UNDERPOTENTIAL DEPOSITION OF COPPER ON AU(III) SINGLE-CRYSTAL SURFACES - A VOLTAMMETRIC AND ROTATING-RING-DISK ELECTRODE STUDY

The underpotential deposition of copper on Au(111) single-crystal substrate was studied using voltammetric and rotating ring-disk electrode measurements (RRDE) in sulfuric acid solutions of different Cu2+ concentrations (5 - 10(-4) less-than-or-equal-to C(Cu2+) less-than-or-equal-to 5 . 10(-2) Mol liter-1) at constant pH 1 and T = 298 K. Cyclic voltammograms and charge isotherms indicate that the ideal metal-monolayer model, characterized by an electrosorption valency value of gamma = z = 2, is valid only at high degree of Cu(ad) coverage at low underpotentials. Significant deviation from the ideal behavior was observed at high underpotentials, where the electrosorption valency was found to be gamma almost-equal-to 1. This can be interpreted in terms of a partial discharge of Cu2+ to Cu(int)+ the adsorption of which is stabilized by co-adsorption of sulfate ions. Transfer of the Cu(int)+ into the solution could only be detected in the transition range from underpotential to overpotential copper deposition using RRDE measurements. The kinetics of the Cu(int)+ formation is mainly determined by the charge-transfer reaction Cu2+ + e- half arrow right over half arrow left Cu(int)+, rather than by the disproportionation reaction Cu2+ + Cu(o) half arrow right over half arrow left 2Cu(int)+.