Morphological evolution of Ba(NO3)2 supported on alpha-Al2O3(0001): an in situ TEM study

A key question for the BaO-based NOx storage/reduction catalyst system is the morphological evolution of the catalyst particles during the uptake and release of NOx. Notably, because the formed product during NOx uptake, Ba(NO3)2, requires a lattice expansion from BaO, one can anticipate that significant structural rearrangements are possible during the storage/reduction processes. Associated with the small crystallite size of high-surface area gamma-Al2O3, it is difficult to extract structural and morphological features of Ba(NO3)2 supported on gamma-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 alpha-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 thermal 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 alpha-Al2O3 possesses a platelet morphology, with the interface and facets being invariably the eight [111] planes. Formation of the platelet structure leads to an enlarged interface area between Ba(NO3)2 and alpha-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 approximately 1/4 of the [111] surface free energy of Ba(NO3)2. The orientation relationship between Ba(NO3)2 and the alpha-Al2O3 is alpha-Al2O3[0001]//Ba(NO3)2[111] and alpha-Al2O3(1-210)//Ba(NO3)2(110). Thus, the results clearly demonstrate dramatic morphology changes in these materials during NOx release processes. Such changes are expected to have significant consequences for the operation of the practical NOx storage/reduction catalyst technology.