DIRECT OBSERVATION OF SURFACE OXIDE FORMATION AND REDUCTION ON PLATINUM CLUSTERS BY TIME-RESOLVED X-RAY-ABSORPTION SPECTROSCOPY

Dispersive X-ray absorption fine structure (XAFS) spectroscopy was used to monitor changes in the Pt charge and the number of O and Pt nearest neighbors during the electrochemical oxidation and reduction of a dispersed Pt catalyst in real time. The oxidation at 1.20 V/SHE follows logarithmic kinetics over a period of 5 min with all three XAFS features (N-O, N-Pt and the absorption peak) changing identically. However, the reduction at 0.10 V is well fitted by a single exponential on a similar time-scale with N-Pt changing at half the rate at which N-O and absorption peak are changing. When combined with the direct quantitative structural information obtained from XAFS, we find that these reactions on clusters apparently proceed by a different mechanism from that on bulk platinum electrodes in aqueous solution and also suggest a mechanism for the platinum restructuring during these reactions. Therefore the behavior of the clusters may differ from that of bulk electrode surfaces.