The Nature of Adsorbed Carbon Dioxide on Immobilized Amines during Carbon Dioxide Capture from Air and Simulated Flue Gas

The structure and nature of adsorbed species on solid amine sorbent under various CO 2 concentrations from ambient air (0.04 vol %) to an CO 2 /air stream (15 vol %) have been studied by using in situ IR spectroscopy. The in situ IR study (i) provided evidence to support the zwitterion pathway for CO 2 adsorption/desorption and (ii) allowed the determination of the nature of the adsorbed species. The IR‐observable zwitterion was deprotonated to ammonium carbamate and carbamic acid reversibly. CO 2 adsorbs on the primary amine site as a strongly adsorbed species in the form of ammonium carbamate and on the secondary amine site as a weakly adsorbed species in the form of carbamic acid. Another form of weakly adsorbed species could be ammonium carbamate on the secondary amine. An increase of the concentration of CO 2 in the stream increased the CO 2 capture capacity, the fraction of strongly adsorbed CO 2 , and the binding energy of adsorbed species. A concentration of 400 ppm CO 2 in air competes over 0.5 % of water vapor for adsorption on Class I amine sorbents.