High mass transport

Worz et al. stress a gain in reaction selectivity as one main chemical benefits of micro-reactor operation [110] (see also [5]). They define criteria that allow one to select particularly suitable reactions for this - fast, exothermic (endothermic), complex and especially multi-phase. They even state that by reaching regimes so far not accessible, maximum selectivity can be obtained [110], Although not explicitly said, maximum refers to the intrinsic possibilities provided by the elemental reactions of a process under conditions defined as ideal this means exhibiting isothermicity and high mass transport. 

Chen SL, Kucemak A. 2004a. Electrocatalysis under conditions of high mass transport rate oxygen reduction on single submicrometer-sized Pt particles supported on carbon. J Phys Chem B 108 3262-3276. 

The kinetics of AgGl dissolution in aqueous solutions without supporting electrolyte have been studied utilizing well-defined and high mass transport properties of the scanning electrochemical microscope [376]. An ultramicroelectrode probe positioned close to the AgGl surface was used to induce and monitor dissolution of the salt via reduction of Ag+ from the initially saturated solution. 

In electrode kinetics, however, the charge transfer rate coefficient can be externally varied over many orders of magnitude through the electrode potential and kd can be controlled by means of hydrodynamic electrodes so separation of /eapp and kd can be achieved. Experiments under high mass transport rate at electrodes are the analogous to relaxation methods such as the stop flow method for the study of reactions in solution. 

Molecules at or near the interface may diffuse back into the bulk solution, particularly if the free energy of adsorption is not very high. Mass transport equations which account for back diffusion are available 3,31,32). 

Compared to conventional (macroscopic) electrodes discussed hitherto, microelectrodes are known to possess several unique properties, including reduced IR drop, high mass transport rates and the ability to achieve steady-state conditions. Diamond microelectrodes were first described recently diamond was deposited on a tip of electrochemically etched tungsten wire. The wire is further sealed into glass capillary. The microelectrode has a radius of few pm [150]. Because of a nearly spherical diffusion mode, voltammograms for the microelectrodes in Ru(NHy)63 and Fe(CN)64- solutions are S-shaped, with a limiting current plateau (Fig. 33a), unlike those for macroscopic plane-plate electrodes that exhibit linear diffusion (see e.g. Fig. 18). The electrode function is linear over the micro- and submicromolar concentration ranges (Fig. 33b) [151]. 

As a consequence of the extremely high mass transport rates attainable at UMEs, HM effects at vibrating UMEs are normally small. However when the electrode is in close proximity to a surface these effects can be greatly enhanced. Although the modulation of the signal is not hydrodyn-... 

In addition since extremely high mass transport rates are achievable at small h values, in excess of 2cms l [76], concentration boundary layer... 

Mass-transport regime. A high mass-transport coefficient that leads to a greater uniformity of pollutant concentration in the reaction layer near the electrode surface and to generally a higher efficiency... 

It has been shown that the oxidation is controlled by the diffusion of the pollutants toward the electrode surface, where the hydroxyl radicals are produced, and the current efficiency is favored by high mass-transport coefficient, high organic concentration, and low current density. Performing electrolysis under optimum conditions, without diffusion limitation, the current efficiency approaches 100%. 

Packed bed electrode — A static three-dimensional - electrode consisting of a restrained bed of electronically conducting particles in continuous intimate contact. Packed Bed Electrodes (PBEs) present high electroactive area per unit electrode volume and moderately high -> mass transport characteristics (the limiting current at a PBE may exceed 100 times the one observed at a two-dimensional electrode of the same volume). 

In the case of sufficiently high mass transport, i.e., the absence of concentration polarization phenomena, and when the electrochemical process is controlled by the charge transfer rate at one electrode, the dependency of the current on the electrode potential, V, , is given by the Butler-Volmer equation (Eq. (10)),... 

Different electrochemical cells have been developed for various applications in the past. According to the criteria of high mass-transport coefficient km and/or large... 

High mass-transport coefficients are obtained in cells with a rotating cylinder electrode (RCE) and a small gap between the anode and the cathode, Fig. 4(a). High rates of mass transport are experienced in the turbulent flow regime, so that RCE reactors allow metal deposition at high speed, even from dilute solutions. RCE reactors have been operated at a scale involving diameters from 5 to 100 cm, with rotation speeds from 100 to 1500 rpm and currents from 1 A to 10 kA [79], It... 

The limiting current density is a function of the concentration of the depositing metal, its diffusion coefficient, and the thickness of the diffusion layer. A high-performance cell needs to have a high mass transport coefficient and enhanced electrode area in order to achieve a high limiting current I ira (13) ... 

In Eq. (13) km is the local mass transport coefficient D/S, that is, diffusion coefficient divided by the thickness of diffusion layer. High mass transport rates can be achieved by electrode movement,... 

Leaching and electrolysis processes can be used for metal recovery from waste electrical and electronic equipment. Metals such as Ag, Au, Cu, Pb, Pd, Sn, are dissolved from shredded electronic scrap in an acidic aqueous chloride electrolyte by oxidizing them with aqueous dissolved chlorine species. In the electrochemical reactor, chlorine is generated at the anode for use as the oxidant in the leach reactor and the dissolved metals are deposited from the leach solution at the cathode. The very low concentrations of the precious metal ions require the use of porous electrodes with high specific surface areas and high mass transport rates to achieve economically adequate reactor productivities and space-time yields [72]. 

This behavior indicates the favorable effect of the high solubility of H2 in SC-CO2 as well as the high mass transport rate and recommends the system for continuous operation in industrial plants. It should be pointed out that the concentration of H2 in a supercritical mixture of H2 (85 bar) and CO2 (120 bar) at 50 °C is 3.2 M, while the concentration of H2 in THF under the same pressure is merely... 

N. V. Rees, J. A. Alden, R. A. W. Dryfe, B. A. Coles, and R. G. Compton. Voltammetry under high mass transport conditions. The high speed channel electrode and heterogeneous kinetics, J. Phys. Chem. 99, 14813-14818 (1995). 

In a solid-liquid two phase system, the solid can be dissolved in the liquid and the liquid can also be dissolved in the solid. A high solubility of the solid in the liquid will facilitate potentially a high mass transport rate through the liquid layer, which is the basic requirement of liquid-phase sintering. However, the dissolving of the liquid in the solid is totally prevented, otherwise the compact would be severely swelling due to the formation of transient liquid phases. The effect of solubUily on densification and swelling of the compact is demonstrated schematically in Fig. 5.33 [73]. 

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