Effects of carbonization atmosphere and subsequent oxidation on pore structure of carbon spheres observed by scanning tu
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Effects of carbonization atmosphere and subsequent oxidation on pore structure of carbon spheres observed by scanning tunneling microscopy M. Inagaki,a) V. Vignal, and H. Konno Graduate School of Engineering, Hokkaido University, Kita-ku, Sapporo 060-8628, Japan
A.W. Morawski Institute of Inorganic Chemical Technology, Technical University of Szczecin, Szczecin, Poland (Received 7 January 1999; accepted 5 March 1999)
The surface of carbon spheres was studied by using field-emission scanning electron microscopy and scanning tunneling microscopy paying particular attention to the effects of atmosphere during carbonization and of subsequent oxidation on shape and size of the entrance of micropores. Commercial spheres of glasslike carbon prepared by carbonization of phenol resin spheres in either N2 or CO2 atmosphere were subjected to the oxidation by immersing into nitric acid and then heating in air at 400 °C. The size distribution of pore entrance at nanoscopic scale was determined from scanning tunneling microscope images. Carbon spheres prepared in CO2 atmosphere had predominantly ultramicropores, but those prepared in N2 had a very low porosity. The behavior during the oxidation process in air was found to be quite different on these two carbon spheres; spheres carbonized in N2 were oxidized heterogeneously, but those in CO2 showed homogeneous oxidation, giving a high density of ultramicropores.
I. INTRODUCTION
A number of studies on activated carbon have been performed by measuring adsorption-desorption isotherms and by calculating surface area and pore size distribution on the bases of gas adsorption from the Brunauer, Emmett, and Teller (BET) method, for example. This method, however, needs some assumptions about the surface structure, such as cylindrical pores, which are not always the case. Therefore, it is also important to study the surface structure of the activated carbons by other techniques, such as a direct observation of pore size and shape on their surfaces. The near-field microscopy [atomic force microscopy and scanning tunneling microscopy (STM)] seems to be the best technique for direct visualization of pores at a nanoscopic scale.1,2 In a series of our work,3–10 the spheres of glasslike carbon derived from a phenol resin have been used. Fundamental studies on carbonization and activation of these carbon spheres have been done.3–7 On the carbon spheres prepared from the same precursor spheres in an industry, an irreversible adsorption of gases, particularly CO2, was a)
Address all correspondence to this author. Present address: Aichi Institute of Technology, Department of Applied Chemistry, Yakusa, Toyota 470-0392, Japan.
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J. Mater. Res., Vol. 14, No. 7, Jul 1999 Downloaded: 20 Feb 2015
found and their adsorption/desorption behavior was studied by both volumetric and gravimetric techniques.3,5,6 With the same spheres, a new procedure for the analysis of shape and size distribution of pore entrance using STM wa
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