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 use d to monitor changes in the Pt charge and the number of O and Pt neare st neighbors during the electrochemical oxidation and reduction of a d ispersed 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 featu res (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 s imilar time-scale with N-Pt changing at half the rate at which N-O and absorption peak are changing. When combined with the direct quantitat ive structural information obtained from XAFS, we find that these reac tions on clusters apparently proceed by a different mechanism from tha t on bulk platinum electrodes in aqueous solution and also suggest a m echanism for the platinum restructuring during these reactions. Theref ore the behavior of the clusters may differ from that of bulk electrod e surfaces.