In Situ UV Raman Study of the NO_x Trapping and Sulfur Poisoning Behavior of Pt/Ba/γ-Al_2O_3 Catalysts

摘要

In situ Raman experiments excited with 244 nm radiation are carried out on Pt/γ-Al_2O_3 and Pt/Ba/γ-Al_2O_3 catalysts. The addition of Ba allows the normal catalyst to store substantial amounts of NO_x as Ba nitrates under lean burn conditions. Dilute amounts of NO and/or SO_2 plus O_2 in N_2 are flowed over the catalysts, with the temperatures and concentrations chosen to simulate the NO_x trapping and sulfur poisoning characteristics encountered in normal operation. Adsorbed species observed on Pt/γ-Al_2O_3 are nitriteitro species and sulfate on γ-Al_2O_3 and NO on Pt. On Pt/Ba/γ-Al_2O_3, in situ UV Raman spectra showed typical NO_x trap behavior: NO_x and SO_x are stored as Ba(NO_3)_2 and BaSO_4 during lean conditions and released at higher temperatures in H_2 flow. Prolonged SO_2 exposure eventually deactivates the catalyst for NO_x storage. The sulfate purged by heating to 500 ℃ in H_2 is not completely removed from the catalyst, since it can reappear at lower temperature without additional exposure to SO_2. On both Pt/γ-Al_2O_3 and Pt/Ba/γ-Al_2O_3, NO_x species form more quickly than sulfates under our reaction conditions, but the sulfates are more stable and eventually block NO_x adsorption, since they occupy the same sites.