Effect of hydrothermal treatment on properties of Ni–Al layered double hydroxides and related mixed oxides

The Ni–Al layered double hydroxides (LDHs) with Ni/Al molar ratio of 2, 3, and 4 were prepared by coprecipitation and treated under hydrothermal conditions at 180 °C for times up to 20 h. Thermal decomposition of the prepared samples was studied using thermal analysis and high-temperature X-ray diffraction. Hydrothermal treatment increased significantly the crystallite size of coprecipitated samples. The characteristic LDH diffraction lines disappeared completely at ca. 350 °C and a gradual crystallization of NiO-like mixed oxide was observed at higher temperatures. Hydrothermal treatment improved thermal stability of the Ni2Al and Ni3Al LDHs but only a slight effect of hydrothermal treatment was observed with the Ni4Al sample. The Rietveld refinement of powder XRD patterns of calcination products obtained at 450 °C showed a formation of Al-containing NiO-like oxide and a presence of a considerable amount of Al-rich amorphous component. Hydrothermal aging of the LDHs resulted in decreasing content of the amorphous component and enhanced substitution of Al cations into NiO-like structure. The hydrothermally treated samples also exhibited a worse reducibility of Ni2+ components. The NiAl2O4 spinel and NiO still containing a marked part of Al in the cationic sublattice were detected in the samples calcined at 900 °C. The Ni2Al LDHs hydrothermally treated for various times and related mixed oxides obtained at 450 °C showed an increase in pore size with increasing time of hydrothermal aging. The hydrothermal treatment of LDH precursor considerably improved the catalytic activity of Ni2Al mixed oxides in N2O decomposition, which can be explained by suppressing internal diffusion effect in catalysts grains.