Bamboo-like MnO₂⋅Co₃O₄: High-performance catalysts for the oxidative removal of toluene

The manganese-cobalt mixed oxide nanorods were fabricated using a hydrothermal method with different metal precursors (KMnO₄ and MnSO₄·H₂O for MnO_x and Co(NO₃)₂⋅6H₂O and CoCl₂⋅6H₂O for Co₃O₄). Bamboo-like MnO₂⋅Co₃O₄ (B-MnO₂⋅Co₃O₄ (S)) was derived from repeated hydrothermal treatments with Co₃O₄@MnO₂ and MnSO₄⋅H₂O, whereas Co₃O₄@MnO₂ nanorods were derived from hydrothermal treatment with Co₃O₄ nanorods and KMnO₄. The study shows that manganese oxide was tetragonal, while the cobalt oxide was found to be cubic in the crystalline arrangement. Mn surface ions were present in multiple oxidation states (e.g., Mn⁴⁺ and Mn³⁺) and surface oxygen deficiencies. The content of adsorbed oxygen species and reducibility at low temperature declined in the sequence of B-MnO₂⋅Co₃O₄ (S) > Co₃O₄@MnO₂ > MnO₂ > Co₃O₄, matching the changing trend in activity. Among all the samples, B-MnO₂⋅Co₃O₄ (S) showed the preeminent catalytic performance for the oxidation of toluene (T_10% = 187°C, T_50% = 276°C, and T_90% = 339°C). In addition, the B-MnO₂⋅Co₃O₄ (S) sample also exhibited good H₂O-, CO₂-, and SO₂-resistant performance. The good catalytic performance of B-MnO₂⋅Co₃O₄ (S) is due to the high concentration of adsorbed oxygen species and good reducibility at low temperature. Toluene oxidation over B-MnO₂⋅Co₃O₄ (S) proceeds through the adsorption of O₂ and toluene to form O*, OH*, and H₂C(C₆H₅)* species, which then react to produce benzyl alcohol, benzoic acid, and benzaldehyde, ultimately converting to CO₂ and H₂O. The findings suggest that B-MnO₂⋅Co₃O₄ (S) has promising potential for use as an effective catalyst in practical applications.