Theoretical Chemistry Study For Adsorption And Catalytic Mechanism Of Typical VOCs Onto δ-MnO₂ Layer

With the development of industrialization,the quality of human life has been greatly improved.In this process,however,enormous environmental challenges have been caused,among which air pollution has been widely concerned.VOCs（volatile organic compounds）is a kind of main pollutant in the air,which can threat human health and ecological environment under certain conditions.In the various of technologies for VOCs treatment,adsorption and catalytic methods are widely used in the treatment of VOCs because of their excellent natures such as simplicity,high efficiency and no secondary pollutants.The cores of these two treatment technologies lie in efficient VOCs adsorption and catalytic materials,respectively.δ-MnO₂ is an excellent VOCs catalyst,which can catalyze CH₂O and other VOCs at room temperature.There are many works on the catalytic performance ofδ-MnO₂,but few studies on its adsorption.In addition,the catalytic mechanism of VOCs on its surface is still ambiguous.Based on this,the adsorption and catalytic behaviors of typical VOCs on layeredδ-MnO₂ surface were studied by using first principles calculation software,VASP and PWmat.The main results are as follows:（1）Firstly,we calculated the adsorption mechanism of typical VOCs（formaldehyde,benzene and chloroform）on pure and Al-modified MnO₂ monolayers.The results show that there are weak physical interactions between VOCs and MnO₂ monolayer,and the modification of Al atom greatly enhances these interactions,which can be transformed into chemisorption.In particular,benzene molecule showed the best adsorption performance on Al-modified MnO₂ monolayer（Eads=-2.416 e V）.In addition,the difference of adsorption energy for CH₂O between pure MnO₂ monolayer and Al-modified MnO₂ monolayer is-2.044e V,which increases nearly 10 times,indicating that Al atom can significantly improve the adsorption performance of MnO₂ layer for CH₂O.In addition,the results of density of states and charge density difference confirm that Al atom acts as a tie between VOCs and MnO₂monolayer,which enhances the adsorption of VOCs.Therefore,Al modified MnO₂ monolayer is expected to be a promising adsorbent for VOCs.（2）By studying the behavior of H₂O and O₂ on layered MnO₂ surface with different surface vacancy conditions,it is found that surface vacancy can promote the formation of oxygen-containing groups on layered MnO₂ surface,and the types and production methods of active groups on different vacancy surfaces are different.The oxygen vacancy on the outer surface can directly activate the oxygen molecule.There are two main cases:the structure with only the oxygen vacancy can activate O₂ to O₂^-group;the structure with O and Mn vacancy can activate O₂ to O₂^2-group.The structure with only Mn vacancy can also generate O₂^2-active group through activating O₂ to split.However,the O vacancy on the inner surface can not activate the surface oxygen molecule.In addition,for the H₂O molecule,the layeredδ-MnO₂material with O and Mn vacancies on the outer surface can activate it to split and produce oxygen-containing active groups.Water can be activated and cleaved completely on the surface of the structure containing only Mn vacancies,and react with the O_2C structure on the surface to form OH and O₂^2-structures.For structure with Mn and O vacancies,they can be split into two OH structures after water adsorption.Therefore,we can conclude that the vacancy on layered MnO₂ surface can activate H₂O and O₂ to form oxygen-containing groups.（3）The oxidation process of HCHO on layeredδ-MnO₂ surface with different vacancy was studied and the results show that the vacancy structure can promote the oxidation of HCHO.The oxidation of HCHO on the surface of layeredδ-MnO₂ containing vacancy is carried out by step dehydrogenation.The layeredδ-MnO₂ with Mn vacancy can effectively activate dehydrogenation of HCHO to produce CO₂.The active site of the reaction is the O_2Catom induced by Mn vacancy.The O_2C atom participates in the reaction process,and finally forms CO₂ with the CO structure of HCHO.The oxidation cycle of HCHO mainly occurs on the surface of the three-vacancy structure（one Mn vacancy and two O vacancies）,which induced by the partial oxidation of HCHO.In this cycle,O₂ mainly plays the role of replenishing O vacancies and consuming surface H atoms produced by dehydrogenation of HCHO.The process of consuming H atoms to generate H₂O is the rate-determing step in the whole cycle,and the co-adsorption of HCHO can promote the process.