First-Principles Study Of Atmospheric Oxidation Reaction Mechanism On The Surface Of Titanium Dioxide Particles

Titanium dioxide（Ti O₂）has been widely studied in the fields of adsorption of organic pollutants,gas survey and so on,because of its stable chemical properties,easy to prepare,non-toxic and harmless.Previous studies have found that surface of the Ti O₂can catalyze solid-liquid,solid-gas multiple chemical reactions in polyphase systems,among which the rutile Ti O₂is a representative material of urban atmospheric mineral dust,and the atmospheric oxidation processes occurring at its interface directly affect air quality and global climate change.Both field observations and laboratory simulations have found that mineral dust particle surfaces can adsorb trace gas in the atmosphere.Among them,sulfur dioxide（SO₂）may be adsorbed on the particle surface with nitrogen dioxide（NO₂）,ozone（O₃）,and then be oxidized to sulfuric acid or sulfate,thus leading to the production of pollutants such as acid rain and haze.Therefore,it is essential to control the oxidation and nucleation of air pollution caused by mineral dust,which requires us to reveal and understand the microscopic reaction mechanism of the heterologous oxidation process after the SO₂is adsorbed on the surface of the mineral particles at the molecular level.In this paper,we take the first principle as the research method,including rutile Ti O₂,one of the main components of mineral dust,and explore several possible oxidation paths about SO₂into sulfuric acid or sulfate by NO₂or O₃on the surface of rutile Ti O₂,providing a theoretical basis for the atmospheric oxidation reaction mechanism on the surface of mineral dust particles.The main research results are as follows:（1）The adsorption energy of three gases(NO₂,O_3,SO₂)adsorbed to the perfect surface or defective surface of rutile Ti O₂was calculated using the density functional theory of PBE0-D3.Rutile Ti O₂showed the most pronounced adsorption effect of NO₂,with an activation energy of 2.03 e V.Among them,the redox reaction occurred when O₃or NO₂adsorbed on the defective surface,but the reaction did occur with SO₂,so oxidation of SO₂on the Ti O₂surface requires the aid of other oxidants.This also confirms the complexity of the atmospheric chemical reactions.Subsequently,we have discussed the six possible oxidative reaction pathways of SO₂on the defective or perfect surface of rutile Ti O₂by oxidative gases（O₃or NO₂）in both dry and wet environments,respectively.（2）Six oxidative reaction pathways were calculated using PBE0-D3 and CI-NEB.When O₃with SO₂,or NO₂with SO₂are co-adsorbed to the rutile Ti O₂particle defective surface,respectively,the redox reaction occurs between the defective surface of oxidized gas（NO₂,O₃）and Ti O₂,eventually forming the perfect surface of adsorbed SO₂and the complex product of oxygen（O₂）or nitric oxide（NO）gas.Thus,the oxidation problem of SO₂occurring on the defective surface is transformed into the oxidation problem of SO₂on perfect surface.When both O₃and SO₂,NO₂and SO₂are co-adsorbed on the perfect surface of rutile Ti O₂particle,SO₂can be oxidized to sulfur trioxide（SO₃）and adsorbed to the rutile Ti O₂particle surface as SO₃,thus further explaining the formation mechanism of bisulfite particles.SO₂adsorbed on the rutile Ti O₂particle surface is more readily oxidized to O₃than to the oxidation process of NO₂,with an activation energy of only 0.22 e V.By analyzing the reaction mechanism of these pathways,the oxidative strength relationship of substances is further demonstrated,that is,oxidability:O₃>NO₂>SO₂>defective rutile Ti O₂.（3）Water molecules are common catalysts in the atmosphere,and according to molecular dynamics simulations,SO₂can spontaneously adsorb on the surface of rutile Ti O₂to form the sulfite acid particles.After SO₂forms a sulfuric acid on the surface of the wet rutile Ti O₂,it can still continue to be oxidized to form a sulfuric acid by O₃or NO₂.SO₂most prone to be oxidized by O₃on the surface of wet Ti O₂particles,with an activation energy of 0.35 e V.In contrast to the drying surface,although the process of O₃oxidation of SO₂is not significantly promoted on the wet surface,this oxidation path is also highly likely to be present in the high humidity atmosphere.By combining CI-NEB transition state search algorithm and PBE0 hybrid functional,we explore six oxidation paths of SO₂on rutile Ti O₂particle surface,study the oxidation reaction mechanism of SO₂on Ti O₂particle surface and find the possible path of SO₂gas-solid heterogeneous oxidation process,which provide a theoretical basis for the microscopic mechanism of sulfate aerosol formation and new ideas for relevant air pollution prevention and control.