Effect of palladium on the reducibility of Mn based materials: correlation with methane oxidation activity (original) (raw)
2008, Physical Chemistry Chemical Physics
Mn-based oxide supports were synthesized using different procedures: (i) carbonate co-precipitation method, leading to the formation of a hexaaluminate crystallized solid (La 0.2 Sr 0.3 Ba 0.5 MnAl 11 O 19 ) and (ii) solid-solid diffusion method, leading to the formation of a doped y-Al 2 O 3 crystallized solid (nominal composition: 60 wt% La 0.2 Sr 0.3 Ba 0.5 MnAl 11 O 19 + 40 wt% Al 2 O 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 1C. It was observed that adding palladium resulted in an important increase in the catalytic activity. The combined use of H 2 -TPR and XPS techniques reveals that only Mn 3+ /Mn 2+ redox ''couple'' is present in the solids, whatever the synthesis procedure used. The fraction Mn 3+ /Mn is proportional to the total Mn content in the solid support, whatever the sample structure (hexaaluminate or doped y-Al 2 O 3 ) and its morphology (large crystals or aggregates of small particles, respectively). Pd impregnation and further calcination at 650 1C has no significant effect on the Mn 3+ /Mn fraction. However, some changes in Mn 3+ 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 (B100 1C). 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/Al 2 O 3 reference sample.
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