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

Abstract

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 similar to 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