Surface Alloying of Lead as a Step in the Cathodic Generation of the Pb₉⁴⁻in KI and RbBr Liquid Ammonia Solutions

Reduction of lead in liquid ammonia solutions of KI and RbBr at −70 °C was studied using voltammetry as well as potentiostatic and galvanostatic techniques. It was found that the only soluble reduction product was the Zintl ion Pb 9 4- . Transitionally two intermetallic phases were also formed via direct incorporation of the alkali metal into lead, but they eventually dissolved in liquid ammonia. The alkali metal (M)-rich phase had the Pb:M molar ratio around 0.4 for both K and Rb, whereas the ratio for the lead-rich phase was between 0.8:1 and 1: 1 for potassium and around 0.7:1 for rubidium. The alkali metal-rich phase was unstable and easily transformed into the lead-rich phase by the reaction with solid lead. This reaction plays a crucial role both in the alkali metal incorporation and in the solid-phase dissolution reaction. A model mechanism of the lead reduction is presented.