Effect Of Light And NH₃ On Heterogeneous Reaction Of SO₂ On Mineral Oxide Surface

Our country faces serious problems of air pollution,mainly manifested as the alternating occurrence of atmospheric fine particles(PM2.5)and ozone(O3)pollution.Thereinto,an atmospheric pollution phenomenon caused by PM2.5 is frequent occurrence of haze in autumn and winter.It is known that the key factor of the haze formation is the explosive growth of secondary aerosol,especially obviously increase of sulfate,which the chemical mechanism of the formation is not fully understood yet.Previous studies have shown that the heterogeneous reaction process is an important source of sulfate in the atmosphere.Because the surface of the mineral particles contains active sites for atmospheric trace gas adsorption and transformation,such as metal atoms,oxygen vacancies,OH groups,defect sites,and adsorbed water,can provide a reaction bed for the heterogeneous formation of sulfate.Furthermore,light-sensitive substances in mineral particles can absorb different wavelengths of light,and then change the process of sulfate formation.Therefore,the study of the heterogeneous photoreaction process on the surface of mineral particles has great significance for understanding the origin of haze.In this paper,heterogeneous reactions of SO2 on the surface of mineral oxides were studied using the in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy(DRIFTS),Flow Tube Reactor,and Ion Chromatography(IC)methods.etc.The main results are as follows:Firstly,the effect of illumination on the heterogeneous reaction of SO2 on various mineral oxides was explored.Results showed that UV light can affect the morphology and adsorption capacity of the surface product.In the dark reaction,the main product on the surface of the particles was sulfite with the relatively low yield.Conversely,mainly generated product was sulfate in light reaction and the total amount of the product is more than dark reaction.Secondly,this research studied the effect of NH3 in the heterogeneous reaction of SOZ on TiO2 particles in dark and light reaction.It was found that NH3 promoted the adsorption of SO2 by increasing the surface alkalinity of the particles.NH3 main absorbed on the Lewis acid site as adsorbed state in the dark condition,while converted to NH4+ adsorbed on the Bronsted acid site when co-reacted with SO2,and the amount of adsorption greatly increased.The photooxidation of NH3 could produce NO3-,which would inhibit the reaction of SO2 on the surface of TiO2,so the promote effect of NH3 would be relatively weakened.Thirdly,the effect of adsorbed water on the heterogeneous reaction of SO2 on TiO2 surface was studied.The results showed that the existence of H2O has an important effect on the heterogeneous reaction of SO2 on the oxide surface.On the one hand,the coexistence of H2O changes the adsorption capacity of the surface species due to the competitive adsorption of H2O and SO2.The pre-adsorbed water molecules occupy the Lewis sites(metal atoms),oxygen vacancies,OH groups and other active sites on the particle surface,thus inhibiting the formation of surface sulfur species.On the other hand,the coexistence of H2O affects the adsorption form of the surface species.For example,with the humidity increased,the surface of the sulfite gradually transformed into water-soluble sulfites and water-soluble sulfate on the surface of TiO2.Through these studies,the mechanism of heterogeneous photochemical reaction of SO2 on the surface of TiO2 was initially revealed,and the key factors affecting the reaction mechanism and rate were obtained,providing a scientific basis for further understanding of the contribution of photochemical reactions to the formation of sulfate in the atmosphere.