Characterisation of geometric and structural properties of pore surfaces of reactivated microporous carbons based upon image analysis and gas adsorption

Abstract

The geometric and structural properties of pore surfaces of microporous carbon specimens were investigated using high resolution transmission electron microscopy (HRTEM) and nitrogen gas adsorption method based upon the fractal theory. The carbon powder specimens with various pore structures were prepared by reactivation of commercially as-activated carbon powder at 1273 K in an atmosphere of a CO 2/CO gas mixture for different activation times. The surface fractal dimensions d F,surf of the carbon specimens were evaluated from the image analysis of the HRTEM images using perimeter–area method. In addition, the pore fractal dimensions d F,pore of the carbon specimens were determined from the analyses of nitrogen gas adsorption isotherms using non-linear fitting method based upon Dubinin–Astakhov adsorption theory in consideration of fractal pore size distribution. From the results of the image analyses of the HRTEM images, it was found that the surface fractal dimension d F,surf of the carbon specimens decreased in value with increasing degree of burn-off, which is ascribed to the more agglomeration of micropores by collapsing the pore walls between the pores. Moreover, from the quantitative coincidence of the nitrogen gas adsorption isotherms experimentally obtained with those theoretically calculated, it is suggested that the pore fractal dimension d F,pore of the carbon specimens is closely related to their porosity, i.e. as the pore fractal dimension d F,pore decreases, the relative volume fraction of the pores with larger size increases as well.