Templating method mesoporous carbons

In this chapter, we provide an overview of the recent research and development in the preparation, characterization, and application of novel porous carbons using both the endotemplate and the exotemplate methods. A discussion of zeolite templates for microporous carbons is followed by that of ordered mesoporous silica templates for OMCs, nanoparticle templates for mesoporous carbons, sol-gel processed porous carbons, self-assembled colloidal crystal templates for ordered macroporous carbons, and colloidal sphere templates for hollow carbon spheres, as well as other templating approaches to preparing carbon nanostructures. Then,... 

Mesoporous carbon materials were prepared using ordered silica templates. The Pt catalysts supported on mesoporous carbons were prepared by an impregnation method for use in the methanol electro-oxidation. The Pt/MC catalysts retained highly dispersed Pt particles on the supports. In the methanol electro-oxidation, the Pt/MC catalysts exhibited better catalytic performance than the Pt/Vulcan catalyst. The enhanced catalytic performance of Pt/MC catalysts resulted from large active metal surface areas. The catalytic performance was in the following order Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It was also revealed that CMK-1 with 3-dimensional pore structure was more favorable for metal dispersion than CMK-3 with 2-dimensional pore arrangement. It is eoncluded that the metal dispersion was a critical factor determining the catalytic performance in the methanol electro-oxidation. 

The principle of the carbon synthesis is shown in Fig. 1. Suitable carbon sources such as sucrose, furfuryl alcohol, phenol-resin monomers and acetylene gas are converted to carbon frameworks inside mesoporous silica template by pyrolysis. An effective method for the restriction of carbonization to inside the template is to incorporate a suitable catalyst such as Al, Sn and Fe onto the silica pore walls prior to the use as template. The template after the carbonization is removed using ethanol-water solution of HF or NaOH. 

CMK-5 is the first example of the ordered tube-type mesoporous carbons that can be characterized with well-defined Bragg diffractions by ordinary XRD instrument [6]. The XRD pattern of the CMK-5 carbon is distinguished from that of CMK-3 by the much lower intensity of the (100) diffraction. The structure of CMK-5 may be described by the substitution of the carbon nanorods in CMK-3 with nanopipes. The CMK-5 carbon is synthesized using SBA-IS, similar to CMK-3, but the carbon source and synthesis condition are somewhat different from those for CMK-3. The synthesis method for the tube-type carbon can be extended to the SBA-16 mesoporous template. The resultant CMK-7 carbon has a bicontinuous mesoporous structure [15]. 

The methods developed for the synthesis of ordered mesoporous carbons are simple and cost-efricient, and the pore size can be tailored. The synthesis process can be scaled up for production in bulk quantities. Recent works on the synthesis of mesoporous silicas brought about much improvement in the cost-efficient and custom-tailored synthesis of the templates [18]. The discovery of new mesoporous silicas is also expected to provide additional promising templates for the synthesis of new mesoporous carbons. The resulting high-surface-area materials with uniform pores promise to be suitable as... 

Among all the precursors studied, precursor 2a appears to be the most useful precursor of nanocrystalline mesoporous Sn02 for application purposes. In comparison with 2d and 2f, compound 2a provides pure cassiterite tin dioxide materials with very low amounts of carbon from calcination at 400 °C, owing to the rather facile elimination of the p-phenylene spacer. After calcination at 400 and 500 °C, the BET surface area (110 and 70 m g ), the mean pore size diameter (5 and 6.5 nm) and the mean crystallite size (4.5 and 7 nm) are close to those reported for tin dioxide materials prepared by the template method after... 

This is where the synthesis of nano-sized molecular sieves is carried out in the template matrix within confined spaces. This is an ideal synthetic route if the space size and uniformity favor the crystallization, and the as-synthesized product is easily isolated from the templates. Mesoporous molecular sieves with uniform mesopore structures can be adopted as the template, such as MCM-41. In 2000, Schmidt et al.[127] first proposed such a route to synthesize ZSM-5 nanocrystals. The synthesis procedure consisted of the impregnation of mesoporous carbon black with reaction solution, followed by treatment with steam at 150 °C, and the combustion of carbon black. Compared with other methods, the advantage of this one is that the nano-sized product is easily isolated and the yield is relatively higher. However, it also has some drawbacks. First, there is a high requirement for the preparation of carbon black as the template matrix, i.e., the mesopore sizes in carbon black must be uniform. Second, the crystallization must be performed in the mesopores, not on the extra surfaces of the carbon black. Third, a large amount of carbon black will be consumed (about four-times that of the nanozeolite product). All of these factors affect the further development of this route to some degree. 

Highly ordered mesoporous silica can be regenerated from a mesoporous carbon CMK-3 that is a negative replica of mesoporous silica SBA-15, indicating reversible replication between carbon and inorganic materials.[193] The advantage of this synthesis method is that it does not need to make the same silica material (template). This method is likely to be a valuable complement to the existing methods for the fabrication of new mesoporous silicas and other composition materials. 

Ordered mesoporous carbon can also be synthesized by a catalytic CVD method.[251] The ordered carbons possess bimodal pores the pores arise from the replica frameworks of the template and the pores correspond to carbon nanotubes formed in the channels of the template. 

Lately, a fascinating strategy has been successfully developed for the preparation of ordered mesoporous carbons. The synthesis procedure of these advanced carbons consists in the infiltration of an organic precursor into the pores of silica or aluminosilicate templates, followed by the subsequent pyrolysis of the precursor and dissolution of the template framework by HF [9—12]. In another process, carbon is directly introduced in the template by a CVD method [86]. The method gives a highly ordered and interconnected network of meso- and micropores [87], where the size of carbon mesopores is defined by the walls thickness of the pristine silica matrix. Such materials are very suitable for better understanding the relationships between the porous characteristics and the supercapacitors performance [88, 89]. 

As mentioned in Section 5.1, the templating method has been especially successfid in the mesopore area. This has been so especially since, in 1999, two Korean groups independently reported the syntesis of ordered mesoporous carbons using mesoporous silicas as templates [106,107]. Their achievements have been described by themselves in excellent recent accounts [92,93], and therefore only an outline of their most significant results will be presented here. 

A flaw of the templafmg method is that the structure of available templates limits the range of achievable mesopores to 2-6 nm in the case of ordered mesoporous carbons and >50 nm in the case of colloidal crystals [124]. As a contribution to overt me this limitation, Li... 

Ordered Mesoporous Carbons The first ordered mesoporous carbons were synthesized by hard templating methods (141). The use of porous inorganic silica templates allowed the preparation of series of ordered mesoporous carbon materials, designated as CMK-x materials (carbon molecular sieves Korean Advanced Institute of Science and Technology). The hrst one, CMK-1, was prepared using MCM-48 as a template and sucrose as the carbon source impregnated in the... 

The other method for preparing ordered mesoporous materials is the so-called hard template method using hard mesoporous silica or replicated carbon templates. The metal precursors are filled into hard templates. In this method, heat treatment can be performed at a higher temperature without structural collapse and highly crystallised materials can be obtainedl ° ] (Figure 3.6). 

Ordered mesoporous crystalline CuO was produced by using a hard carbon template (CMK-3) prepared by using an SBA-15 template. This two-step templating method produced well-ordered hexagonal (similar... 

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