Controllable Synthesis of Different MnO₂ Phases by a Precipitation Method for Effective Ozone Decomposition

It is challenging to controllably synthesize manganese dioxide (MnO2) in a facile routine for structure-oriented applications. Herein, gamma-, alpha-, amorphous (a-), and delta-MnO2 are synthesized by the same liquid phase precipitation of KMnO4 and MnSO4 with tailored pH by KOH dosage and pH = 1 HCl washing. An ozone decomposition reaction is adopted as the catalytic activity evaluation probe, and a-MnO2 showed high efficiency of 80-90% to 100 ppm ozone during a 10 h test at weight hours space velocity of 1,200,000 mL g(-1) h(-1). The high activity can be attributed to its high specific surface area, low average Mn oxidation state, and abundant surface oxygen vacancy content. In addition, the activity decrease of a-MnO2 can be attributed to the accumulation of oxygen intermediates, as illustrated by in situ infrared spectroscopy, which however can be reactivated by simple 60 degrees C oven heating. These results provide a facile way to controllably synthesize differently crystallized MnO2, especially for highly efficient ozone decomposition catalysts.