Surface coordination chemistry of noble-metal electrocatalysts: Oxidative addition and reductive elimination of iodide at iridium,platinum and gold in aqueous solutions

The oxidative addition and reductive elimination of the iodo ligand has been compared at smooth polycrystalline gold, platinum and iridium surfaces in aqueous solutions. On these three metals, the iodo species undergoes spontaneous oxidative chemisorption to form a close-packed monolayer of zero-valent iodine, the saturation coverage of which is limited by the van der Waals radius of the iodine atom; this oxidative addition process is further manifested by evolution of hydrogen gas from proton reduction. Elimination of iodine from these surfaces can be achieved by its reduction back to the anion either by application of sufficiently negative potentials or by exposure to ample amounts of hydrogen gas. On Pt and Ir, the reductive desorption of iodine is coupled with reductive chemisorption of hydrogen; consequently, the reaction is a two-electron, pH-dependent process. A plot of E_1/2, the potential at which the iodine coverage is decreased to half its maximum value, against pH yields information concerning the redox potential of the I_(ads)/I^?_(ads) couple in the surface-coordinated state. On Au, where dissociative chemisorption of hydrogen does not occur, the iodine-stripping process is a pH-independent, one-electron reaction. The difference in the redox potentials E^o_I(ads) -E^o_I(aq] for the I_(ads) and I_2(aq)/I^?_(aq) redox couples was found to be ?0.90 V on Au, ? 0.76 V on Pt, and ?0.72 V on Ir. These values imply that the ratio of the formation constants for surface coordination of the iodine and iodide species (K_f,I/K_f,I?) is 2 × 10²⁸ on Au, 1 × 10²⁶ on Pt, and 2 × 10²⁵ on Ir.