Morphological Effects of Manganese Dioxide on Catalytic Reactions for Low-Temperature NH3-SCR

alpha-MnO2 nanorods, gamma-MnO2 nanosheets, and delta-MnO2 nanofilm-assembled microspheres were prepared using a hydrothermal method and evaluated as catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx). They were also structurally characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), N-2 adsorption-desorption, temperature programmed reduction with hydrogen (H-2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The gamma-MnO2 nanosheets performed the best for the reduction of NOx and selectivity of N-2, while the alpha-MnO2 nanorods performed the worst. Structural analysis indicated that the main factor determining the catalytic activities of the nanomaterials was not the specific surface area but the crystal structure and the exposed active crystals. The gamma-MnO2 nanosheets performed best because their exposed (131) planes contained multiple Mn cations in coordinatively unsaturated environments, which formed numerous strongly acidic sites. They also benefited from active oxygen species. The active sites allowed the activation of NH3 and NO at lower temperatures. Moreover, high concentrations of liquid oxygen and Mn cations at high oxidation states facilitated the redox reactions.