The influence of the cooling down step in the heat treatment on the stability of activated carbons hydrophobicity

Two samples of an activated carbon are heat treated at 500 °C for 2 h under a flow of inert gas. The only difference between the treatments of the two carbons is the cooling down step. After these treatments, the two carbons were hydrophobic and presented similar adsorption properties and an identical behavior toward water and cyclohexane uptakes. After being stored in ambient conditions for 20 months, the stability of oxygen functional groups is studied. The quantification of various oxygen groups is done by Boehm's titration and by thermogravimetry-mass spectroscopy analysis. It is found that the creation of oxygen groups, especially carboxylic acids, which are very attractive to water molecules, depends on the cooling down step. This is confirmed by both water isotherms and cyclohexane breakthrough measurements. Cyclohexane breakthrough times show that one of the heat treated carbons does not preserve its hydrophobic character compared to the other carbon, which presents a breakthrough time value close to that obtained before the storage.