Supported Ni catalysts prepared by intercalation of layered double hydroxides: investigation of acid-base properties and nature of Ni phases

Supported nickel catalysts were obtained by exchange of Mg/Al Layered Double Hydroxides (LDHs) compensated with NO3- ions with negatively charged Ni complexes, followed by thermal reduction. With this aim, suspensions of Ni complexes were prepared by controlled hydroxylation of Ni2+ cations in the presence of citrate (obtaining [Ni(C6H5O7)(OH)]y2y- species) or chloride (obtaining [NiCl4]2- species) complexing ions. For comparative purposes, other two supported Ni catalysts were prepared starting from Mg/Al LDHs compensated with Cl- ions and thereafter exchanged with [Ni(C6H5O7)(OH)]y2y- species (nominal degree of exchange: 100% and 20%). XRD and HRTEM techniques were employed for studying nickel phases and dispersion. CH3CN adsorption at RT followed by FT-IR spectroscopy gave information on surface acid–base properties. CO adsorption at 77 K followed by FT-IR spectroscopy was employed to deepen the characterization of the metal phase. On the basis of the results obtained, it is possible to evidence a correlation between the metal phase morphologies and the surface acid–base properties of the reduced catalysts. High surface basicity revealed by the presence of strongly basic O2- sites stabilizes small nickel particles (<4 nm), in spite of the high metal loading (>11 Ni wt.%), whereas, when the basicity is suppressed by Cl- ions, a dramatic Ni sintering occurs even at low metal loading (in the range 3–10 Ni wt.%).