The kinetics of redox processes of unadsorbed species at noble-metal electrodes are altered to varying degrees by pretreatment of the electrode surface with a full monolayer of iodine. For example, the rate of the quinone/hydroquinone redox couple at pH 4 is increased significantly at Rh, Pd, Ir, and Pt but decreased slightly at Au after these electrodes were coated with iodine. These observations provided the motivation to pursue a comparative study of the surface electrochemical properties of iodine coordinated to these metals and their bimetallic alloys. Experimental measurements were based upon thin-layer electrochemistry, low-energy electron diffraction, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The findings accumulated to date indicate the following: (i) Iodine is spontaneously and oxidatively chemisorbed as iodine atoms on these metals, (ii) I is covalently bonded to the surface metal atoms; that is, only little or no ionic character exists in the I-metal chemical bond, (iii) Adsorbate-adsorbate interactions within the close-packed I layer are negligible with respect to the I-metal bond, (iv) I can be reductively eliminated from the surface either by exposure to electrogenerated hydrogen or by application of sufficiently negative potentials, (iv) The surface binding strength of I at the subject electrodes decreases in the order Ag > Au > Pt > Ir. (v) The close-packed I layer is not insulating; electron transfer can occur directly from the I adatom, (vi) The conductivity of an I-coated surface is slightly lower than that for a surface which does not contain any chemisorbed material, (vii) The profound dependence of the surface electrochemical properties of I on the surface composition of the metal electrodes makes it a suitable electrochemical tracer in the study of mixed-metal interfaces.

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