Supported Ni catalysts (2 wt.% Ni) were investigated by FT-IR and UV–vis-NIR spectroscopy, using CO₂, CH₃CN and CO as probe molecules. The supports studied range from acidic (SiO₂), via amphoteric (Al₂O₃, Mg(Al)O) to basic oxides (MgO, CaO).

CO₂ adsorption experiments allowed to obtain the following qualitative scale for the basic strength of O²⁻ sites and Mⁿ⁺O²⁻ pairs: Ni/CaO > Ni/MgO ≥ Ni/Mg(Al)O ≫ Ni/Al₂O₃. On Ni/SiO₂ these sites are absent.

CH₃CN adsorption allowed to reveal acidic and strong basic Lewis sites with the following results: Ni/SiO₂ does not contain basic or acidic Lewis sites. Ni/Al₂O₃ contains acidic Lewis sites but no strong basic Lewis sites. The ratio between the strong basic and the acidic Lewis sites decreases in the order: Ni/MgO > Ni/CaO > Ni/Mg(Al)O. The strength of Lewis acid sites decreases in the order Ni/Al₂O₃ ≫ Ni/Mg(Al)O > Ni/MgO > Ni/CaO.

Interaction of CO at room temperature results into formation of Ni(CO)₄ and/or Ni(CO)_y < 4 sub-carbonyls. In the case of Ni/SiO₂ and Ni/Al₂O₃, the sub-carbonyls are weakly interacting with the support. For the other supports, Ni(CO)₄ and Ni(CO)_y < 4 sub-carbonyls are stabilized on the metal oxide and give rise to mono- and poly-nuclear complexes (containing 2 or 3 Ni atoms). On Ni/Mg(Al)O, even larger Ni clusters seem to be formed. The support capability of stabilizing mono- and poly-nuclear complexes can be ranked as follows: SiO₂ ≪ Al₂O₃ < Mg(Al)O < CaO < MgO. A reactivity scale of the Ni⁰ particles with CO is also proposed: Ni/SiO₂ ≪ Ni/Al₂O₃ < Ni/Mg(Al)O ≈ Ni/CaO < Ni/MgO.