| Nitrous oxide (N2O, global warming potential, GWP≈310) has been considered as the third greenhouse gas ranked after carbon dioxide and methane, and also as a contributor (ozone-destroying potential, ODP≈0.017) to the destruction of ozone in the stratosphere. It has been reported by IPCC (Intergovernmental Panel on Climate Change) that the atmospheric concentration of N2O had been relatively constant (～270 ppb) before the industrial revolution. However, the concentration of N2O in the atmosphere was 319 ppb in 2005, and its concentration currently increases at an annual growth rate of ca.0.2-0.3%, mainly caused by anthropogenic activities such as energy production and chemical processing. With the growing awareness of environmental protection, the emissions of N2O to the-atmosphere are emphasized on controlling, and N2O was set to reduce emissions of main greenhouse gases in the Kyoto protocol. Therefore, research and development on N2O elimination technologies are of great importance.At present the technologies of N2O elimination mainly include thermal decomposition, selective catalytic reduction, and direct catalytic decomposition, wherein direct catalytic N2O decomposition into N2 and O2 is identified as an effective and economic method. In this thesis, the effects of various parameters such as different synthesis methods and modification of isomorphously substituted FeZSM-5 on direct catalytic N2O decomposition over FeZSM-5 have been systematically investigated. The catalysts have been characterized by the XRD, SEM, UV-vis and N2 adsorption techniques. The main results are summarized as follows: 1. Compared with wet ion-exchange and solid ion-exchange methods, steam-activated FeZSM-5 catalysts using an isomorphously substituted method show the highest activity. The isomorphously substituted FeZSM-5 catalysts exchanged with either hydrochloric acid or ammonium nitrate show very similar properties in direct N2O decomposition. However, the activity of FeZSM-5 catalysts can be significantly improved after steam treatment. In addition, the FeZSM-5 zeolites obtained via different ion-exchange methods can lead to different states of the extra-framework Fe species formed during steam conditions.2. The isomorphously substituted FeZSM-5 catalysts have been modified via steam treatment. The effects of steam-treatment conditions like temperature and partial steam pressure on their activity in direct catalytic N2O decomposition have been systematically studied. The appropriate activation conditions are 30 vol.% H2O at 873 K for 5 h for the steam treatment. Furthermore, the composition of FeZSM-5 zeolites is also an impact factor on the catalytic activity, with increasing the ratio of Fe/Si, the catalytic activity is enhanced to some extent. The investigation on the effects of FeZSM-5 zeolite crystal size on the catalytic activity shows that an increase in the zeolite crystal size results in an increase of the reaction temperature for the complete decomposition of N2O. This, for the first time, demonstrates that there is an internal mass-transfer limitation available in N2O decomposition over FeZSM-5 catalysts. Additionally, the study of the FeZSM-5 catalyst with 20μm in crystal size used in N2O decomposition indicates that the higher activity can be obtained via the catalyst treated with a combined method, i.e., alkaline and steam treatments. Mesopores in the FeZSM-5 crystals can be created by the alkaline treatment and Fe active species can be normally generated by the steam treatment, confirmed by the characterizations with the UV-vis and N2 adsorption techniques, and the better activity can be obtained via the isomorphously substituted FeZSM-5 catalysts first treated with alkaline and then with steam. |
PDF Full Text Request