XPS studies of MoO3/Al2O3 and MoO3/SiO2 systems

The detailed analysis of the experimental XPS envelopes of the model MoO3/Al2O3 and MoO3/SiO2 systems was performed using a curve fitting procedure. It appeared possible to fit the XPS spectrum of MoO3/Al2O3 system with a single Mo 3d5/2-Mo 3d3/2 doublet which indicates the presence of only one type of molybdenum(VI) oxo-species In contrast, curve fitting for MoO3/SiO2 system required two individual Mo 3d5/2-Mo 3d3/2 doublets. The first Mo 3d5/2-Mo 3d3/2 doublet with the binding energy close to that of bulk MoO3 was attributed reflects the to three-dimensional molybdenum(VI) oxo-species. The second Mo 3d5/2-Mo 3d3/2 doublet, shifted to higher binding energy, presence of monomeric or two-dimensional molybdenum(VI) oxo-species which strongly interact with silica surface.