(216d) Understanding of Nox Storage/Release Mechanism over Pt-Bao/Al2o3 Lean Nox Trap Catalysts

The LNT (lean NOx trap) is generally considered as one of the promising solutions for the exhaust control of gasoline lean burn and diesel engine in order to meet future requirements on emission levels. In the LNT technology, an active (alkali and/or alkaline earth) oxide material takes up NOx under lean engine operation conditions and stores them as nitrates [1]. In a brief rich cycle these nitrates are released form the active oxide catalyst component, and reduced to N2 on the precious metal component of the catalyst. Both the storage and release of NOx have been extensively studied on these BaO-based systems. Although the NOx storage mechanism is fairly well understood today [2], the nature of the nitrate species formed is poorly characterized. Here we report the results of our multi-spectroscopy study in which we set out to understand the nature of different nitrate species formed during NO2 uptake on BaO/Al2O3 NOx storage materials. To this end, we investigated NO2 adsorption and desorption on Al2O3, 2wt%-, 8wt%-, and 20wt%-BaO/Al2O3 catalysts using TPD, and Fourier transform infrared (FTIR) and 15N solid state NMR spectroscopies. We also followed the changes in catalyst morphology during these NOx uptake and release processes using synchrotron time resolved XRD (TR-XRD ? carried out at the National Synchrotron Light Source at Brookhaven National Laboratory) and transmission electron microscopy (TEM). In addition, we try to elucidate the role of Pt and BaO components, and the interaction between these two components during NOx storage/release process by performing the reaction at different conditions over Pt-BaO/Al2O3 catalysts with various BaO loading.

(1) Epling, W.S., Campbell, L.E., Yezerets, A., Currier, N.W., Parks II, J.E., Cat. Rev.-Sci. Eng. 2004, 46, 163. (2) Prinetto, F., Ghiotti, G., Nova, I., Lietti, L., Tronconi, E., and Forzatti, P., J. Phys. Chem. B. 2001, 105, 12732.

We acknowledge the financial support from the U.S. Department of Energy (DOE), Office of FreedomCar and Vehicle Technologies. This work was performed in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory (PNNL). The EMSL is a national scientific user facility and supported by the US DOE's Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under contract number DE-AC06-76RLO 1830.