Question

4. (a) Show the relationship between current density 0) and difisional flux U). (b) Write down the Fick's first law. (c) Write down the equation showing the relationship between reactant concentration at catalyst layer (CR) and bulk reactant concentration ??. (use for diffusion layer thickness). (d) Find out limiting current density (i), (ef) Find out Nernstian loss in terms of CR/CR and also in terms of j. (30 pts)

Answer 1

(a) Fick's law states that the particle current density is proportional to the concentration gradient and the magnitude of the diffusive flow.

(b) Fick's first law relates to diffusive flow with concentration under the assumption of a steady state. This law postulates that the flow goes from a region of high concentration to regions of low concentration, with a magnitude that is proportional to the concentration gradient (spatial derivative), or in simpler terms the concept that the solute will move from a region of high concentration to one of low concentration crossing a concentration gradient.

(c) The following equation, which relates the concentration of reactant in the catalyst layer and the mass reagent concentration, is the equation of the diffusion velocity:


Where:

Wr = Diffusion Speed
Dra = Difusion Coefficient
Cr0 = bulk Reactant Concentration
Crº = Reagent Concentration in catalyst layer

(d) 1. In non-homogeneous media, the diffusion coefficient varies in space.

2. In anisotropic media, the diffusion coefficient depends on the direction.