Adsorption–desorption of water vapour on chars prepared from commercial wood charcoals, in relation to their chemical composition, surface chemistry and pore structure

We have studied the chemical composition, the surface chemistry and the pore structure of holm-oak chars and eucalyptus chars prepared at final heating temperature in the range of 250–1000 °C from two commercial wood charcoals. Our study also includes adsorption–desorption of water vapour at 25 °C on the resulting chars. The samples were characterised by chemical analyses, Fourier transform infrared spectroscopy, adsorption (carbon dioxide, 273 K), mercury porosimetry and scanning electron microscopy. The Dubinin–Serpinsky equation (DS-2) and the Dubinin–Astakhov (DA) equation were applied to analyse the water adsorption data. The final heating temperature affects the chemical composition, the surface chemistry and the pore structure of each char series differently; the differences are of less significance at the highest temperature. The adsorption–desorption of water vapour at 25 °C reflects well the changes produced in the chars; the adsorption occurs in the micropores, and in the non-micropore structure at relative pressures higher than 0.95. The shape of the adsorption isotherms is largely dependent on the microporosity; it changes gradually from type II to type V for the chars with poorly developed microporosity (holm-oak chars), and it is more of type V for the chars with highly developed microporosity (eucalyptus chars). The DS-2 equation fails if type V is less defined, while the DA equation is always applicable and provides micropore volumes close to those determined at p/p0 = 0.95. Hysteresis is always observed; chemisorption occurs: the desorption line begins at saturation pressure and in general tends to meet near zero relative pressures.