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Abstract:
The redox properties of perovskites play an important role in their use in thermo- and electrocatalytic applications. Here, we report the effects of substituting copper for manganese in A(Mn1 xCux)O3 (A = La, Pr; 0 ≤ x ≤ 0.4) perovskites on structural and chemical properties when lanthanum and praseodymium are used as A cations, whereas the synthesis of Pr-based materials has not been reported so far. On the one hand, we investigate the influence of the Cu content on the crystal structure and the surface composition of the materials and, on the other hand, how the composition affects the redox properties of the materials under reduction with H2 and reoxidation. The detailed refinement of X-ray diffraction data revealed that complex structural changes occur with increasing substitution, which, however, do not correlate linearly with the copper content and proceed differently for the La- and Pr-based systems with octahedral tilting as the main degree of freedom. While the structure of La-based perovskites changes with Cu content from the rhombohedral LaAlO3 structural type (x < 0.2) to the orthorhombic GdFeO3 type (x > 0.2), Pr(Mn1–xCux)O3 perovskites form an O′-orthorhombic GdFeO3-type structure over the whole composition range. In the temperature range up to 300 °C, which is important for low-temperature catalysis, the perovskites studied are structurally stable in redox reactions with hydrogen and oxygen in the sense that segregation of secondary phases essentially does not occur. However, the perovskites take up and release oxygen reversibly, which is reflected in the reversible changes of the normalized unit cell volume and can be pictorially compared to the breathing in and out of lattice oxygen, the capacity of which clearly correlates with the copper content. Nevertheless, the readily available lattice oxygen neither leads to higher activity in propane oxidation, which is used as a probe reaction, nor does it have a negative effect on the selectivity of the valuable product propene, which is in clear contrast to common concepts in oxidation catalysis. In addition, there is no simple correlation between the structural parameters of the bulk and properties in catalysis. The perovskites thus serve as a carrier and reservoir for a catalytically active Mn-oxide surface phase in low-temperature redox catalysis.
