Structure and properties of a model oxide-supported catalyst under redox conditions: WO _x/α-Fe ₂O ₃ (0001)

Relaxed structures and the related electronic environments of atomic monolayers and half-monolayers of tungsten with varying degrees of oxidation on the (0001) surface of hematite (α-Fe ₂O ₃) are modeled using first-principles density functional theory (DFT). This report focuses on the effect of nominally oxidizing and reducing chemical environments on surface structure and chemistry. By considering the position of W atoms relative to the substrate, calculated surface structures are compared to synchrotron X-ray standing wave (XSW) imaging results recently reported for this system. The question of W valence state, previously reported as nominally W ⁵⁺ or W ⁶⁺ in reducing or oxidizing surroundings, respectively, is addressed and discussed in light of X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (XAFS) results to clarify the relationship between valence state, oxygen coordination, and bond lengths.