Study On Deactivation Of Mn-Ce/AC Catalyst With Low Temperature Denitrification

Nitrogen Oxide is a prominent pollutant discharged from human production activities.A large amount of NO_x is generated by the burning of fossil fuels such as oil and coal and industrial activities such as the production of nitric acid.Excessive NO_xemissions cause great environmental pressure.Selective catalytic reduction（SCR）,which uses NH₃ to neutralize nitrogen oxides with support of active components in the catalyst,is a relatively mature nitrogen oxide removal scheme at present and has been widely developed and applied in coal-fired power plants and iron enterprises.The SCR process is designed around the technical indicators of the catalyst,so the development level of the catalyst determines the removal efficiency of the SCR system.The temperature of sintered flue gas is low and the composition is complex.Currently,commercial SCR catalysts have not been applied in iron and steel enterprises because they need to heat the flue gas to reach the starting temperature window and are prone to poisoning and inactivation.A new type of SCR denitrification catalyst with low temperature,high toxicity resistance and high denitrification efficiency has been developed with carbon as the carrier and transition metal oxides and rare earth oxides as the active components.Among them,Mn-Ce activated carbon（Mn-Ce/AC）catalyst is expected to be a new type of catalyst applied in low-temperature industrial flue gas denitration due to its excellent catalytic performance at low temperature and environmentally friendly characteristics.Therefore,it is necessary to analyze the influence of toxic components such as alkali metals,alkaline earth metals and heavy metals on the activity and mechanism of catalysts in flue gas,so as to provide theoretical basis for catalyst improvement.In this paper,the potassium salt,Ca（NO₃）₂ and Pb（NO₃）₂,which are very common in sintering flue gas,are loaded on the fresh Mn-Ce/AC catalyst by impregnation method.The typical components in the sintered flue gas were simulated in the laboratory to study the effect of toxic components on the denitrification activity of the catalyst.SEM,BET,XRD,XPS,NH₃-TPD,FTIR are used to investigate the influence of toxic component on physicochemical property of the catalyst.Potassium salt has the most toxic effect on the catalyst and makes the optimal reaction temperature transfer to the high temperature zone.Potassium salt in flue gas can lead to the blockage and collapse of micro-pores of catalyst,decrease of specific surface area and decrease of gas adsorption efficiency.Potassium salt will also make the Mn⁴⁺and Ce⁴⁺in the catalyst change to the Mn²⁺and Ce³⁺,crystallize the active components,destroy the oxygen-containing functional groups on the surface of the catalyst,reduce the amount of coordination ammonia on the surface,prevent the conversion of NO to the intermediate NO₂,and reduce the catalytic reaction rate.The effect of Ca（NO₃）₂ on catalyst activity was lower than that of sylvite.Ca（NO₃）₂in flue gas had little effect on specific surface area of Mn-Ce/AC catalyst,but promoted the transformation from Ce⁴⁺to Ce³⁺and formed crystallization products.As the proportion of Ce⁴⁺involved in the reaction decreased,the total amount of chemosorption oxygen in the decreasing system decreased,leading to the decrease of catalyst activity.The influence of Pb（NO₃）₂ on the denitrification process of the catalyst was mainly concentrated in the low temperature region and the effect was obvious.Pb（NO₃）₂ will obviously change the surface morphology of the catalyst,make the catalyst surface smooth and deposit in the pores,forming the"ineffective specific surface area"with low ammonia adsorption efficiency.Pb（NO₃）₃ will promote the transformation of Mn⁴⁺and Mn³⁺in the catalyst to Mn²⁺,reduce the acidity level in the system,and reduce the number of oxygen holes obtained by Ce,leading to the decrease in the content of chemosorption oxygen and the inactivation of the catalyst.