Correlation of TPD and impedance measurements on the desorption of NH3 from zeolite H-ZSM-5

Abstract We compare the characteristic features obtained in TPD and in impedance measurements on the desorption of ammonia from zeolite H-ZSM-5. We show that the ammonia desorption peaks observed by TPD correspond to characteristic changes in the proton transport mechanism. While at low temperature an almost temperature independent Grotthus-like proton conductivity in the presence of ammonia is observed, the on-set of the low temperature desorption peak in TPD coincides with a change in the transport mechanism to a vehicle-mediated transport of protons. Furthermore, at higher temperature, the desorption of ammonia from the Bronsted acid sites, as indicated by the high temperature peak in TPD, causes again a characteristic change in the temperature dependent conductivity, reflected by a decreasing conductivity with increasing temperature in an intermitted range, which then turns over again to simply activated proton hopping.

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