Soran Shwan

Effect of post-synthesis hydrogen-treatment on the nature of iron species in Fe-BEA as NH3-SCR catalyst

Post-synthesis treatment of Fe-BEA with hydrogen has previously been shown to improve the low-temperature activity for NO reduction during standard NH3-SCR. Here, a 2 wt.% Fe-BEA sample was prepared by incipient wetness impregnation and calcined in air at 450 °C for 3 h. The fresh sample was then treated with 5% H2 at 650 °C for 5 h. The evolution of different iron species in Fe-BEA before and after H2-treatment was studied using in situ DRIFT spectroscopy with NO as a probe molecule and by UV-Vis spectroscopy. The DRIFTS results show that the relative intensity of the absorption peak representing isolated iron species increases significantly after H2-treatment of the fresh Fe-BEA sample. Furthermore, the UV-Vis results show a significant decrease in the relative intensity in the UV region representing larger iron particles, whereas the relative intensity representing smaller iron species increases after treatment of Fe-BEA with hydrogen. The results show that the low-temperature NO reduction during standard NH3-SCR can be increased for Fe-BEA by redispersion of smaller iron species into the zeolite structure by hydrogen-treatment.

Influence of hydrothermal ageing on NH3-SCR over Fe-BEA -Inhibition of NH3-SCR by ammonia

The decay in ammonia adsorption capacity and the amount of active iron sites are important to consider in order to understand the deactivation processes of Fe-BEA for NH3-SCR catalyst applications. NH3 and NO storage capacity experiments together with kinetic modeling have been used to evaluate ammonia inhibition during NH3-SCR before and after hydrothermal treatment of H-BEA and Fe-BEA. The kinetic model shows that at least four types of acid sites for H-BEA and one additional site for Fe-BEA are required to predict the NH3 desorption well. NH3-TPD experiments together with simulations show that the strongest adsorption sites are the sites that are most affected by the hydrothermal treatment. For H-BEA a clear correlation between the ammonia storage capacity and the improved NOX conversion after NH3 cut-off during NH3-SCR is observed. However, for Fe-BEA an inhibiting effect of ammonia after NH3 cut-off is seen but no significant difference (i.e. increased NOX conversion time) between fresh and aged samples can be observed, indicating that the inhibiting effect is unaffected by the hydrothermal treatment.

Impact of Thermal and Chemical Ageing of Fe-BEA SCR Catalyst on NOx Conversion Performance

Emissions of nitrogen oxides (NOx) from heavy-duty diesel engines are subject to more stringent environmental legislation. Selective catalytic reduction (SCR) over metal ion-exchanged zeolites is in this connection an efficient method to reduce NOx. Understanding durability of the SCR catalyst is crucial for correct design of the aftertreatment system. In the present paper, thermal and chemical ageing of Fe-BEA as NH3-SCR catalyst is studied. Experimental results of hydrothermal ageing, and chemical ageing due to phosphorous and potassium exposure are presented. The catalyst is characterized by flow reactor experiments, nitrogen physisorption, DRIFTS, XRD, and XPS. Based on the experimental results, a multisite kinetic model is developed to describe the activity of the fresh Fe-BEA catalyst. Furthermore, the model can predict deactivation of the catalyst well by decreasing the number of active sites, representing loss of active iron sites due to migration or chemical blockage of the sites. By performing a systematic study of different deactivation mechanisms, a deactivation expression for the active sites can be formulated.