Transport of adsorbed species occurs in the tangential direction along the surface. The Surface Reactions interface models the tangential flux in the surface dimension, the surface molar flux, Nt,i (SI unit: mol/(m·s)) (the subscript t refers to the tangential direction), as governed by diffusion according to Fick’s law:

where Ds,i (SI unit: m2/s) is the surface diffusion coefficient for species i.

where Rs,i (SI unit: mol/(m2·s)) is the sum of all sources due to surface reactions and adsorption/desorption phenomena.

Of frequent interest for surface reaction kinetics are the fractional surface coverages, θi (dimensionless), of the species (with index i).

Writing the density of sites of the surface as Γs (SI unit: mol/m2) and the site occupancy number for each species as σi (dimensionless), the surface coverages can be calculated from:

For the case of monolayer adsorption, the sum of all fractional coverages of free and adsorbed sites is unity, and hence the fraction of free sites on the surface, θ*, can be calculated from: