Mechanistic Insight Into The Effect Of Chemical Functionalization On The CO₂/CH₄ Adsorption On Carbonaceous Porous Materials

Uncontrolled massive CO₂ emission into atmosphere is becoming a huge threat to our global climate and environment.Carbon capture and storage（CCS）provides a promising approach to alleviate this issue.The major challenge for CCS is to explore efficient adsorbent materials with high storage capacity and selectivity.A diversity of state-of-the-art adsorbent materials has the significant advancement in gas adsorption science field.Carbonaceous porous materials are widely utilized in CCS due to its unique properties.The practical strategies to enhance CO₂ capture and separation based on the current adsorbent materials mainly involve topological structure design,chemical doping,chemical functionalization,open metal sites,and electric field,etc.These strategies paved constructive ways for the design and synthesis of novel adsorbent materials.Among them,chemical functionalization is one of the most efficient strategies to enhance CCS performance for carbonaceous porous materials.The effects of chemical and structural surface heterogeneity on the CH₄ adsorption behaviour on microporous carbons have been investigated using a hybrid theoretical approach,including density functional theory（DFT）,molecular dynamic（MD）,and grand canonical Monte Carlo（GCMC）simulations.Bader charge analysis is first performed to analyze the surface atomic partial charges.The CH₄ adsorption densities in defective and functionalized graphite slit pores are lower than that in the perfect pore according to the MD simulations.Finally,the CH₄ adsorption isotherms for the perfect,defective and functionalized slit pores are analyzed by the GCMC simulations in combination of the DFT and MD results.For pores with defective surface,the adsorption capacities decrease;while the embedded functional groups decrease the adsorption capacity at low pressure and enhance it at high pressure.Our results demonstrate the significant effects of chemical and structural surface heterogeneity on the CH₄ adsorption and provide a systematic approach to understand the gas adsorption behaviour.The effect of edge-functionalization has been investigated,for the first time,on the competitive adsorption of binary CO₂/CH₄ mixture in nanoporous carbons（NPCs）by combining density functional theory（DFT）and grand canonical Monte Carlo（GCMC）simulation.Our results show that edge-functionalization has a more positive effect on the single-component adsorption of CO₂ than CH₄,and therefore significantly enhancing the selectivity of CO₂ over CH₄,in the sequence of NH2–NPC > COOH–NPC > OH–NPC > H–NPC > NPC at low pressure.The enhanced adsorption originates essentially from the effects of（1）the conducive environment with large pore size and effective accessible surface area,（2）the high electronegativity/electropositivity,（3）the strong adsorption energy,and（4）the large electrostatic contribution,due to the inductive effect/direct interaction of the embedded edge-functionalized groups.The larger difference from these effects results in the higher competitive adsorption advantage of CO₂ in binary CO₂/CH₄ mixture.Temperature has a negative effect on the gas adsorption,but no obvious influence on the electrostatic contribution on selectivity.With the increase of pressure,the selectivity of CO₂ over CH₄ first decreases sharply and subsequently flattens out to a constant value.This work highlights the potential of edge-functionalized NPCs in competitive adsorption,capture,and separation for binary CO₂/CH₄ mixture,and provides an effective and superior alternative strategy in the design and screening of adsorbent materials for carbon capture and storage.