Preparation Of Iron-manganese Oxide Catalysts In Microreactor

Fe-Mn oxide catalyst is a kind of non-noble metal catalyst that can be used for the treatment of volatile pollutants(VOCs).Coprecipitation is a common method to prepare Fe-Mn catalysts.Compared with stirred tank,microreactors have more abundant control means for coprecipitation process,and have unique advantages in preparing catalysts with different structural characteristics and understanding the relationship between catalyst structure and activity.In this paper,microreactors are used to study the structure evolution process of Fe-Mn catalysts,and to understand the path of various factors in the coprecipitation process affecting the structure and performance of the catalyst,so as to provide a basis for the development of high-performance Fe-Mn catalysts.The paper determined the direct calcination path of the precipitate without aging process as the main research object through comparative experiments,and analyzed the structure of Fe-Mn precipitate and catalyst using characterization methods such as XRD,TGA,XPS,etc.In the study of the influence of Fe content,it was found that Fe can inhibit the crystallization of MnCO3 in the precipitate and lower its thermal decomposition temperature,thereby facilitating the formation of more highly active Mn3O4 after calcination;At the same time,the added Fe will enter MnOx to form Fe-Mn-O solid solution,which is also conducive to improving the catalytic activity.There is a limit value of Fe content in Fe-Mn-O solid solution,and the catalyst activity is the best when it is the highest.The limit content of Fe is related to the uniformity of Fe-Mn in the precipitate.The microreactors can prepare co/precipitates with higher uniformity.The maximum content of Fe in solid solution is about 25%,higher than 16.7%in the stirred tank in the literature,which also provides a basis for its high activity.At the optimal Fe content of 25%,the effects of mixing on Fe-Mn precipitates and catalysts were investigated by adjusting the flow rate and selecting different configurations of microreactors.In the three channel reactor where Fe and Mn are separated and precipitated,the structural characteristics of Fe-Mn precipitates and catalysts are similar to those of pure component Mn precipitates and catalysts,the effect of Fe is very weak.In the caterpillar microreactor,as the flow rate increases,TEM and TGA characterization shows that the uniformity of Fe-Mn in the precipitates improves,and the decomposition temperature of MnCO3 decreases.The Fe-Mn catalysts prepared under better mixing effect have more Fe-Mn-O solid solution and Mn3O4 phases,better uniformity of Fe and Mn elements,stronger Fe-Mn interaction,higher surface Mn3+ content and better catalytic activity.Based on the reaction characteristics of microreactors,Fe-Mn coprecipitates without aging were prepared,and the structural changes during aging were studied.TEM analysis showed that within 5 minutes of aging,the rapid crystallization of MnCO3 resulted in the separation of Fe and Mn components in the precipitate,similar to the Cu-Mn system;However,prolonged aging does not allow Fe to enter the MnCO3 crystal,which is completely different from the Cu-Mn system.Therefore,as aging progresses in the Fe-Mn system,the uniformity of Fe and Mn monotonically decreases.On the other hand,TGA analysis found that the aged Fe-Mn precipitates require a higher temperature during calcination,which is not conducive to the activity of the catalysts.Under the combined influence of the uniformity of Fe-Mn elements and calcination temperature,the activity of the final Fe-Mn catalysts significantly decreases with aging.Due to the fact that the Fe-Mn catalysts prepared by the stirred tanks inevitably undergoes a certain degree of aging,this may be the reason why the activity of Fe-Mn catalysts has been considered poor in the literature.