Thermal evolution and catalytic activity of Pd/Mg/Al mixed oxides obtained from a hydrotalcite-type precursor

The synthesis, thermal evolution and catalytic activity in the total oxidation of methane of a novel Pd/Mg/Al (5:66:29, as atomic ratio) catalyst, obtained from a hydrotalcite-type (HT) anionic clay [or layered double hydroxide (LDH)] was investigated. The HT phase was prepared by co-precipitation at pH 10.0 from an aqueous solution of the corresponding nitrates. The XRD powder patterns of the sample obtained by calcination at 923 and 1173 K showed, together with MgO-type and/or MgAl2O4-type phases, the presence of increasing amounts of Pd-containing (PdO or metallic Pd) segregated phases. Further information was collected with an in-situ temperature-programmed experiment, where the evolving phases were investigated by neutron diffraction. Moreover, the effect of different heating atmospheres (oxidizing, reducing or inert) on the phase segregation was also investigated using differential thermal analysis. The catalyst always showed high activity in the total oxidation of methane, regardless of the heating atmosphere. However, PdO enhanced the activity at low temperature and low methane conversion, while the complete oxidation of methane was achieved at lower temperatures in the presence of large amounts of metallic Pd.