Nitrate Reduction Catalyzed by Underpotentially Deposited Cd on Au(111):  Identification of the Electroactive Surface Structure

The electrocatalysis of the nitrate reduction to nitrite by Cd underpotential deposition (upd) on Au(111) was studied. The structures of the Cd adlayer underpotentially deposited on a Au(111) surface with nitrate in acidic sodium sulfate electrolyte were investigated by scanning tunneling microscopy (STM). Several ordered adlattices were observed, with the catalytically active structure being a (1 × 1) Cd adlayer. This adlayer exhibited a Moiré pattern rotated by 9° from the Au(111) lattice direction. Thiocyanate and ethanethiol were used to investigate the mechanism of the catalysis by the Cd adlayer. The addition of thiocyanate did not affect the nitrate reduction activity. On the other hand, ethanethiol was a poison toward the nitrate reduction. STM images revealed surface roughening due to ethanethiol at the potential of the reduction of nitrate. We suggest that nitrate association with the (1 × 1) Cd adlattice is a requirement for electroreduction activity.