The Electronic Structure of Vanadium Oxides as Catalysts in the Selective Oxidation of Small Alkanes

The present work considers vanadium oxides catalysts in the selective oxidation of small alkanes. The dynamics of their (surface) electronic structure modulated by the chemical potential of reaction gases were investigated regarding charge carrier dynamics, surface valence/conduction band structure and work function modifications. The charge carrier dynamics were studied with the in situ microwave cavity perturbation technique allowing the determination of the catalyst conductivity in a contact free manner in a fixed bed reactor geometry. An evaluation program based on the transmission line theory was developed for precise conductivity determination. The validity of the evaluation methods was tested with the n-type semiconducting vanadium pentoxide in the oxidation of n-butane. In agreement with literature, the experiments revealed an n-type conductivity. The addition of n-butane in the reaction feed leads to an increased conductivity corresponding to the abundance of electronically active V4+ defect states (corresponding to oxygen vacancies) in the forbidden bandgap of vanadium pentoxide increasing the mobile electron density. Based on results of the reference study, the selective propane oxidation catalyst MoVNbTeOx M1-phase was investigated in the selective oxidation of ethane, propane and n-butane. Also the impact of water in the propane feed, triggering the abundance of the industrially important key product acrylic acid, on the MoVNbTeOx M1-phase electronic structure was studied. The in situ microwave cavity perturbation studies at ambient pressure were complemented with near ambient pressure X-ray photoelectron and X-ray absorption spectroscopy investigations at 0.25 mbar to understand the charge transfer processes according to semiconductor physics. The conductivity of MoVNbTeOx M1-phase increased with increasing propane to oxygen ratio identifying MoVNbTeOx M1-phase as an n-type semiconductor. In the alkane (ethane, propane and n-butane) exchange experiment, the number of electrons transferred to MoVNbTeOx M1-phase increased from ethane, to propane and finally to n-butane oxidation resulting in an increased conductivity. The X-ray photoelectron spectroscopy reveals that the exchange of the alkane leads to a modulation of the V4+/V5+ redox couple at the surface corresponding to shifts of the valence band edge and electron affinity. Thus the surface of MoVNbTeOx M1-phase, being in dynamic equilibrium with the bulk electronic structure, is modified by the compositions (corresponding to the chemical potential) of the gas phase. The bulk charge carrier density is triggered by the barrier height of the surface induced space charge layer resulting in a modified conductivity. In contrast the modulated electron affinity can be explained by a change of the surface dipole. Water in propane feed leads to a decreased conductivity of MoVNbTeOx M1-phase without a modification of the space charge layer. A drastic change of the surface elemental composition, in particular the abundance of V5+, is induced by water, observable in the valence band, core level and vanadium L2,3-edges X-ray absorption spectra. The surface modifications were accompanied with a decreased electron affinity corresponding to a decreased surface dipole. The drastically changed valence and conduction band structure likely affects the charge carrier mobility explaining the decreased conductivity in steam containing propane feed. However, results from low pressure in situ photoelectron studies are debated according to their relevance for "real" catalysis at ambient pressures. In particular the oxygen pressure controls the oxidation state of transition metal oxide surfaces. The vanadium L2,3 X-ray absorption edges of vanadyl pyrophosphate were investigated in the selective n-butane oxidation at 10, 100 and 1000 mbar to identify a possible pressure gap using the surface sensitive conversion electron mode. As a result, at low pressures the oxidation of the surface is controlled by the oxygen pressure. In contrast at higher pressures, the surface state of oxidation is triggered by the catalytic reaction providing a steady state between reduction of the catalyst during n-butane conversion and re-oxidation by molecular oxygen.