Experimental Research On Iron Oxide-based Catalysts For The Low-temperature Selective Catalytic Reduction Of NO With NH₃

Selective catalytic reduction（SCR）is now an established nitrogen oxide（NO）removal technology for industrial flue gas as well as for diesel engine exhaust gas.However,it is still a big challenge to develop novel low-temperature catalysts（LTC）for NH₃-SCR of NO,especially at a temperature below 300 ^oC.Generally,SCR De-NO_x is carried out at a medium to high temperatures（350–450 ^oC）.Whereas,the temperature of the boiler’s discharged flue gas is reduced to around 200 ^oC,after passing through an economizer and preheater.That is why in practical situations,it is quite difficult for a tail-end configuration to apply SCR NO_x abatement technologies developed under medium to high-temperature conditions.In addition to this,the De-NO_x catalyst could be poisoned by SO₂,so the SO₂ should be removed before the De-NO_xprocess proceeds.But the flue gas temperature after desulfurization is generally below 200 ^oC.In such conditions,if the operating temperature window of the De-NO_x catalyst is from medium to high-temperature range,then the flue gas must be reheated by a heat exchanger,which will further increase the energy consumption and should not be preferred due to economics.Hence,it is vital to develop low-temperature De-NO_x catalysts that can perform a good catalytic activity in the low-temperature range.In this way,the SCR system could be located downstream of the desulfurizing section and the particulate removal device without reheating the gas.In this thesis,we have done a performance analysis of iron oxide-based catalysts for the low-temperature NH₃-SCR of NO.Firstly,we have summarized the recent progress and performance of Fe-based catalysts for low-temperature NH₃-SCR of NO.Catalysts are divided into three categories:single Fe_xO_y,Fe-based multi-metal oxide and Fe-based multi-metal oxide with support catalysts.The catalytic activity and selectivity of Fe-based catalysts are systematically analyzed and summarized in light of some key factors such as activation energy,specific surface area,morphology,crystallinity,preparation method and precursor,acid sites,calcination temperature,other metal dopant/substitute and redox property of catalysts.As well,H₂O/SO₂ tolerance and the NH₃-SCR reaction mechanism over Fe-based catalysts,including Eley-Rideal（ER）and Langmuir-Hinshelwood（L-H）mechanism,are emphasized.Lastly,the perspective and future research directions of low-temperature NH₃-SCR of NO are also proposed.Secondly,we have used coal ash samples as a catalyst for the selective catalytic reduction of NO with NH₃.Coal ash samples from different regions of the People Republic of China were collected and afterward processed to be used as the SCR catalysts.The influences of different coal ash catalysts,as well as the influences of calcination temperatures on the NO conversion of catalysts,are analyzed.The methods of（X-ray fluorescence spectrometry）XRF,X-ray powder diffraction（XRD）,and Brunauer-Emmett-Teller（BET）are used to characterize the materials.It is found that the calcined sample（CA-800）exhibited decent NH₃-SCR activity with low GHSV and at high reaction temperatures 400-550 ^oC.As well,the influences of[NH₃]/[NO]molar ratio,oxygen concentration,and residence time on the SCR activity are also studied.Thirdly,the selective catalytic reduction of NO with NH₃ at low temperatures has been investigated with natural iron ore catalysts.Four iron ore raw materials from different locations were taken and processed to be used as catalysts.The methods of XRD,XRF,BET,X-ray photoelectron spectroscopy（XPS）,Hydrogen temperature-programmed reduction（H₂-TPR）,Ammonia temperature-programmed desorption（NH₃-TPD）,Scanning electron microscopy（SEM）,and Fourier transform infrared spectroscopy（FT-IR）were used to characterize the materials.The results showed that the sample A（comprised mainly ofα-Fe₂O₃ andγ-Fe₂O₃）calcined at 250 ^oC achieved excellent SCR activity above 90%from 200–290 ^oC and excellent N₂ selectivity at low temperatures.Suitable calcination temperature,large surface area,high concentration of surface-adsorbed oxygen,good reducibility,lots of acid sites,and adsorption of the reactants