Evidence for the effect of the cooling down step on activated carbon adsorption properties

Two activated carbons are prepared by chemical activation of olive residue using zinc chloride. The first sample is cooled down slowly after the carbonization in the furnace under a flow of nitrogen, while the second is directly removed from the furnace just after the carbonization and put at ambient temperature and pressure. The textural and structural characteristics of the two samples are determined by nitrogen adsorption at -196 °C and X-ray diffraction (XRD). The oxygen functional groups are characterized via Boehm's titration and Fourier transformed infrared spectroscopy (FTIR). The cooling down step at different rates is followed by thermogravimetry-mass spectroscopy analysis (TG/MS). Slow cooling down of the sample after carbonization leads to CO₂, O₂, and CO consumption creating more active sites, which are responsible of the formation of surface oxygen groups, and especially carboxylic groups. The latter dramatically enhance water vapor adsorption at low relative humidity. The cyclohexane adsorption on activated carbons, which is carried out by thermogravimetric analysis (TGA), shows that the cooling down step has an effect on both the initial rate of adsorption and the adsorption uptake at equilibrium.