Templating agent-mediated Cobalt oxide encapsulated in mesoporous silica as an efficient oxone activator for elimination of toxic anionic azo dye in water: Mechanistic and DFT-assisted investigations

While Azorubin S (AZRS) is extensively used as a reddish anionic azo dye for textiles and an alimentary colorant in food, AZRS is mutagenic/carcinogenic, and it shall be removed from dye-containing wastewaters. In view of advantages of SO₄·⁻-related chemical oxidation technology, oxone (KHSO₅) would an ideal source of SO₄·⁻ for degrading AZRS, and heterogeneous Co₃O₄-based catalysts is required and shall be developed for activating oxone. Herein, a facile protocol is proposed for fabricating mesoporous silica (MS)-confined Co₃O₄ by a templating agent-mediated dry-grinding procedure. As the templating agent retained inside the ordered pores of MS (before calcination) would facilitate insertion and dispersion of Co ions into pores, the resulting Co₃O₄ nanoparticles (NPs) would be grown and confined within the pores of MS after calcination, affording Co@MS. On the contrary, another analogue, Co/MS, is also prepared using the similar protocol without the templating agent-mediated introduction of Co, but Co₃O₄ NPs seriously aggregate as clusters on MS. Therefore, Co@MS outperforms Co/MS for activating oxone to eliminate AZRS. Co@MS shows a noticeably lower activation energy of AZRS elimination than the existing catalysts, revealing its advantage over the reported catalysts. Moreover, the mechanistic investigation of AZRS elimination by Co@MS-activated oxone has been also elucidated for identifying the presence of SO₄·⁻, ·OH, and ¹O₂ in AZRS degradation using scavengers, electron paramagnetic resonance spectroscopy, and semi-quantification. The AZRS decomposition pathway is also investigated and unveiled in details via the DFT calculation. These results validate that Co@MS appears as a superior catalyst of oxone activation for AZRS degradation. Copyright © 2022 Published by Elsevier Ltd.