Temperature sensitive long-term stability of MnOx/TiO2 catalysts for selective catalytic reduction of NOx with NH3 at low temperature

Abstract

A number of efforts have been made to improve the NH₃-SCR activity of MnO_x based catalysts at low temperature, but few studies have focused on their long-term stability. In this study, MnO_x/TiO₂ (MnTi) catalysts prepared via wet impregnation were subjected to 30-h long-term stability tests under laboratory conditions to investigate their stability for the selective catalytic reduction (NH₃-SCR) of NO_x at low temperatures (80–200 °C), and combined with XRD, BET, XPS, H₂-TPR, and in situ DRIFTS analysis to elucidate deactivation mechanisms. The results show that the reaction temperature has a great influence on the formation, decomposition, and transformation of nitrate species. Nitrate species, especially stable nitrate species, continuously accumulate on the catalyst surface at relatively low temperatures (≤160 °C in this work), which might block the active sites and hinder the conversion of Mn³⁺ to Mn⁴⁺, thereby leading to deactivation of the MnTi catalyst. In contrast, nitrate species on the catalyst surface can react or decompose rapidly at a high enough temperature (≥200 °C in this work), which facilitates the exposure of active sites and dynamic equilibrium of the Mn⁴⁺/Mn³⁺ redox cycle, resulting in the high long-term stability. We expect that this work will arouse researchers’ attention toward the long-term stability of NH₃-SCR catalysts in the study of improving their catalytic activity at low temperature.