Porous nanocarbon

Zhong M, Natesakhawat S, Baltrus JP, Luebke D, Nulwala H, Matyjaszewski K, Kowalewski T (2012) Copolymer-templated nitrogen-enriched porous nanocarbons for CO2 capture. Chem Commun 48 11516-11518... 

The morphology and structure of porous nanocarbons are very sensitive to the synthesis method [3]. Figure 8.2, for example, demonstrates the typical morphology of porous nanocarbon films made up of connected carbon nanoparticle aggregates obtained by a solution synthesis method. This structure is made up of closely packed carbon nanoparticles and differs considerably from the flame synthesized carbonaceous films. 

Physisorption (i.e., adsorption of hydrogen) of molecular hydrogen by weak van der Waals forces to the inner surface of a highly porous material. Adsorption has been studied on various nanomaterials, e.g., nanocarbons, metal organic frameworks and polymers. 

Nanocarbons can also be deposited onto surfaces via electrochemistry, such as electrophoretic deposition described earlier. A method for one-step electrochemical layer-by-layer deposition of GO and PANI has been reported by Chen et al. [199]. A solution of GO and aniline was prepared and deposited onto a working electrode via cyclic voltammetry. GO was reduced on the surface when a potential of approx. -1 V (vs. SCE) was applied compared to the polymerization of aniline which occurred at approx. 0.7 V (vs. SCE). Repeated continuous scans between -1.4 to 9 V (vs. SCE) resulted in layer by layer deposition [199]. A slightly modified method has been reported by Li et al. who demonstrated a general method for electrochemical RGO hybridization by first reducing GO onto glassy carbon, copper, Ni foam, or graphene paper to form a porous RGO coating [223]. The porous RGO coated electrode could then be transferred to another electrolyte solution for electrochemical deposition, PANI hybridization was shown as an example [223]. 

Another important consideration involves the hybridization of porous carbon with hierarchical 3D architectures, such as fibers or arrays. Wet chemical techniques are often useless as the mandatory solvent removal/drying typically results in the at least partial collapse of the nanocarbon pore structure. Gas phase deposition is a... 

Nanocarbon emitters behave like variants of carbon nanotube emitters. The nanocarbons can be made by a range of techniques. Often this is a form of plasma deposition which is forming nanocrystalline diamond with very small grain sizes. Or it can be deposition on pyrolytic carbon or DLC run on the borderline of forming diamond grains. A third way is to run a vacuum arc system with ballast gas so that it deposits a porous sp2 rich material. In each case, the material has a moderate to high fraction of sp2 carbon, but is structurally very inhomogeneous [29]. The material is moderately conductive. The result is that the field emission is determined by the field enhancement distribution, and not by the sp2/sp3 ratio. The enhancement distribution is broad due to the disorder, so that it follows the Nilsson model [26] of emission site distributions. The disorder on nanocarbons makes the distribution broader. Effectively, this means that emission site density tends to be lower than for a CNT array, and is less controllable. Thus, while it is lower cost to produce nanocarbon films, they tend to have lower performance. 

For application in flow reactors the nanocarbons need to be immobilized to ensure ideal flow conditions and to prevent material discharge. Similar to activated carbon, the material can be pelletized or extruded into millimeter-sized mechanically stable and abrasion-resistant particles. Such a material based on CNTs or CNFs is already commercially available [17]. Adversely, besides a substantial loss of macroporosity, the use of an (organic) binder is often required. This material inevitably leaves an amorphous carbon overlayer on the outer nanocarbon surface after calcination, which can block the intended nanocarbon surface properties from being fully exploited. Here, the more elegant strategy is the growth of nanocarbon structures on a mechanically stable porous support such as carbon felt [15] or directly within the channels of a microreactor [14,18] (Fig. 15.3(a),(b)), which could find application in the continuous production of fine chemicals. Pre-shaped bodies and surfaces can be... 

Abstract. Nanocarbon materials and method of their production, developed by TMSpetsmash Ltd. (Kyiv, Ukraine), are reviewed. Multiwall carbon nanotubes with surface area 200-500 m2/g are produced in industrial scale with use of CVD method. Ethylene is used as a source of carbon and Fe-Mo-Al- mixed oxides as catalysts. Fumed silica is used as a pseudo-liquid diluent in order to decrease aggregation of nanotubes and bulk density of the products. Porous carbon nanofibers with surface area near 300-500 m2/g are produced from acetylene with use of (Fe, Co, Sn)/C/Al203-Si02 catalysts prepared mechanochemically. High surface area microporous nanocarbon materials were prepared by activation of carbon nanofibers. Effective surface area of these nanomaterials reaches 4000-6000 m2/g (by argon desorption method). Such materials are prospective for electrochemical applications. Methods of catalysts synthesis for CVD of nanocarbon materials and mechanisms of catalytic CVD are discussed. 

RDE and RRDE are very convenient voltammetric methods for studying the mechanism and kinetics of ORR and are by far the most widely used methods. However, it is important to bear in mind that the underlying mathematical formulations of these methods are theorized for smooth electrode surfaces under laminar flow hydrodynamics. There are many examples in recent literature where RDE and RRDE have been used to study catalyst films for which turbulent flow hydrodynamics is quite obvious. The collection efficiency of RRDE for microscopically disordered films, for example, very porous materials and irregularly built-up films (as may be the case for catalysts modified with nanocarbons such as carbon nanotubes and graphenes), is likely to be determined erroneously due to sporadic hydrodynamics. Therefore, the quality of a given catalyst film has a great influence on the correctness of results obtained from RDE and RRDE. It is generally recommended that catalyst films for RDE and RRDE studies should be as thin as... 

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