Theoretical study of CO oxidation on Au nanoparticles supported by
MgO(100).

Theoretical study of CO oxidation on Au nanoparticles supported by MgO(100).
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by L. M. Molina and B. Hammer

Phys. Rev. B **69**, 155424 (2004).

Abstract
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We present a density-functional-theory (DFT) study of the reactivity
towards CO oxidation of Au nanoparticles supported by MgO(100). We model
two geometrical aspects of the Au particles, the low index facets of the
Au particles, and the Au-MgO interface boundary. The precise structure
of the interface boundary depends on the size of the Au particles, and
different models with either small or large Au-MgO contact angles are
introduced. For all Au systems, we find that the CO oxidation reaction
proceeds via CO adsorption, trapping of O\ :sub:`2`, leading to the
formation of a metastable CO-O\ :sub:`2` reaction intermediate, which
dissociates into CO\ :sub:`2` and adsorbed atomic oxygen. The atomic
oxygen reacts directly with gas phase CO. No separate O\ :sub:`2`
molecular or dissociative adsorption is found to be favorable. Important
differences were found in the reactivity of the various Au-MgO interface
boundaries. This is explained in terms of two properties: the Au-Au
coordination determining the local reactivity of the Au atoms and the
presence of the MgO support that, besides providing excess electrons to
the Au clusters, forms ionic bonds to the peroxo part of the
CO-O\ :sub:`2` reaction intermediate. We find that the type of interface
boundary likely to be predominant for medium-sized nanoparticles
provides the optimal degree of low-coordinated Au atoms in the
neighborhood of the MgO support. Our DFT study therefore provides a
rational for why the reactivity per site may reach a maximum at a
critical particle size as has been observed experimentally for similar
systems.