Roles of N-Vacancies over Porous g-C ₃ N ₄ Microtubes during Photocatalytic NO _x Removal

The development of catalysts that effectively activate target pollutants and promote their complete conversion is an admirable objective in the environmental photocatalysis field. In this work, graphitic carbon nitride (g-C ₃ N ₄ ) microtubes with tunable N-vacancy concentrations were controllably fabricated using an in situ soft-chemical method. The morphological evolution of g-C ₃ N ₄ , from the bulk to the porous tubular architecture, is discussed in detail with the aid of time-resolved hydrothermal experiments. We found that the NO removal ratio and apparent reaction rate constant of the g-C ₃ N ₄ microtubes were 1.8 and 2.6 times higher than those of pristine g-C ₃ N ₄ , respectively. By combining detailed experimental characterization and density functional theory calculations, the effects of N-vacancies in the g-C ₃ N ₄ microtubes on O ₂ and NO adsorption activation, electron capture, and electronic structure were systematically investigated. These results demonstrate that surface N-vacancies act as specific sites for the adsorption activation of reactants and photoinduced electron capture, while enhancing the light-absorbing capability of g-C ₃ N ₄ . Moreover, the porous wall structures of the as-prepared g-C ₃ N ₄ microtubes facilitate the diffusion of reactants, and their tubular architectures favor the oriented transfer of charge carriers. The intermediates formed during photocatalytic NO removal processes were identified by in situ diffuse reflectance infrared Fourier transform spectroscopy, and different reaction pathways over pristine and N-deficient g-C ₃ N ₄ are proposed. This study provides a feasible strategy for air pollution control over g-C ₃ N ₄ by introducing N-vacancy and porous tubular architecture simultaneously.