Quantitative assessment of pores in oxidized carbon spheres using scanning tunneling microscopy
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Quantitative assessment of pores in oxidized carbon spheres using scanning tunneling microscopy V. Vignal Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
A. W. Morawski Institute of Inorganic Chemical Technology, Technical University of Szczecin, ul. Pulaskiego 10, 70-322 Szczecin, Poland
H. Konno and M. Inagaki Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan (Received 12 May 1998; accepted 2 July 1998)
Surface heterogeneity, particularly shape and size of pores on the surface of activated carbon spheres were studied by using scanning tunneling microscopy (STM) and field-emission type scanning electron microscopy (FE-SEM). Spheres were carbonized either in N2 or CO2 atmosphere and oxidized ones were used as samples. A new numerical method based on the determination of contour maps from STM images was proposed in order to determine the size distribution in micropores. These results were discussed with respect to the adsorption of gas and liquid molecules. A good correlation between Brunaner, Emmett, and Teller (BET) surface area determined from adsorption isotherms of N2 at 77 K and the number of pores with the size of 0.5–1.8 nm was observed, indicating that the proposed procedure to analyze the pore size distribution is effective.
I. INTRODUCTION
Activated carbons are widely studied because of a great interest in gas and liquid filtration technology.1 Standard techniques used in the characterization of porosity are volumetric and gravimetric gas adsorption measurements,2,3 small-angle x-ray scattering,4 and transmission and scanning electron microscopies.5 Numerous works have been devoted to the influence of the precursor used and the preparation conditions (carbonization and activation). However, most of activated carbons have an amorphous structure and a wide distribution in pore size which is not explicitly determined by these measurements. The results also depend principally on the model of pores proposed in the analysis; for example, cylindrical pores are assumed in BET analysis. Pores are roughly classified into three groups according to their width: macropores (size more than 50 nm), mesopores (between 2 and 50 nm), and micropores (less than 2 nm). The former two classes have little interest from the viewpoint of the adsorption of various gaseous molecules. The last class constitutes the majority of an internal surface of an activated carbon, and they represent 90 to 95% of the total surface area. It has been noted6 that the enhancement of the interaction potential responsible for the micropore filling becomes insignificant when the pore width is larger than 2s (3s in the case of regular pores), s being the diameter of adsorbate molecules. This limit would appear to 1102
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 3, Mar 1999
Downloaded: 20 Feb 2015
restrict the micropore filling of small molecules (N2 , CO2 , . . . ) and low molecular weight hydrocarbons, such as methane and ethane
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