N-rich porous carbons with tunable affinity for CO2 adsorption achieve size-sieving CO2/N2 selectivity in turbostratic interlayers

Abstract

While nitrogen- (N-) rich porous carbons hold promise to enable efficient carbon capture from post-combustion flue gas via temperature swing adsorption, a widespread tradeoff of surface chemistry with textural properties has resulted in a lack of detailed understanding in the relationships between a carbon adsorbent's material properties and its propensity for selective CO₂ adsorption. This study details a comprehensive screening of the CO₂ and N₂ adsorption performance for a set of turbostratic porous carbons with a wide range of microporosity (0–0.7 cm³ g⁻¹) and N-content (0–22 at%). By employing a temperature-dependent dual site Langmuir isotherm model, the energetics and capacities of CO₂ adsorption were related to the carbons' textural and surface chemical properties. The isosteric heats of adsorption were dependent on the N content, with the most N-rich carbons exhibiting zero-loading isosteric heats upwards of 45 kJ mol⁻¹ compared to around only 20 kJ mol⁻¹ for unfunctionalized carbons. On the other hand, the N₂ adsorption was a strong function primarily of the surface area and microporosity. As a result, the selectivity and predicted carbon capture performance for a simplified temperature swing adsorption process model were shown to be highly tunable by enhancing the N content (and limiting the N₂-accessible porosity) of the carbon adsorbents. This work provides useful structure–function relationships and helps to answer the longstanding question of the validity of N-incorporation for tuning the CO₂ adsorption performance of N-rich porous carbon adsorbents.