Heterogeneous reactions may occur at the interface between a solid surface and an adjacent gas. Such reactions are central to many important chemical processes including thin-film deposition, chemical or plasma etching, catalytic oxidation, and catalytic conversion processes. Ansys Chemkin Surface Kinetics utilities are based on general and systematic conventions that allow users to describe any level of complexity for such gas-surface interactions.

Surface reactions can involve gas-phase species, surface-site species, and bulk-phase or condensed species. There can be multiple surface (site) phases and multiple bulk phases. Surface reactions may involve conversion between any phases or may involve only one phase. The definitions of these phases and the species that reside in them are provided in more detail in Thermodynamic Expressions of this manual. As with gas-phase reactions, surface reactions must conserve mass, elements, electronic charge, and (usually) surface sites.

This chapter describes basic rate expressions and default assumptions, as well as the wide array of options available to users for describing different types of reactions. While the default formulation assumes elementary reactions, where law of mass action is assumed, users may also easily specify global reaction rates. Global rate expressions can use arbitrary reaction orders and non-integer stoichiometric coefficients. In addition, users can directly input parameters to express Langmuir-Hinshelwood and Eley-Rideal rate expressions.