Theoretical Study On CO Oxidation Catalysed By Monoatomic And Diatomic Catalysts

Automobile exhaust and insufficient combustion of carbon materials are the main source of CO,which seriously affects human health and the ecological environment.Oxidizing CO to CO₂ through O₂ under the catalysts is the simplest and most effective way to eliminate CO.Single atom（SACs）or metal dimer（DACs）doped graphene can improve the catalytic activity for CO oxidation.Based on density functional theory,this paper studies the detailed CO oxidation mechanism on the following metal-doped graphene systems:Mn andN co-doped graphene with single-vacancy and double-vacancy（MnNx）,VIIIB non-noble metal dimer doped graphene system（M1M2@C6）,N coordination VIIIB non-noble metal dimer doped graphene system（M1M2@N6）.Through systematic research on structural stability,adsorption performance and catalytic reaction mechanism,the following conclusions are drawn:According to the adsorption energy,MnN₂C₂,MnN₃C and MnN₄ in the MnNx system are predicted to be suitable as structures for CO oxidation,and the detailed CO oxidation mechanism in the above structure are studied.CO oxidation along the LH（Langmuir-Hinshelwood）mechanism on MnN₂C₂ is more advantageous than the ER（Eley-Rideal）mechanism.CO oxidation on MnN₂C₂-opp along the TER（termolecular Eley-Rideal）mechanism is the most favorable reaction path,and the barrier energy of rate determining step is 0.342 e V.Considering the catalytic mechanism of CO oxidation on MnN₃C,CO oxidation reaction through the LH mechanism is the best reaction path,and its energy barrier of rate determining step is the lowest（0.925 e V）.CO oxidation on MnN₄ tends to TER mechanism,and its energy barrier of rate determining step is 0.218 e V.The low reaction energy barrier indicates that MnN₂C₂-opp and MnN₄ are efficient catalysts for CO oxidationThe best reaction pathways for CO oxidation on FeFe@C6,FeCo@C6 and FeNi@C6 are*O₂+2CO→2*O²+2CO→*CO₃+CO→*O+CO₂+CO→*CO₂+CO₂→2CO₂,2*O+2CO→*CO₃+CO→*O+CO₂+CO→*CO₂¹+CO₂→2CO₂,*O₂+2CO→*O+CO₂+CO→*CO₂²+CO₂→2CO₂（“*”represents chemical adsorption）,the energy barrier of rate determining step are 0.79 e V,0.49 e V,0.42 e V,respectively.The synergism of the heteronuclear metal dimer makes FeCo@C6 and FeNi@C6 have higher catalytic activity for CO oxidation.CO oxidation on FeFe@N6 along the ER and TER mechanisms needs to overcome the low energy barriers of 0.513 and 0.299 e V,which is smaller than the energy barrier of rate determining step for CO oxidation along the LH mechanism on its single metal counterpart VFe@N6（0.584 e V）,FeFe@N6 has higher catalytic activity for CO oxidation than VFe@N6.DACs have higher catalytic activity than SACs,mainly due to the following points:First,DACs can provide more electrons for the reaction gas O₂,which promotes its high activation and easy decomposition.Second,SACs are affected by the active site and cause the second CO₂generated only through the ER mechanism,while CO oxidation on DACs to generate the second CO₂ can also follow the LH mechanism.CO oxidation through the LH mechanism to generate the second CO₂ has a lower reaction energy barrier.Third,the metal dimer in the DACs has a large disturbance to the electronic structure,and the synergistic effect of the metal dimer can effectively improve the catalytic activity.