Investigation On The Preparation And Application In Low-temperature Selective Catalytic Reduction Of NO_x With NH₃ Over Novel Fe-based Catalysts

Nitrogen oxides(NOx)are the main atmospheric pollutants that cause environmental problems such as acid rain haze and photochemical smog,which would seriously harm the ecological environment and human health.With the increasingly strict national policies,it is urgent to carry out NOx control of low-temperature flue gas including metallurgical and mineral industries.Selective catalytic reduction of NOx with NH3(NH3-SCR)is one of the promising early stage technologies to control NOx discharged from mobile and stationary sources,and the exploitation of efficient,stable and environmentally-benign catalyst is the key for low-temperature NH3-SCR reaction.Fe-based catalysts have received much attention due to their potentially high activity,excellent resistance to H2O and SO2,and outstanding environmentally friendly performance.However,some problems still remain,such as poor low-temperature activity,unclear structure-activity relationship and reaction mechanism.Aiming at these shortcomings of Fe-based catalyst,a novel Fe-V composite catalyst with superior low-temperature NH3-SCR activity was developed in this work.By using comprehensive characterizations,the structures of catalyst were analyzed,and the inner relationship between structure and properties,performance,as well as reaction mechanism was deeply investigated.Meanwhile,the influences of calcination temperature on the structure and catalytic activity of the catalyst was also inquired into.Additionally,a novel highly dispersed Fe-Mn/Ti catalyst was developed and the activity enhancement mechanism was revealed.The main achievements are as follows:1.A series of vanadium doped Fe2O3 catalysts Fe1-xVxO?(x=0,0.1,0.25,0.4,0.5,1)were synthesized using the homogeneous precipitation method.The composite catalysts showed excellent catalytic activity and N2 selectivity.The optimal Fe0.75V0.25O? catalyst exhibited superior catalytic performance,achieving 100%NOx conversion at 200? over a wide temperature window from 175 to 400?(GHSV=50000 h-1),and also showed strong durability to SO2 and H2O.The characterization results revealed that the incorporation of V had led to the formation of dispersed ?-Fe2O3 and new FeVO4 species,and the composite catalyst showed highly amorphous morphology,lower crystallinity and higher oxygen mobility.The strong electron inductive effect between Fe and V in the composite catalyst led to the charge transfer from Fe to V,which greatly boosted the oxidability of Fe,effectively promoted the oxidation of NO to NO2,and thus improved the catalytic performance at low temperatures.Meanwhile,the strong interaction between the two phases of ?-Fe2O3 and FeVO4 resulted in the serious electron deviation and a higher valence state of V element,which effectively promoted the adsorption and activation of NH3.2.The mechanism of NH3-SCR reaction and the contributions of Lewis acidic site and Br(?)nsted acidic site to the reaction over the representative catalyst(Feo.75V0.25O?)were deeply investigated by in situ Diffuse Reflection Infrared Fourier Transform Spectroscopy(DRIFTS)and reaction kinetics.The SCR reaction mainly followed the Langmuir-Hinshelwood mechanism below 200?,since the consumption of adsorbed NH3 species could be divided into the explicit "standard SCR" and "fast SCR" stages;while an Eley-Rideal mechanism proceeded dominantly at and above 200?,in which the adsorbed NH3 species were eliminated by gaseous NO directly and linearly.Moreover,O2 was proved to be activated and participated in the reaction cycle through Mars-van Krevelen mechanism.Both of the Br(?)nsted and Lewis acid sites played equivalently significant roles in different pathways of the NH3-SCR reaction.3.The influences of calcination temperature on the structure and catalytic properties of Fe0.75V0.25O? catalyst were investigated to simultaneously clarify the temperature-driven structural evolution of the catalyst and the relationship between structure and activity in NH3-SCR process.It was found that the structure of catalyst underwent the evolution process from the highly dispersed configuration to the crystalline framework with surface FeVO4 amorphous layer and further to the extremely crystalline structure with the increase of treating temperature.With lower calcination temperatures(?600?),the reduction of redox property was the main reason affecting the catalytic performance,so only the low-temperature catalytic performance was inhibited.However,with higher calcination temperatures(>600?),the reduction of both redox property and surface acidity seriously affected the overall DeNOx efficacy of the catalyst.The greatly amorphous a-Fe2O3 species was beneficial for the enhancement of redox property,NO activation ability as well as the suppression of high-temperature NH3 oxidation,and consequently boosting the DeNOx performance;while the highly dispersed FeVO4 species with abundant VOx structure should be the main active species for NH3-SCR reaction.4.A novel Fe2O3-MnO2/TiO2 catalyst was synthesized using a conventional impregnation method assisted with ethylene glycol.The structure-activity relationship and activity enhancement mechanism were analyzed via systematic characterizations.It was found that the prepared Fe-Mn/Ti(M)catalyst exhibited highly dispersed feature with regular nanoparticles.Consequently,a novel Fe-O-Ti structure with strong interaction was formed on the surface of the catalyst,which could enhance the electron inductive effect.Due to the improvement of surface dispersion,the adsorption capacity of NO and NO oxidation performance of the catalyst were greatly boosted,which was conducive to improving the low-temperature NH3-SCR activity.Compared with the unmodified Fe-Mn/Ti catalyst,the highly dispersed Fe-Mn/Ti(M)catalyst exhibited superior catalytic activity at low temperatures,achieving more than 90%of NOx conversion from 100 to 325?(GHSV=30000 h-1).Meanwhile,the catalyst also showed lower apparent activation energy and good long term stability.In summary,the novel Fe-V and Fe-Mn/Ti catalysts developed in this paper exhibited excellent DeNOx efficiency and stability at low temperatures(<200?),which could provide positive support for NOxcontrol of low-temperature flue gas.Meanwhile,the analysis of structure-activity relationship and reaction mechanism would be helpful for the further optimization or development of efficient catalysts,also could provide useful information for the study of NH3-SCR reaction.