Performance Regulation Of Two-Dimensional Mn Based Nanosheets And Catalytic Oxidation Of N-Heptane

As a typical long carbon alkane,N-heptane(C₇H₁₆)is a volatile organic compound（VOCs）from a wide range of sources,and its pollution control has attracted much attention.Catalytic oxidation technology is widely studied in VOCs purification,and the research and development of efficient and stable catalysts is particularly critical.At present,Mn base catalyst showed good VOCs purification effect,compared with traditional bulk material,two-dimensional（2D）MnO₂ nanosheets with an atomic layer thickness,can make up for the active site quantity limitations and the shortage of surface defect abundance of the bulk catalysts,and can provide a certain reference for the study of catalytic oxidation VOCs mechanism of bulk catalysts.In this paper,taking C₇H₁₆ as the research object,a 2D MnO₂ nanosheet was developed for the catalytic purification and performance optimization of C₇H₁₆.The effects of thickness regulation and element doping on the catalytic performance of C₇H₁₆ were investigated,and the respective regulatory mechanisms and corresponding C₇H₁₆ reaction mechanism were analyzed.The main research conclusions are as follows:Two-dimensional Birnesite-MnO₂ nanosheets with a thickness of 1.7-2.5 nm were prepared by direct redox-hydrothermal synthesis method.Compared with bulk Mn O_x,it has a larger specific surface area and more abundant surface oxygen vacancies.The catalytic oxidation of n-heptane conforms to the MVK mechanism,and the increase of reactive oxygen species on the surface of 2D MnO₂ nanosheets reduces the activation energy of the reaction.The catalytic activity is improved,and the complete transformation of C₇H₁₆ can be realized at 253℃,which promotes the transformation of C₇H₁₆to ketone and carboxylic acid species,and finally the deep oxidation to CO₂and H₂O.The thickness of 2D MnO₂ was controlled by secondary hydrothermal method,and ultra-thin 2D MnO₂ nanosheets with a thickness of 0.9-1.2 nm were synthesized.The secondary hydrothermal temperature and time affect the interlayer force and lamellar stripping.Ion exchange is the key mechanism for the formation of ultrathin 2D MnO₂.DFT calculation confirmed the possibility that the oxygen vacancy is more likely to form with the decrease of thickness.With the increase of specific surface area and the increase of surface defects（oxygen vacancies）of ultrathin 2D MnO₂,more gaseous oxygen was adsorbed and activated,which increased the catalytic activity(T₅₀=162℃,T₉₀=194℃,T₁₀₀=239℃).In addition,Fe and Zn elements were doped in the preparation process of 2D MnO₂ to prepare 2D M-MnO₂ nanosheets with metal doping.The doping of Fe and Zn regulates the internal electronic structure of 2D MnO₂,induces the occurrence of lattice distortion and the formation of surface defects,and increases the catalytic activity at low temperature.The difference between Fe and Zn is that Fe also plays the dual role of increasing surface active sites,and has better activity at low temperature(T₅₀=142℃,T₉₀=190℃,T₁₀₀=247℃).The key to the performance optimization of both thickness control and metal doping is the increase of oxygen vacancy,but the combination of these two has no superposition on performance optimization.Thickness control can optimize the thickness and performance of 2D MnO₂,and reduce the temperature when C₇H₁₆is fully converted.While metal doping is more conducive to improve the low temperature catalytic activity,and the regulation method is more convenient.