Electrocatalytic dioxygen reduction on underpotentially deposited T1 on Au(111) studied by an active site blocking strategy

Electrochemical measurements and in situ scanning tunneling microscopy (STM ) are performed to establish a structure-reactivity correlation for peroxid e or dioxygen reduction on underpotentially deposited (upd) TI on Au(111) i n 0.1 M HClO4. At the potential of catalytic activity toward the Oz reducti on, STM reveals the presence of Tl islands on the Au(111) terrace with heig ht Of 0.24 +/- 0.03 nm. These islands expand in size and number; as the pot ential becomes more negative, but the full TI monolayer formed at -0.2 V vs NHE is catalytically inactive. Ethanethiol(EtSH) significantly inhibits th e H2O2 reduction, and the kinetics and thermodynamics of EtSH adsorption on the Tl upd are quantitatively analyzed. STM shows that EtSH introduction l eads to the formation of a 0.15 nm high terrace along the edges of the TI i slands. This terrace is assigned to EtSH bound to the Au surface near the T l islands with the alkyl chain oriented roughly perpendicular to the surfac e. These results show that edge sites around the Tl island are the active s ite of catalytic O-2 reduction by Tl upd on Au(111).