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^δ+ (δ ∼ 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 ℃), while the Re⁴⁺, Re²⁺ and Re^δ+ species remain stable in oxidizing conditions up to at least 450 ℃. In a reducing atmosphere of H₂, the Re²⁺ species remain on the surface up to a temperature of 330 ℃, while Reδ+ species can be detected even at 550 ℃. 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