Study On Formaldehyde Catalytic Performance Of Nano Manganese Dioxide Composites

Formaldehyde is the main organic pollutants in indoor air.Prolonging exposure to high concentrations of formaldehyde may lead to cancer,therefore it is urgent to seek an appropriate way to degrade formaldehyde.Manganese dioxide possess multiple crystal forms and is prone to the gain and loss of electrons.In this paper,four different crystalline forms of manganese oxides were prepared by hydrothermal synthesis:?-,?-,?-and?-MnO₂.The MnO₂-C composites with different loading ratios were synthesized by co-precipitation method.The catalytic oxidation experiments of HCHO were carried out at high temperature and room temperature respectively,in order to test the performance of catalysts.Meanwhile,the catalytic mechanism were investigated by means of In situ infrared diffuse reflection.The main achievements of this paper are as follows:?1?The micro-structures of?-,?-,?-and?-MnO₂ are considered as nanorods,while?-MnO₂ has a nanospheres structure.The structure-activity relationship of the catalysts shows that the crystal structure,specific surface area,surface ion valence state of the catalysts,the surface oxygen vacancy concentration and the binding energy of oxygen species all have great influence on the catalytic activities.?2??-MnO₂ has the best catalytic activity among the four kinds of crystalline manganese oxides,its activity window is 75?-155?,and the maximum conversion of HCHO is 100%which appears at 155?.The catalyst is reversible deactivated when 1%H₂O was added.In the case of coexistence of sulfur dioxide and water,H₂O can accelerate the reduction of catalytic activity of?-MnO₂ through the formation and deposition of ammonium sulfate.?3?We gained active carbon materials with high specific surface area through charring,soaking in alkali solution and activating at high temperature.The?-MnO₂was loaded on the surface of the activated carbon with different proportions?MnO₂:C=5,10,20,30,50?.The active temperature window of 50%MnO₂-C composites was50?-125?,which is mainly due to the defects in the formation of oxygen vacancy clusters.?4?At room temperature,the removal rate of HCHO by 50%MnO₂-C composites has been kept above 80%all the time,and there is no decreasing trend.However,fresh activated carbon was rapidly deactivated at the beginning of the reaction within200min,indicating that the Mn-O structure on the surface of the MnO₂-C catalysts enhanced the overall activity of formaldehyde.?5?At high temperature,we believe the main intermediate products are DOM,formate and carbonate,and then they were degraded to CO₂ and H₂O by surface hydroxyl group and lattice oxygen of catalysts.At room temperature,the main intermediate products are POM,DOM and formate,which were further decomposed into CO or CO₃^2-.In the presence of surface oxygen,intermediate products were oxidized to CO₂ and H₂O,while the active carbon could not further degrade substrates owing to the lack of surface oxygen.It all led to the accumulation of the intermediate products and accelerated the deactivation of the catalysts.