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1 2008

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Mezoporowate węgle : synteza i właściwości

Choma J., Jaroniec M., Zawiślak A.

Wiadomości Chemiczne

2008

[Z] 62, 5-6

373-402

Porous carbonaceous materials are widely used in everyday life and industry because they possess very high surface area, pore volume and unique physicochemical properties, including high adsorption capacity for many organic molecules. Porous carbons have been prepared for hundreds of years, however traditional methods used for their preparation allow rather for a limited control of pore sizes and volumes of micropores and mesopores, which in fact results in a broad distribution of pore sizes. The most common applications of porous carbons include adsorption, catalysis, gas storage, purification of air and water and electrochemical applications [1,2]. This article presents a survey of literature devoted to new methods of synthesis and characterization of mesoporous carbonaceous materials. One of the major topics reviewed in this article is the synthesis of ordered mesoporous carbons (OMC) with a help of ordered mesoporous silicas (OMS) (Fig. 1) [7, 25], colloidal silica (Fig. 2 and 3) [11, 29, 40, 59] and colloidal silica crystals as hard templates [57]. In addition to the synthesis of OMC, this paper presents an overview of physicochemical properties of OMC, especially adsorption properties. The exemplary characteristics, which include BET surface area, pore volume and pore size distribution, are provided for selected carbon materials [29]. A special emphasis was placed on the method, which employs monolithic siliceous templates obtained by pressing colloidal silica followed by their impregnation with oxalic acid (catalyst), resorcinol and formaldehyde (carbon precursors), polymerization of carbon precursors, carbonization and template removal (Fig. 4) [60]. This method affords carbons with uniform spherical mesopores as well as carbon composites with inorganic nanoparticles (e.g., silver), which after additional activation (e.g., with KOH) give micro and mesoporous carbons with superior structural characteristics. A set of basic parameters for one of these carbons includes: the diameter of spherical pores of 26,8 nm, the BET surface area of 1300 m2/g and the single-point (total) pore volume of 4,3 cm3/g; however, only 0,19 cm3/g of the total pore volume belongs to micropores. After KOH activation the volume of micropores increased to 0,8 cm3/g, the BET surface area increased to 2300 m2/g, but the volume of mesopores was reduced to 1,8 cm3/g [60]. Another important topic reviewed in this article is the synthesis of mesoporous carbons from thermosetting polymers (e.g., phenolic resins) used as carbon precursors in the presence of block copolymers used as soft templates (Fig. 5) [29, 78, 79]. This new synthetic strategy affords mesoporous carbons in the form of monoliths, fibers and films and permits to scale up this process. Additional activation of the resulting carbons with KOH (Fig. 7) affords micro and mesoporous carbons with high surface area and large volumes of both types of pores, improving significantly their adsorption properties. A set of basic parameters for one of the soft-templated carbons includes: the BET surface area of about 500 m2/g, the total pore volume of 0,7 cm3/g and the pore width of about 9,7 nm. After KOH activation the BET surface area increased to about 900 m2/g, the total pore volume and the volume of micropores increased to 0,9 cm3/g and 0,22 cm3/g, respectively [79]. Novel ordered mesoporous carbons are not only attractive materials for various advanced applications but also for the development of accurate methods for characterization of porous solids. This article shows that in addition to such important characterization methods as transmission electron microscopy (TEM), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD), adsorption of nitrogen, argon and other adsorbates (Fig. 8) is one of the key methods for the pore structure analysis, especially for the assessment of global properties of porous materials.