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.
The reversible quinone/diphenol redox of 2,5-dihydroxythiophenol (DHTP) chemisorbed on smooth polycrystalline platinum and gold electrodes has been studied by thin-layer electrochemistry. The packing density and mode of binding (through the SH moiety) of the subject compound were found to be identical on both surfaces. However, the width of the redox peak of the pendant diphenolic group, an indicator of intermolecular quinone-diphenol interactions, is at least twice as large on Pt as it is on Au. In the absence of the SH moiety, hydroquinone itself is irreversibly adsorbed on platinum but not on gold. These results suggest that the adsorbate-adsorbate interactions occurring on Pt are principally substrate mediated.
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