Acidic Support Design And NH₃-SCR Performance Of Vanadia-based Catalysts

Nitrogen oxides?NO_x?has several adverse effects on the environment.The selective catalytic reduction?SCR?of NO with NH₃has been proven to be a mature technology of controlling NO_xemissions in terms of removal efficiency,stability and economics.V₂O₅supported on anatase TiO₂,which is promoted by WO₃or MoO₃,is currently being commercialized for stationary applications owing to its stability and high catalytic activity at 200-450°C Nevertheless,this catalyst exhibits relatively low NH₃-SCR activity at low temperatures and insufficient sulfur resistance.Aiming at these issues,V-based de NO_xcatalysts with high performance were developed by designing different acidic supports?WO₃and MoO₃-TiO₂?.Furthermore,the structure-activity relationship,especially the influence of acidity and redox ability,was investigated by analyzing catalyst structure and surface properties.On catalyst aspect,it was observed for V₂O₅/WO₃catalyst that WO₃with?001?dominant exposed facet showed higher NH₃-SCR activity than the?100?dominant one.The maximal NO_xconversion of V₂O₅/WO₃with?001?facet reached 72%at 400°C and was elevated to 96%after loading 5%V₂O₅.The temperature window?with NO_xconversion exceeding 80%?of V₂O₅-MoO₃/TiO₂catalyst was 250-400°C in the presence of 10%H₂O and 300 ppm SO₂,and its maximal NO_xconversion was close to 100%.The NO_xconversion increased sigfniantly from 70%to 88%over V₂O₅-MoO₃/TiO₂catalyst during a continuous NH₃-SCR activity test at 250°C for 30 h,indicating good sulfur resistance,while a slight deactivation occurred for V₂O₅-WO₃/TiO₂catalyst.On reaction mechanism aspect,more unsaturated W⁶⁺atoms existed on?001?facet of hexagonal WO₃,which could adsorb hydroxyl to generate more Br?nsted acid sites.After vanadia impregnation,more V⁴⁺were generated via the electron transfer from high proportioned W⁶⁺.Furthermore,the better matching of bond length between V-O bridge in vanadia and W-O bridge in W-O-W circle on?001?exposed facets promoted the formation of polymeric vanadates.This,as well as more chemisorbed oxygen and V⁴⁺,benefited NH₃-SCR reaction.For V₂O₅-MoO₃/TiO₂catalyst,the SO₂oxidation activity was low and less sulfates were formed.Meanwhilie,the reactivity of NH₄HSO₄with NO was accerelated over this catalyst.Therefore,less NH₄HSO₄was deposited on V₂O₅-MoO₃/TiO₂catalyst surface than on V₂O₅-WO₃/TiO₂.Besides,the high proportioned stable tridentate sulfates on the sulfated V₂O₅-MoO₃/TiO₂catalyst surface acted as strong Br?nsted acid sites.This liberated more active polymeric vanadates to activate the adsorbed NH₄⁺to form–NH₂species as a crucial intermediate for NH₃-SCR reaction.The above mechanisms offered insight into designing low-temperature vanadium-based SCR catalysts with high activity and high durability in the presence of sulfur.