In Situ Visualization of Lattice‐Coherent Phase Oscillations and Active Brownian Motion of a Copper Catalyst During Hydrogen Oxidation

Abstract Structural dynamics govern the catalytic activity of metal nanoparticles (NPs), yet their atomic‐scale mechanisms remain unclear. Using in situ transmission electron microscopy, we reveal redox‐driven lattice‐coherent Cu↔Cu 2 O phase oscillations in individual Cu NPs during hydrogen oxidation conditions. These oscillations generate active Brownian particles, wherein asymmetric H 2 oxidation leads to directional motion that results in particle collisions and sintering. Crucially, the same active Brownian motion also triggers particle splitting, counteracting surface area loss and deactivation. Such active matter behavior arises from the formation of a head–tail morphology at critical H 2 :O 2 ratios (e.g., 5:1), featuring a metallic‐rich head and an oxide‐dominated tail, with their volumetric balance dynamically shifting through competitive oxidation‐reduction cycles. Quantitative analysis establishes a direct correlation between migration velocity and redox dynamics, revealing that the oxidation process significantly enhances particle mobility while the followed reduction process slows the velocity. Molecular dynamics (MD) simulations demonstrate that particle elongation and oxide tail fragmentation, accompanying particle migration, can be explained by asymmetric adhesion forces between the metallic/oxide phases and the silicon nitride support, alongside the redox reactions occurring on the particles. This work provides atomic‐scale insights into catalyst dynamics under operando redox conditions, offering foundational knowledge for designing stable, high‐performance catalytic systems.