Coulostatic Deposition of Metals at Underpotential I. Potential Relaxation in the Absence of Chemical Reactions

Under commonly occurring conditions, coulostatic processes can lead to underpotential deposition of a metal on a foreign metal substrate. These dep-osition processes are described quantitatively in terms of the isotherm for underpotential metal deposition, the double layer capacitance of the elec-trode surface, and the initial potential of %he electrode. Coulostatically con-trolled underpotential deposition is demonstrated for silver on gold and platinum and for mercury on gold. The process of underpotential deposition (UPD) of metals has been studied by a number of workers (1) since the original studies of Haissinsky (2) and Rogers (3). An understanding of UPD can give insight into the nature of the interactions occurring at the inter-face between two metals. Such insight can be used for a variety of purposes, such as to produce electrocata-lytic surfaces of controlled activity. Thus, it is impor-tant to know when UPD has occurred. To date, UPD phenomena have been assumed to occur only when charge flows in the external circuit and no recognition was given to the possibility of unsuspected UPD proc-esses occurring under zero current conditions. Several years ago we discovered a zero current UPD phenome-non in the course of rotating plat inum r ing-plat inum disk electrode studies. It was noted (4) that some shielding of Ag + ion could be detected at the ring electrode of a rotating plat inum ring-disk electrode when it was immersed in ~ I0-5M Ag + dissolved in 0.2M H2SO4, even though the disk electrode was at open circuit. In addition, after the shielding ceased, electrooxidation f the disk produced a collection cur-rent at the ring for a species having the properties of Ag +. As is shown below, these phenomena are the re-sult of UPD of silver occurring via coulostatic dis-Charge of the double layer capacitance. The purpose of this work is to quantitatively describe the coulostatic mechanism of UPD. Our theoretical de-scription has been confirmed using the rotating r ing-disk electrode (RRDE) on the systems Ag+/Ag on Pt, Ag +/Ag on Au, and Hg 2 +/Hg on Au