Experimental and Theoretical Characterization of Superoxide Complexes [W2O6(O2-)] and [W3O9(O2-)]: Models for the Interaction of O2 with Reduced W Sites on Tungsten Oxide Surfaces

摘要

High-valent transition-metal oxides (for example, VV, MoVI, WVI) are employed in a variety of important oxidation processes, and atmospheric dioxygen is commonly employed as a terminal oxidant.[1] Reoxidation of the metal oxide catalyst proceeds by a complex series of redox reactions, with the initial step generally assumed to be electron transfer from a reduced metal site to dioxygen to form a bound superoxo complex.[2] Metal oxide clusters are being actively studied as model systems to obtain molecular-level information for surface and catalytic processes.[3], [4] Herein we present a joint experimental and theoretical study of two O-rich tungsten oxide clusters, [W2O8]- and [W3O11]-. Their electronic and geometric structures and chemical bonding were investigated by using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The two anionic clusters are characterized as [W2O6(O2-)] and [W3O9(O2-)], respectively, that is, a superoxide species interacting with the neutral clusters [W2O6] and [W3O9] (chemisorption). In contrast, the neutral [W2O8] and [W3O11] clusters are found to contain an O2 molecule weakly interacting with the [W2O6] and [W3O9] clusters (physisorption). The [W2O8]- and [W3O11]- clusters can be considered to be formed by nondissociative electron transfer of the single W 5d electron in [W2O6]- and [W3O9]- to dioxygen (W 5dO2 *) and are thus ideal molecular models for understanding the importance of reduced metal sites for the activation of O2 on metal oxide nanostructures and surfaces.