Metal Ions Modified Cu-SSZ-13 Zeolites And Their Catalytic Performance On NH₃-SCR Reaction

Nitrogen oxides(NO_x),from industrial waste gas and vehicle exhaust emissions,are one of the major atmosphere pollutants,which can endanger the human health and the surrounding environment.Despite of their small quantity(10%),diesel vehicles can emit nearly 90%NO_x emissions of the total NO_x emissions of vehicles.Therefore,controlling the NO_x emissions of diesel vehicles is the main task at present.Currently,because of its efficient reduction of NO_x and less secondary pollution emissions,ammonia selective catalytic reduction(NH₃-SCR)is the most promising technology,which has been applied in diesel vehicle exhaust treatment.With the development of environmental regulations on strict control of NO_x emissions,NH₃-SCR catalysts have gone through several major innovations,from the earliest precious metal catalysts,metal oxide catalysts to current commercial zeolite catalysts.Nowadays,due to their superior catalytic activity and high-temperature hydrothermal stability,copper ion-exchanged SSZ-13 zeolite(Cu-SSZ-13)catalysts have been commercialized in Europe and the U.S.SSZ-13,a chabazite-type(CHA)zeolite,possesses three-dimensional eight-membered ring(8MR)channels with the pore diameter of 0.38 nm×0.38 nm.It is well known that there exist two types of active Cu sites in Cu-SSZ-13,i.e.,Cu²⁺ions located at the 6-membered rings(6MRs)(Z₂Cu²⁺,Z stands for a zeolite framework negative charge)and Cu²⁺ions located at the 8MRs([ZCu(OH)]⁺).The former sites have higher hydrothermal stability,while the latter sites possess better low-temperature activity.Due to strong solvation effects from NH₃ and H₂O for Cu sites at low temperatures(?200°C)and the formation of inactive Cu O_x particles as well as dealumination during high-temperature hydrothermal ageing,Cu-SSZ-13zeolite still suffers from some issues such as poor low-temperature activity and low hydrothermal stability.To deal with the above challenges,several strategies have been proposed.For example,making Cu-SSZ-13-core with hydrophobic shell,fabricating composites of metal oxides and zeolites and introducing extra metal cations.Compared with the former two strategies,introducing secondary metal ions into Cu-SSZ-13,such as K⁺,Na⁺,Ce³⁺,can regulate the distribution and redox capacity of the two Cu sites,which can enhance the catalytic activity of Cu-SSZ-13.Moreover,the metal cations can remove a portion of Br(?)nsted acid sites,which can alleviate hydrolysis of the zeolite under high-temperature hydrothermal conditions,thereby promoting the hydrothermal stability of the catalyst.Therefore,the introduction of secondary metal ions into Cu-SSZ-13 is a simple and effective method to solve the above problems.In this thesis,we aimed to synthesize high performance zeolite catalysts for NH₃-SCR by introducing extra metal ions into Cu-SSZ-13.Transition metal Fe³⁺and rare earth Sm³⁺,La³⁺ions were introduced into SSZ-13 zeolite by in-situ or ion-exchanged method to achieve the improvement of low-temperature catalytic activity(NO conversion above 90%at 200°C)and hydrothermal stability of Cu-SSZ-13.Meanwhile,the working mechanism of extra metal ions and the effect of metal ion content on the catalytic activity of Cu-SSZ-13 were revealed.These researches will provide reference and guidance for the preparation of high-performance Cu-SSZ-13zeolite catalysts.The main contents of this thesis are as follows:1.SSZ-13-D zeolites were successfully synthesized with a silicon source of Fe-containing diatomite,and Fe species existed in the form of extraframework Fe³⁺ions.To investigate the functions of Fe³⁺ions,SSZ-13-S zeolite was synthesized with a silicon source of silica sol.Because of the intrinsic six-membered ring in diatomite,the crystallization time of SSZ-13-D was shortened,which was only half of that of SSZ-13-S.SSZ-13-D and SSZ-13-S samples were ion-exchanged with a similar Cu content,and Cu-SSZ-13-D exhibited better catalytic activity and hydrothermal stability than Cu-SSZ-13-S.The NO conversion of Cu-SSZ-13-D was higher than that of Cu-SSZ-13-S at the temperature range of 175-450°C despite of hydrothermal aging.The experimental characterizations demonstrate that Fe³⁺ions can favor the dispersion of Cu species and promote the transformation of[ZCu(OH)]⁺to Z₂Cu²⁺during hydrothermal treatment,thereby inhibiting the formation of Cu O_x.The extra roles of Fe³⁺ions before and after hydrothermal aging are firstly elucidated in this work,which provides guidance and reference for the preparation of efficient Cu Fe-zeolite catalysts.2.A series of Cu-Sm-SSZ-13 zeolites were prepared by incorporating Sm³⁺as a secondary ion where Sm ions were first ion-exchanged into SSZ-13 zeolite followed by ion-exchanging with Cu ions.The NO conversion of Cu-Sm-SSZ-13 was nearly10%higher than that of conventional Cu-SSZ-13(175-250°C)even after hydrothermal ageing,showing enhanced low-temperature activity and hydrothermal stability.The Sm ions were found to occupy the six-membered rings(6MRs)of SSZ-13,which can facilitate the formation of more active[ZCu(OH)]⁺ions at 8MRs.There existed electron transfer from Sm³⁺to[ZCu(OH)]⁺ions,which promoted the activity of[ZCu(OH)]⁺ions by decreasing the activation energy of intermediates formation(NH₄NO₂ and H₂NNO).Meanwhile,the electrostatic interaction between Sm³⁺and[ZCu(OH)]⁺can inhibit[ZCu(OH)]⁺ions transformation into inactive Cu O_x species,thus enhancing the stability of[ZCu(OH)]⁺ions.The influence of the ion-exchanging sequence of Sm³⁺and Cu²⁺ions into SSZ-13 demonstrated that only ion-exchanging Sm³⁺first can regulate the distribution,activity,and stability of Cu active sites.This work provides a mechanistic insight of secondary ions,which is helpful for the design of high-performance Cu-SSZ-13 catalysts for NH₃-SCR reaction.3.On the basis of the above work,Cu-La-SSZ-13 zeolites with different La and Cu contents were prepared.Cu-La-SSZ-13 exhibited higher hydrothermal stability than Cu-SSZ-13 when La content was low.The NO conversion of Cu-La-SSZ-13 was5-10%higher than that of Cu-SSZ-13 at the range of 450-550°C after hydrothermal ageing.The characterization experiments revealed that La³⁺ions can regulate the Cu distribution and promote the formation of more stable Z₂Cu²⁺sites when La content was low,thereby improving the hydrothermal stability of the catalyst.However,more Cu O_x can be formed accompanying with the destruction of framework when La content was high.In addition,the low-temperature activity of Cu-SSZ-13 with low Cu content can be enhanced by introducing a small amount of La³⁺.The experimental results showed that La³⁺ions can improve the redox ability of Cu active sites in this case.This work demonstrates that rational regulation of the content of secondary metal cations can greatly improve the catalytic performance of the catalyst,which provides a guidance for the design of high-performance bimetallic zeolite catalysts.