Oxidation Mechanism Of Sulfur Dioxide On Rutile Titanium Dioxide In The Atmosphere

In the present work,we employed the generalized gradient approximation(GGA)based density functional theory(DFT)in the Vienna Ab-initio Simulation Package(VASP),combined with the slab periodic model,as well as the CL-NEB and Dimer transition state search algorithms to systemically study the adsorption and catalytic oxidation mechanism of sulfur dioxide on the 110 crystal surface of titanium dioxide(rutile),which provides important theoretical understanding of the dust induced atmospheric oxidation mechanism.The specific conclusions are listed as follows.1)We have investigated the different adsorption structures of SO2,NO2,and O3 on rutile(110)surface,and found their optimal adsorption structures:SO2 forms a three-coordination structure on the surface,where the S atom is close to O atom at the bridge position of the substrate surface,and one oxygen atom in SO2 molecule is closed to the 5-coordination Ti atom on the surface.The adsorption energy of the whole process is 0.38 eV.However,O3 and NO2 prefer to form bidentate adsorption on the surface with adsorption energies of 0.41 and 0.21 eV,respectively.2)Coadsorption of SO2 and O3/NO2 occurs on the rutile(110)surface.Besides the single adsorption of SO2,O3 and NO2 on rutile(110),we studied the co-adsorption of SO3 and O3,SO3 and NO2 theoretically,and found out a relatively reasonable co-adsorption structure:compared with the single adsorption,the adsorption sites of SO2 molecules have no obvious change,while the oxygen atoms at both ends of O3/NO2 are close to the five coordinated Ti atoms on the surface.Under the condition of co-adsorption,the co-adsorption energies of SO2/O3 and SO2/NO2 are about 0.75 and 0.58 eV,respectively.3)In the case of single molecular adsorption,O3 on rutile(110)surface oxidizes SO2.After SO2 is stably adsorbed on rutile(110)surface,O3 in the gas phase gradually approaches SO2 on the surface,where the O atom at one end of O3 molecule attacks the S atom,and then the O-O bond in O3 molecule breaks to form O2 and O.This O atom bonds to the SO2 molecule to form SO3,which is attached to the structure and forms a tetrahedral geometry.The activation energy of the whole process is 0.18 eV,and the potential energy of the product system is lowered by 2.39 eV.4)In the case of single molecular adsorption,the process of SO2 oxidation by NO2 on rutile(110)surface is similar to the process of O3 oxidation.The oxygen atom in the NO2 attacks the S atom in the SO2 to form stable SO3 adsorbed on the surface.The activation energy of the whole reaction process is 0.73 eV,and the total energy of the product system is lowered by 0.31 eV.5)In the case of bimolecular co-adsorption,the process of SO2 oxidation by O3 on rutile(110)surface.In the approaching process of SO2 and O3,the oxygen atom at the far end of O3 away from SO2 first desorbs,and then the O-O bond in O3 nearby the SO2 breaks,SO2 near the exposed O atom on the surface and O2 far away from the surface.The two processes almost occur at the same time,and the activation energy of the reaction is 0.38 eV with the released heat of 1.82 eV.The product of SO3 has very weak interaction with the TiO2 surface,which is relatively easy to be removed from the surface and participate in the gas-phase reaction.6)Under the condition of bimolecular co-adsorption,the process of SO2 oxidation by NO2 on rutile(110)surface is similar to that of O3.The products are NO and SO3.The reaction is endothermic.The activation energy of the reaction reaches 0.93 eV,and the total energy of the reaction product system is increased by 0.36 eV.7)On the rutile(110)surface containing H2O,the dissociated product of H2O can effectively promote the formation of sulfite.There is no reaction barrier in the hydration process of SO2,and then the O3 molecule in the gas phase oxidizes S(Ⅳ)to S(Ⅵ)leading to the generation of stable adsorbed H2SO4 on the surface.The activation energy of the reaction process is 0.44 eV,and the energy of product system is lowered by 2.97 eV.In summary,we found that the dry rutile TiO2 surface has an outstanding ability to promote the oxidation of SO2 by O3.However,SO2 oxidation by NO2 has a relatively higher energy barrier,which is not easy to occur at room temperature.Water in the atmosphere does not promote the heterogeneous oxidation process.In the previous experimental observations,it was found that the traditional OH radical oxidation mechanism was not enough to account for the rapid growth of sulfuric acid concentration in the production process of new particle formation.Through our calculations,it is found that the process of gas-solid heterogeneous oxidation induced by atmospheric dust is an important source of sulfuric acid and sulfur trioxides in the atmosphere,which can be an important supplement to the source of OH radical oxidation.This also reveals the theoretical mechanism of dust promoting aerosol and haze.