Effect of praseodymium and europium doping in La1−xLnxMnO3+δ (Ln: Pr or Eu, 0 ≤ x ≤ 1) perosvkite catalysts for total methane oxidation

• A series of La1 − xLnxMnO3 + δ (Ln = Pr, Eu; 0 ≤ x ≤ 1) are obtained by combustion synthesis.
• Studied solids exhibit oxygen over-stoichiometry and a mixture of Mn3+ and Mn4+.
• La, Pr, Eu and Mn segregation to the surface depend on Ln content.
• The best catalytic activity is related to 20% Eu-doped lanthanum manganite.
• Surface Mn composition is the most influencing factor.

Perovskite-type La1 − xLnxMnO3 + δ (Ln: Pr or Eu, 0 ≤ x ≤ 1) oxides were prepared using solution combustion synthesis, characterized by X-ray diffraction, oxygen desorption, X-ray photoelectron spectroscopy and tested for methane deep oxidation. XRD patterns showed perovskite structure over the whole range of Ln substitution degree. Compared to praseodymium, introduction of europium increased the perovskite lattice distortion in relation to its smaller ionic radius. Chemical titration results revealed oxygen over-stoichiometry (δ > 0) for all solids and the presence of Mn3+ and Mn4+ mixture in B sublattice. The catalyst activity was found to depend on the degree of substitution and the Ln nature in the A sites. Among the investigated compositions, the 20% containing europium compound showed the highest catalytic activity. It was possible to correlate catalytic performances with the structural characteristics of the oxides. The released α2-oxygen (i.e. species transported through the lattice to the surfacial reduction site leading to temporary Mn4+ → Mn3+ reduction) and the La, Pr, Eu and Mn segregation at the surface, showed differences along the series and seem to be the most important determining factors concerning the catalytic activity. The relatively low catalytic performances of Pr-modified catalysts should be due to the high segregation of Pr3+ toward the surface. The stability of Mn4+ and the existence of a local minimum of La and Eu and a maximum of Mn segregations for the x = 0.2 catalyst should be considered as the key reasons enhancing the catalytic activity in methane deep oxidation.

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