Direct Catalytic Decomposition Of N₂O Over Modified Fezsm-5Zeolites

Nitrous oxide (N2O, global warming potential GWP=300; ozone depletion potential ODP=0.017) has a significant impact on the destruction of the ozone layer besides its greenhouse effect, consequently regulated by the Kyoto protocol. In recent years, along with strengthening of people’s environmental protection awareness, the control of the emissions of N2O to the atmosphere has attached more attention. From both theoretical and application points of view, the development of the N2O abatement techniques is crucial.Currently, the abatement of N2O mainly involves of three techniques:thermal decomposition, selective catalytic reduction, and direct catalytic decomposition. The last one is reported to be the most promising method due to its merits such as the simple operation and avoiding secondary pollution etc., in which N2O can be directly decomposed into N2and O2as the only products over catalysts without the addition of reductant. It has been reported that FeZSM-5, especially the steam-activated one, is a potential catalyst for the direct decomposition of N2O. In this thesis, an alkaline-steam treatment was proposed to activate FeZSM-5. The prepared catalysts were characterized by N2adsorption, UV-vis, and N2O-CO titration techniques. The content of this thesis mainly consists of two parts as follows:1) FeZSM-5with uniform shape and a crystal size of8μm was employed and activated with either alkaline steam or steam for comparison. The effects of the partial pressure of each component and treatment time and temperature in the alkaline-steam treatment on the N2O decomposition activity over FeZSM-5were investigated. The results indicate that the alkaline-steam treatment is a good protocol for the activation of FeZSM-5. Furthermore, the higher temperature in the alkaline-steam treatment (in the range of773-923K), the lower the reaction temperature required for the complete decomposition of N2O. Various techniques such as XRD, SEM, N2sorption, UV-vis, N2O-CO titration, and so on were employed to investigate the alkaline-steam activated FeZSM-5, showing that the alkaline-steam treatment on FeZSM-5could bring forth more active Fe sites and thus improve the activity of the catalysts.2) Three steam-activated FeZSM-5zeolites with crystal sizes of300nm,8μm, and20μm were employed as the catalysts for the N2O decomposition. It was found that the larger the zeolite crystal size, the higher the reaction temperature required for the complete decomposition of N2O. This is due to the internal mass transfer limitations available in the N2O decomposition over FeZSM-5, especially at high conversions of N2O (high temperatures). Moreover, N2O decomposition over FeBeta and FeMOR was investigated as well for comparison. Similar to the case of FeZSM-5, after the steam treatment, FeBeta and FeMOR showed greatly improved catalytic properties. The complete decomposition of N2O could be achieved at798K over FeBeta catalyst, which possesses a better mass-transport property.