Effect Of Alkali Metal Ions On Layered Manganese Dioxide Catalyst And Its Catalytic Performance Of Toluene

Volatile organic compounds（VOCs）are important precursors of fine particles and ozone,which have a prominent impact on the atmospheric environment.Catalytic oxidation can selectively convert volatile organic compounds into carbon dioxide and water,and is a promising technology for the treatment of VOCs.The performance of non-precious metal catalysts is expected to reach the level of precious metal catalysts,thus replacing expensive precious metal catalysts and becoming a research hotspot in this field.Due to the characteristics of cheap and easy-to-obtain,diverse prices,abundant resources,and environmental friendliness,Mn Ox is widely studied.In particular,manganese dioxide with a layered special crystal structure exhibits good catalytic activity and thermal stability in the removal of volatile organic compounds.There are a large number of water molecules and alkali metal cations between the layers of this type of manganese dioxide material.Especially,the interlayer cations represented by potassium ions have been confirmed to affect the specific surface area,morphology,element valence and reducibility of the catalyst.This further affects the catalytic performance of the material.Therefore,this thesis studies the effects of the types of interlayer alkali metal ions and ion concentration on the catalytic oxidation performance of toluene,and carries out a series of characterizations of its structural properties.The main research content and research results are as follows:First,three catalysts of sodium type（Na-bir）,potassium type（K-bir）and mixed type（Na&K-bir）were prepared by hydrothermal method,and their toluene catalytic oxidation activity and carbon dioxide selectivity were compared.The study found that the type of alkali metal has a significant impact on the performance of layered manganese dioxide in the catalytic oxidation of toluene.Its catalytic performance follows the order of Na-bir>Na&K-bir>K-bir,indicating that sodium-based catalysts can also have excellent toluene catalytic activity.Under the conditions of a toluene concentration of 1000 ppm,a mass space velocity of 30000 m L/（g·h）,and a moisture content of 7%,the low-temperature catalytic oxidation temperature T₅₀ and T₉₀ of Na-bir are respectively 190℃and 215℃,and has a good stability and CO₂ selectivity.X-ray diffraction results show that the crystal structure of the above catalyst has the same two-dimensional layered structure,and its morphology is a flower-like structure composed of two-dimensional nanosheets,which will not cause significant differences in catalytic performance.The normalized reaction rate analysis shows that the change in specific surface area caused by alkali metal cations in the layer is not a decisive factor for the catalytic performance.According to XPS analysis of the chemical state of surface elements,Na-bir has higher Mn³⁺and O_latt content,and higher Mn³⁺means more oxygen vacancy defects.In addition,the reducibility and oxygen species analysis results show that,compared with potassium-type and mixed-type catalysts,sodium-type catalysts have the highest lattice oxygen activity and redox properties,and thus have the highest catalytic activity.The Na-type layered manganese dioxide prepared by the hydrothermal method also has the same advantages as potassium-type layered manganese dioxide,such as high catalytic activity,good catalytic stability,and water resistance.On this basis,the ion exchange method was used to prepare layered manganese dioxide catalysts with different sodium ion concentrations by changing the concentration of sodium hydroxide solution,named Na₀-Kbir,Na_0.5-Kbir,Na₁-Kbir,Na_2-Kbir.In addition,the toluene catalytic oxidation activity and carbon dioxide selectivity of the four catalysts be investigated.The catalytic activity of Nax-Kbir is Na₁-Kbir>Na₂-Kbir>Na_0.5-Kbir>Na₀-Kbir.The research results show that the catalytic performance of toluene changes with the increase of the concentration of the replacement solution,and the sodium-type layered manganese dioxide of moderate concentration exhibits relatively optimal performance in the catalytic oxidation of toluene.Combined with XRD,SEM,BET,ICP-AES,XPS,H₂-TPR,O₂-TPD and other characterization analysis,the Nax-Kbir catalyst prepared by ion exchange method still maintains a layered structure.With the increase of ion concentration,the size of layered manganese dioxide flower-like nanosheets becomes smaller and the specific surface area increases.The results of elemental analysis indicate that the surface sodium ion concentration may be a key factor affecting the catalytic performance,which affects the oxygen vacancy content and lattice oxygen content of Nax-Kbir.Under the conditions of a toluene concentration of 1000 ppm,a mass space velocity of 30000 m L/（g·h）,and a moisture content of 7%,the toluene low-temperature catalytic oxidation temperature T₅₀ and T₉₀ of catalyst Na₁-Kbir are respectively 210℃and 220℃,at the same time has the best the carbon dioxide selectivity.Therefore,by adjusting the concentration of Na ions,the surface properties of the layered manganese dioxide catalyst can be affected,and the toluene catalytic oxidation performance can be improved.