Room Temperature Catalytic Decomposition Of Formaldehyde By Birnessite-type Manganese Dioxide

With the development and enhancement of the society and people's living standard,indoor air pollution is very common caused by the wide use of decoration materials,furniture and so on.Formaldehyde is one of the main gaseous pollutants in indoor air.In order to improve indoor air quality as well as people's health,it is of great significance to develop low-cost and efficient formaldehyde removal materials.This research focused on the birnessite-type manganese dioxide?Bir?by adjusting its water content,manganese vacancy of slab and interlayer cations to enhance its formaldehyde catalytic decomposition activity.In addition,the mechanism of formaldehyde oxidation at room temperature by birnessite was also investigated.The main results are as follows:?1?It was found that birnessite-type manganese dioxide could decompose formaldehyde into CO₂ and H₂ O at room temperature and water played a key role in the formaldehyde oxidation process.The results showed that the activity of birnessite for formaldehyde oxidation was positively associated with its water content,i.e.,the higher the water content,the better activity it had.The bonded water on birnessite surface not only improved the adsorption of formaldehyde,but also could compensate the consumed hydroxyl groups during formaldehyde oxidation via reacting with surface adsorbed oxygen.In addition,water in air stimulated desorption of carbonate species via competitive adsorption and accordingly the birnessite recovered its activity.?2?It is well known that oxygen defects have great influence on manganese oxide catalytic activities,but the function of manganese vacancy?VMn?in catalytic reaction has rarely been reported.It was observed that VMn has a dramatic promotion effect on birnessite for formaldehyde catalysis.Higher content of VMn exhibited higher activity.The presence of VMn can induce more unsaturated oxygen species,which bonded with high electronegativity H+ or K+,therefore leads to higher content of surface hydroxyl and K+.The existence of K+facilitated the activation of surface hydroxyl and promoted transformation from formate to carbonate,therefore resulting in the high performance of VMn birnessite for ambient formaldehyde decomposition.?3?A series of birnessite samples with different interlayer cations?K-Bir,Mg-Bir,Ca-Bir and Fe-Bir?were synthesized through ion exchange method.The activity of as-synthesized birnessite toward HCHO oxidation at ambient temperature wassignificantly influenced by the interlayer cations.Fe-Bir showed highest formaldehyde removal efficiency due to its highest content of hydroxyl while K-Bir showed highest CO₂ conversion efficiency due to its more active surface oxygen.The introduction of K+enhances the activity of surface oxygen,which can react with H₂ O to compensate the consumption of hydroxyl,therefore,resulting in the continuous and complete oxidation of formaldehyde.?4?Birnessite was uniformly loaded on porous polyester fibers made of polyethyleneterephthalate?PET?via a simple one-step method without assistance of any templates or surfactants,which showed good activity for HCHO decomposition at room temperature.The removal of formaldehyde was kept over 94% after 10 h continuous reaction under the conditions of inlet formaldehyde concentration 0.6 mg/m3,space velocity 17,000 h-1and relative humidity 50%.In summary,this composite material showed promising application in indoor air purification due to its light,porous and good removal performance of formaldehyde.