| Summary: | Layered Ni-Mg-Al hydroxides with (Ni + Mg)/Al = 2.5 differing in Mg/Ni ratios and related oxide systems have been synthesized and characterized. Ni-Mg-Al hydroxides were prepared by the coprecipitation method. It was found that the samples dried at 110 °C were layered Ni-Mg-Al hydroxides with a hydrotalcite-type structure. After the heat treatment at 600 °C, the formation of Ni-Mg-Al-mixed oxides with a specific nanostructure, an intermediate between a NaCl and spinel structure, took place. According to XRD data, it had the unit cell parameter a = 4.174–4.181 Å, and a crystallite size of 4.0 nm. The specific surface area of the Ni-Mg-Al samples dried at 110 °C was 45–54 m<sup>2</sup>/g, and that of those calcined at 600 °C was 156.1–209.1 m<sup>2</sup>/g. In agreement with HRTEM data, in all the synthesized nickel catalysts reduced at 700 °C (H<sub>2</sub>), particle size was mainly distributed between 15–20 nm. The catalyst activity of LDH-derived Ni-Mg-Al catalysts in ammonia decomposition was studied in a fixed-bed flow-type reactor at an atmospheric pressure within the temperature range 500–700 °C. The synthesized catalysts overcame existing analogues in catalytic performance. At a process temperature of 500 °C, the Ni<sub>2</sub>Mg<sub>3</sub>Al<sub>2</sub>-HT catalyst showed that the H<sub>2</sub> productivity was 23.8 mmol/(g<sub>cat</sub>·min), exceeding the respective value of nickel catalysts reported in the literature.
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