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

The morphology and reactivity of mass-selected titania clusters, Ti₃O₆ and Ti₃O₅, deposited onto Au(111) were studied by scanning tunneling microscopy and temperature programmed desorption. Despite differing by only one oxygen atom, the stoichiometric Ti₃O₆ and the sub-stoichiometric ("reduced") Ti₃O₅ clusters exhibit very different structures and preferred binding sites. The Ti₃O₆ clusters bind at step edges and form small assemblies (2-4 clusters) on Au terraces, while the "reduced" Ti₃O₅ clusters form much larger fractal-like assemblies that can extend across step boundaries. Annealing the Ti₃O_5,6/Au(111) systems to higher temperatures causes changes in the size-distributions of cluster assemblies, but does not lead to the formation of TiO_x nanoislands for temperatures ≤700 K. Reactivity studies show that the reduced Ti₃O₅ cluster has higher activity than Ti₃O₆ 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_3c sites in the supported clusters.