(169c) Crystallization of NiO Exposing High-Index Facets By Molten Salt Synthesis

Altering metal oxide crystals morphology and their surface terminations is crucial for controlling various surface properties and studying different behaviors, e.g., absorption and catalytic reactions. In this presentation, we will discuss the preparation of rock-salt metal oxides, like NiO, by molten salt synthesis (MSS) methods.^1-3 We will show that MSS enables a high level of morphological control by altering the pathways of metal oxide crystallization. We will illustrate the wide morphological diversity that can be attained using alkali chloride salts, including NiO octahedra and trapezohedra exposing NiO(311) and (611) facets. Density functional theory (DFT), used to examine the impact of growth media on the exposed NiO crystal facets, predict results which are in line with the experimental findings, showing that molten‐salt synthesis in alkali chlorides (KCl, LiCl, and NaCl) imposes shape selectivity on NiO particles.⁴ While the production of NiO octahedra in absence of added salt is attributed to the dissociative adsorption of H₂O, the formation of trapezohedral particles could be associated with the control of the growth kinetics exerted by ordered salt structures on high‐index facets. Our findings also reveal that NiO(311) trapezohedral particles grow through a nonclassical crystallization pathway, and facets 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.² Finally, we will show that the judicious selection of alkali nitrates also allows for the synthesis of NiO cubes, cuboctahedra, and octahedra. 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.

[1] M. D. Susman, H. N Pham, A. K. Datye, S. Chinta, J.D. Rimer; Chem. Mater. 2018 30, 2641–2650 “Factors governing MgO (111) Faceting in the thermal decomposition of oxide precursors”

[2] M.D. Susman, H.N. Pham, X. Zhao, D.H. West, S. Chinta, P. Bollini, A.K. Datye; Angew. Chem. Int. Ed. 2020 59, 15119–15123 “Synthesis of NiO Crystals Exposing Stable High‐Index Facets”

[3] M. D. Susman, S. Chinta, J. D. Rimer; Chem. Mater. 2021, 33, 9, 3155–3163, “High-Index (Ni,Mg)O Crystallization during Molten Salt Synthesis”

[4] R. Cheula, M. D. Susman, D. H. West, S. Chinta, J. D. Rimer, M. Maestri; Angew. Chem. Int. Ed. 2021, 60, 25391–25396, “Local Ordering of Molten Salts at NiO Crystal Interfaces Promotes High‐Index Faceting”