Optimization of Photothermal Catalysis for Formaldehyde Oxidation Through Modulating Crystal Phase of MnO2

MnO2 based photothermal catalysis has been demonstrated to be a promising technology to eliminate pollutants in air. However, the relationship between catalytic activity and MnO2 crystal phase has not been revealed. Herein, we hydrothermally synthesized four MnO2 catalysts, alpha-MnO2, beta- MnO2,gamma-MnO2 and delta-MnO2, and systematically studied their structural properties and photothermal conversion behavior. The alpha-MnO2 was proved to simultaneously possess large specific surface area ( 54.28 m(2) g(-1)), developed < 10 nm pores, and high low-valence manganese ( Mn2+ + Mn3+) content, as well as the robust photothermal conversion ability. The photothermally catalytic behavior was evaluated through degrading gaseous formaldehyde in both batch and fixed bed experiments. As we expected, the alpha-MnO2 showed the best activity for formaldehyde removal and the highest CO2 yield under xenon light illumination. In the meantime, the alpha-MnO2 had good stability of photothermal catalysis that was verified by the repeated cycling tests in the batch reactor and the durable test in the fixed bed reactor. Through detecting the intermediates and reactive oxygen species, a plausible mechanism of photothermal catalysis was proposed to illustrate the degradation route of formaldehyde on MnO2. [GRAPHICS] .

Automated summary

The properties and photothermal conversion behavior of four different crystal phases (α-MnO2, β-MnO2, γ-MnO2, and δ-MnO2) of MnO2 catalysts were studied. It was found that α-MnO2 has a large specific surface area, developed pores, high low-valence manganese content, and robust photothermal conversion ability. Under illumination, α-MnO2 exhibited the best activity for formaldehyde removal and the highest CO2 yield. It also showed good stability of photothermal catalysis.