Adlayer Structure and Electrochemical Reduction of O2 on Self-Organized Arrays of Cobalt and Copper Tetraphenyl Porphines on a Au(111) Surface

Abstract

Adlayers of 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) and copper(II) (CoTPP and CuTPP) formed on a Au(111) electrode by immersion into a benzene solution containing either CoTPP or CuTPP were investigated in 0.1 M HClO4 by cyclic voltammetry and in situ scanning tunneling microscopy (STM). Highly ordered arrays of CoTPP and CuTPP molecules were observed on the Au(111) surface. High-resolution STM images revealed the characteristic shape, internal molecular structure, and molecular orientation of individual CoTPP and CuTPP molecules in ordered arrays. The adlayers of CoTPP and CuTPP formed on Au(111) were in an identical packing arrangement with the same orientation. The center cobalt ion in CoTPP appeared as the brightest spot in the STM image, whereas the copper ion in CuTPP was dark. On the adlayers of highly ordered CoTPP and CuTPP molecules, O2 reduction was carried out in 0.1 M HClO4 saturated with O2. Voltammetric results indicated that CuTPP did not catalyze the reaction, whereas the highly ordered CoTPP adlayer on Au(111) enhanced the two-electron reduction of O2 to H2O2.