Theoretical Study On The Microscopic Reaction Mechanism Of BiOBr Photoctalytic Reduction Of CO₂

Fossil energy is non-renewable,and in the process of burning fossil energy,a large amount of CO2 will be emitted,causing serious environmental problems.Photocatalytic reduction of CO2,using inexhaustible solar energy,through the photocatalyst,can convert CO2 in the air into energy molecules such as CH4,CH3OH,etc.,thereby achieving the carbon cycle.Therefore,photocatalytic reduction of CO2 has become a very popular research direction in the fields of physics,chemistry,and materialsHowever,currently the conversion efficiency of photocatalysts is generally low,there are many types of CO2 reduction products,and the selectivity is not easy to control.It is particularly important to explore the influencing factors and reaction mechanism that afect the photocatalytic reduction of CO2,so as to achieve the purpose of designing and discovering new high-efficiency photocatalysts.Among them,the absorption and utilization of light and the mechanism of surface absorption and desorption are two of the most important factors.The BiOX(X=Cl,Br,I)photocatalytic material has a band gap of visible light absorption and a unique layered structure,and it was found in experiments that the photocatalytic efficiency of the BiOX material can be adjusted by surface regulation and the construction of self-doping defects And selectivity,it is very representativeTherefore,this paper chose BiOBr as the research object,aiming to use the first-principle density functional(DFT)calculation method to compare and analyze the surface properties of BiOBr(001)and(010)crystal planes,as well as CO2 molecules on their clean surfaces,Adsorption properties of defective surfaces.The details are as follows(1)The first-principles planar pseudopotential method was used to study the BiOBr bulk crystal structure and the surface relaxation,surface energy,surface energy band,and electronic density of states of(010)and(001)different crystal planes.The research results show that the valence band of BiOBr crystal material is contributed by O 2p orbital and Br 4p orbital,and the conduction band is contributed by Bi 6p orbital,which belongs to the p-to-p charge transfer type.The(001)surface with Br atom as its terminal has the smallest relaxation degree,the lowest surface energy,and the highest thermodynamic stability.It is easier to be exposed in a larger proportion during the crystal growth process,and its band structure is similar to the bulk structure.Secondly,the(010)surface with BiOBr atoms as the terminal has a small difference in surface energy from the surface energy of(001)Br.It is also one of the most stable surfaces,but its conduction band moves toward the high energy direction and the band gap becomes wider.The(001)O surface and the(001)Bi surface are complementary surfaces.The conduction band moves to high energy and low energy directions respectively,but the band gap becomes smaller,which is beneficial to increase the absorption range of visible light.However,the surface energy of these two surfaces is very large,and the thermodynamic stability is very low,and they are very likely to be gradually replaced by other crystals during the crystal growth process.In comparison,(001)O thermodynamic stability is stronger than(001)Bi thermodynamic stability.The(001)BiO surface and the(001)2Br surface are complementary,the thermodynamic stability is at an intermediate level,and the band gap is reduced.The valence band and conduction band of the former move largely in the lower energy direction,and the conduction band passes through the Fermi level.The conduction band of the latter moves in the direction of high energy,and the valence band just crosses the Fermi level.(2)Study on the microscopic mechanism of CO2 molecular adsorption and activation on BiOBr catalyst clean surface.This chapter mainly studies the adsorption geometry,adsorption energy,differential charge density,and Bader charge analysis of CO2 molecules on each crystal plane.Here we have selected two types of crystal faces based on the thermodynamic stability and redox ability of each crystal face.One is two surfaces with high thermodynamic stability(001)Br surface and(010)BiOBr surface;The other is the surface with relatively weak thermodynamic stability,but relatively strong redox ability—(001)BiO surface;in addition,because(001)O surface is covered with high density O atoms,it provides a large number of active sites,So it is also taken into account.Studies have shown that:1)The surface M-O-M structure is more conducive to the bending of CO2 molecules,and the adsorption configuration is a monodentate C coordination adsorption model;2)The absolute value of the adsorption energy of the curved adsorption configuration is significantly larger than that of the linear adsorption configuration,of which(001)The surface adsorption energy of OH has the highest negative value(-0.61eV),followed by the(010)BiOBr surface,with an adsorption energy of-0.44eV,which makes it easier for the adsorbed molecules to desorb from the surface.(001)Br atoms in the curved adsorption configuration on the Br surface shift,the surface is reconstructed,the system absorbs heat,and is in a metastable state,the adsorption energy is positive(1.00 eV);3)C atomcan obtain electrons from the surface Lewis base center,(010)BiOBr has the largest surface electron transfer.(3)The BiOBr material was modified by the method of constructing defects,and the structural characteristics,electronic structure,adsorption structure,adsorption energy,differential charge density,and Bader charge analysis of different defects were analyzed.Here,we first construct Bi,O,Br,Bi-O,Bi-O-Bi defects for the bulk structure,and calculate the electronic density of states and energy band based on the stable structure.The results show that the defect band edges of Bi,Bi-O,and Bi-O-Bi are close to the clean bulk material.Both the Br defect valence band and conduction band move to the lower energy direction,the valence band just crosses the Fermi level,and the band gap width does not change much.The O defect valence band and conduction band move to the lower energy direction greatly,and a new energy level is generated at the Fermi level.Secondly,we control the(010)and(001)surfaces of BiOBr materials,namely(001)O surface,(010)BiOBr surface structure O vacancy,(001)Br surface structure Br vacancy,(001)BiO surface structure Bi vacancy,Bi-O-Bi vacancies.The calculation results show that the defect surfaces all move to the low-energy direction,which is similar to the structural changes of bulk defects,and intermediate energy levels appear on the surfaces of(010)BiOBr_Vo and(001)BiO_VBiOBi.Finally,we calculated the CO2 adsorption on different defective surfaces.Compared with clean surfaces,the defective surfaces significantly increased the number of electrons transferred during the reaction.Among them,(010)BiOBr_Vo has the most surface electron transfer,but the adsorption energy is less than(010)BiOBr clean surface,which makes it easier to desorb CO2 molecules.