Formation of Nanometer-Sized Surface Platinum Oxide Clusters on a Stepped Pt(557) Single Crystal Surface Induced by Oxygen: A High-Pressure STM and Ambient-Pressure XPS Study

We studied the oxygen-induced restructuring process on a stepped Pt(557) single crystal surface using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) at O-2 pressures up to 1 Torr. HP-STM has revealed that nanometer-sized clusters are created on Pt(557) at 1 Torr of O-2 and at room temperature. These clusters are identified as surface Pt oxide by AP-XPS. The appearance of clusters is preceded by the formation of 1D chain structures at the step edges. By using a Pt(111) surface as a reference, it was found that the step sites are the nucleation centers for the formation of surface oxide clusters. These surface oxide clusters disappear and the stepped structure is restored on Pt(557) after evacuating O-2 to 10(-8) Torr. Changes in the surface oxide concentration in response to variations in the O-2 gas pressure are repeatable for several cycles. Our results that small clusters are initiated at step sites at high pressures demonstrate the importance of performing in situ characterization of stepped Pt catalysts under reaction conditions.