(483a) Are Undercoordinated Sites Poisoned? a Kinetic Monte Carlo Study of NO Direct Decomposition on Pt55
To gain insights into this question, we chose as our model system nitric oxide (NO) direct decomposition on a cuboctahedral Pt55 nanoparticle. Pt55 presents a variety of undercoordinated edge and corner sites which can bind and dissociate NO, a notorious pollutant found in vehicle exhaust. However, the mean-field approximation breaks down when looking to model such a small (~1 nm) nanoparticle due to the strong spatial correlations between its various site types. In order to obtain meaningful reaction kinetics, the use of Kinetic Monte Carlo (KMC) simulations, as opposed to mean-field microkinetic models, is thus necessary. KMC also allows us to accurately take into account coverage effects, which were found to be significant under the studied conditions. This was implemented using a cluster expansion, which parameterizes adsorbate-adsorbate interaction energies terms of point and multibody interactions.3,4 The Zacros graph-theoretical KMC code5,6was used to perform the KMC simulations.
Our results show that undercoordinated sites can protect themselves from being poisoned by exerting strong repulsive interactions on nearby sites. Under our reaction conditions, NO is the most abundant surface intermediate. At high NO coverages, occupation of the sites on Pt55by NO does not follow the Langmuir adsorption isotherm, in which the strongest binding site type is saturated much faster compared to the more weakly binding site types. Instead, we find that site types are occupied in a staggered fashion, such that no one site type is saturated and poisoned, even if certain site types bind NO much more strongly in the dilute limit. The insights provided by our work will provide an interesting dimension to understanding how undercoordinated sites are able to achieve high activity under catalytic conditions.
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