Effect of the Mg/Al ratio of the hydrotalcite-type precursor on the dispersion and activity of Rh and Ru catalysts for the partial oxidation of methane

Active and stable catalysts for the partial oxidation of methane based on Rh and Ru nanoparticles strongly interacting with a MgO or spinel matrix have been prepared by calcination and reduction of hydrotalcite-type precursors. A Rh catalyst containing an high metal loading was used to calculate, by Rietveld analysis, the distribution of the Rh in the MgO-type and spinel-type phases obtained after calcination. The Rh particle size distribution analysed after reduction by HRTEM, has evidenced that the size of Rh particles supported on MgO is smaller than that of Rh particles supported on spinel matrix. The differences in the catalytic activity of the two type of matrix have been studied changing the Mg/Al ratio of the precursor. High Mg/Al ratio depressed the formation of the spinel and led to a narrow distribution of the particle size. The sample with the highest percentage of the MgO phase showed the highest activity. Similar tests have been carried out with Ru containing samples and in this case the increase of the MgO phase decreases the formation of RuO2.