Selectivity overpotential dependence

It was found that both the catalytic rates and the selectivity to the various products can be altered significantly (rate changes up to 250% were observed) and reversibly under NEMCA conditions. Depending on the product, electrophobic or electrophilic behaviour is observed as shown in Fig. 8.57. In addition to the selectivity modification due to the different effect on the rate of formation of each product, acetaldehyde, which is not produced under open circuit conditions is formed at negative overpotentials (Fig. 8.58). Enhancement factor A values up to 10 were observed in this complex system.59... 

Volmer turned his attention to processes at - nonpo-larizable electrodes [iv], and in 1930 followed the famous publication (together with - Erdey-Gruz) on the theory of hydrogen - overpotential [v], which today forms the background of phenomenological kinetics of electrochemistry, and which resulted in the famous - Butler-Volmer equation that describes the dependence of the electrochemical rate constant on applied overpotential. His major work, Kinetics of Phase Formation , was published in 1939 [v]. See also the Volmer reaction (- hydrogen), and the Volmer biography with selected papers [vi]. 

It should be recognized that the derived parameter set is not unique because of the scatter in experimental consumption transients (i.e. Figure 2.22), equally good fits to the q-i and i-t SPS-PEG-C1 data yield a locus of kinc(q) and q(q) parameters [12]. Further study and refinement of the derivatization procedure should help clarify the source of the experimental dispersion. In addition, different combinations of parameters yield distinct predictions for the catalyst evolution so that surface analytical experiments should help narrow the selection. Despite these uncertainties, a correct description of the SPS-PEG-C1 system clearly includes both potential dependent adsorption and deactivation processes. This conclusion is reinforced by multicycle voltammetry where the potential dependence of adsorption that is most significant at large overpotentials is convolved with consumption that dominates catalyst evolution at low overpotentials. 

Among examples of the electrochemical reduction of CO2 by metal complexes having macrocyclic ligands, the case of [Ni(cyclam)] adsorbed on an Hg electrode of especial interest from the viewpoint of high current efficiency and CO generation (almost 100% current efficiency) in H2O under relatively low overpotential conditions (64). The selectivity of CO/H2 formation in the CO2 reduction is largely dependent on the substituents of the Ni macrocycle complexes. Recently,... 

The reduction of CO2 at metallic cathodes has been studied with almost every element in the periodic table °. This reaction can be driven electrochemi-cally or photochemically " and semiconductors have been used as cathodic materials in electrochemical or photoelectrochemical cells . The aim of these studies has been to find cathodes that discriminate against the reduction of H2O to H2 and favor the reduction of CO2 and also to find a cathode selective for one product in the reduction of CO2. A fundamental requirement is that the latter process occurs at a lower overpotential on such electrodes. However the purposes mentioned before in metallic cathodes depends on a series of factors such a solvent, support electrolyte, temperature, pressure, applied overpotential, current density, etc. (we will see the same factors again in macrocyclic electro-catalysis). For instance when protons are not readily available from the solvent (e.g., A,A -dimethylformamide), the electrochemical reduction involves three competing pathways-oxalate association through self-coupling of COj anion radicals, production of CO via O-C coupling between and COj and CO2, and formate generation by interaction of C02 with residual or added water. ... 

To induce this reaction, the kinetic inhibition of the reaction must be overcome by applying an overpotential, which must be minimized. This reaction, in which electrons are transferred across the metal-solution interface with a resulting nitrate reduction, is called a faradaic process. Also, the complexity of the interfacial system is such that other phenomena do occur that can affect the electrode behavior. These processes include adsorption, desorption, and charging of the interface as a result of changing electrode potential these are called non-faradaic processes. Both the efficiency and the selectivity of nitrate electroreduction strongly depend on several parameters such as the electrode composition, physicochemical properties of the electrolyte (pH, coexisting species, temperature, etc.) and the applied potential. 

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