Sorption dynamics of N2 and O2 in carbon monoliths from spruce, beech and oak affected by activation

Three series of activated carbon monoliths have been prepared from logs of different trees (spruce (Picea abies), beech (Fagus silvatica), oak (Quercus robur)) by pyrolization in nitrogen flow at 700 °C and then by steam activation at 900 °C for different duration of the conversion. The characterization of carbon skeleton in the samples was performed by small angle X-ray scattering (SAXS), atomic force microscopy (AFM) and scanning electronmicrography (SEM). The carbonized xylem retained the original tracheal structure of the wood. Biological diversity of wood precursors reflected in the nanometer range likewise than in micrometer range. The evolution of the microporosity has been followed by static gas adsorption (N2 at −196 °C). The characterization of dynamic adsorption properties took advantage of the unique potentials of the frequency response (FR) technique (N2 or O2 at −78 °C). The obtained three-dimensional carbon frames exhibit low transport diffusional resistance due to the wide straight transport channels of micron size. All of the three wood types displayed nearly identical dependence of burn-off upon activation time, which in view of their similar cellulose composition, is not entirely unexpected. The shape of the isotherms indicates the presence of both micro and mesopores and the history of the activation is crucial for the developing porosity. The changes in the adsorption capacities of micropores reflect the differences of precursors and length of activation time. Compared to other methods FR technique seems to be more sensitive to distinguish such differences.