Effect of palladium on the reducibility of Mn based materials: Correlation with methane oxidation activity
Résumé
Mn-based oxide supports were synthesized using different procedures: (i) carbonate co-precipitation method, leading to the formation of a hexaaluminate crystallized solid (La0.2Sr0.3Ba0.5MnAl 11O19) and (ii) solid-solid diffusion method, leading to the formation of a doped θ-Al2O3 crystallized solid (nominal composition: 60 wt% La0.2Sr0.3Ba 0.5MnAl11O19 + 40 wt% Al2O 3). Impregnation of 1.0 wt%Pd was carried out on both oxides. The solids were tested for the catalytic methane combustion up to 700°C. It was observed that adding palladium resulted in an important increase in the catalytic activity. The combined use of H2-TPR and XPS techniques reveals that only Mn3+/Mn2+ redox "couple" is present in the solids, whatever the synthesis procedure used. The fraction Mn3+/Mn is proportional to the total Mn content in the solid support, whatever the sample structure (hexaaluminate or doped θ-Al 2O3) and its morphology (large crystals or aggregates of small particles, respectively). Pd impregnation and further calcination at 650°C has no significant effect on the Mn3+/Mn fraction. However, some changes in Mn3+ reduction profile are observed, depending on the solid structure. Indeed, palladium addition strongly affects the manganese reducibility with an important shift of the reduction process to lower temperatures (∼100°C). On the basis of redox properties observed for the different catalysts, a Mars-van-Krevelen redox mechanism, with oxygen transfer from support oxides to palladium particles, is proposed to explain the difference in terms of catalytic conversion and stability with respect to a 1.0 wt%Pd/Al2O3 reference sample.