Hajime Tamon

Porous structure of organic and carbon aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde

Abstract Organic aerogels were synthesized via the sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and followed by supercritical drying with carbon dioxide. As a result of characterization by nitrogen adsorption, the resorcinol-formaldehyde (RF) aerogels were mesoporous materials with high surface areas and had few micropores. The surface area of the RF aerogel was controlled by the mole ratio of resorcinol to basic catalyst ( R C ). On the other hand, the mesopore volume of RF aerogel changed greatly with R C or the ratio of resorcinol to water ( R W ). The RF aerogel having a monodisperse porous structure was prepared under the conditions of low R C or high R W . As R C increased or R W decreased, the porous structure of the aerogel became dispersed. Carbon aerogels were obtained by pyrolyzing the RF aerogels at 1223 K, and the aerogel had larger surface areas than the RF aerogels. It was found that micropores in the carbon aerogels were formed during pyrolysis of the RF aerogels. When the RF aerogel having a monodisperse porous structure was pyrolyzed, the mesopore volume of the carbon aerogel was smaller than that of the original RF aerogel. The mesopore volume increased by the pyrolysis of the RF aerogel having a dispersed porous structure.

Preparation of mesoporous carbon by freeze drying

Abstract Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t -butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm 3 /g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m 2 /g and large mesopore volumes >0.55 cm 3 /g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.

Preparation and characterization of carbon cryogel microspheres

Abstract Carbon cryogel microspheres (CC microspheres) were successfully synthesized by an inverse emulsion polymerization of resorcinol with formaldehyde, followed by freeze drying and pyrolysis in an inert atmosphere. CC microspheres were characterized by scanning electron microscopy, elemental analysis and various gas adsorption measurements. By changing both the temperature for preparing the emulsion and pyrolysis temperature, it was possible to prepare both mesoporous microspheres and microspheres covered with ultramicroporous surfaces which pore sizes were smaller than the minimum molecular dimensions of ethane or carbon dioxide. Hydrophobicity of the obtained CC microspheres increased with the increase in pyrolysis temperature. The possibility of using the obtained mesoporous CC microspheres as column packing materials for high-performance liquid chromatography was also shown.

Influence of acidic surface oxides of activated carbon on gas adsorption characteristics

Abstract An activated carbon was oxidized by HNO3 at boiling temperature. The influence of acidic surface oxides of the activated carbon was experimentally studied on the adsorption characteristics of eleven different gases or vapors. In the adsorption of cyclohexane, benzene, 2-propanol and 2-butanol, the adsorption capacity decreased greatly with oxidizing the carbon by 13.2 N HNO3. This was because the surface area and micropore volume had decreased by the strong oxidation as suggested by the t-plot analysis of oxidized carbons. When methanol, ethanol, acetone, acetonitrile and sulfur dioxide were adsorbed on the carbons, it was found that the isotherms on the carbon oxidized by 13.2 N HNO3 were much lower than those on the original carbon except in the low partial pressure range. On the other hand, the adsorption capacity of ammonia or water increased greatly with increasing surface oxides on the carbon. Especially, ammonia was strongly adsorbed on the surface oxides, and irreversible adsorption appeared. The experimental results suggest that the adsorption sites increase greatly with the surface oxides for the polar molecules whose polarizability is very small.

Activated carbon from municipal waste

Abstract A refuse derived fuel (RDF) was carbonized by partial combustion at 623 K and the carbonized RDF (cRDF) was steam-activated at 1123 K. The cRDF was also treated by 3.3 or 5.2 N nitric acid at a boiling temperature for 3 h prior to the steam-activation. Porous properties of the activated carbons prepared were determined by the nitrogen adsorption method. The porous structure of the carbon prepared without the pretreatment of nitric acid was not so developed as a commercial carbon for dioxin adsorption. On the other hand, the carbons prepared via the nitric acid treatment had BET surface areas of 515 and 828 m2/g and more mesopore volumes than the commercial carbon. During the acid treatment, the ash dissolution from cRDFs to nitric acid was observed and resulted in the formation of activation sites for developing mesopores. Hence, the carbons prepared could be used to remove dioxins from the incinerators.

Formation of monolithic silica gel microhoneycombs (SMHs) using pseudosteady state growth of microstructural ice crystals

Monolithic silica gel microhoneycombs, which have an array of straight macropores within their structure and developed micro/mesopores inside their walls, were prepared using pseudosteady state growth of ice crystals which occurs during the unidirectional freeze-gelation of freshly gelled aqueous silica hydrogels, followed by a pore-protecting drying method, freeze drying.

