The promotional effect of ZnO addition to supported Ni nanocatalysts from layered double hydroxide precursors on selective hydrogenation of citral

A series of highly-dispersed, ZnO-modified supported nickel nanocatalysts (Ni-ZnO/C) were prepared via in situ self-reduction of hybrid Ni-Zn-Al layered double hydroxide/carbon (NiZnAl-LDH/C) nanocomposite precursors. The materials were characterized by X-ray diffraction (XRD), transmission electronic microscopy (TEM), scanning transmission electron microscopy (STEM), ammonia temperature-programmed desorption (NH3-TPD), and X-ray photoelectron spectroscopy (XPS). The effect of ZnO addition on the catalytic properties of as-synthesized Ni-ZnO/C catalysts for liquid phase selective hydrogenation of citral to citronellol was examined. It was found that ZnO addition significantly modified their catalytic hydrogenation properties for citral, inducing an improved selectivity toward citronellol. A maximum yield of citronellol (similar to 92%) was achieved when the bulk Zn/Ni atomic ratio was 0.25 in the catalysts. This promotional effect was mainly related to the existence of a ZnO-metal interaction, which was proposed to be responsible for enhanced adsorption of the C=O bond in the citral molecule on the surface of catalysts and thus for activation of the C=O bond.