The Oxidation of Rhenium and Identification of Rhenium Oxides During Catalytic Partial Oxidation of Ethylene: An In-Situ XPS Study

Rhenium is catalytically active for many valuable chemical reactions, and consequently has been the subject of scientific investigation for several decades. However, little is known about the chemical identity of the species present on rhenium surfaces during catalytic reactions because techniques for investigating catalyst surfaces in-situ such as near-ambient-pressure X-ray photoemission spectroscopy (NAP-XPS) have only recently become available. In the current work, we present an in-situ XPS study of rhenium catalysts. We examine the oxidized rhenium species that form on a metallic rhenium foil in an oxidizing atmosphere, a reducing atmosphere, and during a model catalytic reaction (i.e. the partial-oxidation of ethylene). We find that, in an oxidizing environment, a Re2O7 film forms on the metal surface, with buried layers of sub-oxides that contain Re4+, Re2+ and Re delta+ (delta similar to 1) species at the Re2O7/Re interface. The Re2+ containing sub-oxide is not a known bulk oxide, and is only known to exist on rhenium-metal surfaces. The Re2O7 film sublimes at a very low temperature (ca. 150 degrees C), while the Re4+, Re2+ and Re8+ species remain stable in oxidizing conditions up to at least 450 degrees C. In a reducing atmosphere of 112, the Re2+ species remain on the surface up to a temperature of 330 degrees C, while Re delta+ species can be detected even at 550 degrees C. Under conditions for partial-oxidation of ethylene, we find that the active rhenium catalyst surface contains no bulk-stable oxides, but consists of mainly Re2+ species and small amounts of Re4+ species. When the catalyst is cooled and inactive, Re2O7 is found to form on the surface. These results suggest that Re2+ and Re4+ species may be active species in heterogeneous rhenium catalysts.