Layered Sodium Mixed Metal Oxides As Pt Support For Improving Formaldehyde Oxidation At Ambient Condition

Formaldehyde(HCHO),one of the most common gaseous indoor pollutants,is harmful to human health known of the potential reproductive and developmental toxicities,genotoxicity and carcinogenic effect.Researchers have proposed various means to convert HCHO into carbon dioxide and water at ambient condition.For now,Pt supported catalysts turn to be one of the most potential candidates,while lots of binary oxides,such as TiO₂,Al₂O₃ and SiO₂,are studied as the noble metal carrier to enhance the catalytic activity.These carriers can affect the reaction interface,surface area and chemical groups,so that occupy an all-important position in the development of the catalysts.Layered sodium mixed metal oxides are characterized by easy preparation,flexible microstructure control,beneficial layered structure and profitable alkali ion,all of which are favorable for HCHO oxidation as Pt carriers.However,there are rare reports about layered sodium mixed metal oxides applied in this area.Herein,we used layered sodium mixed metal oxides as the carrier for Pt nanoparticles,especially focusing on NaInO₂,NaTi₂O₄·OH and NaCo₂O₄,to study the effect of carrier in HCHO ambient oxidation.First of all,we prepared mesoporous coralloid NaInO₂ with In₂O₃ as minor impurity through a facile secondary calcination reaction.After 0.8 wt.%Pt supported,the catalyst reduced the concentration of HCHO from 150 ppm to 5.5 ppm in 1 h,with T50 of 4 min.Compared with other literatures,it showed an extra low final residues as well as a quite high catalytic efficiency.Thus we can see that layered sodium mixed metal oxides are potential for HCHO ambient oxidation as Pt carrier.Meanwhile,the domination of Pt nanoparticles in HCHO oxidation are confirmed,and it is supposed that superoxide(O₂^·-)leads the key role in the reaction via the characterization of O₂-TPD,FTIR and ESR.Next,taking mesoporous NaInO₂ as the carrier,we proved the abundant surface oxygen vacancies via DFT and EPR test,and simultaneously proposed its promotion mechanism for HCHO oxidation.This is to say,O₂^·-generated on Pt nanoparticles can be trapped by oxygen vacancies to the carrier surface,and thus much more active sites can be offered for the reaction.Then,by the study of NaTi₂O₄·OH nanotube,the adsorption energies of gaseous HCHO on three typical planes(001),(010)and(100)were calculated via DFT simulation respectively.According to this,we proposed the requirement for surface hydroxyl adsorbing HCHO molecules in the reaction:taking plane(001)as reference,the hydroxyls on surface is in high density and the O atom in hydroxyl is very close with the other two Na atoms.The distances between each two atoms are less than 3.1(?),leading to a three-atom cooperative adsorption to HCHO molecule,with an exothermic energy of 1.62 eV.Finally,we took NaCo₂O₄ quantum dot as the focus,compositing with SiO₂ nanosheet.Taking advantage of the teeny diameter of quantum dot,the relatively free ions in sodium layer can easily migrate to the crystal surface.This migration eventually led to local Na-rich on the surface and has been proved by XPS and EDS-Mapping measurement.Based on further DFT simulation,we proposed the promotion mechanism of local Na-rich for HCHO oxidation.The point is that,additional Na atoms on the surface can enhance the adsorption of O atom,thus causing the enhance adsorbing of O₂^·-and HCHO.Superoxide adsorbing would provide much more active sites to improve the catalytic efficiency,while HCHO adsorbing could further increase the reactive rate.To sum up,as Pt carrier,layered sodium mixed metal oxides can efficiently capture O₂^·-or HCHO to enhance the catalytic activity via building oxygen vacancies,inserting dense hydroxyls or inducing surface local Na-rich.The feature is to transfer the reactive center from Pt nanoparticles to carrier surface.Besides adding active sites and increasing reactive rate,this center transfer also has a significant meaning in improving the stability of catalysts.