Beschreibung:
<jats:title>Abstract</jats:title>
<jats:p>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).</jats:p>
<jats:p>We find that, in an oxidizing environment, a Re<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> film
forms on the metal surface, with buried layers of sub-oxides that
contain Re<jats:sup>4+</jats:sup>, Re<jats:sup>2+</jats:sup> and Re<jats:sup>
<jats:italic>δ</jats:italic>+</jats:sup> (<jats:italic>δ</jats:italic> ∼ 1)
species at the Re<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub>/Re interface. The Re<jats:sup>2+</jats:sup> containing
sub-oxide is not a known bulk oxide, and is only known to exist on
rhenium-metal surfaces. The Re<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> film sublimes at a very
low temperature (<jats:italic>ca.</jats:italic> 150 ℃), while the
Re<jats:sup>4+</jats:sup>, Re<jats:sup>2+</jats:sup> and Re<jats:sup>
<jats:italic>δ</jats:italic>+</jats:sup> species remain stable in
oxidizing conditions up to at least 450 ℃. In
a reducing atmosphere of H<jats:sub>2</jats:sub>, the Re<jats:sup>2+</jats:sup> species remain on the
surface up to a temperature of 330 ℃, while
Re<jats:sup>
<jats:italic>δ</jats:italic>+</jats:sup> species can be detected even at
550 ℃.</jats:p>
<jats:p>Under conditions for partial-oxidation of ethylene, we find that the
active rhenium catalyst surface contains no bulk-stable oxides, but
consists of mainly Re<jats:sup>2+</jats:sup> species and small amounts of Re<jats:sup>4+</jats:sup>
species. When the catalyst is cooled and inactive, Re<jats:sub>2</jats:sub>O<jats:sub>7</jats:sub> is
found to form on the surface. These results suggest that Re<jats:sup>2+</jats:sup> and
Re<jats:sup>4+</jats:sup> species may be active species in heterogeneous rhenium
catalysts.</jats:p>