(483a) Are Undercoordinated Sites Poisoned? a Kinetic Monte Carlo Study of NO Direct Decomposition on Pt55

Undercoordinated sites are commonly invoked to explain the high activity of small nanoparticles, as well as the structure sensitivity exhibited by some reactions. However, it is unclear if the higher activity of undercoordinated sites is generalizable to most reactions and reaction conditions. Sabatier reminds us that the ideal catalyst is not the one with exceptionally high or low reactivity, but instead the one that binds all species modestly. Undercoordinated sites are especially active for bond breaking as they bind the products of a reaction strongly, increasing the exothermicity of the reaction.^1,2However, precisely due to their strong binding affinity, they can be easily saturated and thus poisoned by reactants or decomposition products. Whether these undercoordinated sites can maintain their intrinsic activity under catalytic conditions, or are instead poisoned and rendered inactive, is a highly important and yet rarely studied issue.

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 code^5,6was 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.

References

1 B. Hammer and J. K. Nørskov, in Advances in Catalysis, 2000, vol. 45, pp. 71–129.

2 R. I. Masel, Catal. Rev., 1986, 28, 335–369.

3 J. M. Sanchez and D. De Fontaine, in Structure and Bonding in Crystals, eds. A. Nsvrotsky and M. O’Keeffe, Academic Press, Inc, New York, 1981, vol. 2, p. 117.

4 D. J. Schmidt, W. Chen, C. Wolverton and W. F. Schneider, J. Chem. Theory Comput., 2012, 8, 264–273.

5 M. Stamatakis and D. G. Vlachos, J. Chem. Phys., 2011, 134, 214115.

6 J. Nielsen, M. D’Avezac, J. Hetherington and M. Stamatakis, J. Chem. Phys., 2013, 139, 224706.