WO3 Modification of MnOx/TiO2 Catalysts for Low Temperature Selective Catalytic Reduction of NO with Ammonia

A series of WO3-modified MnOx/TiO2 catalysts were prepared by three different impregnation methods and were investigated by specific surface area measurement, X-ray diffraction, laser Raman spectroscopy, H-2 temperature-programmed reduction, high-resolution transmission electron microscopy, and in situ Fourier transform infrared spectroscopy. The three-component catalysts obtained with the one-step impregnation exhibited the best catalytic activity. The characterization data revealed that a synergism between tungsten and manganese oxide existed in the catalysts when tungsten was loaded either prior to or simultaneously with manganese, which made the active components better dispersed and blocked the transformation of TiO2 from the anatase to rutile structure. The manganese oxide existed in the form of Mn2O3 on all the samples but was accompanied by a small amount of MnO2 for 15%MnOx-5%WO3/TiO2. WO3 improved the reducibility and enhanced the amount and strength of the surface acid sites, especially the Bronsted acid sites and promoted the formation of the active intermediate (-NH2). Lewis acid sites had the major role in the low temperature selective catalytic reduction reaction while surface -NH2 was an important intermediate species.