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Abstract

The selective oxidation of light alkanes from natural gas to olefins and oxygenates might become increasingly important for the chemical industry due to the increasing shortage of crude oil resources. In general, metal oxides are used to catalytically oxidize alkanes to olefins or oxygenates. During the reaction charge carriers (electrons, holes, oxygen atoms) are transferred. The influence of charge transport properties on the catalytic performance can be studied by electrical conductivity measurements. In the past, these investigations were usually done by contacting pressed powders by two or four electrodes and detecting the DC or AC electrical resistance or conductance, respectively. However, the measured data are highly dependent on the quality of the contact between powder and electrodes, which might even change under reaction conditions, and thus falsify the data. In contrast, the microwave cavity perturbation technique (MCPT) [1], which was pioneered by Slater[2], can be used to measure the catalyst without electrode contacting. The contact-free technique allows the measurement of powder catalysts and their catalytic performance under real reaction conditions in a fixed bed reactor, thereby completely avoiding contact resistance and mass transport limitations. The principle of the measurement is based on the fact that the resonance frequency and the quality factor change after insertion of a sample into a microwave resonator due to its dielectric properties (ε= ε₁+iε₂). From these changes, the electrical conductivity can be directly determined, σ= ε₀·ε₂·ω (with ε₀ being the vacuum permittivity, and ω the angular frequency). We are studying the microwave conductivity of different heterogeneous catalysts for the selective oxidation of light alkanes, e.g. (VO)₂P₂O₇ for n-butane oxidation[3] or MoVTeNbOx (M1) for propane oxidation[4]. As a result, active and selective catalysts show a strong and reversible conductivity response on the applied oxidative and reducing reaction feeds, respectively [5]. The results could be rationalized by X-ray photoelectron spectroscopy proving that the catalysts can be described as semiconducting gas sensors with gas phase dependent work functions and band bending[4]. Furthermore, we are investigating the frequency dependence[6] of the conductivity to elucidate the influence of grain boundaries, as well as of bound and free, bulk and surface charge carriers on the interfacial charge transport between catalyst and gas phase, with the aim to find physical descriptors for active and selective catalysts in order to predict and rationally design better catalysts.