Optical reflectance studies on the oxidation of chemisorbed sulfur at the Pt(111) electrode

Abstract

The oxidation of chemisorbed sulfur on Pt(111) was studied in 0.1 M H2SO4 using combined electrochemical and in situ optical reflectance. The adsorbed sulfur adlayer, prepared by immersion of the Pt electrode in Na2S solution, was successively removed by potential cycling between 0.05 and 1.22 V vs. RHE. Correlation of the electrochemical data with the parallel measured changes in optical reflectance provides insight into the intermediate steps of the sulfur oxidation process. In addition to features related to sulfur oxidation and Pt oxide formation and reduction, two transient cathodic current peaks at rather negative potentials (0.19 and 0.41 V) are observed the first cycles, which can be assigned to intermediates of the sulfur oxidation process. The sulfur coverage oxidized in each cycle is initially large but then decreases rapidly, becoming negligibly small after about 10 cycles. Furthermore, a nonlinear change of the reflectance with decreasing sulfur coverage is observed, which suggests changes in the nature of the S-Pt bond with coverage.