The importance of outgassing conditions when using water vapour to characterize activated carbons

The singular mechanism of water adsorption in carbon pores has been roughly elucidated due to the intense research conducted in this field in the last years. Thus, it has been demonstrated that the surface chemistry of the carbon material plays a crucial role in the initialization of the water sorption process. However, the potential changes induced in the surface properties, and therefore in the water isotherm, during the outgassing of the sample prior to the water sorption measurement, have not been addressed yet. On the contrary, the same preparation protocol used for the analysis of non-associating simple fluids (N₂, CO₂, Ar) is commonly followed. Aiming at filling this gap, two activated carbons with different textural and surface properties have been outgassed under different conditions of time and temperature and subsequently characterized by H₂O, N₂ and CO₂ isotherms and thermogravimetric analysis. The obtained results show that the optimal conditions of the outgassing treatment are not necessarily the same for all the adsorbates tested and will highly depend on the characteristics of the sample. In this regard, the recommended outgassing temperature for water sorption isotherms of nanoporous carbons seems to be lower than for other adsorbates and may not exceed 100 °C.