Efficient monolithic MnO_x catalyst prepared by heat treatment for ozone decomposition

Area of Science:

• Materials Science
• Environmental Chemistry
• Catalysis

Background:

• Ozone (O₃) pollution is a significant environmental concern, particularly at ground level.
• Manganese oxide (MnOₓ) catalysts show promise for ozone decomposition, but their performance is sensitive to preparation conditions and environmental factors like humidity.
• High humidity can lead to catalyst deactivation by surface water accumulation, hindering ozone removal efficiency.

Purpose of the Study:

• To investigate the impact of calcination temperature on the properties and ozone decomposition activity of manganese oxide catalysts.
• To identify an optimal manganese oxide catalyst for efficient ozone removal under high-humidity, low-ozone conditions.
• To elucidate the structure-activity relationships governing the catalyst's performance and stability.

Main Methods:

• Synthesis of manganese oxide catalysts (MnOₓ) at various calcination temperatures.
• Evaluation of ozone decomposition activity under controlled high-humidity (up to 90% RH) and low-ozone concentrations.
• Characterization using X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Temperature-Programmed Desorption of O₂ (O₂-TPD), and X-ray Photoelectron Spectroscopy (XPS).

Main Results:

• An outstanding catalyst, MnOₓ (260 ℃), was prepared, achieving over 90% ozone decomposition at 0% RH and 70% at 90% RH after 6 hours.
• XRD analysis revealed MnOₓ (260 ℃) primarily consists of Mn₃O₄ with some MnO₂.
• TEM showed the exposure of highly active MnO₂ (110) crystal planes, with overlapping lattice fringes of MnO₂ (110) and Mn₃O₄ (103).
• In situ DRIFTS indicated removal of surface hydroxyl groups, preventing deactivation from water accumulation.
• O₂-TPD and XPS results demonstrated good oxygen migration ability and a high concentration of adsorbed oxygen in MnOₓ (260 ℃).

Conclusions:

• Appropriate calcination temperature is crucial for developing highly active and stable manganese oxide catalysts for ozone decomposition.
• The optimal catalyst (MnOₓ (260 ℃)) exhibits a beneficial coexistence of multiple phases, exposed active crystal planes, and effective management of surface hydroxyl groups.
• These properties contribute to enhanced oxygen vacancy exposure and inhibited deactivation, leading to superior performance, especially under humid conditions.