Adsorption of hazardous gases on Cyclo[18]carbon and its analogues

Density functional theory calculations were performed to study the adsorption behavior of hazardous gases (NH₃, CO₂, CO, and NO) on twelve adsorbents (cyclo[18]carbon (C₁₈) and its analogues Al₉N_9, B₉N_9, C₆B₆N₆, C₁₂B₃N₃, C₁₄B₂N₂, d-C₁₄B₂N_2, C₁₆BN, C₁₇Si; cyclo[12]carbon (C₁₂) and its analogues Al₆N₆ and B₆N₆). The molecular electrostatic potential (MESP) maps of C₁₈ revealed its polyynic structure with alternating electron-rich and electron-deficient regions. The most negative-valued MESP point (V_min) indicates that the replacement of carbon atoms of C₁₈/C₁₂ by BN/AlN/Si units increases the electron-rich environment in the molecules. In general, NH₃ and CO₂ are found to be physisorbed on C₁₈/C₁₂ and its analogues. An important result is that the V_min of C₁₈/C₁₂ and its analogues is linearly correlated with the NH₃ and CO₂ adsorption energies. The quantum theory of atoms in molecules (QTAIM) results indicate that the interactions of NH₃ and CO₂ with C₁₈/C₁₂ and its analogues are non-covalent in nature. In general, CO and NO are found to be chemisorbed on C₁₈/C₁₂ and its analogues. In contrast to the cases of NH₃ and CO₂ adsorption, the V_min of C₁₈/C₁₂ and its analogues is generally inversely related to the CO and NO adsorption energies. The QTAIM results indicate the strong covalent character of the bonding for CO/C₁₈, CO/C₆B₆N₆, CO/d-C₁₄B₂N_2, CO/C₁₂, NO/C₁₈, NO/C₁₇Si_, and NO/C₁₂ systems. The adsorption process significantly influenced the chemical potential and/or hardness, especially for CO– and NO-adsorbed C₁₈/C₁₂ and its analogues.