CO oxidation on nanostructured SnO _x/Pt(111) surfaces: Unique properties of reduced SnO _x

We have investigated surface CO oxidation on "inverse catalysts" composed of SnO _x nanostructures supported on Pt(111) using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS) and temperature-programmed desorption (TPD). Nanostructures of SnO _x were prepared by depositing Sn on Pt(111) pre-covered by NO ₂ layers at low temperatures. XPS data show that the SnO _x nanoparticles are highly reduced with Sn(ii)O being the dominant oxide species, but the relative concentration of Sn(ii) in the SnO _x nanoparticles decreases with increasing Sn coverage. We find that the most active SnO _x/Pt(111) surface for CO oxidation has smallest SnO _x coverage. Increasing the surface coverage of SnO _x 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 SnO _x 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-SnO _x interface, as well as unique chemical properties of the SnO _x nanoparticles.