Kinetic modeling of H-BEA and Fe-BEA as NH3-SCR catalysts—Effect of hydrothermal treatment

Kinetic modeling and flow reactor experiments were used to study the dynamic behavior of the active sites of H-BEA and Fe-BEA during NH3-SCR before and after hydrothermal aging of the catalysts. To capture transient changes it was crucial to describe NH3 and NO adsorption. For the H-BEA model the storage of ammonia was simplified to proceed on two zeolite sites representing weak and strong Brønsted acid sites, while NO only is adsorbed on Brønsted acid sites. Furthermore, the oxidation of NH3 and NO, and the NH3-SCR reaction are assumed to proceed over the Brønsted acid sites. To model Fe-BEA, monomeric and dimeric iron, and iron oxide particles represent sites of three different types of iron species. Ammonia and NO adsorb on monomeric iron which is assumed to be the governing site for low temperature SCR. Dimeric iron species provide the activity for high temperature SCR and NH3-oxidation. Furthermore, iron particles, Fe2O3, are not active for NH3-SCR but for oxidation of NO. A spillover mechanism of ammonia adsorbed on Brønsted acid sites to monomeric iron sites is included in the model to simulate the inhibiting effect of NH3 during the SCR reaction. The spillover rate was found to be independent of the site density and depends only on the fraction of free sites, indicating that a constant number of Brønsted acid sites buffer each active iron site, unaffected by the hydrothermal treatment. The model describes the experiments well for H-BEA and Fe-BEA before and after hydrothermal treatment by decreasing the site density and keeping the kinetic parameters constant except the binding energy of ammonia to Brønsted sites, which was found to decrease with hydrothermal treatment. Finally, the model was validated in separate ammonia inhibiting experiments, not used in the fitting process of the kinetic parameters. The aging model describes the validation experiments well.