Insight into the pH effect on the oxygen species and Mn chemical valence of Co–Mn catalysts for total toluene oxidation

Abstract

Mn-Based metal oxides have shown promising performance in catalytic oxidation of toluene due to the mixed Mn³⁺ and Mn⁴⁺ valences on the surface. Herein, a series of Co–Mn catalysts were synthesized by adjusting the redox precipitation pH to explore the effect of pH on Mn³⁺, Mn⁴⁺ and the surface adsorbed oxygen species when applied for toluene combustion. The Co–Mn-12 catalyst fabricated at pH = 12 displayed excellent catalytic activity and stability for the toluene oxidation of which the T₉₀ at 20 000 mL g⁻¹ h⁻¹ was 208 °C. The toluene conversion could be maintained at around 100% even with the introduction of 10 vol% H₂O at 230 °C. We found the appropriate pH for the promotion of the generation of more Mn³⁺ and Co³⁺ ions, which were conducive to the formation of oxygen vacancy clusters and resulted in more adsorbed oxygen species generated on the catalyst surface. Moreover, in situ DRIFTS spectra displayed that the existing high valence metal ions and oxygen vacancy clusters facilitated the CC breaking of aromatic rings and the formation of maleic anhydride in the oxidation process. Therefore, Co–Mn-12 demonstrated the best catalytic activity and water-resistance, illustrating that it is an efficient potential industrial catalyst for removing toluene.