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46835-AC5
Improved Heterogeneous Catalysts for Oxygen Electroreduction

W. Ronald Fawcett, University of California (Davis)

          Research has focused on the decoration of gold by platinum, and using the decorated electrode as cathode in a cell in which oxygen is reduced. Our experiments have followed the procedure outlined by Brankovic et al. [1]. Initially copper is underpotential deposited on gold from a solution containing 0.1 M CuSO₄ and 0.1 M HClO₄. Deposition is carried out at a fixed voltage for the time necessary to produce a monolayer. The quality of the copper monolayer is quite good as seen from STM images. The copper coated gold is then transferred to a solution containing PtCl₆^2- where the following redox reaction occurs:

          2 Cu + PtCl₆^2-  ¾®  2 Cu²⁺ + Pt + 6 Cl^-                         (1)

Thus, two copper atoms on the surface are replaced by one platinum atom. Assuming that deposition of a monolayer of copper requires a charge of  445 μC cm^-2, the maximum coverage achievable by platinum is ~ 222 μC cm^-2. However, in practice, the coverage by platinum was always less than 100 μC cm^-2 because of oxidation of copper by dissolved oxygen during transfer.

          Because of more favorable thermodynamics, lead was considered as a possible intermediate for underpotential deposition. However, the lead atom is considerably larger than a gold atom. As a result a commensurate monolayer cannot be achieved in the deposition of lead on gold.

          The experiments involving gold made use of single crystal surfaces. It is well known [2] that the underpotential deposition of copper and lead on gold depends on the nature of the single crystal surface.  The structure of the deposits was studied using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The platinum deposits tended to be very nonuniform in thickness. Nevertheless, it is clear that the gold electrode decorated by platinum catalyzes the reduction of oxygen in acidic media. At present the reduction curves for oxygen are being compared at pure platinum and at gold electrodes decorated by platinum in order to quantitate the effect.

1. S. R. Brankovic, J. X. Wang, and R. R. Adzic, Surf. Sci., 474 (2001) L173.

2. J. W. Schultze, and D. Dickertmann, Surf. Sci., 54 (1976) 489.

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