Water Enhancement in CO 2 Capture by Amines: An Insight into CO 2 –H 2 O Interactions on Amine Films and Sorbents

Water, a component in flue gas, plays a significant role in CO 2 capture through a complex interaction between water molecules and adsorbed CO 2 on amine sorbents. To determine how the H 2 O–CO 2 –amine interactions affect amine efficiency and the binding energy of adsorbed CO 2, we used in situ infrared spectroscopy (IR) to determine the structure of adsorbed CO 2 and H 2 O as well as their relations to adsorption/desorption kinetics and CO 2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents. H 2 O enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO 2 and promoting the formation of hydronium carbamate and carbamic acid. CO 2 adsorbed on the surface of the TEPA film as a weakly adsorbed CO 2 in the form of hydronium and ammonium–carbamate with a low IR intensity of hydrogen bonding (−OH··· – OOC or −NH··· – OOC) between hydronium/ammonium ions and carbamate ions. CO 2 adsorbed on the middle layers (i.e., 0.2–0.4 μm below the surface) of TEPA films produced a strongly adsorbed species that exhibits an intensive hydrogen bonding band of ammonium–water–carbamate desorbing at temperatures above 120 °C. Comparison of IR spectra shows that the kinetic behaviors of adsorbed CO 2 on amine films are correlated well with those of adsorbed CO 2 on Class I amine sorbents. Thick amine films and high-amine-loading sorbents contain high-density amine sites that produce mainly strongly adsorbed CO 2 . Adsorbed H 2 O further increased amine efficiency and the binding energy of strongly adsorbed CO 2 through the formation of hydronium carbamate.