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Reduction process diffusion limited

The induced co-deposition concept has been successfully exemplified in the formation of metal selenides and tellurides (sulfur has a different behavior) by a chalcogen ion diffusion-limited process, carried out typically in acidic aqueous solutions of oxochalcogenide species containing quadrivalent selenium or tellurium and metal salts with the metal normally in its highest valence state. This is rather the earliest and most studied method for electrodeposition of compound semiconductors [1]. For MX deposition, a simple (4H-2)e reduction process may be considered to describe the overall reaction at the cathode, as for example in... [Pg.80]

The driving force for the transfer process was the enhanced solubility of Br2 in DCE, ca 40 times greater than that in aqueous solution. To probe the transfer processes, Br2 was recollected in the reverse step at the tip UME, by diffusion-limited reduction to Br . The transfer process was found to be controlled exclusively by diffusion in the aqueous phase, but by employing short switching times, tswitch down to 10 ms, it was possible to put a lower limit on the effective interfacial transfer rate constant of 0.5 cm s . Figure 25 shows typical forward and reverse transients from this set of experiments, presented as current (normalized with respect to the steady-state diffusion-limited current, i(oo), for the oxidation of Br ) versus the inverse square-root of time. [Pg.323]

The book focuses on three main themes catalyst preparation and activation, reaction mechanism, and process-related topics. A panel of expert contributors discusses synthesis of catalysts, carbon nanomaterials, nitric oxide calcinations, the influence of carbon, catalytic performance issues, chelating agents, and Cu and alkali promoters. They also explore Co/silica catalysts, thermodynamic control, the Two Alpha model, co-feeding experiments, internal diffusion limitations. Fe-LTFT selectivity, and the effect of co-fed water. Lastly, the book examines cross-flow filtration, kinetic studies, reduction of CO emissions, syncrude, and low-temperature water-gas shift. [Pg.407]

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. [Pg.37]

This method is sometimes abbreviated to LSV. In this method, a static indicator electrode (A cm2 in area) is used and its potential is scanned at constant rate v (V s-1) from an initial value ( ) in the positive or negative direction (Fig. 5.18). A typical linear sweep voltammogram is shown in Fig. 5.19. In contrast to DC polar-ography, there is no limiting current region. After reaching a peak, the current decreases again.9 For a reversible reduction process, the peak current ip (A) is expressed by Eq. (5.26), where D and C are the diffusion coefficient (cm2 s 1) and the concentration (mol cm-3) of the electroactive species ... [Pg.130]

The presence of a base is also essential for the efficient reductive dehalogenation of RX by 1-benzyl-1,4-dihydronicotinamide (BNAH) via photoinduced electron transfer [121,122], Since the one-electron oxidation potential of the singlet excited state of BNAH ( BNAH ) is —3.1 V (vs. SCE) [50], which is more negative than the one-electron reduction potential of benzyl bromide (PhCH2Br), photoinduced electron transfer from BNAH to PhCH2Br occurs efficiently with the diffusion-limited rate [122]. This fast process needs no base catalyst to accelerate the electron transfer rate further. However, the photoinduced electron transfer results in... [Pg.140]

Fig. 10.12. General principles of the SECM feedback mode. The UME, normally a disk electrode of radius r, is used to generate a redox mediator in its oxidised or reduced form (a reduction process is shown here) at a diffusion-controlled rate. As the UME approaches an insulating surface (a) diffusion of Ox to the electrode simply becomes hindered and the recorded limiting current is less than the steady-state value measured when the electrode is placed far from the surface, in the bulk of the solution, /( >). This effect becomes more pronounced as the tip/substrate separation, dKcm, is decreased. As the UME approaches a conducting surface (b) the original form of the redox mediator (Ox) can be regenerated at the substrate establishing a feedback cycle and an additional flux of material to the electrode. Fig. 10.12. General principles of the SECM feedback mode. The UME, normally a disk electrode of radius r, is used to generate a redox mediator in its oxidised or reduced form (a reduction process is shown here) at a diffusion-controlled rate. As the UME approaches an insulating surface (a) diffusion of Ox to the electrode simply becomes hindered and the recorded limiting current is less than the steady-state value measured when the electrode is placed far from the surface, in the bulk of the solution, /( >). This effect becomes more pronounced as the tip/substrate separation, dKcm, is decreased. As the UME approaches a conducting surface (b) the original form of the redox mediator (Ox) can be regenerated at the substrate establishing a feedback cycle and an additional flux of material to the electrode.
The concentration changes at the electrode surface due to mass transport limitations are responsible for the concentration overvoltages. When a reduction process takes place (e.g. Zn + + 2e Zn), a concentration of the oxidized species at the electrode surface (Cox,e) lower than that in the bulk makes the current, at a given potential, lower than that in the absence of an ion diffusion limitation, and to achieve the same current value an overvoltage (concentration overvoltage) must be imposed. This concentration (or diffusion) overvoltage can be calculated from Eq. (16) ... [Pg.3825]


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Diffusion limit

Diffusion limitation

Diffusion limiting

Diffusion process

Diffusive limit

Limiting diffusivity

Limiting processes

Process diffusion-limited

Process limitations

Processing limitations

Processing process limitations

Reduction process

Reduction processing

Reduction-diffusion

Reduction-diffusion process

Reductive processes

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