Design And Reaction Mechanism Study Of High Efficiency Cerium-based NH₃-SCR Catalyst

Selective catalytic reduction of NO_x with NH₃（NH₃-SCR） is one after-treatment technologies for NO_x abatement.The catalyst plays a key role in NH₃-SCR system.The traditional industrial NH₃-SCR catalysts are TiO₂ supported V₂O₅ and WO₃.However,several drawbacks still limit the practical application of vanadia catalysts,such as the poor low-temperature SCR activity,the high N₂O production at high temperatures,and the toxicity of vanadium.Therefore,environment-friendly SCR catalysts should be developed with high catalytic performance in a broad temperature range.China has a very rich rare earth resource.As ceria（CeO₂） with the fluorite structure can release oxygen in a reducing condition while store oxygen in an oxygen-rich environment,it has been widely used in environmental catalysis.However,CeO₂ shows a poor SCR activity due to its poor ability for NH₃ adsorption.To improve the SCR performance of CeO₂,it was prepared with different method to choose the best,or mixed with other metal oxides.It is found that Ce bansed catalysts have a good middle temperature NH₃-SCR activity and high N₂ selectivity.However,there are still some disadvantages,such as poor low temperature activity,a poor activity in present of water and sulfur.Therefore,the development of Ce based NH₃-SCR catalyst with a good low temperature SCR activity,a wide temperature window,and a strong resistance for water and sulfur is of great significance for the efficient utilization of rare earth resources and the application of NH₃-SCR system.Compare to the solution evaporation method,homogeneous precipitation method was the better method for Ce based NH₃-SCR catalysts.Based on the results of SCR activity and characterization over Ce-W-Ti catalysts,the relationship of the structure and activity was built.In this work,HPA was grafted on CeO₂ to prepare the SCR catalysts with high catalytic performance in a broad temperature range.Different kinds of HPA were used to study the effects of HPA on the structure and SCR activity over CeO₂.The function of HPA and CeO₂ and the reaction mechanism were also studied.The mechanism of potassium poisoning and regeneration on the SCR performance was studied.The novelties and important conclusions of this work are summarized as follows:（1）The Ce_0.2W_0.1O_x catalyst prepared by homogeneous precipitation method can achieve an excellent SCR performance at 200-425℃ with NO_x conversion higher than 80%with a high GHSV of 400,000 h^-1.The supported W_0.1TiO_x provide acid sites,and the highly dispersed active species CeO₂ could improve the reducibility.That is the reason for the excellent NH₃‐SCR activity of Ce_0.2W_0.1O_x catalyst.（2）The CeO₂ catalysts grafted with heteropoly acid（i.e.,HPA）could improve their catalytic performance for selective catalytic reduction of NO_x with NH₃（NH₃-SCR）.The amount of HPA is the main impact for the SCR activity over HPA/CeO₂ catalysts.In comparison to HSi W/CeO₂,HPMo/CeO₂,and commercial 0.1%V₂O₅-WO₃/Ti O_x catalysts,HPW/CeO₂ catalysts showed the best SCR activity.（3）Both HPW and CeO₂ on/in HPW/CeO₂ played their functions to the greatest limit for the SCR reaction.CeO₂ in HPW/CeO₂ played the role in the activation of adsorbed NH₃ and NO,and the grafted HPW on HPW/CeO₂ acted as the active sites for NH₃ adsorption.As a result,HPW/CeO₂ showed an excellent SCR performance at 200-450℃ with NO_x conversion higher than 90%and N₂ selectivitly close to 100%with a high GHSV of 120,000 cm³ g^-1 h^-1.（4）In comparison with Ce_0.2W_0.1TiO_x,CeO₂-WO₃/TiO₂ and CeO₂-WO₃,the NH₃-SCR activity of HPW/CeO₂ was much better.Furthermore,the usage of W with the high cost in HPW/CeO₂ was much less than those used in CeO₂-WO₃/TiO₂,Ce_0.2W_0.1TiO_x and CeO₂-WO₃.Therefore,HPW/CeO₂ may be a cost-effective SCR catalyst with a broad temperature window to control NO_x emission.