Ambient-Pressure Soft X-ray Absorption Spectroscopy of a Catalyst Surface in Action: Closing the Pressure Gap in the Selective n-Butane Oxidation over Vanadyl Pyrophosphate

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 (hv <= 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-3-edge and catalytic performance of polycrystalline vanadyl pyrophosphate in the selective oxidation of n-butane to maleic anhydride between 10 and 1000 mbar at 400 degrees C. As a result, major gas phase and pressure dependent spectral changes are observed at energies attributed to V 2p-3d(z)(2) excitations assigned to vanadium atoms square-pyramidally coordinated to oxygen atoms. This can be interpreted in terms of a shortened vanadyl bond (V=O) 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.