Confined-space-promoted non-dissociative O₂ activation for SO₂ oxidation in N-doped carbon nanotubes

Efficient activation of ground-state triplet molecular oxygen (O₂) under mild conditions remains a major catalytic challenge. Herein, spin-polarized density functional theory calculations were employed to investigate a confined-space-assisted non-dissociative O₂ activation pathway for SO₂ oxidation on pristine and N-doped single-walled carbon nanotubes (SWCNTs). The results reveal that co-adsorption of H₂O and SO₂ promotes the formation of H₂SO₃ inside the confined nanotube channels, followed by direct oxidation to H₂SO₅ through non-dissociative molecular O₂ activation. Among the investigated systems, pyridinic N-doped SWCNTs significantly enhanced adsorption and facilitated H₂SO₃ formation, while smaller nanotube diameters effectively lowered the oxidation energy barrier by strengthening electronic confinement effects. Notably, the energy barrier for the oxidation reaction decreased from 1.21 eV in free space to 0.69 eV in pristine SWCNT-(6,6). Electronic structure analyses demonstrate that spatial confinement promotes charge transfer and activates adsorbed O₂ without requiring O-O bond dissociation, thereby providing an energetically favorable alternative to conventional dissociative pathways. This work provides mechanistic insights into confined-space-enhanced oxygen activation and offers theoretical guidance for the design of efficient metal-free carbon catalysts for oxidative desulfurization applications.