Effects of phase structure of MnO2 and morphology of δ-MnO2 on toluene catalytic oxidation

Different phase structured MnO2, including beta-, alpha-, gamma- and three kinds of delta-MnO2, are prepared and their catalytic activities are compared through catalytic oxidation of toluene at different temperature. The as-prepared MnO2 are characterized by X-ray diffraction and scanning electron microscope. Toluene conversion results show that catalytic oxidation activity follows the sequence of beta- < alpha- < gamma- < delta-MnO2. When MnO2 owns the same delta phase structure, the crystallinity degree and morphology also affect toluene catalytic oxidation activity, but the effect is not so significant as that of phase structure. The results are further explained by Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, H-2 temperature programmed reduction and in situ Fourier transform infrared. The weakest toluene catalytic oxidation activity of beta-MnO2 mainly results from its extreme small surface area. In comparison with gamma- and delta-MnO2, the less amounts of Mn4+ and lattice oxygen on alpha-MnO2 surface result in its weaker catalytic oxidation activity. The special hierarchical structure of delta-MnO2 makes toluene more likely to be adsorbed on its surface, and thus the catalytic oxidation activity is better than that of gamma-MnO2. The effect of weight hourly space velocity and the stabilities of as-prepared MnO2 are also investigated in this research.