Scanning tunnelling microscopy studies of the reactivity of the TiO2(110) surface: Re-oxidation and the thermal treatment of metal nanoparticles

Abstract

We have employed variable temperature scanning tunnelling microscopy (STM) to probe the surface structure of the TiO₂(110) surface with clean, adsorbate and metal covered terminations. The aim of the work is to understand the nature of catalysis on supported metal oxide catalysts for which a good model is an admetal on a single crystal oxide surface. For Pd overlayers, annealing in vacuum shows the formation of metal particles with nanometer sized dimensions, which are comparable to those seen in real catalysts. The clean TiO₂(110) surface has two commonly observed terminations, the (1×1) bulk truncation, the (1×2) reduced and reconstructed surface. Less commonly, for very reduced crystals, the formation of ordered defects occurs leading to crystallographic shear planes. We have explored all of these surfaces by low energy electron diffraction (LEED) and STM to provide structural information, while we have employed dynamic imaging of the surface in reactive conditions at elevated temperature to assess the chemistry. We find that oxygen rich atmospheres promote a re-growth of the surface that has important consequences for the surface chemistry and morphology. The oxidation and reduction of the support in this system has been shown to modify the reactive properties of the supported metal and we relate our observations to the strong metal support interaction (SMSI).