COMPARISONS BETWEEN SCANNING TUNNELING MICROSCOPY AND OUTER-SPHERE ELECTRON-TRANSFER RATES AT PT(111) SURFACES COATED WITH ORDERED IODINE ADLAYERS

Rate parameters are reported for the electroreduction of eight Co(III)(NH3)5X complexes at ordered Pt(111) surfaces coated with iodine adlayers whose structures are characterized by scanning tunneling microscopy (STM) in order to explore possible correlations between the outer-sphere electron-transfer kinetics and the spatially resolved adlattice properties as revealed by STM. The sixth ligands, X = NH3, F-, OSO3(2-), OH2, acetate, and three cyclic organic carboxylates, were selected so to vary the reactant charge, and hence the magnitude of electrostatic double-layer effects, and to examine the effect of potential organic mediators. The ordered Pt(111) surfaces were prepared by flame annealing, followed by cooling in a stream of nitrogen over iodine crystals (cf. ref 5). Three types of iodine adlayer structures could be formed, one having a square-root 7 x square-root 7)R19.1-degrees unit cell and two coexisting structures with (3 x 3) symmetry, as identified by STM. The real-space iodine adlattice structures extracted from these data are discussed (cf. ref 4) along with spatially dependent electron-tunneling parameters for each iodine adsorption site, also obtained from STM. For reactants containing only inorganic ligands, the observed (apparent) rate constants k(app) are markedly (3-5-fold) larger on the (square-root 7 x square-root 7) adlayer. This more facile electron mediation provided by the (square-root 7 x square-root 7) versus the (3 x 3) adlayers is rationalized in terms of the preponderance of threefold hollow iodine atoms in the former structure. Somewhat more facile electroreduction on the iodine adlayer surfaces is observed for complexes containing aromatic carboxylate substituents, although the kinetics in these cases are insensitive to the adlayer structure. This is attributed to the presence of specific interactions between the aromatic rings and the iodine adlayer. Comparisons are also made with corresponding rate parameters obtained at unmodified mercury and Pt(111) electrodes.