Gadolinium Modified MnO_x/ZSM-5 For Low-Temperature Selective Catalytic Reduction With NH₃:Catalytic Performance And Poisoning Resistance Mechanism

Selective catalytic reduction with ammonia（NH₃-SCR）is one of the most extensive and effective methods for the removal of nitrogen oxides（NO_x）from stationary sources,in which the catalyst plays a very important role.Since the catalyst exhibits relatively high NO_xconversion and wide active-temperature range when the reaction temperature is below 250℃,low-temperature SCR has a potential industrial application prospect.However,the low-temperature SCR catalyst is readily poisoned under the residual SO₂and H₂O in the actual flue gas,which is till now not effectively solved.Therefore,based on the perspective of environmental and economic benefits,it is of great practical significance to develop a novel catalyst with the high performance for sulfur and water resistance as well as high catalytic activity.Mn O_xshows good low-temperature SCR activity due to the abundant variable valence and strong redox properties.ZSM-5 molecular sieve possesses the good hydrothermal stability,suitable acid properties,regular pore structure and high specific surface area,which is generally used as SCR catalyst carrier.Doping modification is an important way to improve the performance of SCR catalyst.In this work,a series of Mn O_x/ZSM-5 catalysts were prepared with Mn O_xas the active component and ZSM-5 as the carrier.The low-temperature SCR catalytic performance were further improved through the modification.Combined with some characterization technologies,the structure-activity relationship,reaction mechanism and SO₂resistance mechanism of the catalysts were investigated.Firstly,Mn O_x/ZSM-5 catalysts with a Mn loading amount of 10 wt%were prepared by various methods,such as impregnation,chemical precipitation and citric acid-ethanol dispersion method,of which the optimal one was determined through low-temperature SCR activity evaluation.Further,the effect of preparation parameters on catalytic activity were also investigated.The results showed that the catalyst prepared by the citric acid-ethanol dispersion method exhibited the best catalytic activity,of which 90%NO conversion was achieved at a reaction temperature of 120～140℃.Besides,when the molar ratio of absolute ethanol to Mn,citric acid to Mn,the Mn loading amount,and the calcination temperature were 450,0.5,15 wt%and 400℃,respectively,the synthesized catalyst demonstrated 100%NO conversion at 140℃.Secondly,Fe,Co,Ni,La,Ce,Pr,and Gd as modified components based on the redox and acid properties were added to the Mn O_x/ZSM-5 catalysts to improve the performance of SO₂and H₂O resistance.The best modified element and its optimal doping amount were screened out by experimental evaluation.The structure-activity relationship of the catalysts before and after the modification were also studied.The results showed that the addition of Gd on Mn O_x/ZSM-5 catalysts presented the best low-temperature SCR performance.When the molar ratio of Gd/Mn was 0.3,the Gd-modified catalyst achieved 100%NO conversion at120℃as well as more than 96%N₂selectivity and high stability.In particular,the modification of Gd significantly enhanced the SO₂and H₂O poisoning resistance.There was a strong interaction between Gd and Mn.The characterization results of BET,XPS,H₂-TPR and NH₃-TPD showed that the doping of Gd greatly increased the specific surface area,ratio of(Mn⁴⁺+Mn³⁺)/(Mn⁴⁺+Mn³⁺+Mn²⁺),surface chemisorbed oxygen concentration,redox capacity,and amount of weak acid and medium-strong acid.XRD and TEM results also illustrated that the active component of Mn O_xwas highly dispersed on the ZSM-5 carrier in a microcrystalline state and the addition of Gd inhibited the crystallinity of Mn O_xin some extent.Finally,in situ DRIFTS was used to explore the low-temperature NH₃-SCR reaction pathway and the SO₂resistance mechanism of the Mn O_x/ZSM-5 catalysts before and after the addition of Gd.The results showed that Gd significantly promoted the ability to adsorb NH₃and NO_xspecies.The NH₃-SCR reaction before and after the catalyst modification followed the E-R and L-H mechanism simultaneously.The adsorption of NH₃on the catalyst surface before and after the modification were not significantly affected by SO_2.The addition of Gd could inhibit the adsorption of SO₂and block the reaction between SO₂and the active sites of Mn O_xto generate sulfate species（such as Mn SO₄）.As a result,the reaction pathway was almost unaffected by SO₂,which thereby significantly promoted the SO₂poisoning resistance of the catalyst.