Toshihide Horikawa

Preparing activated carbon from various nutshells by chemical activation with K2CO3

We have prepared activated carbons by chemical activation with K2CO3 from five kinds of nutshells: almond shell (AM), coconut shell (CN), oil palm shell (OP), pistachio shell (PT) and walnut shell (WN). When prepared at 1073 K, the activated carbons from all the nutshells had the maximum specific surface areas. According to the maximum values of specific surface areas, the activated carbons prepared were classified into two groups: Group-L and Group-S; the former group included activated carbons with high specific surface area and the latter included those with lower specific surface area, respectively. It was found that K2CO3 effectively worked as an activation reagent, but differently in the temperature ranges below 800 and above 900 K. Due to impregnation, cellulose and hemi-cellulose were modified by K2CO3 and accordingly the weight loss behaviors of the nutshells were changed in the temperature range below 800 K. In the temperature range above 900 K, carbon in the chars was removed as CO gas by the reduction of K2CO3 to increase the specific surface area and the pore volume. It was deduced that the difference between the specific surface areas of Group-L and those of Group-S correspond to the difference between weight loss behaviors in the temperature range above 900 K.

Capillary condensation of adsorbates in porous materials.

Hysteresis in capillary condensation is important for the fundamental study and application of porous materials, and yet experiments on porous materials are sometimes difficult to interpret because of the many interactions and complex solid structures involved in the condensation and evaporation processes. Here we make an overview of the significant progress in understanding capillary condensation and hysteresis phenomena in mesopores that have followed from experiment and simulation applied to highly ordered mesoporous materials such as MCM-41 and SBA-15 over the last few decades.

Activated carbon from chickpea husk by chemical activation with K2CO3: preparation and characterization

Activated carbon was prepared from chickpea husk by chemical activation with K2CO3. At 1073 K, the specific surface area of activated carbon prepared with an impregnation ratio of 1.0 yielded the maximum value of 1778 m2/g. From the results of the yield of the activated carbon and the reagent recovery ratio, it was concluded that the carbon involved in the husk char was removed as CO by reduction of K2CO3 above 1000 K. The fractal dimension changed slightly between 773 and 973 K, and it decreased rapidly between 973 and 1173 K. It was deduced that this decrease of the fractal dimension was due to the decomposition of the cross-linked structure and the small crystallite structure. The micropore volume and the specific surface area increased by the release of plugged pore due to the decomposition of the cross-linked structure. It was further deduced that the mesopore volume increased and the micropore volume decreased by combination of micropores due to the decomposition of small crystallites.

Synthesizing activated carbons from resins by chemical activation with K2CO3

We prepared activated carbons from phenol–formaldehyde (PF) and urea–formaldehyde (UF) resins by chemical activation with K2CO3 with impregnation during the synthesis of the resins. The influence of carbonization temperature (773–1173 K) on the pore structure (specific surface area and pore volume) and the temperature range at which K2CO3 worked effectively as an activation reagent, were investigated. The specific surface area and micropore volume of PF–AC and UF–AC increased with an increase of carbonization temperature in the range of 773–1173 K. We prepared activated carbon with well-developed micropores from PF, and activated carbon with high specific surface area (>3000 m2/g) and large meso-pore volume from UF. We deduced the activation mechanism with thermogravimetry and X-ray diffraction. In preparing activated carbon from PF, K2CO3 was reduced by carbon in the PF char. The carbon was removed as CO gas resulting in increased specific surface area and pore volume above 1000 K. In preparing AC from UF, above 900 K the carbon in UF char was consumed during the K2CO3 reduction step.

Characteristics and humidity control capacity of activated carbon from bamboo.

Activated carbons were prepared from bamboo by chemical activation with K2CO3 or physical activation with CO2. The structural and surface chemical characteristics of the activated carbons were determined by N2 adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the activated carbons with various pore structures (preparation conditions) were examined. The relationship between water vapor adsorption capacity and pore properties, and the humidity control capacity of the prepared activated carbons are also discussed. The water adsorption isotherms show a region of rapidly increasing uptake of water vapor, and the relative humidity corresponding to those regions was different according to the preparation conditions, especially activation temperature. Water vapor adsorption capacity was improved with larger pore volume and surface area, but the humidity control capacity in a certain specific humidity region differed greatly according to the relative humidity corresponding to the steeply rising regions of the isotherms. In the typical operating conditions of an adsorption heat pump, RH 10-35%, the bamboo-sourced activated carbon that was prepared at 873K by potassium carbonate activation with impregnation ratio 1.0 had the highest humidity control capacity.

Preparation and characterization of the carbonized material of phenol–formaldehyde resin with addition of various organic substances

Abstract We prepared carbonized materials of phenol–formaldehyde resins (PF) synthesized with the addition of organic substances such as ethylene glycol (EG), 1,6-hexanediol (1,6HD), polyethylene glycol (PEG), etc. We investigated the influence of the carbonization temperature, the organic additive species, and the additive ratio in synthesizing the PF resins on the pore structure of the carbonized PFs. Variation in the length of the added organic substance caused differences in the pore sizes of the carbonized PFs; when the length was short, the pore size of the carbonized PF became gradually smaller with increasing carbonization temperature, but when the length was long, the pore size gradually increased with increasing carbonization temperature. The difference in the additive organic species gave different pore sizes in the carbonized PFs such that the longer the length of the organic additive the greater the pore size in the carbonized PF. The additive ratio of EG did not give a large change in the pore size of the carbonized PF, but the variance in the average pore sizes were sharper than that of the carbonized PF without any additives. On the other hand, the pore sizes of the carbonized PFs with a high additive ratio of PEG20K were greater, and the carbonized PFs had mesopores. The BET surface area of the carbonized PFs with a 50% additive ratio of PEG20K was about 500 m2/g.

