Hierarchically porous carbon foams coated with carbon nitride: insights into adsorbents for pre-combustion and post-combustion CO₂ separation

Adsorption is fundamental to many industrial processes, including separation of carbon dioxide from other gases in pre- or post-combustion gas mixtures. Adsorbents should have high capacity and selectivity, which are both intimately linked with surface area, pore size distribution, and surface energy. Porous carbons are cheap and scalable adsorbents, but greater understanding of how their textural properties and surface chemistry affects their performance is needed. Here, we investigate the effect of nitrogen doping on CO₂ adsorption. Microporous carbon foams with large surface area (>2500 m² g⁻¹) and pore volume (1.6 cm³ g⁻¹) are synthesized, then coated with varying amounts of carbon nitride (up to 17 at% nitrogen) to achieve high CO₂ uptake (25.5 mmol g⁻¹) and selectivity (CO₂:N₂ = 21), whilst also giving insights into the relationship between structure and function. At low pressure (relevant to post-combustion capture), moderate carbon nitride loading leads to enhanced uptake and selectivity by combining large ultramicropore volume with the introduction of Lewis base sites, leading to high isosteric heat of adsorption. Higher carbon nitride loading further increases selectivity but lowers uptake by blocking micropores. Conversely, at high pressure (relevant to pre-combustion capture) the uncoated carbon foam displays superior uptake, because mesoporosity is the dominant factor in this regime, rather than the presence of ultramicropores. Finally, the samples displayed excellent regeneration under repeated adsorption–desorption cycles, and breakthrough curves were measured. These results underscore the delicate balance required for optimal material design when applying porous carbon adsorbents to CO₂ separation processes. Moving forward, improved adsorbents will contribute to the proliferation of carbon capture and storage (CCS) and carbon capture and utilisation (CCU) technologies, ultimately contributing reduced anthropogenic CO₂ emissions.