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.

The use of in situ Raman spectroscopy to study the molecular structures of supported metal oxide catalysts under different environments is reviewed. The molecular structures under ambient (hydrated) and dehydrated conditions are presented. The effect of moisture at elevated temperatures is also presented and discussed with regard to its implications for catalytic phenomena. The molecular structural transformations during C 2 -C 4 lower alkane (LPG) oxidation, methane oxidation, methanol oxidation and selective catalytic reduction of NO with NH 3 reaction conditions are presented. In situ spectroscopy during catalytic reaction with simultaneous activity/selectivity measurement ('operando' spectroscopy) is emphasized owing to its contribution to the fundamental understanding of catalytic performance. The reducibility of the different surface metal oxide species, the relevance of surface coverage (surface monomeric vs polymeric species) and the specific oxide support are discussed when LPG, methane, methanol or hydrogen is the reducing agent. In situ Raman spectroscopy provides molecular-level information about the surface metal oxide species: structures, stability and transformations under different environments. In many cases, the use of complementary spectroscopic techniques results in a more complete understanding of the molecular structure-activity/selectivity relationships for supported metal oxide catalysts.

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A Ce 2 Zr 2 O 8 mixed oxide was prepared, and the influence of redox treatments on its reduction behavior was systematically analyzed. The temperature of reduction was found to change on application of redox cycles under severe conditions, with the final appearance of temperature-programmed reduction profiles dictated by the exact conditions employed in the redox cycle (temperature of treatment and concentration of H 2 ). In addition, the conditions necessary in the reducing and oxidizing parts of the redox cycle showed interdependence. The reduction was found to vary from a single high-temperature peak to a single lowtemperature peak, via double-peak profiles with various relative intensities. Structural analysis of the sample after application of analogous treatments indicated that the reduction behavior follows a pathway, with t′ and κ phases at the two extremes (high-and low-temperature single-peak profiles). The latter phase contains pyrochlore-type cation ordering. However, bulk ordering is not necessary to promote low-temperature reduction. The evidence suggests that the two-peak profiles are observed when cation ordering is very limited, and a single low-temperature peak is observed when domains of the κ phase coexist with larger amounts of the t′ phase. Thus, even the presence of partial ordering at the surface is sufficient to produce an effect.

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