Unraveling the Structure and Binding Energy of Adsorbed CO2/H2O on Amine Sorbents

One critical issue in the design of amine-based sorbents and solvents for next-generation CO2 capture technology is the effect of H2O on the behavior of adsorbed CO2. To gain an insight into the effect of the complex interaction between adsorbed CO2/H2O on the amine efficiency and binding strength of the adsorbed species, the surface and subsurface molecular interactions of H2O, CO2, and H2O + CO2 were studied over tetraethylenepentamine (TEPA) films by in situ infrared spectroscopy (IR), mass spectrometry, and density functional theory (DFT). The TEPA films were used because they emulate the immobilized amine on solid amine sorbents. The H2O molecules in the TEPA film serve as a proton acceptor during the coadsorption of CO2 and H2O, allowing the formation of hydronium carbamate and producing a ratio of 1:1 for the CO2/amine site. The DFT-simulated structures and IR spectrum enabled the assignment of the experimental IR spectrum for hydronium carbamate that was produced during the interaction of CO2 on the premixed TEPA/H2O and TEPA/D2O films. Adsorption of CO2 on a TEPA/H2O (weight ratio of 5:1) film produced strongly adsorbed species: a hydrated hydronium carbamate with a binding energy >−130 kJ/mol and a cluster of the water molecule with a binding energy >−180 kJ/mol, which endothermically desorbed from the TEPA/H2O film at temperature above 90 °C. In addition to these strongly adsorbed species embedded in the bulk of the TEPA/H2O film, weakly adsorbed CO2 + H2O species (desorbed at temperature below 70 °C) on the surface of TEPA film were elucidated to be single H2O molecule dispersed on amine sites and water coordinated zwitterion. H2O controlled the CO2 capture capacity, the binding energy of adsorbed CO2/H2O, and their adsorption/desorption kinetics through the intermolecular interactions of CO2 and H2O with the specific spatial locations of amine sites. The observation of weakly adsorbed CO2/H2O suggests the spatial arrangements of amine sites in the amine sorbents and solvents can be used to fine-tune their regeneration temperature.