Adsorption of water vapor and microporous structures of carbonaceous adsorbents Communication 10. The effect of preliminary preparation conditions and the modification of activated charcoals on their adsorption properties

Conclusions

• 1. Preliminary desorption under vacuum of activated charcoals at an elevated temperature affects their adsorption capacity for water vapor. The greater the loss of adsorbent sample mass as a result of heating, the more significantly the water-vapor adsorption isotherms change.

• 2. In the adsorption of water vapor by activated charcoals heated at 673 K, chemisorption of water molecules proceeds simultaneously with physical adsorption. The greater the extent of rehydration of the activated charcoals, the greater the extent to which their capacity for adsorbing water vapor is regenerated. The results obtained indicate that chemisorbed water serves as primary adsorption sites in the physical adsorption of water vapor.

• 3. Demineralization of the activated charcoals leads to an increase in the total number of primary adsorption sites, basically by increasing the number of labile surface oxygen-containing groups (in particular, chemisorbed water) that are removed from the surface of the activated charcoal even upon moderate heating.

• 4. Oxidation of the activated charcoals in a stream of moist air at 623 K increases the concentration of relatively thermally stable adsorption sites, the numbers of which correlate with the number of acid surface groups neutralized by Na₂CO₂, by reducing the number of unstable oxygen-containing groups while leaving practically unchanged the number of primary adsorption sites.