The Tuning Of Manganese Dioxides Structures And Its Catalytic Performance For Formaldehyde Decomposition

In recent years,people have been staying indoors longer due to the frequent extreme weather and the COVID-19 pandemic.Indoor air quality is getting more and more attention.Formaldehyde,as the most common indoor air pollutant,has also attracted widespread concern for its potential health risks.Therefore,it is very significant for requirements and practicality to solve the problem of indoor formaldehyde pollution economically and efficiently.In this thesis,δ-Mn O₂ with two-dimensional layered structure was regulated by site-selective nitrogen doping,and the effect of the doping sites on the catalytic decomposition performance of formaldehyde was elucidated.Furthermore,based on the two-dimensional structureδ-Mn O₂,a three-dimensional network framework was constructed through self-assembly to obtain high-efficiency formaldehyde catalytic decomposition materials at room temperature.The mechanism of formaldehyde catalytic decomposition was analyzed in depth by means of experimental characterization.The main contents and achievements are as follows:Two-dimensional layered structureδ-Mn O₂ was synthesized through a rapid redox reaction between KMn O₄ and D-（+）-Glucose at room temperature.Meanwhile,the selective doping of nitrogen atoms at the substituent or interstitial sites in theδ-Mn O₂ lattice was achieved for the first time by controlling the nitrogen plasma treatment time.The substitutional or interstitial sites of the lattice were selectively doped with nitrogen atoms.In detail,the effects of nitrogen doping sites on crystal structure,valence state and electronic structure were studied.The results show that both substitutional and interstitial nitrogen doping can significantly reduce the oxygen vacancy formation energy.The doping sites of nitrogen atoms in the lattice significantly affect the properties ofδ-Mn O₂.In comparison with substitutional nitrogen doping,interstitial nitrogen doping can effectively narrow the band gap and enhance electron transport,which is conducive to the formation of oxygen vacancies.Due to its more oxygen vacancies and stronger oxygen activation ability,the interstitial nitrogen doped Mn O₂ exhibited improved catalytic activity of formaldehyde than substitutional nitrogen doped Mn O₂,which can completely catalytic decomposition of formaldehyde at 80°C.In order to further enhance the catalytic activity of manganese oxide in decomposing formaldehyde at room temperature,a high-purity three-dimensional manganese dioxide（3D-Mn O₂）framework was successfully self-assembled by ice-templating approach starting from two-dimensional layered structureδ-Mn O₂ without assistance of functionalization or stabilization organics.It is found that the main assembly force of 3D-Mn O₂ is the van der Waals force between Mn O₂ nanoparticles enhanced by ice extrusion.Compared with the non-three-dimensional structure of Mn O₂,3D-Mn O₂ framework exhibits significantly excellent catalytic decomposition activity of formaldehyde,which could continuously and efficiently convert formaldehyde into carbon dioxide at room temperature and high space velocity.Further study on the catalytic decomposition mechanism of formaldehyde shows that the excellent activity of 3D-Mn O₂ framework could be attributed to more active sites exposure and strong water adsorption/activation ability.3D-Mn O₂ can not only rapidly supplement the surface hydroxyl groups consumed in the process of formaldehyde oxidation,but also increase desorption of carbonate species and reduce accumulation of intermediary species.