In situ XAS investigation of transformation of Co monolayer on carbon-supported platinum clusters underpotential control

A complete electrochemical oxidation-reduction cycle for a Co monolayer on carbon-supported Pt clusters in alkaline solution has been monitored in situ by X-ray absorption spectroscopy (XAS). The near range order and oxidation state of the Co monolayer as a function of potential has been evaluated by in situ XAS and it was found that the oxidation state of the Co monolayer remains +2 even at a potential of -0.8 V vs Ag/AgCl and becomes +3 at +0.5 V vs Ag/AgCl. No desorption of Co was observed during the oxidation-reduction cycle. Two main peaks in the Fourier transformed Extended X-ray Absorption Fine Structure (EXAFS) spectra were found. The first peak is attributed to the nearest oxygen. The second peak is ascribed to the nearest Co neighbors upon reduction and to the nearest Co and Pt neighbors upon oxidation. It is interesting that the deposited cobalt appears to form a structure similar to that of Co(OH)(2) at the reduced conditions but the incorporation of cobalt oxide into the platinum oxide/hydroxide surface layer was found at the oxidized conditions. The transformation between the reduced Co and the incorporated cobalt oxide is reversible during the reduction-oxidation cycle.