One-pot synthesis of dual-phase manganese dioxide for toluene removal: Effect of crystal phase blending level on oxygen species and activity

Abstract

A series of highly active manganese oxide with different oxygen species were synthesized by one pot method, wherein the active species were induced by in-situ combination of crystal phase. The scanning electron microscope (SEM) showed the successful combination of rodlike MnO2 and flowerlike MnO2. The x-ray diffraction (XRD) indicated an increase in the oxygen defects due to the in-situ assembly. Compared with that of pure α-MnO2 and β-MnO2, the combined-phase xα/yβ-MnO2 ((x, y represents the molar ratio of α and δ) presented the higher activities for the catalytic oxidation of toluene. Through the analysis of X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction with hydrogen (H2-TPR) technologies, the order of catalytic activity was completely consistent with the change trend of the hydrogen consumption and the molar ratio of adsorbed oxygen/lattice oxygen (Oads/Olatt), showing that the high catalytic activities of catalysts were main attributed to their more low-valent manganese and high content of adsorbed oxygen. Moreover, the intermediates of toloune oxidation over α/β-MnO2 with the best catalytic activity were detected by in-situ diffuse reflectance FTIR spectroscopy (DRIFTS), and the by-products in the exhaust gases were detected by a thermal desorption/gas chromatograph mass spectrometer (TD/GC–MS), which confirmed the presence of the intermediates. Furthermore, the catalytic mechanism and degradation path of toluene oxidation have been studied in depth. Finally, the effect of moisture on catalytic activity of α/δ-MnO2 for toluene oxidation were also discussed.