Mass transport processes ultrasound

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The effect of ultrasound on electrochemical processes 69 Sonovoltammetric experiments practical considerations 70 Mass transport effects a simple description 71 Sonotrodes 77... 

It is fair to say that the effect of ultrasound upon the fundamental electron transfer processes at an electrode have been less widely studied than the effects upon mass transport phenomena. Electrode kinetics is defined by the Butler—Volmer equation, which by a series of practical assumptions reduces to the Tafel equation [44],... 

An aim of this volume is to highlight rapidly developing areas of electroanalyt-ical chemistry and electrochemistry. In this context, the application of ultrasound on electrochemical processes is a topic of particular interest. In a series of three chapters, Compton and coworkers provide a treatment of the underlying physical aspects connected with the coupling of ultrasound to electrochemical systems (Chapter 2.8) and applications in electroanalysis (Chapter 2.9). The first of these chapters considers the effect of ultrasound on mass transport, on the electrode surface and on chemical reactions in solution, while the second chapter looks at the use of sonoelectrochemical methods in... 

Representative organic electrosynthetic reactions of different characteristics, in which power ultrasound proves itself to be beneficial, will be described in this section as examples. In some cases, ultrasound dramatically enhances mass transport rates, thus leading to shorter reaction times and enhanced current efficiencies by means of minimizing background processes thanks to the massive decrease in... 

Ultrasound also presents the capacity to emulsify a mixture of immiscible liquids due to cavitational processes occurring at the liquid/liquid phase boundary effectively dispersing the biphasic system. This sonoemulsification allows product extraction from the aqueous phase, but at the same time may also prevent electrode passivation whilst keeping very fast rates of mass transport. The reduction of MG in the presence of a sonoemulsion of toluene (see Sect. 2.10.3.4) is one fine example of this. Another example of successful electroorganic process in a sonoemulsi-fied mixture is the oxidation of carboxylic acids, known as Kolbe processes (see Sect. 2.10.3.5). 

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