were responsible for the excellent SCR performance of the iron ore.However,the addition of H₂O and SO₂ in the feed gas showed some adverse effects on the SCR activity.The FT-IR analysis indicated the formation of sulfate salts on the surface of the catalyst during the SCR reaction in the presence of SO₂,which could cause pore plugging and resulted in suppression of the catalytic activity.After that,we have synthesized a Fe₂O₃ catalyst prepared by the co-precipitation method and tested the catalytic activity.Two different precipitants were used to synthesize the Fe₂O₃ catalyst by the co-precipitation method and their effects on the catalytic activity were also noted.It was found that the Fe₂O₃ catalyst prepared by the co-precipitation method lacked high surface area and good surface chemistry that was why the catalytic activity and the resistance to SO₂poisoning were not good.The existence of theγ-Fe₂O₃phase in the iron ore study was found crucial for the high NH₃-SCR performance of the iron ore sample.This urges us to find some other suitable synthesis method for aγ-Fe₂O₃ catalyst to obtain purity in phase as well as good surface chemistry.So,an environmentally benignγ-Fe₂O₃ catalyst prepared by a facile one-step synthesis approach has been examined for the reduction activity of NO in the low-temperature NH₃-SCR reaction.The NH₃-SCR activity and selectivity of theγ-Fe₂O₃ catalyst were compared with theα-Fe₂O₃ catalyst prepared by the same method.The results demonstrated that theγ-Fe₂O₃ catalyst showed excellent NO reduction activity and N₂ selectivity.The methods of XRD,BET,Transmission electron microscopy（TEM）,Energy Dispersive X-Ray Spectroscopy（EDS）,XPS,H₂-TPR,NH₃-TPD,and In-situ diffuse reflectance infrared Fourier transform spectroscopy（in-situ DRIFTS）were used to characterize the catalyst.A large surface area could be obtained by synthesizing theγ-Fe₂O₃catalyst by the facile one-step synthesis approach.In addition,the high concentration of surface-adsorbed oxygen,good reducibility,a lot of acid sites,and the existence of unstable nitrates on the surface of theγ-Fe₂O₃ were responsible for the high NH₃-SCR activity and selectivity.NO reduction overγ-Fe₂O₃followed both the Eley-Rideal and Langmuir-Hinshelwood mechanisms,whereas,overα-Fe₂O₃catalyst NO reduction only followed the Eley-Rideal mechanism.At last,we compared the low-temperature NH₃-SCR performance of maghemite（γ-Fe₂O₃）catalysts prepared by a facile one-step synthesis method and the conventional co-precipitation method.The methods of XRD,BET,TEM,EDS,and FT-IR were used to characterize the catalysts.The result demonstrated that theγ-Fe₂O₃ catalyst prepared by the facile method exhibited superior NH₃-SCR performance than the catalyst prepared by the co-precipitation method.This superior NH₃-SCR performance was credited to the existence of the regular spherical particles,smaller particle diameter,larger surface area,better pore connections,a higher concentration of surface-adsorbed oxygen,and adsorption of the reactants.As well,the facile method also inhibited the formation of theα-Fe₂O₃ in the catalyst providing the phase purity,which positively influenced the low-temperature SCR activity and selectivity of the catalyst.The SO₂ tolerance of theγ-Fe₂O₃ nanoparticles prepared by the facile method was found better than the conventional co-precipitation method.However,the FT-IR analysis indicated the formation of sulfate salts on the surface of the catalyst during the SCR reaction in the presence of SO₂,which could cause pore plugging and resulted in suppression of the catalytic activity.In summary,our study not only provides a first-ever comprehensive review available so far on the recent progress and performance of Fe-based catalysts for low-temperature NH₃-SCR of NO.But also,we have examined two abundantly available materials（coal ash and iron ore）as potential catalysts for the selective catalytic reduction of NO with NH₃.In addition,we have synthesized an environmentally benignγ-Fe₂O₃ catalyst by facile one-step synthesis approach to be used as a low-temperature catalyst in the NH₃-SCR reaction.At last,we compared the NH₃-SCR performance of the maghemite catalysts prepared by a facile method and the conventional co-precipitation method.