Carbon nanomaterials

This group of carbon materials includes nanotubes and nanofibers. These materials have been attracting attention because of their rather unique properties, i.e. unusual strength as well as a high electrical and thermal conductivity. 

Carbon nanotubes can be readily dispersed in a solvent using ultrasound. However, because of a strong van der Waals forces, they can quickly aggregate and precipitate. This problem can be alleviated by various pretreatments. For example, Table 5 shows the effect of HNO3 on properties of nanotubes. An increase in the content of carboxylic, lactone and hydroxyl groups was noted. At the same time, the total amount of base was decreased to zero. However, the CNT prepared by the template technique could be dispersed in water without requiring any pretreatment. ° 

Nanostructured carbon materials have achieved global attention due to the uniqueness in their properties. Their individual properties are effectively utilized in various devices. These materials are inevitable candidates for the preparation of advanced nanocomposites. The ECP/CNM nanocomposites have attracted the attention of various advanced composite technologies. In several nanocomposites, CNMs are widely used as reinforcing fillers to improve dimensional stability, enhance electrical conductivity, increase strength, enhance chemical and temperature stabilities, etc. But one of the disadvantages is their high production cost. 

and two-dimensional CNMs have attracted the attention of researchers. These include fullerenes, carbon nanofibers, carbon nanotubes, and graphene. The advancements in the various synthesis procedures have envisaged the preparation of various CNMs with different shapes and sizes. Nanotechnology has paved the way to utilize these nanomaterials either individually or as nanocomposites for cutting-edge applications in chemical industry, materials science, biology, medicine, and other sectors. 

Various synthesis methods for the preparation of CNMs are well developed. A brief discussion on the synthesis of CNMs is given here. 

Fullerenes are carbon allotropes discovered in 1985 by Harold W. Kroto, Robert E Curl and Richard E. Smalley. These carbon nanostructures possess icosahedral symmetry and are sp hybridized. Fullerenes have a closed cage-like structure and are examples for zero-dimensional CNMs. Depending on the number of carbon atoms that a cluster possesses, these are named (contains 60 carbon atoms), C (contains 70 carbon atoms), Cg (contains 84 carbon atoms), etc. The unique morphology of these CNMs possess large surface area to volume ratio and is suitable for a wide variety of applications. Synthesis methods of fullerenes are well developed [7-9]. 

Carbon nanofibers or vapor-grown carbon nanofibers are sp hybridized one-dimensional carbon nanostructures. Three types of carbon nanofiber structures classified based on the angle of graphene sheets are stacked, cup-stacked, and nanotubular [10]. The diameter of carbon nanofibers lies in between carbon nanotubes (100 nm) and carbon fibers (1000 nm). The synthesis procedures used for carbon nanofibers include chemical vapor deposition (CVD). 

---

Hurt, R.H., Monthioux, M., and Kane, A. (2006) Toxicology of carbon nanomaterials status, trends, and perspectives on the special issue. 

Challeng es for assessing carbon nanomaterial toxicity to the skin. Carbon, 44 (6), 1070-1078. 

Herzog, E. et al. (2009) Dispersion medium modulates oxidative stress response of human lung epithelial cells upon exposure to carbon nanomaterial samples. Toxicology and Applied Pharmacology, 236 (3), 276-281. 

Jia, G. et al. (2005) Cytotoxicity of carbon nanomaterials single-wall nanotube, multi-wall nanotube, and fullerene. Environmental Science and Technology,... 

The process of producing the new kind of carbon nanomaterials namely carbon nanoscrolls from graphite nitrate is proposed which differs in advance from other techniques in its simplicity, availability of raw material, low level of energy consumption. 

Nakagawa, K. et al., Oxidized diamond as a simultaneous production medium of carbon nanomaterials and hydrogen for fuel cell, Chem. Mater., 15, 4571, 2003. 

Jorda-Beneyto, M., F. Suarez-Garci, D. Lozano-Castello, D. Cazorla-Amoro, A. Linares-Solano, Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures. Carbon 45, 293-303, 2007. 

Carbon Nanomaterials as Supports for Fischer-Tropsch Catalysts... 

