Low-temperature sonoelectrochemical processes Part 1. Mass transport and cavitation effects of 20 kHz ultrasound in liquid ammonia

Sonoelectrochemical processes in liquid ammonia in a temperature range betw een -70 and -35 degrees C in the presence of 20 kHz power ultrasound are st udied with the aim of improving low temperature electrosynthetic procedures . The one and two electron reductions of nitrobenzene and para-chloronitrob enzene are investigated as model systems. Placing an immersed ultrasonic ho rn emitter 'face-on' to a platinum disc electrode in liquid ammonia is show n to result in extreme mass transport enhancements with a resulting diffusi on layer thickness of approximately delta = 2 mu m. This limit of the diffu sion layer thickness is shown to be essentially temperature independent and correspondingly, the highest limiting currents can be observed near the bo iling point of liquid ammonia. Cavitation processes are detected even at - 70 degrees C and result in a considerable fluctuation in the observed mass transport controlled limiting current. Further, the deposition of ionic pro ducts formed in the second reduction step for both nitrobenzene and para-ch loronitrobenzene reduction and the associated drop in current, can be shown to be affected by sonication. Ultrasound has been found to be beneficial b y (i) causing extremely fast mass transport; (ii) enhancing the mixing and dissolution kinetics at low temperature; and (iii) affecting the formation of solid products at the electrode surface. (C) 1999 Elsevier Science S.A. All rights reserved.