Rh, Ru and Ir catalysts obtained by HT precursors: effect of the thermal evolution and composition on the material structure and use

Abstract

The crystal structure refinements of six M/Mg/Al mixed oxides containing cations in the ratio M∶Mg∶Al = 5∶71∶24 (where M is Ru, Rh or Ir) have been carried out using both neutron and X-ray powder full pattern diffraction analysis. The samples were obtained by thermal decomposition of hydrotalcite-type anionic clays at 923 or 1173 K. MgO-type and spinel-type oxides were detected along with IrO₂ or RuO₂. Because of the presence of crystal defects, a more flexible spinel-like structure was used to fit the MgO-type phase, too. The three elements that were investigated exhibit different solubilities in the various oxide lattices but invariably substitute in the 16d site of the spinel structure. The variation of the Mg∶Al ratio was utilised to change the distribution of the Rh and Ru in the present phases. The Mg∶Al ratio affects significantly the structure and the distribution of the noble metal in the calcined material. The activity in the partial oxidation of methane carried out after reduction has changed as a function of the Mg∶Al ratio and has been found to be higher at high Mg∶Al ratios.