The Research Of Fe₂O₃-WO₃ Catalyst On The NH₃-SCR Reaction Mechanism And Low-temperature Activity Regulation

As one of the major air pollutions,NO_x posed a serious threat to the ecological environment and the health of life.NH₃ selective catalytic reduction technology was widely used in industrial fixed source emission control due to its advantages of high NO_xremoval efficiency,high N₂ selectivity and wide active temperature window.The emissions of SO₂,NO_x and other pollutants in the sintered flue gas of steel in China were relatively large,and the SO₂ concentration in the sintered flue gas often exceeded 1000 mg/Nm³,resulting in severe sulfur poisoning of the catalyst in the traditional SCR technology.Therefore,the denitration unit was usually installed after the activated carbon dry desulfurization facility,but at this time the flue gas temperature was low（150～250℃）,the conventional SCR denitration catalyst had low activity,and it was difficult to meet the NO_x efficient purification requirements.Therefore,there was an urgent need for a highly active,low temperature SCR catalyst that matched this system.In this paper,a novel preparation method was used to regulate the wide activity window of Fe₂O₃-WO₃ catalyst,and the low-temperature NH₃-SCR activity of Fe₂O₃-WO₃ catalyst was further regulated by MnO_x doping.Various characterization techniques were used to analyze the relationship between the physicochemical properties of the catalyst and the NH₃-SCR activity,and the in-situ DRIFTS technology was used to explore the adsorption and reaction behavior of the reactants on the catalyst.The main results obtained followed:Firstly,Fe₂O₃-WO₃ catalyst was synthesized by a simple and solvent-free one-step method for selective catalytic reduction of NO.The results showed that the Fe₂O₃-WO₃ catalyst achieved near 100%NO_x removal efficiency and excellent N₂selectivity over a wide temperature range of 225-500℃.Raman and XPS results showed that the introduction of WO₃ changed the electronic environment of Fe₂O₃,induced the formation of Fe₃O₄(Fe²⁺)and surface active oxygen species,and promoted the SCR reaction.In situ DRIFTS analysis showed that the NO_x reduction reaction on the Fe₂O₃-WO₃ catalyst proceeded synchronously through the Eley-Rideal and Langmuir-Hinshelwood mechanisms,but the latter was the main route.This was because the interaction between WO₃ and Fe₂O₃ not only enhanced the adsorption capacity of NH₃ on the catalyst,but also promoted the formation of NO_x adsorbed species.The adsorbed NO₂ became the main species on the Fe₂O₃-WO₃ catalyst.This facilitates the“fast SCR”.Subsequently,doping MnO_x into Fe₂O₃-WO₃ catalyst modified its low-temperature activity.The results showed that the MnO_x-Fe₂O₃-WO₃ catalyst showed an SCR activity of more than 90%at 175℃and excellent N₂ selectivity in the reaction temperature range（50～250℃）.XRD,Raman and XPS results showed that MnO_x promoted the formation ofγ-Fe₂O₃ structure and Mn WO₄ crystal phase in the catalyst,resulting in a change in the coordination environment of iron and manganese and the formation of more adsorbed oxygen on the surface,thereby promoting the SCR reaction.The in situ DRIFTS results showed that the MnO_x-Fe₂O₃-WO₃ catalyst in the low-temperature NH₃-SCR reaction mainly followed the Eley-Rideal reaction mechanism,that is,the adsorbed NH₃ dissociated into-NH₃（a）/-NH₂（a）and reacted with NO（g）.The reaction produced an intermediate NH₂NO,and then decomposed to form N₂ and H₂O.At the same time,the over-oxidation of-NH₃ species to N₂O by-products was inhibited.In summary,a Fe₂O₃-WO₃ catalyst with excellent wide activity temperature window and N₂ selectivity was prepared in this paper.The low-temperature activity and N₂ selectivity of Fe₂O₃-WO₃ catalyst were further enhanced by MnO_x doping.Through systematic analysis and research,the primary factors affecting the SCR activity of this series of catalysts and the main pathways of the reaction were initially recognized.However,the stability of MnO_x-Fe₂O₃-WO₃ catalyst in complex actual flue gas needs further study to explore its industrial applicability.