Mn/beta and Mn/ZSM-5 for the low-temperature selective catalytic reduction of NO with ammonia: Effect of manganese precursors

Abstract

Two series of Mn/beta and Mn/ZSM-5 catalysts were prepared to study the influence of how different Mn precursors, introduced to the respective parent zeolites by wet impregnation, affected the selective catalytic reduction (SCR) of NO by NH3 across a low reaction temperature window of 50–350 °C. In this study, the catalysts were characterized using N2 adsorption/desorption, X-ray diffraction, X-ray fluorescence, H2 temperature-programmed reduction, NH3 temperature-programmed desorption and X-ray photoelectron spectroscopy. As the manganese chloride precursor only partially decomposed this primarily resulted in the formation of MnCl2 in addition to the presence of low levels of crystalline Mn3O4, which resulted in poor catalytic performance. However, the manganese nitrate precursor formed crystalline MnO2 as the major phase in addition to a minor presence of unconverted Mn-nitrate. Furthermore, manganese acetate resulted principally in a mixture of amorphous Mn2O3 and MnO2, and crystalline Mn3O4. From all the catalysts screened, the test performance data showed Mn/beta-Ac to exhibit the highest NO conversion (97.5%) at 240 °C, which remained >90% across a temperature window of 220–350 °C. The excellent catalytic performance was ascribed to the enrichment of highly dispersed MnOx (Mn2O3 and MnO2) species that act as the active phase in the NH3-SCR process. Furthermore, together with a suitable amount of weakly acidic centers, higher concentration of surface manganese and a greater presence of surface labile oxygen groups, SCR performance was collectively enhanced at low temperature.