  Using density-functional theory we assess the stability of bulk and surface
oxides of the late 4d transition metals in a ``constrained equilibrium'' with a
gas phase formed of O2 and CO. While the stability range of the most stable
bulk oxide extends for ruthenium well into gas phase conditions representative
of technological CO oxidation catalysis, this is progressively less so for the
4d metals to its right in the periodic system. Surface oxides could
nevertheless still be stable under such conditions. These thermodynamic
considerations are discussed in the light of recent experiments, emphasizing
the role of (surface) oxides as the active phase of model catalysts formed from
these metals.
