Study On The Catalytic Formation Mechanism Of Nitrate In Fine Particles By Metal Oxides Emitted From Diesel Vehicles

The emission of gaseous pollutants and carbon particles from diesel vehicles is one of the factors that lead to frequent air pollution in large and medium-sized cities,especially the air pollution with fine particles as the primary pollutant.In this paper,the formation mechanism of NO,NO₂,SO₂and their coexistence on the surface of metal oxides（Fe₂O₃,MgO,MnO₂,Al₂O₃,ZnO,Cu O,SiO₂ and Cr₂O₃） was investigated by using diffuse reflectance infrared spectroscopy（DRIFTS） Combined with the micro reaction phenomenon,the paper analyzes whether there is synergistic or antagonistic effect between different reaction gases and metal oxides on the chemical reaction,looks for the key steps for diesel vehicle emission pollutants to form atmospheric fine particulate nitrate intermediates,and establishes the reaction mechanism of diesel vehicle emission pollutants to form nitrate.The results show that the intermediate products and ways of diesel vehicle emission pollutants to form atmospheric fine particles are of great significance to reduce the contribution of diesel vehicle emission pollutants to atmospheric fine particles and air pollution prevention and control.The conclusions are as follows:（1）By analyzing the element composition of carbon particles accumulated in the exhaust pipe of diesel vehicle,it is found that the particles are mainly composed of C,Fe,Zn,Cu,Si,Cr,Al,Mg,Mn and other metal elements.The main component is C,the relative content is more than 90%,in addition to C,there are other elements such as Fe,Zn,Cu,Si,the content of which is less than 10%.Because the proportion of metal elements varies with the use of different lubricants,but the types of elements are basically the same,so they have similar chemical composition.（2）Under different dry and wet conditions,NO,NO₂ and SO₂can not only form adsorbed nitrate on Fe₂O₃ surface,but also increase acidic Fe₂O₃ particles.The introduction of SO₂ into the diffuse reflection tank can produce reductive adsorbed sulfite.The adsorbed nitrate interacts with the formed adsorbed sulfite on the acidic surface,the adsorbed sulfite and bisulfite are oxidized to sulfate,and HONO is formed at the same time.In addition,the heterogeneous reaction of SO₂ with Fe₂O₃ can not only produce Fe²⁺,but also increase the surface acidity.The formed Fe²⁺ can also interact with nitrate adsorbed on acidic surface to produce HONO.The results are of great significance to reveal the micro nature of the formation of atmospheric fine particles from diesel vehicle emissions and to reduce the contribution of nitrate in the formation of atmospheric fine particles from diesel vehicle emissions.（3）Under different mixed gas conditions,the species（nitrate and sulfate）formed by the reaction of NOx and SO₂ with different metal oxides all promote the formation of atmospheric particles.The main products of NOx and metal oxides are nitrate and nitrite;The formation product of SO₂ on the surface of metal oxide is sulfate.SO₂ and NO₂ can react synergistically on the surface of metal oxides to promote the formation of sulfate and nitrate.In addition,from the formation of species on the surface of metal oxides,the heterogeneous reaction ability of each metal oxide is different.The heterogeneous reaction of Cr₂O₃ is stronger than that of SiO₂.It shows that the catalytic performance of metal oxides promotes the formation of secondary components in atmospheric particles to a certain extent.Through experiments and discussion of reaction mechanism,basic data are provided for the formation of nitrate,sulfate and other species by the components of particles emitted by diesel vehicles.（4）Under the condition of oxygen enrichment,the reaction between metal oxides and NOx and SO₂accelerates the formation rate of sulfate and nitrate,which indicates that the enhancement of atmospheric oxidation plays an important role in this process.The total nitrate species content of fine particles formed on the surface of metal oxides is Cr₂O₃>Fe₂O₃>MgO>MnO₂>Al₂O₃>ZnO>CuO>SiO₂.Because Cr₂O₃ has a strong ability to adsorb and activate acid gas,more surface species are generated in the micro reaction than other metal oxides,which indicates that the surface properties of metal oxides play an important role in the synergistic effect between SO₂and NOx.In the presence of NOx,the metal oxides containing iron,magnesium and manganese can convert SO₂ to sulfate more effectively,but the sulfate produced by Cr₂O₃,CuO and Al₂O₃ decreases.The difference of metal oxide activity is due to the change of metal surface properties.Therefore,in the atmospheric environment,the reaction between metal oxides emitted by diesel vehicle exhaust and NOx and SO₂ makes a significant contribution to nitrate and sulfate in the atmosphere.