Pb And SO₂ Poisoning And Regeneration Of Mn-Ce/TiO₂ Catalysts For SCR At Low Temperature

Nitrogen oxides is one of the main pollutants in the atmosphere,which can lead to acid rain and photochemical smog,seriously endanger environment of the natural and health of human.Selective catalytic reduction（SCR）is the most efficient and widely used technology for eliminating NO_xfrom flue gas.Among them,Manganese-Cerium catalysts have been extensively studied due to their good low temperature activity.At present,poisoning and regeneration are few studied of Manganese-Cerium catalysts,researching more performance of catalysts.Pb is one of the heavy metals in the flue gas of coal-fired boilers,and there is also SO₂of a certain concentration.Related studies show that SO₂and Pb have strong toxicity to SCR catalyst,which greatly reduces the activity of the catalyst,and little research on the Pb poisoning and regeneration of Mn-Ce/TiO₂catalysts for SCR at low temperature.Specification SO₂and Pb poisoning causes of Mn-Ce/TiO₂catalyst and searching for effective regeneration method can provide some support for the industrial application of the catalyst.Firstly,the effect of Pb on the SCR activity of Mn-Ce/TiO₂catalysts for SCR at low temperature was studied.The catalysts of Pb poisoning was regenerated.The catalysts were characterized by BET,SEM,XRD,FT-IR,H₂-TPR and NH₃-TPD,and the reason of Pb poisoning and activities recovery of regenerative catalysts are analyzed.As result that Pb doping could greatly reduce the low-temperature SCR activity of the Mn-Ce/TiO₂catalysts.When the mass concentration of Pb is 11%,the activity of catalyst was reduce from 100%to 44%at 180℃.The specific surface area and the amount of active components Mn⁴⁺and Ce³⁺of the Mn-Ce/TiO₂catalyst were reduces by the doping of Pb,affecting the redox cycle reaction in Mn-Ce/TiO₂(Mn⁴⁺+Ce³⁺（?）Mn³⁺+Ce⁴⁺).Pb doping could destroy the acidic sites of the catalyst,hindering adsorption and activation of NH₃by the catalyst.The activity of the catalyst after regeneration by nitric acid is almost completely recovered,and even exceeds the activity of the fresh catalyst at a reaction temperature of 80℃to 150℃.The main reason for the recovery of catalyst activity is that the regeneration of the catalyst restoring the redox capacity,increasing the specific surface area,and adding the new acid sites,thereby recovering the activity of the catalyst.Secondly,the effect of SO₂on the SCR activity of Mn-Ce/TiO₂catalyst was also investigated.The reason of catalyst SO₂poisoning was analyzed by means of characterization.It was found that SO₂has obvious inhibitory effect on the SCR activity of Mn-Ce/TiO₂catalyst.The deposition of ammonium sulfate Salt on catalyst surface,the adsorption of SO₂at the active center,competition with NO or NH₃,and the sulfation of the active phase were proved to be the reasons for the catalyst deactivation.The catalyst for SO₂poisoning was regenerated by water washing regeneration and thermal regeneration,and the SCR activity of the catalyst was almost completely recovered.This indicates that the deposition of ammonium sulfate is the main reason for the deactivation of the catalyst,and the ammonium sulfate formed by the reaction of SO₂and NH₃can be simple Washed and heated to remove.Finally,the effect of SO₂and Pb on the SCR activity of the catalyst was studied.It was found that the combination of SO₂and Pb made the SCR activity of Mn-Ce/TiO₂catalyst lower.The SO₂reacted with NH₃and Pb to form ammonium sulfate and lead sulfate,and the interaction between SO₂or Pb and the active phase of catalyst were proved to be the main reasons for the catalyst deactivation.The catalyst for co-poisoning of SO₂and Pb was regenerated by washing with water-washing and pickling,and the SCR activity of the catalyst was almost completely recovered.The recovery reasons of SCR activity of catalyst are that the specific surface area and micro-porous of catalyst were portion recovered and SO₄^2-increased the strong acid sites on the catalyst surface,etc.