The Superior NH₃-SCR Activity And Poison Resistance Of Titanate And Ceria Nanotubes Supported DeNO_x Catalysts

Nitrogen oxides（NO_x）is one of the main air pollutants that can cause acid rain,photochemical smog and haze weather.To control the emission of NO_x,selective catalytic reduction（SCR）of NO_x by NH₃ in the presence of O₂ has been considered as the most efficient technology.For deNO_x catalysts,the commercial vanadia-based catalysts（V₂O₅-WO₃/TiO₂ and V₂O₅-MoO₃/TiO₂）have shown excellent catalytic activity,but their serious deactivation by alkali/alkaline earth metals（hereafter referred to as alkaline metals）,phosphate,and heveay metals is still a great challenge.Besides,the catalysts encounter more serious deactivation in the application of biomass boiler,refuse incinerator,cement furnaces,glass furnaces,which caused by much higher amount of alkali/alkaline earth metals,phosphate and heveay metals.Thus,titanate nanotubes and ceria nanotubes confining metal oxides with superior SCR activity,N₂ selectivity,stability,SO₂ and H₂O tolerance,and alkaline metals/phosphate/heveay metals resistance are developed in this thesis to solve the serious deactivation issue of deNO_x catalysts.Firstly,ion-exchangeable titanate nanotubes synthesized by a treatment of ethanol washing（TNTs-eth）are utilized as supports for ceria（CeO₂）catalysts.The prepared "core-shell" catalysts showed excellent SCR activity and resistance to alkaline metal poisoning in the NH₃-SCR application,where the NO conversion could be kept at 97%,88%and 95%（at 350 ℃）when Na,K or Ca was added,respectively.It was found that this resistance was mainly attributed to the well-maintained nano-tubular structure,significantly increased structural ion-exchangeable OH groups,enhanced acidity,unchangeable Ce³⁺ content and oxygen defects that all resulted from the ethanol treatment.The nano-tubular structure of TNTs physically prevented the CeO₂ from alkaline poisoning and increased OH groups would help to stabilize the poisons in the interlayer of TNTs by chemical ion-exchange.Besides,increment of acid sites not only promoted the NH₃ adsorption but also neutralized a part of alkaline metals.Based on the modification mechanism of ethanol modified titanate nanotubes catalyst,superior SCR activity and enhanced resistance against alkaline poisoning are both achieved due to an appropriate sulfation modification on the titanate nanotubes catalysts.The optimal H₂SO₄ concentration for sulfation process should be below 0.5 mol/L.Ceria doped 0.1 mol/L sulfated titanate nanotubes catalyst exhibited～100%NH₃-SCR catalytic activity and alkaline metals resistance in the temperature region of 290-470℃.It was revealed that the enhanced alkali resistance was greatly relevant with the increased OH groups,surface acid sites and chemisorbed oxygen species.Due to the excellent ion-exchange capability of protonated titanate nanotubes,they were performed as potential support material to synthesize deNO_x catalysts with different vanadium precursors（NH₄VO₃ and VOSO₄）.The results showed that VOSO₄ exhibited better synergistic effect with titanate nanotubes than NH₄VO₃,which was caused by the ion-exchange reaction between titanate nanotubes and VOSO₄.Then high loading content of vanadium,uniformly active phase distribution,and tuned chemical property of vanadium species were achieved.When the load content of V₂O₅ was 4 wt.%,the SCR activity and alkaline metals resistance of vanadia-titanate nanotubes were above 95%and 90%,respectively.Furthermore,molybdenum is applied as an additive for vanadium-titanate nanotubes（V-TNTs）catalysts to alleviate the dual poisoning by potassium and phosphate.The introduced Mo prefered to interacte with phosphate,following with the regeneration of active and acidic sites.Moreover,in-situ DRIFT spectra showed that Mo not only enhanced the NH₃ and NO adsorption but also contributed to the activation of adsorbed NO species,making them react with NH₃ more easily over Mo-contained catalysts then directly influencing the SCR activity as well as dual resistance.The optimal molar ratiof of Mo/V was 0.25.The modified catalyst yielded above 95%SCR activity and dual resistance against potassium and phosphate in the temperature region of 300-450℃.Moreover,excellent N₂ selectivity,stability,SO₂ and H₂O tolerance were all achieved.According to the superior SCR activity and poisoning resistance performance of titanate naotubes catalysts,a novel ceria nanotubes supporting material with high redox property was synthesized.Nb,Cr,Mn,Co,Cu were selected as acitive phase to prepare the "core-shell" deNO_x catalyst.Among these,Nb showed special synergistic effect with ceria nanotubes and achieved superior SCR activity and alkaline metals/phosphate/heveay metals resistance.The NO_x removal efficiency was 98%and resistance against K,P,and Pb was 78%,90%,and 95%,respectively.After subjected to a range of characterization techniques,it was found that the remarkable deNO_x activity and resistance performance were mainly attributed to the special nanotubular structure of ceria nanotubes,smooth Nb⁵⁺/Nb⁴⁺ and Ce⁴⁺/Ce³⁺ redox cycles,superior redox capacity,and abundant surface Br（?）nsted acidic sites.