Morphology and reactivity of size-selected titanium oxide nanoclusters on Au(111).

The morphology and reactivity of mass-selected titania clusters, TiO and TiO, deposited onto Au(111) were studied by scanning tunneling microscopy and temperature programmed desorption. Despite differing by only one oxygen atom, the stoichiometric TiO and the sub-stoichiometric ("reduced") TiO clusters exhibit very different structures and preferred binding sites. The TiO clusters bind at step edges and form small assemblies (2-4 clusters) on Au terraces, while the "reduced" TiO clusters form much larger fractal-like assemblies that can extend across step boundaries. Annealing the TiO/Au(111) systems to higher temperatures causes changes in the size-distributions of cluster assemblies, but does not lead to the formation of TiO nanoislands for temperatures ≤700 K. Reactivity studies show that the reduced TiO cluster has higher activity than TiO for 2-propanol dehydration, although both clusters exhibit substantial activity for dehydrogenation to acetone. Calculations using DFT+U suggest that the differences in aggregate morphology and reactivity are associated with the number of undercoordinated Ti sites in the supported clusters.