Formation Mechanism And Sulfation Mechanism Study Of MnO_x/TiO₂ Catalysts For Low Temperature NH₃ Selective Catalytic Reduction Of NO_x

Selective catalytic reduction of NOx with NH3 (NH3-SCR) is now considered to be one of the most effective way to remove NOx in the flue gas from stationary sources. In recent years, low-temperature SCR technology has bacome a research hotspot because of its low energy consumption and low operating cost. Based on the research progress of domestic and foreign, the low-temperature SCR still has the follwing shotcomings:poor SO2 resistance of catalysts and formation of N2O. In order to solve these problems, MnOx/TiO2 catalyst was used to study the low-temperature SCR reaction in this disseration systematically.In this study, a MnOx/TiO2 catalyst was prepared by impregnation method, and the DeNOx activity of catalyst has been investigated. In addition, in situ DRIFTS、XPS、TPD、 H2-TPR were used to investigated the reaction mechanism, N2O formation mechanism and mechanism of sulfation. Lastly, we introduced kinetic equation and the experimental phenomena were further confirmed by the kinetic equation. The main acquired research results were as follows:The NH3-SCR activity of MnOx/TiO2 catalyst increased with the increase of temperature during 100℃～250℃ under a GHSV(gas hourly space velocity) of 75,000h-1(i.e.60,000 cm3 g-1 h-1), and over 90% NOx conversion was achieved at 250℃, indicating that the MnOx/TiO2 catalyst is a very promising catalyst for the removal of NOx in the flue gas from stationary sources.With further research, we found that N2O selectivity of the low temperature SCR reaction over MnOx/TiO2 was related to the concentrations of gaseous NO in the inlet, and The N2O selectivity of MnOx/TiO2 decreased with the increase of gaseous NO concentration.Through the transient reaction at 150℃, we found that both Langmuir-Hinshelwood mechanism and Eley-Rideal mechanism were contributed to N2O formation. Then we changed the reaction temperature to 250℃, and we found that N2O formation at 250℃ mainly resulted from the Eley-Rideal mechanism. we also noticed that the concentration of formed N2O from the transient reaction at 250℃ was much higher than that at 150℃, it suggests that the formation of N2O from the Eley-Rideal mechanism was obviously promoted with the increase of reaction temperature.At higher temperature, as gaseous NO concentration increased, more-NH2 was used to reduce gaseous NO to form N2 and the further oxidization of-NH2 to-NH was restrained, leading to an obvious decrease of N2O selectivity. In addition, kinetic equation was introduced, and the equation we obtained through derivation could well confirm the conclusion that the N2O selectivity of MnOx/TiO2 catalyst decreased with the increase of the concentration of gaseous NO in the flue gas.The sulfation showed a dual effect on the selective catalytic rduction reaction over MnOx/TiO2. The adsorption of NOx, and the activation of adsorbed NH3 on MnOx/TiO2 were both restrained after the sulfation. Therefore, the SCR reaction through the Langmuir-Hinshelwood mechanism and that through the Eley-Rideal mechanism were both restrained, resulting in an obvious decrese of NO conversion below 300℃. During NO reduction over MnOx/TiO2 above 300℃, there was a competition among the SCR reaction, the nonselective catalytic reduction reaction(NSCR) and the catalytic oxidation of NH3 to NO(C-O). The NSCR reaction and the catalytic oxidation of NH3 to NO over MnOx/TiO2 at higher temperatures were both restrained after the sulfation, so more NH3 adsorbed on sulfated MnOx/TiO2 was used to reduce NO to N2(SCR). Therefore, the N2 selectivity and SCR activity of sulfated MnOx/TiO2 at higher temperatures were much higher than those of MnOx/TiO2. In addition, we introduced kinetic equation and the equation we obtained through derivation could well confirm the conclusion.