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

The application of MnOx-CeO2 as the low temperature selective catalytic reduction (SCR) catalyst to control NOx 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 MnOx-CeO2 may be possible to control NOx emission from biomass-fired power plants. However, a very small amount of alkali metals showed a seriously negative effect on NO reduction over MnOx-CeO2 such that both NOx conversion and N2 selectivity obviously decreased. In this work, the mechanism of NO reduction over MnOx-CeO2 and K-MnOx-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 MnOx-CeO2 was discovered. The decrease of the SCR activity of MnOx-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-MnOx-CeO2 through the Langmuir–Hinshelwood mechanism.