Combined In Situ STEM and SEM Investigation of Fe-Ru Nanocatalysts

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It is critical to develop innovative catalysts to improve the synthesis of chemicals and reduce the cost of industrial processes. To this end, bimetallic nanoparticles are of great importance as the synergy between the two metals can create unique catalytic properties [1, 2]. However, changes in morphology and composition in a reactive environment can significantly alter the chemical properties of the catalysts [3-5]. In this work, we synthesized and studied Fe-Ru nanoparticles supported on Al2O3 for catalysis of the reverse water gas shift (rWGS) reaction (Figure 1). Catalytic measurements show a decrease in activity after pre-treatment with H2 at 600 °C for 1 hour. However, the catalytic activity could be recovered after a pre-treatment with O2 at 600 °C for 1 hour. We correlated these catalytic measurements with aberration-corrected in situ SEM and STEM recorded simultaneously using the multiple detector capability of a Hitachi HF5000-IS instrument operated at 200kV. When exposed to gas flow of nominally 10 Pa at elevated temperatures of 400-600 °C, the particles showed segregation and mixing of Ru combined with changes in facets depending on the dynamic environment (Figure 2). In oxidative conditions, Ru tends to mix with Fe oxide at the surface of the nanoparticles, and the Fe-Ru particles show facets. The mixing of Fe oxides and Ru and the formation of faceted surfaces appears beneficial for improving the catalytic properties and the synergy between both metals. In contrast, reductive conditions cause the aggregation of Ru atoms into larger clusters, implying that fewer Ru atoms are exposed to the surface and mixed with Fe. Hence, this work underlines the advantages of a combined STEM and SEM diagnostic to track surface structure and compositional changes of bimetallic nanocatalysts upon oxidative or reductive pre-treatment. We conclude that combined in situ STEM and SEM diagnostics are powerful approaches to understanding the structure-property correlations of nanomaterials and guiding their rational design [6]. Analysis of the Fe-Ru nanoparticles. (a) HAADF-STEM image of a single Fe-Ru nanoparticle. The Fe oxide shell is distinguishable from the Fe-Ru core. (b) HAADF-STEM image of a group of Fe-Ru particles on amorphous carbon. (c) Diffraction pattern of Fe-Ru nanoparticles, consistent with the structure of magnetite Fe3O4. (d) Size distribution of 200 nanoparticles. In situ HAADF-STEM images demonstrating segregation of Ru nanoparticles in Fe-Ru particles upon reduction. Pressure of gas was 10 Pa.