Probing the role of O-containing groups in CO₂ adsorption of N-doped porous activated carbon

Porous activated carbons (PACs) are promising candidates to capture CO₂ through physical adsorption because of their chemical stability, easy-synthesis, cost-effectiveness and good recyclability. However, their low CO₂ adsorption capacity, especially low CO₂/N₂ selectivity, has limited their practical applications. In this work, an optimized PAC with a large specific surface area, a small micropore size, and a large micropore volume has been synthesized by one-step carbonization/activation of casein using K₂CO₃ as a mild activation agent. It showed a remarkably enhanced CO₂ adsorption capacity as high as 5.78 mmol g^-1 and an excellent CO₂/N₂ selectivity of 144 (25 °C, 1 bar). Based on DFT calculations and experimental results, the coexistence of adjacent pyridinic N and -OH/-NH₂ species was proposed for the first time to make an important contribution to the ultra-high CO₂ adsorption performance, especially CO₂/N₂ selectivity. This work provides effective guidance to design PAC adsorbents with high CO₂ adsorption performance. The content of pyridine N combined with -OH/-NH₂ was further elevated by additional nitrogen introduction, resulting in a further enhanced CO₂ adsorption capacity up to 5.96 mmol g^-1 (25 °C, 1 bar). All these results suggest that, in addition to the well-defined pore structure, pyridinic N with neighboring OH or NH₂ species played an important role in enhancing the CO₂ adsorption performance of PACs, thus providing effective guidance for the rational design of CO₂ adsorbents.