Carbon Gels in Catalysis

An important feature of these materials is that metal-doped monolithic carbon gels can readily be prepared (see below). These are emergent materials in the field of heterogeneous catalysis, and their use will grow in the near future, due 

Carbon Materials for Catalysis, Edited by Philippe Serp and Josd Luts Figueiredo Copyright 2009 John Wiley Sons, Inc. 

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In addition to mesostructured metal oxide molecular sieves prepared through supramolecular assembly pathways, clays, carbon molecular sieves, porous polymers, sol-gel and imprinted materials, as well as self-assembled organic and other zeolite-like materials, have captured the attention of materials researchers around the globe. Clays, zeolites and sol-gel materials are still very popular because of their extensive and expanding applications in catalysis and separation science. Novel carbons and polymers of ordered porous structures have been synthesized. There are almost unlimited opportunities in the synthesis of new organic materials of desired structural and surface properties via self-assembly or imprinting procedures. 

Cytochrome c dissolved in a film of polyethyleneoxide (PEO) or polyacrylamide (PAA) and immobilized at the surface of carbon electrode in this way yields a well-defined cyclic voltammogram (Fig. 16). The parameters of the electrode process are almost identical to those of the process in an aqueous medium. This finding can be apparently related to a high content of water in the gel. This system can be used in investigations of electron transfer properties of semirigid polymers suitable for constructing biosensors and to study solid-state enzyme catalysis. [221]. 

Zeolites. In heterogeneous catalysis porosity is nearly always of essential importance. In most cases porous materials are synthesized using the above de.scribed sol-gel techniques resulting in so-called amorphous catalysts. Porosity is introduced in the agglomeration process in which the sol is transformed into a gel. From X-ray Diffraction patterns it is clear that the material shows only weak broad lines, characteristic of non-crystalline materials. Silica and alumina are typical examples. Zeolites are an exception they are crystalline materials but nevertheless exhibit high (micro) porosity. Zeolites belong to the class of molecular sieves, which are porous solids with pores of molecular dimensions, i.e., typically the pore diameter ranges from 0.3 to 10 nm. Examples of molecular sieves are carbons, oxides and zeolites. 

Attempted catalysis. A number of experiments were carried out to test the possible catalytic activity of substances such as potassium carbonate, cupric carbonate, ammonium chloride, ammonium sulfate, potassium silicate, boron phosphate, and silica gel, but in no case was there any indication that the reaction could be catalyzed. 

Krocher, O., Koppel, R.A., Froba, M. and Baiker, A. (1998) Silica hybrid gel catalysts containing group(VIII) transition metal complexes preparation, structural, and catalytic properties in the synthesis of N, N-dimethylformamide and methyl formate from supercritical carbon dioxide. Journal of Catalysis, 178, 284-298. 

Fig 6 shows the single-stage system, which is referred to as plasma-driven catalysis [77]. In the PDC process, catalysts arc directly placed in the NTP reactor. These catalysts arc activated by NTP at low temperature region, where the thermal catalysis docs not occur. The shape of catalyst is cither of honeycomb, foam or pellet. In contrast to the PEC system, all reactions of gas-phase, surface and their interaction lake place simultaneously. In this sense, it is quite complicate to understand and optimize the chemical reactions in the PDC system. In an early USA patent, Henis proposed a PDC reactor for NO.r removal. Figure 7 shows the schematic diagram of the PDC reactor proposed by Henis [78], which is quite similar to those used in recent studies. The gases arc introduced to the reaction zone through the contact materials for heat transfer purpose. The catalysts listed in the patent are alumina, zirconium silicate, cobalt oxide, Thoria, activated carbon, molecular sieves, silica gel etc. 

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