Molecular characterization, reactions electrodes

The development of scanning probe microscopies and x-ray reflectivity (see Chapter VIII) has allowed molecular-level characterization of the structure of the electrode surface after electrochemical reactions [145]. In particular, the important role of adsorbates in determining the state of an electrode surface is illustrated by scanning tunneling microscopic (STM) images of gold (III) surfaces in the presence and absence of chloride ions [153]. Electrodeposition of one metal on another can also be measured via x-ray diffraction [154]. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

We carried out potentiometric investigations with a Cl sensitive electrode [40] to determine the degree of conversion, i.e., the release of the low molecular counterions in the reactions between sodium poly(styrene sulfonate) (NaPSS) and poly(diallyldimethylammonium chloride) (PDADMAC) and its copolymers with acrylamide of various compositions (for synthesis and characterization of the samples see [41]). The basic idea of these studies on PEC formation is the change of the Cl ion activity coefficient due to the release of the counterions. According to Manning s theory [31], the activity coefficient of the counterions of a polyelectrolyte is given by... 

The study of dissociative chemisorption of gas-phase molecules on metal surfaces is important to the understanding of a myriad of processes such as heterogeneous catalysis, electrode reactions, and corrosion, to name but a few (179-181). Recent advances in molecular beam, laser, and surface detection technologies have made it possible to study the reactions of monoenergetic molecular beams with clean, well-characterized metal surfaces. 

Infrared spectroscopy has continued to support the study of adsorption and reactivity at well-defined electrode surfaces. Single crystals are employed to probe active site models for catalytic reactions and as templates for the deposition and growth of other phases. Infrared spectroscopy has played an important role in enabling in-situ detection and molecular-level characterization of species present at these surfaces. The sections below highlight some recent areas of apphcation. 

Stationary hanging mercury drop electrodes (h.m.d.e.) are suitable for evaluating a slow equilibrium of adsorption and subsequent reduction. Solid metal or graphite electrodes are used mainly for oxidation. From the molecular biophysical point of view such measurements are performed for characterization of structural and conformational transitions caused by physical and electrochemical influences, such as heat, light, electrical fields, solvents, ions, and other ligands. In all cases, one can distinguish between reversible (allosteric and conformational modifications) and irreversible (denaturation, strand break, enzyme reactions) processes. Besides these investigations, biochemical analysis, clinical tests, and electrochemical synthesis are fruitful applications. 

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