DFT Study On The Mechanism Of O₂ Activation And Photocatalytic Decarboxylation Of Methyl Stearate And CO₂ Reduction On BiOBr（001） Surfaces

Waste oil is a kind of biomass resource,which has not been used effectively at present.It could be made into biodiesel through transesterification or thermochemical process.However,as the main component of biodiesel,fatty acid methyl ester has high viscosity,easy solidification and low combustion value,which are disadvantageous to direct utilization.Therefore,the green catalytic hydrodeoxidation or direct decarboxylation of fatty acid methyl ester into hydrocarbon fuel without oxygen and with high calorific value have become one of the most urgent scientific problems in the field of comprehensive utilization of waste oil.Photocatalysis technology using of the solar energy has attracted much attention due to its advantages such as mild reaction,easy process control and green environmental protection.It can make full use of the active species generated by photocatalyst and the interaction of fatty acid methyl ester to generate a variety of hydrocarbon compounds.Studies have shown that the generation of hydroxyl radical（·OH）which is a key active species for the photocatalytic decarboxylation of fatty acid methyl esters,and the reduction of CO₂ which is a by-product of decarboxylation are all associated with effective photoelectrons.And the effective separation of photoinduced carriers is an important prerequisite for the increase of effective photoelectrons and the enhancement of photocatalytic performance.Therefore,from the molecular or atomic level,it is of practical significance to deeply understand the migration behavior of photogenic carriers of catalysts,effective separation and the interaction process with fatty acid methyl ester and CO₂ to promote the theoretical study of fatty acid methyl ester decarboxylation.Bismuthyl bromide（BiOBr）is a kind of visible light responsive high-activity catalyst,and its indirect band gap,unique layered structure and the presence of internal electric field（IEF）are more conducive to the effective separation of photogenic carriers.In this paper,the Dmol3 module,based on density functional theory from first principles,was employed to comparative analyse the photoinduced carrier behaviors and microscopic characteristics on BiOX（001）surfaces.The BiOBr with rich electron（001）surfaces and methyl stearate as models of reactants were chosen to further study the reaction mechanism and process for effective electron affecting activation O₂ to form·OH and CO₂reduction.Meanwhile,the thermodynamic and kinetic analysis were hired to reveal the control steps and key factors of the methyl stearate decarboxylation through·OH.The details are as follows:（1）Theoretical study on the electronic structure and photogenic carrier migration behavior of BiOX（X=F,Cl,Br,I）（001）surfaceIn order to determine the photocarrier transmission and surface behavior,the BiOX（001）surfaces were employed as the research object and the band structure,electron density of states,Mulliken population analysis,electron density difference,electronegativity and bonding analysis were calculated.Meanwhile,the effect of oxygen vacancy on electron properties was also discussed.The results showed that the top of the valence band of BiOX（001）was composed of O 2p and X np（X np=F 2p,Cl 3p,Br 4p,I 5p）hybrid states,while the bottom of the conduction band was mainly contributed by Bi 6p states.Mulliken population analysis and electron density difference displayed that the electrons preferred to migrate around the interface Bi atoms.More importantly,the electronegativity and bonding analysis results were similar to the experimental results:there were Bi–F ionic bonds in BiOF,while BiOCl,BiOBr and BiOI were all covalent crystals.The calculation of carrier drift velocity revealed that X atom acted as a"bridge"connecting the upper and lower layers in the process of electron transmission.Under the action of IEF,photogenerated electrons were transmitted from the bulk phase to the（001）interface through the transmission migration path of O→Bi→X→X→Bi→O and photogenerated holes will diffuse to the hole-rich surface through the interface O atom.The presence of oxygen vacancy would increase curvature of the band structure for（001）surfaces,these meant that the effective mass of electrons would be decreased and the migration ability could be increased,that is to say,the electrons preferred to enrich around the oxygen vacancy.Compared with BiOF,BiOCl and BiOI semiconductor,BiOBr showed the best effective electron migration ability.（2）Theoretical study on the mechanism of adsorption-activated O₂ for the generation of decarboxylated reactive oxygen species·OH on BiOBr（001）Taking BiOBr（001）as the research object,the electronic properties of（001）surfaces with different exposed terminal atoms Bi,O and Br（denoted as 001Bi,001O and 001Br,respectively）were theoretically calculated and analyzed,and the mechanism of effective electronic activation of O₂ to form·OH was also studied.The work functions of 001Bi,001O and 001Br were calculated to be 5.22eV,5.88 eV and 6.69 eV,respectively,indicating that surface electrons of 001Br were the most difficult to form effective free electrons.The adsorption energy of O₂ on（001）surfaces showed that the adsorption stability of O₂ on 001Bi surface was the strongest（O-O2:-1.32 eV,BiU-O2:-2.55 eV and BiD-O2:-2.55 eV）,while the adsorption energies of O₂ on 001Br were positive（O-O2:0.60 eV,Bi-O2:0.62 eV and Br-O2:0.61 eV）,showing the weakest adsorption capacity of O₂.Based on the band center from electron