Effect of MnO2 polymorphs structure on the selective catalytic reduction of NOx with NH3 over TiO2-Palygorskite

MnO2 polymorphs such as alpha-, beta-, gamma-, and delta-MnO2 were prepared and measured for the selective catalytic reduction of NOx with NH3 respectively. Subsequently, gamma-MnO2/TiO2-Pal ternary composites were fabricated as an efficient catalyst for low-temperature NH3-SCR and characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), temperature-programmed desorption of NH3 (NH3-TPD), temperature-programmed reduction of H-2 (H-2-TPR), isothermal N-2 adsorption/desorption (BET), and X-ray photoelectron spectroscopy (XPS). TEM images show that m(Pal):m(TiO2) = 2:1 exhibits well dispersed particles and is the best ratio as a carrier of gamma-MnO2. Weak peaks of gamma-MnO2 in the XRD pattern suggest that gamma-MnO2 is considered to be in a poorly crystalline phase. It is evidenced that gamma-MnO2 catalysts present the best performance compared with others, and the optimum loading amount of gamma-MnO2 over the surface of TiO2/Pal is about 5 wt.%. NH3-TPD patterns show that 5 wt.% gamma-MnO2/TiO2-Pal catalyst calcined in the range of (80-300 degrees C) displays the excellent NH3 adsorption by the Bronsted acidic sites. H-2-TPR patterns show that the manganese oxide species occur change when calcination temperature is over 400 degrees C. The XPS spectrum of 5 wt.% gamma-MnO2/TiO2-Pal reveals the high existing amount of Mnz(4+) ions over the surface of TiO2/Pal, the results of which indicate that MnO2 is the dominant phase with respect to Mn2O3 and MnO phases. The NH3-SCR experiments demonstrate that 5 wt.% gamma-MnO2/TiO2-Pal catalysts can attain almost 100% NOx conversion at lower temperature scope (200-300 degrees C). (C) 2015 Elsevier B.V. All rights reserved.