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.
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