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 CO2 species on monoethanolamine (MEA) thin films in a CO2 and direct air capture cycle at 25 °C have been studied by in situ infrared spectroscopy. CO2 adsorption produced two types of adsorbed CO2: (i) hydrogen-bonded ammonium carbamate, as strongly adsorbed CO2, which can only be removed by heating above 25 °C and (ii) carbamic acid, as weakly adsorbed CO2, 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 CO2. Weakly adsorbed CO2 and H2O species were observed for the first time on MEA and nonaqueous MEA thin films. Decreasing the concentration of CO2 from 100% to 420 ppm of CO2/1.5% H2O (atmospheric CO2) for direct air capture decreased the CO2 capture capacity and increased the adsorption half-time while the CO2 desorption half-time (i.e., rate) remained at the same level. Controlling the type and structure of adsorbed CO2 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.