Enhanced activity and SO2 tolerance in low-temperature NH3-SCR of NOx over MnCr catalyst by hetero-oxide modification

Abstract

Developing NH3-SCR catalysts with high humid-SO2 resistance is of great significance for NOx emission control of the two-stroke low-speed ship. For further optimizing the catalytic performance and SO2 tolerance of Mn-Cr catalyst, Al2O3, CeO2 and Nb2O5 were doped by coprecipitation. Through structural analyses, it was determined that the incorporation of Al and Ce effectively increased the surface area of the catalysts and promoted the dispersion of various components throughout them. Moreover, the introduction of Nb led to the formation of secondary particle aggregates, ultimately strengthening the structural stability of the catalysts. The catalytic performance, N2 selectivity and SO2 tolerance of the co-doped catalyst containing Al2O3, CeO2 and Nb2O5 showed remarkable improvement. The NOx conversion of MnCrCeAlNb was exceeding 90% and N2 selectivity above 97% at 175–300 °C and remained around 88% after 10 h reaction under 150 ppm SO2 and 10 vol% H2O at 225 °C. Co-doping Al2O3, CeO2 and Nb2O5 greatly improved the reducibility and significantly increased the number of surface acid sites. And the XPS results demonstrated that the addition of niobium enhanced the redox properties and adsorbed oxygen through the electron transfer between Mn4+ and Cr3+, resulting in a significant promotion in the NH3-SCR performance and humid-SO2 tolerance. In particular, the cause of the excellent humid-SO2 tolerance of the MnCrCeAlNb was investigated by TG and FT-IR, demonstrating that the Nb addition significantly reduce the formation of (NH4)2SO4, NH4HSO4 and sulfate on the surface of MnCr catalyst. These results suggested that the doping of Hetero-oxide greatly enhanced the low-temperature NH3-SCR and SO2 tolerance of Mn-Cr based catalysts.