| Selective catalytic reduction of NOx with NH3(NH3-SCR)is the most effective and widely used method for the abatement of NO,from stationary sources.In this respect,the commercial catalyst is V2O5-WO3(MoO3)/TiO2.Because of the narrow operation temperature window with 300-400 ℃,This type of catalyst must be placed upstream of the particulate collector and desulfurization unit thus,the catalyst is easy to be to be suffered from blocked and poisoned by the particles and sulfates in the flue gas.The fine dust and sulfur dioxide in the flue gas are easy to inactivate the catalyst.the metal vanadium contained in the catalyst is biotoxic and volatile,and the failure of catalyst disposal is difficult.In addition,V2O5-WO3(MoO3)/TiO2 catalyst is difficult to adapt to the low smoke exhaust temperature of the industrial funace.Therefore,it is of great significance and interest to develop novel active NH3-SCR catalysts working in the low temperature as a potential substitution for the conventional V2O5-WO3(MoO3)/TiO2 catalyst.Manganese based catalysts have attracted wide attention due to their excellent low temperature NH3-SCR activity.However,the current studies of manganese based catalysts are mainly concentrated on manganese based metal oxide catalysts and supported manganese based catalysts.There are few studies on the structure-activity relationship of Mn nano oxide catalysts.Therefore,four types of MnO2 nano-catalysts with various crystal structure(α-,β-,γ-and δ-MnO2)were synthesized by hydrothermal method,and their low temperature NH3-SCR activity were measured.The results indicated that catalysts with different structures showed various activity which followed the sequence of γ->α->β->δ-MnO2.It could be found that γ-MnO2 expressed highest catalytic activity and its NOx conversion rate surpassed 90%at the temperature range of 150～260 ℃.It was inferred that the morphology of the a-andβ-MnO2 were nanorods,while y-and 8-MnO2 with the structures of nanoneedles.The specific surface area of the catalyst was not the dominant factor affecting the NH3-SCR activity at low temperature.The decent pore structure,strong redox property,abundant chemisorption oxygen and Lewis acid sites were responsible for high low temperature NH3-SCR activity of y-MnO2 nano-catalyst.α-MnO2,also namely OMS-2 were selected as the research object because of the high catalytic activity,easy modification and low cost.And then,the preparation conditions of the OMS-2 catalyst were optimized by experiments.In order to further improve the low temperature activity of OMS-2 catalyst,the Ce-OMS-2 catalyst with different Ce loading position was prepared by hydrothermal method and impregnation method.The impregnation method loaded Ce on the surface of the catalyst,and when the hydrothermal method was prepared,Ce entered the catalyst.The modified of Ce greatly improved the low temperature activity of the catalyst,which is because the doping of Ce can improve the redox ability of the catalyst,which is helpful to the "fast-SCR" reaction.After Ce modification,there is a certain interaction between Ce and Mn.When Ce enters into the inner part of the catalyst,the interaction between Ce and Mn is stronger.Finally,the ability of catalyst to resist alkali metal poisoning and the performance of water and sulfur dioxide poisoning were studied.The results show that the OMS-2 catalyst has a good ability to resist alkali metal poisoning.The toxic effect of alkali metal on the catalyst is related to the type of alkali metal.The stronger the alkali metal is,the greater the toxicity of the catalyst to the catalyst.Sulfur poisoning of catalyst is mainly caused by sulfation of active component Mn.Since Ce entered the internal structure of the catalyst,the strongly interaction between Ce and Mn protects the Mn.The OMS-2 catalyst modified by hydrothermal method has the strongest resistance to water and sulfur dioxide poisoning. |
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