The Coordination Chemistry of the Oxide-Electrolyte Interface; The Dependence of Surface Reactivity (Dissolution, Redox Reactions) on Surface Structure

Many heterogeneous processes (formation and dissolution of solid phases, redox and photochemical processes at the solid- -water interface are 'kinetically controlled by a reaction step at the surface (and not by a transport step). Obviously the surface reactivity depends on the surface species and their structural identity, which — in turn — depend on the coordination chemical interactions that occur at the solid water interface. We discuss these processes for oxide-water interfaces in terms of a unifying rate law: R = kxa P} S, where R is the reaction rate [imolm'2 s'1], xa denotes the mole fraction of the reaction-active surface sites [—], Pj represents the probability to find a specific site in a suitable coordinative arrangement [—], and S is the surface concentration of sites [molm'2]. Thus, for example, the dependency of the dissolution rate of an oxide mineral on pH can be explained in terms of surface protonation (or surface deprotonation). Similarly the effect of ligands such as oxalate on the dissolution can be accounted for by the concentration of ligand surface complexes. We extend this concept to the reductive dissolution of iron(III) hydroxides, to the oxidation of transition metal ions and other redox- and photorerox-reactions occurring at hydrous oxide surfaces and illustrate the dependence of reaction rates on specifically adsorbed oxidants and feductants. The cycling of iron as it occurs in natural systems (water, sediments, soils and atmosphere), dis used to illustrate the various redox processes, including photocatalyzed reactions, that are mediated by surfaces. Furthermore, we try to illustrate that the concepts of surface reactivity should be applicable to the interpretation of corrosion reactions, specyiifically the passivity of iron oxide films.