The oxidation and reduction of an Ir(100) surface using 2.5, 5, and 10 mbar O2partial pressure and a sample temperature of 775 K have been studied by using high-energy surface X-ray diffraction (HESXRD) which allowed to record large volumes of reciprocal space in short time periods. The complex 3D diffraction patterns could be disentangled in a stepwise procedure. For the 2.5 mbar experiment the measurements indicate the formation of an Ir(100)-O c(2 × 2) oxygen superstructure along with the onset of epitaxial IrO2(110) bulk oxide formation. For the 5 and 10 mbar O2partial pressures the formation of additional IrO2bulk oxide epitaxies with (100) and (101) orientations as well as of polycrystalline IrO2was observed. Upon CO reduction, we found the IrO2islands to be reduced into epitaxial and metallic Ir(111) and (221) oriented islands.
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