Effect of palladium on the reducibility of Mn based materials: correlation with methane oxidation activity

Abstract

Mn-based oxide supports were synthesized using different procedures: (i) carbonate co-precipitation method, leading to the formation of a hexaaluminate crystallized solid (La_0.2Sr_0.3Ba_0.5MnAl₁₁O₁₉) and (ii) solid–solid diffusion method, leading to the formation of a doped θ-Al₂O₃ crystallized solid (nominal composition: 60 wt% La_0.2Sr_0.3Ba_0.5MnAl₁₁O₁₉ + 40 wt% Al₂O₃). Impregnation of 1.0 wt%Pd was carried out on both oxides. The solids were tested for the catalyticmethane 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-Al₂O₃) 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.