CO oxidation on nanostructured SnOx/Pt(111) surfaces: unique properties of reduced SnOx

We have investigated surface CO oxidation on "inverse catalysts'' composed of SnOx nanostructures supported on Pt(111) using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS) and temperature-programmed desorption (TPD). Nanostructures of SnOx were prepared by depositing Sn on Pt(111) pre-covered by NO2 layers at low temperatures. XPS data show that the SnOx nanoparticles are highly reduced with Sn(II)O being the dominant oxide species, but the relative concentration of Sn(II) in the SnOx nanoparticles decreases with increasing Sn coverage. We find that the most active SnOx/Pt(111) surface for CO oxidation has smallest SnOx coverage. Increasing the surface coverage of SnOx reduces CO oxidation activity and eventually suppresses it altogether. The study suggests that reduced Sn(II)O, rather than Sn(IV)O-2, is responsible for surface CO oxidation. The occurrence of a non-CO oxidation reaction path involving reduced Sn(II)O species at higher SnOx coverages accounts for the decreased CO oxidation activity. From these results, we conclude that the efficacy of CO oxidation is strongly dependent on the availability of reduced tin oxide sites at the Pt-SnOx interface, as well as unique chemical properties of the SnOx nanoparticles.