In-plasma catalytic degradation of toluene over different MnO2 polymorphs and study of reaction mechanism

Abstract

α-, β-, γ- and δ-MnO2 catalysts were synthesized by a one-step hydrothermal method, and were utilized for the catalytic oxidation of toluene in a combined plasma-catalytic process. The relationship between catalytic performance and MnO2 crystal structures was investigated. It was noted that the toluene removal efficiency was 32.5% at the specific input energy of 160 J/L when non-thermal plasma was used alone. The α-MnO2 catalyst showed the best activity among the investigated catalysts, yielding a toluene conversion of 78.1% at the specific input energy of 160 J/L. For β-MnO2, γ-MnO2 and δ-MnO2, removal efficiencies of 47.4%, 66.1% and 50.0%, respectively, were achieved. By powder X-ray diffraction, Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, Brunauer-Emmett-Teller, H2 temperature-programmed reduction and X-ray photoelectron spectroscopy analyses, it was concluded that the tunnel structure, the stability of the crystal in plasma, the Mn–O bond strength of MnO2 and the surface-chemisorbed oxygen species played important roles in the plasma-catalytic degradation of toluene. Additionally, the degradation routes of toluene in non-thermal plasma and in the plasma-catalytic process were also studied. It was concluded that the introduction of MnO2 catalysts enabled O3, O2, electrons and radical species in the gas to be adsorbed on the MnO2 surface via a facile interconversion among the Mn4+, Mn3+ and Mn2+ states. These four species could then be transported to the toluene or intermediate organic by-products, which greatly improved the toluene removal efficiency and decreased the final output of by-products.