Study On SAPO-18 Zeolite Based Catalysts For The Selectively Catalytic Reduction Of NO_x By NH₃

Nitrogen oxides?NO_x?emission from the processing and utilization of fossil fuel such as coal and oil are a major type of air pollutants,which bring photochemical smog,acid rain,and harm to human health.Thus the environmental concern stimulates the more and more stringent regulations aimed at confining the emissions of NO_x.NH₃-SCR is one of the most effective technology for NO_x elimination.Zeolite-based catalysts with small pores have been attracted much attention from the researchers due to their advantages for NH₃-SCR reaction in recent years.They are more active and thermally stable in catalyzing NH₃-SCR reaction.In this thesis works,a series of zeolite-based catalysts were designed and prepared,using silicoaluminophosphate SAPO-18 zeolite with small pore as the support object and transition metals such as Fe,Mn and Cu as the active components.The catalytic activitives of these catalysts for NO_x elimination were evaluated by NH₃-SCR technology,while the physiochemical properties were characterized.The structure-activity relationships between structures and NH₃-SCR performcances were discussed,while the intracrystalline diffusion limitations were investigated.The SCR mechanism were also proposed on the basis of the in-situ DRIFTs experiments and DFT calculations.The main conclusions are as follows:?1?Fe-SAPO-18 catalysts were synthesized by impregnation,ion exchange and one-pot methods,respectively.The NH₃-SCR activities for impregnation method and one-pot method prepared catalysts are dissatisfactory.Fe-SAPO-18 catalyst prepared by ion exchange method exhibited excellent high-temperature NH₃-SCR activity.NO conversion were higher than 80%between 350 ? to 500 ?,while the N₂ selectivity were higher than 90%.This is mainly attibuted to the more active Fe³⁺ions in SAPO-18zeolite.After Mn loading,the active temperature window of MnFe-SAPO-18 catalysts moves towards lower temperature.The low-temperature activity was greatly increased,yet the high-temperature activity was decreased.Mn₅Fe_0.42Z catalyst maintained over80%NO conversion between 275 ? and 425 ?,but its N₂ selectivity decreased,mainly due to the generation of N₂O.Mn species in the form of amorphous MnO_x were the active centers of MnFe-SAPO-18 catalysts,which are mainly distributed on the surface of zeolite.In-situ DRIFTs experiments suggested the different ways for catalyzing NH₃-SCR reaction by Fe and Mn active sites.The activation of NO_x species on the Fe centers were the important steps for SCR reaction.The activation and dissociation of NH₃ species were facilated on Mn centers,causing the formation of N₂O via the reaction with NO_x species.?2?A series of Cu-SAPO-18 catalysts with various Cu loadings were prepared and tested for NH₃-SCR reaction.Isolated Cu²⁺ions are confirmed to be the catalytic active sites of Cu-SAPO-18.Cu-4.42 catalyst exhibits high NO conversion?>90%?at the lowest temperature of 180 ? among all catalysts.It can be attributed to the maximum amount of isolated Cu²⁺ions in Cu-4.42 catalyst.DFT calculations show that the isolated Cu ions are located in the pear shaped cavity and exhibit a preference for the neighboring of 6R planes of Cu-SAPO-18.NH₃-SCR mechanism over Cu-SAPO-18catalyst is elucidated by a combination of in-situ DRIFTS technique and DFT calculations,in which the dissociation of NH₃ and the oxidation of NO are shown to be key steps in reaction.?3?Cu-based SAPO-18 zeolite was successfully synthesized by adding Cu source into the synthetic gel of SAPO-18 zeolite.The NH₃-SCR activivity of fresh Cu-F-500catalyst was low,for which NO ceoversion reached to 80%only in the temperature range of 250-350 ?.Through high temperature aging,the SCR activity increased,and the low-temperature and high-temperature activity were both improved.Among them,the NO conversion Cu-T-800 catalyst is over 80%in the temperature range of 200-450?.This was maily due to the transformation of CuO clusters into active Cu ion via the substitution reaction between CuO and protons in SAPO-18 framwork under high temperature aging.It could enhanced the adsorption and activation of reactant NH₃ and NO_x species.The NH₃-SCR reaction activity was influenced by the intracrystalline diffusion,which is also related to the state and location of Cu species in the prepared Cu-SAPO-18 zeolites.The diffusion limitation was weaken due to the transformation of CuO clusters into active Cu ion.But under the more severe condition of high temperature,the diffusion limitation would be slightly enhanced due to the migration of active Cu ions.?4?A series of CuZ-Ce_n catalysts with thin CeO₂ film on the external surface of Cu-SAPO-18 were prepared by the means of liquid deposition.The thickness of CeO₂film was controllably tuned,and the different film thickness significantly influenced the catalytic activities of CuZ-Ce_n catalysts for NH₃-SCR reaction.The protection of CeO₂thin film not only inhibits the active sites from aggregating into copper oxides at high temperature,but also suppresses H₂O from competitively adsorbing on the acid sites and Cu active sites over CuZ-Ce_n catalysts.Thus CuZ-Ce_n catalysts exhibit preferable resistance to H₂O.Moreover,CeO₂ thin film can suppress the formation and deposition of sulfate species from blocking the active sites over CuZ-Ce_n catalysts.It leads to that CuZ-Ce_n catalysts possess the higher SO₂ resistance than Cu-SAPO-18.