Study On The Mechanism Of SO₂ Resistance And The Catalytic Performance In Low-temperature Selective Catalytic Reduction Of No By NH₃ Over Fe-based Catalysts

The de-NOx in non-electric industries has been the most difficult problem for the control of atmospheric pollution in China.The selective catalytic reduction of NOx with ammonia(NH3-SCR),which widely used in fossil-fuel power industries,is an optimal method for the denitration in non-electric industries.The catalyst is the core of the NH3-SCR technology.However,the fuel gas temperature in non-electric industries is relatively low(80-300 oC),and it is difficult to match with the operating temperature window(300-420 oC)of the commercial V2O5-WO3(Mo O3)/Ti O2 catalyst.Meanwhile,the composition of flue gases from non-electric industries is complex,especially the influence of SO2/H2 O poisoning is severe.Therefore,it is the primary task to develop an outstanding low-temperature NH3-SCR catalyst with efficient activity and high resistance to SO2/H2 O.In this thesis,we focus on investigating the mechanism of the sulfur tolerance on γ-Fe2O3 at low temperature and further studying the influence of the modification of additives and optimizing preparation methods on the catalytic performance over Fe-based catalysts,which can provide theoretical guidance and technical support for developing Fe-based catalyst which is suitable for industrial application.The details are as follows:(1)The activity tests in the presence of SO2 were performed from 200-300 oC on γ-Fe2O3 catalyst.The study found that even at 225 oC,γ-Fe2O3 catalyst still exhibited stable SCR activity.And with the extension of SO2 resistance test time,the catalytic activity exhibited the tendency that slight decreased shortly and then increased slowly.The textual properties of catalysts that underwent different test times with the presence of SO2 were studied via XRD and BET.And the effects of the introduction of SO2 on the adsorption of the active NH3/NOx species were explored by in situ DRIFTs.It was proved that the deposition of ferric sulfate species on the surface of catalyst inhibited the adsorption of NOx and hindered the L-H reaction pathway,leading to the decrease of SCR activity in the early stage.The evolution of ABS on the surface of γ-Fe2O3 catalyst was observed through the ion chromatography and TPSR experiments.It was found that the ABS formed on the catalyst surface could be continuously consumed by reacting with NO/O2.In other words,the formation and decomposition of ABS reached a dynamic equilibrium,which protected the catalytic activity from the ABS.Combined with NH3-TPD,XPS and DFT,it was verified that the formation of ferric sulfate species enhanced the Br?nsted acid and Lewis acid strength and increased the contents of surface active oxygen species,providing more reductant for the E-R reaction pathway and improving the SCR activity.TG-DTA and SO2-TPD tests for the samples after SCR reaction in the presence of SO2 at different temperature were carried out.The results demonstrated that the SO42-accumulated on the surface of catalyst gradually increased with the test temperature rising,thereby supplying more surface acid sites and surface active oxygen species,which could be the important reason leading to the different trends of SCR activity at different test temperature.In addition,the activity was continuously improved when tested at 300 oC,(2)The WO3 modified modified iron-titanate mixed metal oxides catalysts were prepared by wet impregnation method and the influence of WO3 content on catalytic performance of Fe Ti Ox was explored.y W/Fe Ti Ox exhibit great NH3-SCR activity and N2 selectivity in the wide operating temperature range and shows outstanding SO2 resistance at 250 oC even 225 oC.The addition of WO3 causes the formation of the Fe(3+n)+-O-W(6-n)+ and Ti(4+n)+-O-W(6-n)+ structures and the strong electronic inductive effect among Fe3+ and W6+,Ti4+ and W6+ lead to the increase of surface active oxygen species and surface hydroxyl sepcies.The existence of W-OH bring with the increased Br?nsted acid sites,which is conductive to the improvement of low temperature activity.Besides,the dominating reaction route in the NH3-SCR reaction over y W/Fe Ti Ox is ER reaction pathway,which may be the main reason that catalyst obtains high SO2 resistance at low temperature.(3)The Fe Mn Ce Ox catalysts were synthesized by sol-gel and co-precipitation methods respectively.It can be found that the catalyst prepared by sol-gel method shows better SCR activity and obtains above 90% NO conversions in the temperature range of 80-150 oC,and possesses more stable SO2 resistance.XRD,BET,H2-TPR and NH3-TPD were used to study the textual properties,redox perfoemance and surface acidity,and the reason of different preparation methods leading to different catalytic performance was revealed.The results show that the Fe Mn Ce Ox catalyst prepared by sol-gel method has better redox ability and surface acidity,thus exhibiting better activity at low temperature.