DeNO_x Performance Of Low Temperature SCR Catalyst Supported On Layered Double Oxide

Nitrogen oxides（NO_x）from the combustion of industrial fossil fuels are one of the major pollutants in the atmosphere,which accelerate the production of secondary pollutants such as fine particulate matter(PM_2.5)and ozone（O₃）.Selective catalytic reduction with ammonia（NH₃-SCR）is a highly efficient denitrification technology.The development of low-temperature NH₃-SCR catalysts can effectively promote the technology as an end-of-pipe treatment unit for flue gas denitrification under different operating conditions.Thus,this work investigates the low-temperature catalytic reduction of NO_x and develops MA/Mg₂Al O_x catalysts with excellent low-temperature activity and H₂O resistance through the modification of the manganese precursor,calcination temperature of the support and the ratio of Mg/Al.Deep investigations on the mechanism of NH₃-SCR and H₂O resistance over MA/Mg₂Al O_x provide innovative insights into the design of efficient catalysts for low-temperature NH₃-SCR.The details are as follows:Firstly,the novel catalysts are prepared by impregnation method with Mg Al-LDO as the support and Mn O_x as the active component.At the same time,the effect of different Mn salt precursors（manganese sulphate（MS）,manganese nitrate（MN）and manganese acetate（MA））on the catalytic performance is investigated.It is shown that the catalysts obtained with different Mn precursors differ in the morphology of the active component Mn O_x,the specific surface area and pore structure properties,redox and surface acidity,which in turn affect the catalytic efficiency.The MA/Mg₃Al O_x-900 and MN/Mg₃Al O_x-900 samples with Mn O₂ as the main crystalline phase show excellent catalytic activity at medium-low temperatures,while the MS/Mg₃Al O_x-900 with Mn SO₄as the main crystalline phase does not show significant activity.The activity tests show that the catalyst prepared with MA exhibts the best denitrification activity with the NO_xconversion higher than 80%at 100-250°C,which is related to the large surface area,the strong reducibility of the surface Mn O₂ and the abundant surface acidity.Then,the effect of the calcination temperature of the support on the catalytic performance is further investigated.The activity tests show that the MA/Mg₃Al O_x-800catalyst has excellent denitrification performance at low temperatures,achieving the maximum NO_x conversion of 99.2%at 200°C.The strong interaction between the high temperature calcined Mg Al-LDO support and the active component allows Mn to exist in the form of Mn O_x species on the one hand and to dope into the spinel phase as Mn³⁺to form Mg Mn₂O₄ on the other hand,enriching chemisorbed oxygen on the catalyst surface.Subsequent comparative studies with Mn/γ-Al₂O₃,Mn_0.5Mg₃Al₁-LDO and existing catalysts verify the superiority of Mg Al-LDO as the support of Mn-based NH₃-SCR catalyst.Based on the special layered structure and compositional variability of LDH,the effect of the Mg/Al ratio on the denitrification performance of the catalysts is investigated.It is shown that the modulation of the Mg/Al ratio results in a wider catalytic activity window.The MA/Mg₂Al O_x catalyst with Mg/Al=2 shows the best catalytic activity with the NO_x conversion higher than 80%at 100-300°C.The abundance of Mn⁴⁺and reactive oxygen species on the catalyst surface accelerate the redox cycle in the SCR reaction.Furthermore,the MA/Mg₂Al O_x catalyst has the outstanding H₂O resistance with the NO_xconversion remaining stable at around 90%in the presence of 5vol.%H₂O after 8h reaction at 200°C.The H₂O molecules weaken the oxidation ability of the catalyst to a certain extent,resulting in the improved N₂ selectivity at high temperatures.At the same time,the large specific surface area and ordered hierarchical structure of the MA/Mg₂Al O_x catalyst prevent the condensation of H₂O molecules in the hole.Finally,in situ diffuse reflectance infrared spectroscopy（in situ DRIFTS）is used to investigate the reaction pathway of catalytic reduction of NO_x over the MA/Mg₂Al O_x.The results show that adsorbed ammonia on the surface of the catalyst and gaseous NO would react according to the Eley-Rideal mechanism to form environmentally friendly N₂and H₂O.Moreover,the modulation of the Mg Al ratio not only enhances the adsorption capacity of the catalyst for the reactants,but also the activation capacity of the adsorbed species,leading to the formation of beneficial intermediate species such as NH₂,which effectively facilitates the low-temperature SCR reaction.