STRUCTURE OF BR ADLAYERS IN THE COURSE OF ELECTROCATALYTIC REACTIONS O-2 REDUCTION ON PT(111) AND AU(100)

Structures of Br adlayers were determined on the Pt(111) and Au(100) e lectrodes in the absence and in the presence of O-2, reduction in 0.1 M HClO4, solutions. On Pt(111), the commensurate (3 x 3) Br adlayer wa s found between 0.3 and 0.8 V. On Au(100), the c(root 2x2 root 2)R45 d egrees and c(root 2 x p)R45 degrees phases were found above 0.35 and 0 .68 V, respectively, in agreement with previous work in neutral soluti ons. O-2 reduction is completely inhibited by the c(root 2 x 2 root 2) R45 degrees phase on Au(100). It takes place only at potentials negati ve of the low potential limit of the existence of the c(root 2 x 2 roo t 2)R45 degrees phase. On Pt(111), O-2 reduction is also completely bl ocked by the (3 x 3) Br adlayer up to 0.5 V. At this potential, the or dered phase vanishes and O, reduction takes place. Adsorbed disordered Br adions change the mechanism of O-2 reduction on Pt(111) from a 4e- into a 2e-reaction. (C) 1998 Elsevier Science B.V.