Evolution of pore structures and fractal characteristics of coal-based activated carbon in steam activation based on nitrogen adsorption method

To investigate the structures and fractal characteristics of micro- and mesopores during steam activation, a series activated carbons were prepared from Datong bituminous coal by physical activation at 750 and 850 °C. Low-temperature nitrogen adsorption and Frenkel-Halsey-Hill model were employed to determine the fractal dimensions of pore structures. The results demonstrate that micro- and mesopore structures were fully developed during steam activation, and activation at 750 °C was conducive to the development of pore structures, especially mesopores. The samples had similar pore size distributions that the micropore diameters mainly centered at 0.4–0.8 nm and 1.2–1.8 nm, and the mesopores mainly distributed in the range of 2–9 nm. The variation of surface and structure fractal dimensions for activated carbons with different burn-offs indicated that the surface roughness and structure complexity changed during activation process. The fractal dimensions of micropore surface (D1) and micropore structure (D2) had positively linear correlation with Smic/SBET and Vmic/Vtot, respectively. While the mesopore surface fractal dimensions (D1’) have a negative association with Sext/SBET, and the structure fractal dimensions (D2’) have a negative but weak association with Vmes/Vtot.