Charge Transport in Vanadia Oxidation Catalysts

An efficient direct functionalization of light alkanes is highly desirable to achieve a more sustainable raw material usage in chemical industry. Our knowledge of heterogeneous selective alkane oxidation catalysts is still largely based on empirical concepts. There is growing evidence that in addition to the traditional (local) concepts the underlying bulk electronic structure should be considered. The aim of this thesis is to further extend current understanding of the selective alkane oxidation over vanadium-based transition metal oxides. Within the framework of a redox mechanism, the role of charge transfer and charge transport in vanadia oxides was investigated under catalytic operation conditions. For this reason, contact-free electrical conductivity or complex permittivity measurements based on the microwave cavity perturbation technique (MCPT) were performed, which enables the non-invasive and highly sensitive investigation of semiconducting powder catalysts under operando conditions. Complementary techniques like near-ambient pressure XPS and resonant photoemission spectroscopy, NEXAFS spectroscopy, or UV-Vis spectroscopy were applied to get additional information about the surface electronic structure and surface composition and the redox response of the catalysts to different gas atmospheres. Non-stoichiometric V2O5−x showed reversible changes in the vanadium oxidation state in n-butane- and O2-containing gas atmospheres, which were not restricted to the surface. A band gap peak caused by occupied V 3d levels adapted to the different gas feeds as was clearly demonstrated by resonant photoemission spectroscopy (resPES). This indicates that a redox mechanism is indeed operative over V2O5−x, but unlike in the selective catalysts VPP or MoVTeNb-oxide also deeper layers may contribute. Different decay channels contribute to the resPES spectra, which were likewise affected by the treatment of the sample in different gas feeds. A comparative study on V2O5−x and VPP in the oxidation of n-butane to maleic anhydride was performed. The electrical conductivity was shown to be a sensitive function of the local chemical potential and closely linked to the catalytic reaction at the surface being also affected by the reactant conversion level. Although many factors are likely contributing to the selectivity or the activity of a partial oxidation catalyst, non-local electronic properties are part of the phenomenon. The following parameters were identified: (i) the conductivity of V2O5−x is ∼50 times higher compared to VPP under isothermal and iso-conversion conditions, (ii) the apparent activation energy of conduction (Ec) of V2O5−x is four to eight times lower than the one of VPP, (iii) the extent of the conductivity/permittivity change with varying conversion levels under isothermal conditions (i.e., with the number of exchanged electrons during the reaction) is much greater in V2O5−x. A stable catalytic performance retaining a high selectivity is related to low conductivity changes at various conversion levels and a “stable” surface layer. These conclusions could further be extended to the mixed-metal MoV-oxide in the orthorhombic M1 phase, whose semiconducting properties were studied in the selective oxidation of ethane, propane, and n-butane. Furthermore, MoV-oxide was found to exhibit p-type semiconducting behavior contrary to MoVTeNb-oxide (M1)[1] as seen in a reversed conductivity response. The thus changed availability of either free holes or electrons may have an influence on their respective catalytic performance. The addition of steam to the propane oxidation feed leads to an enhanced acrylic acid formation. It further resulted in the modification of the surface termination and a significant decrease of the work function by 0.2 eV. This may rationalize the conductivity or permittivity decrease in the wet propane oxidation feed, which was observed at the same time and is probably not caused by a change in the charge carrier density. Also, the simultaneously changed chemical potential of the gas phase including the changed product distribution (increased selectivity) is suggested to be intricately linked to the conductivity/permittivity response. Taken together, the findings of this thesis show the relevance of electronic properties for the catalytic performance of heterogeneous selective oxidation catalysts