Discerning molecular-level CO₂ adsorption behavior in amine-modified sorbents within a controlled CO₂/H₂O environment towards direct air capture

Sorbents designed for direct air capture (DAC) play a crucial role in the pursuit of achieving net-zero carbon dioxide emissions. This study elucidates CO₂ adsorption from dilute, humidified CO₂ streams onto an amine-modified benchmark DAC adsorbent via solid-state NMR spectroscopy. Various NMR techniques, including 1D ¹H MAS, ¹³C MAS, 2D ¹H-¹³C HETCOR NMR, and ¹H R₂ and R_1ρ relaxometry reveal the impact of CO₂ partial pressure and H₂O on CO₂ adsorption behavior. We find that CO₂ concentration governs the stepwise formation of ammonium carbamate, carbamic acid, and physisorbed CO₂, where relative humidity (RH) at a desired low (<400 ppm) CO₂ loading affects total CO₂ uptake. The relaxation studies reveal the cooperative or competitive nature of H₂O-CO₂ sorption in CO₂-dilute humid gas, and in particular polymer swelling upon humidification. From those results, we demonstrate that the observed absorption capacity enhancement by humidity is caused by pore opening due to sorbent swelling, and not by bicarbonate formation. This NMR-discerned speciation provides insights into sorption behavior at different RHs in dilute CO₂ gas streams, simulating real-world atmospheric conditions, and governs the design of efficient and adaptable material-process combinations for solid sorbent DAC.