Reductive adsorption of Cd²⁺ on a nickel surface in aqueous solution: Characterization of surface adatoms by in situ EXAFS

Cd²⁺ can be removed from water by reaction with hydrogen adsorbed on the surface of alumina supported nickel particles. A redox reaction between Cd²⁺ and adsorbed hydrogen occurs with formation of Cd ⁽⁰⁾ and two protons. The in situ EXAFS studies were carried out in water at the Cd K-edge during the grafting reaction, and at various nickel coverages by cadmium. At low coverage, the Cd⁽⁰⁾ is interacting with ca. four surface nickel atoms at a distance of 2.64 Å. Cd completes its coordination sphere by interacting with ca. two water molecules at a distance of 2.27 Å. At higher coverage, Cd is bonded to only three surface nickel at a distance of 2.61 Å and to ca. three water molecules, at a distance of 2.25 Å. The rather small Cd-Ni distance found by EXAFS may result from a slight polarization of the Cd-Ni bond. Cadmium deposition is thermodynamically driven by the formation of these Cd-Ni bonds (underpotential deposition) and is then limited to a monolayer of cadmium atoms deposited onto the nickel surface.