Novel Ni‐Mn Bi‐oxides Doped Active Coke Catalysts for NH ₃ ‐SCR De‐NOx at Low Temperature

Abstract

Abstract Various 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 NH 3 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 Mn 2 NiOx/AC−N (DP, 97 %) > Mn 2 NiOx/AC−N (IM, 94 %)≈Mn 2 NiOx/AC−N (HS, 93 %). The activity test results indicate that Ni‐modification has a clear promoting effects on the SCR activity and resistance to SO 2 , which might be attributed to more (Mn 3+ +Mn 4+ ), high contents of Mn 3+ /Mn 4+ and Ni 3+ /Ni 2+ . 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 NiMn 2 O 4 spinel and MnO 2 phases, Mn 2 NiOx/AC−N (DP) might present the highly uniform dispersion, and Mn 2 NiOx/AC−N (HS) has the strong interaction between carbon supports and surface active components. Importantly, Mn 2 NiOx/AC−N (HS) catalyst has the great advantages of high selectivity to N 2 and good resistance to SO 2 (with the lowest loss proportion of NOx conversion). The spinel‐structured NiMn 2 O 4 probably configured by [Ni 2+ Mn 4+ ] tet [Ni 2+ Ni 3+ Mn 3+ Mn 4+ ] oct O 4 might be one important reason for their good activity and high resistance to SO 2 , in which the Mn 3+ and Ni 3+ ions might be filled into the inside spinel‐crystal wrapped by a large number of outside Ni 2+ and Mn 4+ . 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 Ni 3+ +Mn 3+ ↔Ni 2+ +Mn 4+ . This electronic transfers might be closely related to the activation of NO and NH 3 during SCR reaction.