Hierarchically Nanoporous Carbon for CO2 Capture and Separation: Roles of Morphology, Porosity, and Surface Chemistry

Nanoporous carbons, as promising adsorbents for CO2 capture and separation from flue gas, span the major challenge of optimizing the physicochemical property to meet the need of CO2 capture and separation. Herein, we rationally engineered sustainable nanoporous carbons with tailorable porosity and rich surface chemistry using an activating agent of small-molecule organic acids. The porosity could be easily tailored by varying the activation temperature and activator dosage. The surface chemistry and morphology could be tuned by changing the dopant concentration. In detail, porosity played a determinant role in CO2 adsorption capacity, especially ultramicroporosity. Narrow micropores brought major contributions to CO2 uptake at low pressure, while mesopores played a great role at high adsorption pressure. Surface chemistry played a critical role in CO2 capture, especially adsorption selectivity and the isosteric heat of adsorption when the ultramicroporosity was optimal. We revealed that the C–OH group (one type of oxygen-containing species) made a relatively high contribution for improving CO2 capture by hydrogen bonding. For nitrogen-containing groups, the pyridinic-N, pyrrolic-N, and amine groups provoked more improvements in CO2 capture by offering more basic CO2-philic sites, while the oxidized-N group made a weak influence. Sheet-like structures exhibited more accessible affine sites for fast CO2 capture and separation. We proposed a valuable reference for rationally engineering nanoporous carbons with the optimal texture to meet the different requirements of CO2 capture. More importantly, the conversion of biowaste into high-added-value adsorbents for effective CO2 capture and separation opened up an elegant route of killing two birds with one stone.