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.