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Surface Reaction And Diffusion

diffusion boundary conditions modeling solid-gas reaction

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#1 angie88


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Posted 23 May 2019 - 01:46 AM



I’m trying to model a heterogeneous reaction and determine a concentration profile on a solid.


I have a gaseous atmosphere in contact with a solid. Gaseous species A adsorbs on the solid. Once adsorbed, it dissociates on the surface and produces species B. Species B then diffuses down the solid.


Basically, at the surface of the solid:

A(g) à A(ads) à B(ads) and then B diffuses


Diffusion can be modeled by Fick’s second law but I am having problems determining the boundary condition at the surface (left boundary). The right boundary condition is a null flux.


The reaction leading from A to B is assumed to be a first order reaction proportional to the concentration of species A in the gaseous atmosphere, r=k[A]. I do not know how to relate the concentration of species A(g) to the concentration of B(ads) at the solid surface.


This is similar to heterogeneous catalytic reactions but in previous courses we never determined what happened in the solid because there was desorption of the products instead of diffusion.


Any help is appreciated, thank you


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