electric arc, and chemical vapour deposition (CVD), are mainly used to synthesize CNT from graphite (Koziol, Boskovic, and Yahya 2010; Dresselhaus, Dresselhaus, and Eklund 1996; Gohardani, Elola, and Elizetxea 2014).

Due to its unique properties, CNT has been utilized in different applications. For the preparation of sensors and actuators, CNT has been frequently used in electronic nanodevices. Biomolecules are often integrated with CNTs for their application in diagnosing disease, drug delivery, and processing of biomaterials (Katz and Willner 2004; Willner and Willner 2010; Gooding 2005; Wang 2005; Allen, Kichambare, and Star 2007; Balasubramanian and Burghard 2006). SWNT-based field-effect transistor is also prepared for detecting biological species in the control environment (Allen, Kichambare, and Star 2007; Balasubramanian and Burghard 2006). Due to its large surface area, high electrical conductivity, mechanical strength, and lightweight, CNT is being used readily as supercapacitor for energy storage applications. Optical transparency and flexibility of CNT are also advantageous as it allows developing flexible electronic devices such as pliable cellphones and computers. Metal oxides are often incorporated into CNT to reduce its contact resistance and improve its energy storage capacity (Li and Wei 2013; Li et al. 2013). CNT has five times higher elasticity and 50-200 times higher tensile strength than steel. In addition, CNT has high lightweight than any metal. These properties make it ideal for use in the preparation of aircraft materials (Gohardani, Elola, and Elizetxea 2014). Threads made of CNT are being used to prepare the bullet-proof jacket (Obradovic et al. 2015). CNT has also been used for the improvement of electrostatic shielding in plastic composites (Pande et al. 2014). CNT has been used for improving the mechanical strength of composites, including biocomposites, and has been applied in tissue engineering, bone, and cartilage replacement (Gupta et al. 2014; Wang et al. 2015).

The use of CNTs, before or after their modification, for CO, capture is a new trend since the last decades. Esteves et al. (2008) indicated that the CO, is adsorbed onto CNTs preferentially compared with CH4 and H,S. CNTs have often been utilized as fillers for the preparation of nanocomposites that are useful for gas separation applications. Yoo et al. (2014) indicated that the incorporation of CNTs dramatically improves the kinetics of gas storage nanomaterials. Recently, Kang et al. (2015) showed that MWCNTs can improve the CO, storage capacity of the nanocomposites and the CNTs. Also, they monitored around 70% increase in CO, captured for the nanocomposites compared with that for CNTs itself. Lu et al. (2008) also modified CNTs with silane reagents and found that the capacity of CO, adsorption improved by 40% compared with unmodified CNTs. Although the use of CNTs for the large- scale storage of CO, is found to be promising, it still needs further researches to be implemented. All the current studies related to gas-capturing properties of CNTs are mainly laboratory based and need to be validated by large-scale gas storage applications. C02-capturing capacities of different CNT-based materials are given in Table 8.3.

Graphene and Its Derivatives

Graphene is one of the exceptional carbon-based nanomaterials with extraordinary chemical, electrical, and mechanical properties. It is a single-layered two-dimensional

TABLE 8.3

CNT-Based C02 Capture (Kang et al. 2015)

Name of the nanomaterials

Capacity (cm3g_1)

MWCNTs

7.4

JUC-32

41.7

Physical mixture

42.8

MWCNTs @JUC32-1

34.5

MWCNTs @JUC32-2

31.0

sheet where carbon atoms are arranged in a hexagonal lattice structure. Graphene is being used in batteries, fuel cells, sensors, supercapacitors, solar cells, and biotechnological processes. Graphene has 2.5 x 105cm2 Vs-1 electron mobility at ambient temperature, more than 1 TPa Young’s modulus, and more than 3000 W mK"1 thermal conductivity. In addition, graphene is impermeable to any gas and sustains a current density far more than that of copper. Also, it can be easily functionalized. These exceptional properties of graphene make it an obvious choice for many advanced technological innovations.

The synthesis process of graphene can be divided into two processes, namely the top-down process and the bottom-up process. In the bottom-up process, graphene sheets are directly synthesized from molecular organic precursors. The precursors contain a benzene ring with highly reactive functional groups. Another process is the use of catalyst substances for graphene synthesis via CVD, arc discharge, or SiC epitaxial growth. However, these processes are expensive and cannot produce a uniform and large amount of graphene. The top-down method is rather easy, simple, and used for the large-scale graphene production through chemical and mechanical exfoliation methods of graphite. However, this method also suffers from limitation like the preparation of two-dimensional graphene sheets. Graphene oxide (GO) is an intermediate of this process. GO is converted into a graphene-like material through the reduction process. Though, the product will have structural differences compared with the pure graphene.

Graphene and its derivatives are being used in many applications, including energy storage and dye-sensitizing solar cell (Dhand et al. 2013). It has been utilized in lithium ion batteries (Dhand et al. 2013). Being transparent, flexible, highly conductive, mechanically resistant, and lightweight, graphene can be an excellent alternative for supercapacitor applications. Graphene can be used in photonics and as a photodetector, polarization controller, insulator, and solid-state laser (Novoselov et al. 2012). Alike CNT, graphene is also used in the synthesis of sensors and biosensors (Kuila et al. 2011; Huang et al. 2010). In addition, it has a promising future in the field of medicine.

