(564c) Molten Salt Synthesis of NiO, MgO, and Their Mixed Oxides: Designing New Methods to Control Crystal Morphology

In several catalytic and absorption processes of industrial relevance, the crystal morphology and surface terminations of metal oxides play an important role; however, controlling these surface properties and studying their different behaviors in order to rationally optimize catalyst performance is not always feasible. In this presentation, we will discuss methods of preparing rock-salt metal oxides like MgO, NiO, and their solid solutions, (Mg,Ni)O, by molten salt synthesis (MSS), Our findings reveal that MSS enables a high level of morphological control by altering the pathways of metal oxide crystallization. We will show that polar MgO(111) can be prepared using either alkali nitrates or chlorides. For NiO, the morphological diversity is much wider, including NiO octahedra and trapezohedra exposing NiO(311) and (611) facets in alkali chlorides. These high-index facets have not been previously reported for this material. Our findings also reveal that NiO(311) trapezohedral particles grow through a nonclassical crystallization pathway, and are stable under steaming and retain catalytic activity in oxidative dehydrogenation of ethane over long time on stream, indicating high morphological stability at reaction conditions without evidence of sintering. We will show that the judicious selection of alkali nitrates allows for the synthesis of NiO cubes, cuboctahedra, and octahedra. Moreover, we have discovered that the formation of (Mg,Ni)O solid solutions is distinctively favored in alkali chlorides whereas in nitrates segregation of the oxides takes place. The crystallization process in chlorides is mediated by the formation of molten phases comprised of K₃NaMCl₆ (M=Mg, Ni) from which solid metal oxides form. Collectively, our findings reveal that diverse morphologies can be achieved through crystal engineering, thus allowing for the establishment of structure-performance relationships in a wide range of industrial catalytic processes.