density of state,it was found that the average position of 001O Bi 6p orbital was closest to the vacuum level,which meant that excited electrons in Bi 6p orbital were more likely to spill out and participate in electron-related reactions.According to the local density of state,the energy band center variation values of 001O and 001Br（3.84 eV and 3.53 eV）were significantly greater than that of 001Bi（1.85 eV）,indicating that the O₂adsorbed on 001Bi has the minimum electron receiving capacity.However,when calculating the effective electron drift velocity,it was found that surface 001Bi（2.595?and 2.664?）with the shortest migration distance presented the maximum electron density diffusion.These meant that the steric steric effect at interface 001O and 001Br increased the migration distance of effective electrons to be adverse to the migration of effective electrons to O₂.The results of the hydrogenation barrier for O₂ showed that the exposure of Bi atoms could enhance the migration of photogenerated electrons to O₂,which could boost the O–O bond dissociation energy decreased to 0.18 eV and facilitated the formation of·OH.However,with the O and Br exposure,the reduced effective electron mobility could induce the O–O bond breakage hardly,while preferring to generate·O₂H.Therefore,from the perspective of thermodynamics,it is confirmed that 001Bi is more beneficial to O₂ forming the active species·OH that was required for photocatalytic decarboxylation.And these provided a good data foundation and guiding significance for the study of photocatalytic decarboxylation of methyl stearate.（3）Decarboxylation of methyl stearate（MS）by reactive oxygen species·OHBased on thermodynamics and kinetic analysis,the reaction mechanism and control steps of MS decarboxylation with·OH were investigated,the calculation about the OH?catalyzed ester hydrolysis reaction were introduced as comparation.The electrostatic potential showed that the electrostatic potential field of carboxyl group position changes obviously,which confirms that the charge property will change obviously in there.At 298.15 K,the calculated dissociation energy of C–C bond was 74.851⁹0.390 kcal/mol,which was consistent with the reported average dissociation energy of C–C bond（83.174 kcal/mol）.The increasing fluctuating of the C–C bond dissociation energy corresponding with the increase of temperature revealed that the temperature rising could promote the formation of short-chain hydrocarbons.The results of thermodynamic and kinetic analysis showed that the OH?inhibited the fracture of C–C bond,but to favor the hydrolysis reaction,which was consistent with that the MS was preferentially hydrolyzed with the OH?rather than generated short-chain compounds in the experiment.Meanwhile,the·OH was benefit for the C–C bond breaking,and the dissociation energy of C–C bond near the carboxyl group decreased sharply,indicating that·OH preferentially induced the carboxyl group of MS to dissociate to form the intermediate methyl bicarbonate（HO–（C=O）–OCH₃,MHC）.Double·OH would be more advantageous to the positive direction of MS decarboxylation reaction,and its key intermediate was the MHC.Therefore,increasing the MHC decarboxylation rate could accelerate the progress of decarboxylation reaction,and the activation energy Ea for the decarboxylation controlled step was 14.446kcal/mol,which was uniform with the reported 15.640 kcal/mol,further proving the reliability of our calculation results.（4）Microscopic properties,reaction path and mechanism of CO₂ adsorption-reduction on BiOBr（001）surfacesThe BiOBr（001）surfaces with different atom terminals（001Bi,001O and001Br）were employed to explore the reduction process of CO₂,which was the by-products from MS decarboxylation process.The electronic properties,CO₂adsorption capacity,reaction paths and reduction reaction mechanism of photocatalytic reduction for CO₂ were investigated to uncover the important scientific significance for this green comprehensive recycling of waste cooking oil and the reduction of CO₂ research advance.Electron-rich site was defined as a more electronic migration to the reactants on the site.The adsorption energy calculations showed that only two adsorption sites of 001O（O–COO）and 001Br（Bi–COO）sites were positive（0.46 eV and 0.36 eV）,while the two sites displayed as high electron density diffusions through the calculations of electron density difference.These meant that the CO₂ adsorption over the electron-rich sites on 001O and 001Br were energy-consuming process.The band center variation of 001O（2.00 eV）was less than 3.48 eV for 001Bi and 3.45 eV for001Br,indicating that the electron capacity of CO₂ adsorbed on 001O was the weakest.The effective electron drift velocity calculations showed that the maximum migration velocity appeared at 001Br surfaces,but the migration distances（3.608?and 3.047?）were larger than 001Bi（2.518?and 2.456?）and 001O（2.430?and 1.492?）,corresponding to the minimum electron density.These signified that the exposure of surface Br atoms at BiOBr（001）increased the steric resistance to arise the electron migration distance,which affected the number of effective electron migration reduction to CO₂.The results of the reaction energy barrier of CO₂ at the electron-rich site showed that the more electrons transferred to CO₂,the lower the dissociation energy of C–O bond.And the formation of intermediate*COOH was related to the surface protonation ability.When the effective electron migration to CO₂ decreased,the dissociation energy of C–O bond increased,which could be used to improve the selectivity of reduction product HCOOH.