Parallel reactions, anodic

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Activationless and barrierless regions cannot be realized in all reactions. Often or 1 are in regions of potentials where measurements are impossible or extremely difficult (e.g., because of parallel reactions). The crossover to the barrierless region has been demonstrated experimentally for cathodic hydrogen and anodic chlorine evolution at certain electrodes. Clear-cut experimental evidence has not yet been obtained for limiting currents appearing as a result of an activationless reaction. 

In the anodic polarization of metals, surface layers of adsorbed oxygen are almost always formed by reactions of the type of (10.18) occurring in parallel with anodic dissolution, and sometimes, phase layers (films) of tfie metal s oxides or salts are also formed. Oxygen-containing layers often simply are produced upon contact of the metal with the solution (without anodic polarization) or with air (the air-oxidized surface state). 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

Some commonly used batteries are shown in Table 15.5, and two are drawn schematically in Fig. 15.10. From these it can be seen that important components are the container, the anode/cathode compartment separators, current collectors to transport current from the electrode material (usually a porous, particulate paste), the electrode material itself, and the electrolyte. It should be noted that the electrode reactions can be significantly more complex than those indicated in Table 15.5, and there will probably be parallel reactions. By stacking the batteries in series, any multiple of the cell potential can be obtained. 

Although the data of Herrero et al. [34] were interpreted in terms of a parallel reaction scheme model, such a model is certainly not established by their treatment, and Vielstich and Xia [36] have criticised such a model on the basis of their Differential Electrochemical Mass Spectroscopy (DEMS) data [37]. At least below a potential of 420 mV, the very sensitive DEMS technique detects no C02 evolved from a polycrystalline particulate Pt electrode surface on chemisorption of methanol indeed, the only product detected other than adsorbed CO, in very small yield (one or two orders of magnitude smaller), is methyl formate from the intermediate oxidation product HCOOH. This is graphically illustrated in Fig. 18.2 in which the clean electrode is maintained at 50 mV, a 0.2M methanol/O.lM HCIO4 electrolyte introduced, and the electrode swept at 10 mV s I anod-... 

During ECM, electrochemical dissolution of anode and cathodic evolution of hydrogen proceeds on the electrodes (the WP and TE, respectively). Along with these basic reactions, parallel reactions proceed concurrently, for example, oxygen anodic evolution, cathodic reduction of nitrate ions, if NaNC>3 electrolyte is used. It is important to note that electrochemical reactions in a narrow IEG result in gas evolution. The temperature of the electrolyte in the IEG and the void fraction increase as the electrolyte flows along the gap. This leads to a variation in the electrolyte conductivity that has an effect on the distributions of current and metal dissolution rate over the WP surface. The electrode processes and the processes in... 

The CE for the cyanide destruction process is obviously determined by the extent of the parallel reaction (7), a factor which becomes much more important as the cyanide levels drops to below 100 ppm [88]. Complete mineralization of the CN-by reaction (8) removes all traces of the offending species, but at the expense of considerably more charge consumption. It may be reasonable to stop the reaction at the CNO- stage, something which is quite feasible since reaction (8) occurs with more difficulty than (6) [89] at a potential of almost 0.5 V more anodic [87]. At a PH < 10, the cyanate ion hydrolyzes on its own according to the following reaction ... 

It would appear that the depletion of OH near the anode could be stopped (and thus changes in pH and zeta potential prevented) during EOD by depolarizing the reaction (18) by another competing, parallel reaction... 

Gupta et al. (2004) studied the cyclic voltammograms (Fig. 10) of ethanol electro-oxidation behavior on CuNi, CuNi/Pt and CuNi/PtRu alloys electro-catalysts in 0.5 M NaOH solution. Fig. 10 (a) shows a steady rise of the anodic peak current for the CuNi/Pt electro-catalyst. The peak current increases substantially from F to the 50 scan. Fig. 10 (b) shows the increase in reaction kinetics for ethanol electro-oxidation when Ru is added in the alloy. They have detected the presence of acetaldehyde and CO2 (as carbonate) with CuNi/PtRu electro-catalyst Authors found carbonate ions because of the cleavage of C—C bond of ethanol molecule. The temperature of ethanol electro-oxidation was not mentioned although the experimental work was done at room temperature. Tripkovic et al. (2001) studied the electro-oxidation of methanol, ethanol, -propanol and n-butanol (C[—C4 alcohol) in alkaline solution at the Pt (111) and vicinal stepped planes Pt (755) and Pt (332). The nature of the oxygen-containing species as well as their role in the alcohol oxidation is proposed. A dual path reaction mechanism as shown by eqs. (4) and (5) is proposed based on the assumptions that RCOa is a reactive intermediate of the main reaction path, while CO2 is a product of the poisoning species oxidation in a parallel reaction path. 

