Predicting multicomponent diffusivities for diffusion on surfaces and in molecular sieves with energy heterogeneity

A simple theory is derived for predicting multicomponent diffusion on solid surfaces and in molecular sieves with energetic heterogeneity. The energetic heterogeneity is represented by the uniform energy distribution and the equilibrium adsorption is assumed to follow the Langmuirian behavior. Multicomponent Fickian diffusivities can be predicted from pure-component Fickian diffusivities. The required information for the calculation includes the concentration-dependent pure-component diffusivities and the pure-component adsorption isotherms. The effects of the energetic heterogeneity can be significant, depending on the mutual direction of diffusion (co-diffusion or counter-diffusion), the initial and final surface coverages, and the relative diffusivities of the components. The effects of heterogeneity are stronger on the faster diffusing component. The effect of heterogeneity becomes stronger as the total surface coverage increases. The theory compares favorably with the available experimental data.