Resistance To H₂O & SO₂ And Reaction Mechanism Of Mn-based Catalysts For Selective Catalytic Reduction Of NO_x With NH₃ At Low Temperature

Low-temperature selective catalytic reduction?LT-SCR,<250??is an urgent approach for NO_x reduction of exhaust gases from steel,coking and other industries.Mn-based catalysts are widely investigated for LT-SCR due to its various valence states and excellent redox performance at low temperature.However,the inhibition and poisoning effects of H₂O or/and SO₂ are one of the severe challenges.In this paper,various characterizations and DRIFTS experiments aimed at elucidating the physicochemical properties,intermediate species,and behavioral effects of H₂O&SO₂ on reaction pathways over numerous Mn-based catalysts during the SCR process.Furthermore,the Mn-based catalysts with good resistances to H₂O&SO₂ were successfully synthesized with special structure and morphology by improving the synthesis methods.Firstly,metals doped MnO_x-CeO₂ catalysts with a molar ratio of Me:Mn:Ce=1:4:5?M= Cu,Co,Cr,Ni,Fe,Sn,Mg?were prepared by a co-precipitation method for LT-SCR.The results showed Co/Ni doped catalysts had high SCR performance and good tolerance to SO₂,which obtained about 78%NO_x conversion in the presence of 150 ppm SO₂ at 175 ?.The reasons for better SCR catalytic activity were the improvement of physicochemical properties such as higher surface area and Ce⁴⁺/Ce³⁺,more Mn³⁺ and Mn⁴⁺,and also the enhancement of reaction pathways with the abundant and enhanced coordinated NH₃,N₂O₄ and bidentate nitrate active species.The good resistance to SO₂ benefited from the suppression of surface ammonium bisulfate and metal-sulphation,the trifling impacts of SO₂ on the reaction pathways of bidentate nitrate species,and also the ER mechanisms.Then,Ni or Co modified MnO_x catalysts prepared by a co-precipitation method were investigated for NH₃-SCR reaction.It was found that Mn?2?Ni?1?O_x and Mn_XCo_3-xO₄ catalysts performed excellent low-temperature SCR activity and good resistance to SO₂,which obtained above 80%NO_x conversions within 15 h in the presence of 150 ppm SO₂ at 175 ?.The main reasons for excellent SCR activity were due to the special spinel structure with the alternate tetrahedrons and octahedrons,and the efficient electronic interaction of different valence state such as Mn³⁺+Ni³⁺<???Mn⁴⁺+Ni²⁺ or Mn³⁺+Co³⁺???Mn⁴⁺+Co²⁺.The essential reaction pathways based on ER mechanism were less affected by the competitive adsorption with SO₂,which was deemed as one of the major reasons for good SO₂-resistance.Furthermore,the NiMn₂O₄ spinel with different morphologies were investigated.The urchin-like and foliated NiMn₂O₄ spinel catalysts with more uniformity,high surface area and little pore size was successfully synthesized by the combined methods of hydrothermal synthesis with co-precipitation and urea-hydrolysis,respectively.The uniform foliated NiMn₂O₄ spinel obtained an outstanding low-temperature activity and excellent resistance to H₂O and SO₂,which obtained 85?90%NO_x conversion in the presence of 10 vol.%H₂O and 150 ppm SO₂ at 150 to 300?.The excellent resistance were associated with the spinel structure and foliated morphology,which an outside configuration of tetrahedrons embed by Ni sites avoided the sulfation of inside octahedrons wrapped Mn active sites,and the foliated morphology retarded the adhesion of sulfur ammonia species resulting in the remission of deposition/inhibition effects.Finally,the inverse CrMn₂O₄ spinel catalyst has a good SCR activity due to the micro-mesoporous CrMn₂O₄ spinel with high surface area,more active sites and effective electron transfer.Besides,the abundant chemisorbed oxygen of CrMn₂O₄ was benefit for NO oxidation to NO₂ promoting the "fast-SCR" reaction.The inverse CrMn₂O₄ spinel showed worse resistance to SO₂ but good to 10 vol.%H₂O with 80?86%NO_x conversion and benign to 10 vol.%H₂O + 150 ppm SO₂ with 69?75%NO_x conversion within 20h at 200 ?.The main reason for SO₂ poisoning was due to the depositions of ammonium bisulfate,resulting the blockage of surface pores.The formation of Cr???sulfate could play an important role in protecting Mn active sites away from sulfating.Besides,in the coexistence of H₂O and SO₂,the transform of-HSO₃ and ?SO₄?^2- to (H...?SO₄?^2-)can provide new Br???nsted acid sites for ionic ?NH₄?⁺,enhancing the SCR activity via fast-SCR.Ni or Co has a significant improvement on the SO₂-resistance of Mn-based catalysts.And then MeMn₂O₄ spinel?Me=Ni/Co?catalysts were prepared,and the favorable SO₂-resistance was attributed to the special construction of spinel crystal and insusceptible ER mechanism to SO₂.Furthermore,good resistance of foliated spinel catalyst was also due to the remission of sulfur ammonia.As a counterexample,the inverse spinel presented poor resistance to SO₂ due to the Mn-external structure.