Influence of coal preoxidation on the porosity of the activated carbons with steam activation

Activated carbons have been prepared from a low ash content anthracite preoxidized in air to different degrees. Steam has been used as activating agent to prepare different burn-off samples. The preoxidation effect on the physico-chemical characteristics of the resulting chars and activated carbons were comparatively studied. The surface area and porosity of sample was studied by N2 adsorption at 77 K. The results show that introduced oxygen in coal structure had a great influence on the carbonization and subsequent activation process. The carbonization of oxidized coal exhibited a broader volatile evolution with respect to temperature, and the resulting chars had a larger microporosity. The porosity of the char is a primary foundation to develop more microporosity upon activation. Activation of char from oxidized coal facilitated development of small scale micropore, however, the micropore widening was also observed at high burn-offs. Compared with development of supermicropore, the evolution of mesoporosity is hindered strongly by preoxidation treatment. The quantity of basic surface sites in activated carbons increased with an increase in oxidation degree, while the quantity of acidic sites appeared equivalent. It seemed that the amount of surface groups and the microporosity mainly developed in a parallel way.