Graphene and its derivatives are relatively inexpensive when compared with the available solid-state gas adsorbents. Graphene nanosheets have been utilized for CO, storage and showed very high CO,-capturing abilities (Mishra and Ramaprabhu 2011). Graphene is often modified with different ways to improve its gas-capturing capacities. Modification of GO with porous MgO (Ning et al. 2012) and doublelayered Mg-Al hydroxide (Garcia-Gallastegui et al. 2012) has been reported for the improvement of the storage capacity of the gas. It was noticed that the CO, storage capacity of the double-layered metal hydroxide can be improved by around 62% by the addition of 7% GO. Also, the use of GO improves the stability of the double-layered metal hydroxide upon reuse and increases the effective surface area. Recently, Fe,0, nanoparticle-coated graphene has been developed by Mishra and Ramaprabhu (2014). The Fe,0,-coated graphene shows very high CO,-capturing capacities. As the sorption of the gas molecule on GO surface depends on electrostatic and van der Waals interactions, charge transfer, and dispersion interactions, the related modifications of the graphene surface can be beneficial for the improvement in its gas sorption ability.

Nanoporous Carbon

Recently, a series of nanoporous carbon materials have been synthesized for their application in the storage of natural gases (Liu et al. 2006). These nanoporous materials have large pore amount, definite surface area, thermal stability, mechanical stability, and organized pore structure. All these properties make them an exceptional choice for use in the storage of gas. Biomasses from different origins have been utilized for the preparation of nanoporous carbon (Lee, Han, and Hyeon 2004; Kumar et al. 2018). For the synthesis of nanoporous carbon structure, different biomasses have been modified either using an activating agent or using some templates like silica.

Non-Carbonaceous

Nanosized Zeolites

Zeolites are highly porous and three-dimensional crystalline aluminosilicates containing alkaline earth metals such as Na+, K+, and Ca+. Due to their highly ordered porous structure, zeolites are being used for many applications, including gas separation (Yang and Xu 1997). Although zeolites favour C02 adsorption, their affinity decreases with increased temperature and under humid condition. To improve the separation ability, zeolites are modified in the nanoscale materials. Jiang et al. (2013) synthesized a nanosized T-type zeolite with the pore size of 0.36 x 0.51 nm. The pore size of T-type zeolite is smaller than those of N, and methane molecules, however larger than the CO, molecule. The researchers found that such modification enhances the separation ability of the nanosized zeolites by 30%. Nanosized zeolite and silica-based composites have also developed for the improvement of gas separation capacity of zeolites (Auerbach, Carrado, and Dutta 2003).

Mesoporous Silica Nanoparticles

Mesoporous silica nanoparticles have gained attention due to their exceptional pore size of 30 nm. This silica nanoparticle has ordered structure of the hexagonal array. This nanoparticle shows around 31% higher gas separation ability than the activated carbon in identical condition (Liu et al. 2006). Qi et al. (2011) studied that amine- functionalized mesoporous silica-based gas separator has been prepared and used for C02 separation. This process is reported to be inexpensive and easy to recover CO, from the reaction. Atriamine-grafted mesoporous silica was also synthesized for CO, separation by researchers (Belmabkhout, De Weireld, and Sayari 2009) and found to separate CO, more efficiently.

Nanoparticle-Embedded Metal-Organic Framework

The organo-metal framework is a new emerging class of nanoporous materials forming a 3D network with a centralized cation surrounded by an organic framework. These materials are getting attention in recent times due to the possibility of using them for catalysis, separation, gas storage, and optics applications. However, Bloch et al. (2013) showed that the metal-organic framework nanoparticle (MOFN) often shows poor gas adsorption capacities in low CO, partial pressure. Many reports are available for the enhancement of gas adsorption properties of MOFNs. In some studies, the pore surface was functionalized with an amine to enhance the gas adsorption capacities of MOFN. Recently, zinc-based MOFN has been synthesized that showed very high C02 adsorption capacities (Millward and Yaghi 2005). Amino- functionalized zirconium-based MOFN has also been developed that showed good C02 storage capacities (Abid et al. 2013). However, it should be mentioned that there are some technical difficulties related to the use of MOFN for packed bed column. CO, adsorption capacities of MOFs were further enhanced by the incorporation of CNT-like nanomaterials (Khdary and Ghanem 2012; Xiang et al. 2011).

Metal Oxide Nanoparticles

Nanostructured metal and metal oxide nanoparticles are attractive options for use in gas separation. Mesoporous MgO with nanostructure has been produced by using mesoporous carbon as a template, and the materials showed CO, adsorption capability (Bhagiyalakshmi, Lee, and Jang 2010). Nano-hollow-structured CaO has also been synthesized and showed significant CO, adsorption properties (Yang et al. 2009). These reports also indicated that CaO nanopods showed higher CO, adsorption capacity than the commercial CaO.

Other metal nanoparticles such as hydroxylated Fe,0„ A1,0„ and TiO, were also tested for C02 adsorption and found to be useful (Baltrusaitis et al. 2011). The reports indicated that hydroxylated Fe,0, and A 1,0, capture CO, in the form of bicarbonate. However, TiO, nanoparticles capture CO, in the form of bidentate carbonates. Recently, Fe,0,-amended activated charcoal has been developed (Hakim et al. 2015) for enhanced CO, adsorption. CO, adsorption was enhanced after the nanoparticle amendment.

 
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