The competitive adsorption mechanism of CO2, H2O and O2 on a solid amine adsorbent

To explore the effect of H2O and O2 on CO2 adsorption for branched polyethylenimine impregnated multi-walled carbon nanotubes (CNTs-PEI), the CO2 capacity and cyclic adsorption performance were measured under different gas mixtures (CO2/N2, CO2/H2O/N2, CO2/O2/N2, CO2/O2/H2O/N2). The competitive adsorption mechanisms among CO2, H2O and O2 were investigated by adsorption isotherms, adsorption energies and diffusion coefficient calculations, and in situ FTIR spectra of three gases adsorbed on CNT-PEI. The results showed that H2O had the highest adsorption energy and diffusion coefficient. This indicated the stronger interaction between H2O and CNT-PEI, resulting in the highest capacity of H2O. Furthermore, it also promoted amino groups reacting with CO2 at a mole ratio of 1:1, leading to the highest CO2 capacity under a humid gas mixture (CO2/H2O/N2) of up to approximately 5 mmol g-1. O2 had the lowest adsorption energy and diffusion coefficient, indicating that a small quantity of active sites was occupied by O2, which caused the less effect on CO2 capacity for the fresh adsorbent. However, it also reflected the lower desorption rate, which prolonged the contact time of amino groups with O2 and increased the probability of amino group oxidation. In situ FTIR spectra of O2 cyclically adsorbed on CNT-PEI proved the presence of amine oxide, which explained the continuous decrease in cyclic adsorption under a humid O2-containing gas mixture (CO2/O2/H2O/N2). CO2 molecules had moderate diffusion coefficient and adsorption energy, which proved excellent cyclic adsorption properties under dry gas mixture.