Template carbonization technique

In summary, we have presented a simple approach to synthesize hollow SiC spheres. The size of the hollow spheres depending on the size of the template carbon spheres. The shell of the spheres consists of a lot of twisted SiC nanowires with length of 5 20 pm and diameter of 50-500 nm, which is formed through a gas phase reaction. This technique presents a convenient method to synthesize hollow SiC spheres and an effective way to utilize fly ash. 

The so-called template-based technique has been found to be particularly suitable for the synthesis of carbons whose porosity is not only uniform in size and shape, but also periodically ordered in some cases. In this approach, the porous carbon is prepared through infiltration of an organic precursor into the nanochannels of an appropriate inorganic material (the template), followed by carbonization and then liberation of the resultant carbon from the template. Different nanospaces in templates have been used to confine the carbon precursors. The first templates used included, e.g., silica gel or porous glass [84,85], layered clays such as montmorillonite ortaeniolite [86,87], or pillared clays [88-90]. Several detailed reviews on this topic have been published [75,91-95] that cover the areas of microporous and, especially, mesoporous solids. Here, some illustrative examples will he presented in some detail rather than reviewing systematically the literature. 

Zhong, D.Y., Liu, S., Wang, E.G., 2003. Patterned growth of coiled carbon nanotubes by a template-assisted technique. Appl. Phys. Lett. 83, 4423—4425. 

Barata-Rodrigues et al. [73] reported that the CVD technique does not help improve the structural regularity of a zeolite-templated carbon. We indeed found that, when wet impregnation is followed by a CVD treatment, the XRD peak at 6° 20 can be observed, indicating the appearance of an ordered pore structure [75]. However, this XRD peak cannot be resolved for the carbon prepared without using the CVD technique [74]. An alternative route to the synthesis of ordered microporous carbons by using zeolite Y as template has also been described [88]. In addition, the use of other zeolite templates such as EMC-2 to improve structural regularity of replicated microporous carbons has been demonstrated... 

The second class of materials, which we will consider herein are carbons with a highly ordered porosity prepared by a template technique [15-18]. The pores are characterized by a well-defined size determined by the wall thickness of the silica substrate used as substrate for carbon infiltration. They can be also interconnected, that is very useful for the charge diffusion in the electrodes. Figure 1 presents the general principle of the carbon preparation by a template technique, where the silica matrix can be, for example, MCM-48 or SBA-15. 

Figure 1. Schematic representation of carbon elaboration by a template technique.

Recently, the LbL technique has been extended from conventional nonporous substrates to macroporous substrates, such as 3DOM materials [58,59], macroporous membranes [60-63], and porous calcium carbonate microparticles [64,65], to prepare porous PE-based materials. LbL-assembly of polyelectrolytes can also be performed on the surface of MS particles preloaded with enzymes [66,67] or small molecule drugs [68], and, under appropriate solution conditions, within the pores of MS particles to generate polymer-based nanoporous spheres following removal of the silica template [69]. 

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

The carbon templated tin incorporated mesoporous silicalite catalysts with MFI structure were successfully synthesized using microwave and well characterized using all the physico-chemical techniques. The catalytic activity of these catalysts was studied for liquid phase Baeyer-Villiger oxidation of various cyclic ketones using hydrogen peroxide. All the catalyst showed high conversion ( 100%) for bicyclic ketones with 100% selectivity to the corresponding lactone. 

By the template technique using anodic oxide films and pyrolytic carbon deposition, one can prepare monodisperse carbon tubes. Since the length and the inner diameter of the channels in an anodic oxide film can easily be controlled by changing the anodic oxidation period and the current density during the oxidation, respectively, it is possible to control the length and the diameter of the carbon tubes. Furthermore, by changing the carbon deposition period, the wall thickness of the carbon tubes is controllable. This template method makes it possible to produce only carbon tubes that are not capped at both ends. Various features of the template method are summarized in Table 10.1.1 in comparison with the conventional arc-discharge method. 

By applying the template technique, Kyotani et al. and Che et al. succeeded in preparing Pt and Pt/Ru metal-filled uniform carbon nanotubes in which the metal is present as either nanorods or nanoparticles. It should be noted that no metal was observed on the outside wall of the tubes. This is due to the preparation procedure, in which the metal precursor was loaded into the carbon-deposited alumina film before the dissolution of alumina by HF (see Fig. 10.1.9). Thus, there is no other space for metal to be loaded except in the channels. 

As demonstrated in this chapter, the template technique using anodic oxide films makes it possible to control the length, diameter, and thickness of carbon lubes and... 

Encapsulation of other material into carbon nanotubes would also open up a possibility for the applications to electrodevices. By applying the template method, perfect encapsulation of other material into carbon nanotubes became possible. No foreign material was observed on the outer surface of carbon nanotubes. The metal-filled uniform carbon nanotubes thus prepared can be regarded as a novel onedimensional composite, which could have a variety of potential applications (e.g novel catalyst for Pt metal-filled nanotubes, and magnetic nanodevice for Fe304-filled nanotubes). Furthermore, the template method enables selective chemical modification of the inner surface of carbon nanotubes. With this technique, carbon... 

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