Effect Of Fe Doping On The NH₃-SCR Performance Of Manganese-based Catalysts At Low Temperature

NO_x as the one of major air pollutants in China could cause a series of environmental problems,such as haze,photo chemical smog,acid rain and so on,which will lead to serious harm to the air environment and human health.The selective catalytic reduction of NO with NH₃（NH₃-SCR）is an effective technology employed to control the NO_x emissions,while low-temperature NH₃-SCR is of great potential value in industry application and has become to a hot topic in the denitrification research field owing to its obvious advantage of energy saving due to the lower reaction temperature.However,there still remains a major obstacle to its practical application resulted from the shortcomings of developed low-temperature SCR catalysts,such as poor low-temperature activity,poor sulfur resistance,etc.Based on these problems,a series of Fe-Mn composite oxide catalysts were synthesized by sol-gel method.The mechanism of effect of Fe modification on low-temperature catalytic activity of Mn-base catalysts were investigated,and the effect of Fe doping on the sulfur tolerance of Mn-based catalysts was further studied.The influence of operating parameters and content of Fe on manganese-based catalyst were investigated at first.It was found that the preparation conditions had an important influence on SCR activity.The catalyst prepared with hydrolysis time of 30 minutes and aging time of 7 days showed the superior SCR performane.The SCR activity and N₂ selectivity of catalysts could be significantly improved by Fe doping,and the T₈₀activity temperature window of the catalyst with Fe/Ti molar ratio of 0.24 was in the range of 75～400℃.BET,XPS,H₂-TPR,NO_x-TPD characterization and NO/NO₂ conversion test of catalysts were carried out to further study the promotional mechanism of Fe doping on the SCR catalytic activity.The results indicated that proper Fe could increase the specific surface area of the catalyst,which is one of reasons why the Fe-doped catalysts exhibited excellent low-temperature SCR activity.Moreover,the addition of Fe led to the increase in the chemisorption oxygen on the surface,which benefited the oxidation of NO to NO₂ therefore promoted the SCR reaction.Additionally,the NO_x adsorption capacity was significantly improved by the modification of Fe.In conclusion,the increase of the specific surface area,chemisorption oxygen,NO_x adsorption capacity and NO/NO₂ conversion were the main reasons for the further improvement of SCR activity of Fe-doped catalysts.The effect of Fe doping on the sulfur resistance of catalysts were further studied.The results showed that Fe doping had a serious effect on the sulfur resistance of the Mn-based catalysts.At low temperature,the sulfur resistance of Fe-Mn/Ti O₂ catalysts were significantly lower than that of Mn/Ti O₂ catalyst,and the NO_x conversion of Fe-Mn/Ti O₂（0.24）catalyst decreased to about 30%within 300 minutes after the introduction of SO₂.On the contrary,at relatively high temperature,the sulfur resistance of Mn-based catalyst could be improved after Fe doped,and the NO_x conversion of Fe-Mn/Ti O₂（0.16）catalyst was about 20%higher than that of Mn/Ti O₂ catalyst.The results of H₂-TPR showed that Fe doping led to the active component of Mn-based catalyst react with SO₂ more easily to form Mn SO₄,which significantly reduced the redox capacity and sulfur resistance.In addition,TG/DSC and BET characterization,measurement of the decomposition behavior of substance generated on the catalyst surface under the real SCR condition were carried out on the poisoned catalyst.The results showed that the doping of Fe resulted in the deposition of more NH₄HSO₄ on the surface of catalysts,which seriously decreased the specific surface area and led to the poor sulfur resistance.All of these factors were the main reasons for the worse sulfur resistance of Fe-modified catalysts.Finally,the influencing factors of sulfur resistance of catalysts were further observed by means of dynamic reaction with SO₂ pretreatment,SO₂+O₂ pretreatment and NH₃pretreatment respectively.It was found that the deactivation of Mn/Ti O₂ catalyst after SO₂+O₂pretreatment was much greater than that of SO₂ pretreatment,indicating that the generated SO₃ was of greater influence to Mn/Ti O₂ catalyst.However,Fe-modified catalysts were more seriously deactivated after SO₂ pretreatment,which further confirmed that Mn-based catalysts could be poisoned by SO₂ easier after Fe doping.On the contrary,different from the results obtained by the two pretreatment methods,the sulfur poisoning effect of catalysts after the NH₃ pretreatment were obviously alleviated,indicated that NH₃ had a certain protective effect on the catalysts.Therefore,it is of great value to improve the sulfur resistance of catalysts by means of pre-adsorbing of NH₃ on the catalysts in future experiments.