Pore structure of chars and activated carbons prepared using carbon dioxide at different temperatures from extracted rockrose

This paper discusses the pore structure of chars and activated carbons prep ared at different temperatures from rockrose (Cistus ladaniferus L.), extra cted previously into petroleum ether. The isothermal temperature of carboni zation in nitrogen ranged from 600 to 1000 degreesC. The starting char for activated carbons was prepared by treating a larger amount of precursor in the atmosphere formed as temperature increased from 30-600 degreesC, at 10 degreesC min(-1), being the total heating time 120 min. This char was heate d in nitrogen before activation, which was carried out using carbon dioxide at 700-950 degreesC to 40% burn-off. Pycnometry (Hg, He), adsorption (N-2, 77 K), mercury porosimetry and scanning electron microscopy techniques hav e been applied to the characterization. In the chars prepared in nitrogen, a shrinkage of the carbon structure is responsible for the pore narrowing i n all the pore ranges, including a micropore closing above 800 degreesC, wh ich is attributed to the disappearance of ether groups. This shrinkage is l ess important in comparison with that occuring in chars prepared from rockr ose without extraction. The starting char of the activated carbons presents a rudimentary pore structure due to the different conditions of its prepar ation. In the activated carbons, the pore volumes (micro, meso and macro) i ncrease up to 750 degreesC. At higher temperatures, the mesopore volume inc reases, whereas the micro- and macropore volumes decrease. These structural changes are discussed considering the starting char as a Ca-supported cata lyst. A shrinkage of the carbon structure also occurs at high temperatures, without causing micropore closing. (C) 2001 Elsevier Science B.V. All righ ts reserved.