The Significance of Gas-Phase Mass Transport in Assessment of kchem and Dchem

In this work, the validity of electrical conductivity relaxation (ECR) as a method for the assessment of chemical surface exchange, kchem, and bulk diffusion, Dchem, coefficients is investigated with respect to mass transport limitations in the gas phase. A model encompassing both the oxygen surface exchange, mass transport in the bulk sample and the gas phase was set up and solved under different conditions using finite element software. It is found that the transport of oxygen in the gas phase is insufficient at low oxygen partial pressures. The effect of mass transport limitations on the computed transport coefficients is shown to be significant and the resulting surface exchange coefficients deviated as much as one order of magnitude from the set values. When mass transport limitations are pronounced, a discrepancy between oxidation and reduction values of the apparent kchem and Dchem is evident and modeled apparent activation energies for kchem are shown to decrease significantly. A criteria for the validity of Dchem is introduced while no such measure could be introduced for kchem. The effect of experimental parameters and material properties on mass transport limitations are presented and general recommendations concerning the assessment of kchem and Dchem are given.