  Reducible oxides are widely used catalyst supports that can increase
oxidation reaction rates by transferring their lattice oxygen at the
metal-support interface. The interfacial oxidation process is typically
described in terms of a Mars-van Krevelen mechanism. However, many outstanding
questions remain unanswered regarding the atomic-scale structure and dynamic
meta-stability (i.e., fluxional behavior) of the interface $\textit{during
catalysis}$. Here, we employ aberration-corrected $\textit{operando}$ electron
microscopy to visualize the structural dynamics occurring at and near
Pt/CeO$_{2}$ interfaces during CO oxidation. Finite element simulations are
performed to develop a reaction rate analysis wherein the atomic-level
structural observations are directly correlated with the catalyst's turnover
frequency for CO oxidation. We show that the increasing frequency of catalytic
turnover correlates with dynamic fluxional behavior that (a) destablizes the
supported Pt particle, (b) marks an enhanced rate of oxygen vacancy creation
and annihilation, and (c) leads to increased strain and reduction in the
surface of the CeO$_{2}$ support. Overall, the results implicate the
interfacial Pt-O-Ce bonds anchoring the Pt to the support as being involved
also in the catalytically-driven oxygen transfer process, and they suggest that
oxygen reduction takes place on the highly reduced nearby CeO2 surface before
migrating to the interfacial perimeter for reaction with CO. The
$\textit{operando}$ electron microscopy approach described here should be
applicable to a large number of nanoparticle catalysts. This technique will
enable the identification of catalytically functional surface structures and
strengthen our ability to establish (dynamic) structure-activity relationships.
