Photocatalytic Degradation Mechanism Of Gaseous Xylene Isomers By TiO₂ Based On Oxygen Isotope Tracing

Volatile organic compounds?VOCs?are a group of air pollutants widely existing in indoor and outdoor environments.Large amounts of VOCs are emitted during industrial production,among which benzene compounds pollutants can cause serious harm to human,the most important pollutants are benzene,toluene and xylenes.They are an important source of urban particulate matter?PM?and have important implications for air quality and human health.Titanium dioxide?Ti O₂?is by far the most investigated photocatalyst for photocatalytic degradation of gaseous benzene compounds.Under ideal conditions,benzene compounds can be completely degraded to H₂O and CO₂ by photocatalysis.However,in the actual process,a large number of intermediates are often produced,which may cause greater harm to the environment and human health than the mother VOCs.Xylene is a special kind of benzene compound,which has three isomers?m-xylene,o-xylene and p-xylene?.Due to the different positions of three methyl groups,the same or different photocatalytic oxidation mechanism may occur.In this paper,the isomers of xylene were selected as the research object,and the mechanism of photocatalytic degradation of xylene by Ti O₂ was studied by combining mass spectrometry analysis and oxygen isotope tracing.The main conclusions are as follows:1.The mechanism of photocatalytic degradation of xylene isomers by Ti O₂ under UV?UV254nm+UV185nm?irradiation were studied.The results of degradation kinetics showed that 11 ppm remained after 120 min degradation of m-xylene,6.2 ppm remained after 120 min degradation of o-xylene and 16.4 ppm remained after 120 min degradation of p-xylene,indicating that xylenes were not completely degraded.Mass spectrometry analysis of intermediates on mineral particles—Ti O₂ surface confirmed that OH-addition and H-abstraction intermediates were formed at the same time.Six H-abstraction and five OH-addition intermediates were clearly observed from total ion chromatogram after m-xylene oxidation.A total of eleven intermediates were detected during 120 min reaction of o-xylene,where seven of them were H-abstraction intermediates and the other four were OH-adducts,while the oxidation of p-xylene on Ti O₂ results in formation of seven H-abstraction products and three OH-addition products.The qualitative results of the products indicated that the degradation products of m-xylene were similar to p-xylene,but the photocatalysis in the degradation products of o-xylene identified benzofuran intermediates?phthalic anhydride and phthalide?on Ti O₂.Further quantitative results showed that the ratio of H-abstraction products and OH-addition products of m-xylene,o-xylene and p-xylene was 98.01%and 1.99%,99.87%and 0.13%,99.94%and 0.06%respectively at 120 min.Methyl-benzoic acid was the most important H-abstraction product and dimethyl phenol was the important OH-addition product.Combined with product analysis and quantum chemical calculation,it was confirmed that H-abstraction pathway was the main process of photocatalytic degradation of xylene isomers by Ti O₂.2.We further studied the mechanism of photocatalytic degradation of xylenes with oxygen isotope tracer.The results showed that the degradation products of the three xylenes under UV254nm irradiation were similar to those of the previous part.It was again verified that H-abstraction pathway was the main process of the photocatalytic degradation of xylene by Ti O₂.The H₂¹⁶O in the reaction system was replaced with H₂¹⁸O to explore the contribution mechanism of oxygen species in the photocatalytic degradation of xylene.The results showed that the presence of H₂O was important for the formation of OH-addition products in the photocatalytic oxidation of xylenes.When the H₂O in the reaction system was reduced,the amount of OH-addition products dimethyl-phenol was significantly reduced.For H-abstraction products,the oxygen atoms in methyl-benzyl alcohol were almost all derived from¹⁶O₂.With the progress of the reaction,although O₂ still dominated the formation of methyl-benzaldehyde,methyl-benzoicacid,1,3-benzenedicarboxaldehyde,1,3-benzenedicarboxaldehyde and 1?3H?-isobenzofuranone,H₂O can accelerate the reaction,which had a significant contribution.