We present here the results of our density-functional-theory-based
calculations of the electronic and geometric structures and energetics of Se
and O adsorption on Ru 93- and 105-atom nanoparticles. These studies have been
inspired by the fact that Se/Ru nanoparticles are considered promising
electrocatalysts for the oxygen reduction reaction (ORR) on the direct methanol
fuel cell cathodes and the oxygen binding energy is a descriptor for the
catalyst activity towards this reaction. We find the character of chemical
bonding of Se on a flat nanoparticle facet to be ionic, similar to that
obtained earlier for the Se/Ru(0001) surface, while in the case of a low
coordinated Ru configuration there is an indication of some covalent
contribution to the bonding leading to an increase in Se binding energy. Se and
O co-adsorbed on the flat facet, both accept electronic charge from Ru, whereas
the adsorption on low-coordinated sites causes more complicated valence charge
redistribution. The Se modification of the Ru particles leads to weakening of
the oxygen bonding to the particle. However, overall, O binding energies are
found to be higher for the particles than for Se/Ru(0001). High reactivity of
the Se/Ru nanoparticles found in this work is not favorable for ORR. We thus
expect that larger particles with well-developed flat facets are more efficient
ORR catalysts than small nanoparticles with a large fraction of
under-coordinated adsorption sites
