NOx Adsorption of Pt/K/Al2O3 in the presence of CO2 and H2O: an In-situ DRIFTS study
Todd Toops, Oak Ridge National Laboratory
A leading solution to the stricter EPA NOx emissions regulations is NOx adsorber catalysts, which are based on the ability of alkali and alkaline elements to trap NOx under lean conditions in the form of a nitrate. The stored nitrate is then reduced in a brief rich cycle to obtain N2 and H2O. To work effectively, these catalysts require an oxidation component, typically a noble metal like Pt, a storage component, commonly Ba, and a high surface area support like ¥ã-Al2O3. Potassium is another element that has shown potential as a storage component, especially in conjunction with Ba; however, reports on the specific contributions of potassium are scarce. This study explores the contribution of K during the NOx storage phase in a Pt/K/Al2O3 powder using in-situ Diffuse Reflectance Fourier Transform Infrared Spectrscopy (DRIFTS). Each catalyst component, i.e. ¥ã-Al2O3, Pt/¥ã-Al2O3, and K/¥ã-Al2O3, is studied to better understand its contributing factor. The effects of CO2 and H2O were also carried out between 150 to 400¢ªC. A free nitrate ion, NO3-, is the primary form of stored NOx on the potassium phase. The Al2O3 support also has storage capabilities, but at saturation, the covalently bound nitrates formed on alumina are 75-95% less than those formed on K. Pt is essential for the expeditious storage of NOx on the potassium phase; its role in storage goes beyond simply oxidation of NO to NO2. When CO2 is introduced to the catalyst system a decrease of 45% is observed in the ionic nitrate formation on potassium due to a competition between carboxylate, CO2-, and nitrate ions; however, when H2O is also included in the feed the decrease is only 15%. This is one illustration of the effects of hydroxyl groups on the surface chemistry of Pt/K/Al2O3. The storage effects of Al2O3 are greatly mitigated in the presence of H2O, as there is a 90% reduction in these nitrates. NO2 chemisorption shows that the surface coverage of stored NOx decreases from 8.9 ¥ìmols/m2 at 25¢ªC to 1.8 ¥ìmols/m2 at 400¢ªC; however, the rate of storage is much slower below ~200¢ªC. For instance, a temporal examination shows that after 5 minutes on stream the maximum ionic nitrate adsorption is 2.5 ¥ìmols/m2 at 300¢ªC with only 0.6 ¥ìmols/m2 at 150¢ªC. The relative amount of CO2- formed is also significantly higher below 200¢ªC as the inefficiency of Pt-catalyzed NO oxidation reaction is apparent.