(93h) Influencing the Surface Morphology of Highly Dilute Alloy Surfaces: An Ab Initio Monte Carlo Approach

Materials with excellent catalytic performance can be produced by the addition of small amounts of a reactive platinum group metal (PGM) into a matrix of a more inert coinage metal (e.g. Cu, Au, Ag).^1,2 However, the catalytic properties of these highly dilute alloys are strongly dependent on the morphology of the surface and more specifically on the “ensemble” size and geometry of the PGM clusters.^3–7 Consequently, if one could discover means to control the ensemble size and geometry, one could also optimize the catalytic performance of the alloy for a specific application.

In this work, we explore the idea of establishing control over the ensemble size of the PGMs within a coinage metal host upon exposure of the highly dilute alloy surface to CO, a strongly binding adsorbate. We extensively study three highly dilute alloy (111) surfaces, Pt/Cu(111), Ni/Cu(111) and Rh/Cu(111), which have been the focal point of numerous theoretical and experimental investigations. Based on energetics that we have previously computed by means of Density Functional Theory (DFT),^8,9 we parameterize on-lattice Monte Carlo (MC) simulations to investigate the effects of dopant loading and CO partial pressure (P_CO) on surface morphology and the arrangement of PGM atoms thereon.

Under conditions resembling those of an ultra-high vacuum system, we find that there is a significant amount of PGM dimers on the Ni/Cu(111) surface. Conversely, we find that under the same conditions the dispersion of PGM atoms as isolated single atoms (i.e. the single atom alloy phase) is favored on Pt/Cu(111) and Rh/Cu(111) surfaces. In all cases, the formation of small PGM aggregates is promoted by applying low P_CO, whilst exposure to high P_CO causes the “breaking” of such aggregates, thereby promoting the single atom alloy phase.

References

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² M.D. Marcinkowski, M.T. Darby, J. Liu, J.M. Wimble, F.R. Lucci, S. Lee, A. Michaelides, M. Flytzani-Stephanopoulos, M. Stamatakis, and E.C.H. Sykes, Nat. Chem. 10, 325 (2018).

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⁸ M.T. Darby, E.C.H. Sykes, A. Michaelides, and M. Stamatakis, Top. Catal. 61, 428 (2018).

⁹ K.G. Papanikolaou, M.T. Darby, and M. Stamatakis, J. Phys. Chem. C 123, 9128 (2019).