Currents neutral bounds

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

Enzymes are proteins that act as biological catalysts. They facilitate chemical modification of substrate molecules by virtue of their specific binding properties, which arise from particular combinations of functional groups in the constituent amino acids at the so-called active site. In many cases, an essential cofactor, e.g. NAD+, PLP, or TPP, may also be bound to participate in the transformation. The involvement of enzymes in biochemical reactions has been a major theme throughout this book. The ability of enzymes to carry out quite complex chemical reactions, rapidly, at room temperature, and under essentially neutral conditions is viewed with envy by synthetic chemists, who are making rapid progress in harnessing this ability for their own uses. Several enzymes are currently of importance commercially, or for medical use, and... 

However, there are many other options to combine electricity with chemistry. One that has been studied intensively for a variety of different applications is plasma chemistry (see Fridman, 2008 for a recent overview). A plasma is a partially ionized gas, in which a certain percentage of the electrons is free instead of bound to an atom or molecule. Because the charge neutrality of a plasma requires that plasma currents close on themselves in electric circuits, a plasma reactor shows resemblance to an electrochemical cell, although due to the much lower ionization degree and conductivity, a plasma discharge will typically be operated in the range of hundreds of volts, compared to a few volts in the case of an aqueous electrochemical cell. 

The term pseudohalogen is used to refer to strongly bound, linear or planar univalent radicals which can form anions, hydracids, neutral dipseudohalogens and interpseudohalogens. Recently the CS2N3 radical has been shown to satisfy all these conditions and so is classed as a pseudohalogen [33]. It is the only cyclic pseudohalogen currently known. 

To define a unique solution, we must specify the corresponding boundary and initial conditions. Normally electrolyte solutions are in contact with or bounded by electrodes. An electrode in its simplest form is a metal immersed in an electrolyte solution so that it makes contact with it. For example, copper in a solution of cupric sulfate is an example of an electrode. A system consisting of two electrodes forms an electrochemical cell. If the cell generates an emf by chemical reactions at the electrodes, it is termed a galvanic cell, whereas if an emf is imposed across the electrodes it is an electrolytic cell (Fig. 6.1.1). If a current is generated by the imposed emf, the electrochemical or electrolytic process that occurs is known as electrolysis. Now whether or not a current flows, the electrolyte can be considered to be neutral except at the solution-electrode interface. There a thin layer, termed a Debye sheath or electric double layer, forms that is composed predominately of ions of charge opposite to that of the metal electrode. We shall examine this double layer in Section 6.4, but for our purposes here it may be neglected. 

In the static case with no leakage across the membrane, there is no current flow inside or outside the cell. At the inside membrane, a surface charge density of free ions neutralizes the electric field in the membrane plus the double layer charges, so that the eeU interior has E = 0 and no current flow. There is electroneutrality in the bulk, but surface charges at the interphase. The intracellular bulk volume has a potential, but zero electric field strength. The membrane has a bound surface charge density qsb, which wfll reduce the membrane E-field to (qgf — qsb)/eo- Thus, the E-field in the membrane is less than the potential difference divided by the membrane thickness. 

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