(788e) Revealing Atomic Restructuring of Co-Based Nanocatalysts During Reactions

Catalytic nanoparticles often change their structure, elemental distribution or shape during different reactive environments. An understanding of the structural restructuring mechanisms is crucial for novel catalytic materials design. We study Co-based nanocatalysts in situ using an environmental transmission electron microscope (ETEM). First, we show restructuring of Co-Pt bimetallic catalysts in oxidizing or reducing environments using a differentially pumped ETEM (DP-ETEM). Our direct observation reveals that during oxidation Co migrates to surface forming patches of cobalt oxide and eventually a PtCo-CoOx or Pt-CoOx core-shell structure. During reduction cobalt migrates back to the bulk leaving a monolayer of platinum on the surface. The atomic pathways of phase segregation have been achieved with a great level of details. The operational pressure in a DP-ETEM is generally limited up to 20 mbar. However, many industrial catalytic reactions are operated under pressures equal or higher than 1 bar - 100 times higher than that in the DP-ETEM. Therefore, we also study gas reactions in a higher pressure range using a gas flow membrane cell TEM holder that allows a pressure up to 4 bar. The built-in membrane heaters enable reactions at a temperature of 95—400 ºC with flowing reactive gases. We demonstrate that, using a conventional thermionic TEM, 2-angstrom atomic fringes can be resolved with the presence of 1 atm O₂ gases in an environmental cell and we show initial studies on oxidation of cobalt nanocatalyts under the atmospheric gas pressure.