Study On Preparation And Modification Progresses Of Mn-Ce-Based Catalysts For Selective Catalysis Reduction Of NO With NH3

Large ocean-going vessels generally adopt large low-speed two-stroke diesel engines as the propulsion plants.The exhaust gas emitted contains a large amount of NO_x,which has caused serious harm to the atmospheric environment and human health.International and domestic regulations have established increasingly stringent emission regulations to limit NO_xemissions from ships.At present,the more mature NO_x emission reduction methods for marine diesel engines mainly include selective catalytic reduction（SCR）technology and exhaust gas recirculation（EGR）technology.Among them,SCR technology has the advantages of high denitrification efficiency,less secondary pollution,and low investment cost.It is a technology for reducing NO_x emissions from ships with great application potential.However,the catalyst used in the SCR denitrification system of the actual ship is still limited to the commercialized V-W（Mo）/Ti O₂ catalyst.Although these catalysts have good high-temperature catalytic activity,they have problems such as poor catalytic activity and poor sulfur and water resistance in the medium and low temperature range.It is difficult to fully meet the requirements of marine diesel engine exhaust gas denitration.Therefore,the research and development of catalysts with wide catalytic activity temperature window,good sulfur and water resistance at medium and low temperature is of great significance for the denitration of ship exhaust gas.Although the manganese-cerium-based catalyst has the advantages of good catalytic activity at medium and low temperature and low cost,it has become a hot spot in domestic and foreign research in recent years.It is still insufficient in terms of sulfur and water resistance.In this paper,Mn Ce Ti O_x catalyst is selected as the research object.Firstly,the effect of two typical preparation processes on the catalytic performance is compared.Then a variety of different transition metals are used to dope and modify the Mn Ce Ti O_x catalyst.Finally,the effect of the preparation process is studied on the transition metal modified Mn Ce Ti O_x.The relationship between structure-efficiency is explored and analyzed.The specific research work carried out in this paper is summarized as follows:（1）The Mn_aCe_bTi O_x catalyst was synthesized by the co-precipitation method and the sol-gel method respectively.The effects of the two preparation processes and different Mn/Ce ratios on the catalytic performance were compared and analyzed,and the related reaction mechanism was discussed and analyzed.The results show that with the increase of the Mn/Ce ratio,the active temperature window of the Mn_aCe_bTi O_x-CP catalyst prepared by the co-precipitation method will shift to the low temperature region.Compared with Mn_0.375Ti O_x-CP and Ce_0.375Ti O_x-CP,the proportions of Mn⁴⁺and Ce³⁺in the Mn_0.075Ce_0.3Ti O_x-CP catalyst are significantly increased,which makes the catalyst easier to be reduced and provides more oxygen vacancies.This is mainly due to Because of the interaction between Mn and Ce.Mn_0.075Ce_0.3Ti O_x-CP catalyst has a higher specific surface area,active component dispersion,and surface acidity,so it shows higher mid-low temperature activity and sulfur resistance.When the reaction space velocity is 30,000 h^-1,the NO_x conversion rate of the Mn_0.075Ce_0.3Ti O_x-CP catalyst in the temperature window of 200-325°C is higher than 99%.Although the Mn_0.075Ce_0.3Ti O_x-SG catalyst prepared by the sol-gel method has a higher proportion of Ce³⁺,its lower specific surface area,redox properties,and surface acidity affects the catalytic activity to a greater extent.The NH₃-SCR reaction on the surface of the Mn_0.075Ce_0.3Ti O_x catalyst prepared by the two processes follows the L-H mechanism and the E-R mechanism at the same time,but the NH₃ species and NO species on the surface of the Mn_0.075Ce_0.3Ti O_x-CP catalyst are more abundant and active,making it show higher catalytic activity.（2）6 different transition metals such as W,Fe,Cu,Co,Ni,Mo were selected,and the Mn Ce Ti O_x catalyst was modified by the co-precipitation method during the synthesis process.And the catalytic performance of different transition metal modifications was compared.And discuss and analyze the related reaction mechanism.The results show that when the reaction space velocity is 150,000 h^-1,compared with the other 5 transition metal-doped Mn Ce Ti O_xcatalysts,the NO_x conversion rate of W_0.15Mn Ce Ti O_x catalyst in the temperature window of220-296°C is higher than 99%.In addition,the active temperature window is relatively wide,and the sulfur resistance is better,thus showing a better modification effect.With the increase of W,the low-temperature activity of the Mn Ce Ti O_x catalyst first increases and then decreases,while the high-temperature activity first decreases and then increases.When the W/Ti ratio is0.15,the activity temperature window is relatively wider.Compared with the Mn Ce Ti O_xcatalyst,the introduction of W will cause the redox property,surface acidity,and specific surface area of the catalyst to decrease to varying degrees,but at the same time it will significantly increase the proportion of Ce³⁺on the catalyst surface,which greatly improves the overall catalyst.The proportion of Mn⁴⁺in the W_0.15Mn Ce Ti O_x catalyst has decreased,which is beneficial to improve the N₂ selectivity of the catalyst.The NH₃-SCR reaction on the surface of the W_0.15Mn Ce Ti O_x catalyst follows the L-H mechanism and the E-R mechanism.W modification makes the NO species on the surface of the catalyst more abundant and more active,which is beneficial to improve the catalytic activity of NH₃-SCR.After the sulfur resistance experiment,the sulfur element on the surface of the catalyst was mainly in the form of sulfate.The deposition of ammonium bisulfate is one of the main reasons for the deactivation of the catalyst.The introduction of W reduces the deposition of ammonium bisulfate on the surface of the Mn Ce Ti O_x catalyst,making it exhibit higher sulfur resistance.（3）WMn Ce Ti O_x catalysts were prepared by co-precipitation method,deposition precipitation method,and sol-gel method respectively.The effect of preparation process on the performance of WMn Ce Ti O_x catalyst was comparatively studied,and the related reaction mechanism was discussed and analyzed.The results show that the catalyst WMn Ce Ti O_x-CP prepared by the co-precipitation method exhibits higher catalytic activity and resistance to water and sulfur.Compared with WMn Ce Ti O_x-DP catalyst,WMn Ce Ti O_x-CP catalyst has higher redox properties,surface acidity,active component dispersion,specific surface area,and the ratio of Mn⁴⁺and Ce³⁺on the surface of the catalyst is also higher,which helps promote NH₃-SCR reaction.The WMn Ce Ti O_x-SG catalyst has a small specific surface area,and its activity,sulfur and water resistance are poor.The in-situ DRIFTS test results show that the NH₃-SCR reaction on the surface of the catalyst prepared by the three processes simultaneously follows the E-R mechanism and the L-H mechanism.The species adsorbed on the surface of WMn Ce Ti O_x-CP catalyst is the most abundant and active,so its activity is also the highest.In the sulfur resistance experiment,the main reason for the degradation of the catalyst’s denitrification performance was attributed to the deposition of ammonium salts and the competitive adsorption between SO₂ and NO species.