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 u pd adlattice on Au(III) has been studied by deliberately poisoning the adla ttice with thiocyanate and ethanethiol during the course of electroreductio n activity. The diminution in reduction activity was monitored using chrono amperometry. For SCN-, the drop in current could be modeled using a Langmui r 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 appr oximately the same. STM images of the surface obtained following introducti on of SCN- revealed a (4 x 4) adlattice, which was partially (6%) defected. The percentage of defects agreed well with the percentage of residual curr ent found at long times (3%) leading us to associate these defects with sit es of catalytic activity. STM images obtained from surfaces poisoned with e thanethiol revealed two lattices: a (8 x 8) structure which was unstable an d a more stable (root 57 x 3) structure which is consistent with an overlay er of thiols lying flat on the surface. IR studies of the SCN--poisoned sur face showed that the SCN- was S-bound to the surface at almost the same ene rgy as that expected from SCN- bound to a bare Au(III) surface. XPS measure ments 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.