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Anomalous small-angle X-ray scattering (ASAXS) of synchrotron radiation with X-ray energies near the Pt-L3 X-ray absorption edge was used to examine the size distribution of nanometer-sized catalyst particles in porous electrodes for electrocatalytic applications. Carbon-supported platinum electrocatalysts with 5-80 wt% Pt were studied in situ in an electrochemical cell with a 1 M sulfuric acid electrolyte. The anodic oxidation was found to shift the particle size distribution from a mean size of 1.7 nm in the reduced state to 2.1 nm for a 10 wt% Pt/C catalyst. From the size increase, one can infer the formation of oxide shells at the particle surfaces with layer thicknesses of about 1 nm.
