(727c) Developing Stability Criteria of Isolated Metal Adatoms on a Well-Defined Copper Oxide Film

Abstract

Developing Stability
Criteria of Isolated Metal Adatoms on a Well-Defined Copper Oxide Film

Authors: Nisa
Ulumuddin, Kyle Groden, Alex Schilling, E. Charles H. Sykes, Jean-Sabin McEwen

Single-site catalysts (SSC) are attractive materials due
their high selectivity and atomic efficiency.¹
In the case of SSC’s consisting of supported metal adatoms, strong interactions
between the metal species and the support are required to anchor the adatom on
the surface. Oxides are found to be effective supports for metal adatoms,
preventing their agglomeration and thus promoting the catalyst’s thermal
stability.^2-3
Despite this, adatoms can still diffuse into the bulk, causing the catalyst to
deactivate. STM measurements of a thin surface oxide film, specifically the “29”
Cu_xO/Cu(111) surface oxide, with atomically dispersed Pt after CO
oxidation,  found that some Pt adatoms previously planted on its surface were
missing, suggesting that Pt was able to go subsurface after the reaction.²
This brings us to the investigation of the segregation tendencies of various
metal adatoms on a thin-film copper oxide surface, using a model “29” Cu_xO/Cu(111)
surface oxide as an example.

In this work, we determine the segregation tendencies of Ag,
Au, Pd, Pt and Rh adatoms on a pristine “29” Cu_xO/Cu(111) surface
from first principles using density functional theory based calculations (schematics
in Figure 1). To deconvolute the properties which govern the segregation of the
metal (M) adatom from the surface, we first correlate the adatom segregation
energy (Figure 1) to the formation energy of a bulk Cu₃M alloy, the bulk
metal surface energy, and the strain energy induced from fitting the metal adatom
into a fixed Cu lattice. We also perform a differential charge analysis and oxygen
binding energy calculations to determine how oxygen adatoms affect the
segregation tendencies on the surface of an oxide film. The oxide film’s influence
is further isolated through a comparison of the segregation energy trends on a
Cu surface in the presence and in the absence of a surface oxide. We find that metals
with surface energies that are lower than that of Cu result in favorable adatom
segregation energetics. The ability of a metal adatom to segregate from Cu increases
in the following order: Rh, Pt, Pd, Au, and Ag, where Ag is the least likely to
diffuse into the bulk. A density of states (DOS) analysis confirms that
segregation is favorable for metal adatoms with d-band centers that are further
away from the Fermi level than Cu. A more segregated adatom also binds weaker
to CO.  We thus propose the difference between the surface energy of the bulk
adatom metal and that of Cu as a descriptor for its segregation tendency on the
“29” Cu_xO/Cu(111) surface. Using this information, we can now
predict stable adatoms to be supported on “29” Cu_xO/Cu(111) before
investing into more computationally expensive calculations.

Figure
1: Schematic of the adatom and alloyed structures used to calculate
segregation energies.

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