| Volatile organic compounds(VOCs)is a class of air pollutants with many sources of emissions,of which industrial production is the main source.VOCs emissions can cause serious environmental problems and are harmful to human health.Toluene in VOCs is widely used as a representative industrial solvent.In this paper,low concentration toluene is used as the target pollutant,and it is degraded into H2O and CO2 using catalytic ozonation technology.The technology operated at normal temperature and had low operating costs for degrading low-concentration VOCs.The key is the selection and preparation of the catalyst,transition metal oxides are preferred because of their lower cost.In this paper,transition metal oxides with high catalytic activity are prepared as catalysts.Using SBA-15(mesoporous molecular sieve)and Mn(NO3)2 solution as templates and precursors respectively,mesoporous MnOx was prepared by template method.?-MnO2,?-MnO2 and Mn2O3 catalysts were prepared at different calcination temperatures(200?,300?,400?,and 500?).The order of the catalytic performance of the catalysts was:?-MnO2>?-MnO2>Mn2O3.Then using KMnO4 and Mn(NO3)2 as materials to prepare mesoporous ?-MnO2 by template-free method.It was found that the mesoporous ?-MnO2 prepared by the template method had better catalytic performance by comparing the catalytic performance of the two kinds of catalysts,therefore,the catalysis was selected for subsequent research.The effects of GHSV,feed ratio(mass concentration ratio of toluene and ozone)and fed toluene concentration on the catalyst performance were investigated.The results showed that when the fed toluene concentration was 140 mg/m3,the reaction space velocity was 10000 h-1 and the optimal fed ratio was 1:15,the CO2 selectivity was 73%and the removal rates of toluene and ozone could reach 100%.When the fed toluene concentration gradually increased from 50 mg/m3 to 225 mg/m3,the removal rates of both toluene and ozone could reach 100%,and the CO2 selectivity dropped from 83%to 69%.Adding water vapor would promote the catalytic reaction to a certain extent.The fed ratio(mass concentration ratio of toluene and ozone)was set to 1:15.When the fed toluene concentration was 80mg/m3 and the relative humidity increased from 0%to 40%,both toluene and ozone could be completely removed and the CO2 selectivity increased from 79%to 81%.When the relative humidity increased from 40%to 90%,the ozone removal rate reduced from 100%to 97%,toluene could be completely removed and the CO2 selectivity increased from 81%to 83%;when the fed toluene concentration was 200 mg/m3 and the relative humidity increased from 0%to 20%,both toluene and ozone could be completely removed,and the CO2 selectivity increased from 67%to 72%.When the relative humidity increased from 20%to 90%,the removal rates of ozone and toluene reduced from 100%to 85%and 90%,respectively,and the CO2 selectivity reduced from 72%to 45%.After purging for 8?12h without water vapor,the catalyst performance could be restored.Through FTIR testing and analysis,it could be seen that the addition of water vapor generated more freeˇOH,the by-products accumulated on the surface of the catalyst were reduced and the reaction process was promoted within a certain humidity range.According to FTIR and GC-MS analysis,benzyl alcohol,benzaldehyde and benzoic acid were generated during the degradation of toluene,and then acetone,acetaldehyde,ethanol and other small molecules were generated through the oxidative ring-opening process,which were finally oxidized into H2O and CO2. |
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