Oxidation Dynamics of Supported Catalytic Cu Clusters: Coupling to Fluxionality

Abstract

Copper oxide nanoclusters have a wide range of catalytic applications, such as the selective oxidation of hydrocarbons. O2 binding to the catalyst, activation, and release upon reagent oxidation are key events in these catalytic chemistries. These events are expected to be accompanied by significant structural changes of the Cu clusters, because O atoms integrate into the cluster, rather than bind to its surface. Topping the complexity of the problem, partially oxidized Cu clusters are known to exhibit strong fluxionality and feature diverse and interconverting structures and oxygen contents in conditions of oxidative dehydrogenation (ODH). Hence, a significant dynamic coupling between the “hot” O2 molecule impacting the cluster at reaction temperatures and the cluster fluxionality can be expected. In this work, we focus on the dynamics of dioxygen integration into a partially oxidized Cu cluster supported on hydroxylated amorphous alumina–a system recently reported to be an exceptionally selective catalyst for cyclohexane ODH with very little CO produced, whose mechanistic underpinnings are of utmost interest. The statistics over a swarm of adsorption and scattering trajectories where O2 hits various sites on the cluster at reaction temperature shows that the O2 binding does not only follow the minimal energy paths. O2 also rarely integrates into the cluster in a single step and instead first binds to a single Cu atom via either an η1-O2 or an η2-O2 mode. Surprisingly, this step often has a higher barrier than the subsequent O2 integration and dissociation, which in turn take multiple steps and complete the oxidation process. Dynamic trajectories starting from the key transition state of integration of the adsorbed O2 can also lead to different intermediate structures during or right after the dissociation, due to the energy released from the transition state and the thermal intracluster effects. From these activated O2 chemisorbed structures, O2 dissociation occurs with moderate barriers (∼0.5 eV), producing multiple final oxidized Cu4O4 states. Hence, a diversity of reaction profiles for the attack of supported Cu cluster by O2 emerges due to the dynamic effects, with implications for mechanisms, kinetic models, and catalyst design principles.