The potential of carbon nanomaterials for the Fischer-Tropsch synthesis was investigated by employing three different nanomaterials as catalyst supports. Herringbone (HB) and platelet (PL) type nanofibers as well as multiwalled (MW) nanotubes were examined in terms of stability, activity, and selectivity for Fischer-Tropsch synthesis (FTS). 

Concerning the Fischer-Tropsch synthesis, carbon nanomaterials have already been successfully employed as catalyst support media on a laboratory scale. The main attention in literature has been paid so far to subjects such as the comparison of functionalization techniques,9-11 the influence of promoters on the catalytic performance,1 12 and the investigations of metal particle size effects7,8 as well as of metal-support interactions.14,15 However, research was focused on one nanomaterial type only in each of these studies. Yu et al.16 compared the performance of two different kinds of nanofibers (herringbones and platelets) in the Fischer-Tropsch synthesis. A direct comparison between nanotubes and nanofibers as catalyst support media has not yet been an issue of discussion in Fischer-Tropsch investigations. In addition, a comparison with commercially used FT catalysts has up to now not been published. 

Prior to functionalization the carbon nanomaterials were washed in concentrated nitric acid (65% Fisher Scientific) for 8 h using a Soxhlet device in order to remove catalyst residues of the nanomaterial synthesis as well as to create anchor sites (surface oxides) for the Co on the surface of the nanomaterials. After acid treatment the feedstock was treated overnight with a sodium hydrogen carbonate solution (Gruessing) for neutralization reasons. For the functionalization of the support media with cobalt particles, a wet impregnation technique was applied. For this purpose 10 g of the respective nanomaterial and 10 g of cobalt(II)-nitrate hexahydrate (Co(N03)2-6 H20, Fluka) were suspended in ethanol (11) and stirred for 24 h. Thereafter, the suspension was filtered via a water jet pump and finally entirely dried using a high-vacuum pump (5 mbar). 

As can be seen from Figure 2.1, cobalt was deposited on the carbon nanomaterials quite homogeneously. Hence, the cobalt particle sizes of the three catalyst types vary only little. The Co/nanofiber materials exhibit cobalt particle diameters of roughly 10 nm. In case of the nanotubes, particle sizes ranging from 5 to 7 nm were observed. 

Results of Carbon Nanomaterial Catalyst Characterization ICP, Chemisorption, Physisorption, Thermogravimetric Analysis (TG)... 

Physisorption measurements showed that carbon nanomaterials exhibit rather meso- and macroporous structures (maximum micropore fraction, 15% see Table 2.1). The lowest specific surface area was measured with the platelet fiber catalyst exhibiting slightly more than 100 m2/g. The Co/HB material offers 120 m2/g of surface area, and the highest BET value was determined with the Co/ MW catalyst featuring nearly 290 m2/g. Carbon nanomaterials, though, are not really porous, as the space between the graphene layers is too small for nitrogen molecules to enter. The only location of adsorption is the external surface of the nanomaterials and the inner surface of the nanotubes. 

FIGURE 2.2 TPR profiles of carbon nanomaterial Fischer-Tropsch catalysts (gas mixture 10% H2 in Ar heating rate 10 K/min). 

FIGURE 2.4 Arrhenius plots of the tested carbon nanomaterial catalysts and commercially used Fischer-Tropsch catalysts (reaction conditions p = 3 MPa, CO/H2 = A, V. = 18.5 1/h (NTP)). "... 

Activation Energy, and Collision Factor, Arlt of Carbon Nanomaterial-Supported Co Catalysts and Commercially Used Fischer-Tropsch Catalysts... 

In initial experiments carbon nanomaterial-supported catalysts showed acceptable activities and comparatively high selectivities toward higher hydrocarbons. Nevertheless, the applicability of these new materials in large-scale fixed bed reactors is limited due to their powdery appearance. Concerning this challenge research has already started, and hopefully carbon nanomaterial pellets will... 

Although research on carbon nanomaterials has by far not yet been finished and the price of carbon nanomaterials remains high, their unique properties justify continuing research on these remarkable materials. 

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. 

---