Modification,Exfoliating And Performance For Formaldehyde Degradation Of Manganese Oxide

With the continuous development of society,people work and live longer in indoor environment.Formaldehyde pollution in the indoor environment is gradually becoming more and more important.As a first-class carcinogen,formaldehyde exists in a certain content in various building materials,decoration materials and furnitures.In recent years,researchers have found that manganese dioxide is the best catalyst for catalytic degradation of formaldehyde at room temperature.Rich source,simple preparation method,green degradation process,no harmful products.Most of the current research is to modify manganese dioxide by doping and other methods to optimize its degradation effect on formaldehyde.On the contrary,less research has been done on manganese dioxide itself and the mechanism of manganese dioxide degradation of formaldehyde.In this paper,four different crystal forms of manganese dioxide were prepared,and they were exfoliated and modified by a top-to-bottom method to try to prepare nano-powders and improve the degradation performance of manganese dioxide to formaldehyde.The main research contents and conclusions of this paper are as follows:（1）Using potassium permanganate,ammonium chloride and manganese sulfate monohydrate as raw materials,α-MnO₂,β-MnO₂ andδ-MnO₂ were synthesized by hydrothermal method,purchased commercialγ-MnO₂.They were characterized and tested for formaldehyde degradation performance.The order of degradation ability from large to small isα-MnO₂≈δ-MnO₂>γ-MnO₂>β-MnO₂.The lower the valence state of Mnin the one-dimensional tunnel-like MnO₂,the better the degradation performance of formaldehyde Layered manganese dioxide has smaller specific surface area before modification,but still has good formaldehyde degradation performance.The present formaldehyde degradation mechanism is briefly described and its advantages and disadvantages are demonstrated.（2）Using H₂O,ethanol（Et OH）,N,N-dimethylformamide（DMF）,and acetonitrile（ACN）as solvents,α-MnO₂,γ-MnO₂,andδ-MnO₂ were subjected to ultrasonic peeling treatment.The characterization was carried out to explore the effect of ultrasonic peeling treatment on the degradation rate of MnO₂ formaldehyde with different crystal forms and structures.The size of the specific surface area is not the decisive factor affecting the efficiency of MnO₂ to degrade formaldehyde.δ-MnO₂ has a unique acoustic cavitation phenomenon when ultrasonically treated in Et OH.After treatment,the BET specific surface area is increased from 5.4 m²/g to 151.9 m²/g,but the degradation effect of formaldehyde is reduced.The pore size has a greater impact on the formaldehyde degradation activity of the catalyst.For example,γ-MnO₂ with a smaller pore size is easily blocked by the intermediate products of formate and carbonate generated by the reaction after several hours of continuous degradation,resulting in the loss of its ability to degrade formaldehyde.Forα-MnO₂ andγ-MnO₂ with one-dimensional tunnel structure,the higher the average oxidation state（AOS）,the better the degradation performance of the catalyst to formaldehyde.For layeredδ-MnO₂,AOS has little effect on its ability to degrade formaldehyde.Ultrasonic peeling treatment is an effective method to improve the surface properties of MnO₂catalysts with different crystal forms and structures,and ACN is the best choice in various solvents.The degradation activity of formaldehyde for each sample was greatly improved after ultrasonic treatment in ACN.（3）α-MnO₂,γ-MnO₂,δ-MnO₂ were swelled by Li⁺,Na⁺,K⁺.The swollen samples and the initial samples were peeled.During the swelling process,Li⁺will dopeα-MnO₂ andδ-MnO₂,thereby changing the structure of the sample.Theα-MnO₂ changed from a one-dimensional tunnel-like structure to a curd-like structure,and theδ-MnO₂ also lost the characteristics of the layered structure.The swelling and exfoliation treatment improved the specific surface area and pore size of each sample,and the exfoliated samples ofα-MnO₂,γ-Na-MnO₂ andδ-Li-MnO₂ had the largest specific surface area among the isomorphous samples.However,their catalytic activity for formaldehyde within 24 h is not the highest,which proves that the specific surface area is not the decisive factor for MnO₂ to catalyze formaldehyde once again.And the higher the content of Mn（III）in the sample,the higher the degradation activity of formaldehyde.Combined with the experimental results of ultrasonic peeling,it can be seen that the ratio of Mn（III）to Mn（IV）has a great influence on the formaldehyde degradation activity of MnO₂.In the case of sufficient surface active sites,when the proportion of Mn（III）is high,the catalytic activity for formaldehyde is stronger.Treating MnO₂ with different structures and crystal forms by swelling exfoliation method can effectively improve its catalytic activity to formaldehyde.