Manganese oxide catalysts supported on zinc oxide nanorod arrays: A new composite for selective catalytic reduction of NOx with NH3 at low temperature

Abstract

Abstract In this study, a new composite of low-temperature selective catalytic reduction (LT-SCR) catalyst is synthesized by impregnating MnOx onto ZnO nanorod arrays integrated with cordierite ceramic substrate (MnOx-ZnO/CC), which exhibits improved activities at 100–250 °C compared to its analogue that fabricated from TiO2 nanorod arrays (MnOx-TiO2/CC). By optimizing the synthesis conditions, the MnOx-ZnO/CC catalyst obtains a maximum NOx conversion efficiency of 96.8% at 200 °C. The introduction of ZnO nanorod arrays not only provides a high surface area, but also results in the formation of amorphous MnOx. A higher content of Mn4+, proper ratio of surface-adsorbed oxygen (Oα), and more Lewis acid sites are found on the surface of the MnOx-ZnO/CC catalyst than that on the MnOx-TiO2/CC counterpart. Additional information about the reaction mechanism is systematically investigated via in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), the results of which demonstrate that the LT-SCR reactions over the MnOx-ZnO/CC catalyst generally occur between NOx and NH3 species in the adsorption status, following the Langmuir-Hinshelwood pathway. The enhanced activities can be attributed to the generation of abundant NH4NO3 originating from NO2 in the “Fast SCR” route, because the synergistic effect between MnOx and ZnO facilitates the oxidation of NO.