Comparative study of α-, β-, γ- and δ-MnO2 on toluene oxidation: Oxygen vacancies and reaction intermediates

The structure-performance relationship of α-, β-, γ- and δ-MnO2 catalysts were studied. The four samples exhibited different activities of toluene oxidation in terms of distinct tunnel sizes, surface-active oxygen and redox properties. δ-MnO2 catalyst with K+ in the mezzanines of its layers presented the highest toluene oxidation activity under a GHSV of 60,000 mL·g−1 h−1, as well as good water resistance. HAADF images and EELS results showed that oxygen vacancies preferred to form on δ-MnO2 lattice with layer stack dislocations via Mn4+ reduction rather than β-MnO2 with good crystallization. These inherent-distorted structures with heterocations K+ improved the emerge-annihilate cycling of oxygen vacancies. In-situ DRIFTS results showed that toluene adsorption was facilitated via rapid dehydrogenation of methyl due to abundant surface adsorbed oxygen on δ-MnO2. In addition, benzoate, maleic and manganese carbonate on δ-MnO2 were the key intermediate species during toluene oxidation at relatively low temperatures.