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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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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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A close-packed monolayer of 2,5-dihydroxythiophenol (DHT) chemisorbed on gold and platinum exclusively through the sulfur atom displays reversible two-electron quinone/hydroquinone redox, due to the pendant diphenol, at the same potential where the unadsorbed molecule reacts. However, the cyclic voltammetric peaks are approximately twice as broad at Pt as at Au. Since the DHT surface packing densities are identical at the two surfaces, the differences in the redox peak widths can only be rationalized in terms of substrate-mediated adsorbate-adsorbate interactions on Pt. The aim of the present study is to obtain empirical information with regards to the origins of this substrate mediation. Experiments were performed in which the coverage of and composition within the chemisorbed DHT layer were varied at smooth Au and Pt surfaces in acid media. When DHT is chemisorbed at submonolayer coverages on Pt, no redox peaks are observed. This signifies that the diphenolic group is no longer pendant but is directly bonded to the surface; an adsorbed molecule orientation which allows DHT to behave like a surface chelate is a strong possibility. In comparision, redox activity is still observed when submonolayer DHT is chemisorbed on Au. Even on a sparsely populated Au surface, the diphenolic moiety remains pendant; this means that diphenol-Au reactivity is not enhanced even by entropic or chelate effects. Reversible redox peaks reappear when DHT is coadsorbed at submonolayer coverages onto an iodine-pretreated Pt electrode. In the presence of coadsorbed iodine, the diphenol group is again pendant; evidently, direct interaction between the diphenol moiety and Pt surface is blocked by the surface iodine. The redox peaks are sharpened when surface iodine is present, indicating that the substrate-mediated DHT-DHT interactions are also suppressed by iodine coadsorption. On Au, essentially no changes in the peak widths are observed for the iodine/DHT mixed layer. The present results suggest that the driving force in the substrate-mediated intermolecular interactions which occur within the close-packed DHT layer is the inherent strong reactivity of the diphenolic moiety with the Pt surface. Although the phenomenon of substrate-mediated adsorbate-adsorbate interactions is not well understood, it may be possible to view it in terms of traditional concepts of mixed-valence metal complexes in which two metal ions separated by a common ligand are still able to interact with each other through the mediation of the delocalized electrons in the ligand.

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