Catalytic oxidation of NO over MnO2 with different crystal structures

alpha-, beta-, gamma- and delta-MnO2 catalysts were prepared by a hydrothermal method and their catalytic properties for NO oxidation were evaluated. There were dramatic differences in the activities among the MnO2 catalysts with different crystal structures. The gamma-MnO2 catalyst exhibited the best activity among the four catalysts and gave rise to 91% NO conversion at 250 degrees C. Moreover, 50% of NO conversion was achieved by gamma-MnO2 even at 160 degrees C. To investigate the factors influencing the catalytic activity, the catalysts were then characterized by X-ray diffraction, N-2 adsorption, scanning electron microscopy, Fourier transform infrared spectrometry, Raman spectrometry, temperature-programmed reduction by H-2 and X-ray photoelectron spectroscopy. On the basis of the characterization results, the tunnel structure and surface chemisorbed oxygen of gamma-MnO2 were proposed to be the main factors that contributed to the excellent performance in NO oxidation. In addition, the four catalysts showed stable catalytic activity. Over gamma-MnO2, H2O had a reversible inhibiting effect on catalytic activity, while the conversion of NO could not recover after removing SO2. In the presence of H2O and SO2, H2O was able to slow the progression of catalyst deactivation resulting from SO2.