|其他摘要||Volatile Organic Compounds (VOCs), one of the main sources of air pollution, pollute the environment and are harmful to human health. Various applicable technologies have been developed in order to efficiently remove VOCs pollutants. Among all, catalytic oxidation has been proven to be an effective and energy-saving process where organic pollutants could be completely decomposed into those harmless products such as carbon dioxide and water at relatively low temperature. The main concern in the field of VOCs catalytic oxidation is to search a proper catalyst with high catalytic activity and stability but low cost. In spite that supported noble metals are highly active in catalyzing VOCs oxidation at low temperature, high costs and easy deactivation are the main defects limiting their wide application. It has been accepted that transition metal oxides are another branch of catalysts for VOCs oxidation because of their good catalytic activities at high temperature and resistant ability to poisoning. In this thesis, catalytic performances of MnOx/HZSM-5 catalysts were evaluated for the catalytic oxidation of toluene, which was typically selected as a model reaction for ordinary VOCs abatement. Numerous characterization techniques including XRD, N2 physisorption, SEM, TEM, H2-TPR and NH3-TPD were conducted to investigate the correlation between the physicochemical properties and the catalytic performances over the prepared catalysts. The main results obtained in the thesis are as follows:
(1) Series of y%MnOx/HZSM-5 (y=2.5, 5.0, 7.5, 10, 15) catalysts were successfully prepared by an incipient impregnation method and used for the catalytic oxidation of toluene. Catalytic testing results indicated that the catalytic activities of HZSM-5 supported MnOx catalysts decreased in the order of 10%MnOx/HZSM-5> 7.5%MnOx/HZSM-5≈15%MnOx/HZSM-5> 5.0%MnOx/HZSM-5> 2.5%MnOx/ HZSM-5, where 10%MnOx/HZSM-5 exhibited the optimum catalytic activity, excellent catalytic durability in dry condition as well as good regeneration capability in humid condition. The dispersion of MnOx species on HZSM-5 surface greatly reduced the formation of coke and simultaneously efficiently accelerated the oxidative elimination of surface coke deposition during the catalytic oxidation of toluene which was the factor for toluene oxidation reaction. Based on a comparative analysis with those SiO2 and Al2O3 supported MnOx catalysts, it was predicated that the Brønsted acid sites of HZSM-5 zeolite make a promotional effect on the catalytic oxidation of toluene. The cooperative action between the redox ability of MnOx species and the acidity property of HZSM-5 zeolite resulted in high catalytic reactivity for toluene oxidation.
(2) Using the ion exchange method, series of 10%MnOx/NaZSM-5-y (y=0.16, 0.31, 0.51) catalysts were prepared. The total acidity of the catalysts was decreased with the increasing of Na/Al ratio and there was no obvious relationship between the acdity and the reductibility for the catalysts. By comparing to the ion changed samples, 10%MnOx/HZSM-5 catalyst showed the best catalytic performance with the lowest Ea value. After a comparative analysis and discussion based on the obtained characterizations and catalytic testing results, it showed that the acidity of catalysts highly improved the catalytic activities in the toluene oxidation reactions by lowering the activation energy.
(3) A range of HZSM-5 zeolite with various Si/Al ratio (25, 50, 75) suppored MnOx catalysts had been successfully prepared via hydrothermal systhesis and impregnation. The Si/Al ratios strongly influenced the acidity of 10%MnOx/ HZSM-5-n (n=25, 50, 75) catalysts. Catalytic tests in toluene oxidation showed that 10%MnOx/HZSM-5-25 exhibits the highest activity (T50=253 ℃ and T90=261 ℃) and the lowest activation energy value among all the catalysts. The catalytic activity was reduced with the increasing of Si/Al ratio, indicating that actidiy of the catalyst is the key factor responsible for the catalytic activity.
(4) In order to investigate the influence of size effect on the catalytic activites, series of catalysts with different partical sizes and inner structure were prepared. Campared with micro-sized sample Micro-10%MnOx/HZSM-5, the Nano-10%MnOx/ HZSM-5 catalyst exhibited a better reducing property which was attribute to its higher specific surface area. In the catalytic tests for toluene oxidation, Hollow-10%MnOx/ HZSM-5 catalysts exhibited the best catalytic activity (T50=253 ℃ and T90=261 ℃) and the lowest activation energy value. The special hollow structure significant improved the dispersion of MnOx on the catalyst surface, lowered the reaction activation energy and promoted the toluene oxidation. In addition, the short transfer route was another advantage of the hollow structure.|