Pd/Mg(Al)O catalysts obtained from hydrotalcites: Investigation of acid-base properties and nature of Pd phases

A series of palladium catalysts with Pd loading in the range 0.05-1.5 wt% were prepared by impregnation with either Pd acetylacetonate or Pd chloride of Mg(Al)O mixed oxides obtained by thermal decomposition of hydrotalcite compounds. For comparative purposes, Pd catalysts supported on MgO and Al ₂O₃ were prepared using the same procedures. The morphology of Pd particles was studied by TEM and HRTEM, while the surface acid-base properties, on the one hand, and the nature of Pd phases and the metal-support interactions, on the other hand, were investigated by FT-IR spectroscopy, using CH₃CN and CO probes, respectively. Pd catalysts obtained by the acetylacetonate precursor exhibited strongly basic oxygen ions, similar to those of MgO, along with Al³⁺ Lewis acid sites. A peculiarity of these catalysts was the presence of Pd sites, isolated or in very small clusters, strongly interacting with the basic oxygen anions of the support, which give carbonyl species showing unusual features. These sites were partially or totally depleted, depending on the amount of remaining Cl, on catalysts prepared with the chloride precursor.