Carbon nanocarbon

Support is one of the important factors affecting the performance of catalyst, which include its type, composition, pore structure, surface properties and mechanical strength etc. The supports used include activated carbon, nanocarbon tube, carbon molecular sieves and carbon fiber, oxide, zeolite molecular sieve, alkali metal exchanged X- and Y-type molecular sieve,carbon covered alumina and boron nitride etc. 

Huczko, A. et al. (2005) Pulmonary toxicity of 1-D nanocarbon materials. Fullerenes, Nanotubes, and Carbon Nanostructures, 13 (2), 141—145. Grubek-Jaworska, H. et al. (2006) Preliminary results on the pathogenic effects of intratracheal exposure to onedimensional nanocarbons. Carbon,... 

In this section, different nanocarbons and their chemical and physical properties are discussed (for more details see Chapters 1 and 2). Furthermore, the types of defects that can be embedded within these carbon nanostructures are explained, as well as their resulting chemical and physical properties. 

Nanocarbon structures such as fullerenes, carbon nanotubes and graphene, are characterized by their weak interphase interaction with host matrices (polymer, ceramic, metals) when fabricating composites [99,100]. In addition to their characteristic high surface area and high chemical inertness, this fact turns these carbon nanostructures into materials that are very difficult to disperse in a given matrix. However, uniform dispersion and improved nanotube/matrix interactions are necessary to increase the mechanical, physical and chemical properties as well as biocompatibility of the composites [101,102]. 

Nanocarbon electrodes can also be prepared by utilizing the n electrons of nanocarbons and glassy carbon (GC) electrodes. For example, Cai et al. simply dropped an aqueous solution of RGO onto a cleaned GC electrode and allowed the solvent to dry. The n-7T interactions were suitable for the subsequent deposition of Pt-Au nanostructures via electrochemical reduction of varying ratios of PtCl and AuC14 [133]. 

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]. 

Nanocarbons other than CNTs and graphene often exhibit similar surface chemistry and can be hybridized in a similar fashion. For example, single-walled carbon nanohorns (SWCNHs) have been oxidized via heat treatment in air atmosphere followed by immersion in a solution containing H2PtCl6. The Pt ions adsorbed to the oxidized SWCNHs and were then reduced via addition of sodium citrate to form Pt NPs [150],... 

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... 

As subsequent chapters will document, the type, structure and quality of the nanocarbon have a considerable impact on the final performance of the nanocarbon hybrid. Currently, most publications on the synthesis of nanocarbon hybrids focus on GO, which is both easy to prepare and simple to hybridize. However, the mechanical and electrical properties of GO (and also RGO) are often inferior to their pristine counterparts and in fact closer to those of activated carbon. Hence, we recommend always synthesizing and comparing various types of nanocarbons with different features and functionalizations. 

Nanocarbon composites can be broadly divided into three kinds, each with some possible subdivisions. Examples of these composites and their schematic representations are presented in Fig. 8.1. The first type corresponds to composites where the nanocarbon is used as a filler added to a polymer matrix analogous, for example, to rubber reinforced with carbon black (CB). The second consists of hierarchical composites with both macroscopic fibers and nanocarbon in a polymer, such as a carbon fiber laminate with CNTs dispersed in the epoxy matrix. The third type is macroscopic fibers based... 

In the third route of integration of nanocarbons in FRPCs, carbon nanotubes are radially attached to the surface of the fibers, typically grown in situ by chemical vapor... 

Mas B, Fernandez-Blazquez JP, Duval, Bunyan H, Vilatela ]]. Thermoset curing through Joule heating of nanocarbons for composite manufacture, repair and soldering. Carbon. 2013 Nov 63(0) 523-9. 

The third family of research grade materials is less well defined and encompasses aerogels of carbon [81,82] designed mesoscopic void structures in C3 with nanostruc-tured fillers [51,83], composites with nanocarbon fillers [24,82,84 88] and carbon-heterostructure [54,89-94] compounds. The references stated here are only examples for a wide range of activities stemming from the efforts to synthesize novel nanostruc-tured composites. These materials often exhibit unusual surface properties and are used in electrochemical and catalytic applications rather in the domain of traditional C3 compounds where mechanical properties dominate the application profile. 

Thus, emission can be increased by using nanodiamond grain boundaries or DLC with conductive tracks. However, these systems suffer from irreproducibility and instability. Eventually, sp2 carbon systems such as carbon nanotubes (CNTs), nanowalls and nanocarbons were studied, which are the best systems, because they are metallic and have the desired shape for field enhancement. 

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. 

Carbon nanowalls are an unusual form of nanocarbon that can be produced by plasma deposition with a certain set of deposition parameters. They are graphitic but with their graphitic planes perpendicular to the substrate. The walls are a finite thickness, not a few layers thick like graphene, and the walls tend to be quite wiggly [33,34]. They have not been extensively studied. 

The field emission properties of carbon nanotube forests and single nanotubes are described. Controlled emission is possible for aligned CNT arrays where the spacing is twice the CNT height, as grown by plasma enhanced chemical vapor deposition. This leads to the maximum field enhancement factor. For random forests, the field enhancement obeys an exponential distribution, leading to a lower emission site density and imperfect current sharing. Ballast resistors can help alleviate this problem. Random nanocarbons perform less well than CNTs. Some applications are covered. Elec-... 

Besides the practical application, the diversity of nanostructured carbon allotropes makes nanocarbon also an ideal model system for the investigation of structure-function correlations in heterogeneous catalysis. Nanocarbons can be tailored in terms of their hybridization state, curvature, and aspect ratio, i.e., dimensions of stacks of basic structural units (BSU), Chapters 1 and 2. The preferred exposition of two types of surfaces, which strongly differ in their physico-chemical behavior, i.e., the basal plane and prismatic edges, can be controlled. Such controlled diversity is seldom found for other materials giving carbon a unique role in this field of basic research. The focus of this chapter is set on the most prominent representatives of the... 

Ill-defined carbon materials that provide a distinct nanostructure, such as spherical particles in the case of soot and carbon black, or hexagonally ordered cylindrical pores in the case of ordered mesoporous carbons, are not discussed here. Surface chemical, thus catalytic properties of these material are closer to carbon black or activated carbon [13], which is frequently reviewed [2-4]. Here, the higher degree of sp3 hybridization often results in a higher reactivity, however, at lower selectivity, as compared to nanocarbons exposing large basal plane fractions of the overall surface. 

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... 

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