  The present density functional theory study addresses the question whether
the presence of H2O influences the catalytic activity of small gold clusters,
Au1-4/MgO(100), towards the oxidation of carbon monoxide. To this end, we
studied the (co-)adsorption of H2O and CO/O2 on these gold clusters. The ground
state structures in the presence of all three molecular species, that we found,
are Au1O2/MgO and Au2-4CO/MgO with H2O adsorbed on the surface in the proximity
of the clusters-molecule complex. In this configuration the catalytic activity
of Au1-4/MgO is indifferent to the presence of H2O. We also found that a
stable, highly activated hydroperoxyl-hydroxyl complex, O2H\dot\dot OH, can be
formed on Au1,3/MgO. For the catalytic active system Au8/MgO, it has been
predicted that this complex opens an alternative catalytic reaction pathway
towards CO oxidation. Our results suggest that this water mediated catalytic
cycle is unlikely to occur on Au1,3/MgO. In the case of Au1/MgO the cycle is
interrupted by the dissociation of the remaining (OH)2 complex after forming
the first CO2 molecule. On Au3 /MgO the O2 H\dot\dot OH complex is likely to
decompose upon the impact of a CO molecule, since three of its bond
dissociation energies are comparable to the reaction barrier of the CO to CO2
oxidation.
