Preparation Of Cerium-based Catalysts And Study On The Structure-activity Relationship For Toluene Oxidation

In recent years,with the rapid development of petrochemical,printing plants and other industries,the emission of volatile organic compounds（VOCs）represented by toluene poses a great threat to the environment and human health.It is urgent to control and reduce thier emission.Catalytic oxidation has become the most important technology for VOCs emission control due to the low energy consumption,high efficiency and no secondary pollution.As the core of this technology,catalyst has also got more attention.At present,noble metal catalysts are limited in practical application due to the high price and poor stability,thus researchers pay attention to the non-noble metal catalysts.Among them,cerium-based catalysts have become a research hotspot for VOCs oxidation due to their low price,excellent oxygen storage performance and thermal stability.In this paper,the cerium-based catalyst is used as the research object.It is modified by ion doping and metal composite to achieve the purpose of improving the catalytic performance of catalyst,and deeply study the reaction mechanism.We hope to provide a theoretical basis for the design of a novel catalyst for toluene oxidation.Firstly,in this paper,F-doped Ce O₂ catalyst（F_x-Ce）was prepared by co-precipitation method to solve the problem of the low catalytic oxidation activity of Ce O₂ catalyst at medium-low temperature.The toluene catalytic oxidation activity test showed that F-doping significantly improved the catalytic activity of Ce O₂ at medium-low temperature（<300℃）.Among them,the toluene conversion of F₃-Ce at 220℃and 240℃is 17%and 26%higher than that of Ce O₂ respectively;and the activity temperature window（>90%toluene conversion）of F₃-Ce was widened by 30℃tower the low temperature in comparision with Ce O₂.Combined with Raman,XPS,H₂-TPR,O₂-TPD and DFT calculation results,it was found that F species exist on the catalyst in the form of surface-F and lattice-F,and there is a strong interaction between surface-F and the Ce who connected to it directly,which leads to the charge imbalance on the adjacent Ce and O and weakens the Ce-O bond.This phenomenon is conducive to the breaking of Ce-O bonds and the O atoms leaving the lattice,which promotes the formation of oxygen vacancies on F_x-Ce.The formation of oxygen vacancies enhances the redox performance of F_x-Ce and promotes the adsorption-activation of O₂,thereby improving the catalytic activity for toluene oxidation.Secondly,the research on Mn-Ce catalyst has been relatively mature for the low-temperature catalytic oxidation of toluene.However,these catalysts also have some problems such as poor stability and water resistance.This paper aimed to improve the stability and water resistance of Mn-Ce catalyst,and further widen the active temperature window.Thus,K-doped Mn-Ce catalyst was prepared.The activity test showed that K_x-Mn-Ce exhibits better catalytic performance,stability and water resistance than Mn-Ce,the activity temperature window of K_0.1-Mn-Ce is widened by 20℃tower the low temperature in comparision with Mn-Ce.Combined with XRD,HR-TEM,XPS and H₂-TPR characterization,it could be found that K-doping enhances the interaction between Mn and Ce ions in K_x-Mn-Ce,and promotes the electron transfer,thereby improving the redox performance of catalyst.At the same time,K-doping can promote the formation of more Mn⁴⁺and Ce³⁺,and induce the formation of more oxygen vacancies on K_x-Mn-Ce.The formation of oxygen vacancies is conducive to the adsorption-activation of O₂ to form more active oxygen species,thereby promoting the low-temperature catalytic oxidation of toluene.Finally,based on the above research of K_x-Mn-Ce catalyst,it is considered that the chemical properties of different alkali metal elements are quite different,and the chemical properties of doping elements have a significant impact on the catalytic performance of catalyst.In this paper,Li,Na,K doped Mn-Ce catalysts were prepared to study the effects of different alkali metal elements doping on the catalytic performance of Mn-Ce.The experimental results showed that Li,Na,and K doping radically enhance the catalytic activity of Mn-Ce.Combined with XRD,XPS,H₂-TPR,O₂-TPD and Toluene-TPD characterization,it was found that the interaction between Li,Na,K and Mn-Ce can increase the Mn⁴⁺and Ce³⁺concentration,and promote the formation of oxygen vacancy,which lead to the increase of surface active oxygen species.In addition,Li,Na,and K doping can also promote the adsorption-activation of toluene on the surface of Mn-Ce,which is beneficial to accelerating the reaction rate of low-temperature toluene catalytic oxidation.Among the Li,Na,K doped Mn-Ce catalysts,K-Mn-Ce has the best catalytic activity,It is determined that K-Mn-Ce shows the optimal catalytic activity,which benefits from its higher content of chemisorbed oxygen and the best toluene adsorption and the best adsorption-activation performance of toluene.