Modification Of Fe₂O₃ Catalysts For The Selective Catalytic Reduction Of No_x By NH₃

At present,selective catalytic reduction of NO_x with NH₃（NH₃-SCR）is one of the most effective NO_x emission control technology.V₂O₅-WO₃/TiO₂ is the most widely-used commercial catalyst,which has the advantages of high catalytic activity and selectivity.However,the V₂O₅-WO₃/TiO₂catalyst has the problems of high cost,heavy metal vanadium loss and secondary pollution.Therefore,it is of great economic and practical significance to develop low-cost and environment-friendly catalysts with high catalytic activity.Fe-based catalysts have been widely investigated for NH₃-SCR reaction due to their high abundance,low price,no-pollution to environment.However,the low-temperature activity and sulfur resistance of Fe-based catalysts are still unsatisfactory.In this paper,the industrial V₂O₅-WO₃/TiO₂ catalysts after running for two and three years,denoted as VWTi-2 and VWTi-3,respectively,were evaluated and the deactivation mechanism was studied.Then,the Fe₂O₃catalysts were modified by phosphotungstic acid（HPW）and doped with Sm to obtain the HPW/Fe₂O₃ and Sm/Fe₂O₃ catalysts,respectively.The low-temperature NH₃-SCR activity and sulfur resistance of Fe-based catalyst were improved and the corresponding mechanisms were discussed.1.The industrial V₂O₅-WO₃/TiO₂catalysts with different running time were characterized.The VWTi-2 catalyst still shows excellent low-temperature activity and sulfur resistance,and its compressive strength and specific surface area all meet the standard of fresh commercial catalyst.However,the low-temperature activity of VWTi-3 catalyst decreases remarkablely.The characterizations show that the generation and deposition of sulfate species was the main reason for the deactivation of V₂O₅-WO₃/TiO₂ catalysts.More VOSO₄,（NH₄）₂SO₄ and NH₄HSO₄ species are covered on the VWTi-3 catalyst,which causes the significant decrease of specific surface area,V⁵⁺concentration and surface adsorbed oxygen（O_α）,restricting the low-temperature activity.2.Flower-like microstructed Fe₂O₃ catalysts were prepared via solvothermal reaction by using FeCl₃·6H₂O,hexamethylene tetramine and ethylene glycol as raw materials,then were modified by HPW via impregnation,rotary evaporation drying and calcination processes.When the mass ratio of HPW to Fe₂O₃ is 0.5 and the calcination temperature is 350 ^oC,the as-obtained HPW/Fe₂O₃-350-0.5 catalyst exhibits nearly 100%of NO conversion at 240-460 ^oC as well as good SO₂ resistance.The number of Br（?）nsted acid sites and Lewis acid sites increases significantly after HPW modification,which promotes the adsorption of NH₃.Moreover,the incorporation of HPW decreases the oxidation ability of Fe₂O₃ catalysts,which effectively inhibits the overoxidization of NH₃,resulting in excellent catalytic activity and N₂ selectivity.In-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）results indicate that the NH₃-SCR reaction on HPW/Fe₂O₃-350-0.5 catalyst mainly follows Eley–Rideal（E-R）mechanism,in addition,the adsorption and oxidization of SO₂ on the surface of HPW/Fe₂O₃-350-0.5 catalyst is suppressed due to high surface acidity and the decrease of oxidation ability,leading to the enhancement of SO₂ tolerance of HPW-modified Fe₂O₃catalysts.3.The Sm-doped Fe₂O₃ catalysts were prepared via coprecipitation by using FeCl₃·6H₂O,FeCl₂·4H₂O,Sm（NO₃）₃·6H₂O and ammonia as raw materials,followed by calcination.When the molar ratio of Sm and Fe is 0.0225 and the calcination temperature is 500 ^oC,the as-obtained0.0225Sm/Fe-500 catalyst shows the best low-temperature activity,with>90%of NO conversion at 175-380 ^oC,as well as the excellent SO₂ resistance.Sm doping inhibits the transformation ofγ-Fe₂O₃ toα-Fe₂O₃ at 500 ^oC,improving the thermal stability ofγ-Fe₂O₃phase.A proper amount of Sm doping can increase the specific surface area,surface acidity,oxidation ability and amount ofO_α,which are beneficial to the improvement of low-temperature activity.However,when the molar ratio of Sm and Fe is above 0.0225,the oxidation ability andO_αcontent of catalyst decrease,suppressing the conversion of NO to NO₂,which is detrimental to the low-temperature NH₃-SCR performance.Moreover,the XPS results demonstrate that SO₂can preferably adsorbed on Sm sites to generate Sm₂（SO₄）₃,which effectively alleviates the sulfidation of Fe active sites,resulting superior SO₂tolerance of Sm/Fe₂O₃ catalysts.