Poisoning the active site of electrochemical reduction of dioxygen on metal monolayer modified electrode surfaces

The four electron electroreduction of dioxygen to water on the (2 x 2) Bi upd adlattice on Au(III) has been studied by deliberately poisoning the adlattice with thiocyanate and ethanethiol during the course of electroreduction activity. The diminution in reduction activity was monitored using chronoamperometry. For SCN-, the drop in current could be modeled using a Langmuir kinetic expression yielding an adsorption rate constant of 1.1 x 10(4) s(-1) M-1. The rate for ethanethiol could not be measured exactly but is approximately the same. STM images of the surface obtained following introduction of SCN- revealed a (4 x 4) adlattice, which was partially (6%) defected. The percentage of defects agreed well with the percentage of residual current found at long times (3%) leading us to associate these defects with sites of catalytic activity. STM images obtained from surfaces poisoned with ethanethiol revealed two lattices: a (8 x 8) structure which was unstable and a more stable (root 57 x 3) structure which is consistent with an overlayer of thiols lying flat on the surface. IR studies of the SCN--poisoned surface showed that the SCN- was S-bound to the surface at almost the same energy as that expected from SCN- bound to a bare Au(III) surface. XPS measurements on emersed samples showed that Bi and S were present on the surface. Analysis of these data suggests that the site of dioxygen association with the (2 x 2) Bi unpoisoned surface is the uncoordinated Au atom in the (2 x 2) unit cell.