In-situ investigation of atomic-scale structure of electrochemical dioxygen reduction catalyst

Abstract

Electrochemical measurements and in situ scanning tunneling microscopy (STM) are performed to establish a structure-reactivity correlation for peroxide or dioxygen reduction on underpotentially deposited (upd) Bi, Tl, and Pb on Au(111) in 0.1 M $HClO_4$. The four-electron electroreduction of dioxygen to water on the Bi upd adlattices has been studied by deliberately poisoning the adlattices during the course of electroreduction activity with thiocyanate and ethanethiol. The diminution in reduction activity was monitored using chronoamperometry. For $SCN^-$ to Bi upd, the drop in current could be modeled using a Langmuir kinetic expression yielding a rate constant for adsorption of $1.1×10^4 s^{-1}M^{-1}$. The rate for ethanethiol could not be measured but must be at least two orders of magnitude faster. STM images of the surface obtained following introduction of $SCN^-$ revealed a (4×4) adlattice, which was partially (6%) defected. The percentage of defects agreed well with the percentage of residual current found at long times (4%) leading us to associate these defects with sites of catalytic activity. STM images obtained from surfaces poisoned with ethanethiol revealed two lattices: a (8×8) structure which was unstable and a more stable ($\surd 57×3$) structure which appeared to exhibit 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 Au(111). 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 (2×2)-Bi unpoisoned surface is the lone uncoordinated Au atom in the (2×2) unit cell. With respect to the Tl upd system, at the potential of catalytic activity toward the $O_2$ reduction, STM reveals the presence of Tl islands on the Au(111) terrace with height of 0.24 ± 0.03 nm. These islands expand in size and n...