Beam-Induced Effects on Platinum Oxidation during Ambient-Pressure X-ray Photoelectron Spectroscopy.

Ambient-pressure X-ray photoelectron spectroscopy (APXPS) is commonly used to identify active phases of Pt-based catalysts. Unavoidable beam-induced chemistry under in situ conditions with high-flux X-rays is important yet has often been disregarded. To evaluate beam effects on Pt oxidation, we revisited surface species on Pt(111) and Pt(110) in O environments using APXPS. The observed X-ray-induced phenomena strongly depended on pressure and surface orientation. Below 1 mbar of O, we found only chemisorbed oxygen species on both surfaces. No significant change in Pt(111) was observed with long-time illumination under ≤2 mbar of O. Under ∼5 mbar with similar oxygen exposure, beam-induced oxidation was apparent on Pt(111) with the formation of abundant surface oxide and chemisorbed oxygen. However, such beam-induced oxidation was strongly suppressed on Pt(110). Understanding these "pressure gap" and surface orientation-dependent beam-induced phenomena is essential for our interpretation of the in situ X-ray results, particularly for higher-pressure experiments with brighter synchrotron sources.