Surface sulfur functionalized defects on the synergistic and competitive effects of CO2 and H2O adsorption: Density functional theory study

Heteroatom doping can significantly enhance the CO₂ adsorption capacity of carbon-based materials, but it also increases the hydrophilicity of the carbon matrix. This can lead to the competitive adsorption of CO₂ and H₂O becoming more pronounced in humid environments. In this study, the co-adsorption behaviors of H₂O and CO₂ on carbon surfaces modified with various sulfur functional groups were investigated at the molecular level using density functional theory. The adsorption mechanisms were comprehensively analyzed through IGMH, QTAIM, EDA-FF, and charge transfer analysis. ESP analysis revealed that the absolute values of the local maxima and minima of the water molecules’ electrostatic potential were higher than those of CO₂, indicating that water molecules exhibit greater adsorption stability and hydrogen bonding capability on highly polar functional groups. QTAIM analysis identified the specific interaction pathways and strengths between the atoms of CO₂ and H₂O, while IGMH analysis showed that the interactions between CO₂, H₂O, and the porous carbon are primarily weak van der Waals forces. EDA-FF results indicate that electrostatic and dispersive interactions are the dominant forces in the co-adsorption of CO₂ and H₂O. The enhanced adsorption stability of CO₂ in the co-adsorption system is mainly due to the additional unsaturated carbon sites created by basal defects, which significantly strengthen the van der Waals interactions of CO₂. Additionally, hydrogen bonding between CO₂ and H₂O further promotes CO₂ adsorption energy. This study underscores the critical role of sulfur functionalization in modulating adsorption behavior on carbon surfaces and provides valuable theoretical insights for designing advanced adsorbent materials capable of synergistically adsorbing multi-component gases.