PNNL

Abstract

One of the key questions for the BaO-based NOx catalyst system is the morphological evolution of Ba(NO3)2 to BaO upon heating for releasing of NOx or vice versa from BaO to Ba(NO3)2 upon uptaking of NOx. However, associated with the small crystallite size of high-surface area ? Al2O3, it can be difficult to extract structural and morphological features of Ba(NO3)2 supported on ?-Al2O3 by any direct imaging method including transmission electron microscopy. In this work, by choosing a model system of Ba(NO3)2 particles supported on single crystal ?-Al2O3, we have investigated the structural and morphological features of Ba(NO3)2 as well as the formation of BaO from Ba(NO3)2 during the release of NOx using ex-situ and in-situ TEM imaging, electron diffraction, energy dispersive spectroscopy (EDS), and Wulff shape construction. We find that Ba(NO3)2 supported on ?-Al2O3 possesses a platelet morphology, with the interface and facets being invariably the 8 {111} planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and ?-Al2O3, indicating that the interfacial energy is lower than the Ba(NO3)2 surface free energy. In fact, Wulff shape constructions indicate that the interfacial energy is ~1/4 of the {111} surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the ?-Al2O3 is: ?-Al2O3[0001]?//Ba(NO3)2[111] and ?-Al2O3(1-2 10)//Ba(NO3)2(110).