Novel Co– or Ni–Mn binary oxide catalysts with hydroxyl groups for NH3–SCR of NOx at low temperature

Abstract

Novel hydroxyl–containing Me-Mn binary oxides (Me = Co, Ni) were prepared for the selective catalytic reduction of NOx with NH3 by a combined complexation–esterification method. The binary oxides of Co–MnOx and Ni–MnOx with mixed crystal phases of Mn3O4 and Co3O4, Mn2O3 and NiMnO3 were obtained at 550 °C. SCR activity decreased in the order of Mn3O4–Co3O4–OH > Mn2O3–NiMnO3–OH > Mn2O3–OH > Mn3O4–OH, benefiting from the high concentration of chemisorbed oxygen and effective electron transformation of cations. Mn2O3–containing catalysts had better selectivity to N2 than those containing Mn3O4. Higher selectivity to N2O over Mn3O4–containing catalysts was attributed to the depth dehydrogenation of coordinated NH3 by the active oxygen species with lower MnO band energy. The typical Eley–Rideal mechanism over Mn3O4–OH and Mn3O4–Co3O4–OH, and the additional formation pathway of NH4NO3 species over Mn2O3–OH and Mn2O3–NiMnO3–OH catalysts were proposed via the in–situ DRIFTS experiments. Although the Co and Ni elements had a good role in delaying the poisoning of SO2, these catalysts were eventually sulfated by SO2 over the postponement, which might due to the metal sulfate and ammonia hydrogensulfite species.