We report the results of a systematic study of the catalytic activity of mass-selected vanadium oxide clusters deposited on rutile TiO2 surfaces under ultrahigh vacuum (UHV) conditions. Our results show that supported V, VO, and VO2 clusters are not catalytically active for the oxidative dehydrogenation of methanol to formaldehyde but can be made catalytically active by postoxidation. In addition, we found that the postoxidized VO/TiO2 produces the most formaldehyde. Scanning tunneling microscopy (STM) imaging of the postoxidized VO/TiO2 reveals isolated clusters with height and width indicative of VO3 bound to the TiO2 surface. Our results are consistent with previous density functional theory (DFT) calculations that predict that VO3 will be produced by postoxidation of VO and that VO3/TiO2 is an active catalyst.

We probe the adsorption of molecular H₂O on a TiO₂ (110)-(1 × 1) surface decorated with isolated VO clusters using ultrahigh-vacuum scanning tunneling microscopy (UHV-STM) and temperature-programmed desorption (TPD). Our STM images show that preadsorbed VO clusters on the TiO₂ (110)-(1 × 1) surface induce the adsorption of H₂O molecules at room temperature (RT). The adsorbed H₂O molecules form strings of beads of H₂O dimers bound to the 5-fold coordinated Ti atom (5c-Ti) rows and are anchored by VO. This RT adsorption is completely reversible and is unique to the VO-decorated TiO₂ surface. TPD spectra reveal two new desorption states for VO stabilized H₂O at 395 and 445 K, which is in sharp contrast to the desorption of water due to recombination of hydroxyl groups at 490 K from clean TiO₂(110)-(1 × 1) surfaces. Density functional theory (DFT) calculations show that the binding energy of molecular H₂O to the VO clusters on the TiO₂ (110)-(1 × 1) surface is higher than binding to the bare surface by 0.42 eV, and the resulting H₂O-VO-TiO₂ (110) complex provides the anchor point for adsorption of the string of beads of H₂O dimers.