Synthesis And Study On FeAlPO-5 Catalyst For Selectively Catalytic Reduction Of N₂O With Methane

Nitrous oxide (N₂O) is considered as an environmental pollutant since it has been recognised as a strong greenhouse gas and as a contributor to stratospheric ozone layer depletion.The concentration of nitrous oxide in the atmosphere continues to increase. And this increase appears to be caused mainly by human activities. It thus appeared quite important to convert N₂O into harmless products. The selective catalytic reduction (SCR) of N₂O with methane has attracted a lot of attention in recent years. Various types of catalysts have been reported to be active for the reaction. However, oxygen and water vapor are typically present in the exhausts and will cause seriously deactivation to the SCR catalysts. Therefore, finding new catalyst which has high activities for the SCR of N₂O in the presence of oxygen and poisoning of water is very important.Using FeAlPO-5 zeolites as catalysts in the N₂O reduction was investigated. Methods of XRD, IR, SEM, DR UV-Vis and DRIFT were used to characterize these catalysts as well as to investigate the mechanism of CH₄-SCR of N₂O. The main experimental results and conclusions are summarized as follows:A series of FeAlPO-5 molecular sieves were synthesized by the hydrothermal method by aluminum source complete and incomplete solubility. The catalysts were characterized by XRD, IR and SEM and evaluated in the selective catalytic reduction (SCR) of N₂O with CH₄. The results reveal that the structure and morphology of FeAlPO-5 were affected by the gel preparation conditions such as aluminum source solubility, iron loading. It is easy to form dense tridymite phase over FeAlPO-5 when other agents was added after the aluminum source at the solubility of 100%. The excess iron loading also led to the transformation of AlPO₄-5 from AFI structure to dense tridymite phase. The activity of FeAlPO-5 zeolite is best for the CH₄-SCR of N₂O reaction. Effect of the amount of Fe loading and proportion of reactants in N₂O reduction over FeAlPO-5 catalysts was investigated. The results indicated that Fe introduce AlPO₄-5 zeolite improve the catalytic activity for N₂O decomposition and N₂O reduction, the activity of FeAlPO-5 catalyst increases with the increment of introduce Fe and conversion of N₂O increases with the increment of the amount of CH₄ reductant.By comparision of the activity of FeAlPO-5 and impregnated Fe/AlPO-5 zeolites in CH₄-SCR of N₂O with the presence of excess oxygen and/or steam, it has found that the FeAlPO-5 zeolite has the higher activities, selectivities and stabilities than other zeolite, the activity was maintained at least for 12 h in the coexist of excess oxygen and water in the feed gas. The analytical result of XDR and DR UV-Vis indicated that the hydrothermal synthesized FeAlPO-5 zeolite Fe introduce to AFI topology structure, impregnated Fe/AlPO-5 zeolite has the presence of scattering Fe_xO_y inside the zeolite pores lead to decrease the pore diameter so oxygen formed by the CH₄-SCR of N₂O is easily hard desorbed from the surface of the present catalyst. This is proving for the stabilities low. The hydrothermal synthesized FeAlPO-5 zeolite iron is introduced at extraframework positions and crystallinity of AFI structure phase is induced in CH₄-SCR of N₂O with the presence of excess oxygen and/or steam. This is valid for the steady-state activities.The adsorption of CH₃OH and the reduction of N₂O with methane were studied with in-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT). The results indicated that CH₄ and N₂O reacted over FeAlPO-5 generate methoxy groups intermediates, these groups residing on the catalyst again reacted N₂O and decomposed when the temperature was raised above 300℃.