A reactive
force field has been developed that is used in molecular
dynamics (MD) studies of the surface transformation of the cobalt
(0001) surface induced by an overlayer of adsorbed carbon atoms. Significant
surface reconstruction is observed with movement of the Co atoms upward
and part of the C atoms to positions below the surface. In a particular
C ad atom coverage regime step edge type surface sites are formed,
which can dissociate adsorbed CO with a low activation energy barrier.
A driving force for the surface transformation is the preference of
C adatoms to adsorb in 5- or 6-fold coordinated sites and the increasing
strain in the surface because of the changes in surface metal atom–metal
atom bond distances with the increasing surface overlayer concentration.
The process is found to depend on the nanosize dimension of the surface
covered with carbon. When this surface is an overlayer on top of a
vacant Co surface, it can reduce stress by displacement of the Co
atoms to unoccupied surface positions and the popping up process of
Co atom does not occur. This explains why small nanoparticles will
not reconstruct by popping up of Co atoms and do not create CO dissociation
active sites even when covered with a substantial overlayer of C atoms.
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