Photocatalytic Degradation Mechanism Of Benzene Compounds By TiO₂ Based On Oxygen Isotope Tracing

Benzene compounds are a series of volatile organic pollutants existing widely in indoor and atmospheric environment,which have neurotoxicity and carcinogenicity,and also has serious influence on human health and body function.Photocatalytic oxidation has become one of the commonly used degradation technologies for benzene compounds due to its advantages of convenient operation,energy saving and no secondary pollution.However,due to the participation of H₂O and O₂ in the reaction process,many oxygen-containing intermediate products will appear.In view of the current mechanism studies on benzene compounds mostly focusing on product characterization and reaction path derivation,it is of great significance to reveal the migration mechanism of oxygen atom for the study of degradation mechanism of benzene compounds.Isotope labeling is the method to trace the atomic process as the core,the transfer path can be clearly observed during the reaction using¹⁸O-labeled oxygen species or substrate.However,with environmental conditions such as relative humidity（RH）,reactant concentration and environmental media changing,the composition of these oxygen-containing products becomes more complex and oxygen atom transfer may be affected.In this paper,styrene and three isomers of trimethylbenzene were selected as the research object.Combining qualitative and quantitative analysis of oxygen-containing products with oxygen isotope labeling,the photocatalytic degradation mechanism of Ti O₂ was studied under different reaction systems.First,the photocatalytic degradation mechanism of styrene under different substrate concentrations was studied.A total of eight Ti O₂ interface products were identified under 100ppmv:benzaldehyde,benzyl alcohol,phenylacetaldehyde,phenylglyoxal monohydrate,benzoic acid,2,3-dihydrobenzofuran,2-hydroxy-1-phenyl-ethanon and 1-phenyl-1,2-ethandiol.Only 1,3-dihydrobenzofuran was formed at substrate concentrations of 200 and 400ppmv,indicating that increased substrate concentration would change the product type.An increase in total product content was also observed(from 9.48×10^-2,8.46×10^-2 to 2.92×10^-2mg).However,further study at different RH（5%,50%and 100%）all identified nine products,but the total product content decreased(from 9.48×10^-2,3.52×10^-2to 2.92×10^-2mg),which revealed RH only affected product content.In the whole process,H₂¹⁸O was used to participate in the reaction,and the results showed that the increase of both would promote the ¹⁸O ratio of products,but substrate concentration did not change the dominant position of O₂（>70%）.While increasing RH not only lead to a higher ¹⁸O ratio of interface products,but also the appearance of H₂O-dominated products such as benzoic acid（54%）and 2,3-dihydrobenzoisofuran（56.8%）.The qualitative and quantitative analysis of reactive oxygen species was further carried out.At 5%RH,the contribution rate of superoxide free radicals and singlet oxygen(¹O₂ and O₂^·-)to the interface products were more than 80%.The increase of RH promoted the participation of（·OH）in the formation of interface products,which in turn inhibited the production of O₂^·-and ¹O₂.Then,the photocatalytic degradation mechanism of trimethylbenzene isomers in gas and liquid phase was studied.In gas phase system,11,10and 5 kinds of interface products were identified,from 1,2,4-trimethylbenzene,1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene,respectively.The interface products of 1,2,4-tritoluene and 1,2,3-tritoluene had similar types,both of which were alcohols,aldehydes,carboxylic acids and benzofuranones,but only the latter produces 3-methyl-phthalic anhydride.Benzofuranones were not involved in 1,3,5-trimethylbenzene intermediates,but hydroxyl addition reaction occured to produce 2,4,6-trimethyl-phenol.In liquid phase system,only 3kinds of interface products were identified by 1,2,4-trimethylbenzene,and 4 kinds of interface products were identified by 1,2,3-trimethylbenzene,all of which were aldehydes and benzofuranones.No new products were formed and the content of products decreased significantly.Under all conditions,aldehydes were the most content of interface products.In gas phase system,carboxylic acids had the most contribution proportion of H₂O in all trimethylbenzenes,and H₂O played a leading role（>60%）.Product 3-methyl-phthalic anhydride of 1,2,3-trimethylbenzene was also contributed mainly by H₂O（65.7%）,while others were contributed more by O₂.In liquid phase system,the contribution rate of H₂O in the interface products of 1,2,4-trimethylbenzene and 1,2,3-trimethylbenzene decreased obviously,and O₂ played a dominant role.Therefore,H₂O was more likely to participate in the formation of Ti O₂ interface products in gas phase system.These results provide a more profound insight for the study of the photocatalytic mechanism of benzene compounds and the regulation of oxygen-containing intermediates.