Preparation of molecular sieving carbon from waste resin by chemical vapor deposition

Abstract Molecular sieving carbons (MSCs) were prepared from carbonized phenol–formaldehyde resin wastes by the chemical vapor deposition (CVD) of the pyrolyzed carbon from hydrocarbon species. The pore size of the MSCs could be controlled in the range 0.37–0.42 nm by changing the hydrocarbon species pyrolyzed, the pyrolyzing temperature, and the processing time. It is shown that some of the MSCs have an excellent selectivity for separating CO2 and CH4, and others for separating C3H8 and C3H6. As the mechanism for controlling the pore size during CVD processing, we elucidated that the adsorption of hydrocarbon molecules first takes place on the pore surface and then the adsorbed hydrocarbons pyrolyze into carbon. Therefore, the pore size of the MSC can be adjusted by controlling the amount hydrocarbon adsorbed on the phenol–formaldehyde resin char.

On the isosteric heat of adsorption of non-polar and polar fluids on highly graphitized carbon black

Isosteric heat of adsorption is indispensable in probing the energetic behavior of interaction between adsorbate and solid, and it can shed insight into how molecules interact with a solid by studying the dependence of isosteric heat on loading. In this study, we illustrated how this can be used to explain the difference between adsorption of non-polar (and weakly polar) fluids and strong polar fluids on a highly graphitized carbon black, Carbopack F. This carbon black has a very small quantity of functional group, and interestingly we showed that no matter how small it is the analysis of the isosteric heat versus loading can identify its presence and how it affects the way polar molecules adsorb. We used argon and nitrogen as representatives of non-polar fluid and weakly polar fluid, and methanol and water for strong polar fluid. The pattern of the isosteric heat versus loading can be regarded as a fingerprint to determine the mechanism of adsorption for strong polar fluids, which is very distinct from that for non-polar fluids. This also allows us to estimate the interplay between the various interactions: fluid-fluid, fluid-basal plane and fluid-functional group.

The interplay between molecular layering and clustering in adsorption of gases on graphitized thermal carbon black--spill-over phenomenon and the important role of strong sites.

We analyse in detail our experimental data, our simulation results and data from the literature, for the adsorption of argon, nitrogen, carbon dioxide, methanol, ammonia and water on graphitized carbon black (GTCB), and show that there are two mechanisms of adsorption at play, and that their interplay governs how different gases adsorb on the surface by either: (1) molecular layering on the basal plane or (2) clustering around very strong sites on the adsorbent whose affinity is much greater than that of the basal plane or the functional groups. Depending on the concentration of the very strong sites or the functional groups, the temperature and the relative strength of the three interactions, (a) fluid-strong sites (fine crevices and functional group) (F-SS), (b) fluid-basal plane (FB) and (c) fluid-fluid (FF), the uptake of adsorbate tends to be dominated by one mechanism. However, there are conditions (temperature and adsorbate) where two mechanisms can both govern the uptake. For simple gases, like argon, nitrogen and carbon dioxide, adsorption proceeds by molecular layering on the basal plane of graphene, but for water which represents an extreme case of a polar molecule, clustering around the strong sites or the functional groups at the edges of the graphene layers is the major mechanism of adsorption and there is little or no adsorption on the basal planes because the F-SS and FF interactions are far stronger than the FB interaction. For adsorptives with lower polarity, exemplified by methanol or ammonia, the adsorption mechanism switches from clustering to layering in the order: ammonia, methanol; and we suggest that the bridging between these two mechanisms is a molecular spill-over phenomenon, which has not been previously proposed in the literature in the context of physical adsorption.

Characterization of palladium and palladium-silver alloy layers on stainless steel support

Pd and Pd/Ag layers were prepared by a technique of electroless plating on stainless steel supports. To form Pd/Ag layers, Pd layer was plated on an activated stainless steel (SS) sheet followed by Ag plating. The Pd/Ag-SS sheet composite was annealed at 873 K-973 K for 10 h -12 h in helium. Before annealing, the thickness of Pd layer and Ag layer on Pd/Ag-SS sheet composite was 7.6 μm and 1.9 μm, respectively. The formation of Pd/Ag alloy was observed on the SS sheet after annealing at 873 K for 12 h in helium. The ratios of Ag/(Pd+Ag) were 0.28 on the surface by XRF and 0.29 in the bulk by XPS. This result showed the formation of homogeneous Pd/Ag alloy on the SS sheet at that annealing condition. Helium permeances were measured at a pressure difference of 1 atm and room temperature. The permeances of porous stainless steel (PSS) tubes without and with 78 μm Pd layer were 100 m3/m2h and 0.35 m3/m2h, respectively. This result showed the obtained membrane was an almost dense membrane.