Takaomi Suzuki

Characterization of porous carbons with high resolution αs-analysis and low temperature magnetic susceptibility

Abstract The effectiveness of integrated characterization of porous carbons with different approaches such as the high resolution α s -method for N 2 adsorption, X-ray diffraction, and magnetic susceptibility measurement is shown. The theoretical basis for the high resolution α s -analysis for N 2 adsorption using GCMC simulation and the background of magnetic susceptibility of carbon are given. The applicability of the integrated characterization to changes of ACF with heating at 1473–3173 K in Ar is shown. The activated mesocarbon microbead (a-MCMB) and activated carbon aerogel (a-CA) were examined by the integrated characterization method. a-MCMB showed an unusual magnetic relaxation below 10 K due to random magnetism. The random magnetism was associated with the mutually isolated network structure of nanographitic units. a-CA also showed random magnetism at low temperature, which was associated with the spin isolated nanographitic structure similar to a-MCMB.

Intrapore field‐dependent micropore filling of supercritical N2 in slit‐shaped micropores

The micropore filling of supercritical N2 on micrographitic carbon fibers having slit‐shaped micropores of different micropore widths from 0.7 to 1.4 nm was examined at 303 K under the high pressure of N2 up to 10 MPa. Supercritical N2 in the micropore was presumed to be transformed into quasivapor by strong micropore field; the Dubinin–Radushkevich equation for a vapor was extended to the quasivapor in the micropore. The determination method of the saturated vapor pressure Psq of the quasi‐N2 vapor in the micropore was proposed and was applied to the experimental results on N2/micrographitic carbon fiber systems; the determined Psq increases with the micropore width. The relationship between the extent of micropore filling of a supercritical gas through the quasivapor state and the interaction energy between a molecule and two parallel micrographites from the Lennard‐Jones potential was theoretically derived, which was evidenced by the simple experimental relationship between the amount of high pressure ...

N2 formation from NO over metal oxide-dispersed microporous carbon fibers

Activated carbon fibers (ACFs) were modified with various kinds of transition metal hydroxides. The surface modified ACFs were characterized by EXAFS. The NO reduction over the surface modified ACFs, which is not poisoned by SO2, was examined. Almost all NO (80%) was converted to N2 at 473 Kover α-FeOOH-dispersed ACF.

Ultramicropores in microporous carbon fibres evidenced by helium adsorption at 4.2 K

The adsorption isotherms of He on activated carbon fibres at 4.2 K have been determined gravimetrically. The He adsorption isotherms were of typical type I and were analysed by the Dubinin–Radushkevich plots, giving the micropore volumes which are greater than those obtained from the N2 adsorption isotherm at 77 K by 20–50%. The excess amount of He adsorption was ascribed to the presence of ultramicropores which cannot be assessed by N2 molecules.

Size evaluation of graphitic crystallites in activated carbon fibers from diamagnetic susceptibility measurements

Abstract Diamagnetic susceptibilities of various kinds of activated carbon fibers (ACFs), nonporous carbon black, and Grafoil were measured under an N 2 atmosphere in the temperature range of 273 to 423 K. An empirical relationship between the diamagnetic susceptibility and molecular structure was derived from the literature for polynuclear aromatic compounds without hydrogen atoms. The unit size of graphitic crystallites in ACFs was evaluated from the derived relationship.

Preformed monolayer-induced filling of molecules in micropores

Abstract The effect of the preformed monolayer on second layer adsorption of N2 molecules in a graphite slit micropore of pore width=1.04−1.24 nm at 77 K was studied using GCMC simulation. The predominant potential comes from the interaction of a molecule with the preformed monolayer and is more than 70% of the total molecular potential. Therefore, the second stage filling after monolayer adsorption on the micropore walls should be called cooperative filling which was suggested experimentally by Sing et al.

Structural Characterization of Heat-Treated Activated Carbon Fibers

Pitch (PIT) and cellulose (CEL)-based activated carbon fibers (ACFs) were heated at 1473–3173 K under an Ar atmosphere. The N2 adsorption, X-ray diffraction (XRD), and magnetic susceptibility of the heat-treated ACFs were measured. The specific surface area of ACF samples was determined by the subtracting pore effect (SPE) analysis using the N2 adsorption isotherm.Both stacking height, Lc and stacking width, La of ACFs began to increase remarkably above 2000 K. The amounts of N2 adsorption on PIT and CEL became nil by heating above 1773 K and 2073 K, respectively. The relationships between the heating temperature and the magnetic susceptibility of ACFs near room temperature were divided into three regions: below 1773 K, from 1773 K to 2473 K, and above 2473 K.The results from gas adsorption, X-ray diffraction and magnetic susceptibilities have been explained with respect to the changes taking place in these three regions.

Surface fractal dimensional change of microporous carbon fibers with partial graphitization

Abstract The surface fractal dimension D a of the micropore walls of activated carbon fibers (ACFs) preheated at 1573 and 1773 K in an Ar atmosphere was determined by precision organic-vapor adsorption measurements. Preheating of ACF caused partial graphitization as demonstrated by X-ray diffraction examination. The D a values of the original cellulose-based ACF and nonporous carbon black were 2.9 and 2.0, respectively. The D a of ACF heated at 1773 K decreased to 2.3, indicating that the micropore wall becomes flatter and approaches a graphitic surface.

The structural change of graphitization-controlled microporous carbon upon adsorption of H2O and N2

Abstract The partial graphitization of activated carbon fibers (ACFs) was controlled by heating at 1773–3173 K in Ar. The growth of graphitic crystallites was examined by high-resolution transmission electron microscopy and X-ray diffraction. The adsorption processes of H2O and N2 on the graphitization-controlled ACFs were investigated by use of the in situ X-ray diffraction. The adsorption of H2O at 303 K and N2 at 165 K led to shrinkage of the interlayer spacing, d002, between the graphitic carbon layers. The shrinkage of more graphitized ACF is less remarkable than that of as-received ACF. The d002 value decreased hardly until 40% of the fractional filling of H2O, then it lowered markedly with the fractional filling. On the other hand, only less than 5% of the fractional filling decreased seriously the d002 value in the case of N2 adsorption. The relationship between the molecular adsorption and the graphitic structural change was discussed on the basis of the adsorption process; N2 molecules are at first adsorbed in the deep sites of the distorted slit-shaped micropores, while H2O molecules are firstly adsorbed near the entrance of the micropore with hydrophilic functional groups.

Micropore filling of supercritical NO on Cu-doped iron oxide dispersed activated carbon fibers

Abstract Cu-doped α-FeOOH dispersed activated carbon fibers were prepared and their NO adsorptivities were examined to elucidate the role of an electronic factor of the dispersed oxide in the micropore filling of supercritical NO. The nitrogen adsorption isotherms of the samples were determined and analyzed by t and Dubinin-Radushkevich plots; doping Cu2+ ions did not change significantly the microporosity of α-FeOOH dispersed activated carbon fibers. The dispersed oxides were characterized by EXAFS spectroscopy; the Cu-doped oxide on the activated carbon fibers have a local structure similar to that of α-FeOOH. The effects on NO micropore filling of doping with Cu2+ ions and preheating Cu-doped samples were examined. Addition of 1% Cu2+ ions enhanced most effectively the NO micropore filling and preheating the 3% Cu-doped α-FeOOH dispersed activated carbon fibers at 773 K enhanced remarkably the NO micropore filling. The enhancement effects on NO micropore filling of Cu doping were presumed to be associated with the defect structural change in the dispersed α-FeOOH.