Reduction sonoelectrochemical

These authors, using a horn system, also noted less striking but still significant switches towards the one-electron products in other sonoelectrochemical reductions [66] including dimethylmaleate at a lead cathode in an aqueous mixed-phosphate buffer, and benzyl bromide at a lead cathode in methanolic tetraethylammonium bromide solution (Tab. 6.16). 

The reduction of benzoic acid at a lead cathode in aqueous sulphuric/citric acids yields the two-electron products benzaldehyde and the four-electron product benzyl alcohol rather than one-electron hydrodimer. In all cases studied by the authors they found that ultrasound favoured the process involving the smaller number of electrons per molecule. This is the opposite of the sonoelectrochemical effect seen in carboxylate electrooxidation [57,59,60] where the process involving the greater number of electrons was favoured by ultrasound. 

Reisse and co-workers [147-149] were the first to describe a novel device for the production of metal powders using pulsed sonoelectrochemical reduction. This device exposes only the flat circular area at the end of the sonic tip to the electrodeposition solution. The exposed area acts as both cathode and ultrasound emitter, named by Reisse et al. as sonoelectrode . A pulse of electric current produces a high density of fine metal nuclei. This is immediately followed by a burst of ultrasonic energy that removes the metal particles from the cathode, cleans the surface of the cathode, and replenishes the double layer with metal cations by stirring the solution. In [145], a list is given of chemically pure fine crystalline powders, mostly metals or metallic alloys, prepared by this method, with particle sizes varying between 10 and 1000 run depending on deposition conditions. 

There is thus a need for caution in the assignment of sonoelectrochemical phenomena. However, since the eventual aim of synthetic chemists is to produce new or improved routes to target molecules, it is not necessarily important in the first instance to establish mechanistic details. The fact is that ultrasonic irradiation has been shown to produce a number of benefits in electrosynthetic oxidations and reductions, and may act before, during, or after the fundamental electrochemical processes. 

Sonoelectrochemical synthesis has recently been used for the preparation of semiconductor nanocrystalline powders. In the sonoelectrochemical method, the ultrasound horn acts as both cathode and ultrasound emitter. This technique was used for preparing metal powders [65] and was extended to CdTe, although details of the CdTe particle size were not given [66]. CdSe nanocrystalline powders have been prepared by pulsed sonoelectrochemical reduction from an aqueous selenosulfate solution. The crystal size could be varied from X-ray amorphous up to 9 nm (sphalerite phase) by controlling the various electrodeposition and sonic parameters [67]. Crystal size was smaller for lower preparation temperatures, higher ultrasound intensity, and shorter current pulse width. These dependencies could be explained based on a pulse of electric... 

In an original approach, reactive metallic powders of copper, cobalt, and zinc have been generated by pulsed sonoelectrochemical reduction. Zinc produced in this manner effects allylation reactions of carbonyl compounds (p. 223) in higher yields than commercial powders... 

Comparing product ratios and limiting current densities from voltammetric data, the authors used their previously reported method for estimation of mass-transport coefficients to conclude that ultrasound was acting simply by agitation and not by cavitation. They further pursue the thesis that simple enhancement of mass transport will produce the same product switch in a more recent paper, but also describe work in which an increase in bulk reaction temperature from 0 to 60 produces a similar shift in products. xhe authors also noted less striking but still significant switches towards the 1-e products in other sonoelectrochemical reductions,67 e.g., dimethyl maleate at a lead cathode in an aqueous mixed-phosphate buffer, and benzyl bromide at a lead cathode in methanolic TEAR solution (Fig. 16). 

The diminution of electrode fouling in oxido-reductive mode is shown in Fig. 22. Other ECL systems were also examined and sonoelectrochemical enhancement was observed for luminol (3-aminophthalhydrazide). This system has been thoroughly studied for the detection of low concentrations of residual hydrogen peroxide in contact-lens cleaning solutions. The ECL intensity of 9,10-diphenyl-anthracene in sonicated DMF solutions was increased by factors of 20-30 depending on the concentration of the solutions, ultrasound power, and fre-quencyT Similar effects were observed from l,5-diphenyl-3-styrylpyrazoline, rubrene, 9,10-dimethyl-anthracene, and perylene. ... 

Durant, A. Delplanche, J.L. Winand, R. Reisse, J. (1995) A new procedure for the production of highly reactive metal powder by pulsed sonoelectrochemical reduction. Tetrahedron Lett., 36,4257-60. 

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