Weakly Adsorbed CO₂ and H₂O Species on Monoethanolamine Films in a Room-Temperature CO₂ and Direct Air Capture Cycle: An In Situ Infrared Study

Abstract

The binding states of adsorbed CO 2 species on monoethanolamine (MEA) thin films in a CO 2 and direct air capture cycle at 25 °C have been studied by in situ infrared spectroscopy. CO 2 adsorption produced two types of adsorbed CO 2 : (i) hydrogen-bonded ammonium carbamate, as strongly adsorbed CO 2 , which can only be removed by heating above 25 °C and (ii) carbamic acid, as weakly adsorbed CO 2 , which can be removed at room temperature from the MEA film. The infrared results suggest that carbamic acid is on an amine site, which is isolated by OH from neighboring MEA molecules and further clarify the sequence of the formation of adsorbed CO 2 . Weakly adsorbed CO 2 and H 2 O species were observed for the first time on MEA and nonaqueous MEA thin films. Decreasing the concentration of CO 2 from 100% to 420 ppm of CO 2 /1.5% H 2 O (atmospheric CO 2 ) for direct air capture decreased the CO 2 capture capacity and increased the adsorption half-time while the CO 2 desorption halftime (i.e., rate) remained at the same level. Controlling the type and structure of adsorbed CO 2 could be effective in bringing down the energy needed for the regeneration of amine solvents and sorbents through enhancing the density of amine sites for carbamic acid.