Structure of Mercury Layer Deposited on Platinum and Hydrogen-evolution Reaction at the Mercury-coated Platinum Electrode

The thermal evaporation of mercury is investigated in order to ascertain the structure of mercury electrodeposited on platinum. At first, Pt2Hg is formed on platinum, and a hydrogen(H)-overpotential at the surface is identical with that at the mercury-free platinum. With the increase in the amount of mercury to more than the amount corresponding to PtHg, the H-overpotential increases. When a large amount of mercury is deposited, the layer is composed of three mercury compounds (Pt2Hg, Pt2Hg2, and PtHg4), metallic mercury, and adatom mercury. Even when a sufficiently large amount of mercury is deposited, e.g., several thousand atomic layers, the H-overpotential is much less than that at a hanging mercury drop electrode (HMDE). From the general relationship between the hydrogen evolution at a metal electrode and the work function of the metal, the change in the H-overpotential with the increase in the amount of mercury is attributable to the work-function change.