Density Functional Theory Study On The Reduction Mechanism Of Nitrogen Oxides In Metal-graphene Catalyst System

In recent years,carbon supported transition metal has become a research focus for denitration catalyst.Specifically,graphene oxide-supported transition metal has excellent denitration performance due to its excellent physical and chemical properties.However,the reaction mechanism and main controlling factors for denitrification still lacked in-depth microscopic understanding.Density functional theory?DFT?is a quantum mechanical method for studying electronic structures of multi-electron systems.It is one of the most commonly used methods in field of computational material science.In this dissertation,density functional theory was used to bulid a model for graphene-supported metal oxide,in order to calculate energy for NO molecule on surface adsorption sites,reaction pathway and transition states.The main contents and conclusions were as follows:?1?In order to screen out a catalyst system with a better adsorption capacity,different metal-oxide surfaces were studied for adsorption of NO molecule.The results showed that adsorption energy of 3.37 eV was the highest when N atom was adsorbed vertically on surface of?-Fe₂O₃?001?.When O atom was adsorbed vertically and NO molecule was adsorbed parallel to metal,adsorption energy of Fe atom with NO was 1.65 and 1.70 eV,respectively.Thus,vertical adsorption of N atom on Fe atom was the best configuration.Meanwhile,adsorption energy of NO were compared on surface of iron oxide,chromium oxide and chromium doped iron oxide.The adsorption energy on chromium oxide was 3.84 eV,which was bigger than that?3.37 eV?on iron oxide,indicating that chromium oxide had a stronger adsorption capacity for NO than iron oxide.When chromium was doped on iron oxide,the adsorption energy increased from 3.37 to 4.34 eV.Moreover,Bader charge calculations show that NO gained 0.36 electrons from surface of iron oxide,smaller than that?0.68 electrons?from surface of iron oxide doped with chromium.As a result,bimetallic doping changed electronic-structure properties of metal oxide,improving its adsorption capacity for NO.?2?A model of graphene-loaded metal oxide was established to investigate denitrification mechanism using graphene as reductant.Density functional theory and transition state theory were used to explore reaction path for NO reduction with different catalyst.Calculations for the reaction path included three stages,namely NO decomposition on catalyst surface?S1?,arbon-atom separation from graphene-carbon ring?S2?,and formation of CO₂ together with N₂?S3?.As a result,The calculation results showed that in the Graphene-Fe₂O₃ catalyst showed maximum energy barrier values of 0.51?S1?,4.70?S2?and 2.92 eV?S3?.Thus,the rate-controlling step was S2,i.e.formation of"C-O"group.By comparsion,maximum energy barrier value of S2was 5.87 and 3.74 eV in chromium oxide and chromium was doped on iron oxide.Therefore,bimetallic doping in metal oxide improved catalytic performance for NO reduction.?3?Environmental factor usually determined performance of metal-carbon catalytic center in a catalyst.Thus,influence of oxygen-containing functional group was investigated on adsorption capacity of Fe-C bond on graphene for NO molecule.As a result,carboxyl,hydroxyl and epoxy group all increased ability of Fe-C bond for NO adsorption.Among these groups,carboxyl group had the greatest enhanement,with adsorption energy being increased from 327.88 to 366.47 kJ/mol.By comparsion,epoxy and hydroxyl group reached 343.33 and 339.47 kJ/mol,respectively.Besides,in order to explore the influence of oxygen-containing functional group on reductibility of Fe-C bond,Mulliken charge distribution was calculated in different systems.When a group was presented,Mulliken charge of Fe atom increased from0.198 to 0.305?hydroxyl?,0.427?epoxy?and 0.364?carboxyl?.In other words,reductibility of Fe-C bond on graphene was enhanced.Therefore,presence of oxygen-containing functional group around a catalytic center increased the abilities of Fe-C bond for adsorption and reduction of NO molecules.