Water vapor adsorption of CaCl₂-impregnated activated carbon

Activated carbons (AC) impregnated with the hygroscopic salt CaCl ₂ were synthesized to enhance the amount of water vapor adsorbed at low relative pressure (P/P₀). Possible applications for these materials are in adsorption heat pumps and desiccant air conditioners. The ACs were synthesized from tissue paper by chemical activation with K ₂CO₃ and the porous properties were investigated of a number of materials prepared under a variety of activation conditions. The highest specific surface area (S_BET) (1820 m²/g) was obtained from the sample prepared with a K₂CO₃/sample ratio = 1/1 by mass, activated at 900 °C for 2 h under flowing N₂ (2 L/min). Impregnation with CaCl₂ was performed using 2.4-6.5 M CaCl₂ solutions and a solution volume equal to the pore volume of the AC. XPS surface chemical analysis revealed that the impregnated CaCl ₂ was mostly successfully impregnated into the pores while the surface CaCl₂ increased gradually with increasing amount of impregnation. The adsorption of water vapor in the P/P₀ range 0.1-0.3 (ΔW_0.1-0.3) was dramatically increased by CaCl₂ impregnation, the ΔW_0.1-0.3 value reaching 0.52 g/g in the sample impregnated with 70 mass% CaCl₂; this value is much higher than previously reported ΔW_0.1-0.3 values.