HRSTEM-EELS investigation of manganese based perovskites for the oxidative dehydrogenation of propane

Session

PSA.5 - Nanoparticles & Catalysts

Authors

Dr. Walid Hetaba (2, 1), Dr. Gregor Koch (1), Dr. Annette Trunschke (1), Prof. Robert Schlögl (2, 1), Dr. Thomas Lunkenbein (1)

Affiliations

1. Fritz Haber Institute of the Max Planck Society
2. Max Planck Institute for Chemical Energy Conversion

Keywords

EELS, HR-STEM, oxidation state, Perovskites, Surface modification

Abstract text

Perovskites have been studied in various catalytic applications [1] and are frequently used to study the structure – function relationship by relating their structural and electronic properties with catalytic properties [2]. This electron microscopical contribution is part of a larger study aiming to investigate whether the selectivity in the oxidative dehydrogenation of propane can be changed by surface modifications.

We studied a set of Mn based perovskites AMnO₃ (A = La, Sm) using high resolution scanning transmission electron microscopy (HR-STEM) and electron energy-loss spectrometry (EELS). STEM-EELS investigations were performed in a double C_S corrected Jeol JEM ARM-200F electron microscope operated at 200 kV acceleration voltage, equipped with a Gatan GIF Quantum energy filter. Using HR-STEM, the structure of the perovskites, especially the surface of the nanoparticles, was examined. Additionally, spatially resolved EELS measurements were employed to study the oxidation state of Mn in the bulk and on the surface. Our results were compared to results from various other techniques, including X-ray photoemission spectroscopy and near edge X-ray absorption fine structure spectroscopy.

We have found an increased amount of Mn²⁺ at the majority of the investigated surfaces of the SmMnO₃ perovskites while in the bulk mostly Mn³⁺ is present. This is evidenced by the changes in the fine structure of the Mn L_2,3 edge as well as the O K edge, shown in Fig. 1 a) and b). This effect was reported as originating from a partial substitution of La atoms with Mn²⁺ atoms that are no longer hybridized with the surrounding oxygen atoms [3]. Additionally, the HR-STEM images concurrently show that the corresponding surfaces are mainly terminated by Mn, as can be seen in Fig. 1 c). Similar results were found for the LaMnO₃ perovskites, however, the SmMnO₃ perovskites showed an overall higher surface Mn concentration. Comparison of our findings with the results of complementary investigation methods show that this enrichment of Mn at the surface leads to an increased selectivity to propene in the studied reaction. Using transmission electron microscopy, we can thus substantially contribute to unveil the local structures directing the functionality of Mn based perovskites in catalytic applications.

Figure 1. EEL spectra and HR-STEM image of a SmMnO₃ nanoparticle. a) Near edge fine structure of the Mn L_2,3 edge. The presence of Mn²⁺ at the surface and Mn³⁺ in the bulk is evident from the changes in the fine structure. b) Oxygen K edge and Mn L_2,3 edge of SmMnO₃, showing differences in the O K edge at the surface and the bulk. c) High angle annular dark field image of SmMnO₃. The preferred B site termination is visible in the top part of the image.

References

[1] R.J.H. Voorhoeve et al., Science 195 (1977), p. 827-833

[2] H. Tanaka, M. Misono, Current Opinion in Solid State and Materials Science 5 (2001), p. 381-387

[3] R. Ignatans et al., The Journal of Physical Chemistry C 123 (2019), p. 11621-11627