High-Index (Ni,Mg)O Crystallization during Molten Salt Synthesis

Abstract

Heterogeneous NiO–MgO materials and (Ni,Mg)O solid solutions have found application in catalysis owing to the combined redox properties of NiO and basic properties of MgO. Examples include the use of (Ni,Mg)O to tune the surface reducibility of Ni(II), which determines the Ni cluster size and the tendency of catalysts to form coke and deactivate in dry reforming and partial oxidation of methane. To this end, control of the metal oxide structure and surface properties, both in terms of composition and faceting, are of paramount importance. In this article, we show that both compositional and morphological control can be achieved by selecting an appropriate molten salt synthesis (MSS) to obtain a solid solution of (Ni,Mg)O or segregated NiO–MgO crystals at low temperature (≤550 °C). Our findings reveal that nitrate precursors in a medium with excess NaCl/KCl (1:1) putatively results in the formation of a reaction intermediate comprised of molten K3Na(Mg,Ni)Cl6. In the presence of exchanged nitrate ions acting as Lux-Flood bases, this molten intermediate forms solid oxide solutions whose morphology is dictated by the composition of the medium, shifting from an octahedral to icositetrahedral (trapezohedral) habit with increasing Ni(II) content. Conversely, when using NaNO3/KNO3 (1:1) as a growth medium, solid solutions are inhibited and we observe segregation of the oxide phases. Under these conditions, the morphologies of the heterogeneous products are predominantly octahedral for MgO and truncated cubic-to-octahedral for NiO, depending on the Ni(II) content. Systematic examination of Ni/Mg mixed metal oxide crystallization highlights the strikingly different pathways followed in nitrate and chloride salt media. Overall, this study highlights the importance of molten intermediates in solid solution synthesis and reveals that the judicious selection of ions in MSS plays a major role in achieving well-defined crystals with high-index facets.