Surface chemistry and catalysis on well-defined oxide surfaces: nanoscale design bases for single-site heterogeneous catalysts

As the surface science of metal oxides has developed over the past two decades, it has made increasing contributions to the understanding of the site requirements for catalysis by oxides. Metal oxide single crystals and thin films represent the systems most often studied by surface science techniques as models of oxide catalysts. We explore an alternative approach—molecular functionalization of surfaces with ordered arrays of discrete, reactive oxide molecules. Heteropolyanions (polyoxometalates) can be deposited to form ordered monolayers that permit site-by-site mapping of chemical functions on the surface, as well as characterization of redox properties of individual molecules by tunneling spectroscopy with the scanning tunneling microscope. These nanoscale oxide clusters exhibit negative differential resistance in their tunneling spectra at potentials that track their reduction potentials. Thus tunneling spectroscopy measurements may provide correlation and prediction tools for catalyst performance in selective oxidation processes. Because polyoxometalate monolayers present uniform catalytic sites whose redox properties can be defined by single molecule spectroscopy, they may serve as a prototype of single site heterogeneous catalysts designed and fabricated on the nanoscale.