Research Of Mn/Ce-ZrO₂for Selective Catalytic Reduction Of NO_x With NH₃at Low Temperature

Selective catalytic reduction (SCR) of NO with NH3is widely used to reduce NO producing in combustion processes because of the low cost and high efficiency. V2O5-WO3(MOO3)/TiO2is the most commonly adopted commercialized catalyst system with high activity in the temperature range of300-400℃. However, the deactivation of catalysts caused by dust and SO2is serious because the unit for SCR catalysts has to be located upstream of the desulfurizer and the electrostatic precipitator when the reaction temperature is considered. In order to avoid reheating the flue gas, the unit of SCR catalysts has to be located upstream of the desulfurizer and electrostatic precipitator. However, the high concentration of dust reduces the performance and longevity of catalysts. Therefore, it’s necessary to develop low-temperature SCR catalysts which can be located downstream of the desulfurizer and electrostatic precipitator. Most of the flue gas contains a few amounts of SO2even after the desulfurizer. Catalysts for low-temperature SCR are generally very sensitive to SO2and the deactivated action of SO2will be more intense when H2O is present. Therefore, the resistance to SO2and H2O is the core of low-temperature SCR study. Base on these facts, we researched the performance of NO removal and the resistance to SO2and H2O of Mn/Ce-ZrO2.A contrastive study was made on Mn/Ce-ZrO2, Mn/P25and Mn/Al2O3. The results showed that Mn/Ce-ZrO2had a better activity at lower temperature. This corresponded to the redox character of the catalysts. The results of H2-TPR showed that Mn/Ce-ZrO2was more easily to initiate reduction reaction. Mn/Ce-ZrO2had the best resistance to SO2and H2O which might due to more acid sites and the instability of ammonia sulfate on the surface of Mn/Ce-ZrO2.A contrastive study was made on Mn/Ce-ZrO2, Mn/CeO2and Mn/ZrO2to study the reason for the good performance of NO removal and the good resistance to SO2and H2O of Mn/Ce-ZrO2. Mn/Ce-ZrO2had higher dispersion of manganese oxides, better redox property and more weakly adsorbed oxygen species, which resulted in the higher activity of Mn/Ce-ZrO2. In addition, Mn/Ce-ZrO2showed a good resistance to SO2and H2O which was due to the weak water absorption and weak sulfation process on the surface of the catalyst.Co was used as a modifier for Mn/Ce-ZrO2and Mn/Co-Ce-ZrO2with Co as a modifier for the support had a higher dispersion of manganese oxides, a better redox property and more surface adsorbed oxygen species and Mn4+. These facts caused a better low-temperature activity for Mn/Co-Ce-ZrO2. In addition, Mn/Co-Ce-ZrO2showed an excellent resistance to SO2and H2O and presented91%NO conversion under SO2and H2O and the activity of Mn/Co-Ce-Zr02totally recovered after cutting off the injection of SO2and H2O. XPS results indicated that the introduction of cobalt as a modifier for the support of catalysts decreased the formation of sulfate salts and hydroxyls on the surface of Mn/Co-Ce-ZrO2.A systematic study was made on Mn/Co-Ce-ZrO2and it showed an excellent performance of NO removal and the resistance to SO2and H2O at different conditions. The results of kinetic study showed that the active energy of Mn/Co-Ce-ZrO2was17543.3J·mol-1and the lower active energy is the reason for the good performance of NO removal for Mn/Co-Ce-ZrO2. To reduce costs, Mn/Co-Ce-ZrO2was coated on cordierite and zeolites. Both catalysts coated on cordierite and zeolites had good performance of NO removal. And the catalyst coated on zeolites had better resistance to SO2and H2O. The treatment with H2SO4for zeolites improved the resistance to SO2and H2O of the catalyst.