Preparation and in-Situ Spectroscopic Characterization of Molecularly Dispersed Titanium Oxide on Silica

The molecularly dispersed TiO<INF>2</INF>/SiO<INF>2</INF> 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<INF>2</INF> was reached at ~4 Ti atoms/nm² with a two-step impregnation procedure. The surface structures of the molecularly dispersed TiO<INF>2</INF>/SiO<INF>2</INF> 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<INF>2</INF>/SiO<INF>2</INF> sample (0.24 Ti atoms/nm²) are predominantly found to be isolated TiO<INF>4</INF> units, whereas at maximum surface coverage (~4 Ti atoms/nm²), two-dimensional polymerized TiO<INF>5</INF> 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<INF>4</INF> ≫ polymerized TiO<INF>5</INF>). These new results provide fundamental insights about molecular structure−reactivity/selectivity relationships of the molecularly dispersed TiO<INF>2</INF>/SiO<INF>2</INF> supported oxides.