Advanced interpretation of CO2 adsorption thermodynamics onto porous solids by statistical physics formalism

Three different activated carbons, namely Aquacarb 207EA (AQ), Organosorb-10 (OR-10) and Organosorb-10AA (OR10-AA) are investigated for carbon dioxide (CO2) adsorption from simulated flue-gas, which is one of the promising approaches to mitigate global warming. Adsorption isotherms are determined at three temperatures (303 K, 323 K and 353 K) and over a wide CO2 partial pressure range (0.03–0.3 bar), in a dynamic lab-scale experimental apparatus. Experimental results show that CO2 adsorption capacity is affected by micropore volume, while the chemical properties (mainly pHPZC and acid functional groups) seem to play a secondary role. The CO2 adsorption isotherms are interpreted by three advanced models derived from statistical physics. The selected model allows retrieving useful information about the sorption process by determining the number of captured CO2 molecules per site (nms), the density of receptor sites (Dsr), the total number of adsorbed layers (Nc) and the energetic parameters (−ε1) and (−ε2). The modeling analysis suggests that the captured CO2 molecules aggregate upon adsorption and are anchored with a perpendicular position on the adsorbents surface. The total numbers of adsorbed layers range from 3.12 to 5.56 for the three adsorption systems, and the value increases with temperature as if the diminution of active sites available for CO2 capture on adsorbent surface was partially counterbalanced by a multilayer adsorption mechanism. The adsorption energies retrieved from data analysis are in the range from −11.39 to −21.10 kJ/mol, confirming that, for the investigated systems, adsorption is exothermic and based on physisorption. Finally, the site energy distribution is also estimated to corroborate the surface heterogeneity and the physical nature of the adsorbate/adsorbent interactions.