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Research Topic:
  Multifrequency noncontact in situ conductivity measurements of heterogeneous catalysts under working conditions
Maria Heenemann

In a simple approximation particles and grains in a powder catalyst could be physically described as plates of a capacitor. In between the two plates (or particles) the electrical conductivity is usually very low, as described by the dielectric medium in the capacitor model. Since the corresponding capacitive resistance Xc of the capacitor with capacitance C decreases with increasing angular frequency ω after Xc = -1/(ωC), the frequency dependent measurement of the conductivity of a powder will allow the differentiation between intrinsic electronic properties of the catalyst and properties due to grain boundaries and particle interfaces. Besides, the obtained frequency dependence will also provide insights into the conductivity mechanism (e.g. band or hopping conduction), the nature of the charge carriers (e.g. free or bound/localized charges) and the contribution of dielectric relaxation effects to the measured microwave conductivity.

For this purpose, microwave resonators with eigenfrequencies between 1 and 20 GHz are to be developed and quartz tube reactors connected to gas delivery manifolds and gas analytic systems are to be implemented (cf. Figure). Cylindrical cavities sustaining the TM010 mode are to be built for the L, S and X band region of the electromagnetic spectrum and the higher TM0n0 modes are to be used to allow measurements at even more frequencies. The project aims at investigating the frequency dependence of the conductivity of powder catalysts for the selective oxidation of light alkanes. The electrical conductivity of the samples is to be deduced after the principles of the microwave cavity perturbation technique. [1]

Quartz tube reactor connected to
gas delivery manifolds
and gas analytic systems.

(to see full size, click on the image)
References
  1. M. Eichelbaum, et al.,
    Phys. Chem. Chem. Phys 2012, 14, 1302-1312.
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