Study On Catalytic Oxidation Of BTX Over The Titania-based Catalysts

Volatile organic compounds (VOCs) are released massively by many industrial processes. Catalytic oxidation has been considered as an effective method to convert the toxic VOCs into relatively harmless H2O and CO2. However, the practical application of this promising technology has been hindered by the lack of high active catalysts with high persistence capability and the short of reaction mechanism for the typical contaminant oxidation process. To address these issues, titania supported mixed transition metal oxides were employed for the catalytic oxidation of BTX (toluene in the most cases) to seek for a prosperous catalyst and to investigate the reaction mechanism regarding their adsorption-oxidation process.Firstly, the optimizations of support, active phase and preparing method were carried out. Titania was found to be the most suitable support among the materials investigated. And the catalysts prepared by sol-gel method performed better than the ones prepared by traditional impregnation method for toluene oxidation. The sol-gel method would facilitate the sample to maintain high specific area, controllable pore system and good dispersion of active phase even at high loading content. More amorphous active phase dispersed well on the surface of the catalyst prepared by sol-gel method, resulting in more available active sites and therefore higher activity. Manganese-based catalyst had higher activity than the other transition metal oxide-based catalysts. The activity and the textural properties of the catalyst could be improved further with the addition of ceria. The structural similarities between ceria and manganese oxide could produce stronger interaction, leading to promoted dispersion of the active phase, higher specific surface area, enrichment of surface mobile oxygen species, et al.After that, the effects of operating conditions during the catalytic oxidation process were investigated. It was found that the toluene conversion decreased with the increments of toluene initial concentration and space velocity, and the decrease of the oxygen concentration. The optimal TiMnCe catalyst showed a relatively steady high activity under typical industrial operating conditions in toluene oxidation. Furthermore, this catalyst could maintain 100% efficiency 240℃within 200 hrs for the oxidation of toluene, without obvious activity decrease or significant changes of chem-physical properties. These finding illustrated that this catalyst was a promising candidate in potential industrial applications. For the binary benzene-toluene mixture system, benzene oxidation was inhibited while the abatement of toluene did not show evident changes. However, in the toluene-xylene binary mixture system, xylene oxidation didnot show much changes compared with the corresponding singe xylene environment. These results lied mainly in the competitive occupations of adsorption sites and active sites.Finally, to study the mechanism of adsorption-oxidation of toluene or BTX mixture system, in situ DRIFT experiments were carried out over these series catalysts. The toluene adsorption capacity was very small over pure TiO2, and the oxidation of adsorbed toluene was very limited as well. The addition of ceria or transition metal oxides would enhance the toluene adsorption and oxidation significantly. Within the catalysts investigated, the toluene decomposed via two pathways. Major toluene was attacked by the surficial active oxygen on the methyl group firstly, to produce intermediates like benzaldehyde and benzoic acid. Via the breakage of the benzene ring, part of benzoic acid decomposed to form maleate species and lower aliphatic acid species, or the final product such as bicarbonate. The oxygen addition or the temperature rising could always result in the further oxidation of benzaldehyde and carboxylic acid species, together with the decomposition and desorption of carbonate species. And it was found that the deep oxidation of benzaldehyde probably played a vital role in toluene oxidation. Besides, the toluene absorption and the mobilization of deep oxidation were also important factors in catalytic oxidation. The DRIFT result also showed that benzene was the most difficult one to be adsorbed and to be oxidized among BTX, while xylene was easy to be adsorbed and its reactivity was relatively high. This could be attributed to the main reason for the inhibition of benzene oxidation when it was present in the mixture pollutants.