De-NO_x Performance Of Fe-SO₄/TiO₂ Catalysts And Its Anti-poisoning Ability

Selective catalytic reduction with ammonia(NH₃-SCR)is one of the most effective technologies for NOx removal,of which catalyst is the core.Vanadium-based catalysts have been used for denitrification of exhaust gas in many industry applications,having achieved remarkable results in controlling the NOx emission.However,its development is limited by disadvantages like narrow range of active temperature and secondary pollution to the environment.Metal oxide-based catalysts have been proved to be with excellent SCR activity,but can not be used in industry because of their vulnerability to sulfation.Sulfate catalysts have become a research hotspot because of its high SCR activity at medium and high temperatures and the ability to avoid sulfur poisoning.With the help of XRD,FESEM,HRTEM,TPR/TPD,BET,XPS,TG-IR,DRIFT and other modern testing methods,the SCR activity,improvement approaches and structure-activity relationship of Fe-SO₄/TiO₂catalysts were studied,in addition the mechanism of sulfur resistance and alkali poisoning resistance was explored.Firstly,the performance and structure of pure FeSO₄,Fe(OH)SO₄ and Fe₂(SO₄)₃were tested,and results showed that Fe₂(SO₄)₃ with the largest specific surface area showed the best high-temperature performance.Fe(OH)SO₄ with the highest content of chemisorbed oxygen and the lowest initial reduction temperature showed the best low-temperature activity.FeSO₄ showed the worst overall activity.Secondly,FeSO₄ and TiO₂ were used as raw materials to prepare Fe-SO₄/TiO₂catalysts by impregnation method,the results showed that the active components of the catalysts were FeSO₄,Fe₂(SO₄)₃ and Fe(OH)SO₄.The optimum performance of Fe-SO₄/TiO₂ catalyst was obtained when FeSO4 loading was 20%and calcination temperature was 300?.For optimized Fe-SO₄/TiO₂ catalyst,light-off temperature was found at around 200?,and over 80%of NO conversion could be reached from280 to 400?.Then,in order to further enhance SCR activity of Fe-SO₄/TiO₂ catalyst and broaden the active temperature window,modification with a series of sub-group metals was carried out on catalysts.The results showed that the synergistic effect between Mn,Ce or Cu and Fe could improve the reduction ability and surface acidity of the catalysts,change the content of different active components,and enhance the SCR activity.In addition,Mn doping could effectively broaden the active temperature window of Fe-SO₄/TiO₂ catalyst and made it have better low temperature activity.At the same time,in order to explore the anti-sulfur poisoning ability of the catalysts,the sulfur resistance of the optimum activity of Fe-SO₄/TiO₂ and its modified catalysts was studied.The results showed that Fe-SO₄/TiO₂,Ce-Fe-SO₄/TiO₂ and Cu-Fe-SO₄/TiO₂ catalysts all had excellent anti-sulfur poisoning ability.SO₂ could increase the acidity of the catalysts surface,the content of chemisorbed oxygen in Fe-SO₄/TiO₂ catalyst and the content of Fe₂(SO₄)₃ in Cu-Fe-SO₄/TiO₂ catalyst.There was interaction between Mn and Fe in Mn-Fe-SO₄/TiO₂ catalyst,so only a small amount of Mn was sulfated and the SCR efficiency decreased by about 10%.When the reaction was complete,the activity tended to be stable gradually.Finally,the ability of anti-alkali and alkaline-earth metal poisoning of Fe-SO₄/TiO₂ catalysts was investigated by introducing three kinds of alkali or alkaline-earth metals(K,Na,Ca).The results showed that Fe-SO₄/TiO₂ catalysts with low contents of K,Na or Ca exhibited weak toxicity,but high loading might result in the decreased of chemi-adsorbed oxygen and Fe³⁺content,reduction ability and specific surface area,which led to serious deactivation of catalysts.The influence order of alkali or alkaline earth metals on catalytic activity was as follows:K>Na>Ca.In addition,Fe-SO₄/TiO₂ catalysts could withstand about 5%,7%and 9%of K,Na and Ca respectively maintaining the maximum NO conversion up to 70%.DRIFTS spectra was used to study the reaction mechanisms of the catalysts.It could be found that there were certain NH₃ and NO adsorption sites on the surface of Fe-SO₄/TiO₂ catalysts prepared under different conditions,which was the basis for SCR activity.The number of sites increased after the doping of sub-group metals,but catalysts poisoned by alkali metals reversed.In addition,this series of Fe-SO₄/TiO₂catalysts mainly followed"L-H"reaction mechanism accompanied by a certain"E-R"mechanism for SCR reaction.