Experimental study of the frequency factor in the Polanyi-Wigner equation: the case of C2H6

Abstract

The C2H6 molecule has been used to determine experimentally, for the first time, the frequency factor present in the Polanyi-Wigner equation and to study how temperature influences this magnitude for a zeroth order desorption. This parameter is necessary to calculate the desorption rates for environments in which this process occurs. The method presented is based on the analysis of a quartz crystal microbalance signal. In the literature the frequency factor is not experimentally obtained but is rather assumed to be K-6.T/h (at 50 K), as proposed by the activated state theory for first order desorption processes, or it is estimated by other methods. Additionally, the factor's variation with temperature has not been experimentally explored to date.

Two different types of zeroth order desorption experiments have been designed for this study. The purpose of the first experiment, carried out at a constant rate of warming, is to obtain the desorption energy, which is compared with previous values reported in the literature. The second group of desorption experiments is performed at constant temperatures and is used to calculate and study the frequency factor. Several temperatures have been specifically selected, enabling us to determine the influence of the temperature on this parameter. We have calculated a relationship for the frequency factor and temperature, obtaining an increase of approximately 50% for the frequency factor for an increase of only 6 K. This result must be taken into account when the Polanyi-Wigner equation is used for desorption rate calculations. (C) 2015 Elsevier Ltd. All rights reserved.