Room Temperature Catalytic Ozonation Of Toluene Over Transition Metal Oxides

Growing pollution caused by volatile organic compounds (VOCs) has draw global attention. Room temperature catalytic ozonation is capable of abating two kinds of air pollutants simultaneously, regarded as a promising technology. However, among the current work concerning the subject, few was focused on the reaction process or mechanism. This paper investigated the structure-activity relationship, interaction between species formed on catalyst surface, as well as the reaction pathway and mechanism of catalytic ozonation of toluene.Five kinds of transition metal oxides supported on alumina and prepared by wetness impregnation were evaluated for the catalytic ozonation of toluene at room temperature and characterized by TPR, TPO, N2 adsorption-desorption and XPS. Influencing factors as loading of active component, concentration ratio of reactants and humidity were taken into consideration. We undertook a preliminary investigation into the reaction pathway by in situ DRIFTS. The main research results and conclusions were as follows:(1) Catalysts with a lower reduction temperature and less H2 consumption showed a higher efficiency for ozone and toluene decomposition, such as NiO/Al2O3, CoO/Al2O3 and MnO2/Al2O3, while lower efficiency were observed on Fe2O3/Al2O3 and CuO/Al2O3. Toluene decomposition efficiency was obviously dependent on ozone. Because of the low ozone to toluene concentration ratio, a CO2 yield of less than 30% was obtained.(2) Among the selected loadings, catalytic activity of MnO2/Al2O3 decreased with the increasing loading, due to higher surface area, less H2 consumption and higher content of Mn4+. Humidity had an influence on both decomposition mechanism and mass transfer of ozone. High ozone and toluene conversion could be obtained under proper humidity. Toluene conversion, byproducts on catalyst surface and content of lattice oxygen increased under higher ozone concentration.(3) Both more kinds and amount of products formed on catalyst surface with higher acticity. With regards to the formation of COO-, C=O and C-O groups the high ozone concentration accelerated the transformation from COO- to C=O and C-O. The substance containing COO- remained unchanged at 570 K, while the substances containing C=O and C-O underwent oxidation at higher than 500 K. A possible reaction pathway is proposed based on these findings.