Study On Hydrotalcites Supported Transition Metal Catalysts For Formaldehyde Removal

Formaldehyde is one of the common indoor volatile organic pollutants.Long-term exposure to formaldehyde seriously harms human health and even causes cancers.Therefore,it takes great significance to find a safe and inexpensive way for formaldehyde removal method.Transition metal catalysts represented by manganese oxides display high formaldehyde removal efficiency as well as stable performance,which is expected to be widely applied.However,transition metal catalysts require high temperature for completely formaldehyde oxidation,and their powder morphology is not conducive to practical application.Therefore,hydrotalcites were applied as a carrier loading manganese oxide in this paper to improve the formaldehyde removal performance with the help of hydroxyl groups.First,the formaldehyde removal efficiencies of the two catalysts prepared via co-precipitation method and in-situ redox method were evaluated.Then,the ratio of Mg²⁺/Al³⁺in hydrotalcites was adjusted to achieve highest catalytic activity,namely 10Mn/Mg₃Al₁-LDH.The structure,composition,formaldehyde reaction mechanism and kinetics of prepared catalysts were studied through XRD,N₂ adsorption-desorption,TG,XPS,SEM,FITR.Finally,the 10Mn/Mg₃Al₁-LDH catalyst was fixed on an aluminum honeycomb to obtain practical a composite catalytic material with excellent room temperature formaldehyde removal performance was got.The main research results were summarized as follows:（1）10Mn/Mg Al-LDH synthesized by in-situ redox proposed a higher formaldehyde removal efficiency than the catalysts prepared via co-precipitation.10Mn/Mg₃Al₁-LDH(i.e,Mg²⁺/Al³⁺was 3)showed the highest catalytic activity.The complete removal and oxidation temperatures of formaldehyde were 80℃and 120℃respectively.Formaldehyde adsorption results presented that adsorption capacity of 10Mn/Mg Al-LDH was positively correlated with the content of Mg²⁺in hydrotalcites carrier,and 10Mn/Mg₅Al₁-LDH showed the largest adsorption capacity.（2）The catalyst characterization results proved that there existed a large amount of hydroxyl groups in the 10Mn/Mg Al-LDH catalysts,and the hydroxyl groups were positively correlated to Mg²⁺content.10Mn/Mg₅Al₁-LDH catalyst proposed the most hydroxyl groups,while the 10Mn/Mg₃Al₁-LDH catalyst displayed the highest surface adsorbed oxygen and Mn⁴⁺/Mn³⁺couple.Abundant surface hydroxyl groups,surface adsorbed oxygen and Mn⁴⁺/Mn³⁺couples provide favorable conditions for formaldehyde oxidation over 10Mn/Mg₃Al₁-LDH catalyst.（3）The main intermediate products of formaldehyde oxidation over 10Mn/Mg Al-LDH catalysts were DOM,formate and carbonate,which would be gradually converted into CO₂ and water with active oxygen and active hydroxyl.Asynchronous conversion of formaldehyde leaded to intermediate products accumulation,thus was responsible for the deactivation of the catalyst.（4）The reaction kinetics study showed that formaldehyde oxidation on the 10Mn/Mg Al-LDH catalyst conformed to the second-order kinetic model.The reaction constants followed the order of10Mn/Mg₃Al₁-LDH>10Mn/Mg₅Al₁-LDH>10Mn/Mg₁Al₁-LDH,which proved that 10Mn/Mg₃Al₁-LDH performed the best catalytic activity.（5）The aluminum honeycomb loaded with 10Mn/Mg₃Al₁-LDH exhibited an admirable formaldehyde removal performance at room temperature.The formaldehyde concentration could be reduced by more than 92%after 4 hours reaction.Regeneration and continuous tests result also demonstrated that the aluminum honeycomb composite material presented excellent stability.