Alkali Metal Deactivation on the Low Temperature Selective Catalytic Reduction of NO_x with NH₃ over MnO_x‑CeO₂: A Mechanism Study

The application of MnO_x-CeO₂ as the low temperature selective catalytic reduction (SCR) catalyst to control NO_x emission from coal-fired power plants is extremely restricted due to the unrecoverable deactivation by SO₂. There is little SO₂ in the flue gas from biomass-fired power plants, and the concentration of alkali metals in the flue gas after the electrostatic precipitator is very low, so the application of MnO_x-CeO₂ may be possible to control NO_x emission from biomass-fired power plants. However, a very small amount of alkali metals showed a seriously negative effect on NO reduction over MnO_x-CeO₂ such that both NO_x conversion and N₂ selectivity obviously decreased. In this work, the mechanism of NO reduction over MnO_x-CeO₂ and K-MnO_x-CeO₂ was investigated by the transient reaction study and the kinetic parameters of NO reduction were obtained from the steady-state kinetic study. After comparison of the kinetic parameters, the mechanism of potassium deactivation on NO reduction over MnO_x-CeO₂ was discovered. The decrease of the SCR activity of MnO_x-CeO₂ after potassium deactivation was mainly attributed to the decrease of acid site and Mn⁴⁺ concentration on the surface, and the increase of N₂O selectivity was mainly related to the occurrence of N₂O formation over K-MnO_x-CeO₂ through the Langmuir–Hinshelwood mechanism.