Anomalous voltammetry

Long-Range Structural Effects in the Anomalous Voltammetry of Platinum(lll)... 

The history of the observation of anomalous voltammetry is reviewed and an experimental consensus on the relation between the anomalous behavior and the conditions of measurement (e.g., surface preparation, electrolyte composition) is presented. The behavior is anomalous in the sense that features appear in the voltammetry of well-ordered Pt(lll) surfaces that had never before been observed on any other type of Ft surface, and these features are not easily understood in terms of current theory of electrode processes. A number of possible interpretations for the anomalous features are discussed. A new model for the processes is presented which is based on the observation of long-period icelike structures in the low temperature states of water on metals, including Pt(lll). It is shown that this model can account for the extreme structure sensitivity of the anomalous behavior, and shows that the most probable explanation of the anomalous behavior is based on capacitive processes involving ordered phases in the double-layer, i.e., no new chemistry is required. 

Prior to the publication in 1980 of Clavilier s historic paper (1) reporting anomalous voltammetry of Pt(lll), there had been a number of studies of the voltammetry of single crystal Pt electrodes, with some using modern methods of surface analysis (e.g., LEED or RHEED) for characterization of the structure of the crystal prior to immersion in electrolyte (2-6). and all were in qualitative agreement with the seminal work (in 1965) on Pt single crystals by Will (7.). 

Table I gives a summary of the observational data base which I shall use here for a discussion of possible interpretations for the anomalous voltammetry. The discussion here is of necessity brief, and only summarizes the detailed discussion presented in our recent paper (Wagner, F.T. Ross, P.N., J. Electroanal. Chem.. in press).

In contrast to the successful implementation of the bead method in studying the anomalous features, the contributions from studies with UHV-electrochemical systems has been limited to just a few. Subsequent work from our apparatus following corroboration of Clavilier s results concentrated on the effect of potential cycling through "oxide formation potential on the surface structure (19). and later on the effect of pH and type of anion (Wagner, F.T. Ross, P.N., J. Electroanal. Chem.. in press) on the anomalous features. Using the system in Yeager s laboratory, Hanson (20) was able to reproduced Clavilier s voltammetry not only for the (111) surface, but also the (100) and (110) surfaces as well. In spite of the relatively small number of contributions to the literature that have come from the UHV-electrochemical systems, they have made and essential validation of the bead method of surface preparation, and have verified the structure sensitivity of the anomalous features inferred from purely electrochemical observations. 

The storage and reactivity of electroactive molecules in polymerized diacetylene vesicles was the subject of studies reported by Stanish, Singh, and coworkers [109, 110], They entrapped ferricyanide in large unilamellar vesicles of photopolymerized PCg PC (1 - palmitoyl - 2 - (tricosa - 10,12-diynoyl)-OT-glycero-3-phosphocholine). Cyclic voltammetry was used to demonstrate that the ferricyanide was electrochem-ically isolated by the poly(lipid) bilayer [110]. At pH7 and 25°C, an anomalously long half-life of 2.4 weeks was calculated for Fe (CN)g- retention in polymerized vesicles. In a subsequent study [109], vesicles with entrapped ferricyanide were prepared from 2-bis(10,12-tricosadiynoyl)-OT-glycero-3-phosphatidylcholine (DCs.gPC) doped with a disulfide-capped lipid (Af-3-(pyridyl-2-dithio)propionyl-2-... 

The main effect of changing the cationic component of the ioific liquid was found to be its effect on the solvent viscosity, as the diffusion coefficient (D) of each species was found to be inversely proportional to the viscosity across the series of ionic liquids, in accordance with Stake s equation [16,17], The only deviation from this relationship arose for the case when [P6,6,6,i4][N(Tf)2] was used as the solvent [17]. Here, though relatively normal voltammetry was observed for the oxidation and immediate re-reduction of xxi (first electron transfer in Eq. 15.22), sweeping the potential at a more positive value to promote the formation of the dication, resulted in an anomalous wave shape. This behavior was rationalized in terms of a dimerization reaction between the dication and neutral xxi (Eq. 15.23), with the further dimer oxidation at a potential more positive than that for the first oxidation of the monomer (Eq. 15.24) [17]. 

Cytochrome c can be strongly adsorbed in its native state on tin oxide electrodes at coverages of several tenths of a monolayer. Measurements of unimolecular electron transfer rate constants for adsorbed cytochrome c can be readily made in the absence of solution cytochrome c using cyclic voltammetry. The kinetic results are consistent with an interfacial model involving electrostatic interaction between the tin oxide and the exposed heme edge of the cytochrome as well as an electrostatically driven adsorbate reorientation capability to account for the anomalous dependence of rate constant on scan rate. Other possible explanations for the... 

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