Preparation And Properties Of Modified Manganese Based Composite Oxides For Catalytic Oxidation Of Formaldehyde

Formaldehyde is mainly derived from decoration materials,which is the main source of indoor pollution.Long-term exposure to formaldehyde can cause irritation of various organs,headaches and other uncomfortable symptoms,and can even cause poisoning and cancer.Therefore,it is urgent to eliminate formaldehyde and reduce the concentration of formaldehyde in the environment.Among many treatment technologies,catalytic oxidation technology can convert formaldehyde into CO₂ and H₂O at low temperature.Due to its advantages of low price and no secondary pollution,it has broad application prospects.The core of this technology is the development and research of catalysts.Compared with precious metal catalysts,transition metal oxide catalysts have been widely studied for their advantages of abundant resources and low price.In particular,manganese based catalysts are used in the field of catalytic oxidation of formaldehyde because of their rich oxidation valence and diversified forms.Such catalysts have good catalytic oxidation performance at high temperature（≥100℃）,but the low temperature oxidation performance of formaldehyde still needs to be further improved.In this paper,a series of MnO₂-Co₃O₄ nanospheres composite oxide catalysts were prepared by using carbon microspheres as sacrificial templates based on the advantages of pore structure and high specific surface area.At the same time,due to the characteristics of Metal-organic frame materials（MOFs）,such as large specific surface area,open structure and porous structure,Ce-MOFs were selected as the sacrificial template to prepare a series of MnO_x-CeO₂ nanorods composite oxide catalysts.By regulating Mn/Co molar ratio and KMnO₄/Ce-MOFs mass ratio,the influence of active component content on catalytic performance was studied.N₂ physical adsorption-desorption,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,hydrogen temperature programmed reduction（H₂-TPR）,oxygen temperature programmed desorption（O₂-TPD）,X-ray photoelectron spectroscopy（XPS）,and in situ infrared spectrum（In situ DRIFTS）and MnO₂-Co₃O₄ nanorods and MnO_x-CeO₂ nanorods are characterized by particle size,morphology,catalyst reducibility,surface active oxygen species,surface element composition and reaction mechanism.Combined with catalytic performance evaluation data,the best performance catalysts are selected.The structure-activity relationship of catalyst was established,and the following conclusions were drawn:（1）The precursor of MnO₂-Co₃O₄ nanospheres was assembled using carbon microspheres as templates,and the composite oxides of MnO₂-Co₃O₄ nanospheres were prepared by drying and roasting.The effects of the content of active components on the performance of the catalysts were investigated by adjusting the molar ratios of Mnand Co(n _Mn:n _Co=1:9,5:5,9:1).The results show that when the molar ratio of Mn/Co is 5:5,5Mn5Co has well low temperature reducibility and plenty surface active oxygen species.5Mn5Co has the best catalytic performance,and it can completely convert formaldehyde at 80℃.It was found that the synergistic effect between Mnand Cewas conducive to the exposure of active sites and the adsorption of formaldehyde.Mnenters the crystal lattice of the cobalt oxide and forms a solid solution.The high prices of manganese and cobalt on catalyst surface promote the formation of catalyst defects,increase the number of oxygen vacancies,and facilitate the migration of active oxygen species.MnO₂-Co₃O₄catalyst surface adsorption oxygen concentration from low to high:MnO₂<Co₃O₄<5Mn5Co-c<9Mn1Co<1Mn9Co<5Mn5Co,which consistent with the catalytic performance of the catalyst.In situ DRIFTS study found that 5Mn5Co formed DOM and HCOO^-intermediate species in the process of catalytic oxidation of formaldehyde,and surface reactive oxygen species and hydroxyl groups accelerated the further oxidation of DOM and HCOO^-intermediate species.The possible reaction path was as follows:HCHO（1）DOM（1）HCOO^-+OH（1）CO₂+H₂O.（2）Using Ce-MOFs as the carrier,the precursor supported by manganese oxide was impregnated,dried and roasted to obtain MnO_x-CeO₂ nanorods.By adjusting the mass ratio of potassium permanganate and Ce-MOFs(m _KMnO4:m _Ce-MOFs=4:5,5:5,6:5),a series of x Mny Cecatalysts were prepared,and the effect of manganese content on the catalyst performance was investigated.It is found that when(m _KMnO4:m _Ce-MOFs=5:5,5Mn5Cehas the best catalytic activity,which can completely convert formaldehyde into CO₂ and H₂O at 80℃.The high specific surface area of 5Mn5Ceis conducive to the full exposure of the active site and the adsorption of the reactants.The strong interaction between Mnand Cein 5Mn5Ceimproved the low temperature reduction performance of the catalyst and promoted the formation of surface reactive oxygen species.The abundant Mn⁴⁺,Ce³⁺and active oxygen species on the surface of5Mn5Cecatalyst are beneficial to the catalytic oxidation of formaldehyde.In situ DRIFTS study found that the intermediate products produced by catalyst 5Mn5Ceduring the catalytic oxidation of formaldehyde were DOM,HCOO^-and CO₃^2-.Therefore,the possible reaction mechanism was inferred as follows:HCHO（1）POM/DOM（1）HCOO^-/CO₃^2-（1）CO₂+H₂O.