Novel Ni‐Mn Bi‐oxides Doped Active Coke Catalysts for NH3‐SCR De‐NOx at Low Temperature

Abstract

AbstractVarious Ni−Mn bi‐oxides doped active coke catalysts were investigated by traditional impregnation method (IM), deposition precipitation (DP), hydrothermal synthesis (HS) and oxidation reduction (OR) method for the selective catalytic reduction of NO by NH3 at low temperature. Compared with the four preparation methods to synthesize MnOx/AC−N catalyst, the test shows that MnOx/AC−N (DP) achieved NOx conversion above 80 % at 200–250 °C and MnOx/AC−N (HS) gave its > 70 % SCR activity at > 225 °C. MnOx/AC−N (IM) achieves 63 % NO conversion at 200 °C. It is worth noting that MnOx/AC−N (OR) exhibits less than 50 % NOx conversion over temperature range. Therefore, the activity of Co‐modified MnOx/AC−N prepared by IM, DP and HS methods was tested. The SCR activities at 200 °C was in the order of Mn2NiOx/AC−N (DP, 97 %) > Mn2NiOx/AC−N (IM, 94 %)≈Mn2NiOx/AC−N (HS, 93 %). The activity test results indicate that Ni‐modification has a clear promoting effects on the SCR activity and resistance to SO2, which might be attributed to more (Mn3++Mn4+), high contents of Mn3+/Mn4+ and Ni3+/Ni2+. These catalysis were characterized by X‐ray diffraction (XRD), Scanning electron microscope (SEM), Raman spectra (Raman), Inductively coupled plasma mass spectrometry (ICP‐MS) and X‐ray photoelectron spectroscopy (XPS). XRD and Raman results indicated those samples showed the NiMn2O4 spinel and MnO2 phases, Mn2NiOx/AC−N (DP) might present the highly uniform dispersion, and Mn2NiOx/AC−N (HS) has the strong interaction between carbon supports and surface active components. Importantly, Mn2NiOx/AC−N (HS) catalyst has the great advantages of high selectivity to N2 and good resistance to SO2 (with the lowest loss proportion of NOx conversion). The spinel‐structured NiMn2O4 probably configured by [Ni2+Mn4+]tet[Ni2+Ni3+Mn3+Mn4+]octO4 might be one important reason for their good activity and high resistance to SO2, in which the Mn3+ and Ni3+ ions might be filled into the inside spinel‐crystal wrapped by a large number of outside Ni2+ and Mn4+. It was suggested that this configstruct was highly conducive to protect active sites from the sulfurization poisoning, benefiting from the efficient electronic interactions between Mn and Ni in VI‐octahedral sites expressed as Ni3++Mn3+↔Ni2++Mn4+. This electronic transfers might be closely related to the activation of NO and NH3 during SCR reaction.