posted on 2021-12-28, 15:38authored byHan Guo, Patricia Poths, Philippe Sautet, Anastassia N. Alexandrova
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.