Direct Observation of Oxygen Evolution and Surface Restructuring on Mn 2 O 3 Nanocatalysts Using In Situ and Ex Situ Transmission Electron Microscopy.

Direct observation of oxygen evolution reaction (OER) on catalyst surface may significantly advance the mechanistic understanding of OER catalysis. Here, we report the first real-time nanoscale observation of chemical OER on Mn ₂ O ₃ nanocatalyst surface using an in situ liquid holder in a transmission electron microscope (TEM). The oxygen evolution process can be directly visualized from the development of oxygen nanobubbles around nanocatalysts. The high spatial and temporal resolution further enables us to unravel the real-time formation of a surface layer on Mn ₂ O ₃ , whose thickness oscillation reflects a partially reversible surface restructuring relevant to OER catalysis. Ex situ atomic-resolution TEM on the residual surface layer after OER reveals its amorphous nature with reduced Mn valence and oxygen coordination. Besides shedding light on the dynamic OER catalysis, our results also demonstrate a powerful strategy combining in situ and ex situ TEM for investigating various chemical reaction mechanisms in liquid.