(269b) Effects of Dopant Loading and CO Adsorption on the Structural Stability of Highly Dilute Alloys

Platinum group metals (PGMs), including Pd, Pt, Rh and Ir, as well as Ni are often employed to catalyse reactions of high industrial importance owing to their excellent activity in a wide range of applications. Yet, their high catalytic activity is often hampered by their susceptibility to CO poisoning that in turn leads to catalyst deactivation.^1,2 Studies have shown that this problem may be effectively addressed by preparing highly dilute alloys of PGMs with less reactive host metals, such as Ag, Au and Cu. In particular, single atom alloys (SAAs) are found to exhibit high tolerance to CO poisoning as a result of the weaker CO binding compared to PGMs,³ whilst offering the economic benefit of minimizing the loading of expensive PGMs. These advantages in conjunction with their excellent catalytic performance, in terms of activity and selectivity in a variety of reactions such as hydrogenations,⁴ dehydrogenations⁵ and hydrosilylation,⁶ make SAAs an extremely attractive alternative to conventional PGM catalysts.

The performance of alloy catalysts is, however, associated with the structure and morphology of the catalytic surface, and structural modifications specific to the application may be required in order to achieve high catalytic performance. The significance of the dopant and host metal loadings, as well as the way these atoms are distributed on the nanoparticle (NP) facet, have been underlined through computational and experimental studies.^7,8 Dopant atom clustering may be beneficial to the catalytic activity,⁸ though the formation of dimers, trimers or islands may result in the deterioration of catalytic performance as a result of the increased susceptibility to CO poisoning.³ It follows that establishing methods to manipulate the surface structure according to the needs of the specific application is imperative. We recently investigated the structural stability of highly dilute alloys in the presence and absence of CO,⁹ demonstrating that the strong binding of CO to dopant metal atoms has a marked influence on the structural stability of highly dilute alloys through effects such as segregation and aggregation.

In this work, we employ Monte Carlo (MC) simulations and Density Functional Theory (DFT) to further examine the most stable structures of PGM alloys with loading up to 5 % in Ag, Au and Cu hosts. We perform DFT calculations on the (111), (100) and corner/edge facets with various dopant atom loadings and CO coverages. We use the energetics determined using DFT to parameterize on lattice MC simulations to elucidate the surface structure of highly dilute NPs of PGM alloys. We establish the most favored facet for dopant atoms, thereby providing a guide for the selection thereof in future theoretical and experimental investigations of catalytic reactions on these alloys. Finally we show how the partial pressure of CO may be manipulated to tune active PGM atom ensemble size and to engineer alloy surface architectures that will be of significant use in catalysis.

References

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