Atomic-Scale In Situ STEM Investigation of Complex Fe Oxide-Ru Nanostructures

Continued development of catalysts is essential 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 creates unique catalytic properties. However, changes in morphology and composition in a reactive environment can significantly alter the chemical properties of the catalysts. In this work, we studied Fe oxide-Ru nanoparticles as catalysts in reactive conditions (oxidative and reductive) to understand the dynamical restructuring effects that affect their catalytic potential. We recorded aberration-corrected <i>in situ</i> scanning transmission electron microscopy (STEM) images while simultaneously acquiring secondary electron (SE) images using the dual detectors available in a Hitachi HF5000-IS environmental transmission electron microscope. Atomic-scale STEM imaging combined with SEM and spectroscopy provided an overview of changes in the materials as a function of the conditions (temperature and gas environment). In particular, the surface-sensitive SE images and the projected Z-contrast sensitivity of dark field STEM allowed us to conclude that both segregation and mixing of Ru occur under different conditions along with changes in facet geometry. In an oxidative environment, Ru tends to mix with Fe₂O₃ at the surface of the nanoparticles. In contrast, a reductive environment causes the aggregation of Ru atoms into larger clusters, causing fewer Ru atoms to be exposed to the surface and mixed with Fe. Based on these results, we suggest protocols for maximizing the exposed expensive Ru and its potential for surface chemistry applications. Hence, this work underlines the advantages of combining atomic-scale STEM imaging with other techniques to track surface structure and compositional changes of bimetallic nanocrystals upon oxidative or reductive treatment.