Structure and properties of a model oxide-supported catalyst under redox conditions: WOx/α-Fe2O3 (0001)

a r t i c l e i n f o Keywords: Oxide surface Metal adsorption Density functional X-ray standing wave XAFS 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 2 O 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. Low-coverage layers of transition metals or their oxides atop oxide substrates of different compositions are of interest for various applications, most notably gas sensing and catalysis [1,2]. For oxide-supported heterogeneous catalysts, the catalytic selectivity and activity for specific reactions are defined by the composition of both the oxide substrate and the metal or metal oxide overlayer and the structure of the interface. The atomic and electronic structures of surface active sites must be well defined in order to understand the catalytic mechanism and for the rational design of reaction pathways with catalysts [3]. In previous work [4,5], density functional theory (DFT) has been combined with surface-sensitive X-ray techniques to characterize monolayer and sub-monolayer coverage vanadium oxides (VO x) on α-Fe 2 O 3 (hematite) (0001) surfaces under relevant chemical conditions. X-ray photoelectron spectroscopy (XPS) was employed to probe chemical changes when the surface was subjected to oxidizing and reducing conditions [6]. X-ray standing wave (XSW) imaging provided a sub-atomic-scale map of V positions relative to the hematite lattice under the same chemical conditions. Each of these methods revealed that chemically induced changes in the surface structure were reversible, indicating " redox reversibility " , an important quality in catalytic behavior. With the use of the same experimental methodology, the structure and chemical properties of a similar heterogeneous catalyst [7], tungsten oxide (WO x) atop the hematite (0001) surface, have recently been elucidated [8]. While XSW has provided insight on the positions of …