Catalytic hot-spots in CO oxidation resolved by operando electron microscopy.

Catalytic reactions over nanoparticle ensembles are often interpreted by intrinsic nanoparticle descriptors. However, the collective behavior that emerges as nanoparticles are densely packed remains rarely addressed. Here, we use operando transmission electron microscopy and spectroscopy to examine the CO oxidation over Pt nanoparticles in a unidirectional gas-flow reactor. The experiments show that regions with high nanoparticle loading develop catalytic hot-spots, sustaining full CO conversion independent of the position in the reactor, whereas low-loaded regions display gradual conversion downstream. The hot-spots correlate with distinct shape-activity relationships and indicate that thermally mediated entanglement among densely packed nanoparticles governs self-sustained activity. Identifying such entanglement of nanoparticles opens up new pathways for engineering nanoparticle ensembles with spatially enhanced catalytic functionalities.