Fluxional Behavior at the Atomic Level and its Impact on Activity: CO Oxidation over CeO$_{2}$-supported Pt Catalysts

Abstract

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.