Adsorbed-iodine-catalyzed dissolution of Pd single-crystal electrodes: Studies by electrochemical scanning tunneling microscopy

Unfettered anodic dissolution occurs in halide-free sulfuric acid solutions for Pd electrodes pretreated with a single chemisorbed layer of iodine atoms; no dissolution takes place in the absence of iodine. Tandem cyclic voltammetry and in situ scanning tunneling microscopy have been employed to investigate the mechanism of this type of corrosion lander low-current conditions. The ordered adlattices studied were those spontaneously formed upon immersion of the Pd single-crystal surface to a dilute solution of iodide: Pd(111)-(root 3 x root 3)-R30 degrees-I; Pd(100)-c(2 x 2)-I; Pd(110)-pseudohexagonal-I. It has been found that (i) adsorbed-iodine-catalyzed corrosion of Pd is a structure-sensitive reaction; it decreases in the order Pd(110)-I > Pd(111)-I greater than or equal to Pd(100)-I. (ii) At Pd(111)-I, dissolution occurs exclusively at step-edges in a layer-by-layer sequence without deterioration of the iodine adlattice structure. (iii) At Pd(100)-I, dissolution takes place anisotropically along a step aligned in the {100} direction but in a layer-by-layer process without disruption of the iodine adlattice structure. (iv) At Pd(110)-I, dissolution transpires predominantly at a step-edge that runs parallel to the {100} direction; pit formation at terraces precluded layer-by-layer dissolution and led to progressive disorder of the substrate structure. Heuristic models are presented to account for these observations.