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. O 2 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' O 2 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 O 2 hits various sites on the cluster at reaction temperature shows that the O 2 binding does not only follow the minimal energy paths. O 2 also rarely integrates into the cluster in a single step, and instead first binds to a single Cu atom via either 𝜂 " -O 2 or 𝜂 # -O 2 mode. Surprisingly, this step often has higher barrier than the subsequent O 2 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 O 2 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 intra-cluster effects. From these activated O 2 chemisorbed structures, O 2 dissociation occurs with moderate barriers (~0.5 eV), producing multiple final oxidized Cu 4 O 4 states. Hence, a diversity of reaction profiles for the attack of supported Cu cluster by O 2 emerges due to the dynamic effects, with implications for mechanisms, kinetic models, and catalyst design principles.

Rate Expressions in Mean Field Microkinetic Models Incorporating Multiple Types of Active Sites

“…This also demonstrates that meta stable sites can dominate the reactive adlayer even though they are weaker binding at low coverages entirely due to macroscopic configurational entropy and realistic coverage effects. This corroborates the recent and ongoing work of Sautet and Alexandrova and co-workers [30][31][32][33][53][54][55][56][57][58][59][60][61][62][63][64] demonstrating the catalytic potential of fluxional and meta-stable sites. Overall, our results are very sensible given the parameters used and substantiate the appropriateness of the rate expressions established here for multiple site types.…”

“…However, it is becoming increasingly clear that multiple active centers exist that are relevant to the kinetics of such systems, 3,4, and recent work has highlighted the critical role of metastable state structures in heterogeneous catalysis. [30][31][32][33] So, a proper formulation is still required.…”

Insights into coverage-affected selective catalytic oxidation of ethylene on Ag(111) from comprehensive microkinetic analyses

“…3, because the structure with the lowest energy should occur most often. In principle, other structures with energies not far away from the lowest energy should also be included, 78,79 but they are beyond the scope of the current microkinetic simulations and will be studied in future work.…”

Theoretical Perspective on Operando Spectroscopy of Fluxional Nanocatalysts

“…Dynamic restructuring of the NPs was on the same time scales as the TOF for CO oxidation, indicating that the fluxional behavior of catalysts may couple quite closely with the reaction they are catalyzing. 59 This suggests that computational approaches where the dynamic fluxionality of the cluster is coupled to reactant activation 12,62 or the chemical step in catalysis 63 may be key to understanding the full scope of the catalyst behavior, despite the computational cost. Extent of Diversity in the Catalyst Ensembles and Factors That Influence It.…”

“…(B) Cu cluster oxide on silica undergoing the attack and integration of O 2 at 400 K: (top) typical dynamic trajectory, showing atomic displacements relative to analogous thermal trajectory without the O 2 attack, (bottom) schematics of a typical minimal energy path, overlaid with several representative dynamic trajectories, and resultant ensemble diversification. Reproduced from ref . Copyright 2021 American Chemical Society.…”