Ambient-Pressure
Soft X‑ray Absorption Spectroscopy of a Catalyst Surface in
Action: Closing the Pressure Gap in the Selective <i>n</i>‑Butane Oxidation over Vanadyl Pyrophosphate
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Abstract
In order to close the pressure gap
in the investigation of catalyst surfaces under real operation conditions
we have developed a variable-pressure soft X-ray (<i>h</i>ν ≤1.5 keV) absorption cell coupled to a gas analysis
system to study the pressure dependency of the electronic and catalytic
properties of catalyst surfaces in reactive atmospheres at elevated
temperatures. With this setup we investigated the vanadium L<sub>3</sub>-edge and catalytic performance of polycrystalline vanadyl pyrophosphate
in the selective oxidation of <i>n</i>-butane to maleic
anhydride between 10 and 1000 mbar at 400 °C. As a result, major
gas phase and pressure dependent spectral changes are observed at
energies attributed to V 2p-3d<sub><i>z</i><sup>2</sup></sub> excitations assigned to vanadium atoms square-pyramidally coordinated
to oxygen atoms. This can be interpreted in terms of a shortened vanadyl
bond (VO) and an increased vanadium oxidation state with higher
pressures. Since this is accompanied by an increasing catalytic activity
and selectivity, it indicates that vanadyl oxygen is actively involved
in the selective oxidation of the alkane