Investigation On The Preparation Of Fece And Few Composite Oxides And Their Performance On Selective Catalytic Reduction Of NO_x With NH₃

As a kind of atmospheric pollutants,nitrogen oxides(NO_x)have significant adverse effects on the environment as well as on humans.NH3-SCR has been accepted as a high potential technology to reduce NO_x emissions from stationary and automotive vehicles sources.Currently,V2O5-WO3/TiO2 catalysts have been used commercially,while it has many shortcomings in practical application,such as the narrow temperature window,the low N2 selectivity at high temperatures,the high manufacturing costs and the secondary pollution to the environment.To solve these problems,a series of environmental-friendly iron based catalysts were developed for NH3-SCR reaction in this paper.Characterization of spectroscopic and theoretical approaches were used to investigate the physical and chemical properties of the Fe based complex oxides,which shed new light on relating structure of catalysts with SCR activity.Meanwhile,the contribution of reactant species to SCR activity and the reaction pathways were studied by the in situ DRIFTS and MS technologies.The detailed results are as follows:(1)A series of mesoporous FexCe1-xO2-? catalysts with high surface area were prepared by a surfactant-assisted method.XRD and Raman results indicated that the doping of Fe into CeO2 led to the formation of iron cerium solid solution(specific Fe-O-Ce structure),which resulted in the increased activity and decreased activation energy for SCR reaction.Among the FeCe complex oxides catalysts,the Fe0.4Ce0.6O2-? catalyst showed the lowest activation energy(26 kJ·mol-1)and the excellent activity with above 80%NO_x conversion from 250 to 350?.XPS and DFT calculation demonstrated that the formed Fe-O-Ce structure increased the number of Lewis acid sites and improved the redox properties through the electronic inductive effect between Fe3+ and Ce4+.Importantly,the formation mechanism was related with the Fe contents.The lower Fe addition(0<x<0.3)resulted in the formation of FeCe solid solution via a vacancy compensation mechanism,leading to a remarkable increase in the oxygen vacancy and thereby the catalytic performance.On the other hand,the higher Fe addition(1>x ? 0.3)created interstitial Fe3+ species via a dopant interstitial compensation mechanism.These iron species with a relatively lower Hirshfeld charge of 0.19 presented the rich electron properties,which were favorable for the NO oxidation to NO2.(2)Iron tungsten catalysts were developed,achieving an excellent performance with a wide operating temperature window.The Fe0.75W0.25O? catalyst exhibited the best activity with above 90%NO_x conversion from 225 to 450? under GHSV of 30000 h-1.Meanwhile,this sample presented high thermal stability and resisted high GHSV.The introduction of tungsten resulted in the formation of smaller Fe2O3 and FeWO4 particles.The synergic effect of two species contributed to SCR activity,because of the increased surface acidity and electronic property.The FeWO4 with octahedral[FeO6]/[WO6]structure acted as the Br(?)nsted acid sites to form highly active NH4+ species.Combining DFT calculations with XPS and UV-vis results,it was found that the fine interfacial electron interaction between Fe2O3 and FeWO4 made the electron more easily transfer from W6+ sites to Fe3+ sites,which promoted the formation of NO2.(3)Moreover,the in situ DRIFTS gave insight into the NH3-SCR reaction mechanisms for FeW catalyst.At the low temperature region(<250?),coordinated NH3,NH4+ and NO2 were the main adsorbed species,which were reacted to form the complexes NO2(NH3)2 and NO2(NH4+)2 as intermediates.These active complexes could react with NO to produce N2 and H2O.Therefore,the reaction path was likely to be a "fast SCR" process,and the oxidation of NO to NO2 was the rate-determining step for the SCR reaction.At the high temperature region(?250?),NH2NO species formed from the reaction between surface coordinated NH3/NH4+species and NO were the key intermediates,which were further reduced to produce N2 and H2O.Therefore,the reaction occurred through E-R reaction mechanism.(4)The impact of sulfuration on catalyst structure,reaction performance and mechanism was systematically investigated through spectroscopic and temperature-programmed approaches.The sulfuration inhibited the NH3-SCR activity at low temperatures(<300°C),while no evident effect was observed at high temperatures(?300?).The FeW catalyst after sulfuration still exhibited an excellent activity above 90%NO_x conversion in a temperatures windows of 275-450?.After sulfuration for FeW oxides catalyst,the organic-like with covalent S=0 bonds sulfate species were mainly formed over the FeW catalysts.Combining TPD with in situ DRIFTS results,it was found that the Lewis and the Br(?)nsted acidity were enhanced by the interaction between metal species and sulfate species due to the strong electron withdrawing effect of the S=0 double bonds.The formation of NO2 was hindered,leading to the "fast-SCR" pathway was partly cut off by the sulfuration process and thereby the loss of SCR activity at low temperatures.However,the Langmuir-Hinshelwood reaction pathway between adsorbed NH3/NH4+ species and nitrate species was facilitated and dominated at high temperatures.