Surface Reactions
The equations solved for the transport for each species need to be supplemented by appropriate boundary conditions. This is often accomplished by way of a set of suitable surface reactions, which describe how species are created or consumed on surfaces.
The rate expression for a surface species k, Rsurf, k (SI unit: mol/(m2·s)), is determined by the stoichiometry of the system as
where νkj are the stoichiometric coefficients, defined as negative for reactants and positive for products, and qj is the reaction rate corresponding to reaction j. The reaction rates depend on the order of the reaction and the molar concentration of each species. In compact form, the equation for the jth reaction rate is
where the following definitions apply for ck. When the Motz–Wise correction option is set to On, the forward rate constant is related to the sticking coefficients via
(5-19)
When the Motz–Wise correction option is set to Off, the forward rate constant is given via
(5-20).
where Γtot is the total surface site concentration (SI unit: mol/m2), m is the sum of all of the surface reactants, and σi is the site occupancy number. The sticking coefficient can be a constant or even temperature dependent through an Arrhenius expression:
where a, b, and e are constants. Since γ is a probability it must be between 0 and 1 to be physically meaningful.