Structure of active adlayers on bimetallic surfaces: Oxygen reduction on Au(111) with Bi adlayers

The structure of Bi adlayers on the Au( I 11) electrode surface during the course Of 02 reduction has been investigated by in situ surface X-ray scattering. Under oxygen reduction conditions, both the low-coverage (2 x 2)-Bi and the close-packed (p x root,3)-2Bi adlayer structures are stable. We developed an electrochemical drop cell that provides a more uniform potential distribution during high current conditions compared with commonly used thin-layer cells. Our results clearly indicate distinctly different catalytic properties for the two ordered bismuth adlayer phases. In the potential region corresponding to the (2 x 2)-Bi phase, 02 reduction is promoted to a four-electron reaction, albeit with relatively slow kinetics. The close-packed (p x root3)-2Bi phase, formed at more negative potentials, appears to have a limited number of sites available for four-electron reduction, but the reaction kinetics on this adlayer is enhanced by an increased overpotential.