Synthesis of MnO2 with Different Crystal Phases via Adjusting pH for Ozone Decomposition Under Various Humidity Conditions and Monolithic Catalyst Development

MnO2 catalysts are recognized as highly efficient materials for ozone decomposition at room temperature. However, the conventional preparation methods, such as the hydrothermal method, typically require critical conditions (100–200 °C for 6–48 h). Moreover, the prepared catalysts are almost powders, which makes them difficult to apply as monolithic catalysts. In this work, a simple pH-adjusted method was developed to in situ prepare MnO2 with different crystal phases (α, amorphous, and δ) under ambient conditions. XRD analysis revealed that decreasing the pH from 13 to 3 induced a gradual phase transformation from δ-MnO2 to amorphous MnO2, while the α-phase appeared at pH = 1.5. The combination of XPS and O2-TPD results shows that amorphous MnO2 exhibited the lowest average oxidation state (AOS) and highest oxygen vacancy concentration. The optimized amorphous MnO2 catalyst (Cat. 2) achieved the highest ozone removal efficiency of 98% with a high relative humidity of 90%. Furthermore, in situ DRIFTS experiments further demonstrated that the prepared Cat. 2 maintained minimal OH accumulation under humid conditions, confirming its excellent water resistance. Finally, the preparation method of amorphous MnO2 was effectively applied to cordierite honeycomb carrier (CHC). The a(amorphous)-MnO2/CHC catalyst module (100 mm × 100 mm × 20 mm) showed stable ozone removal efficiency of 60% during a 60 h evaluation in an air duct (O3: 400 ± 30 ppb, T: 25 ± 5 °C, gas velocity: 1 m s−1). This study innovatively developed a simple pH-adjusted method to prepare MnO2 with different crystal phases under ambient conditions and successfully applied it to the cordierite honeycomb carrier for monolithic catalyst development.