<b>In Situ Characterizations of Nanostructured SnO</b>_<i>x</i><b>/Pt(111) Surfaces Using Ambient-Pressure XPS (APXPS) and High-Pressure Scanning Tunneling Microscopy (HPSTM)</b>

We have conducted in situ measurement of "inverse catalysts" of SnOx nanostructures supported on Pt(111) using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling microscopy (HPSTM) techniques under CO exposure at room temperature and 450 K. Nanostructures of SnOx were prepared by depositing Sn on Pt(111) precovered by O2 layers at liquid nitrogen temperature. APXPS data show that the prepared SnOx nanoparticles are highly reduced, with Sn2+ being the dominant oxide species. The relative Sn2+concentration, compared to Sn4+ and Sn0, in the SnOx nanoparticles decreases slightly with increasing Sn coverage. In situ study of SnOx/Pt(111) inverse catalyst shows that for lower coverage of SnOx (0.25 monolayers (ML)), the amount of Sn2+ decreased steadily, while Sn0 amount steadily increased with negligible Sn4+ amount, as the surface was heated under CO exposure at 450 K. Meanwhile, for the higher coverage (1.0 ML), the decrease of Sn2+ is followed by sharp increase in the amount of Sn4+ and Sn0. HPSTM images show that small islands of SnOx are randomly formed on the substrate, with the size and density increasing with SnOx coverage. HPSTM images show morphology differences between low and high coverages of SnOx on Pt(111) under both UHV and CO exposure conditions.