Water Enhancement in CO2 Capture by Amines: An Insight into CO2–H2O Interactions on Amine Films and Sorbents

Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water molecules and adsorbed CO2 on amine sorbents. To determine how the H2O–CO2–amine interactions affect amine efficiency and the binding energy of adsorbed CO2, we used in situ infrared spectroscopy (IR) to determine the structure of adsorbed CO2 and H2O as well as their relations to adsorption/desorption kinetics and CO2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents. H2O enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO2 and promoting the formation of hydronium carbamate and carbamic acid. CO2 adsorbed on the surface of the TEPA film as a weakly adsorbed CO2 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. CO2 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 CO2 on amine films are correlated well with those of adsorbed CO2 on Class I amine sorbents. Thick amine films and high-amine-loading sorbents contain high-density amine sites that produce mainly strongly adsorbed CO2. Adsorbed H2O further increased amine efficiency and the binding energy of strongly adsorbed CO2 through the formation of hydronium carbamate.