Direct atomic-scale observation of redox-induced cation dynamics in an oxide-supported monolayer catalyst: WO_x/α-Fe₂O ₃(0001)

(Figure Presented) For the case of one-third of a monolayer of tungsten grown by atomic layer deposition on a hematite α-Fe₂O ₃(0001) surface, we report direct atomic-scale observations of the structural and chemical changes that occur as this model interfacial system evolves from the as-deposited state to the oxidized state, then to the reduced state, and finally back to the oxidized state. In situ X-ray standing-wave atomic images relative to the hematite lattice show dramatic (but redoxreversible) changes to the W cation locations with respect to the occupied and unoccupied Fe surface sites. These structural changes are concurrent with X-ray photoelectron spectroscopy chemical shift changes, where W is observed to go from the 6+ to 5+ oxidation state. These novel observations of redox-induced cation dynamics in an oxide-supported catalyst are explained by models that account for W incorporation at the interface in Fe sites with various coordination schemes. Our proposed structure models are supported by our X-ray absorption fine structure measurements.