Straw-based biochar prepared from multi-step KOH activation and its structure-effect relationship of CO₂ capture under atmospheric/pressurized conditions via experimental analysis and MD/DFT calculations

Porous carbon-based CO₂ capture holds great promise. However, the preparation process, pore structure and oxygen/nitrogen functional groups of porous carbon on CO₂ adsorption still remain unclear. Herein, we develope straw-based biochar with ultra-high specific surface area(SSA), high micropore ratio and different O/N content by modifying the two-step KOH activation procedure and confirm the KOH activation mechanism, CO₂ adsorption characteristics, and the structure-effect relationship of CO₂ capture. The KOH activation process is controlled by the activation temperature and is divided into five temperature zones. The pickling treatment enhanced the activation effect of biochar, with a SSA of 3567.33 m²/g and a microporous ratio of 96.05 %. The potassium-containing active components reacts with the defective carbon structure above 800 °C catalyzed by alkali and alkaline earth metals(AAEMs), but reacts with the aliphatic hydrocarbon group after pickling. H-4–9-4-m has a maximum CO₂ adsorption capacity of 24.77 mmol/g (25 °C/30 bar), whereas 4–9-2-m has a maximum capacity of 3.46 (25 °C/1 bar) and 5.42 mmol/g (0 °C/1 bar) and outstanding CO₂/N₂ selectivity (123 at 0.02 bar). The pressurized CO₂ adsorption capacity is positively correlated with total pore volume. While under atmospheric pressure, the optimal adsorption site of biochar at 25 °C is ultramicroporous(0.5 nm) and oxygen-containing functional groups, and transforms into ultramicroporous(0.7 nm) and N-5 groups at 0 °C. The simulation results show that 0.5 nm is the pore size of CO₂ adsorption from monolayer to bilayer, and both oxygen/nitrogen-containing groups could increase the CO₂ adsorption capacity of biochar. This study provides guidance for the preparation of excellent carbon-based CO₂ adsorbents according to application scenarios.