Oxide formation at the surface of late 4d transition metals: insights from first-principles atomistic thermodynamics

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. (orig.)