Synthesis and thermal evolution of hydrotalcite-type compounds containing noble metals

M/Mg/Al hydrotalcite-type (HT) anionic clays [or layered double hydroxides (LDHs)] (M2+/M3+ atomic ratio≈2.5; M=Rh3+, Ir3+, Ru3+, Pd2+ and Pt2+) with the amount of M ranging from 0.04% to 5.0% (as atomic ratio) were successfully prepared by co-precipitation in an aqueous solution at pH 10.0, although for the Pt-containing sample about a 20% loss of the noble metal ions occurred. Key factors in the synthesis were the ionic radius and preferential coordination, the latter playing a more important role. Calcination of the HT precursors at 923 K gave rise mainly to cubic mixed oxides, with significant increases in surface area. As a function of their nature and amount, the noble metal ions were also detected as segregated oxide or metal side-phases, with the following scale of stability: Rh>Ir>Ru>Pd>Pt. This segregation was only partial, as evidenced by both quantitative XRD analysis and the crystallographic parameters of the mixed oxides. Calcination of the HT precursors at 1173 K formed mainly MgO- and spinel-type phases, with significant increases in the amount of segregated oxide or metal side-phases and decreases in surface area, which, however, were always higher than 45 m2/g. On the basis of the crystallographic parameters, it was concluded that the not segregated noble metal ions were always present inside the spinel-type phases. The scale of stability inside these phases was the same already observed for the samples obtained at 923 K and may be related to the oxidation state and the preferential structure of the noble metal ions.