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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.
