Cobalt oxide decked with inorganic-sulfur containing vanadium oxide for chromium(vi) reduction and UV-light-assisted methyl orange degradation

Abstract

Cobalt oxide (CoO_x) derived from a zeolitic imidazolate framework (ZIF) was modified with sulfur-doped vanadium oxide (VO_x) using an impregnation method. The presence of inorganic sulfur in the mixed metal oxide matrix was confirmed through various physicochemical and spectroscopic techniques. The hybrid material designated as CoO_x–S–VO_x was applied as a catalyst for rapid reduction of toxic chromium, Cr(VI), in the presence of sodium borohydride, NaBH₄. CoO_x–S–VO_x + NaBH₄ (1 mmol) reduced Cr(VI) to Cr(III) within three minutes (3 min). The same catalyst possessing a band gap of ∼3.14 eV served as an effective photocatalyst for the degradation of methyl orange (MO) dye on irradiating with UV-light. The optimization of different parameters indicated that the degradation of photocatalytic dye proceeded very efficiently within 15 min with a total mineralization of 80.06%. The dye degradation experiments carried out under both light and dark phase conditions truly signified the impact of light irradiation on MO degradation. Mott–Schottky analysis and ultraviolet photoelectron spectroscopy (UPS) were performed to calculate the band edge positions of the material. Both the aforementioned analyses predicted that the hybrid catalyst possessed a perfect band structure that could provide high activity to the material. The mechanistic study of the dye degradation process suggested that the MO decomposition was mostly caused by free radicals. The presence of sulfur was considered to play an active role in enhancing the dye degradation process by reducing the electron–hole recombination rate. The catalyst was recyclable for ten consecutive cycles without any significant loss in catalytic activity.