Crystals 2021, 11, 1429. https://doi.org/10.3390/cryst11111429 www.mdpi.com/journal/crystals Article Effect of Synthesis Method on Properties of Layered Double Hydroxides Containing Ni(III) Olga V. Nestroinaia, Irina G. Ryltsova and Olga E. Lebedeva * Institute of Pharmacy, Chemistry and Biology, Belgorod State National Research University, 308015 Belgorod, Russia; nestroynaya@bsu.edu.ru (O.V.N.); ryltsova@bsu.edu.ru (I.G.R.) * Correspondence: olebedeva@bsu.edu.ru Abstract: Unstable oxidation state +3 of nickel can be stabilized in the structure of layered double hydroxides, the resulting crystallinity and properties being dependent on the synthesis method. Three different wet chemical methods (co-precipitation at variable pH, co-precipitation followed by hydrothermal treatment, co-precipitation with microwave treatment) were used to synthesize Mg/Ni–Al layered double hydroxides containing triply charged nickel cations. Lattice parameters of the samples synthesized by various methods were found to differ from each other by about 1.5%. The most crystallized sample was obtained by hydrothermal synthesis. The oxidation state of nickel in the LDH samples was confirmed by XPS. TEM mapping gave evidence of the uniform distribution of nickel in all the samples. The LDHs’ reduction with hydrogen and thermal trans- formations of the phase composition and morphology of LDHs were studied in detail. The prop- erties of the samples synthesized by the different methods were shown to be quite similar. Keywords: layered double hydroxides; nickel (III); co-precipitation; hydrothermal treatment; mi- crowave treatment; XRD; XPS; TPR-H2 1. Introduction Layered double hydroxides (LDHs) or hydrotalcite-like compounds are layered basic salts. The general formula for this class of compounds is [ ] O mH A OH M M n x n х x x 2 / 2 3 2 1 ) [( ) ( ⋅ - + + + - , where M 2 + and M 3 + are metal ions located in octahedral positions of brucite-like layers, and A n- stands for inorganic or organic an- ions that compensate the positive charge of brucite-like layers [1,2]. The structural fea- tures of hydrotalcite-like compounds give them a number of specific ion-exchange [3–5], sorption [6–8], electrical [9–11], catalytic [12–14], and magnetic properties [15–17]. The properties of LDHs are known to be significantly affected by cation–anion composition. Due to the easily reconstructed structure, it is possible to introduce cations of various metals into brucite-like layers, which makes it possible to regulate the proper- ties of layered double hydroxides [18–21]. From the other point of view, LDHs are able to influence the properties of cations, changing them or, on the contrary, preserving them. The typical example is the stabilization of the unstable oxidation state of metals in a ma- trix of layered double hydroxides. Previously, our scientific group synthesized and characterized layered double hydroxides with unstable Ce (III) [22], Sn (II) [23], or Ni (III) [24] incorporated into brucite-like layers of LDHs. Compounds containing trivalent nickel, however exotic they seem to be, are con- sidered to be promising catalysts for the carbonation of polypropylene [25], the hydro- carboxylation of acetylene [26], the photocatalytic generation of hydrogen from water [27], and the electrocatalytic production of biodiesel [28]. The current study was aimed at the optimization of the synthesis method for ob- Citation: Nestroinaia, O.V.; Ryltsova, I.G.; Lebedeva, O.E. Effect of Synthesis Method on Properties of Layered Double Hydroxides Containing Ni(III). Crystals 2021, 11, 1429. https://doi.org/10.3390/ cryst11111429 Academic Editor: Aivaras Kareiva Received: 23 October 2021 Accepted: 18 November 2021 Published: 21 November 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional claims in published maps and insti- tutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses /by/4.0/).