Electrochemical Oxidation of Molybdenum Metal in 0.5 M H₂SO₄ Studied by Core and Valence Band X-ray Photoelectron Spectroscopy and Interpreted by Band Structure Calculations

The behavior of molybdenum metal in 0.5 M sulfuric acid treated at various potentials has been studied using core and valence band X-ray photoelectron spectroscopy in a previously described anaerobic cell. Valence band spectra of molybdenum and its oxidized species were interpreted by ab initio band structure calculations. The study indicates that Mo(IV) and Mo(VI) (including possibly Mo(V) in some cases) were formed, in a mixed film that included some metal, as the potential of the system was increased within the active range of molybdenum. Three regimes were found. At potentials at or below 0.0 V ( vs the saturated calomel electrode) the metal remained practically unoxidized. At potentials between 0.5 and 2.0 V about 75% of the mixed film consisted of oxidized molybdenum with the predominant oxide being Mo(IV). When the polarization time at 2.0 V increased the mixed film continued to consist of about 80% of oxidized molybdenum, but the predominant oxide was Mo(V/VI), and the film became mechanically unstable, eventually falling off the metal.