XPS study of as-prepared and reduced molybdenum oxides

The surface properties of Mo oxides (MoO2 and MoO3) were investigated using X-ray photoelectron spectroscopy (XPS). As-prepared MoO3 showed two well resolved spectral lines at 232.5 and 235.6 eV, which were assigned to the Mo 3d(5/2), 3d(3/2) spin-orbit components, respectively. After the sample was reduced with H-2 at 673 K for 3 h, 76% of the MoO3 present on the surface was transformed to MoO2. Hydrogen reduction of MoO2 produced a new state (Mo-delta+, 0 < delta < 4) at a binding energy of 228.7 +/- 0.2 eV. We assigned this new state to Mo3+ based on our binding energy assignments for Mo-0, Mo4+, Mo5+, and Mo6+. The concentration of Mo-delta+ state increased with increasing reduction time and this state was suggested to contain hydrogen, which may be a necessary requisite for desulfurization and denitrogenation reactions.