Work continues on the Jacques cell in various configurations by SARA, Inc. (Cypress, CA) (Zecevic, 2004 Pesaventeo, 2001). Current work features a cell with a porous separator, with a hydroxide electrolyte in the air-sparged cathode half-cell, and a mixed hydroxide and carbonate electrolyte on the anode side. (Patton, 2005). Two parallel net anode reactions are posited, although no mechanism is proposed ... 

Design possibilities for electrolytic cells are numerous, and the design chosen for a particular electrochemical process depends on factors such as the need to separate anode and cathode reactants or products, the concentrations of feedstocks, desired subsequent chemical reactions of electrolysis products, transport of electroactive species to electrode surfaces, and electrode materials and shapes. Cells may be arranged in series and/or parallel circuits. Some cell design possibiUties for electrolytic cells are... 

Even in good alloys and under favorable conditions, the a value does not lie above about 0.6. In enamelled storage tanks where the current requirement is low, the a value can fall to as low as about 0.1. The cause of the high proportion of selfcorrosion is hydrogen evolution, which occurs as a parallel cathodic reaction according to Eq. (6-5b) or by free corrosion of material separated from the anode on the severely craggy surface [2-4, 19-21]. 

The SEI is formed by parallel and competing reduction reactions and its composition thus depends on i0, t], and the concentrations of each of the electroactive materials. For carbon anodes, (0 also depends on the surface properties of the electrode (ash content, surface chemistry, and surface morphology). Thus, SEI composition on the basal plane is different from that on the cross—section planes. 

Our experimental techniques have been described extensively in earlier papers (2, 13). The gamma ray irradiations were carried out in a 50,000-curie source located at the bottom of a pool. The photoionization experiments were carried out by krypton and argon resonance lamps of high purity. The krypton resonance lamp was provided with a CaF2 window which transmits only the 1236 A. (10 e.v.) line while the radiation from the argon resonance lamp passed through a thin ( 0.3 mm.) LiF window. In the latter case, the resonance lines at 1067 and 1048 A. are transmitted. The intensity of 1048-A. line was about 75% of that of the 1067-A. line. The number of ions produced in both the radiolysis and photoionization experiments was determined by measuring the saturation current across two electrodes. In the radiolysis, the outer wall of a cylindrical stainless steel reaction vessel served as a cathode while a centrally located rod was used as anode. The photoionization apparatus was provided with two parallel plate nickel electrodes which were located at equal distances from the window of the resonance lamp. 

The set of all intermediate steps is called the reaction pathway. A given reaction (involving the same reactants and products) may occur by a single pathway or by several parallel pathways. In the case of invertible reactions, the pathway followed in the reverse direction (e.g., the cathodic) may or may not coincide with that of the forward direction (in this example, the anodic). For instance, the relatively simple anodic oxidation of divalent manganese ions which in acidic solutions yields tetrava-lent manganese ions Mn +— Mn -l-2e , can follow these two pathways ... 

Methods have been developed for perchloric acid synthesis which involve the electrolysis of solutions containing hydrogen chloride or molecular chlorine. These processes occur at high anode potentials (2.8 to 3.0 V vs. SHE), when oxygen is evolved at the anode in parallel with perchloric acid formation. The current yields of perchloric acid will increase considerably when the reaction is conducted at low temperatures (e.g., 20°C). 

Each of these reactions occurs in its own typical potential range. Several reactions may occur in parallel. The oxidation of solution components and the evolution of oxygen and chlorine are discussed in Chapter 15, the formation of surface layers in Section 16.3. In the present section we discuss anodic metal dissolution. 

V to +0.7 V vs. RHE for a Pd surface. Normally, this is anodic, or positive, with respect to the Em value of the electroless reaction (Fig. 1). Following removal of the oxide species from the catalyst surface, whether deposition subsequently initiates or not depends on the interplay between the kinetics of the parallel metal ion and O2 reduction reactions, and oxidation of the reducing agent. Once an appropriate Em value is reached, metal deposition will occur. 

At sufficiently high anodic potentials, only the anodic reaction (1) wiU proceed at the experimental electrode. Then on the counter electrode the reactions (2) or (3) causing CMT measurements will proceed at the same electrical rate. These CMT measurements should coincide with the value of current measured electrically. The only restriction in this case (other than those discussed in Section II.2) is that dissolved metal ions must not be plated onto the counter electrode in a cathodic reaction in parallel with (2) or (3). 

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