In Situ UV Raman Study of the NOx Trapping and Sulfur Poisoning Behavior of Pt/Ba/γ-Al2O3 Catalysts

In situ Raman experiments excited with 244 nm radiation are carried out on Pt/γ-Al2O3 and Pt/Ba/γ-Al2O3 catalysts. The addition of Ba allows the normal catalyst to store substantial amounts of NOx as Ba nitrates under lean burn conditions. Dilute amounts of NO and/or SO2 plus O2 in N2 are flowed over the catalysts, with the temperatures and concentrations chosen to simulate the NOx trapping and sulfur poisoning characteristics encountered in normal operation. Adsorbed species observed on Pt/γ-Al2O3 are nitrite/nitro species and sulfate on γ-Al2O3 and NO on Pt. On Pt/Ba/γ-Al2O3, in situ UV Raman spectra showed typical NOx trap behavior: NOx and SOx are stored as Ba(NO3)2 and BaSO4 during lean conditions and released at higher temperatures in H2 flow. Prolonged SO2 exposure eventually deactivates the catalyst for NOx storage. The sulfate purged by heating to 500 °C in H2 is not completely removed from the catalyst, since it can reappear at lower temperature without additional exposure to SO2. On both Pt/γ-Al2O3 and Pt/Ba/γ-Al2O3, NOx species form more quickly than sulfates under our reaction conditions, but the sulfates are more stable and eventually block NOx adsorption, since they occupy the same sites.