Combined Theoretical and Experimental Model of an Oxide-Supported Heterogeneous Catalyst: WO$_{x}$/$\alpha$-Fe$_{2}$O$_{3}$(0001)

Spin-polarized density functional theory (DFT) calculations were combined with surface-sensitive experimental techniques to evaluate models of monolayers and sub-monolayers of catalytic WO$_{x}$ on the (0001) surface of $\alpha$-Fe$_{2}$O$_{3}$ (hematite). Relaxed structures for various surface configurations were calculated, taking into account the surface lattice position and oxygen coordination of W as well as the presence of hydroxyl groups or adsorbed water. These structures were compared to surface atomic density maps generated by synchrotron x-ray standing wave (XSW) imaging under reducing and oxidizing conditions to deduce the most plausible atomic configurations. Theoretical ionicity of W atoms increased with oxygen coordination; as expected, the formal charges (W$^{5+}$ and W$^{6+}$) inferred from x-ray photoelectron spectroscopy (XPS) were not found. Using charge density maps and local densities of states, surface W-O and W-W interactions were studied and compared to Fe-O interactions in the bulk.