Adsorption of halide ions from aqueous solutions at a Cd(0Â 0Â 0Â 1) electrode surface: quantum chemical modelling and experimental study

Adsorption of the halide ions X (X = Cl−, Br−, I−) at a cadmium monocrystalline face (0 0 0 1) from aqueous solutions has been studied by the impedance spectroscopy. Analysis of the impedance data does not predict specific adsorption for Cl− and shows that the Gibbs adsorption energy increases from Br− to I− in accordance with the decrease of the hydration energy of anions. The adsorption of halide ions and their atomic forms on a Cd(0 0 0 1) electrode at the potential of zero charge (pzc) has been studied with the use of the cluster model. The quantum chemical calculations were performed at the DFT and the SCF levels involving the hydrated species X−(H2O)3 and X−(H2O)6 besides the bare anions. The effective distance-dependent solvation energy of halide ions constructed on the basis of molecular dynamics simulations was used to build the adsorption terms. For all three halides contact adsorption was found to be the most favourable state. The partial charge transfer (PCT) from the adsorbed species to the metal was analysed in terms of the Anderson-Newns model. A combined consideration of the solvation and PCT effects enables prediction of the experimentally observed metal-ion interaction order (Cl− < Br− < I−).