The molecularly dispersed V 2 O 5 /SiO 2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of V-isopropoxide. The experimental maximum dispersion of surface vanadium oxide species on SiO 2 was achieved at ∼12 wt % V 2 O 5 (∼2.6 V atoms/nm 2 ). The surface structures of the molecularly dispersed V 2 O 5 /SiO 2 samples under various conditions were extensively investigated by in situ Raman, UVvis-NIR DRS and XANES spectroscopies. The combined characterization techniques revealed that in the dehydrated state only isolated VO 4 species are present on the silica surface up to monolayer coverage. Interestingly, the three-member siloxane rings on the silica surface appear to be the most favorable sites for anchoring the isolated, three-legged (SiO) 3 VdO species. Hydration dramatically changes the molecular structure of the surface vanadium oxide species. The specific structure of the hydrated surface vanadium oxide species is dependent on the degree of hydration. The molecular structure of the fully hydrated vanadium oxide species closely resembles V 2 O 5 ‚nH 2 O gels, rather than V 2 O 5 crystallites. The fully hydrated surface vanadium oxide species are proposed to be chain and/or two-dimensional polymers with highly distorted square-pyramidal VO 5 connected by V-OH-V bridges, which are stabilized on the silica surface by the sixth neighbor of Si-OH hydroxyls via Si-OH‚‚‚V hydrogen bonds. In analogy to the hydration process, alcoholysis occurs during methanol chemisorption, and similar molecular structures are proposed to interpret the interaction between methanol molecules and the surface vanadium oxide species on silica.

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The molecularly dispersed TiO 2 /SiO 2 supported oxides were prepared by the incipient wetness impregnation of 2-propanol solutions of titanium isopropoxide. Experimental monolayer dispersion of surface titanium oxide species on SiO 2 was reached at ∼4 Ti atoms/nm 2 with a two-step impregnation procedure. The surface structures of the molecularly dispersed TiO 2 /SiO 2 under various environments were extensively investigated by in-situ spectroscopic techniques (e.g., Raman, UV-vis-NIR DRS, and XANES) as well as XPS. The combined characterization techniques revealed the consumption of surface Si-OH groups and the formation of Ti-O-Si bridging bonds. In the dehydrated state, the surface Ti atoms in the 1% TiO 2 /SiO 2 sample (0.24 Ti atoms/nm 2 ) are predominantly found to be isolated TiO 4 units, whereas at maximum surface coverage (∼4 Ti atoms/nm 2 ), two-dimensional polymerized TiO 5 units are dominant on the silica surface. The in-situ spectroscopic studies demonstrated that the coordination and ligands of the surface Ti cations change upon hydration/dehydration as well as during methanol oxidation. Methanol oxidation showed that the molecularly dispersed surface titanium oxide species exhibit completely different catalytic behavior (predominantly redox products) compared to bulk titanium oxide (predominantly dehydration products). Furthermore, the TOF of the surface titanium oxide species is strongly dependent on their local structures and varies by 1 order of magnitude (isolated TiO 4 . polymerized TiO 5 ). These new results provide fundamental insights about molecular structure-reactivity/selectivity relationships of the molecularly dispersed TiO 2 /SiO 2 supported oxides.