Porous properties of silica gels with controlled morphology synthesized by unidirectional freeze-gelation

Abstract In this work, the unidirectional freezing method was applied to a wide variety of silica gel precursors; from silica sols which do not readily gel, to thoroughly aged silica hydrogels. It was found that in addition to the well-known structure of fibers with polygonal cross-sections which are commonly obtained in the unidirectional freezing method, porous silica gels having unique morphologies such as honeycomb, lamellar and flat fiber structures could also be obtained by changing the state of the precursor sol or gel. The obtained silica hydrogels were freeze-dried after exchanging the water included in its structure to t -butanol, and finally dry samples maintaining their wet state structures were obtained. The morphology and the porous properties of the obtained silica gels were systematically analyzed and the influences of preparation conditions, including pH, aging time before freezing, SiO 2 concentration, freezing temperature, and storage time at the frozen state on both factors were examined in detail. It was found that the simultaneous controlling of both factors could be easily conducted by simply adjusting preparation conditions.

Gas adsorption on activated carbons from PET mixtures with a metal salt

Abstract Activated carbons were prepared from carbonized PET by steam activation via pretreatment by mixing PET with a metal salt [Ca(NO 3 ) 2 ·4H 2 O, Ca(OH) 2 , CaCO 3 , ZnO, and AlNH 4 (SO 4 ) 2 ·12H 2 O], and with acid treatment after carbonization. The porous properties of the activated carbons were determined by the nitrogen adsorption method. The adsorption isotherms of CO 2 , C 2 H 6 , nC 4 H 10 and iC 4 H 10 at 298 K on the prepared activated carbons were measured to determine practical applications and to obtain a better understanding of the porous structure of the prepared carbons. Steam-activated carbons via pretreatment have a larger mesoporosity than carbons with no pretreatment. The metal salt used in the pretreatment for steam activation has no influence on the microporous structure, but it does influence the mesoporous structure of the prepared carbons. Activated carbons prepared via pretreatment show a large adsorption capacity for nC 4 H 10 and iC 4 H 10 . These carbons are suitable as adsorbents for canisters, etc. Application of the potential theory to adsorption data for the prepared carbons suggests that the pretreatment contributes to the formation of pores larger than 0.50 nm at high burnoff.

Preparation of monolithic SiO2–Al2O3 cryogels with inter-connected macropores through ice templating

Macroporous monoliths of SiO2–Al2O3 cryogels were prepared. Macropores were generated by using ice crystals as the template, while the walls which surround the macropores were tailored as porous cryogels by freeze drying. Macropores and walls formed honeycomb-like structures, which were confirmed from scanning electron microscopy images of cross-sections of the samples. It was confirmed that the sizes of the macropores and the wall thicknesses were respectively in the ranges of 10–20 µm and 200–500 nm. Al mapping analysis by energy dispersive X-ray diffractometry showed that Al atoms were homogeneously dispersed throughout the samples without local aggregation. Moreover, Raman spectroscopy and 27Al NMR spectroscopy indicated that Al atoms were incorporated into the silica framework by forming an Al–O–Si polymeric network. Nitrogen adsorption–desorption measurements indicated that the walls were micro/mesoporous with high BET surface areas (>700 m2 g−1) and large pore volumes (>0.45 cm3 g−1). Moreover, NH3-TPD measurements revealed that the samples had acid sites, which allowed this material to be used as a solid acid catalyst.

A theoretical study on storage states of Li ions in carbon anodes of Li ion batteries using molecular orbital calculations

Abstract Semi-empirical molecular orbital calculations were carried out to clarify storage states of Li ions in amorphous carbon anodes of Li ion batteries. Storage states of Li ions between two graphene sheets were investigated and a favorable structure for a carbon anode to produce large reversible and small irreversible capacities is discussed. A polycyclic hydrocarbon molecule, C 54 H 18 , was used as a model of a graphene sheet. Relations between the interlayer distance of two graphene sheets and the storage state of Li ions were investigated, and preferable interlayer distances for specific numbers of Li ions were estimated. In particular, storage states with all Li ions on the basal area were treated, because the amount of basal carbons should be larger than that of edge carbons. The charge distribution of Li ions was also investigated. Calculated results suggested that a storage state in which a double Li ion layer was formed was preferable to achieve a larger capacity than the theoretical maximum capacity of graphitic carbons (372 mAh/g) and to reduce hysteresis in the charge–discharge process. Moreover, suitable distance between edges of graphene sheets to prevent the intercalation of electrolyte species was discussed. A recommended structure of carbon anodes suitable for the double Li ion layer storage and prevention of the intercalation of electrolyte species is proposed.