Activated carbons with degrees of burn-off between 20% and 80% have been prepared from a semianthracite char by gasification in steam at 1123 K. The adsorption isotherms of nitrogen at 77 K and CO, at 273 K have been determined.
The pore size distribution has been calculated from t plots, and helium and mercury densities. The ungasified char has a narrow pore size distribution and mainly consists of very fine microporosity.
The pores have constrictions that do not allow adsorption of nitrogen, but the microporosity can be measured by the adsorption of CO, at 273 K. S,i, and CO, (D-R) surface areas of the ungasified char are about the same. The gasification of the char broadens the micropore distribution. The small microporosity calculated from t plots increases with burn-off between 20%-35%. Gasification to higher degrees of burn-off causes burning of the walls between the adjacent pores and eliminates pore constrictions.
This results in widening the micropores, creating meso and macroporosity and decreasing microporosity. When the degree of gasification is 80%, the small microporosity disappears almost completely, although the total porosity has been increased almost three times. While the nitrogen surface area increases with burn-off, the CO, surface area decreases at higher degrees of burn-off.