Theoretical Study On The Mechanism Of The Selective Oxidation Of Sulfide By Single Atom Catalysts

Sulfoxide compounds are an extremely important intermediate,so the synthesis of sulfoxide compounds has always been a hot spot in scientific research.At present,how to control the selective oxidation of sulfide to sulfoxide has become a key issue in research.Therefore,it is particularly important to find a catalyst that can catalyze the oxidation of sulfide to sulfoxide with high efficiency and selectivity.Single-Atom Catalysts（SACs）are a kind of supported catalysts that only contain isolated single atoms as catalytic active centers.Compared to traditional nano-and sub-nano catalysts,single-atom catalysts have high activity,large surface free energy and high atom utilization.For the last decade,it has gradually become the most potential material in the field of catalysis.SACs materials can be used in a variety of catalytic reactions,such as photocatalytic reactions,electrocatalytic reactions,oxidation reactions,hydrogenation reactions,etc.Single-atom catalysts also have the characteristics of both heterogeneous catalysts and homogeneous catalysts,and are considered as the bridge connecting heterogeneous catalysis and homogeneous catalysis.At present,a large number of research results have been achieved in the synthesis and catalysis of single-atom materials,but the understanding of the microstructure of materials is still lacking.The density functional theory（DFT）can help us further reveal the microstructure of single atom materials in the carrier,simulate the reaction path in the catalytic reaction process,and calculate the different paths using different single atom catalyst models.The feasibility of the reaction provides theoretical guidance and support for the subsequent design of high-activity and high-selectivity single atom materials.In this paper,the representative transition metals V,Cr,Mn,Fe,and Co were selected as the active centers to be anchored on the two-dimensional material graphene,and were studied for their stable configurations,oxidation models,and corresponding catalytic oxidation reaction mechanisms.The main research contents of this paper are as follows:（1）A series of transition metal single atoms V,Cr,Mn,Fe,Co are anchored on nitrogen-doped graphene,and the method of DFT combined with the method of dispersion correction（DFT-D3）was employed to systematically study the configuration of single atoms materials in two different coordination environments（three-coordination and four-coordination environment）,and their formation energies have been compared to obtain the most stable configuration model.The calculation results show that the configuration of single atom V,Cr,Mn,Fe,Co coordinated with four nitrogen atoms has higher thermodynamic stability than that coordinated with three nitrogen atoms.In addition,the stable configurations with the single atom coordinated with four nitrogen atoms are all planar.（2）The highly stable MN₄-Gr（M=V,Cr,Mn,Fe,Co）were selected as the catalyst model,and use the DFT combined with the dispersion correction method was used to simulate the interaction process between the oxidizer tert-butyl hydrogen peroxide（TBHP）and the single-atom catalysts.The first model takes into account the positional effect of the two oxygen atoms after the oxidant TBHP interacts with the single-atom catalyst twice.It is found that there are two model structures,namely the"oxygen-oxygen same side model"and"oxygen-oxygen different side model".The calculation results show that the"oxygen-oxygen same side model"of single atom V is thermodynamically more stable than the opposite side structure,while the"oxygen-oxygen different side model"of single atom Cr,Mn,Fe,and Co is more thermodynamically stable than the same side structure.The second model has an hydroxyl（-OH）group on the MN₄-Gr configuration,which was optimized to obtain the thermodynamically stable model—HO-MN₄-Gr.This model,was used as a catalyst model after interacting with the oxidant TBHP,an oxygen-containing model with hydroxyl modification is formed（HO-MN₄-Gr-O）.The calculation results show that the existence of hydroxyl makes the interaction between the oxidant TBHP and the single-atom catalysts stronger,that is,the presence of hydroxyl may promote the single-atom catalysts undergo catalytic oxidation reactions.（3）With the theoretical method of DFT-D3 combined with the NEB method,and the catalytic models of the high-stability V"oxygen-oxygen same side structure"and Cr,Mn,Fe,Co"oxygen-oxygen different side structure"and oxygen-containing modified with hydroxyl model（HO-MN₄-Gr-O）,we discussed the reaction mechanism of the selective oxidation of methyl phenyl sulfide to methyl phenyl sulfoxide.The calculation results show that the reaction mechanism of thioether oxidation is mainly divided into 4 processes.The first process is the interaction of single-atom catalyst and oxidant TBHP to form an oxidation model（O=MN₄=O/HO-MN₄-O）.The second process is the nucleophilic attack of the S atom on the thioether on the O atom on the single-atom oxidation model to form a sulfoxide product.The third process is that the oxygen atom of the oxidant activates the metal single atom to form the oxidation model intermediate.The final process is that the further oxidation of sulfoxide to sulfone.When the oxygen-oxygen model（O=MN₄=O）is selected as the oxidation model of the catalyst,the NEB calculation results showed that the sulfoxide product could be easily converted to sulfone in the reaction of sulfide oxidation with V,Cr,Mn,Fe and Co as the single-atom catalysts.When the oxygen-containing model with hydroxyl modification（HO-MN₄-Gr-O）is selected,the NEB calculation results show that the V and Fe single-atom catalyzed the oxidization of thioether to stay in the stage of sulfoxide formation,achieving selective oxidation.However,the sulfoxide generated in the catalytic oxidation reaction of single atomic catalysts Cr,Mn and Co can still be easily converted into sulfoxide products,indicating a poor selective oxidation.